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Poling and Characterization of Nonpolar and Polar Polymersfor Electromechanical and Optical Applications

Dissertation zur Erlangung des akademischen Grades Doktor der technischen

Wissenschaften (Dr. techn.)

in der Studienrichtung Technische Physik

Angefertigt am Institut für Experimentalphysik

Betreuung:

Prof. Dr. Siegfried Bauer

Von:

DI Michael Lindner

Gutachter

Prof. Dr. Günther Bauer

Linz, Jänner 2003

„Aufgabe der Naturwissenschaft ist es nicht nur, dieErfahrung zu erweitern, sondern in diese Erfahrung eine

Ordnung zu bringen.“

Nils Bohrdänischer Physiker (1885 - 1962)

Contents I

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Kurzreferat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Part I: Optical Characterization of Nonlinear Optical (NLO) Polymers . . 5

1. Amorphous Dipole Electrets . . . . . . . . . . . . . . . . . . . . . . . 71.1 Polymeric Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2 Dielectric Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.3 Piezo- and Pyroelectricity . . . . . . . . . . . . . . . . . . . . . . . . 101.4 Linear and Nonlinear Optical Properties . . . . . . . . . . . . . . . . . 11

2. Cold Poling Methods Probed With Second Harmonic Generation (SHG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1 Photoinduced Poling (PIP) . . . . . . . . . . . . . . . . . . . . . . . . 152.2 Nonlinear Optical Bimorphs . . . . . . . . . . . . . . . . . . . . . . . 172.3 Memory Poling (MP) . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.4 Surface Deformation after Poling . . . . . . . . . . . . . . . . . . . . . 19

3. Experimental Results and Discussion . . . . . . . . . . . . . . . . . 213.1 Efficiency of Memory Poling (MP) . . . . . . . . . . . . . . . . . . . . 213.2 Temperature Dependence of Photoinduced Poling (PIP) . . . . . . . . . 243.3 Depth Controlled Poling of Bimorph Structures . . . . . . . . . . . . . 263.4 Formation of Surface Gratings . . . . . . . . . . . . . . . . . . . . . . 28

4. Conclusion of Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . 35References of Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Part II: Plasma-Induced Charging of Nonpolar Cellular Materials . . . . . 41

5. Charge Electrets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.1 Fluorinated and Non-Fluorinated Polymers . . . . . . . . . . . . . . . . 435.2 Polymer Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.3 Artificial Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.4 Dielectric Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.5 Piezo- and Pyroelectricity in Nonpolar Cellular Electrets . . . . . . . . . 48

6. Dielectric Barrier Microdischarges . . . . . . . . . . . . . . . . . . . 516.1 “Cold” Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516.2 Paschen Breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556.3 Charging of Cellular Polymer Electrets . . . . . . . . . . . . . . . . . . 55

7. Experimental Results and Discussion . . . . . . . . . . . . . . . . . . 577.1 Preparation of Cellular Electret Systems . . . . . . . . . . . . . . . . . . 577.2 Time-Resolved Optical Detection of Discharges . . . . . . . . . . . . . . 587.3 Characterization of Breakdown Events . . . . . . . . . . . . . . . . . . . 62

II Contents

7.4 Temperature Dependence of the Charge Stability . . . . . . . . . . . . . 63

8. Conclusion of Part II . . . . . . . . . . . . . . . . . . . . . . . . . . 65References of Part II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Part III: Ferroelectric-Like Effects in Nonpolar Polymers . . . . . . . . . 69

9. Semicrystalline Dipole Electrets . . . . . . . . . . . . . . . . . . . . 719.1 Crystal Structure and Ferroelectricity . . . . . . . . . . . . . . . . . . 719.2 Dielectric Properties of Ferroelectric Polymers . . . . . . . . . . . . . . 749.3 Piezo- and Pyroelectricity . . . . . . . . . . . . . . . . . . . . . . . . . 759.4 Nonlinear Optical Properties of Ferroelectric Polymers . . . . . . . . . 76

10. Techniques for the Investigation of Ferroelectric Materials . . . . . 7710.1 Dielectric Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7710.2 Electromechanical Effects . . . . . . . . . . . . . . . . . . . . . . . . . 7910.3 Nonlinear Optical (NLO) Effects . . . . . . . . . . . . . . . . . . . . . 80

11. Experimental Results and Discussion . . . . . . . . . . . . . . . . . . 8311.1 Hysteresis in the Dielectric Function . . . . . . . . . . . . . . . . . . . 8311.2 Switching of the Electromechanical Effect . . . . . . . . . . . . . . . . 8611.3 Switching of the Nonlinear Optical (NLO) Effects . . . . . . . . . . . . 9011.4 The Electret Microphone Reconsidered . . . . . . . . . . . . . . . . . . 94

12. Conclusion of Part III . . . . . . . . . . . . . . . . . . . . . . . . . . 97References of Part III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Appendix - Material Data . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Own Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Curriculum Vitae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108Eidesstattliche Erklärung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Abstract - Kurzreferat 1

Abstract

Functional polymers have emerged as novel materials for high-technologyapplications in several fields, like transducing devices, optical data storage, andphotonics. An important class of functional polymers are polymer electrets, dielectricmaterials exhibiting a quasipermanent electric charge. Polymer electrets can bedivided into charge electrets with real charges and dipole electrets with polarizationcharges due to molecular dipoles. These materials have found applications inelectronics, sensors, and photonics due to their piezo- and pyroelectric as well asnonlinear optical properties. In this work several methods and techniques for electret formation, such as chargingand poling and characterization have been developed and applied to selected modelsystems.In the first part, amorphous polymers with polar molecular dipoles are investigated.Such polymers are employed wherever large second-order nonlinear optical propertiesare required. Special emphasis is given on the “cold” photoinduced poling process.The experimental results obtained with second-harmonic generation of light wereinterpreted in the frame of a modified Eisenbach free-volume model. Based on thefree-volume model, predictions on the thermal stability after “cold” poling can beenvisaged. The results might have practical implications in the preparation ofnonlinear optical devices. Nonpolar cellular polymers with large piezoelectric properties are the topic of thesecond part of this thesis. Internal charging of the air-filled voids is shown to arisefrom dielectric barrier microdischarges. As a consequence of the internal charges,analogies to ferroelectric materials can be found.The similarities of charged nonpolar cellular electrets to ferroelectric materials aredemonstrated and discussed in the third part of this work. Here, it is shown thattypical effects known from ferroelectric materials can be also found in chargedpolymer foams, examples are hysteresis loops in displacement, electromechanicalstrain and optical second-harmonic generation versus applied voltage. Due to theresults presented in this thesis, cellular polymers are suggested to be called“ferroelectrets” and the material properties “ferroelectretic” henceforth.By reconsidering electret microphones, ferroelectretic behavior has been found inhysteresis loops in dielectric displacement and electromechanical strain. These resultsmay pave the way to micromachined electrostatic devices for air-borne ultrasound.

2 Abstract - Kurzreferat

Kurzreferat

Funktionspolymere haben sich als neuartige Materialien für hoch-technologischeAnwendungen in Bereichen der Sensorik und Aktorik, optischen Datenspeicherungund Photonik erwiesen. Eine wichtige Klasse von Funktionsmaterialien sindPolymerelektrete, dielektrische Materialien mit einer quasipermanenten elektrischenLadung. Polymerelektrete können in Ladungselektrete mit echten Ladungen undDipolelektrete mit Polarisationsladungen auf Grund molekularer Dipole unterteiltwerden. Diese Materialien haben wegen ihrer piezo- und pyroelektrischen bzw.nichtlinear optischen Eigenschaften Anwendungen in der Elektronik, Sensorik undPhotonik gefunden.In dieser Arbeit wurden verschiedene Polungs-, Aufladungs- undCharakterisierungsmethoden für die Erzeugung von Elektreten entwickelt und dieseauf ausgewählte Modellsysteme angewendet.Im ersten Teil wurden amorphe Polymere mit molekularen Dipolen untersucht. DiesePolymere werden dort eingesetzt, wo große nichtlineare optische Eigenschaftenzweiter Ordnung benötigt werden. Der Schwerpunkt in diesem Teil der Arbeit liegtauf der Betrachtung der “kalten” lichtinduzierten Polung. Die experimentellenErgebnisse - ermittelt mit Frequenzverdopplungsexperimenten - wurden im Rahmeneines modifizierten Eisenbach Modells für das freie Volumen interpretiert. Mit diesemModell konnten Informationen über die thermische Stabilität nach der “kalten”Polung gegeben werden. Die Ergebnisse könnten praktische Einflüsse in derHerstellung nichtlinearer optischer Bauteile haben.Unpolare zellulare Polymere mit großen piezoelektrischen Eigenschaften sind dasThema des zweiten Teiles der Arbeit. Die interne Aufladung der mit Luft gefülltenHohlräume lässt sich mit dielektrisch behinderten Mikroentladungen verstehen. AlsKonsequenz dieser internen Aufladung lassen sich Analogien zu ferroelektrischenMaterialien finden.Die Ähnlichkeiten zwischen geladenen, unpolaren zellularen Elektreten undferroelektrischen Materialien werden im dritten Teil dieser Arbeit aufgezeigt unddiskutiert. Es wird demonstriert, dass für Ferroelektrika typische Effekte auch ingeladenen Polymerschäumen gefunden werden können. Beispiele sind Hysteresen inder dielektrischen Verschiebung, mechanischen Verformung undFrequenzverdopplung als Funktion der angelegten Spannung. Auf Grund der Erkenntnisse aus dieser Arbeit könnten zellulare Polymere durchausals “Ferroelektrete” und deren Eigenschaften “ferroelektretisch” bezeichnet werden.Bei der Betrachtung von Elektretmikrofonen wurde ferroelektretisches Verhaltendurch dielektrische und elektromechanische Hysteresen nachgewiesen. Dadurchöffnen sich Wege für die Entwicklung mikromechanischer elektrostatischer Bauteilefür den luftgestützten Ultraschall.

Introduction 3

Introduction

Within the last 40 years, polymer materials have played an active role in hightechnology applications ranging from semiconductor technology, sensors, optics,photonics, and data storage devices. Polymers have received increased attention sincethey can be easily fabricated and processed into any desired shape. One example for aclass of high technology polymers are polymer electrets, which are dielectricsexhibiting either oriented dipoles or quasi-permanently stored real charges. Different polymer electret concepts have been identified for a large variety ofapplications, ranging from sensors and actuators to photonics. Polymer electrets range from amorphous polymers for nonlinear optical applications[Shi00] to semicrystalline, nonpolar [Jacobs01] or polar [Nalwa95, Zhang98,Zhang02] polymers for sensors and actuators. Material properties, which are relevantfor these applications, are for example piezo- and pyroelectricity, ferroelectricity, andsecond-order nonlinear optical effects. In order to generate these effects, polymershave to be functionalized by poling, this means, they have to be charged or apermanent parallel orientation of polar groups has to be ensured. In this work, different techniques for poling and the characterization of materialproperties are demonstrated on selected polymer systems in order to functionalizepolymer electrets into electromechanical sensors and actuators as well as intoelectrooptical and photonic devices. Three distinct polymer classes have been used asexamples for the various properties and application fields of polymer electrets: (i)amorphous polar systems, (ii) cellular nonpolar materials, and (iii) ferroelectricpolymers.

In the first part of this work, amorphous polar polymers are investigated, containingmolecular dipoles (chromophores). These materials are suitable for photonicapplications which are based on second-order nonlinear optical effects, such aselectrooptical modulators, multiplexers, spatial light modulators, frequency doublersand mixers, and optical data storage devices. The required symmetry breaking withpermanently oriented dipole molecules is generated by poling processes [Bauer-Gogonea01]. Usually, poling is performed by heating the polymer above its glasstransition temperature where the dipoles become mobile. Orientation of thechromophores along a preferential direction is achieved by an external electric field,and a permanent orientation is established by decreasing the temperature below theglass transition temperature where the oriented dipoles become frozen. Other polingtechniques, which can be performed below the glass transition temperature - so-called“cold” poling processes - are based on the optical excitation of the chromophores.Cold poling is investigated by means of second-harmonic generation. The opticalexcitation of the chromophores below the glass transition temperature results in achange of the shape of the chromophores and induces memory effects in the polymer.Issues addressed are the achieved polar order, as well as the thermal and temporalstability of the dipolar orientation.

In the second part of the thesis, the attention is drawn on nonpolar cellular polymers.These materials have recently been found to exhibit surprisingly large dynamicpiezoelectric responses and therefore emerge as a novel class of polymers forelectromechanical sensors and actuators [Neugschwandtner00, Peltonen00].Functionality in this polymer class is shown to be achieved by internal chargingarising from dielectric barrier microplasma discharges within the voids of the cellular

4 Introduction

polymers. Here, the charging process is investigated, methods for the characterizationare introduced, and the electromechanical properties are determined.

In the third part of the work, striking analogies between nonpolar cellular polymersand polar ferroelectric polymers are identified and discussed. Properties offerroelectric polymers like hysteresis in displacement, electromechanical strain andsecond-harmonic generation are compared with experimental results obtained fromcellular electret systems. The results in this section are surprising, since ferroelectric-like behavior would have been (in the past) excluded to be obtained in nonpolarmaterials. Therefore, even daily-life technological devices like electret microphonescan be reconsidered in the frame of these results. This might yield new devices, likemicromachined, electrostatically-driven electromechanical transducers.

References:

[Bauer-Gogonea01] S. Bauer-Gogonea and S. Bauer, Polymer electrets forelectronics, sensors, and photonics, Handbook ofAdvanced Electronic and Photonic Materials and DevicesVol. 10, pp. 185-231, Academic Press (2001).

[Jacobs01] H. O. Jacobs and G. M. Whitesides, Submicrometerpatterning of charge in thin-film electrets, Science Vol.291, pp. 1763-1766 (2001).

[Nalwa95] H. S. Nalwa (Ed.), Ferroelectric Polymers: Chemistry,Physics and Applications, Marcel Dekker, New York(1995).

[Neugschwandtner00] G. S. Neugschwandtner, R. Schwödiauer, M. Vieytes, S.Bauer-Gogonea, S. Bauer, J. Hillenbrand, R. Kressmann,G. M. Sessler , M. Paajanen, and J. Lekkala, Large andbroadband piezoelectricity in smart polymer-foam space-charge electrets, Appl. Phys. Lett. Vol. 77, pp. 3827-3829(2000).

[Peltonen00] J. Peltonen, M. Paajanen, and J. Lekkala, Determination ofthe actuator sensitivity of electromechanical polypropylenefilms by atomic force microscopy, J. App. Phys. Vol. 88,pp. 4789–4793 (2000).

[Shi00] Y. Shi, C. Zhang, H. Zhang, J. H. Bechtel, L. R. Dalton, B.H. Robinson, and W. H. Steier, Low (Sub-1-Volt) HalfwaveVoltage Polymeric Electro-optic Modulators Achieved byControlling Chromophore Shape, Science Vol. 288, pp.119-122 (2000).

[Zhang98] Q. M. Zhang, Vivek Bharti, and X. Zhao, GiantElectrostriction and Relaxor Ferroelectric Behavior inElectron-Irradiated Poly(vinylidene fluoride-trifluoroethylene) Copolymer, Science Vol. 280, pp. 2101-2104 (1998).

[Zhang02] Q. M. Zhang, H. Li, M. Poh, F. Xia, Z.-Y. Cheng, H. Xu,and C. Huang, An all-organic composite actuator materialwith a high dielectric constant, Nature Vol. 419, pp. 284-287 (2002).

M. LindnerPoling and Characterization of Nonpolar and Polar Polymers for Electromechanical and Optical

ApplicationsPhD Thesis

_____________________________________________________________________

Part I

Optical Characterization of NonlinearOptical (NLO) Polymers

Part I. Optical Characterization of Nonliner Optical (NLO) Polymers 7

Chapter 1

Amorphous Dipole Electrets

Amorphous polymers are attractive for photonic applications, like integrated opticalwaveguide devices or polymer optical fibres [Bauer-Gogonea01, Chemla87, Zyss94].They are highly transparent and especially exhibit transparency windows at therelevant telecommunication wavelengths. Two classes of polymer electrets areemployed: nonpolar or weakly polar materials (as cladding layers and in passivephotonic devices) and strongly polar polymers for second-order nonlinear opticalapplications. The nonpolar materials exhibit a low dielectric constant and are therefore alsoinvestigated as low-k polymers in semiconductor industry.Polar photonic polymers contain dipolar molecules with delocalized π-electronsystems (usually chromophores) [Burland94]. The functionality in this polymer classis obtained by poling, i. e. by the noncentrosymmetric, quasi-permanent orientation ofthe dye molecules in the polymer matrix [Bauer-Gogonea96]. Additionally, thesematerials became recently interesting in the technologically important areas of opticaldata storage and burnable CD-ROMs [Coufal00].

1.1 Polymeric Systems

Fig. 1.1 shows, how polar chromophore molecules can be linked with the polymermatrix [Bauer-Gogonea95]. Large dipole moments are achieved by using strong donor(D) and acceptor (A) groups and by enlarging the conjugation length of thedelocalized π-electron system (A-π-D chromophores). Typical examples are azo- orstilbene dyes.

guest-host side-chain main-chain cross-linking

Figure 1.1: Amorphous polymers for optical applications contain dipolar dye molecules exhibiting largeoptical nonlinearities [Bauer-Gogonea95].

Guest-host polymers are systems where the chromophore is simply dissolved in thepolymer matrix. Common host materials are for example polymethylmethacrylate(PMMA), polycarbonate (PC) or polyimide (PI). Problems with guest-host systemsare the limited solubility of NLO molecules, phase separation, and the relatively lowthermal stability of the dipole orientation.

8 Chapter 1. Amorphous Dipole Electrets

In side-chain polymers, the chromophores are attached via covalent bonds with thepolymer matrix, which enables larger dipole densities and enhanced stability oforiented dipoles due to the covalent binding. Chemistry has identified a large numberof side-chain polymer systems, one example is poly(styrene-maleic-anhydride) withattached disperse-red 1 azo-dyes (P(S-MA)-DR1) [Ahlheim94]. A problem with side-chain polymers is the possible appearance of liquid crystalline phases due to themesogenic groups within the side-chain.In contrast to side-chain polymers, the NLO dipoles in main-chain polymers aredirectly located within the polymer matrix. For this type of polymeric system both theorientation and the relaxation of the chromophore molecules is more difficult to beachieved. During poling, the motion of large segments of the matrix is required for thenoncentrosymmetric orientation of the chromophores. Moreover, liquid crystallinephases may also occur in main-chain systems.In the case of cross-linking polymers, cross-linking of polymer chains is initiatedeither by heating processes or by photochemical reactions. The objective of the cross-linking process is to reduce the mobility of dye molecules which is equivalent to anincrease of the glass-transition temperature ( gT ) of the material concerned. The dipole

orientation must be performed during the cross-linking process because thechromophores become immobile after cross-linking. Usually, simple guest-hostsystems with reactive groups at the matrix and at the chromophores can be used forcross-linking polymers. Cross-linked side-chain polymers usually achieve an evenbetter long-term stability of oriented dipoles in comparison to cross-linked guest-hostsystems. An example of a cross-linking polymer - Red-Acid-Magly - can be found in[Liang94].The measurements and experimental results discussed in Chapter 2 and 3 are based onthe side-chain polymer poly(styrene-maleic-anhydride) P(S-MA)-DR1, producedsynthetically in a polymer-analogue reaction by SANDOZ Optoelectronics. The 9511-type of this class of amorphous polymers has a dye content of 93 mol-% and a glass-rubber-transition temperature of 137°C. Its chemical structure is illustrated in Fig. 1.2[Ahlheim94] including the backbone, the linker (C3H6), and the chromophore.

Figure 1.2: Chemical structure of the side-chain polymer P(S-MA)-DR1; after [Ahlheim94].

* CH2 CH CH CH *

C

NOC

O

C3H6

N C2H5

CH3

N

N

n

NO2

Hauptkette

Verbindungsglied

Farbstoffdye

linker

backbone


1.2 Dielectric Function

Poled NLO polymers are glass-forming systems and exhibit different relaxationprocesses. Relaxation phenomena in polymers are often labelled with Greek letters.The α -relaxation is strongly connected with the glass-transition and occurs at thehighest temperature for a given frequency or at the lowest frequency for a giventemperature. Further relaxation processes ( β , γ , …) occur in the glassy region below

gT and are related to molecular motions of the polymer side-chains. In this work the

discussion concentrates mainly on the α -relaxation process.An important material property for the characterization of NLO polymers is thedielectric function. The dipolar relaxation in the dielectric function is described by thedielectric relaxation strength 1ε∆ , which is defined as the increment of the dielectricpermittivity during the transition from the glassy into the rubber state and is given bythe Kirkwood-Fröhlich equation [McCrum91]:

kT

gNu

ur

rur

0

20

2

1 33

2

2

3

εµε

εεεεεε

+

+=−=∆ (1.1)

Here, rε and uε are the permittivitiy at low and high frequencies or temperatures,

respectively. N is the number of dipoles, 0µ is the dipole moment of the dipolar

groups, and g is a correlation function based on the cosine of the orientation anglebetween the dipoles.The temperature and frequency dependence of the dielectric function is described by arelaxation process characterized by the phenomenological Havriliak-Negami (HN)-function [Havriliak67]:

qpu Tii

]))((1['''1 ωτ

εεεεε

+∆

+=−= (1.2)

'ε and ''ε are the real and imaginary part of the complex permittivity, )(Tτ is thetemperature-dependent mean relaxation time, ω is the angular frequency, and p , qare parameters describing the width and asymmetry of the dielectric loss peak ''ε .The mean relaxation time is strongly non-Arrhenius and can be described by theAdam-Gibbs theory [Kaatz96, Winkelhahn96]:

−=

)/1(exp)(

2 fTTT

BATτ (1.3)

where A is a pre-exponential time factor, B an activation temperature, 2T is the

“static” glass-transition temperature and fT is the so-called fictive temperature which

describes the deviation of the glass from the equilibrium state. Above gT , the fictive

temperature is equal to the sample temperature ( TT f = ) and Eq. (1.3) reduces to the

Vogel-Fulcher-Tamann-Hesse (VFTH) expression [McKenna89]:


−

=2

exp)(TT

BATτ (1.4)

In the vicinity of gT , where the polymer is in a non-equilibrium state, the fictive

temperature deviates from the actual sample temperature and thus, the meanrelaxation time deviates from the VFTH expression. Finally, well below gT , the

fictive temperature reaches a final limiting value above 2T and the Adam-Gibbsexpression Eq. (1.3) reduces to an Arrhenius relation [Cheng98]. In the time domain, dipole relaxation processes are often described by thephenomenological Kohlrausch-Williams-Watts function (KWW or stretchedexponential function) [Williams85]:

p

T

tt

−=Φ

)(exp)(

τ (1.5)

where p is the stretching factor between 0 and 1.

The dielectric relaxation strength ε∆ is a measure for the polarization that can beobtained during the poling process, e. g. for the orientation of the chromophoremolecules. Usually, an electric field pE is applied above the glass-transition

temperature where the chromophores are mobile, thereby a polarization

( ) pEP εεε ∆+= ∞0 (1.6)

is introduced. Upon cooling below gT with the field pE applied, a frozen polarization

PEP 10 εε ∆= (1.7)

is obtained after removing the electric poling field [Mopsik75].

1.3 Piezo- and Pyroelectricity

After poling, amorphous dipole electrets become piezo- and pyroelectric. The directpiezoelectric effect is the charge response of the material upon a change in themechanical stress, whereas the pyroelectric effect is the charge response upon achange in temperature. Although the piezoelectric and pyroelectric coefficients aredefined as derivatives of the dielectric displacement, it is quite common in polymerphysics to adopt the definition

Ξ∂∂

=Q

A

1ξ (1.8)


where ζ is either the piezo- or pyroelectric coefficient, Ξ is the pressure ortemperature, and A and Q are the electrode area and the charge on the measuringelectrode, respectively. In a simple view, the piezo- and pyroelectric effects describethe pressure and the temperature dependence of the frozen polarization, since the maincontribution to these effects arise from the dipole density change upon compression orthermal expansion. Generally, piezoelectricity is represented by a third-rank tensor( 33d in contracted notation), while pyroelectricity is a vector quantity ( 3p ). The

subscript 3 denotes the direction of the poling field. The coefficients 33d and 3p are

directly proportional to the frozen polarization P. Typical values for the 33d

coefficient of NLO polymers are between 1 and 10 pC/N and therefore, these polymersystems are less suitable for piezoelectric or pyroelectric applications, since thecoefficients are too small to be of practical interest. In Part II and III, classes ofpolymers will be presented which show much larger piezoelectric responses requirede. g. for sensor applications.

1.4 Linear and Nonlinear Optical Properties

In linear optics, the dielectric polarisation of a medium iP is linearly related to the

electric field jE .

jiji EP χε 0= (1.9)

where ijχ are the components of the susceptibility tensor χ . In the nonlinear regime,

Eq. (1.9) has to be replaced by

...)( )3()2()1(0 +++= lkjijklkjijkjiji EEEEEEP χχχε (1.10)

where )(nχ is the n -th order susceptibility, represented by a tensor of rank )1( +n .For a fundamental understanding of the nonlinear optical properties it is sufficient touse a scalar representation of Eq. (1.10). The interaction between a monochromaticlight wave

)cos(0 kztEE −= ωvv

(1.11)

and the material leads to the emission of electromagnetic waves of both thefundamental and higher harmonic frequencies.Inserting Eq. (1.11) into Eq. (1.10) and considering noncentrosymmetric materialswith a nonvanishing )2(χ yields the polarization at 0=z :

)coscos( 220

)2(0

)1(0 tEtEP xxx ωχωχε += (1.12)

Analogue equations can be derived for yP and zP . With )2cos21(2/1cos2 xx += the

polarization


++= tEtEEP xxxx ωχωχχε 2cos

2

1cos

2

1 20

)2(0

)1(20

)2(0 (1.13)

can be separated into a constant term 20

)2(02

1xEχε (optical rectification), a linear

optical part, and a term oscillating with ω2 (second-harmonic generation, SHG) (seeFig. 1.3 (a) [Gross01]). Translated into the picture of photons, this can be explainedby the recombination of two photons with the frequency ω into one photon with thefrequency ω2 . The intensity of the second-harmonic (SH) light is proportional to thesquare of the intensity of the incident light and to the square of the second-ordersusceptibility )2(χ . )2(χ and therefore the intensity of the SH light depends on theamount of dipole orientation. SHG provides a method for the contact-lessinvestigation of the polarization distribution of oriented dipoles in transparent NLOpolymers. The efficiency of SHG emitted from the polymer is related to theconstructive or destructive interference of the SH waves generated within thematerial. Finding conditions for an optimal interference of SH light generated indifferent parts of the material is part of investigations on phase matching techniqueswhich are discussed in [Khanarian90, Rikken93, Seppen91, Si01]. For opticalexperiments on thin transparent polymer films, aspects of phase matching can beneglected.

A further second-order nonlinear optical effects is for example frequency mixing (seeFig. 1.3 (b) [Gross01]), where light is emitted with the sum or the difference of thefrequencies of the incident light waves.

Figure 1.3: Schematic illustration of second order nonlinear optical effects: (a) second harmonicgeneration and (b) frequency mixing; after [Gross01].

Since second-order nonlinear optical effects require noncentrosymmetry, in materialswith a center of symmetry the third-order nonlinear term is the lowest non-vanishingterm. A typical third-order nonlinear optical effect is characterized by an intensitydependent refractive index, yielding effects like self focusing, self phase-modulation,and four-wave mixing [Bergmann78]. Also based on third order nonlinearities iselectric-field-induced SHG (EFISH). Here, second-harmonic light is generated from asample when an external electric field is applied, e. g. during poling. Since SHG andEFISH appear simultaneously in a poled sample with additional bias field, the emittedlight depends on the direction of the oriented dipoles and the applied electric field. Itis therefore possible to determine the phase of the SH light. Thereby, in this work atechnique for the determination of the phase of SH light based on a combination ofSHG and EFISH was developed and applied on bimorph structures, described in detailin Chapter 3.


Poled amorphous polymers belong to the ∞C symmetry class of materials. For thissymmetry class, the second-order susceptibility tensor is - far away from resonances -given by [Nye37, Schlaich92, Singer87]:

=000

00000

00000

)2(33

)2(31

)2(31

)2(31

)2(31

)2(

χχχχ

χχ (1.14)

Until now, a macroscopic description of nonlinear optical properties was given. Onthe molecular basis, the large nonlinearity of NLO polymers is related to the A-π-D-molecules. Here, the strong charge-transfer interaction between acceptor and donorcomponents provides the necessary source for strong second-order effects. For asingle molecule, the molecular polarization is given by

( )...0 ++++= lkjijklkjijkjijii EEEEEEp γβαεµ (1.15)

with the dipole moment iµ , the linear polarizability ijα , and the hyperpolarizabilities

ijkβ and ijklγ .

The charge transfer provides a contribution to the hyperpolarizability β and can bedescribed by a quantum mechanical two-level model where the nonlinearity isdetermined by the ground state dipole moment 0µ of the chromophore, the transition

dipole moment µ∆ from the ground state to the lowest excited state and the energy

0ωh of the excited state [Chemla87]:

))()((

)3(),;(

23

20

22

20

21

20

2321

20

20

2

20

3

213 ωωωωωωωωωωωµµωωωβ

−−−−+∆

=−h

e (1.16)

1ω , 2ω and 213 ωωω += are the light frequencies coupled by the nonlinear optical

process, respectively. Near the resonance frequency 0ω the hyperpolarizability

becomes a complex value (not included in Eq. (1.16) for simplicity). The )2(χ susceptibility tensor elements are directly proportional to the poling field,the proportionality being given by the hyperpolarizability β .

Finally, an example for a typical linear optical effect based on the orientation ofchromophores is presented: Birefringence is caused by the linear optical anisotropy

α∆ of molecular dipoles. The birefringence achieved after poling is quadratic in thepoling field [Wu91]:

kt

F

n

VNn u

453

2

2

/ 20

2

0 µεα

ε

+

∆=∆ (1.17)

Eq. (1.17) is valid for poling fields 1/ <kTFµ , where )2/( urr EF εεε += is defined

as the local poling field. Linear optical anisotropy plays an important role e. g. inoptical data storage devices [Coufal00], however, in this case, the chromophore


orientation is performed optically. This process is related to the cold poling processdescribed in the next chapter.


Chapter 2

Cold Poling Methods Probed with Second HarmonicGeneration (SHG)

Several techniques have been developed for orienting the dipolar chromophoreswithin NLO polymers, in order to achieve the required noncentrosymmetry within thematerial. Poling is usually based on the preferential orientation of chromophoredipoles along the electric poling field, in a state where the chromophores are mobile.The techniques can be distinguished with respect to the mechanism for temporarilymaking the dipoles mobile during the poling process: heat (thermal poling, electron-beam poling, corona poling), pressure (gas-assisted poling) or light (all-optical poling,photo-induced poling). The poling process is completed by freezing the dipoleorientation in the glassy state of the polymer. The techniques which do not employheat for achieving dipole mobility have been called “cold” poling techniques. Ageneral feature of “cold” poling is the local change of the free volume within thepolymer around the chromophores. Thereby, memory effects occur in the polymerfilm, due to the relaxation of the free volume. In this work, photoinduced poling (PIP)was modified in order to demonstrate and investigate the memory effects after “cold”poling (memory poling - MP).

2.1 Photoinduced Poling (PIP)

The photoinduced poling process (PIP) was introduced by Sekkat and Dumont in1992 [Sekkat92]. Hereby, the mobility of the molecular dipoles is increased far belowthe glass transition temperature gT by locally enhancing the free volume around the

chromophores. In PIP, this is achieved by irradiating the polymer with light within theabsorption range of the chromophores, causing a reversible trans-cis conformationchange of the chromophore molecule (Figs. 2.1 and 2.2). Fig. 2.1 schematically showsthe trans-cis isomerization of the chromophore dipoles. The dipole is straight in the trans-state and bent in the cis-state as shown in Fig. 2.1for the two isomeric forms of azo-dyes [Sekkat96].After excitation with photons, the azo-dipoles undergo a transition from the trans- tothe metastable cis-state. The probability for this process is pΘ2cos where pΘ is the

angle between the direction of the electric field of the pump light and the molecularaxis. Therefore, dipoles oriented parallel to the light field are preferentially excited[Binks00]. The cis-state thermally relaxes to the trans-state (Fig. 2.1 (b)). The thermalrecovery to the trans-state can occur with and without reorientation of the dipole.After several isomerization cycles, most of the chromophores become orientedperpendicular to the light field. Since photoisomerization is related to a structuralchange of the chromophores, it leads to an increase of the free volume around the dyedipoles, as depicted schematically in Fig. 2.2 [Eisenbach79]. Due to the enhanceddipole mobility they can be oriented noncentrosymmetrically by an externally appliedelectric poling field.

16 Chapter 2. Novel (Cold) Poling Methods Probed with Second Harmonic Generation (SHG)

(b)

Figure 2.1: (a) trans- and cis-state of chromophores and (b) energy diagram for the photoisomerizationprocess; after [Sekkat96].

Figure 2.2: Schematic picture for the increase of the local free volume around the azo-dye dipolesduring photoisomerization cycles [Eisenbach79].

A problem with the “cold” poling is the thermal stability of the dipole orientation.During trans-cis-trans-isomerization cycles, the free volume around thechromophores is locally changed. This leads to a relaxation of the free volume andtherefore introduces a memory in the polymer film.

PIP is spatially selective, since only chromophores within the penetration depth of thepump light are excited (Fig. 2.3). Dipole orientation is impossible in the non-


irradiated region, since the mobility of the chromophores is too low. By using lightsources with different wavelengths, and thus different absorption lengths, structuredpoling across the sample thickness is possible. This feature of PIP can be used toadvantage for the preparation of step-like dipole orientation profiles (bimorphs),interesting for phase-matching in waveguide devices.

Figure 2.3: PIP is only possible within the penetration depth of the pump light into the polymer film.

2.2 Nonlinear Optical Bimorphs

The spatial selectiveness of PIP allows the fabrication of amorphous polymers withinverted dipole orientation profiles (inverted- )2(χ ) (bimorphs). These NLO bimorphsare important for phase-matching of SHG [Khanarian90, Rikken93, Seppen91].Usually, inverted- )2(χ systems can be made by a two-layer structure of two polymerfilms of different glass transition temperatures, where the layers are thermally poled ina two-step poling process at their individual glass-transition temperatures gT [Bauer-

Gogonea94, Bauer-Gogonea99]. In this work, the fabrication of inverted- )2(χstructures in a single polymer film is demonstrated. This is possible by making use ofthe advantage of PIP to be effective only within the penetration depth of theirradiating light into the material. In the first step (Fig. 2.4 (a)), the NLO polymer filmis thermally poled uniformly across the thickness. In the second step (Fig. 2.4 (b)), alayer with inverted polarization (thickness defined by the optical absorption length ofthe pump light) is created within the same film by PIP with a reversed poling field atroom temperature.

+ + + +

- - - -

+++

+++

+ +

+ +

+

+

---

---

- -

-

-

-

-Vdc

pump light

18 Chapter 2. Novel (Cold) Poling Methods Probed with Second Harmonic Generation (SHG)

Figure 2.4: Fabrication of inverted- )2(χ structures in a single polymer film. Step one (a): The sample

is thermally poled above gT ; step two (b): After cooling down, the sample is photoinduced poled at

room temperature with the electric poling field inversed.

In order to prove the fabrication of NLO bimorphs, the relative phase of the emittedSH light is measured by employing EFISH.

2.3 Memory Poling (MP)

Memory effects induced in the polymer during PIP are demonstrated by the followingexperiments which can also be interpreted as a variation of PIP (memory poling -MP).As already discussed, both the pump light and the electric poling field are applied onthe sample simultaneously during PIP. For memory poling , the electric poling field isapplied after the photo-excitation of the molecular dipoles as depicted in Fig. 2.5.

Figure 2.5: During PIP, pumping and poling are performed simultaneously. In MP, the dipoleorientation is performed after pumping. The memory in the polymer allows for dipole orientation evenif the electric field is applied hours after pumping.

Pu

mp

ligh

tE

l. F

ield

Pump on Pump off

Time

Time

PIP MP

+ + + +

- - - -

+++

+++

+ + + +

+

+

---

---

- - - -

-

-Vdc

- - - -

+ + + +

+++

+++

+ + + +

+

+

---

---

- - - -

-

-

Vdc

photoinduced poling below Tg

thermal poling above Tg


In side-chain polymers, the dipole mobility decreases only slowly after lightirradiation. This memory effect enables poling even several hours after the excitation.It is a challenging task to investigate the speed and efficiency of MP as well as thestability of the generated polarization. A model will be introduced and described inorder to understand qualitatively the memory effects in the azo-chromophore basedside-chain polymer.

2.4 Surface Deformation after Poling

Thin films of polymers containing azobenzene chromophores have attracted attentionas photorefractive and photochromic materials for optical data storage [Delaire00]. Itis known, but not yet fully understood that trans-cis-trans-isomerization cycles causea strong deformation of the material surface. It has been demonstrated that surfacerelief gratings (SRGs) can be inscribed onto amorphous polymer films usingholographic techniques with visible light [Rochon95]. Experimental results areillustrated in Fig. 2.6 (a) [Geue00]. The patterns can be erased optically below gT or

by heating the sample above gT . In order to understand the kinetics of SRG

formation, single beam experiments have been performed in which a visible Gaussianlaser-beam of different intensities writes deformation patterns on polymer surfaces(Fig. 2.6 (b) [Bian99]).

(a) (b)

Figure 2.6: AFM images of surface deformation patterns induced by (a) holographic recording[Geue00] or (b) a single Gaussian beam [Bian99].

The question is whether surface deformations are also observed during thermal polingprocesses. The answer is given in Chapter 3 together with a model describing theprocess of grating formation.


Chapter 3

Experimental Results and Discussion

Experimental results obtained with second-harmonic generation (SHG) measurementson PIP and MP are presented for P(S-MA)-DR1 NLO side-chain polymers of type9511 (see Chapter 1). Attention is drawn on the characteristics of PIP and MP withinthe frame of a free volume model. In order to show that PIP is a spatially selectivemethod, the preparation of bimorphs will be explained together with techniquesmeasuring the relative phase of the SH light. Furthermore, the formation of surfacegratings during thermal poling processes will be discussed.

3.1 Efficiency of Memory Poling (MP)

As already explained in Chapter 2, the free volume created around the chromophoresof NLO polymers decreases only slowly after irradiation with the pump light.Therefore, poling is possible even hours after pumping. Both PIP and MP have beenperformed on 2 µm thick polymer samples spin-coated on ITO-glass substrates. Anelectric poling field of 35 V/µm was applied to the ITO and aluminum electrodes. ForPIP, the sample was irradiated by green light from a diode-pumped solid state laser(model LCL-LCM-T-11ccs from Laser 2000 with a wavelength of 532 nm and amaximum radiation power of 10 mW). The set-up for the SGH measurements isshown in Fig. 3.1. The fundamental beam is generated with a pulsed Nd-YAG laser(Newwave Minilaze I) with a pulse length of 6 ns and a pulse energy of 25 mJ/pulse.The intensity of the SH light ( SHGI ) is detected by a Hamamatsu photomultiplier

(H6779). The experimental SHG results are depicted in Fig. 3.2, where PIP and MP

experiments are compared. Fig. 3.2 shows SHGI , since the SHG light intensity

detected is proportional to 2P .

Figure 3.1: Setup for SHG measurements in reflection.

Nd-YAG, =1064 nmλ

Filter for =1064 nmλMirror

Attenuator

Aperture

Heating stage

Al

ITO

Polymer

Filter for =532 nmλ

Photomultiplier

OSC.Bias voltage

22 Chapter 3. Experimental Results and Discussion

For the recording of a SHG intensity data point, poling was briefly interrupted byswitching off the voltage source in order to avoid contributions from EFISH. Themeasurement data were recorded and stored with a Tektronix TDS520 C digitalstorage oscilloscope with a bandwidth of 500 MHz.

Figure 3.2: Rise and decay of polarization during and after PIP and MP. Optical pumping wasperformed for 80 minutes. The SHG efficiency for a thermally poled sample is marked for comparison.

The circles in Fig. 3.2 denote the rise and decay during and after PIP while thetriangles represent MP, when the electric field was immediately applied on the sampleafter 80 minutes of optical pumping. The diamond symbols illustrate that MP is evenpossible when the poling voltage was switched on 80 minutes after pumping.

The results can be interpreted in terms of the free volume model of polymers. In PIP,pumping and dipole orientation are performed simultaneously. The rise of thepolarization is rather slow, since the free volume must be increased around thechromophores by a large number of trans-cis-trans-isomerization cycles. Saturation isapproximately achieved only after 80 min of poling and pumping. After switching offthe pump light, the free volume around the chromophores is still large, so orienteddipoles relax fast. The relaxation is, however, slowing down significantly due to therelaxation of the free volume. If pumping is performed for 80 min prior to poling, thefree volume in the polymer already exists before the poling process is started.Therefore, dipoles can be oriented much faster, even though the pumping light is off.However, the orientation degree that can be achieved is smaller than in PIP due to thesimultaneous relaxation of the free volume, which prevents the further orientation ofchromophores. Even 80 min after pumping, a significant mobility of chromophores ispresent in the polymer, but the saturation polarization that can be achieved alsodecreases. For comparison, the SHG efficiency for a thermally poled sample is alsoincluded in Fig. 3.2. Reasons for the lower efficiency of PIP are twofold: (i) theefficiency of the trans-cis isomerization strongly depends on the angle between thechromophore and the electric field; (ii) PIP addresses only dipoles within theabsorption length of the pump light.

0 100 200 300 400 5000,0

0,2

0,4

0,6

0,8

opt

ical

pum

pin

g

√(ISHG

) for a thermally poled sample

PIP Memory-poling after 80 min pumping Memory-poling after 80 min pumping

+ 80 min delay

√ (I S

HG)

(a.

u.)

time (min)


The observed rise and decay of polarization can be described by the KWW function(Chapter 1), as shown by the good agreement between measurement and fit curves inFig. 3.2.

The stability of dipole orientation after PIP and MP can be significantly increasedwhen poling and pumping are performed at elevated temperatures (but still below gT )

while the relaxation after poling is recorded at room temperature. In Fig. 3.3, thecircles represent the results for PIP performed at 110°C (open symbols) and at 80°C(closed symbols). The triangles and diamonds show the behavior of MP. Again, opensymbols denote poling at 110°C, closed symbols poling at 80°C. In both cases, therelaxation of the dipole orientation is measured at room temperature.

Figure 3.3: Thermal stability of the dipole orientation after PIP and MP at elevated temperatures.

By fitting the results with KWW functions a relaxation time τ of 103 minutes fororiented dipoles after PIP and MP at room temperature is determined, whereas afterpoling at elevated temperatures (80°C and 110°C) values up to 104 and 105 minutesare achieved for τ . As already known from the first experiment, the efficiency of PIPat higher temperatures is still lower than that of thermal poling. Again the results canbe understood in the frame of free volume considerations. At increased temperature(but still below gT ), the free volume also increases. The additional free volume

necessary for the trans-cis isomerization is thereby smaller. Upon cooling to roomtemperature, the free volume decreases and the relaxation stability increases.

Fig. 3.4 summarizes the poling efficiencies for PIP and MP as a function oftemperature. Each data point represents the maximum value of the SHG intensity afterPIP and MP. An increase of the poling efficiency is observed for both PIP and MPwith the same slope. This can be understood, because the mechanism for enhancingthe dipolar mobility is the same. The results are compared to the thermally inducednormalized polarization.

0 100 200 300 400 5000,0

0,2

0,4

0,6

0,8

1,0

optic

al p

umpi

ng

PIP @ 80°C PIP @ 110°C√(I

SHG) for a thermally

poled sample Memory-poling @ 80°C after 80 min pumping Memory-poling @ 110°C after 80 min pumping Memory-poling @ 80°C after 80 min pumping

+ 80 min delay

√(I S

HG)

(a.

u.)

time (min)


Figure 3.4: Comparison of the poling efficiency after PIP and MP at room temperature ( 0T =30°C) and

elevated temperatures (80°C and 110°C). The results are normalized to the poling efficiency of a

thermally poled sample.

3.2 Temperature Dependence of Photoinduced Poling (PIP)

Photochemical reactions like photoisomerization in amorphous side-chain polymersare related to the cooperative motion of molecules in the material. Based on thekinetics of photochromic reactions the free volume theory yields a conclusiveunderstanding of relaxation processes after PIP and MP. Relations between therelaxation of photochromic azo benzenes and their polymer properties, based onthermally induced isomerization below and above the glass-transition temperature,have been investigated intensively by Eisenbach [Eisenbach80a, Eisenbach80b]. The temperature dependence of the ratio Ta of mechanical or dielectric relaxation

times )(Tτ at the temperature T to its value 0τ at a reference temperature rT in

polymers can be described by the Williams-Landel-Ferry (WLF) equation[Williams55] which is equivalent to the Vogel-Fulcher-Tamann-Hesse (VFTH)expression introduced in Chapter 1:

r

rT TTC

TTCa

−+−

−=2

1 )(log (3.1)

The numerical values for 1C and 2C are called WLF parameters and can bedetermined graphically as explained in [Eisenbach80a]. The connection with the freevolume in a polymer is seen when deriving Eq. (3.1) from the Doolittle-equation[Doolittle51]

f

BA += lnlnη (3.2)

0 20 40 60 800,0

0,5

1,0

Normalized √(ISHG

) for thermal poling

PIP MP

nor

mal

ize

d √(

I SH

G)

temperature T-T0 (°C)


which relates the viscosity η of a liquid to the fractional free volume f

0v

vf f= (3.3)

where fv is the free volume and 0v is the total volume of the liquid. In this equation

f is assumed to be a linear function of temperature and with rT as the reference

temperature for relaxation processes below gT

)( rr TTff −+= α (3.4)

where rf is the fractional free volume at rT , and α the thermal expansion coefficientof the amorphous polymer in the glassy state. For the ratio of the viscosities of thematerial at a temperature T to its value at rT one obtains from Eq. (3.2)

−==

rT

T

T

ffBa

11lnln

0ηη

(3.5)

and by inserting Eq. (3.4) in Eq. (3.5)

−+

−=

)(

)(ln

rr

rT TTf

TTBa

αα

(3.6)

with B and rf as free volume parameters, which are related to the WLF-constants 1C

and 2C .

In the following, the free volume model is adapted to the discussion of PIP. Withoutpumping, the free volume in the polymer is assumed to be temperature dependentaccording to Eq. (3.4). However, during pumping, additional free volume is generatedlocally around the chromophores within the polymer film, due to the trans-cisisomerization cycles. The total fractional free volume achieved during PIP is assumedto be independent of the sample temperature PIPfTf =)( . This means that the amountof free volume created by trans-cis isomerization cycles decreases with increasingtemperature. Upon cooling to room temperature 0T , the fractional free volume in the

polymer decreases due to the decreasing sample volume according to:

)()( 0TTfTf PIP −−= α (3.7)

Therefore, Eq. (3.6) has to be modified for describing the relaxation behaviour of thesamples at room temperature

−−

−=

)(

)(ln

0

0, TTf

TTBa

PIPPIPT α

α (3.8)


Eq. (3.8) is used to describe the dipole relaxation of PIP samples at 0T =40°C, where

pumping and poling has been performed at different temperatures below gT .

Fig. 3.5 (a) shows the SHG results for the dipole orientation and relaxation togetherwith KWW fit-functions. The values for the relaxation times can be taken from thefigure.From preceding measurements (not shown in Fig. 3.5 (a)) a relaxation time of 0τ =700

minutes was determined at 0T =40°C. The ratio of the average dipole relaxation time

0, /)( ττ Ta PIPT = is depicted in Fig. 3.5 (b) as a function of 0TT − . The result is in

good agreement with the model for PIPTa , (Eq. (3.8)). The value for the thermal

expansion coefficient is 51021.7 −⋅=α 1/K and has been taken from independentmeasurements, the only fit parameter is the fractional free volume PIPf =0.021,corresponding well to literature values [Eisenbach80a].

(a) (b)

Figure 3.5: (a) Long-term investigation of the dipole relaxation after PIP performed at various

temperatures. (b) Ratio 0/ττ as a function of the sample temperature 0TT − together with fit

functions according to the modified Eisenbach free volume model.

The simple model derived above demonstrates that the dipole relaxation after PIP canbe interpreted within the frame of free volume models in polymers. Thereby,predictions on the thermal stability after “cold” poling can be envisaged.

3.3 Depth Controlled Poling of Bimorph Structures

For the fabrication of NLO bimorphs (polymer film with inverted )2(χ ) thermal andphotoinduced poling is combined in a two-step poling process. During thermal poling,the sample is heated to 160°C (significantly above gT ) with an applied electric field

of 35 V/µm. Cooling down the sample to room temperature leads to a frozen dipoleorientation uniformly across the thickness of the polymer. In the second step, PIP isperformed with the electric field reversed. After PIP for two hours, the poling field

0 20 40 60 801

10

100

lnaT,PIP

=Bα(T-T0)/[f

PIP-α(T-T

0)]

T0=40°C, α=7.21*10

-5 1/K, B=15, f

0=0.021

Ratio

τ/τ

0

Temperature T-T0 (°C)

1 10 100 1000 100000,0

0,2

0,4

0,6

0,8

PIP @ 50°C τ=1300 min

PIP @ 70°C τ=4000 min

PIP @ 90°C τ=15000 min

PIP @ 110°C τ=80000 min

√(I S

HG)

(a.

u.)

time (min)


was still kept on the sample for two hours in order to enhance the dipole orientationstability. SHG is used for monitoring the dipole orientation distribution in the polymerbimorphs. The experimental setup is similar to that of Fig. 3.1, but instead of thereflection geometry, the measurements are performed in transmission. The SH light isrecorded for the two cases shown in Fig. 3.6. The absorption length of the secondharmonic at 532 nm is only around 100 nm for the polymer investigated. Therefore,the SH light transmitted from the sample stems solely from the thermally poled regionor from the PIP region.The experiment shown in Fig. 3.6 is intended to verify theexistence of two differently poled layers within the single polymer film. For this,thermally stimulated depolarization (TSD) is performed by heating the sample at aconstant rate of 5°C/min. During heating, the oriented dipoles within the “PIP layer”start to relax far below gT , while the polarization of the thermally poled region

disappears rapidly in the vicinity of gT . Therefore, the SHG signal in Fig. 3.6 (top)

stems only from the thermally poled layer, as expected from the absorption length ofthe second harmonic. In Fig. 3.6 (bottom), transmitted SHG light stems solely fromthe PIP region. For this reason, the observed SHG intensity already drops at 60°C,since the orientation stability of PIP at room temperature is much lower as comparedto thermal poling. In order to be of practical interest, bimorphs must be prepared withhigh- gT polymers, so that PIP can be performed at elevated temperatures.

Figure 3.6: Thermally stimulated depolarisation of an NLO polymer film with a step-like dipoleorientation profile across the thickness of the film. Due to the strong absorption at the second-harmonicwavelength, the transmitted SH light stems only from the thermally poled region (top) or from the PIPregion (bottom).

The results for the SH light transmitted from the PIP layer are shown with negativesign, in order to indicate the step-like dipole orientation profile. However, it is clearthat the experiments shown in Fig. 3.6 cannot reveal the phase of the SH light.Therefore, a simple experiment was defined to measure the phase of the SH lighttransmitted in the experiment. The experiment is based on EFISH on the poledsamples. Hereby, depending on the direction of the dc-bias field with respect to thedipole orientation, the SH light generated in the polymer film increases or decreases.

PIP

Thermal p.

ω ω 2ω+

0 40 80 120 160-1,2

-0,8

-0,4

0,0

0,4

0,8

1,2

norm

aliz

ed √

(IS

HG)

temperature (°C)

Thermal p.

PIP

ωω 2ω+


Fig. 3.7 shows the change of the SH light intensities when the direction of the dc-fieldrelative to the dipole orientation is reversed.

Figure 3.7: The interaction between )2(χ -SHG and )3(χ -EFISH offers the possibility to determine the

phase of the SH light.

The colors of the data points denoting the emitted SHG intensity correspond to theindividual direction of the external electric field as depicted schematically in the insetof Fig. 3.7. The )2(χ -intensity without applied bias shows again the enhancedefficiency of thermal poling to PIP.

3.4 Formation of Surface Gratings

For the description of the photoinduced pattern formation on polymer surfaces severalmodels are under discussion. One model is based on the Quincke effect, which dealswith a dielectric material that is drawn into a capacitor when applying an electric field[Baldus01]. A further model is related to the volume changes caused by trans-cisisomerization cycles of the azobenzenes. In this optical-field gradient force model theinternal pressure induced by local volume changes leads to a viscoelastic masstransfer forming the surface relief gratings (SRGs) [Bian99, Geue00]. In agreementwith this model, it has been reported recently that light-induced softening ofazobenzene dye-doped polymer films favours the mass transport [Srikhirin00]. FromNewtonian dynamic relations of viscous fluids by using the Navier-Stokes equation anexpression for the time-dependent film thickness can be derived [Barrett98]:

tx

xPhhth

2

230

0

)(

3

1)(

∂∂

+=η

(3.9)

where 0h is the film thickness at time 0=t , η is the kinematic viscosity of the liquid

and 22 /)( xxP ∂∂ represents the change of the pressure gradient at any point in the

0,00

0,02

0,04

0,06

0,08

0,10

0,12

no bias

no bias

I SH

G (

a. u

.)

Th. p.

PIP

ω ω 2ω+

Th. p.

PIP

ωω 2ω+


interference pattern. Eq. (3.9) is valid only for short irradiation times, since surfacetension has been neglected in the consideration.As shown here, SRGs can also be created during thermal poling at a temperature inthe vicinity of gT . The advantage of this method is to generate surface patterns rather

fast because no time-consuming irradiation of the polymer film is required. Theproblem is that the size of surface structures is restricted to the geometry of theelectrode pattern.In order to apply Eq. (3.9) for SRGs generated by thermally-assisted poling, thepressure gradient has to be replaced by the electric field gradient:

tx

xEhhth

2

230

0

)(

3

1)(

∂∂

+=η

ζ (3.10)

with ζ as proportionality factor that converts electric field to pressure. The appliedelectric field represents the mass-driving force and the electric field distributionbetween the poling electrodes as well as the fringing field at the electrode boundarieshave to be taken into account.

For the writing of gratings, amorphous polymers are spin-coated as thin films on ITO-coated samples and an electrode grid consisting of small strips with varying widthswas placed on the film, as schematically depicted in Fig. 3.8. The arrangementrepresents a series of parallel-plate capacitors for the formation of SRGs withdifferent widths during one poling step.

Figure 3.8: Experimental arrangement for the formation of surface relief gratings with different widths.

Fig. 3.9 shows the AFM-image of a surface profile on a 2 µm thick polymer filmgenerated by a 3 µm electrode strip after poling at 160°C with a poling field of 100V/µm.

patterned upper electrode

NLO-polymer film

ITO-electrode

substrate

poling voltage


Figure 3.9: Surface profile in a NLO polymer film after thermal poling with a 3 µm top electrode.

In order to use Eq. 3.10 for a theoretical fit of the experimental results, the electricfield distribution must be calculated. For a parallel-plate capacitor the calculation ofthe electric fringing field is a standard problem in electrostatics. The exact solutioncan be obtained e. g. with the theory of conformal mapping.A numerical approach is based on the solution of the Laplace equation )(2 r

vϕ∇ forthe electric potential. The area between the capacitor electrodes is sub-divided intosquare domains (square grid with distance d ) with unknown potential values at theedges. Any point ),( yx within the grid with the potential ),( yxϕ has fourneighbouring points with the potentials ),( ydx −ϕ , ),( ydx +ϕ , ),( dyx −ϕ , and

),( dyx +ϕ . The first derivative is achieved with differential quotients:

d

yxydx

x dx

),(),( ϕϕϕ −−=

∂∂

−

and d

ydxyx

x dx

),(),( +−=

∂∂

+

ϕϕϕ (3.11)

Analogue steps can be made for the second derivative:

2

2

2

),(),(2),(

),(),(),(),(11

d

ydxyxydx

d

ydxyx

d

yxydx

dxxdx dxdx

++−−=

+−

−−−

=

∂∂

−∂∂

=∂∂

+−

ϕϕϕ

ϕϕϕϕϕϕϕ

(3.12)

This yields the Laplace operator

22

2

2

2 ),(4),(),(),(),(

d

yxdyxdyxydxydx

yx

ϕϕϕϕϕϕϕ −++−+++−=

∂∂

+∂∂

(3.13)


Finally, with Eq. (3.13) equal to zero, the potential in ),( yx is the average of theneighbouring potentials:

4

),(),(),(),(),(

dyxdyxydxydxyx

++−+++−=

ϕϕϕϕϕ (3.14)

Points of the grid within the electrodes have a fixed potential, while all the otherunknown potential values of the grid are assumed to be zero. Using Eq. (3.14)iteratively yields a potential distribution that rapidly converges. Fig. 3.10 shows thecalculated potential lines of a parallel-plate in front of a conducting plane surfacewhere the equipotential lines are indicated by solid lines. The fringing potential

distribution follows a y/1 -law along the y-direction and a decay, related to the

exponential function along the x-direction. The latter will be interesting for thecalculation of fit-curves.

Figure 3.10: Potential distribution in a parallel-plate capacitor obtained by solving the Laplaceequation.

If it is assumed that the total electric field )(xE is proportional to 22 )()( wxwx ee −−+− + ,

where w is the width of the upper electrode, Eq. (3.10) leads to a calculated surfacedeformation as shown in Fig. 3.11.

-40 -20 0 20 400

50

100

150

200

250

ele

ctro

de

dis

tan

ce (

y-d

ire

ctio

n)

(a. u

.)

po

ten

tial (

a. u

.)

electrode width (x-direction) (a. u.)


Figure 3.11: Calculated surface profile for the experimental conditions shown in Fig. 3.9.

In the following, several examples will be presented in order to demonstrate theformation of complicated surface profiles during thermal poling based on the electric-field gradient force model described before.In these experiments, 1.5 µm thick NLO side-chain polymer films (P(S-MA)-DR1,9511) have been spin-coated on ITO-glass substrates. An electric poling field of 100V/µm is applied on the electrodes during heating the sample up to 160°C. From Fig.3.12 (a)-(d), it can be noted that rather complex surface patterns can be created bypoling. When the top electrodes are broad the viscoelastic deformation of the polymerlayer within the electrode area is difficult to describe. Furthermore, the behavior of theviscoelastic mass transfer due to the electrostatic fringing field changes during thegrating formation, since the rigid surface electrodes do not follow the sample surfacedeformation.

The approximation 22 )()( wxwx ee −−+− + for the electric field distribution )(xE describes

qualitatively the main mechanism behind the formation of mass-flow induced surfacepatterns, although additional deformation effects at the electrode boundaries are nottaken into account. A possible way to describe these effects would be to considersinusoidal potential distribution [Tepe87, Tepe88]. Moreover, many more questionsconcerning the formation of surface profiles are still open like the dependence of thegrating height from the applied electric field strength.

For nonpolar polymers, no surface profiles were obtained under similar experimentalconditions. It seems that the dipoles within the polymer matrix are necessary togenerate a large Maxwell stress in the sample and therefore large surface profiles. The results shown here have strong implications for waveguide fabrication. The largesurface profiles after poling can significantly enhance light scattering in thewaveguide device. However, by the use of nonpolar cladding layers, the formation ofsurface reliefs can be significantly suppressed.

-3 -2 -1 0 1 2 3-4

-3

-2

-1

0

1

2

3

gra

ting

heig

ht (y

-direct

ion)

(µm

)

grating width (x-direction) (µm)


(a) (b)

(c) (d)

Figure 3.12: AFM images of surface profiles generated by (a) 1 µm, (b) 4 µm, (c) 10 µm, and (d) 13µm electrode strips together with calculated deformations.

0 5 10 15-400

-200

0

200

400

scanned result model calculation

h

eig

ht (n

m)

scanning length (µm)

0 5 10 15

-400

-300

-200

-100

0

100

200

300


he

igh

t (nm

)


0 10 20 30

-500

-400

-300

-200

-100

0

100

200

300


hei

ght

(n

m)


0 10 20 30 40-500

-400

-300

-200

-100

0

100

200

300


hei

ght

(n

m)



Chapter 4

Conclusion of Part I

In Part I of the thesis, amorphous nonlinear optical polymers with azo-benzenechromophores have been investigated for their potential use in photonic applications,by means of second-harmonic generation of light. Special emphasis was given onpoling techniques, especially on the “cold” photo-induced poling process. In a seriesof experiments, it was demonstrated that “cold” poling introduces memory effects inthe polymer material, which limit the thermal stability of the dipole orientation. Inorder to overcome this problem, “cold” poling was investigated by varying the polingtemperature, but still far away from the glass transition. The increase in the thermalstability of the dipole orientation was explained within the frame of a modifiedEisenbach free volume model. Based on the free volume model, predictions on thethermal stability after “cold” poling can be envisaged. This might have practicalimplications in the preparation of nonlinear optical devices.

An advantage of “cold” poling was demonstrated in the fabrication of step-like dipoleorientation profiles by combining photo-induced and thermal poling in a two-steppoling process. A technique for measuring the relative phase of the second-harmoniclight generated within the polymer. Such bimorph structures are under discussion forphase-matched frequency-conversion devices.

Under suitable illumination conditions, photoinduced processes in polymers areknown to generate surface relief profiles. In this work, the problem of surface profilesafter thermal poling with electrodes was addressed. It was shown that surface profilesare observed for polar polymers, whereas nonpolar polymers show no surfacedeformation upon poling. The experimental findings were explained with a viscousfluid model, where the driving force for surface profile formation is the change of thepoling electric field gradient within the polymer. The results shown might have strongimplications for waveguide fabrication, since the large surface profiles after polingsignificantly enhance light scattering in the waveguide device.

36 References Part I

References of Part I

[Ahlheim94] M. Ahlheim and F. Lehr, Electrooptically active polymers.Non-linear optical polymers prepared from maleic anhydridecopolymers by polymer analogous reaction, Macromol. Chem.Phys. Vol. 195, pp. 361-373 (1994).

[Baldus01] O. Baldus, A. Leopold, R. Hagen, T. Bieringer, and S. J. Zilker,Surface relief gratings generated by pulsed holography: Asimple way to polymer nanostructures without isomerizing side-chains, J. Chem. Phys. Vol. 114, pp. 1344-1349 (2001).

[Barrett98] Ch. J. Barrett, P. L. Rochon, and A. L. Natansohn, Model oflaser-driven mass transport in thin films of dye-functionalizedpolymers, J. Chem. Phys. Vol. 109, pp. 1505-1516 (1998).

[Bauer-Gogonea94] S. Bauer-Gogonea, S. Bauer, W. Wirges, and R. Gerhard-Multhaupt, Preparation of inverted )2(χ -structures with NLOpolymers, Polymer Preprints Vol. 35, pp. 261-262 (1994).

[Bauer-Gogonea95] S. Bauer-Gogonea, Strukturierte Polung von nichtlinearoptischen Polymeren und deren dielektrische undpyroelektrische Charakterisierung,Dissertation, TU Berlin 1995.

[Bauer-Gogonea96] S. Bauer-Gogonea and R. Gerhard-Multhaupt, Nonlinearoptical polymer electrets, IEEE Transactions on Dielectrics andElectrical Insulation Vol. 3, pp. 677-705 (1996).

[Bauer-Gogonea99] S. Bauer-Gogonea, S. Bauer, and W. Wirges, In-situ profiling ofdipole polarization distributions in poled nonlinear opticalpolymers with electrothermal and optical techniques, Chem.Phys. Vol. 245, pp. 297-310 (1999).

[Bauer-Gogonea01] S. Bauer-Gogonea and S. Bauer, Polymer electrets forelectronics, sensors, and photonics, Handbook of AdvancedElectronic and Photonic Materials and Devices Vol. 10, pp.185-231, Academic Press (2001).

[Bergmann78] L. Bergmann, C. Schäfer, Lehrbuch der Experimentalphysik,Band III: Optik, Walter de Gruyter, Berlin (1978).

[Bian99] Sh. Bian, J. M. Williams, D. Y. Kim, L. Li, S.Balasubramanian, J. Kumar, and S. Tripathy, Photoinducedsurface deformations on azobenzene polymer films, J. Appl.Phys. Vol. 86, pp. 4498-4508 (1999).

[Binks00] D. J. Binks and D. P. West, Dispersive rotation of dipoles inamorphous media, Appl. Phys. Lett., Vol. 77, pp. 1108-1110(2000).

[Booth89] B. L. Booth, Low loss channel waveguides in polymers, J.Lightw. Technol. Vol. 7, 1445-1453 (1989).

[Burland94] D. M. Burland, R. D. Miller, and C. A. Walsh, Second-ordernonlinearity in poled-polymer systems, Chem. Rev. Vol. 94, pp.31-75 (1994).

[Chemla87] D. S. Chemla and J. Zyss (Eds.), Nonlinear optical propertiesof organic molecules and crystals, Vol. 1 and 2, AcademicPress, New York (1987).


[Cheng98] Z.-Y. Cheng, . Yilmaz, Werner Wirges, S. Bauer-Gogonea,and S. Bauer, Temperature-domain analysis of primary andsecondary relaxation phenomena in a nonlinear optical side-chain polymer, J. Appl. Phys. Vol. 83, pp. 7799-7807 (1998).

[Coufal00] H. Coufal and D. Psaltis (Eds.), Holographic Data Storage,Springer, Berlin-Heidelberg (2000).

[Delaire00] J. A. Delaire and K. Nakatani, Linear and Nonlinear OpticalProperties of Photochromic Molecules and Materials, Chem.Rev. Vol. 100, pp. 1817-1845 (2000).

[Doolittle51] A. K. Doolittle, Studies in Newtonian Flow. II. TheDependence of the Viscosity of Liquids on Free-Space, J. Appl.Phys. Vol. 22, pp. 1471-1475 (1951).

[Dumont91] M. Dumont, Y. Levy, and D. Morichère, in Organic Materialsfor Nonlinear Optics and Photonics, edited by J. Messier,Kluwer, Dordrecht (1991)

[Eisenbach79] C. D. Eisenbach, New Aspects of Photochromism in BulkPolymers, Photographic Science and Engineering Vol. 23, pp.183-190 (1979).

[Eisenbach80a] C. D. Eisenbach, Relation between Photochromism ofChromophores and Free Volume Theory in Bulk Polymers, Ber.Bunsenges. Phys. Chem. Vol. 84, pp. 680-690 (1980).

[Eisenbach80b] C. D. Eisenbach, Comment on the Matrix Effect onPhotochromism of Spiropyrans in Bulk Polymers, PolymerBulletin Vol. 2, 169-176 (1980).

[Geue00] Th. Geue, M. Schultz, J. Grenzer, U. Pietsch, A. L. Natansohn,and P. Rochon, X-ray investigation of the molecular mobilitywithin polymer surface gratings, J. Appl. Phys. Vol. 87, pp.7712-7719 (2000).

[Goldberg90] H. A. Goldberg, A. J. East, I. L. Kalnin, R. E. Johnson, H. T.Man, R. A. Keosian, and D. Karim, Mater. Res. Soc. Symp.Proc. Vol. 175, p. 113 (1990).

[Gross01] R. Gross, Physik III: Optik, Quantenphänomene, Aufbau derAtome, Vorlesungsskriptum, Technische Universität München(2001).

[Havriliak67] S. Havriliak and S. Negami, A complex plane representation ofdielectric and mechanical relaxation processes in somepolymers, Polymer Vol. 8, pp. 161-210 (1967).

[Kaatz96] P. Kaatz, P. Pretre, U. Meier, U. Stadler, Ch. Bosshard, P.Günter, B. Zysset, M. Stähelin, M. Ahlheim, and F. Lehr,Relaxation Processes in Nonlinear Optical Polyimide Side-Chain Polymers, Macromolecules Vol. 29, pp. 1666-1678(1996).

[Khanarian90] G. Khanarian, R. A. Norwood, D. Haas, B. Feuer, and D.Karim, Phase-matched second-harmonic generation in apolymer waveguide, Appl. Phys. Lett. Vol. 57, pp. 977-979(1990).

[Liang94] J. Liang, R. Levenson, C. Rossier, E. Toussaere, J. Zyss, A.Rousseau, B. Boutevin, E. Foll, and D. Bose, Thermally stablecross-linked polymers for electro-optic applications, J. Phys. III(France) Vol. 4, p. 2441 (1994).

38 References Part I

[Liu00] X. Liu, G. Xu, J. Si, P. Ye, Z. Li, and Y. Shen, Effect oftemperature on the optical poling process in a side-chainpolymer system, J. Appl. Phys. Vol. 88, pp. 3848-3852 (2000).

[McCrum91] N. G. McCrum, B. E. Read, and G. Williams, Anelastic andDielectric Effects in Polymer Solids, Dover Publ., New York(1991).

[McKenna89] G. B. McKenna, Glass formation and glassy behaviour, in C.Booth and C. Price (Ed.), Polymer properties, Comprehensivepolymer science Vol. 2 (Pergamon, Oxford 1989), pp. 311-362.

[Mopsik75] F. I. Mopsik and M. G. Broadhurst, Molecular dipole electrets,J. Appl. Phys. Vol. 46, pp. 4204-4208 (1975).

[Nye37] J. F. Nye, Physical Properties of Crystals, Claredon Press,London (1937).

[Rochon95] P. Rochon, E. Batalla, and A. L. Natansohn, Optically inducedsurface gratings on azoaromatic polymer films, Appl. Phys.Lett. Vol. 66, pp. 136-138 (1995).

[Rikken93] G. L. J. A. Rikken, C. J. E. Seppen, E. G. J. Staring, and A. H.J. Venhuizen, Efficient modal dispersion phase-matchedfrequency doubling in poled polymer waveguides, Appl. Phys.Lett. Vol. 62, pp. 2483-2485 (1993).

[Schlaich92] H. Schlaich, Die Bestimmung der absolutenFrequenzverdopplungs-koeffizienten gepolter nicht-linearoptisch aktiver Polymere, diploma thesis, Marburg (1992).

[Sekkat92] Z. Sekkat, D. Morichere. M. Dumont, R. Loucif-Saibi, and J.Delaire, Photoisomerisation of azobenzene derivatives inpolymeric thin films, J. Appl. Phys. Vol. 71, pp. 1543-1545(1992).

[Sekkat96] Z. Sekkat, J. Wood, E. F. Aust, W. Knoll, W. Volksen, and R.D. Miller, Light induced orientation in a high glass transitiontemperature polyimide with polar azo dyes in the side chain, J.Opt. Soc. Am. B Vol. 13, pp. 1713-1724 (1996).

[Seppen91] C. J. E. Seppen, G. L. J. A. Rikken, E. G. J. Staring, S. Nijhuis,and A. H. J. Venhuizen, Linear Optical Properties ofFrequency Doubling Polymers, Appl. Phys. B Vol. 53, pp. 282-286 (1991).

[Si01] J. Si, J. Qiu, K. Kitaoka, and K. Hirao, Photo-induced phse-matched second-harmonic generation in azodye-doped polymerfilms, J. Appl. Phys. Vol. 89, pp. 2029-2032 (2001).

[Singer87] K. D. Singer, S. L. Lalama, J. E. Sohn, and R. D. Small, inNonlinear Optical Properties of Organic Molecules andCrystals Vol. 1, D. S. Chemla and J. Zyss (Eds.), AcademicPress, Orlando (1987)

[Srikhirin00] T. Srikhirin, A. Laschitsch, D. Neher, and D. Johannsmann,Light-induced softening of atobenzenecdye-doped polymer filmsprobed with quartz crystal resonators, Appl. Phys. Lett, Vol.77, pp. 963-965 (2000).

[Tepe87] R. Tepe, Theoretical analysis of an electrically addressedviscoelastic spatial light modulator, Opt. Soc. Am. Vol. 4, pp.1273-1282 (1987)


[Tepe88] R. Tepe, Steuerbare Lichtmodulatoren für optischeInformationsverarbeitung am Beispiel viskoelastischerSchichten: Theorie und Anwendung (Teil 1), ntzArchiv Bd. 10,pp. 269-302 (1988).

[Williams55] M. L. Williams, R. F. Landel, and J. D. Ferry, The temperaturedependence of relaxation mechanisms in amorphous polymersand other glass-forming liquids, J. Am. Chem. Soc. Vol. 77, p.3701 (1955).

[Williams85] G. Williams, Dielectric relaxation behaviour of amorphouspolymers and related materials, IEEE Trans. Electr. Insul. Vol.20, pp. 843-857 (1985).

[Winkelhahn96] H. J. Winkelhahn, Th. K. Servay, and D. Neher, Ber.Bunsenges., Phys. Chem. Vol. 100, p. 123 (1996)

[Wu91] J. W. Wu, Birefringent and electro-optic effects in poledpolymer films: steady-state and transient properties, J. Opt.Soc. Am. B Vol. 8, pp. 142-152 (1991).

[Zyss94] J. Zyss (Ed.), Molecular nonlinear optics, Academic Press, SanDiego (1994).



_____________________________________________________________________

Part II

Plasma-Induced Charging of NonpolarCellular Materials

Part II: Plasma-Induced Charging of Nonpolar Cellular Materials 43

Chapter 5

Polymer Charge Electrets

In general, charge electrets are employed wherever large external fields are required[Gerhard-Multhaupt99]. Polymer charge electrets are nonpolar or weakly polarinsulators, which carry a quasi-permanent excess charge trapped either at the surfaceor in the bulk of the material. Charge trapping within the bulk of nonpolar polymers ispossible by chemical traps at specific sites of the chain or by traps in cages betweenadjacent molecular groups or chains. Charge electrets are excellent insulatorscharacterized by a low dielectric constant and extremely small dielectric losses. Sofar, these materials have been extensively used in microphones, air filters, dosimeters,etc. but they have been excluded for electromechanical sensors and transducerapplications [Gerhard-Multhaupt99].

In this work, a specific class of polymer charge electrets is investigated: chargedcellular materials. The “raw” materials are nonpolar and therefore, they cannot exhibitfeatures like piezoelectricity or ferroelectricity. The unexpected identification of hugeelectromechanical effects in internally charged cellular materials was therefore asurprise [Neugschwandtner00a, Neugschwandtner00b, Sessler99]. This chapter provides a short synopsis of the most often employed polymer chargeelectrets. In the following chapters, dielectric barrier discharges (DBDs) are identifiedas the mechanism for the internal charging of the materials. Based on this mechanism,analogies to ferroelectric materials are outlined, discussed in detail in Part III of thiswork.

5.1 Fluorinated and Non-Fluorinated Polymers

The best polymer electrets identified so far are fluorinated polymers, likesemicrystalline polytetrafluoroethylene (PTFE or Teflon), its copolymersfluoroethylenepropylene (FEP), amorphous PTFE (e. g. Teflon AF 1600 fromDupont) and perfluorinated cyclobutene (PFCB), a low dielectric constantfluoropolymer with a glass transition temperature ( gT ) above 300°C. Their chemical

structures are illustrated in Fig. 5.1 (a)-(d).PTFE or Teflon is primarily known as low-cost, anti-stick coating on frying pansdue to its hydrophobicity. Therefore, PTFE is unique for water-resistant textiles andbiomedical coatings. PTFE and its copolymers show an outstanding charge stabilitywhich is well-established up to a temperature of 200°C. Unfortunately, the depositionof Teflon on substrates is rather difficult and still a challenging task[Schwödiauer98]. In contrast to PTFE, Teflon AF and PFCB are very suitable polymers for spin-coating. Furthermore, these electret materials show remarkable thermal chargestability. The only disadvantage is their high price.

44 Chapter 5. Polymer Charge Electrets

(a) (b)

(c)

(d)

(e)

(f)

Figure 5.1: Fluorinated charge electrets exhibiting excellent charge stability: (a)polytetrafluoroethylene (PTFE or Teflon), (b) fluoroethylenepropylene (FEP), (c) amorphous PTFE(Teflon AF), and (d) perfluorinated cyclobutene (PFCB). Furthermore, two examples of promisingnon-fluorinated electret materials: (e) cyclo-olefine polymers and (f) benzocyclobutene (BCB); after[Bauer-Gogonea01].

Non-fluorinated polymers are another class of nonpolar electret materials, althoughthe electret properties are inferior compared to the PTFE family. Polyethylene (PE) iswidely used as low-cost material for cable coatings and as packaging material , but the

C C* *

F F

F Fn C C*

F F

F Fx C C *

F CF3

F F n

1-x

C

CH3

* O O *

F F

F F

FF

O

F F

F

FF

O F

n

C C*

F F

F Fx C C *

F F

O O n

CF3C CF3

1-x

1-xC C*

H H

H R1x C C *

H H

n

R2 R2

SiMe2

SiMe2

O

SiMe2

SiMe2

OSiMe2


charge stability is not useful for technical applications. The main reason is theincorporation of impurities and softeners within the polymer matrix. Better suited ispolypropylene (PP), also a cheap polymer used in cable coatings and as packagingmaterials. The charge stability is reasonable, but much inferior to polymers from thePTFE family. However, remarkable charge stability has been found in cyclo-olefinepolymers and in network-forming low dielectric constant dielectrics likebenzocyclobutene (BCB) as schematically shown in Fig. 5.1 (e) and (f).

Charge trapping in nonpolar charge electrets may occur at different molecular andmorphological levels. First-level trapping occurs on chemical traps at specific sites ofthe chain, and second-level trapping on traps in cages between adjacent moleculargroups or chains. In semicrystalline polymers, trapping is observed at the interfacebetween the amorphous and crystalline phase (tertiary-level trapping). Furthermore,improved charge storage is also observed in materials with excellent chemical purityof the polymer species, highly symmetric chemical structures, and the incorporationof strongly electronegative atoms, like oxygen and fluorine.

5.2 Polymer Foams

Most recently, foam polymers have been investigated as charge elecret with strongelectromechanical effects. Foams contain a large number of voids filled with air orother gases. An example of such a foam is shown in Fig. 5.2. The foam is produced bybiaxial stretching a polypropylene (PP) polymer preform with inorganic fillerparticles. Foaming is achieved in a continuous process with void dimensions on theorder of 100×100×10 µm³. PP foams were introduced in Finland and furtherdeveloped by VTT Processes [Savolainen89]. The cross-section of such a 70 µm thickporous PP foil is shown in the top of Fig. 5.2 together with a schematic depiction ofthe internal charge distribution after charging [Neugschwandtner01]. Fig. 5.2(bottom) depicts a second example of a PP foam with a smaller degree of foaming.

On these two examples, the effect of the degree of foaming on the electromechanicalproperties can be investigated. The illustration shows charges trapped at the interfacesbetween polymer and air. Cellular PP foils can be internally charged for example bycorona discharges. In Chapter 6, electrode charging is demonstrated for the internalcharging of the foams and for clarifying the microscopic mechanism for charging.


Figure 5.2: Top: Cross-section of a microporous HS01 polypropylene foam and a schematic illustrationof the charge distribution after charging [Neugschwandtner01]. Bottom: Cross-section of a PP O01 foilwith a sample thickness of 40 µm.

Besides PP foams, fluorinated cellular polymers are presented. Cellular FEP cableswith thicknesses ranging from 95 to 200 µm and with closed cells of variousdimensions have been investigated (Fig. 5.3). The core of the cable is a simplemetallic wire with a diameter of 700 µm and the cable surface is coated with a thinmetallic film. After charging, this cable is strongly piezoelectric and can be used e. g.as sensor cable placed below a carpet for human detection or in traffic countingsystems.

Figure 5.3: Image of a microporous FEP cable.

charging


5.3 Artificial Structures

In this work, also artificial structures have been investigated. Fig. 5.4 shows thestructure of a sandwich, consisting of two charge electrets, separated by an air-gap.For the charge electrets, excellent materials like amorphpus PTFE and perfluorinatedbenzocyclobutene (PFCB) can be used. The system is an ideal model system forfundamental investigations of the charging process in voided materials. It may alsohave practical implications in microelectromechanical devices. In addition, the hightransparency of the stack arrangement allows for optical investigations, e.g. forsecond-harmonic generation (SHG) on the nonpolar structures. Besides such artificialstructures with an air-gap, hybrid systems consisting of electrets with stronglydifferent mechanical properties have been employed [Kacprzyk95,Neugschwandtner00a, Schwödiauer00a, Schwödiauer00b].

Figure 5.4: Artificial air-gap structure as model cellular electret.

In the following a short discussion of the relevant material properties of chargeelectrets is given.

5.4 Dielectric Function

The linear dielectric properties of isotropic nonpolar polymers may be described tofirst approximation by the Clausius-Mossotti equation [McCrum91]:

∑=+−

n

N

TVT

T

03)(

1

2)(

1)(

εα

εε

(5.1)

Here, ε is the dielectric function, 0ε the dielectric permittivity of vacuum, V the

temperature dependent volume, N the number of induced dipoles, and α is themolecular polarizability. The dielectric constant is low in nonpolar polymers because only the polarizability ofthe chemical bonds is involved. A typical low-dielectric constant material is PTFEwith a dielectric constant ε =2.1, free of dispersion up to frequencies in the GHzrange. Amorphous PTFE - Teflon AF - with a smaller density than thesemicrystalline PTFE shows a lower dielectric constant ε =1.9, the lowest value of εof all condensed materials.The dielectric constant of cyclo-olefine polymers is 2.35 while in network-forminglow-k dielectrics like PFCB and BCB values of 2.4 and 2.6 can be found.

glass

glass

ITO

ITOpolymer

polymer


The temperature dependent dielectric function can be expressed by

∑+≈n

N

TVT

03)(

31)(

εαε (5.2)

Here, the dielectric constant follows the specific volume of the polymer, which showsan increase in the vicinity of the phase and glass-transition temperature as shown inFig. 5.5 for PTFE. Measuring the temperature dependent dielectric function revealsthe glass I and II and the structural phase transition from a triclinic to hexagonal andpseudohexagonal phase at at 20°C and 30°C, respectively [Schwödiauer99]. The dielectric function of nonpolar polymers decreases with increasing temperature,since the molecular polarizability and the induced dipole density are also decreasing.In weakly polar materials the behavior of )(Tε with increasing temperature dependson the relation between the increase of the molecular polarizability and the decreaseof the dipole density. For comparison, a strongly polar polymer shows an increasingdielectric function due to the domination of the rising orientation polarization withincreasing temperature (see Chapter 1).

Figure 5.5: Temperature dependent dielectric constant of a PTFE film indicating the glass I and II aswell as the structural phase transitions; after [Schwödiauer99].

5.5 Piezo- and Pyroelectricity in Nonpolar Cellular Electrets

Heterogeneous polymer systems such as polymer foams and hybrid systems becomestrongly piezo- and pyroelectric after charging. The thermodynamic definition ofpiezoelectricity couples the dielectric displacement or electric field with stress orstrain and is represented by a third-rank tensor. Pyroelectricity relates thedisplacement with the sample temperature and is thus a vector quantity [Mason64]:

m

j

j

mmj E

SDd

∂

∂=

Σ∂∂

= , m

mm ET

Dp

∂Ξ∂

=∂∂

= (5.3)


Here, =j 1,…,6 and =m 1, 2, 3; Σ is the stress, S the strain, D the dielectricdisplacement, E the electric field, T the temperature, and Ξ is the entropy. (For thetensor description contracted notation is used).More “practical” definitions for the piezo- and pyroelectric coefficients of soft mattersystems have been given in Chapter 1, where the coefficients are defined as thederivatives of the change of charge induced on the electrode area by mechanical stressor temperature.

Piezo- and pyroelectric effects in nonpolar cellular polymers require anoncentrosymmetric structure. In the foam shown in Fig. 5.2, noncentrosymmetry isachieved by bipolar charging. Piezoelectricity is a result of the strong deformation of the macroscopic dipolesformed by the charges in the voids. The piezoelectric signal cannot be distinguishedfrom a “true” piezoelectric effect based on noncentrosymmetry in polar materials andtherefore, charged cellular polymer electrets can be undoubtedly considered aspiezoelectric materials. The same arguments hold for pyroelectricity. Themeasurement of piezoelectric coefficients will be discussed in Part III of this work.The microscopic mechanism behind the internal bipolar charging of nonpolar polymerfoams will be further discussed in Chapter 6.


Chapter 6

Dielectric Barrier Microdischarges

Cellular polymers are based on nonpolar materials and are therefore uncharged. Forelectromechanical applications, the polymers must be charged internally. Themechanism behind the internal charging process of cellular materials can be explainedby dielectric barrier microdischarges. The dielectric barrier discharge (DBD), also known as silent discharge, is widely usedin industry for ozone synthesis, as a source of UV light (for example in excimerlamps) and for the maskless etching of substrates [Hippler01, Sankaran01]. In DBDsthe discharge gap is separated from the electrodes by at least one dielectric layer,typical arrangements for DBDs are shown in Fig. 6.1. DBDs can be operated even atatmospheric pressure (high-pressure discharges). Above the threshold voltage forbreakdown of the gap, self-extinguishing short microdischarges that transfer a sheetcharge across the air-gap are ignited. The barrier discharge is always accompanied bythe emission of a short light pulse from the discharge gap. The plasma generated byDBDs is in an extreme non-equilibrium state with “hot” electrons (electrontemperature corresponds to an energy of several eV) and “cold” ions (remaining atambient temperature).

6.1 “Cold” Plasma

Plasmas are quasi-neutral particle systems consisting of gaseous and fluid-likemixtures of free electrons, ions, and neutral particles like atoms and molecules. Themean kinetic energy of the electrons and the other plasma components lies between0.2 eV and 2 MeV per particle. The particles within the plasma are interacting eitherdue to Coulomb forces or by the formation of macroscopic space charges.Plasmas can be subdivided into low-temperature plasmas (LTP) and high-temperatureplasmas (HTP). LTP can be again subdivided into thermal LTP and non-thermal(“cold”) LTP. Tab. 6.1 gives an overview of possible plasma types together with thetemperatures of the different plasma species [Hippler01].

In order to provide a theoretical description of plasma discharges, various simulationmodels have been proposed based on kinetic and statistical theory as well as onhydrodynamic relations (e. g. equilibrium fluid model [Li97]). An overview aboutthese theoretical approaches is given in [Hippler01].

52 Chapter 6. Dielectric Barrier Microdischarges

Low-temperature plasma(LTP)

High-temperature plasma(HTP)

Thermal LTP

KTTT ie4102 ⋅≤≈≈

e. g. arc plasma at normalpressure

Non-thermal (“cold”) LTP

KTTi 300≈≈

KTT ei510≤<<

e. g. low-pressure glowdischarge

KTT ei710≥≈

e. g. fusion plasmas

Table 6.1: Subdivision of plasmas according to their different species energy after [Hippler01].

Dielectric barrier discharges (DBDs) are self-sustained non-equilibrium electrical gasdischarges. The non-equilibrium state is related to the different temperatures betweenthe electrons and the ions or neutral particles (see Tab. 6.1) and therefore, “hot”electrons are indicated by a much higher kinetic energy than the “cold” species.DBDs are usually characterized by insulating layers on one or both electrodes, butalso structures with dielectric layers inside the discharge gap are reported, asillustrated in Fig. 6.1 [Hippler01]. Materials for the dielectric barriers can be glass,quartz, ceramics, silicon rubber, and polymer films. DBDs were proposed first byWerner von Siemens in 1857 [Siemens57].

Figure 6.1: Common planar and cylindrical configurations for the formation of dielectric barrierdischarges [Hippler01].

DBDs at atmospheric pressure consist of many tiny parallel current filaments knownas partial discharges or microdischarges. The breakdown event itself can be describedby four different discharge stages, demonstrated schematically in Fig. 6.2 [Gibalov92,Hippler01].


Figure 6.2: Development of a dielectric barrier microdischarge described by four phases [Hippler01].

If the electric field within the gas gap is sufficiently high to initiate discharges, thebreakdown event starts with the Townsend phase. This is followed by the formationof a streamer in which a conducting channel - the filament - is generated (phase 2).Through this channel, charges can be transferred and they get accumulated on thedielectric surface (phase 3). In this situation, the voltage across the filament or theelectric field between the electrodes is compensated and the discharge stops. Thisstage is denoted as phase 4.The first electrical breakdown event requires an electric field above a certain criticalvalue. The corresponding voltage value is usually called breakdown or thresholdvoltage bV . After the first breakdown event, charges are deposited on the dielectric

surface and the development of further discharges is determined by these residualcharges.

Periodic breakdown events are generated by a sinusoidal input voltage. The situations,if an ac-voltage is applied on the electrodes, are depicted in Fig. 6.3 [Hippler01].Beginning at the stage 1t of the voltage signal, an increasing electric field is observed

within the discharge gap. At 2t , microdischarges are established simultaneouslygenerating charges on the dielectric surfaces. The discharges extinguish and the gapvoltage becomes compensated. This breakdown lasts only for a few ns (usuallybetween 5 and 10 ns), indicating a rather fast charge transfer [Klein01, Xu98]. At 3t ,

the applied electric voltage has been lowered and the electric field across the gas gapis now dominated by the memory charges deposited at the dielectric surface[Callegari00]. Therefore, at 4t - when the input voltage approaches to zero - newmicrodischarges of opposite polarity occur. These discharges generate filamentswithin the residual channels of the previous breakdown events, if the frequency of theapplied ac-voltage is high enough and the residual channels have not fully recoveredin the meantime.


Figure 6.3: Generation of discharge filaments by using high sinusoidal voltage; after [Hippler01].

During the atmospheric pressure microdischarges discussed above, current densitieson the order of 100 A/cm² are observed in the cylindrical filaments with a radius ofabout 100 µm each [Xu98]. It must be noted here that the gaps in most investigationsare on the order of mm, the situation at tiny gaps like that in cellular materials has notyet been investigated.

The high energetic electrons generated by the non-equilibrium or “cold” plasma arenecessary for the excitation, dissociation and ionization of species of the backgroundgas which are then used for plasma chemical reactions or charging processes.Thereby, dissociation by electron collisions is the most important process leading toplasma chemical synthesis like ozone formation. Fig. 6.4 shows a photograph of adischarge plasma within a gas-gap filled with He/Xe and a gap width of 1.5 mm at aac-voltage frequency of 78 kHz, recorded by a special digital camera [Akishev98].

Figure 6.4: Plasma due to dielectric barrier microdischarges at a high frequency of the appliedalternating voltage; after [Akishev98].

Further potential applications for dielectric barrier microdischarges are found in thesurface modification of materials, in high-power CO2 lasers, excimer lamps, and inplasma display panels (PDPs) [Eliasson97, Hippler01].


6.2 Paschen Breakdown

In dielectric discharge gap structures, a gas mixture such as Ne, Xe or simply air (air-gap structures - AGS) is ionized by a high voltage, depending on the gas pressure andthe gap width. The generated self-sustaining plasma discharge follows Paschen’s lawfor electrical breakdown [Elsbergen00, Hippler01, Lieberman94, Raizer97] and thebreakdown voltage bV as a function of gas pressure and gap width ( pd ) is given by

[ ])/11ln(lnln seb Apd

BpdV

γ+−= (6.1)

where seγ is the secondary electron emission yield coefficient. The constants A and

B are related to specific gases [Liebermann94]. The calculated Paschen curves for afew typical gases used e. g. in PDPs are illustrated in Fig. 6.5 [Hippler01].

Figure 6.5: Paschen breakdown curves in gas-gap structures filled with different gases of technologicalimportance in the plasma display technology [Hippler01].

Knowledge of the breakdown behavior, which depends on the gas species andpressure (e. g. [Oversluizen02]) and the type and structure of the dielectric layers isimportant for the improvement of the discharge efficiency in discharge applications.

6.3 Charging of Cellular Polymer Electrets

DBDs within the air-gaps of cellular polymer materials arise at atmospheric pressureand are therefore related to low-temperature or “cold” plasmas. In the following,DBDs are used for the formation of charged cellular polymer electrets.The idea behind the charging technique is the generation of bipolar charges within thevoids by dielectric barrier microdischarges. The charges are transferred across thegaps and trapped at the cell walls. A schematic description of the internal chargingprocess of piezoelectric polymer foams and AGS is given in Fig. 6.6 (a) and (b).


Figure 6.6: Schematic description of the internal charging process of cellular polymer materials (a)polymer foams (for example porous PP) and (b) dielectric AGS.

After charging, perfectly oriented macroscopic “dipoles” have been created in thematerial. This situation can be compared for example with polar materials, where afterthe application of an external electric field molecular dipoles are preferentiallyoriented in the direction of the poling field. The macroscopic “dipoles” areestablishing a “polarization” within the material. This “polarization” isthermodynamically metastable, while polarization in polar materials can bethermodynamically stable (for example in single crystals) or thermodynamicallymetastable (like in amorphous polymers with a frozen dipole orientation). The“polarization” within the cellular polymer is related to strong piezo- and pyroelectriceffects as well as to second-order nonlinear optical (NLO) properties of chargedpolymer foams. Experimental results on these properties will be shown in Part III.Furthermore, close analogies of cellular materials and ferroelectrics will be discussedin the final part of this thesis.

V

UNCHARGED

CHARGING

ELECTRET


Chapter 7


In this chapter experimental results will be presented to show that dielectric barriermicrodischarges are responsible for the internal charging of cellular materials. Thebreakdown events are indicated by light emission from the voids or from thedischarge gap, as shown with porous PP foils and dielectric AGS. The Paschen lawfor electrical discharges will be applied to describe dielectric barrier discharges(DBDs) at different atmospheric pressures and with different air-gap geometry.

7.1 Preparation of Cellular Electret Systems

For the following investigations, samples with polypropylene foams and artificialstructure like those described in Section 5.3 were prepared. Poling experiments on PPfoams were performed with the structure shown in Fig. 7.1 (a). Polymer foils or filmsare sandwiched between transparent ITO-coated glass substrates, which are connectedwith a high voltage source. The structure has been chosen for two reasons: (i) thepossibility to perform electrode poling and (ii) the feasibility to investigate themicroplasma discharges within the voids. Polymer foils are sandwiched between theITO-coated substrates by means of thin paraffin layers. Fig. 7.1 (b) depicts a photo ofthe electret sandwich being mounted on a sample holder. An analogue samplepreparation technique is used for electret systems with porous FEP foils.

(a)

(b)

Figure 7.1: (a) Schematic view of a porous PP foil sandwiched between two ITO-coated glass slides bymeans of a thin sticking paraffin layer. (b) Top view of the electret sample mounted on a sampleholder.

As already described in Section 5.3, artificial air-gap structures (AGS) are a perfectmodel system for a fundamental study of internal charging processes in voidedmaterials. The structure described in Fig. 5.4 consists of two polymer layers separatedby a thin air-gap (see also Fig. 5.4 in Section 5.3).

glassITO

PP foam

ITOglass


Figure 7.2: Schematic sample geometry for artificial air-gap structures (AGS).

PFCB and BCB films with thicknesses around 5 µm thick were obtained by spin-coating and by subsequent annealing up to 300°C for solvent removal and cross-linking. Thermal curing of PFCB films was performed in a nonreactive nitrogenatmosphere in order to avoid oxidation during the annealing process. In the followingexperiments, the air-gap is defined by a PET foil with a circular hole, which is fixedto the polymer layer by two-component glue. For less rigid sample sandwiches, whichare required e. g. for the detection of electromechanically induced surfacedeformations, high-viscous liquids (e. g. Vaseline) are used. The hole defines thearea of the air-gap after placing and fixing the second part of the sandwich on thepolymer foil.

7.2 Time-Resolved Optical Detection of Discharges

During the application of voltage signals above the breakdown voltage bV of the air in

the gap, micro-discharges are initiated within the voids of cellular polymers. Themicro-discharges are accompanied by the emission of short light bursts from thevoids. The light emission can be seen as an evidence for the formation of anextremely non-equilibrium plasma at atmospheric pressure. The self-extinguishingmicro-discharges transfer a sheet charge density σ across the air-gap, asschematically shown in Fig. 7.3 together with the depiction of emitted light burstsfrom the discharge gap [Lindner02].

glass

ITOpolymer

spacer(schematically)


Ub

-Ub

time

time

σ

ac-v

olta

gelig

ht in

tens

ity

Figure 7.3: Dielectric barrier microdischarges in a foam (schematically). Self-extinguishing dischargesare ignited above the threshold voltage evidenced by the emission of short light pulses [Lindner02].

The sinusoidal ac-voltage is produced with a Philips PM 5193 function generatorconnected to a broadband high-voltage amplifier Trek 610D. For the time-resolveddetection of the emitted light bursts a fast photomultiplier from Hamamatsu (H6779)with a response speed of 1 ns has been used which allows recording of individualbreakdown events. The photomultiplier data are displayed and stored with theTektronix TDS520C digital oscilloscope with an electrical time constant of 2 ns. Asillustrated in Fig. 7.4, a short light burst with a duration of about 5 ns is emittedduring a single breakdown event, a clear sign of self-extinguishing microdischarges aswell as an indication for a very fast charge transfer across the air-gap.

Figure 7.4: Transient of the light emission from an individual breakdown event, showing amicrodischarge with a duration of a few ns.

The transient nature of the light emission can be recorded at different voltage levelswith time-resolved light detection over several periods of the applied sinusoidal ac-voltage. For comparison, Fig. 7.5 shows the photomultiplier recordings of a porous PPHS01 sample (Fig. 7.5 (a)) and a dielectric AGS (Fig. 7.5 (b)) over a time period of

0 5 100

5

10

15

PD

sig

nal (

a. u

.)

time (ns)


500 µs per half cycle of the applied voltage. Also shown in Fig. 7.5 are photographsof the blue light emission recorded at 500 Hz of the ac-voltage by a conventionalcamera. In the case of porous PP, the emitted light of a 1.5×1.2 cm² sample has beendetected, for AGS, dielectric barrier microdischarges occur within a circular area witha diameter of 8 mm. It can be clearly seen in Fig. 7.5 (top), that a certain thresholdvoltage is necessary for the ignition of electrical breakdowns. For PP foams, the onsetof discharges occurs at a voltage level of 1.5 kV, for AGS at about 600 V. The valuesare in good agreement with the Paschen breakdown mechanism in air at atmosphericpressure. Monitoring the light emission visualizes quite well the local distribution of the voidswithin the porous material. Therefore, a simple tool for the in-situ opticalinvestigation of the internal charging processes in cellular electret materials isprovided. Furthermore, the size distribution of the micropores can be obtained in anondestructive way.Fig. 7.6 illustrates the light emission from Dyneon materials. The left image (Fig. 7.6(a)) depicts a photo of a 140 µm thick FEP foil indicating large, elliptically shapedclosed microvoids. It is obvious from the photo that the void density is rather low inthis foil compared to PP. Discharges were performed with an ac-voltage signal of 3.5kV and 500 Hz. On the right (Fig. 7.6 (b)), light emission from a cellular cable can beobserved at 8 kV and 500 Hz of the input voltage.

The typical emission spectrum of dielectric barrier microdischarges buring in airranges from 300 to 440 nm, where the largest intensities of emitted light are found inthe UV range [Trunec00].


(a) (b)

Figure 7.5: Transient light emission during sinusoidal excitation with different voltage levels (topfigures) and photographs of the blue light emission from (a) voids of a PP foam and (b) a circular areaAGS observed during electrical breakdowns.

-1,0

-0,5

0,0

0,5

1,0

ap

plie

d a

c-vo

ltag

e (

no

rma

lize

d)

-1,0

-0,5

0,0

0,5

1,0

ap

plie

d a

c-vo

ltag

e (

no

rma

lize

d)

0 1 2 30

10

20

30

40 500 V 600 V 650 V 700 V

PM

re

spo

nse

(a.

u.)

time (ms)

0 1 2 30

5

10

15

PM

re

spo

nse

(a

. u

.)

time (ms)

1.00 kV 1.50 kV 1.75 kV 2.00 kV


(a) (b)

Figure 7.6: Photographs taken from cellular FEP polymer foams (a) microporous FEP foil and (b)porous polymer cable.

7.3 Characterization of Breakdown Events

The gas within the gap of AGS is ionized by a high voltage. The threshold voltage forthe ignition of dielectric barrier breakdown events within the gap depends on the typeof the gas and its pressure as well as on the gap width. These aspects have alreadybeen summarized by the Paschen’s law for electrical breakdowns in Chapter 6.In order to apply the Paschen model on simple cellular electrets, dischargeexperiments on air-gap sandwich structures with Teflon AF layers have beenperformed. The gaps were defined by spacers (plastic foils) of different thicknessranging from 3 to 300 µm. The air pressure was kept constant at atmospheric pressure.The threshold was determined by increasing the level of the ac-voltage with afrequency of 500 Hz, while the onset of light emission was detected with thephotomultiplier. The experimental result is shown in Fig. 7.7 together with an inset,demonstrating the theoretical results for the Paschen law for noble gas atmospheres[Hippler01]. The numbers in the graph denote the distance between the dielectriclayers.

Figure 7.7: Experimental Paschen curve of AGS at atmospheric pressure. The inset compares themeasurement with results discussed in [Hippler01].

1 10 1000

400

800

1200

1600

300 µm

150 µm

60 µm

25 µm6 µm

3 µm

Bre

akd

ow

n v

olta

ge (

V)

760*d (Torr*mm)


Unfortunately, it was not possible to figure out more accurately the left branch of thePaschen curve, i. e. the regime of the very fast decrease of the breakdown voltagewith increasing gap width. Experiments with different air-pressure may reveal resultsfor a better correlation between breakdown phenomena in AGS and the Paschentheory in the low pd -regime. The intention is, to charge not only the large voids butalso the small voids within cellular polymers. With an increased pressure it evenseems feasible to produce microplasmas in nanoscopic voids (with a pressure of morethan 5 bar, the air-gap thickness may be below 1 µm).

7.4 Temperature Dependence of the Charge Stability

Cellular electrets must show good charge stability in order to be interesting forcommercial applications. At room temperature, porous PP foams remain internallycharged even for years, while charged AGS discussed in this work become partiallydischarged after a few days, most probably caused by the humidity content of the airwithin the gap. At elevated temperatures, the charge stability of cellular polymermaterials decreases rapidly. Porous PP starts to discharge at temperatures above 50°C[Turnhout99]. For AGS with PFCB layers, the temperature dependent charge stabilityis depicted in Fig. 7.8.The experiment has been performed with a transparent sandwich sample on a LinkamTMS 90 heating stage. The experiment was performed with a heating rate of 2°C/min.During heating, the decrease of “polarization” - i. e. the decrease of the surfacecharges - is observed with SHG. The black curve shows the thermally stimulated discharge recorded directly after thecharging process while the red curve depicts a measurement performed 14 h aftercharging. The result demonstrates that the charge layers are unstable immediatelyafter charging, a quasi-stationary state is achieved after several hours. These deeplytrapped charges remain stable up to a temperature of 80°C, before the onset ofthermally stimulated discharges can be seen.

Figure 7.8: Thermally stimulated discharge of AGS immediately after charging (black curve) and 14 hafter charging (red curve).

0 20 40 60 80 100 120 140 160 180 2000,0

0,1

0,2

0,3

charge stability immediately after charging

charge stability 14 h after charging

SH

G-s

igna

l (a. u

.)

temperature (°C)

Part II. Plasma-Induced Charging of Nonpolar Cellular Materials 65

Chapter 8

Conclusion of Part II

In Part II of the thesis, the mechanism for the internal charging of cellular polymershas been investigated on cellular polypropylene foams and on artificial structures ofdielectric layers separated by an air-gap (AGS). Dielectric barrier microdischarges(DBDs), which are widely known in a variety of industrial applications, have beenidentified during the charging of cellular polymers. DBDs are self-extinguishingmicrodischarges that transfer a sheet charge across the voids in the polymer. Thecharges are trapped at the walls of the voids, so they form large macroscopic“dipoles”, responsible for the strong electromechanical effects in cellular materials.

DBDs have been investigated by monitoring the light emission from voids duringbreakdown. This technique also provides a simple means for the visualization of voidswithin cellular structures. The barrier discharges in the voids are shown to be inagreement with the Paschen law. Experiments indicate that microplasmas may begenerated in nanoscopic gap sizes at pressures above atmospheric pressures. Thereby,also small voids in the foam may be charged, which may result in a significantimprovement in the electromechanical properties.

The thermal stability of charged AGS has been recorded by SHG. The results indicatethat charged surfaces may nondestructively investigated by optical means. In an SHGmicroscope, even the lateral distribution of charges may be measured.

66 References Part II

References of Part II

[Akishev98] Y. S. Akishev, A. V. Dem’yanov, N. I. Trushkin, andM. V. Pan’kin, A transverse spatial structure of thebarrier discharge in rare gases, ICPP & 25th Conf. onContr. Fusion and Plasma Physics, Praha, ECA Vol.22C, pp. 2489-2492 (1998).

[Bauer-Gogonea01] S. Bauer-Gogonea and S. Bauer, Polymer electrets forelectronics, sensors, and photonics, Handbook ofAdvanced Electronic and Photonic Materials andDevices Vol. 10, pp. 185-231, Academic Press (2001).

[Callegari00] Th. Callegari, R. Ganter, and J. P. Boeuf, Diagnosticsand modeling of a macroscopic plasma display panelcell, J. Appl. Phys. Vol. 88, pp. 3905-3913 (2000).

[Eliasson97] B. Eliasson and Ulrich Kogelschatz, EquilibriumVolume Plasma Chemical Processing, Trans. Plasma.Sci. Vol. 19, pp. 1063-1077 (1997).

[Elsbergen00] V. v. Elsbergen, P. K. Bachmann, and T. Juestel, Ion-Induced Secondary Electron Emission: A ComparativeStudy, Philips Research Laboratories SID 00 Digest, pp.220-223 (2000).

[Gerhard-Multhaupt99] R. Gerhard-Multhaupt, Electrets, in Wiley Encyclopediaof Electrical and Electronics Engineering Vol. 6, J. G.Webster (Ed.), pp. 220-229, John Wiley & Sons, NewYork (1999).

[Gibalov92] V. Gibalov and G. Pietsch, Proc. Int. Conf. of GasDischarges and their Appl., Swansea, GB, pp. 552-555(1992).

[Hippler01] R. Hippler, S. Pfau, M. Schmidt, K. H. Schoenbach(Eds.), Low Temperature Plasma Physics –Fundamental Aspects and Applications, John Wiley andSons (2001).

[Kacprzyk95] R. Kacprzyk, E. Motyl, J. B. Gajewski, and A.Pasternak, Piezoelectric properties of nonuniformelectrets, J. Electrostat. Vol. 35, pp. 161-166 (1995).

[Klein01] M. Klein, N. Miller, and M. Walhout, Time-resolvedimaging of spatiotemporal patterns in a one-dimensional dielectric barrier discharge system,Physical Review E Vol. 64, pp. 1-5 (2001).

[Li97] J. Li and S. K. Dhali, Simulation of microdischarges ina dielectric-barrier discharge, J. Appl. Phys. Vol. 82,pp. 4205-4210 (1997).

[Lieberman94] M. A. Lieberman, A. J. Lichtenberg, Principle ofPlasma Discharges and Materials Processing, JohnWiley and Sons, New York (1994).

[Lindner02] M. Lindner, S. Bauer-Gogonea, and S. Bauer, Dielectricbarrier microdischarges: Mechanism for the chargingof cellular piezoelectric polymers, J. Appl. Phys. Vol.91, pp. 5283-5287 (2002).

Part II. Plasma-Induced Charging of Nonpolar Cellular Materials 67

[Mason64] W. P. Mason (Ed.), Physical Acoustics, AcademicPress, New York (1964).

[McCrum91] N. G. McCrum, B. E. Read, and G. Williams, Anelasticand Dielectric Effects in Polymer Solids, Dover Publ.,New York (1991).

[Neugschwandtner00a] G. S. Neugschwandtner, R. Schwödiauer, S. Bauer-Gogonea, and S. Bauer, Giant piezoelectricity incharged heterogeneous fluoropolymer electrets,Appl. Phys. A Vol. 70, pp. 1-4 (2000).

[Neugschwandtner00b] G. S. Neugschwandtner, R. Schwödiauer, M. Vieytes,S. Bauer-Gogonea, S. Bauer, J. Hillenbrand, R.Kressmann, G. M. Sessler , M. Paajanen, and J.Lekkala, Large and broadband piezoelectricity in smartpolymer-foam space-charge electrets, Appl. Phys. Lett.Vol. 77, pp. 3827-3829 (2000).

[Neugschwandtner01] G. S. Neugschwandtner, R. Schwödiauer, S. Bauer-Gogonea, and S. Bauer, Piezo- and pyroelectricity of apolymer-foam space-charge electret, J. Appl. Phys. Vol.89, pp. 4503-4511 (2001).

[Oversluizen02] G. Oversluizen, M. Klein, S. de Zwart, S. van Heusden,and T. Dekker, Improvement of the discharge efficiencyin plasma displays, J. Appl. Phys. Vol. 91, pp. 2403-2408 (2002).

[Raizer97] Y. P. Raizer, Gas Discharge Physics, Springer, Berlin(1997).

[Sankaran01] R. M. Sankaran and K. P. Giapis, Maskless etching ofsilicon using patterned microdischarges, Appl. Phys.Lett. Vol. 79, pp. 593-595 (2001).

[Savolainen89] A. Savolainen, K. Kirjavainen, Electromechanical film,Pt. I, Design and characteristics, J. Macromol. Sci.Chem. A Vol. 26, pp. 583-591 (1989).

[Schwödiauer98] R. Schwödiauer, S. Bauer-Gogonea, S. Bauer, J. Heitz,E. Arenholz, and D. Bäuerle, Charge stability of pulsed-laser deposited polytetrafluoroethylene film electrets,

Appl. Phys. Lett. Vol. 73, pp. 2941-2943 (1998).[Schwödiauer99] R. Schwödiauer, J. Heitz, E. Arenholz, S. Bauer-

Gogonea, S. Bauer, and W. Wirges, Plasma- andpulsed-laser-deposited polytetrafluoroethylene (PTFE)-like thin films : A comparative study on PTFE-specificproperties, J. Polym. Sci. B: Polym. Phys. Vol. 37, pp.2115-2125 (1999).

[Schwödiauer00a] R. Schwödiauer, G. S. Neugschwandtner, K.Schrattbauer, M. Lindner, M. Vieytes, S. Bauer-Gogonea, and S. Bauer, Preparation andcharacterization of novel piezoelectric and pyroelectricpolymer electrets, IEEE Trans. Diel. Electr. Insul. Vol.7, p. 578 (2000).

68 References Part II

[Schwödiauer00b] R. Schwödiauer, G. S. Neugschwandtner, S. Bauer-Gogonea, and S. Bauer, Dielectric and electretproperties of nanoemulsion spin-on polytetrafluoro-ethylene films, Appl. Phys. Lett. Vol. 76, pp. 2612-2614(2000).

[Sessler99] G. M. Sessler and J. Hillenbrand, Electromechanicalresponse of cellular electret films, Appl. Phys. Lett.Vol. 75, pp. 3405-3407 (1999).

[Siemens57] W. Siemens, Poggendorffs Ann. Phys. Chem. Vol. 102,pp. 66-122 (1857).

[Trunec00] D. Trunec, A. Brablec, and J. Buchta, Efficiency ofOzone Production in Atmospheric Pressure Glow andSilent Discharges, Dep. of Phys. Electronics, Faculty ofScience, Masaryk University, Czech Republic (2000).

[Turnhout99] J. v. Turnhout, R. E. Staal, M. Wübbenhorst, and P. H.Haan, Distribution and Stability of Charges in PorousPolypropylene Films, 10th International Symposium onElectrets, IEEE (1999).

[Xu98] X. P. Xu and M. J. Kushner, Multiple microdischargedynamics in dielectric barrier discharges, J. Appl.Phys. Vol. 84, pp. 4153-4160 (1998).



_____________________________________________________________________

Part III

Ferroelectric-Like Effects in NonpolarPolymers

Part III. Ferroelectric-Like Effects in Nonpolar Polymers 71

Chapter 9

Semicrystalline Dipole Electrets

Semicrystalline dipole electrets contain polar crystallites within a polar amorphousmatrix. The crystallites in most of these polymers are ferroelectric. Typical examplesof technologically important ferroelectric polymers are polyvinylidene fluoride(PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)). Ferroelectric polymers are functionalized by poling for piezo- and pyroelectricapplications. Hereby, molecular dipoles within the polar crystallites are oriented. Incontrast to the polar amorphous polymers investigated in Part I of this thesis,semicrystalline dipole electrets show quite strong piezo- and pyroelectric effects, butweak nonlinear optical (NLO) effects.

9.1 Crystal Structure and Ferroelectricity

In semicrystalline ferroelectric polymers, the ferroelectric crystallites are embedded inan amorphous, polar matrix. The chemical structure of the ferroelectric polymerpolyvinylidene fluoride (PVDF) and of its copolymers with trifluoroethylene (P(VDF-TrFE)) are schematically depicted in Fig. 9.1. These polymers are partially fluorinatedwith a dipole moment of 2.3 Debye (D) per repeat unit for PVDF. For P(VDF-TrFE)the dipole moment is lower due to the incorporation of TrFE monomers.

(a) (b)

Figure 9.1: Chemical structures of fluorinated ferroelectric polymers: (a) polyvinylidenefluoride(PVDF) and (b) polyvinylidene trifluoroethylene copolymer (P(VDF-TrFE)).

For semicrystalline PVDF, at least five different crystalline phases (α , β , γ , δ , andε -phase) have been identified. The degree of crystallinity in PVDF is around 50%. Inthe crystalline form, the polymer shows regular chain conformations in which thesubstituents are located at 180° to each other (trans- or t-conformation) or at ±60°(gauche±- or g±-conformation). The most stable crystalline phase of PVDF is thenonpolar α -phase with a tg+tg- chain conformation. The α -phase can be transformedinto the polar, ferroelectric β -phase with an all-trans-conformation by mechanicalstretching. The δ - and the ε -phases are the polar counterparts of the α - and the γ -phases, respectively. The different crystalline phases together with the correspondingchain conformations are shown in Fig. 9.2.

CH2

CF2n

CH2

CF2

x

CH2

CHFy

n

y=1-x

72 Chapter 9. Semicrystalline Dipole Electrets

Figure 9.2: Chain conformation and crystalline structure of the five different crystalline phases ofPVDF; after [Schilling88].

Copolymers of PVDF with a TrFE content exceeding 20 mol% crystallize directlyinto the polar β -phase. For PVDF and its copolymers, the glass-transition of theamorphous phase is around -40°C. The amorphous matrix is in the rubbery state atroom temperature.After poling, the ferroelectric crystallites are no longer split in domains, and trappedcharges at the interface between crystallites and amorphous matrix are necessary forthe compensation of the depolarizing field and for stabilizing the polarization (see Fig.9.3 [Bauer-Gogonea01]).

Figure 9.3: Ferroelectric crystallite with compensation charges; after [Bauer-Gogonea01].


The size of the crystallites is typically around 50 nm. Dielectric, ferro-, piezo-, andpyroelectric properties of ferroelectric polymers depend strongly on the crystallinityand on the morphology of the material.

The phase diagram of P(VDF-TrFE) copolymers is summarized in Fig. 9.4[Furukawa97]. The ferroelectric polymers show a phase transition from a high-temperature paraelectric to a low-temperature ferroelectric phase from 50 to 80 mol%VDF. Above 80 mol% VDF, no phase transition is observed, since the extrapolatedCurie temperature CT of 205°C is higher than the melting temperature mT of 180°C.

Between 60 and 80 mol% VDF, the copolymers reveal a discontinuous first-orderphase transition accompanied by a thermal hysteresis upon heating and cooling (seeFig. 9.5 [Heiler94]). This thermal hysteresis is related to the large size distribution ofthe crystals in the lamellar chain. Continuous second-order phase transitions withoutthermal hysteresis occur between 50 and 60 mol% VDF. Finally, below 50 mol%VDF content, P(VDF-TrFE) undergoes a transition from the high-temperatureparaelectric to the low-temperature antiferroelectric state. In electron-beam irradiatedcopolymers a new phase has been observed recently, the transition from ferroelectricto relaxor ferroelectric behavior [Zhang02].

Figure 9.4: Phase diagram of P(VDF-TrFE) as a function of the VDF content and temperature[Furukawa97].

Ferroelectricity in ferroelectric materials can be revealed by the switching ofpolarization in an alternating electric field. This switching results in a polarizationhysteresis loop. Switching of polarization in ferroelectric polymers can be performedfast but the switched polarization may not be stable, since the transport ofcompensation charges is a slow process.


9.2 Dielectric Properties of Ferroelectric Polymers

The dielectric response of semicrystalline polymers is rather complex to describe,since it is a heterogeneous system consisting of amorphous and crystalline phases.The dielectric function of the amorphous phase may be described according to thediscussion on photonic polymers in Chapter 1 (Section 1.2). The ferroelectric-to-paraelectric phase transition in copolymers with 50 to 80 mol% VDF is evidenced bya sharp increase below and a sharp decrease of the dielectric function above the phasetransition temperature.Ferroelectricity in polymers is an effect related to the crystallites. The temperaturedependence of the linear dielectric function of ferroelectric crystallites can bephenomenologically described by the Landau-Devonshire theory of phase transitions[Lines77]. Thereby, the free energy is given by

6420 6

1

4

1

2

1DDDFF δγα +++= (9.1)

where α , γ , and δ are the Landau parameters. The properties of most ferroelectricsare well described by a temperature dependent coefficient α (Devonshireapproximation)

)( 0TT −= βα (9.2)

and by temperature independent parameters γ and 0>δ . 0T is the Curie temperature

upon cooling for first-order phase transitions (thermal hysteresis) and upon heatingand cooling for second-order phase transitions ( CTT =0 ). The electric field E as a

power series of D follows from the derivative TDFE )/( ∂∂= :

53)( DDDTE δγα ++= (9.3)

Minima of the free energy F are obtained with E equal to zero. The sign of γdetermines the order of the phase transition. For 0<γ , the phase transition is of firstorder. The five different solutions for the free energy F and a detailed discussion ofthe different temperature regimes are found in [Bauer-Gogonea01]. For 0>γ , asecond-order phase transition is observed. Herein, the linear dielectric function ε isobtained from the derivative ( TED )/( ∂∂∝ε ) with TDFE )/( ∂∂= :

2

2

D

F

∂∂

∝ε (9.4)

The dielectric function in the paraelectric phase is described as a function of theLandau parameters by:

)(

1110

CTT −==βα

εε (9.5)


Eq. (9.5) is known as the Curie-Weiss law.

Fig. 9.5 [Heiler94] summarizes experimental results on first- and second-order phasetransitions of ferroelectric polymers by showing the temperature dependent dielectricfunction for two different P(VDF-TrFE) copolymers. The thermal hysteresis in thedielectric function of the 70/30 copolymer upon heating and cooling, characteristic forfirst-order phase transitions, is clearly detected.

Figure 9.5: Temperature-dependent dielectric function of two different P(VDF-TrFE) copolymers. Afirst-order phase transition of a (70/30) sample is observed on the left, the (56/44) sample on the rightshows a typical second-order phase transition [Heiler94].

The Landau-Devonshire approach describes the dielectric function of ferroelectricmaterials only on a phenomenological basis and provides no physical insight into themechanisms behind the nonlinear dielectric response. Microscopic models based on adetailed discussion of intrachain and interchain interactions in ferroelectric polymerswere developed to provide insight into the phase transitions of ferroelectric polymers[Zhang93].

9.3 Piezo- and Pyroelectricity

On a phenomenological basis, piezoelectricity is electrostrictive in origin, biased bythe spontaneous polarization [Furukawa89]. The most important contribution topiezo- and pyroelectricity in ferroelectric polymers arises from dimensional changesof the amorphous and crystalline phase due to mechanical stress and thermalexpansion. The thermal stability of polarization in e. g. semicrystalline PVDF isdetermined for example by measuring the pyroelectric response with cyclic increaseand decrease of the temperature as shown in Fig. 9.6 [DeRossi82]. It can be observedthat the pyroelectric signal decreases irreversibly already at temperatures below theCurie temperature CT . The observed depolarization process is ascribed to the

thermally induced detrapping of compensation charges required for the stabilizationof polarization.


Figure 9.6: Pyroelectric response of a poled PVDF sample during cyclic in- and decrease of the sampletemperature indicating detrapping of compensation charges [DeRossi82].

Typical values for the piezo- and pyroelectric coefficients of PVDF and itscopolymers are presented in the Appendix.

9.4 Nonlinear Optical Properties of Ferroelectric Polymers

Poled ferroelectric polymers are birefringent and dichroitic as a result of the oriented,optically anisotropic molecular dipoles. Birefringence and dichroism are stronglynonlinear and show butterfly hysteresis loops. The Electro-optical (EO) Pockels and Kerr effects in semicrystalline ferroelectricpolymers are mainly of piezoelectric and electrostrictive origin. Values for the EOPockels coefficient of around 1 pm/V are obtained for PVDF and P(VDF-TrFE)copolymers, only weakly dependent on the wavelength throughout the near infraredand visible region. The EO response is highly nonlinear as a function of the polingfield and shows also a hysteresis loop. For details about the determination of Pockelsand Kerr coefficients the reader is referred to [Ballato02].Like EO effects, second-harmonic generation (SHG) in materials like PVDF is rathernon-efficient. Nevertheless, SHG techniques provide potential methods for thecharacterization of materials, since no conductivity contribution is affecting themeasurement [Aktsipetrov00]. For example, SHG can be used in order to gaininformation about the orientation distribution of polymer chains in ferroelectricpolymer films.


Chapter 10

Techniques for the Investigation of Ferroelectric Materials

In a ferroelectric material, the spontaneous polarization SP can be switched between

equilibrium directions with an experimentally accessible electric field [Ballato02].Ferroelectric materials are primarily characterized by the spontaneous polarization

SP , at zero electric field the remanent polarization is retained. In ideal ferroelectric

materials, SP is equal to rP , but e. g. due to bulk defects rP is usually smaller than

SP . The dipoles within the crystallites of semicrystalline ferroelectric polymers are

oriented by an external electric field exceeding the coercive field CE . After removing

the poling field, the polymers show a remanent polarization rP . Above the Curie

temperature CT , the spontaneous polarization SP vanishes and a transition from the

ferroelectric to the paraelectric phase is observed. In this chapter, several techniques for the investigation of ferroelectric materials willbe discussed.

10.1 Dielectric Hysteresis

If the polarization P or the dielectric displacement D is recorded as a function of analternating electric field, a hysteresis loop is obtained. From the hysteresis, values for

rP , SP , and the coercive field CE can be derived. Typical examples of a )(EP and

)(ED hysteresis are shown in Fig. 10.1 (a) and (b) [Furukawa80], where 10.1 (a)depicts a schematic sketch of a hysteresis loop, while 10.1 (b) shows experimentalresults on the ferroelectric polymer PVDF.

(a) (b)

Figure 10.1: (a) Typical hysteresis curves for ferroelectric materials below the Curie temperature

CTT < and the proportionality of the polarization to the electric field for CTT > ; (b) Experimentally

obtained hysteresis in the dielectric displacement demonstrated on the ferroelectric polymer PVDF[Furukawa80].

78 Chapter 10. Techniques for the Investigation of Ferroelectric Materials

The shape of the hysteresis depends on the switching of domains, which are parts ofthe material with the same polarization. Typical dimensions of domains inferroelectric materials are between 0.1 and 100 µm separated by domain walls with athickness of 1 to 10 lattice parameters [Ballato02]. The polarization is switched by thedisplacement of domain walls and the switching of domains. The energy, which isnecessary to perform one dielectric hysteresis cycle corresponds to the area within thecurve. Details about switching properties of domains can be found e. g. in [Ballato02,Dougherty72, Lines77, Sapriel75].

The standard technique for a direct extraction of the ferroelectric hysteresis is theSawyer-Tower circuit, which is schematically shown in Fig. 10.2 together with theequivalent circuit diagram [Dragosits01].

(a) (b)

Figure 10.2: (a) Sawyer-Tower circuit for the measurement of dielectric hysteresis loops offerroelectric materials and (b) its equivalent circuit diagram [Dragosits01].

The alternating input signal )(tVi is produced by a function generator and a high-

voltage amplifier. The circuit itself consists of the ferroelectric capacitor FC

(ferroelectric sample with top and bottom electrode) and a capacitor SC . The voltage

between the two capacitors is measured with a digital oscilloscope. The dielectricdisplacement D has the dimension of a charge (Q ) density: == AQ /σ [µC/cm²].

For a iVD − hysteresis loop, the output voltage outV has to be multiplied by ACS / :

====

2cm

CD

A

Q

A

CV S

out

µσ (10.1)

In the equivalent circuit diagram, the serial resistance FR and the input impedance iR

of the digital oscilloscope are considered. FR is preferably quite high, reasonable

values are in the range of 1000 MΩ. The typical value for the input impedance FR is

1 MΩ. For the serial capacitor SC the condition FS CC >> is required.


In Part II of the work, nonpolar cellular electrets have been demonstrated withoriented macroscopic “dipoles” after charging. On these materials, dielectrichysteresis measurements are applied in order to investigate the switching properties ofthe “polarization”. In the next chapter, dielectric barrier discharges (DBDs) at high ac-voltages will be shown to be responsible for the formation of hysteresis loops in thedielectric displacement of cellular materials.

10.2 Electromechanical Effects

Dielectric hysteresis loops have been obtained in nonpolar dielectrics without beingaccompanied by piezo- and pyroelectric effects [Gross50, Wegener01]. In order toextend ferroelectric(-like) properties in heterogenous nonpolar polymer electrets, themechanical strain is recorded as a function of an ac-voltage applied to the sample.Thereby, the sample is sandwiched between two conducting electrodes. The thicknesschanges are measured in-situ with a DekTak3 stylus surface profilometer in staticmode. The experimental arrangement is illustrated schematically in Fig. 10.3. TheDekTak has been modified in order to allow for the evaluation of the profilometersignals directly with an oscilloscope.

Figure 10.3: Experimental setup for the detection of the surface displacement of ferroelectric(-like)materials during the application of an ac-voltage.

The problem with the experimental setup is that periodic thickness changes can berecorded only at low-frequencies between 50 and 100 mHz. At higher frequencies, themechanical system of the stylus profilometer does not follow the periodicdisplacement of the sample surface. Nevertheless, with this technique, hysteresiscurves of the surface displacement versus the applied electric field or voltage can bemeasured with charged nonpolar cellular polymer electrets. Furthermore,electrostrictive and converse piezoelectric effects of these materials will bedetermined in Chapter 11.

In ferroelectric polymers, the piezoelectric response can be switched by poling, due tothe proportionality to the poling field PE . Electrostriction, which is related to 2E[Yuki98], is observed in any solid dielectric material. In dielectrics without a center ofsymmetry, the quadratic electrostrictive effect can be neglected against the linearpiezoelectric effect. Switching of the piezoelectric response of nonpolar cellularpolymers will be also demonstrated in the next chapter.The direct piezoelectric coefficient ( 33d ) is obtained by quasi-static measurements

with the experimental setup shown in Fig. 10.4 [Schwödiauer00].

ac-voltage

Osc./PC

sample

80 Chapter 10. Techniques for the Investigation of Ferroelectric Materials

Figure 10.4: Schematic view of the experimental setup for quasi-static piezoelectric measurements onpolymer samples; after [Schwödiauer00].

A mass m of 150 g is periodically loaded to the sample, and the charge response ofthe piezoelectric sample is recorded with a Keithley 1017 charge amplifier anddisplayed on a digital storage oscilloscope (Tektronix TDS520C). The mass mgenerates a force mgF = , with the charge Q accumulated on the sample electrodesduring loading, the direct piezoelectric coefficient can be determined from

== FQd /33 [pC/N].

10.3 Nonlinear Optical (NLO) Effects

In transparent ferroelectric polymer films, experimental techniques based on second-harmonic generation (SHG) have already been reported for the investigation oforiented, optically anisotropic molecular dipoles [Boyd87]. Since the opticalnonlinearities depend on the polarization of the sample, hysteresis loops indicating theswitching of polarization can also be generated with nonlinear (NLO) optical methodslike SHG [Wicker89a, Wicker89b] and electro-optics.

A typical example for a SHG hysteresis in transparent ferroelectric copolymer films isshown in Fig. 10.5 [Wicker89a, Wicker89b]. The experiment was performed on a 21µm polymer film with an alternating electric field of 1 mHz. In the next chapter, SHG will be used for the investigation of cellular polymers byshowing hysteresis loops in the SH response.


Figure 10.5: Hysteresis in SHG performed on thin films of P(VDF-TrFE) copolymers [Wicker98a,Wicker89b]. The immediate decrease of the SH intensity during polarization switching is seen in thepicture.

In addition to SHG, hysteresis loops are recorded for the electro-optical (EO) effect.The experimental arrangement for the measurement of EO hysteresis loops is depictedin Fig. 10.6.

Figure 10.6: Schematic experimental setup for the detection of the electric-field-induced change of therefractive index for the formation of hysteresis loops of EO response.

Linearly polarized light with a wavelength of 532 nm is produced by a 10 mW laserdiode. The analyzer is mounted in a way, that the light intensity after the analyzer isalmost zero when no sample is mounted. Finally, the air-gap sample is placed betweenthe polarization filters and an alternating poling voltage is applied. Due to the EOeffect, the plane of polarization of the light is turned and the light intensity on thephotomultiplier increases and decreases periodically.

laser diode

polarizer analyzer

sample sandwich

PM

ac-voltage

osc.

Part III: Ferroelectric-Like Effects in Nonpolar Polymers 83

Chapter 11


In the literature, a ferroelectric material is usually defined by a number of propertiessuch as the spontaneous polarization, the Curie transition from the ferroelectric to theparaelectric state, the Curie-Weiss law, and the hysteresis in polarization. Newcrystalline materials have shown that not all of the above mentioned features arenecessary conditions to make a system ferroelectric [Ballato02]. Considering physicaland engineering aspects, a material is assumed to be ferroelectric if it showsreorientation between domain states i. e. a controllable switching of polarization.The switching of dielectric, electromechanical and optical properties are going to bediscussed in the following in order to proof ferroelectric-like effects in chargednonpolar cellular polymer electrets.

11.1 Hysteresis in the Dielectric Function

A hysteresis loop in ferroelectric single crystals is obtained if they are used asdielectric material in a capacitor such that the ferroelectric axis is parallel to theapplied electric field. Nonpolar cellular electrets are investigated in a capacitorstructure with top and bottom electrodes. In order to monitor the “polarization”reversal, the Sawyer-Tower circuit, described in the last chapter, was used. Theexperimental method yields the dielectric displacement (or polarization) as a functionof the applied voltage.Fig. 11.1 shows a series of dielectric hysteresis loops recorded with a Sawyer-Towercircuit with a serial capacitor SC of 210 pF. In Fig. 11.1 (a), a porous polypropylene

(PP) HS01 foil was sandwiched between two ITO-coated glass substrates and asinusoidal ac-voltage with a frequency of 500 Hz was applied. Below the breakdownvoltage bV of ±1.5 kV, only the capacitive response is observed since no charges are

transferred across the air-filled voids (blue line). Increase of the voltage up to ±5 kVdemonstrates hysteresis loops in the displacement due to the “switching” of charges(red curve).PP foams show a large size-distribution of the micropores related to differentbreakdown voltages of individual voids. This means that discharges in the differentvoids appear at different times. Therefore, the hysteresis in Fig. 11.1 (a) is broad likein ferroelectric materials with a large number of defects. In Fig. 11.1 (b), hysteresisloops obtained on an AGS structure with a 50 µm air-gap and PFCB dielectric layersare shown. The breakdown voltages are ≈bV ±600 V. Below the threshold voltage,

only the capacitive response is seen, above bV at a voltage level of ±800 V, a typical

ferroelectric-like ED − hysteresis is observed.


(a) (b)

(c) (d)

Figure 11.1: Dielectric hysteresis loops measured with the Sawyer-Tower circuit: (a) Porous PP HS01,(b) PFCB AGS, (c) porous FEP cable, and (d) frequency dependence of hysteresis loops in PFCB AGS.

A slim dielectric hysteresis is found in cellular FEP cables, as depicted in Fig. 11.1(c). The insulator thickness in the cable has a thickness of 550 µm, and at ±4 kV ofthe input voltage, the hysteresis loop indicates the voids and a low void-density in thepolymer bulk.Fig. 11.1 (d) compares the hysteresis loops of PFCB AGS generated at differentfrequencies of )(tVi . At 500 Hz, the highest values for D and CE are achieved,

above this frequency (see the inset), the results show a decrease of the dielectricdisplacement D . It might be possible that higher frequencies of the electric field leadto a substantial temperature rise in the sample [Ballato02, Dickens92]. At lowfrequencies in the range between 10 and 100 Hz, a decrease of CE is observed.

Experiments at high frequencies are only possible up to frequencies of 10 kHz, due tothe bandwidth limitation of the high voltage amplifier.

From the experimental results shown above, values for CE between 25 and 30 V/µm

are obtained for PP HS01 and around 10 V/µm for PFCB AGS. Typical values forsingle crystals and ceramics lie in the range 0.01-0.1 V/µm and for ferroelectricpolymers between 30-100 V/µm (e. g. 60 V/µm for PVDF).

-4 -2 0 2 4-0,12

-0,08

-0,04

0,00

0,04

0,08

0,12

1 kV 5 kV

die

lect

ric

dis

pla

cem

en

t D

(µ

C/c

m2 )

applied ac-voltage (kV)

-1000 -500 0 500 1000-0,2

-0,1

0,0

0,1

0,2

500 V 800 V

dis

pla

cem

ent

D (

µC

/cm

2 )

applied ac-voltage (V)

-4 -2 0 2 4-1,0

-0,5

0,0

0,5

1,0

4 kV

die

lect

ric

dis

pla

cem

ent

D (

µC

/cm

2 )


-600 -400 -200 0 200 400 600-0,2

-0,1

0,0

0,1

0,2 1 kHz 500 Hz 100 Hz 50 Hz 10 Hz

die

lect

ric

dis

pla

cem

en

t D (

µC

/cm

2 )



Tilted or asymmetric loops may occur in samples with material defects in the bulk orin near surface layers. An example of such a distorted hysteresis loop is shown in Fig.11.2. The microscopic mechanism behind the formation of the “steps” in thehysteresis curve depicted in Fig. 11.2 [Su97] is not completely understood yet.

Figure 11.2: EP − hysteresis for a ferroelectric with a damaged surface layer (steps in hysteresiscurves; after [Su97].

In Fig. 11.3 (a), “ferroelectric” bulk defects are simulated by inserting a 3 µm thickloose polymer foil (PET) into a BCB air-gap sandwich system. The double hysteresisof this sample is shown in Fig. 11.3 (b). An interpretation of this experimental resultcannot be given at the moment.

(a)

(b)

Figure 11.3: (a) Schematic sample geometry for simulating “macroscopic bulk defects” in order togenerate (b) a tilted ED − hysteresis loop.

-1500 -1000 -500 0 500 1000 1500-0,15

-0,10

-0,05

0,00

0,05

0,10

0,15

1250 V

die

lect

ric

dis

pla

cem

en

t D

(µ

C/c

m2 )


polymerfoil (PET)


11.2 Switching of the Electromechanical Effect

Hysteresis in displacement vs. field ( ED − ) is necessary but not sufficient toestablish ferroelectricity. Important physical properties coupled to “polarization” arethe electromechanical strain and the piezoelectric effect. If at least one of thecomponents of the mechanical strain tensor ijS is proportional to the electric field

within the discharge gap, the relation strain versus applied field roughly copies that ofthe ED − hysteresis loop. The strain is related to the square of “polarization”(electrostrictive in nature), so that the ESij − dependence shows a butterfly-like

hysteresis. Fig. 11.4 summarizes experimental results on the switching of electromechanicalproperties of cellular electrets. In Fig. 11.4 (a), the surface displacement of a low-density PP sandwich sample is recorded as a function of an applied voltage with afrequency of 0.1 Hz. It can be noted that below the breakdown voltage bV of ±2 kV

only the quadratic electrostrictive response is observed (blue curve). Above bV , the

strong influence of the piezoelectric effect is responsible for the hysteresis behavior(red curve). The butterfly loop in this experiment illustrates very well the linearpiezoelectric and the quadratic electrostrictive contributions. For comparison, Fig.11.4 (b) demonstrates switching of the mechanical strain on a cellular high-density PPsample (less voids in comparison to the low-density PP foam). It can be seen that dueto the smaller porosity the effects in the electromechanical strain are smaller.Furthermore, a strong asymmetry in the loop is obtained at a voltage level of ±4 kV.This may be explained by inhomogeneous charge trapping effects. Results, similar to the low-porosity PP are obtained with a 95 µm thick FEP foil (seeFig. 11.4 (c)). Since the voids are larger than in the low porosity PP foam, the valuefor bV is lower.

AGS with PFCB dielectric layers display excellent symmetric electromechanicalhysteresis loops, as depicted in Fig. 11.4 (d). This is a proof for the homogeneity ofthe dielectric surface morphology and the charge storage properties. Up to ±400 V,the surface displacement is characterized only by electrostriction (blue curve), at ±900V, the mechanical strain shows piezoelectricity at low voltage levels below ±200 Vand a butterfly hysteresis (red curve). The shape of the butterfly loop showsirregularities above ±400 V during the increase of the applied voltage. Theseirregularities show the electrical breakdown events, which occur above bV . The small

mechanical disturbances due to these breakdowns are detected by the highly-sensitivesurface profilometer.


(a) (b)

(c) (d)

Figure 11.4: Experimental results of switching of the electromechanical material properties performedon porous PP foams with a different degree of porosity (a) high porosity (b) low porosity, (c) cellularFEP foam, and (d) PFCB AGS. The butterfly loop looks similar to the electromechanical strain loops offerroelectric materials.

A common feature of all the electromechanical hysteresis loops above is that thestrain becomes minimal when the quasi-polarization reverses (i. e. 0=D ) andapproaches a maximum at the saturation of D . In general, this is explained byintrinsic piezoelectricity where the molecular dipoles become stretched or stressedduring mechanical sample deformation. Intrinsic piezoelectric effects with a positivepiezoelectric 33d coefficient are usually restricted to ferroelectric crystals and

ceramics. In ferroelectric polymers, piezoelectricity is a dimensional effect ( 33d is

negative) in which the molecular dipoles retain fixed moment and orientation duringsample deformation [Furukawa90]. The piezoelectric charge response of PVDF andits copolymers is - for example - related to macroscopic dimensional changes, sincethe dipolar groups are rigidly connected with the main-chain carbons. In contrast tocrystals and ceramics, ferroelectric polymers become thickest when their polarizationis reversed (compare Fig. 11.5 [Li-Jie95]).

-8 -4 0 4 8-6

-4

-2

0

2 kV 7 kV

su

rfa

ce d

ispl

ace

me

nt (

µm

)

applied ac-voltage (kV)-4 -2 0 2 4

-800

-600

-400

-200

0

2 kV 4 kV

surf

ace

dis

pla

cem

en

t (n

m)


-1000 -500 0 500 1000

-600

-400

-200

0

400 V 900 V

surf

ace

dis

pla

cem

en

t (n

m)


-4 -2 0 2 4

-600

-400

-200

0

1 kV 4.5 kV

surf

ace

dis

pla

cem

en

t (n

m)



Figure 11.5: Hysteresis in the mechanical strain of a thin P(VDF-TrFE) copolymer film; after[Li-Jie95].

The piezoelectric third-rank tensor relates either the dielectric displacement tomechanical stress or the mechanical strain to the electric field (see Chapter 5). Forpiezoelectric sensors, the piezoelectric 33d coefficient is important. From a practical

point of view, it is much more comfortable to describe piezoelectricity withexperimentally related properties, like electrode charge Q , mechanical force F ,sample thickness x , and applied voltage V :

FV V

x

F

Qd

∂∂

=

∂∂

=33 (11.1)

Direct piezoelectricity is defined as the generation of an electric signal due tomechanical stress or strain (pC/N), whereas the converse piezoelectric effect is thechange of the sample thickness due to an externally applied electric voltage (pm/V).Values for the 33d coefficient of nonpolar cellular electrets can be obtained by

measuring the converse or direct piezoelectric response. In the first method, thesample is charged by a dc-voltage above bV (barrier discharges), and a low frequency

ac-voltage below bV is used for the detection of the periodic surface displacement.

The displacement is measured by the Dek Tak surface profilometer and the results areshown in Fig. 11.6. In Fig. 11.6 (a), a 500 V ac-voltage with 0.4 Hz was applied on ahigh porosity PP sample sandwich after previous charging with +7 kV. In the nextstep, the electric field was reversed in order to ”switch” the “polarization”. Measuringthe converse piezoelectric signal again reveals a phase shift of 180° in comparison tothe previous result. Therefore, the piezoelectric response can be switched due to itslinear dependence on the poling field.Analogue experiments have been performed with dielectric AGS with BCB layers(Fig. 11.6 (b)). Here, the sample was charged with ±800 V before the conversepiezoelectric responses were recorded at a voltage level of 100 V (0.4 Hz).


Figure 11.6: DekTak recordings of the converse piezoelectric response of (a) a 7 kV charged PP foamand (b) a 800 V charged dielectric air-gap sandwich. Top figures show the sinusoidal excitation withthe individual peak values below the threshold voltage for electrical breakdowns.

From the results, 33d coefficients of 50 pm/V for charged porous PP films and 1500

pm/V for dielectric AGS are obtained. The latter value is rather high, demonstratingthat air-gap cellular electrets are potentially interesting for potential a large number ofpiezoelectric applications.

The experimental technique for measuring the direct 33d coefficient was discussed in

Chapter 10 and the results obtained with the technique are illustrated in Fig. 11.7. Thesignal peaks correspond to the periodically loaded mass on the sample. In Fig. 11.7(a), charged PP foams show a large 33d coefficient of 270 pC/N, comparable to that

of ferroelectric ceramics [Neugschwandtner01]. In Fig. 11.7 (b), a porous FEP foilwith a thickness of 180 µm was charged 10 seconds at -10 kV. The piezoelectriccoefficient for this system is ten times lower than the value of charged PP foams andcan be compared with 33d coefficients of ferroelectric polymers (e. g. =33d -30 pC/N

-100

-50

0

50

100

ap

plie

d a

c-vo

ltag

e (

V)

-500

-250

0

250

500

a

pp

lied

ac-

volta

ge (

V)

-20

0

20

40

surf

ace

dis

pla

cem

en

t (n

m)

0 1 2 3 4 5

-20

0

20

40

surf

ace

dis

pla

cem

en

t (n

m)

time (s)

-100

-50

0

50

100

150

200

250

surf

ace

dis

pla

cem

en

t (n

m)

0 1 2 3 4 5

-100

-50

0

50

100

150

200

surf

ace

dis

pla

cem

ent (

nm)

time (s)


for PVDF). However, since FEP is thermally more stable than PVDF the results pavethe way for thermally stable cellular piezoelectric materials.

Figure 11.7: Quasi-static measurement of the direct 33d coefficient for (a) charged PP foam

[Neugschwandtner01] and (b) a charged 180 µm thick cellular FEP foil.

The quasi-static piezoelectric coefficient is used only in low-frequency piezoelectricapplications. In ultrasound generation or detection, dynamic piezoelectric coefficientsare applied. In foams, it was found that the quasi-static and dynamic piezoelectriccoefficients are largely different, a detailed understanding of this effect is at presentnot available.

11.3 Switching of the Nonlinear Optical (NLO) Effects

The principle behind the formation of Second-harmonic generation (SHG) signals incharged nonpolar air-gap structures (AGS) is illustrated in Fig. 11.8. The fundamentalbeam is provided by a pulsed Nd-YAG laser (see Chapter 3). During the charging ofAGS by dielectric barrier microdischarges, oriented macroscopic “dipoles” aregenerated in the air-gap and in the polymer layers. In general, the square root of the

SHG intensity is directly related to polarization ( SHGIP ∝ ). The nonlinearity in the

polymer layers in Fig. 11.8 is much larger than in the air-gap and so, second harmonic(SH) light stems mainly from EFISH in the dielectric layers. Hysteresis loops in theSH intensity are generated by an ac-voltage applied on the sample with transparentelectrodes.

0 2 4 6 8 100

10

20

30

d3

3-co

eff

icie

nt (p

C/N

)

time (s)0 2 4 6 8 10

0

50

100

150

200

250

300

time (s)

piez

oele

ctric

res

pons

e (p

C/N

)


Figure 11.8: Description of SH light generation in charged AGS based on the formation of orientedmacroscopic “dipoles” due to “polarization”.

Electrooptical (EO) effects refer to ways in which electric fields applied to a materialmay affect the velocities v , with which light waves propagate. In general, for acrystalline material, two different plane-polarized waves having two differentvelocities but the same wave-normal propagate. The value of vc / for each wave iscalled the refractive index for that wave [Ballato02].The linear Pockels EO effect describes the change of the refractive index n of amaterial by a change of the external electric field ( En ∝ ). At high poling fields, theEO response becomes highly nonlinear and shows hysteresis behavior.

Like in electromechanical techniques, the charges in polymer foams and AGS areperiodically “switched” by an applied ac-voltage above bV . SHG yields information

about the “polarization” states by measuring the intensity changes of the SH lightduring voltage cycling. In the following, this method is applied on AGS with PFCBlayers. Evaluation of the intensity signal together with the input voltage shows a SHGhysteresis as displayed in Fig. 11.9. The hysteresis loops were obtained at a frequencyof 0.1 Hz. Below the threshold voltage (blue curve), practically no SHG signal isdetected, since no “polarization” is formed. For voltages exceeding the threshold forbreakdown, a clear SHG hysteresis is established (red curve). Since the SHG signal isrelated to the “polarization” the SHG signal decreases rapidly in the vicinity of thethreshold voltages for breakdown (the analogy of the coercive field in ferroelectricpolymers).

V

ω 2ω (SH)

+ + + + +

- - - - -

- - - - - - - - -

+ + + + + + + + +


Figure 11.9: SHG hysteresis demonstrating switching of the “dipoles” .

In Part I, a technique was introduced in order to determine the relative phase of theSH light with NLO bimorphs. In the following, an alternative method is demonstrated.The combination of a poled transparent ferroelectric polymer and a periodicallycharged air-gap sample allows the formation of a phase-dependent SHG hysteresisloop. The sample setup is schematically depicted in Fig. 11.10. A ferroelectric 76/24P(VDF-TrFE) copolymer with a thickness of 5 µm was spin-coated on an ITO-glasssubstrate. Poling of the material was performed by electrode poling at 30 V/µm. Thisrather small electric poling field establishes a polarization that generates a SHintensity comparable to the SH intensity of charged AGS. The poled ferroelectricsample was mounted on a dielectric AGS (PFCB).

Figure 11.10: Schematic sample geometry combining ferroelectric and ferroelectric-like materialsystems in order to investigate the phase of the SH light.

-800 -600 -400 -200 0 200 400 600 8000,00

0,05

0,10

0,15

400 V 800 V

√(I SHG) (a. u.)


glass ITO P(VDF-TrFE)

PFCB-AGS

fundamental beam


With the experimental SHG technique described above, a SHG hysteresis has beengenerated with an ac-voltage of ±750 V and 0.5 Hz applied on the AGS electrodes.The result is shown in Fig. 11.11.

Figure 11.11: NLO SHG hysteresis loop demonstrating the switching of the sign of )2(χ effects

according to the direction of “polarization”.

It can be seen that the emitted SHG intensity is a result of constructive and destructiveinterference of the SH light from the ferroelectric polymer and from the AGS.Constructive interference is observed on the right part of the loop in Fig. 11.11. Themaximum SH intensity corresponds approximately to the doubled intensity in Fig.11.9. Therefore, SHG efficiency is nearly equal for the poled ferroelectric sample andthe charged AGS. During the switching of quasi-polarization in the AGS (left part ofthe loop in Fig. 11.11) EFISH is compensated by SHG from the ferroelectric film. For the electrooptical (EO) effect, the linear Pockels and the quadratic Kerr effectcontribute to the hysteresis behavior. The experimental procedure was described inChapter 10. The experimental result in Fig. 11.12 reveals the quadratic EO Kerr effectat an ac-voltage of 0.1 Hz and ±400 V (far below bV ). Switching of the macroscopic

“dipoles” above bV is evidenced by the butterfly hysteresis in the EO properties.

-800 -400 0 400 8000,0

0,1

0,2

0,3

0,4

750 V

√(I SHG) (a. u.)



Figure 11.12: Experiment to demonstrate the switching of the linear EO Pockels effect (red curve) andthe quadratic Kerr effect (blue curve).

A detailed discussion on EO effects in nonpolar cellular electrets is not outlined yet.Quantitative NLO experiments are necessary in order to provide a picture for themechanisms behind the NLO properties of nonpolar cellular polymer systems.

11.4 The Electret Microphone Reconsidered

It has been demonstrated that charged AGS show strong piezoelectric effects.Therefore, these material systems may be interesting as internally chargedelectrostatic microelectromechanical (MEMS) devices. MEMS with large applied dc-bias fields have already emerged as efficient sources for air-borne ultrasoundgeneration and detection [Minami99]. Internally charged MEMS devices would beattractive, since the charges provide an internal dc-bias field.Closely related to the discussions provided in this and the preceeding chapter areelectret microphones. A photograph of a typical electret microphone as well as aschematic illustration about its configuration is given in Fig. 11.13. For the followinginvestigation of ferroelectric-like properties, commercially available electretmicrophones from Peiker Acoustics Germany have been employed.The circular microphone with a diameter of 8 mm consists of a 1 mm thick metallicgrid covered with a 15 µm thick charged fluoropolymer (probably FEP) layer and ametallized uncharged polymer diaphragm with a thickness of about 1-2 µm. Thediaphragm and the dielectric layer are separated by a 35 µm thick air-gap. Voltagesignals during mechanical deformations of the top layer are amplified with a FETimpedance converter. The dielectric layer on the back plate is usually charged beforethe fabrication of the microphone, for example by employing corona discharges.

-1000 -500 0 500 10000,0

0,5

1,0

1,5

400 V 900 V

tra

nsm

itted

ligh

t in

ten

sity

(a

. u.)



Figure 11.13: Photograph of a commercial electret microphone with a diameter of 8 mm and aschematic description of the microphone geometry.

Figure 11.14 (a) and (b) show hysteresis loops in the dielectric displacement and themechanical strain of electret microphones. A sinusoidal high voltage with a frequencyof 500 Hz for the dielectric and 0.1 Hz for the electromechanical hysteresis wasapplied on the sample after removing the FET converter.

(a) (b)

Figure 11.14: (a) Hysteresis loop of dielectric displacement versus applied voltage and (b) hysteresisloop of the electromechanical effect (mechanical strain).

The experiments prove that electret microphones can be charged by dielectric barriermicrodischarges even after fabrication. Moreover, a surface displacement of about 6µm is obtained during the switching between two charged states which is more than10% of the original sample thickness (without the metallic grid) of 55 µm. Theoscillating motion of the polymer diaphragm during the experiment can be easilyobserved with the naked eye (Fig. 11.15).

back plate (grid)

spacer

metallized polymerdiaphragm

dielectric layer

FET impedanceconverter

outpoutvoltage

-800 -400 0 400 800-0,2

-0,1

0,0

0,1

0,2

800 V

die

lect

ric

dis

pla

cem

ent

D (

µC

/cm

2 )


-1200 -600 0 600 1200-6

-4

-2

0

1200 V

surf

ace

dis

pla

cem

en

t (µ

m)



Figure 11.15: Mechanical surface deformations of the microphone diaphragm during electromechanicalhysteresis loops. At zero voltage, the diaphragm shows no surface displacement (top picture); at ±1.2kV, the surface is displaced by more than 10% of the original sample thickness (left and right images).

In analogy to air-gap sandwich structures investigated above, these experimentsprovide a first step towards internally charged electrostatic MEMS devices, with ahuge potential for applications in acoustic technology.

Part III. Semicrystalline Dipole Electrets 97

Chapter 12

Conclusion of Part III

In the last part of the thesis, analogies between cellular polymers and ferroelectricmaterials have been outlined. Cellular polymers show “polarization” that can beswitched by the application of electric fields. Switching of polarization is outlined inhysteresis loops in displacement versus applied voltage. The threshold voltage forbreakdown in cellular materials is the analogy of the coercive voltage in ferroelectricmaterials. Analogies to ferroelectric materials are far more reaching, cellular polymersalso exhibit hysteresis loops in the electromechanically induced strain versus voltage.The results obtained can be interpreted in terms of intrinsic piezoelectricity andelectrostrictively biased electromechanical strains. A consequence ofelectromechanical hysteresis loops is the possible switching of the piezoelectricresponse. Hysteresis loops have been also identified in nonlinear optical effects thatare related to the “polarization” of cellular materials, like the electro-optical effect andsecond-harmonic generation of light. The multitude of effects shown in this part of thethesis has been found so far only in ferroelectric materials. In view of the results, wesuggest to call cellular space charge electrets henceforth “ferroelectrets” and thematerials properties “ferroelectretic”.

In a reconsideration of the electret microphone, ferroelectretic behavior has beenshown by hysteresis loops in displacement and electromechanical strain. The resultsmay pave the way for micromachined electrostatic transducers for the generation ofair-borne ultrasound.

98 References Part III

References of Part III

[Aktsipetrov00] O. A. Aktsipetrov, T. V. Misuryaev, T. V. Murzina, L.M. Blinov, V. M. Fridkin, and S. P. Palto, Opticalsecond-harmonic-generation probe of two-dimensionalferroelectricity, Opt. Lett. Vol. 25, pp. 411-413 (2000).

[Ballato02] J. Ballato, J. Fousek, A. Bhalla, R. Guo, P. Bloomfield,R. E. Newnham, W. Cao, C. Randall, L. E. Cross, Q.Zhang, J. P. Dougherty, S. Pilgrim, IEEE StandardDefinitions of Terms Associated with Ferroelectric andRelated Materials, Draft 14, IEEE Piscataway (2002).

[Bauer-Gogonea01] S. Bauer-Gogonea and S. Bauer, Polymer electrets forelectronics, sensors, and photonics, Handbook ofAdvanced Electronic and Photonic Materials andDevices Vol. 10, pp. 185-231, Academic Press (2001).

[Boyd87] G. T. Boyd, Optical second-harmonic generation as anorientational probe in poled polymers, Thin Solid FilmsVol. 152, pp. 295-304 (1987).

[DeRossi82] D. DeRossi, A. S. DeReggi, M. G. Broadhurst, S. C.Roth, and G. T. Davis, Method of evaluating thethermal stability of the pyroelectric properties ofpolyvinylidene fluoride: Effects of poling temperatureand field, J. Appl. Phys. Vol. 53, pp. 6520-6525 (1982).

[Dickens92] B. Dickens, E. Balizer, A. S. DeReggi, and S. C. Roth,Hysteresis measurements of remanent polarization andcoercive field in polymers, J. Appl. Phys. Vol. 72, pp.4258-4264 (1992).

[Dougherty72] J. P. Dougherty, E. Sawaguchi, and L. E. Cross,Ferroelectric Optical Resolution Domains in Single-Crystal Pb5Ge3O11, Appl. Phys. Lett. Vol. 20, pp. 364-365 (1972).

[Dragosits01] K. Dragosits, Modeling and Simulation of FerroelectricDevices, PhD thesis, Technical University of Vienna(2001).

[Furukawa80] T. Furukawa, M. Date, and E. Fukada, Hysteresisphenomena in polyvinylidene fluoride under highelectric field, J. Appl. Phys. Vol. 51, pp. 1135-1141(1980).

[Furukawa89] T. Furukawa, Piezoelectricity and pyroelectricity inpolymers, IEEE Trans. Electr. Insul. EI-24, pp. 375-394(1989).

[Furukawa90] T. Furukawa and N. Seo, Electrostriction as the Originof Piezoelectricity in Ferroelectric Polymers, Jap. J.Appl. Phys. Vol. 29, pp. 675-680 (1990).

[Furukawa97] T. Furukawa, Structure and Functional Properties ofFerroelectric Polymers, Adv. Colloid Interface Sci.Vol. 183, pp. 71-72 (1997).

[Gross50] B. Gross, Br. J. Appl. Phys. Vol. 1, p. 259 (1950).


[Heiler94] B. Heiler and B. Ploss, in Proceedings: InternationalSymposium on Electrets, ISE Vol. 8, p. 662, J. Lewiner,C. Alquie, and D. Morisseau (Eds.), IEEE Piscataway(1994).

[Li-Jie95] Li-Jie, C. Baur, B. Koslowski, and K. Dransfeld, Studyof the microscopic properties of copolymer P(VDF-TrFE) films, Physica B Vol. 204, pp. 318-324 (1995).

[Limes77] M. E. Lines and A. M. Glass, Principles andApplications of Ferroelectrics and Related Materials,Clarendon Press, Oxford (1977).

[Minami99] K. Minami, H. Morishata, and M. Esashi, A bellows-shape electrostatic microactuator, Sensors & ActuatorsVol. 72, pp. 269-276 (1999).

[Neugschwandtner01] G. S. Neugschwandtner, R. Schwödiauer, S. Bauer-Gogonea, and S. Bauer, Piezo- and pyroelectricity of apolymer-foam space-charge electret, J. Appl. Phys. Vol.89, pp. 4503-4511 (2001).

[Sapriel75] J. Sapriel, Domain-wall orientations in ferroelastics,Phys. Rev. B Vol. 12, pp. 5128-5140 (1975).

[Schilling88] D. Schilling, Dissertation Konstanz, Germany (1988).[Schwödiauer00] R. Schwödiauer, G. S. Neugschwandtner, K.

Schrattbauer, M. Lindner, M. Vieytes, S. Bauer-Gogonea, and S. Bauer, Preparation andcharacterization of novel piezoelectric and pyroelectricpolymer electrets, IEEE Trans. Diel. Electr. Insul. Vol.7, p. 578 (2000).

[Su97] Q. Su, T. A. Rabson, and M. Robert, Integ. Ferroelec.Vol. (18), p. 415 (1997).

[Wegener01] M. Wegener, IEEE Trans. Diel. Electr. Insul. Vol. 8, p.494 (2001).

[Wicker89a] A. Wicker, B. Berge, J. Lajzerowicz, and. J. F. Legrand,Non-linear optical investigation of the bulk ferroelectricpolarization in thin films of VF2-TrFE copolymers,Ferroelectrics Vol. 92, pp. 35-40 (1989).

[Wicker89b] A. Wicker, B. Berge, J. Lajzerowicz, and. J. F. Legrand,Nonlinear optical investigation of the bulk ferroelectricpolarization in a vinylidene fluoride/trifluoroethylenecopolymer, J. Appl. Phys. Vol. 66, pp. 342-349 (1989).

[Yuki98] T. Yuki, E. Yamaguchi, T. Koda, and S. Ikeda,Electrostrictive Phenomena Associated withPolarization Reversal in Ferroelectric Polymers, Jpn. J.Appl. Phys. Vol. 38, pp. 1448-1453 (1998).

[Zhang93] R. Zhang and P. L. Taylor, J. Appl. Phys. Vol. 73, pp.1395 (1993).

[Zhang02] Q. M. Zhang, H. Li, M. Poh, F. Xia, Z.-Y. Cheng, H.Xu, and C. Huang, An all-organic composite actuatormaterial with a high dielectric constant, Nature Vol.419, pp. 284-287 (2002).

Prospects 101

Prospects

In the thesis, several techniques for charging and poling polymer electrets have beendeveloped in addition to novel techniques for the characterization of these materials.With ferroelectrets, a new class of nonpolar materials with ferroelectric-likeproperties has been introduced. Future work on this material class will certainlyprovide more new and probably unexpected results. However, the field of functionalpolymers is much broader, and extensive research is going on in other directions aswell.Photonic polymers like those discussed in Chapter 1 are currently investigated foroptical data storage. Azo-chromophore containing polymers are already employed inCD’s. By combining several amorphous side-chain polymers, the capability of opticaldata storage media can be increased significantly [Baldus01]. Liquid crystallineazobenzene side-chain polymers have been found to be strongly nonlinear opticallyactive and to be useful in reversible optical data storage applications [Joo00].Furthermore, azo-dye containing polymers easily allow the fabrication ofbirefringence and surface relief gratings upon exposure to an interference pattern oflaser beams [Hattori00].Novel polymer electrets like nonpolar cellular space-charge electrets have emerged asa new class of materials for electrostatic microelectromechanical devices. Theincreasing demand for small and efficient device structures raises the problem of acompromise between sample processing and electret quality. PTFE as actually one ofthe best electret materials cannot be easily produced in foams or deposited onsubstrates. A series of excellent nonpolar polymers have been sythesized which arestill very expensive. Therefore, efforts are made on the generation of cheap, low-dielectric constant charge electrets with excellent charge stability for high-frequencymicroelectronic and transducer applications. Recently, paraxylylene (Parylene®) -usually known as a protective coating material - was demonstrated as a cheap electretmaterial with a low dielectric constant and promising thermal charge stability[Mitu02]. Further materials may be found that may be processed by standardtechniques known in industry, such as plasma polymerization.Dielectric barrier microdischarges have been shown to be a simple and potentialcharging tool for cellular polymers and an in-situ technique for the visualization of thevoids in foams. On a macroscopic level, the internal charging process is reasonablywell understood. On the microscopic level, the temporal investigation of dischargesmust be performed in order to observe e. g. multiple breakdown events in one void.This may allow to prove ferroelectretic domains - the analogy of ferroelectricdomains - in cellular materials. The reduction of the void height towards onemicrometer or even below would enable novel nanoporous piezoelectric materials.Such a reduction seems feasible by studying dielectric breakdown in tiny gaps aboveambient pressure. Ferroelectric polymers of the PVDF family combine highelectromechanical coupling and low acoustic impedance required for ultrasonictransducer applications and medical instruments. It has been demonstrated that theincorporation of chlorotrifluorethylene units into a P(VDF-TrFE) copolymerestablishes a stable ferroelectric phase with reduced Curie temperature and with asmall energy barrier for the phase transition [Chung02]. Such copolymers, with astatistical distribution of monomers on the nano-scale and extremely small nano-crystallites have shown huge electrostrictive and relaxor-ferroelectric beahviour. Asimilar effect is known from high energy electron irradiated P(VDF-TrFE)

102 Prospects

copolymers which exhibit giant broadband electrostrictive responses coupled with alarge elastic energy density [Xu02]. Functional polymers for sensors, actuators, and photonics still represent a very activefield of research, with a large scope for practical applications.

References:

[Baldus01] O. Baldus, A. Leopold, R. Hagen, T. Bieringer, and S. J. Zilker,Surface relief gratings generated by pulsed holography: Asimple way to polymer nanostructures without isomerizing side-chains, J. Chem. Phys. Vol. 114, pp. 1344-1349 (2001).

[Chung02] T. C. Chung and A. Petchsuk, Synthesis and Properties ofFerroelectric Fluoroterpolymers with Curie Transition atAmbient Temperature, Macromol. Vol. 35, pp. 7678-7684(2002).

[Hattori00] T. Hattori, T. Shibata, S. Onodera, and T. Kaino, Fabrication ofrefractive index grating into azo-dye-containing polymer filmsby irreversible photoinduced bleaching, J. Appl. Phys. Vol. 87,pp. 3240-3244 (2000).

[Joo00] W. J. Joo, H. D. Shin, C. H. Oh, S. H. Song, P. S. Kim, B. S.Ko, and Y. K. Han, Novel mechanism of fast relaxation ofphoto-induced anisotropy in a poly(malonic esters) containingp-cyanoazobenzene, J. Chem. Phys. Vol. 113, pp. 8848-8851(2000).

[Mitu02] B. Mitu, S. Bauer-Gogonea, H. Leonhartsberger, M. Lindner, S.Bauer, and G. Dinescu, Plasma-deposited parylene thin films:Process and material properties, Surface and CoatingsTechnology, in print.

[Xu02] T. B. Xu, Z. Y. Cheng, and Q. M. Zhang, High-perfromancemicromachined unimorph actuators based on electrostrictivepoly(vinylidene fluoride-trifluoroethylene) copolymer, Appl.Phys. Lett. Vol. 80, pp. 1082-1084 (2002).

Appendix - Material Data 103

Appendix - Material Data

P(S-MA)-DR1

Type 9511: Dielectric constant ε : 4Dye-content (mol-%): 93Glass transition temperature gT (°C): 137

Specific heat capacity pc (kJ/kgK): 0.96

Thermal conductivity κ (W/mK): 0.16Thermal diffusivity D (m²/s): 10-7

Absorption maximum (nm): 497

Type 9512: Dielectric constant ε : 4Dye-content (mol-%): 61Glass transition temperature gT (°C): 164


Thermal conductivity κ (W/mK): 0.16Thermal diffusivity D (m²/s): 10-7

Absorption maximum (nm): 530

PTFE (Teflon®)

Dielectric constant ε : 2.1Mass density ρ (g/cm³): 2.15

Glass transition temperature gT (°C): -33

Melting temperature mT (°C): 330


Thermal conductivity κ (W/mK): 0.4

Teflon AF® 1600

Dielectric constant ε : 1.9Glass transition temperature gT (°C): 160


PFCB

Dielectric constant ε : 2.4Glass transition temperature gT (°C): 300

104 Appendix - Material Data

BCB

Dielectric constant ε : 2.6

Cellular Polypropylene (PP)

Type HS01: Foil thickness (µm): 70Void dimensions (µm): 100 lateral, 5-10 verticalDielectric constant ε : 1.2Glass transition temperature gT (°C): -20


Mass density ρ (g/cm³): 0.33

Piezoelectric 33d coefficient (pC/N): 300

Type O01: Foil thickness (µm): 37Dielectric constant ε : 1.6Glass transition temperature gT (°C): -20


Mass density ρ (g/cm³): 0.55

Piezoelectric 33d coefficient (pC/N): 30

Cellular FEP

Type IC 35/01: Foil thickness (µm): 95Melting temperature mT (°C): 260

Type IC 29/01: Foil thickness (µm): 140Melting temperature mT (°C): 260

Appendix - Material Data 105

PVDF



Curie temperature CT (°C): 205°C (extrapolated)

Dipole moment µ (Debye): 2.3Dielectric constant ε : 9Piezoelectric 33d coefficient (pC/N): -31

Pyroelectric 3p coefficient (µC/m²K): 25

Electrooptical coefficient r (pm/V): 1Coercive field strength CE (V/µm): 60

Specific heat capacity pc (kJ/kgK): 80

Thermal conductivity κ (W/mK): 0.12Thermal diffusivity D (m²/s): 6·10-8

P(VDF-TrFE) 75/25



Curie temperature CT (°C): 118

Dipole moment µ (Debye): 1.9Dielectric constant ε : 9Pyroelectric 3p coefficient (µC/m²K): 35

Piezoelectric 33d coefficient (pC/N): -43

Electrooptical coefficient r (pm/V): 1Specific heat capacity pc (kJ/kgK): 80

Thermal conductivity κ (W/mK): 0.12Thermal diffusivity D (m²/s): 6·10-8

106 Own Publications

Own Publications

M. Lindner, K. Schrattbauer, R. Schwödiauer, S. Bauer-Gogonea, and S. Bauer, Newdevelopments of electro-thermal techniques for material characterization,Proceedings, 10th International Symposium on Electrets ISE10, pp. 461-464 (Delphi1999).

S. Bauer, S. Bauer-Gogonea, M. Lindner, and K. Schrattbauer, Piezo-, pyro-, andferroelectric polymers, in C. Galassi, M. Dinescu, K. Uchino, and M. Sayer (Eds.),Piezoelectric Materials: Advances in Science, Technology and Applications, NATOScience Series: High Technology Vol. 76, (Kluwer Dordrecht), pp. 11-19 (2000).

R. Schwödiauer, G. S. Neugschwandtner, K. Schrattbauer, M. Lindner, M. Vieytes, S.Bauer-Gogonea, and S. Bauer, Preparation and characterization of novelpiezoelectric and pyroelectric polymer electrets, IEEE Trans. Diel. Electr. Insul. Vol.7, p. 578 (2000).

M. Lindner, S. Bauer-Gogonea, S. Bauer, M. Paajanen, and J. Raukola, Dielectricbarrier microdischarges: Mechanism for the charging of cellular piezoelectricpolymers, J. Appl. Phys. Vol. 91, pp.5283-5287 (2002).

M. Lindner, S. Bauer-Gogonea, and S. Bauer, Ferroelectric-like behavior in nonpolarcellular electrets, Transducing Materials and Devices, part of SPIE’s PhotonicsFabrication Europe, Brugge, Belgium (2002).

B. Mitu, S. Bauer-Gogonea, H. Leonhartsberger, M. Lindner, S. Bauer, and G.Dinescu, Plasma-deposited parylene thin films: Process and material properties,Surface and Coatings Technology, in print.

Acknowledgement 107

Acknowledgement

Zu Beginn möchte ich mich bei allen Mitarbeitern der Abteilung für AngewandtePhysik bedanken, die mir in einem angenehmen und heiteren Arbeitsklima jederzeitmit Rat und Tat beiseite standen und mit deren Unterstützung es mir gelang, Problemezu lösen und Schwierigkeiten zu beseitigen. Besonders hervorheben möchte ichhierbei unseren Elektronik-Spezialisten Alfred Nimmervoll, der mir des öfterengeduldig die Grundprinzipien und Grenzen der Elektronik, die man im Normalfallnicht überschreiten sollte, aufzeigte. Heidi Piglmayer-Brezina danke ich für dieUnterstützung im Bereich der Probenpräparation und Chemikalienbereitstellung. Alljene, die mit Handwerkzeug und Bohrmaschine besser umgehen konnten als ich,verdienen meinen höchste Anerkennung.

Bei Prof. Dr. Siegfried Bauer und Dr. Simona Bauer-Gogonea bedanke ich mich, dassich meine Dissertation unter ihrer Aufsicht anfertigen konnte und dass sie selbst in derEndphase meiner Ausbildung noch immer mit Geduld und Optimismus versuchten,mir klarzumachen, dass unerwartete Messergebnisse überprüft und ungewöhnlichhohe Effekte sehr kritisch betrachtet werden sollten. Bedanken möchte ich mich andieser Stelle auch bei beiden für anregende Diskussionen und für die kritischeDurchsicht dieser Arbeit.

Besonderen Dank gilt dem Zweitgutachter dieser Arbeit, Prof. Dr. Günther Bauervom Institut für Halbleiterphysik, der bereits den Vorsitz bei meiner Diplomprüfungübernahm.

Weiters danke ich meinen Eltern für die Möglichkeit, mein Physikstudium zuabsolvieren, obwohl sie vermutlich bis heute keine Vorstellung darüber besitzen,welches Ziel meine Arbeit wirklich verfolgt und wie man damit Geld verdienenkönnte (ironisches Zitat meines Vaters während der Sponsionsfeier).

Zum Schluss möchte ich noch meiner kleinen Marlene danken, die so mancheUnzufriedenheit nach demotivierenden Arbeitstagen im Labor miterleben und michbeim Schreiben der Dissertation des öfteren regelrecht vom Computer zerren musste,um mein Frühstück nicht zu vernachlässen. Es gelang ihr dann glücklicherweise auchimmer wieder, meine Gedanken von Zeit zu Zeit auf angenehmere Dinge des Lebenszu lenken, um neue Ideen zu schöpfen und diese in einer mehr oder wenigewissenschaftliche Sprache auf Papier zu bringen.

108 Curriculum Vitae

Curriculum Vitae

Persönliche Daten

Name: Michael LindnerGeboren am: 12.05.1976 in Linz, ÖsterreichEltern: Rudolf Lindner und Theresia Lindner, geb. FrühwirthFamilienstand: ledig

Schulbildung

1982-1986: Volksschule Pregarten1986-1994: Bundesgymnasium/Bundesrealgymnasium FreistadtJuni 1994: Matura am BG/BRG Freistadt

Diplomstudium

1994-1999: Technische Physik an der Johannes Kepler Universität LinzFebruar 1999 bisDezember 1999: Diplomarbeit bei Prof. Dr. Siegfried Bauer an der Abteilung für

Angewandte Physik an der Johannes Kepler Universität Linzmit dem Thema: Wärmepulsverfahren zur Charakterisierung und Optimierungder lichtinduzierten Polung von Photonikpolymeren

22. Dezember 1999: Diplomprüfung

Doktoratsstudium

Seit Februar 2000: Assistent in der Arbeitsgruppe bei Prof. Dr. Siegfried Bauer,Forschungsschwerpunkt: Polymere Funktionsmaterialien.Arbeiten im Rahmen eines vom FWF (Fond zur Förderung derwissenschaftlichen Forschung in Österreich) gefördertenForschungsprojektes.

Eidesstattliche Erklärung 109

Eidesstattliche Erklärung

Ich erkläre an Eides statt, dass ich die vorliegende Dissertation selbstständig und ohnefremde Hilfe verfasst, andere als die angegebenen Quellen und Hilfsmittel nichtbenutzt bzw. die wörtlich oder sinngemäß entnommenen Stellen als solche kenntlichgemacht habe.

Linz, 28.01.2003 ……………………………… (DI Michael Lindner)

poling and characterization of nonpolar and polar …familielindner.net/phd-thesis.pdfpoling and...

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