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Doctoral Thesis

Nucleation and Clarification of Polyethylenes

Author(s): Aksel, Seda

Publication Date: 2015

Permanent Link: https://doi.org/10.3929/ethz-a-010580676

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DISS. ETH NO. 22972

Nucleation and Clarification

of Polyethylenes

A thesis submitted to attain the degree of

DOCTOR OF SCIENCES of ETH ZURICH

(Dr. sc. ETH Zurich)

presented by

SEDA AKSEL

Master of Science ETH in Materials Science, ETH Zurich

born on 12.04.1987

citizen of Turkey

accepted on the recommendation of

Prof. Dr. Paul Smith, examiner

Prof. Dr. Hans-Werner Schmidt, co-examiner

Prof. Dr. Jan Vermant, co-examiner

2015

Contents

Summary 1

Zusammenfassung 3

Chapter I Introduction 5

Chapter II Influence of Polymer Chain Architecture 21

on Opto-thermo-mechanical Properties of Polyethylenes

Chapter III Efficiency of Commercial Additives for 33


Chapter IV New “Designer” Nucleating/Clarifying Agents for Polyethylene 55

Chapter V Phase Behavior of Polyethylene and 79

Selected Nucleating/Clarifying Agent Binary Systems

Chapter VI Influence of Polyethylene Macromolecular Structure on 97

Crystallization onto 1,2,3-trideoxy-4,6:5,7-bis-O-

[(4-propylphenyl)methylene]-nonitol

Chapter VII Conclusions and Outlook 113

Appendix 119

Acknowledgements 125

Curriculum Vitae 129

1

Summary

In general, polymers that can crystallize most often do so in the form of spherulitic structures, which are

known to efficiently scatter visible light (wavelength 400-700 nm). Thus, semi-crystalline polymers

generally exhibit high values of so-called “haze”, which correspond to translucent or even opaque

specimens. “Nucleating agents” – known to enhance heterogeneous nucleation of semi-crystalline

polymers – in some cases can yield small crystalline non-spherulitic entities in the final solid-state

structures which are not of the order of visible light range and reduce haze, in which case are termed as

“clarifying agents”.

Among semi-crystalline polymers, toughness under a wide range of environmental conditions due to its

low glass transition temperature (as opposed to, for instance, isotactic polypropylene, i-PP), and

relatively low melting temperature permitting relatively low energy-consuming production of industrial

artifacts make polyethylenes (PEs) attractive notably for the major packaging industry. Therefore, the

present thesis explores the efficient enhancement of the crystallization process by “nucleating agents”,

seeking optical transparency and, superior thermal and mechanical properties of PEs.

In a first approach, optical, thermal and mechanical properties of neat PEs of widely different chain

architectures were investigated. It is shown that while transparency can be readily achieved for the PEs

with a high degree of branching, a penalty incurs with a major reduction in their equally-relevant thermal

(i.e. melting temperature) and mechanical (i.e. stiffness) properties. In order to seek a compromise

between optical and thermo-mechanical properties, a range of nucleating/clarifying agents were

investigated for the PEs that intrinsically possess superior thermal and mechanical properties (i.e. high-

density polyethylene (HDPE) and linear low-density polyethylene (LLDPE)) to obtain advantageous

optical characteristics such as those observed for the low-density resins. In this study, the widely-used

“sorbitol”- and “1,3,5-benzene trisamide”- based commercial additives for i-PP, and newly designed

molecules based on “1,3,5-benzene trisamides” and “1,4-phenylene bisamides” were explored as

potential nucleating/clarifying agents for polyethylene.

In this study it was found that the sorbitol derivatives 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol

(DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol (TBPMN) enhanced

nucleation of LLDPE evidenced by increasing its crystallization peak temperature by up to 9 °C and,

in the case of TBPMN clarification imparted by decreasing the haze value of 1 mm thick injection-

molded plaque to values as low as 15 % as observed in commercial clarified i-PP. Furthermore, the

newly designed molecule, aramid-based N,N’-bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide

(BCPCA) – which circumvents the disadvantage of the thermal instability of aldehyde-based sorbitols

– yielded almost the same reduction in haze as for DMDBS and at substantially lower concentrations

2

(i.e. a bulk haze of 30 % with only 0.1 % w/w BCPCA compared with 25 % bulk haze with 0.25 % w/w

DMDBS).

Investigations of the temperature/composition phase behavior of binary PE/additive systems revealed

that favorable dispersion of the additive in the polymer melt at a concentration range, i.e. hyper-eutectic

regime but below the onset of liquid-liquid phase separation, yields fine fibrils of the additive for the

polymer chains to subsequently crystalize onto them into a non-spherulitic arrangement as observed for

i-PP. Further studies on the microstructure of the solid-state material at those favorable additive

concentrations revealed that addition of sorbitol-based DMDBS and TBPMN, as well as the newly

designed additive BCPCA, prevents spherulitic growth of PE during crystallization – instead a molecular

arrangement featuring random rod-like shish-kebab-type structures, which reduce light scattering was

found to form. However, further analysis of experiments with different PEs possessing different

macromolecular structures proved that preventing spherulitic growth is not sufficient for obtaining

improved clarification, and that the additive fibril-widths as well as the polymer lamellar-widths play an

important role to reduce haze.

3

Zusammenfassung

Polymere, die kristallisieren, bilden meist Sphärolithe. Solche Strukturen streuen sichtbares Licht (400-

700nm). Teilkristalline Polymere sind deshalb transluzent oder gar opak, d.h. weisen hohe sogenannte

“Haze”-Werte auf. “Nukleierungsmittel” werden verwendet für die Erhöhung der heterogenen

Nukleierung von teilkristallinen Polymeren. In bestimmten Fällen bewirkt deren Verwendung die

Bildung kleiner, nicht-sphärolithischer Einheiten, welche nicht der Grössenordnung der sichtbaren

Wellenlänge entsprechen. Nukleierungsmittel, welche auf diese Weise eine Reduzierung des Haze-

Wertes ermöglichen, werden auch “Klärungsmittel” genannt.

Die Zähigkeit von Polyethylen (PE) unter verschiedensten Umgebungsbedingungen – bedingt durch

eine tiefe Glasübergangstemperatur (z.b. verglichen mit dem weit verbreiteten isotaktischen

Polypropylen, i-PP), sowie dessen ebenfalls tiefe Schmelztemperatur und die damit verbundene

sparsame Produktion, machen es zu einem attraktiven Material für die Verpackungsindustrie. Motiviert

durch diese Vorteile untersucht die vorliegende Dissertation die Optimierung des

Kristallisationsprozesses von Polyethylen durch Nukleierungsmittel mit dem Fokus auf die

Verbesserung der Lichtdurchlässigkeit sowie der thermischen und mechanischen Eigenschaften.

In einem ersten Teil werden die optischen, thermischen und mechanischen Eigenschaften purer PEs mit

verschiedensten Kettenarchitekturen aufgezeigt. PEs mit hoher Verästelung können bereits transparent

hergestellt werden, jedoch mit signifikanten Einbussen bezüglich Schmelztemperatur und Steifigkeit.

Mit dem Ziel, einen vergleichbar attraktiveren Kompromiss zu finden, wird im Anschluss die

Verwendung einer Reihe von Nukleierungs-/Klärungsmittel mit PEs mit intrinsisch besseren

thermischen und mechanischen Eigenschaften (z.b. Hochdichte-Polyethylen (HDPE) und lineares

Polyethylen niederer Dichte (LLDPE)) analysiert, um deren Lichtdurchlässigkeit zu verbessern. Weit

verbreitete “Sorbitol”- und “1,3,5-Benzoltrisamid”-basierte kommerziell erhältliche Zusatzstoffe für i-

PP, sowie neuentwickelte Moleküle basiert auf “1,3,5-Benzoltrisamid” und “1,4-Phenylenbisamid”

werden untersucht im Hinblick auf deren Potential als Nukleierungs-/Klärungsmittel für PEs.

Für LLDPE wird eine Erhöhung der Kristallisationstemperatur um ~9°C erreicht unter Verwendung der

Sorbitol-Derivate 1,3:2,4-Bis(3,4-Dimethylbenzylidene)Sorbitol (DMDBS) und 1,2,3-Trideoxy-

4,6:5,7-Bis-O-[(4-Propylphenyl)Methylen]-Nonitol (TBPMN). Im Falle von TBPMN wurde eine

Minderung des Haze-Werts auf bis zu ~15% beobachtet, ähnlich zu kommerziell geklärtem i-PP.

Darüber hinaus bewirkt das neuentwickelte Aramid-basierte N,N’-Bis(cyclohexylmethyl)-1,4-phenylen

dicarboxamid (BCPCA) – welches den Nachteil der thermischen Instabilität von Aldehyd-basierten

Sorbitolen umgeht – fast dieselbe Haze-Reduktion wie DMDBS, jedoch bereits bei substantiell kleineren

Konzentrationen (z.b. Bulk Haze 30% bei 0.1% w/w BCPCA, Bulk Haze 25% bei 0.25% w/w DMDBS).

4

Aus einer Untersuchung des Phasenverhaltens im Hinblick auf Temperatur und Mischung für binäre

PE/Zusatzsoff-Systeme geht hervor, dass die optimale Dispersion des Zusatzstoffes in der

Polymerschmelze bei Konzentrationen im hyper-eutektischen Bereich, jedoch unter der flüssig-flüssig-

Phasentrennung geschieht. Dabei bildet der Zusatzstoff feine Fäserchen, auf welchen die Polymerketten

kristallisieren und nicht-sphärolithische Strukturen bilden, ähnlich zu Beobachtungen für i-PP.

Weitergehende Studien über die entstehenden Mikrostrukturen zeigten zudem, dass die Verwendung

von Sorbitol-basiertem DMDBS und TBPMN, sowie dem neuentwickelten BCPCA unter den genannten

bevorzugten Konzentrationen die Bildung von stäbchenförmigen Molekülarrangements vom

sogenannten “Shish-Kebab-Typ” hervorruft, welche die Streuung reduzieren. Experimente mit

verschiedenen makromolekularen Strukturen der PEs bringen anschliessend hervor, dass die

Vermeidung von sphärolithischem Kristallwachstum alleine nicht genügt, um die Lichtdurchlässigkeit

zu verbessern, sondern dass darüber hinaus die Breite der Zusatzstoff-Fäserchen und Polymer-Lamellen

eine entscheidende Rolle in der Haze-Reduktion spielen.

5

Chapter 1

Introduction

6

1 Preface

Whilst during the past decades much progress has been made in reducing processing cycles of

thermoplastic polymers, and in special cases – most notably isotactic polypropylene (i-PP) – enhancing

their optical properties, through the use of small-molecular additives (“nucleating” and “clarifying”

agents), applying those advances to the most-common, most-produced and most-used polymer, i.e.

polyethylene (PE), has been largely unsuccessful for reasons ill- or not understood. Due to its favorable

chemical resistance and thermal properties, among other things, a very low glass transition temperature

– which offers toughness under a wide range of environmental conditions (as opposed to i-PP), and a

relatively low melting temperature permitting relatively low energy-consuming production of artifacts,

it would be of high benefit to enhance nucleation and – ideally – also clarification of polyethylene,

especially for applications in the packaging industry.

Hence, the objective of the research described in this thesis is to explore the use of known, as well as

novel additives for enhancement of nucleation of polyethylenes, as well as improving their optical

characteristics – in particular reducing scattering of light, which is of obvious interest for the above-

mentioned use – while maintaining their advantageous mechanical properties; and if unsuccessful,

clarify the underlying cause(s).

2 Background

1) Nucleation

When an equilibrated molten crystallizable, flexible polymer solidifies, for instance by cooling below

its crystalline melting temperature, at a certain supercooling the initially randomly coiled chain

molecules form embryonic ordered entities (“nuclei”) that above a critical size grow into semi-

crystalline structures often in the form of radially oriented lamellae, commonly referred to as

“spherulites” 1-4. Importantly, the size, shape, orientation, molecular connectivity of these crystalline

entities and the overall degree of crystallinity dictate the macroscopic physical properties of the

solidified polymer 5. Therefore, understanding the nucleation and crystallization processes, and

controlling the final microstructure are of critical interest in order to direct these characteristics.

As mentioned above, the formation of (semi-)crystalline structures commences with the formation of

entities in which chain molecules are locally ordered, so called “primary nucleation”, until a critical-

size nucleus is reached. Once supercritical-size nuclei are formed, additional chains deposit onto the

surface of the primary nuclei, – a process referred as “secondary nucleation or crystal growth”. Primary

nucleation requires an energy barrier to be overcome for the generation of stabile nuclei. In the absence

7

of foreign matter such as solid particles, critical-size nuclei are formed by the polymer chains

themselves, in the event known as “homogeneous nucleation”. In this case, the energy barrier is

overcome when the polymer melt is supercooled to a temperature well below the crystalline melting

temperature of the polymer. The presence of foreign bodies in the molten phase may reduce the energy

barrier by acting as pre-existing nucleation sites onto which chains can deposit; in such a case the term

“heterogeneous nucleation” is used. The aforesaid foreign species, called “nucleating agents”, initiate

crystallization at higher temperatures, i.e. at reduced supercoolings, and may increase the number of

nucleation sites which results in increased solidification rates 6-12, and therewith reduce processing cycles

of industrial production of artifacts. For instance, in manufacturing injection-molded products, higher

crystallization temperatures require less cooling of the mold and higher rates of crystallization permits

faster removal of the articles 13. In addition to the above-mentioned processing advantages, nucleating

agents can cause changes in the polymer solid-state structure which affects its macroscopic physical

properties. For instance, increased nucleation and resulting decreased size of spherulitic structures were

reported to cause improvement in mechanical properties such as elastic modulus, tensile strength, yield

strength and impact strength of the material 14-18. N.B. Apart from nucleating agents, predetermined

nucleation can also be achieved by polymer crystallites themselves which are not completely molten and

can act as a foreign surface, in the polymer melt (self–seeding) 5. For nucleating agents to be most

efficient, they should exhibit sufficient thermal stability, be well dispersed in the polymer melt and form

a large solid surface area to provide polymer chains to nucleate on it 19-22.

The overall rate of solidification of crystallizable polymers is controlled by a competition between

thermodynamic and kinetic factors. At high temperatures (i.e. close to the equilibrium melting

temperature), solidification is slow due to the small thermodynamic driving force – which is required to

overcome the energy barrier for the formation of stable nuclei and subsequently to drive secondary

nucleation (crystal growth). On the other hand, at very low temperatures, the mobility of chains to diffuse

into favorable positions is highly reduced, leading to low rates of crystallization. In the case of

polyethylene, nucleation and crystallization occur extremely quickly due to its very simple molecular

structure, especially for unbranched resins such as high-density polyethylene (HDPE) 5. These high

nucleation and crystallization rates render interference with and control of the solidification of this

particular polymer difficult. This is most clearly illustrated by the fact that known nucleating agents

were found to increase the crystallization temperature of polyethylene by 2-5 °C only, while this values

has been recorded as high as ~20 °C for the case of i-PP 23.

8

2) Analysis

There exist a number of analytical methods to examine and quantify the effect of the addition of

nucleating agents on the crystallization process. Besides observation of the altered morphology by,

among others, optical microscopy (e.g. decreased size of spherulites – see Figure 1), crystallization can

conveniently be analyzed with differential scanning calorimetry (DSC), for instance by recording the

increase in peak or onset crystallization temperature (Tc) and a concomitant decrease in the supercooling,

(∆T) of the polymer during cooling at a particular rate (cf. Figure 2).

Figure 1.

Polarized optical micrographs of a solidified compression-molded film of a neat linear low-density polyethylene

(LLDPE) (left) and the same polymer comprising the nucleating agent 1,2,3-trideoxy-4,6:5,7-bis-O-

[(4propylphenyl)methylene]-nonitol (TBPMN) (right). The images illustrate the dramatic influence of the additive

on the size and number of the polymer spherulites; see Chapter 3.

Figure 2.

Differential scanning calorimetry (DSC) thermographs of a neat linear low-density polyethylene (LLDPE)

( ─ ─ ) and that polymer comprising the nucleating agent, 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)

methylene]-nonitol (TBPMN) ( ▬ ) during dynamic crystallization (cooling rate 10 °C/min); see Chapter 3.

9

3) Clarification

Amongst the above-referred nucleating agents, certain additives (unfortunately, only selected few) are

known to not only affect processing cycles and mechanical properties of polymers, but also can

dramatically enhance their optical characteristics as expressed in terms of “haze” and “clarity” – hence

the denotation “clarifying agents” 21, 24-26. In the following, basic issues of these optical characteristics

are addressed in some detail.

Inhomogeneous regions in matter may lead to scattering of incoming light and thus resolution details of

an object seen through the material can be affected. Inhomogeneity can stem from either the differences

in refractive index between adjacent regions, or the roughness on the surface of the material. In order to

characterize optical properties, commonly three terms are employed: i.e. “haze” (related to scattered

light), “clarity” (related to unscattered light transmission) and “gloss” (associated with reflectivity) 5.

In this thesis, the phenomenon of haze is the principle mode of characterization; it quantifies the fraction

of light transmitted through a specimen, which is scattered between the angles of 2.5° and 90° 27.

It is important to note that haze is affected by scattering of light by both internal/bulk and

external/surface inhomogeneities of an object.

Differences in the molecular arrangement in crystalline and disordered phases of typical semi-

crystalline polymers are the main cause of the “internal/bulk haze”. As stated before, most of such

materials, including polyethylene, form lamellar crystallites oriented in a radial manner (i.e. spherulites)

that due to their highly anisotropic structure, efficiently scatter light. Naturally, once diameters of such

spherulites approach the wavelengths of visible light, scattering becomes even more pronounced. Hence,

as was demonstrated by Bernland 28 for the case of i-PP, reduction of the size of spherulitic entities is

not the cause of reduced haze by “clarifying agents”. But, prevention of the formation of that structure

is a path reducing light scattering.

In addition to scattering of light by inhomogeneities in the solid state, surface irregularities often due to

processing issues are another contributor to scattering of light, referred to as “external/surface haze”.

The latter expectedly becomes of increasing importance for objects of decreasing thickness, such as

blown films 29-32, as discussed in the following in some more detail. But before entering into that, it is

important to point out that – most conveniently indeed – the two types of contributions to the overall

haze (bulk and surface) can be readily distinguished and separated by eliminating the haze due to

external/surface issues by, for instance, applying a liquid, which has a similar refractive index as the

material of interests, onto the material surface as shown in Figure 3 5, 33-35.

10

Figure 3.

Illustration of haze measurement with a hazemeter according to ASTM Standard D 1003 27. The hazemeter records

the “overall haze” of the specimen with surface irregularities (top). Immersion oil, which has similar refractive

index as the specimen has, eliminates “external/surface haze” and hazemeter measures “internal/bulk haze”

(bottom).

Returning to the issue of haze caused by surface roughness, two causes have been proposed in production

technologies of, for instance, melt-blown films – known as free-surface flow processes: 1) “extrusion

roughness” resulting from flow-induced irregularities and 2) “crystallization roughness” due to the

formation of crystalline entities on, or close to the surface. In an in-depth and most revealing study,

Sukhadia et al. investigated the correlation between surface haze of melt-blown films of a variety of

polyethylenes and their diverse melt elasticities 34. These authors reported three different regimes of

haze in a parabolic dependence of recoverable shear strain (Υ∞) of the polyethylenes processed: at very

low values of Υ∞ of the resins, blown films exhibited high haze values due to the development of distinct

spherulitic superstructures, which result in pronounced surface roughness and hence haze

(crystallization haze/regime I); at somewhat higher values of Υ∞, films featured the lowest haze values

due to a change in morphology from the aforementioned spherulitic structure into a fibrillar, row-

nucleated type texture (intermediate haze/regime II); in resins of higher values of Υ∞, i.e. those of high

melt elasticity (for instance due to long-chain branching or a broad molecular weight distribution) a very

fine-scale, elasticity-driven surface roughness was induced, which increased haze (extrusion

haze/regime III). In order to minimize surface roughness due to the latter rheological instabilities

changes in processing conditions have been explored, such as repeatedly extruding the polymer melt

prior to film blowing, lowering the throughput or increasing the extrusion temperature 29, 30, 36-38.

Proposed solutions to reduce growth of spherulitic entities, as well as reduction of surface roughness

resulting from flow instabilities are discussed in the following section with the principal focus on

nucleation and clarification of polyethylenes.

11

3 State-of-the Art of Nucleating and Clarifying Polyethylenes

The very many reported attempts to enhance nucleation and/or clarification of polyethylenes – based on

fundamentally different approaches – can be classified as follows:

1) Modification of the macromolecular structure

2) Polymer additives

3) Inorganic additives

4) Organic additives

5) Processing aids

6) Orientation

7) Lamination

In the following each of these approaches will be briefly reviewed.

1) Modification of the Macromolecular Structure

The term “polyethylene” in the literature may refer a polymer made of 100 % ethylene-monomer repeat

units, the homopolymer, or to copolymers produced with ethylene and minor amounts of other

monomeric moieties such as α-olefins, e.g. propylene, 1-butene, 1-pentene, 1-hexene or 1-octene, etc.

This modification of the regular linear polyethylene chain macromolecules via incorporation of unlike

repeat units inevitably leads to different molecular order in their solid state and changes the degree and

nature of crystallinity and therewith, among other characteristics, optical properties of the material. In

addition, related effects can be obtained by employing synthetic conditions yielding longer chain-

branched polyethylenes 39. However, as will be demonstrated in Chapter 2, the above modifications of

the main-polyethylene chains – whilst at times leading to enhanced optical properties in a most

impressive manner – are often at an unacceptable expense in terms of reduced melting (i.e. use-)

temperature and mechanical characteristics.

2) Polymer Additives

Polymers of a relatively high degree of crystallinity and high crystalline melting temperature relative to

a host polymer have been used as a nucleating agent in different polyethylene matrixes. For instance,

addition of high-density polyethylene (HDPE) to low-density polyethylene (LDPE) has been shown to

provide a high nucleation density, yielding a shish-kebab-type structure comprising HDPE pre-nucleated

fibrils which LDPE lamellae crystallize onto it 40. Similar nucleating fibrillar entities (shish) and

lamellae (kebabs) were observed by cooling of bimodal HDPE under a pulse of shear. Under such

conditions, flow-induced nucleation of the high molecular weight tail of the bimodal grade becomes a

substrate for the nucleation of lower molecular weight material 41. Furthermore, friction fibrillated ultra-

12

high molecular weight poly(tetrafluoroethylene) (PTFE) added to HDPE, generated during controlled

shear in compounding, resulted in HDPE crystallizing onto the PTFE nanofibrils 42. In another polymer-

polymer blend study, linear low-density polyethylene (LLDPE) was mixed with i-PP which itself was

pre-nucleated with nucleating agents 43.

In addition, LLDPE produced with a chromium catalyst (Cr-LLDPE) which features a high molecular

weight tail was introduced into metallocene LLDPE (m-LLDPE) to manufacture blown films 34. Prior

to this study, it was found that spherulitic-like superstructures are the dominant factor that contributes

to surface roughness (hence surface haze) of m-LLDPE in comparison to different LLDPE resins of

similar melt index 33. It was proposed that the relatively narrow molecular weight distribution of m-

LLDPE results in fast relaxation of flow-induced orientation and yielding spherulitic-like

superstructures and decrease of the melt elasticity. In order to broaden the molecular weight distribution

to increase the macromolecular relaxation time, a high molecular weight tail material was added to m-

LLDPE, and indeed it was found that by increasing the melt elasticity of the polymer blend, the surface

haze decreased as described in aforesaid parabolic “recoverable shear strain (Υ∞)” versus “haze” curve’s

regime II in the section on Clarification 34.

In still another approach, it has been tried to decrease the surface haze of m-LLDPE by adding high-

pressure, long-chain branched low-density polyethylene (HP-LDPE) to increase melt elasticity,

therewith reducing the formation of spherulitic-like superstructures 33. However, long-chain branched

structures can promote directional orientation which may lead to imbalances in mechanical properties

such as reduced impact- and tear resistance.

In alternative studies, HDPE was blended with plastomer-type ethylene (PEP) and a copolymer of

ethylene/vinyl acetate (EVA). Whilst EVA alone showed the same effect as LDPE in terms of decreasing

the haze, it deteriorated the dart impact- and tear strength; therefore PEP was added to attempt to

maintain mechanical properties of HDPE 44.

13

3) Inorganic Additives

The nucleation efficiency of a vast collection of small-particle sized minerals such as talc

(3MgO.4SiO2.H2O) 45, calcium carbonate (CaCO3) 46, 47 or whiskers (B2Mg3O6, SiO2-MgO-CaO) 48 have

been investigated as nucleating agents for PEs. Selected, prominent results of these studies are listed in

Table 1.

Table 1.

Inorganic additives as nucleating/clarifying agents for PE: polymer type used; molecular composition of the

additive; maximum peak crystallization temperature (Tc), increase in Tc with respect to the neat PE (ΔTc) and its

corresponding additive concentration; minimum haze (Hazemin), haze difference between neat PE and Hazemin

samples (Δhaze) and its corresponding additive concentration; sample type according to processing; reference

(ref). n.a. = not available.

14

4) Organic Additives

A variety of small organic molecules (e.g. anthracene, p-terphenyl) 49, organic acid derivatives (as listed

in Table 2) 50-54, sorbitol derivatives (as listed in Table 3) 55, 56 and phosphorus containing species (i.e.

phenyl phosphate compound) 57 have been investigated for improving nucleation and clarification of

polyethylenes.

Table 2.

Organic acid derivatives as nucleating/clarifying agents for PE; molecular composition of the additive; maximum

peak crystallization temperature (Tc), increase in Tc with respect to the neat PE (ΔTc) and its corresponding

additive concentration; minimum haze (Hazemin), haze difference between neat PE and Hazemin samples (Δhaze)

and its corresponding additive concentration; sample type according to processing; reference (ref). n.a. = not

available.

15

Table 3.

Sorbitol derivatives as nucleating/clarifying agents for PE: polymer type used; molecular composition of the

additive; maximum peak crystallization temperature (Tc), increase in Tc with respect to the neat PE (ΔTc) and its

corresponding additive concentration; minimum haze (Hazemin), haze difference between neat PE and Hazemin

samples (Δhaze) and its corresponding additive concentration; sample type according to processing; reference

(ref). n.a. = not available.

5) Processing Aids

Processing aids have been widely used in commercial polymer processing operations to eliminate,

among other things, flow instabilities, e.g. melt-fracture, which leads to distortions of the surface of

extruded products 58-62. The commercially most applied example is fluoropolymer-based materials for

eliminating melt-fracture induced sharkskin formation of LLDPE 63-68. In terms of decreasing the

extrusion roughness and hence the surface haze, Pruss et al. examined such boron nitride based

processing aids in film-blowing of m-LLDPE and showed improved clarification 69.

6) Orientation

Prevention of the formation of the efficiently-light-scattering spherulitic structures, or destruction

thereof, can also be achieved by inducing orientation by stretching polyethylene films to enhance

clarification. Solid-state drawing of blown films between two heated rolls at the temperatures below the

crystallization melting point is one method to achieve reduced haze 70-72. A similar method was also

employed with three rolls to provide temperature differences between the rolls to enhance rapid cooling

73. Apart from solid-state stretching by heated rolls, cast films of LLDPE blended with a nucleating agent

16

(i.e. Hyperform HPN-20E from Milliken & Company) were stretched to a 4x4 areal draw ratio resulting

in improved haze relative to that of the unoriented blend films 74. Furthermore, gel sheets, obtained by

dissolving and quenching polyethylene from a solvent, were drawn in another study with the same

objective 75.

7) Lamination

As it was previously reported in the section on “Modification of the Macromolecular Structure”, the

trade-off between the optical and thermo-mechanical properties of polyethylenes is one of the major

issues for further development of the polyethylene market. Therefore, also a multi-layer approach,

comprising extrusion of two or more materials through a single die with two or more orifices to merge

laminar structured blown films with different properties, has been explored in several studies 76-78.

4 Objective and Scope of the Thesis

From the above introductory review it is evident that a vast amount of research has been conducted to

generate polyethylenes with feature both the superior mechanical properties of HDPE and the attractive

optical characteristics of, for instance, ultra-low-density polyethylene (ULDPE). Whilst the mechanisms

to reduce haze, and maintain mechanical characteristics, such as stiffness, are now reasonably well

understood, a solution to achieve the above desired combination of properties for polyethylene has still

not materialized – particularly in blown film production or typical molding operations where the material

cannot be quenched fast enough to restrict crystal growth – which is, hence, the principal objective of

this thesis. In order to elucidate the trade-off issue between haze and stiffness in more detail; optical,

thermal and mechanical properties of PEs with different chain architectures are investigated in Chapter

2. In Chapter 3, widely-used “sorbitol”- and “1,3,5-benzene trisamide”-based commercial additives

for i-PP are explored as a nucleating/clarifying agent for the polyethylenes, possessing superior thermal

and mechanical properties (i.e. LLDPE and HDPE). In addition to the commercial compounds, further

studies in Chapter 4 explore newly designed molecules based on “1,3,5-benzene trisamides” and “1,4-

phenylene bisamides” as a potential nucleating/clarifying agent for PEs.

In Chapter 5, the phase behavior of different PEs with selected additives – which display efficient

clarification in previous chapters – is presented and the solid-state microstructure of these binary systems

is investigated. In more elaborate studies, influence of the macromolecular structure of PEs on the final

solid-state microstructure and, associated optical properties of the most effective additive and the

polymers are investigated in Chapter 6.

Finally, thesis is terminated with general conclusions and an outlook in Chapter 7.

17

5 References

1. Hannay, N. B. Crystalline and Noncrystalline Solids. Plenum Press: New York, 1976; p 497.

2. Wunderlich, B. Macromolecular Physics. Academic Press: New York, 1973; Vol. 2, p 1.

3. Chiu, G.; Alamo, R. G.; Mandelkern, L. J. Pol. Sci. Pol. Phys. 1990, 28, (8), 1207-1221.

4. Hoffman, J. D.; Miller, R. L. Polymer 1997, 38, (13), 3151-3212.

5. Peacock, A. J. Handbook of Polyethylene: Structures, Properties, and Applications. Marcel

Dekker: New York, 2000; p 67.

6. Turnbull, D. J. Chem. Phys. 1950, 18, (2), 198-203.

7. Last, A. G. M. J. Polym. Sci. 1959, 39, (135), 543-545.

8. Beck, H. N. J. Appl. Polym. Sci. 1965, 9, (6), 2131-2142.

9. Schonhor, H. J. Polym. Sci. Pol. Lett. 1967, 5, (10PB), 919-924.

10. Zettlemoyer, A. C. Nucleation. Marcel Dekker: New York, 1969; pp 405-488.

11. Binsbergen, F. L. J. Polym. Sci. Pol. Sym. 1977, (59), 11-29.

12. Zweifel, H.; Amos, S. E. Plastics Additives Handbook. 5th ed.; Hanser Gardner Publications:

Cincinnati, OH, 2001; p 949.

13. Fairgrieve, S. Rapra Review Reports 187: Nucleating Agents. In Pergamon: Oxford; New York,

2007; p 6.

14. Shepard, T. A.; Delsorbo, C. R.; Louth, R. M.; Walborn, J. L.; Norman, D. A.; Harvey, N. G.;

Spontak, R. J. J. Polym. Sci. Pol. Phys. 1997, 35, (16), 2617-2628.

15. Pukanszky, B.; Mudra, I.; Staniek, P. J. Vinyl Addit. Techn. 1997, 3, (1), 53-57.

16. Kristiansen, M.; Tervoort, T.; Smith, P.; Goossens, H., Macromolecules 2005, 38, (25), 10461-

10465.

17. Zhang, Y. F.; Xin, Z. J. Appl. Polym. Sci. 2006, 100, (6), 4868-4874.

18. Zhang, Y. F. J. Macromol. Sci. B 2008, 47, (6), 1188-1196.

19. Beck, H. N. J. Appl. Polym. Sci. 1967, 11, (5), 673-685.

20. Uhlmann, D. R.; Chalmers, B. Ind. Eng. Chem. 1965, 57, (9), 19-31.

21. Jackson, K. A. Ind. Eng. Chem. 1965, 57, (12), 28-32.

22. Gornick, F.; Hoffman, J. D. Ind. Eng. Chem. 1966, 58, (2), 41-53.

23. Tolinski, M. Additives for Polyolefins. William Andrew: Oxford, 2009; p 158.

24. Blomenhofer, M.; Ganzleben, S.; Hanft, D.; Schmidt, H.-W.; Kristiansen, M.; Smith, P.; Stoll,

K.; Mader, D.; Hoffmann, K. Macromolecules 2005, 38, (9), 3688-3695.

25. Abraham, F.; Ganzleben, S.; Hanft, D.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys.

2010, 211, (2), 171-181.

26. Kristiansen, M.; Werner, M.; Tervoort, T.; Smith, P.; Blomenhofer, M.; Schmidt, H.-W.

Macromolecules 2003, 36, (14), 5150-5156.

18

27. Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ASTM

Standard D 1003-07el, 2007.

28. Bernland, K. M. Nucleating and Clarifying Polymers. Ph.D. Dissertation, Swiss Federal Institute

of Technology Zurich, Nr 19388, Zurich, 2010.

29. Stehling, F. C.; Speed, C. S.; Westerman, L. Macromolecules 1981, 14, (3), 698-708.

30. Ashizawa, H.; Spruiell, J. E.; White, J. L. Polym. Eng. Sci. 1984, 24, (13), 1035-1042.

31. Larena, A.; Pinto, G. Polym. Eng. Sci. 1993, 33, (12), 742-747.

32. Bernabeu, E.; Boix, J. M. J. Mater. Sci. 1993, 28, (21), 5826-5830.

33. Johnson, M. B.; Wilkes, G. L.; Sukhadia, A. M.; Rohlfing, D. C. J. Appl. Polym. Sci. 2000, 77,

(13), 2845-2864.

34. Sukhadia, A. M.; Rohlfing, D. C.; Johnson, M. B.; Wilkes, G. L. J. Appl. Polym. Sci. 2002, 85,

(11), 2396-2411.

35. Kretzschmar, E.; Wolkowicz, M. D.; Enderle, J.-f.; Lilge, D. US Patent 8,029,888 B2, 2011.

36. Smith, P. F.; Chun, I.; Liu, G.; Dimitrievich, D.; Rasburn, J.; Vancso, G. J. Polym. Eng. Sci.

1996, 36, (16), 2129-2134.

37. Edmondson, M. S.; Pirtle, S. E. J. Plast. Film. Sheet. 1993, 9, (4), 334-343.

38. Shang, S. W.; Kamla, R. D. J. Plast. Film. Sheet. 1995, 11, (1), 21-37.

39. Foster, G. N.; Chen, T.; Vogel, R. H.; Wasserman, S. H.; Lee, D.-c.; Reichle, W. T.; Karol, F.

J.; Whiteker, G. T. US Patent 6,159,617, 2000.

40. Wendt, U. J. Mater. Sci. Lett. 1988, 7, (6), 643-645.

41. Balzano, L.; Rastogi, S.; Peters, G. Macromolecules 2011, 44, (8), 2926-2933.

42. Bernland, K.; Smith, P. J. Appl. Polym. Sci. 2009, 114, (1), 281-287.

43. Ealer, G. E. US Patent 5,149,484, 1992.

44. Li, R.; Chen, P.; Yang, A.; Singh, R.; Lin, C. H. US Patent 5,714,547, 1998.

45. Helland, I.; Nilsen, J.; Myhre, O. J.; Nummila-Pakarinen, A.; Lehtinen, A. European Patent

1,740,651, 2007.

46. Addiego, F.; Martino, J. D.; Dahoun, A.; Godard, O.; Patlazhan, S. J. Eng. Mater. Technol.

2011, 133, (3), 030904-(1-7).

47. Edwards, W. L.; Schiavone, R. J. US Patent 6,727,306 B2, 2004.

48. Ning, N. Y.; Deng, H.; Luo, F.; Wang, K.; Zhang, Q.; Chen, F.; Fu, Q. Compos. Part B-Eng.

2011, 42, (4), 631-637.

49. Wittmann, J. C.; Lotz, B. J. Polym. Sci. Pol. Phys. 1981, 19, (12), 1837-1851.

50. Lindahl, A. K.; Oysaed, H.; Goris, R. European Patent 1,146,077 A1, 2001.

51. Narh, K. A.; Odell, J. A.; Keller, A.; Fraser, G. V. J. Mater. Sci. 1980, 15, (8), 2001-2009.

52. Sadamitsu, K.; Ishikawa, M.; Kobayashi, T. US Patent 5,998,576, 1999.

53. Horrocks, M.; Kerscher, C. Plastics & Rubber Singapore Journal 2008, 15, 29-39.

54. Schmidt, H.-W.; Blomenhofer, M.; Stoll, K.; Meier, H.-R. US Patent 0,149,663 A1, 2007.

19

55. Cheruvu, S.; Lo, F. Y.-K.; Ong, S. C.; Su, T.-K. World Intellectual Property Organization

95/13317, 1995.

56. Miley, J. W.; Carroll, C. C.; Lever, J. G.; Mehl, N. A.; Salley, J. M. US Patent 5,973,043, 1999.

57. Su, T.-K.; Youngjohn, N. R. US Patent 4,585,817, 1986.

58. Ramamurthy, A. V. J. Rheol. 1986, 30, (2), 337-357.

59. Denn, M. M. Annu. Rev. Fluid. Mech. 2001, 33, 265-287.

60. Larson, R. G. Rheol. Acta. 1992, 31, (3), 213-263.

61. Han, C. D. Rheology in Polymer Processing. Academic Press: New York, 1976; p 304.

62. Kontopoulou, M. Applied Polymer Rheology: Polymeric Fluids with Industrial Applications.

John Wiley & Sons: Hoboken, New Jersey, 2012; p 29.

63. Rudin, A.; Worm, A. T.; Blacklock, J. E. Plast. Eng. 1986, 42, (3), 63-66.

64. Priester, D. E.; Stika, K. M.; Chapman, G. R.; Mcminn, R. S.; Ferrandez, P. Proc. SPE ANTEC

1993, 39, 2528-2533.

65. Kanu, R. C.; Shaw, M. T. Polym. Eng. Sci. 1982, 22, (8), 507-511.

66. Buckmaster, M. D.; Henry, D. L.; Randa, S. K. US Patent 5,688,457, 1997.

67. Kurtz, S. J.; Blakeslee, T. R.; Scarola, L. S. US Patent 4,282,177 1981.

68. Stewart, C. W.; Randa, S. K.; Hatzikiriakos, S. G.; Rozenbaoum, E. E.; Buckmaster, M. D. US

Patent 0,048,179 A1, 2001.

69. Pruss, E. A.; Randa, S. K.; Lyle, S. S.; Clere, T. M. Proc. SPE ANTEC, 2002, 46, 2864-2868.

70. Taka, T.; Shishido, K.; Ohkubo, T. US Patent 4,913,977, 1990.

71. Taka, T.; Shishido, K.; Okhubo, T. US Patent 5,110,686, 1992.

72. Taka, T.; Shishido, K.; Okhubo, T. US Patent 5,294,398, 1994.

73. Kotani, T.; Taka, T.; Saito, Y. US Patent 4,954,391, 1990.

74. McLeod, M.; Ashbaugh, J.; Chevillard, C.; Guenther, G.; Curtis, R. L.; Nguyen, J.; Aguirre, J.;

McBride, R.; Hicks, B. US Patent 8,026,305, 2011.

75. Kono, K.; Mori, S.; Miyasaka, K.; Tabuchi, J. US Patent 4,600,633, 1986.

76. Sukhadia, A. M.; Coutant, W. R.; Byers, J. D.; Moore, L.; Welch, M. B.; Palackal, S. J.; Cowan,

K. D.; Rohlfing, D. C.; Janzen, J.; DesLauriers, P. J.; Whitte, W. M. US Patent 6,355,359, 2002.

77. Saavedra, J. V.; Patel, R.; Ratta, V. US Patent 8,092,920, 2012.

78. Perdomi, G. US Patent 6,159,587, 2000.

21

Chapter 2

Influence of Polymer Chain Architecture on

Opto-thermo-mechanical Properties of Polyethylenes

23

1 Introduction

It is well-known that modification of the degree of crystallinity, as well as the size and shape of

crystalline entities, have a major effect on the macroscopic physical properties of polymers. As

mentioned in the previous chapter, one rather simple manner to manipulate the above-mentioned

structural elements of a given polymer is through modification of the macromolecular structure; that is

introduction of “foreign” repeat units in an otherwise structurally regular chain molecule (i.e.

homopolymer). This approach inevitably leads to altered molecular order in the solid state,

accompanied by changes in the degree and nature of crystallinity and therewith, among other

characteristics, the mechanical and optical properties, and melting temperature of the material 1, 2.

Focusing on polyethylene (PE), comonomers can be added in the polymerization reactor at a judiciously-

chosen ratio with the ethylene monomer. By incorporation of a specified amount of an α-olefin

comonomer, the molecular order of the polymer can be changed and its crystallinity in the solid state

can be controlled from ~ 0 % (i.e. plastomer) up to ~ 90 % in the case of ethylene homopolymer 3. As

summarized in Figure 1, linear, essentially unbranched PE chain molecules are termed “high-density

polyethylene” (HDPE); PE backbones with shortly alkyl groups typically attached at random intervals,

are termed “linear low-density polyethylene” (LLDPE); highly-branched polymers with, on the order of

50 branch points per 1000 carbon atoms, are termed “low-density polyethylene” (LDPE); a particular

form of LLDPE that has a much higher concentration of short or longer chain branches, are termed

“very low-density polyethylene” – also known as “ultralow-density polyethylene” (VLDPE/ULDPE) 3,

4.

Figure 1

Schematic representation of different classes of PE structures (reproduced from Ref. 3, 4)

In this chapter, polyethylenes comprising a range of comonomers and degrees of branching, were

examined to develop an “opto-thermo-mechanical matrix”, with a focus on four “base” polyethylene

resins supplied by The Dow Chemical Company.

24

2 Experimental

1) Materials

The base polyethylene resins used throughout this study were supplied by The Dow Chemical Company

and used as received. Selected properties of these resins are listed in Table 1, Section 3.1. In addition, a

number of other PE resins from different polyethylene suppliers were also included to explore possible

trends in the “opto-thermo-mechanical matrix”: polyolefin plastomer with ethene –1– octene

copolymer, AFFINITY EG 8100 G, melt index (MI) (190 °C/2.16 kg) = 1 g/10 min, density (d) = 0.870

g/cm3 (The Dow Chemical Company); polyolefin plastomer with ethene –1– octene copolymer,

AFFINITY PL 1280 G, MI (190 °C/2.16 kg) = 6 g/10 min, d = 0.900 g/cm3 (The Dow Chemical

Company); LDPE, 42,803-5, MI (190 °C/2.16 kg) = 7 g/10 min, d = 0.918 g/cm3 (Aldrich); HDPE,

Seetec CJ563, MI (190 °C/2.16 kg) = 4.7 g/10 min, d = 0.955 g/cm3 (Lotte Daesen Petrochemical Corp.)

and HDPE, Hostalen GC 7260, MI (190 °C/2.16 kg) = 8.0 g/10 min, d = 0.960 g/cm3 (LyondellBasell).

2) Processing

Samples for mechanical testing were prepared by melt-compression molding at 220 °C, followed by

quenching to room temperature in a cold press, yielding films of ~100 µm thickness. For optical

characterization, plaque samples were prepared by injection molding. As shown in Figure 1, neat

polyethylene resins were processed at 220 °C under a nitrogen blanket in a laboratory co-rotating mini-

twin-screw extruder (Xplore (DSM), 15 ml) at 40 r.p.m. for 5 min. Subsequently the molten polymers

were extruded into a laboratory mini-injector (Xplore (DSM), 12 ml), followed by injection into a mold

kept at room temperature, to yield circular plaque samples (thickness 1 mm, diameter 25 mm).

Figure 1.

Processing scheme for the injection-molded plaques: co-rotating mini-twin-screw extruder (a), mini-injector (b),

laboratory-scale injection-molding machine (c), final circular plaque sample (d).

25

3) Analysis

Optical characteristics

Haze of the injection-molded samples was determined at room temperature with a Haze-Gard Plus®

instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 5. In addition to

“overall haze”, in order to eliminate the effect of surface scattering, “bulk haze” measurements were

also conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner GmbH,

Germany) with non-drying immersion oil (Cargille Series A refractive index oil, n = 1.5150 ± 0.0002)

which has a refractive index similar to the polymer plaques. Haze values reported here correspond to the

average of measured for five samples.

Thermal analysis

Thermal analysis was conducted using a differential scanning calorimeter (DSC 822e, Mettler Toledo,

Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at standard

heating and cooling rates of 10 °C/min; the sample weight was typically 5 to 10 mg. In order to ensure

complete melting of the polymer and to prevent self-nucleation, the samples were kept for 5 min at the

maximum temperature (i.e. 220 °C) prior to cooling. The reported melting temperatures correspond to

the peak temperatures in the DSC thermograms. The melting temperature values of the resins other than

the base PEs were obtained from the technical data sheets of their respective suppliers. The degree of

crystallinity of the polymer was calculated from the enthalpy of fusion, derived from the endothermic

peak, adopting a value of 293 J/g for 100 % crystalline polyethylene 6.

Mechanical properties

Uniaxial tensile testing of the base polyethylene resins was performed on an Instron 5864 tensile testing

machine equipped with pneumatic clamps. The instrument was set up with a ±100 N static load cell and

was used in constant rate of elongation mode (i.e. 12 mm/min). All tests were carried out at room

temperature on dogbone-shaped specimens of ~100 µm thickness, 2 mm width and 12 mm gauge length.

All reported Young’s modulus (E) values of the “base resins” correspond to an average of five

measurements. The Young’s modulus values of the other resins were obtained from the technical data

sheets of their respective suppliers.

Scanning electron microscopy

Samples for scanning electron microscopy (SEM) studies were prepared by casting low concentration

solutions (~1 % w/w) of neat PE in p-xylene, yielding thin films after evaporation of the solvent; these

were subsequently molten at 220 °C and quenched to room temperature. The solidified films were coated

with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini scanning electron

microscope (LEO Elektronenmikroskopie GmbH, Germany).

26

3 Results and Discussion

1) Properties

Characteristics of the various “base” polyethylenes used in this study, including the grades of

copolymers comprising ethylene-butene (C4), hexene (C6) or octene (C8) comonomers, are presented in

Table 1.

Table 1.

Summary of principal characteristics of the “base” PE resins studied: density; melt index; number-average

molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI, Mw/Mn);

comonomer type and comonomer content; peak melting temperatures (Tm); crystallinity; Young’s modulus;

“overall haze” and “bulk haze”.

27

Density is one of the most commonly used measures for characterization of polyethylene resins. Its value

is a function of molecular weight, crystallization kinetics and, most importantly, comonomer and branch

content. In the case that all other factors are constant, generally the density of a specimen decreases as

the branch content, molecular weight or rate of crystallization increase 1, 7. In the following, a broad

range of polyethylenes possessing different branch content and molecular weight will be assessed and

expressed in the form of density on prominent macroscopic physical material properties, i.e. optical,

thermal and mechanical, will be presented.

2) Optical Properties

Figure 2 shows a plot of “overall haze” of 1 mm thick injection-molded plaques versus the density of

various PEs. Clearly, haze is generally found to dramatically increase with their density.

Figure 2.

Plot of “overall haze” of 1 mm thick injection-molded polyethylene plaques versus density of the polymer. Solid

symbols denote experimental data obtained for the “base” PE resins and open symbols refer to data from Ref. 8.

The solid line is a guide to the eye only.

On the other hand, resins possessing approximately the same density can feature a relatively large spread

in the value of haze (cf. Figure 2: ~ 0.92 g/cm3 and ~ 0.95 g/cm3). In this context, it should be noted that

“surface haze” resulting from processing imperfections affects the overall haze values. As can be seen

from Table 1, plaques produced with pellets of LLDPE – 6 and LLDPE - 8, 1MI resins, which have

similar comonomer content and molecular weight characteristics, exhibit similar bulk haze (57% and

62%, respectively). However, their “overall haze” values are different and much higher than their “bulk

haze” (i.e. 81 % and 73%). The influence of “surface haze” on the optical properties of the selected

injection-molded plaque samples is illustrated in Figure 3. For instance, while resolution details of the

numbers seen through the plaques reveal variation for the low melt index resins such as LLDPE – 6,

28

LLDPE – 8, 1MI (cf. Figure 3. a, b, c), LLDPE – 8, 25MI possessing a high melt index has a

homogeneous appearance for the resolution of numbers. This result can be contributed to the varying

extent of surface imperfections in the final product due to issues in processing, which is affected by the

molecular architecture of PE.

Figure 3.

Illustration of the effects of inhomogeneous surface imperfections on “overall haze” (O) and “bulk haze” (B)

values of the samples. The 1 mm thick, injection-molded plaques of neat LLDPE – 6, powder; LLDPE – 6, pellet;

LLDPE – 8, 1MI; LLDPE – 8, 25MI (a, b, c, d, respectively) have respectively: O 80%, B 54; O 81%, B 57; O

73%, B 62; O 90%, B 88. Images on the right with white-written text background are the identical samples shown

on the left.

It must be also taken into account that, apart from the branch content, molecular weight characteristics,

which is another parameter that affect the density, can also alter the haze. As can be seen in Table 1,

LLDPE – 8, 1MI and LLDPE – 8, 25MI resins, which have identical comonomer content and side chain

length, higher haze is found for the LLDPE of the lower molecular weight.

Furthermore, additives in the resin (i.e. stabilizers, anti-oxidants, etc.) can also affect haze. For instance,

injection-molded plaques of HDPE – 4, powder resin, which is free of additives, show ~15 % lower haze

in comparison with those produced with its pellet version. Therefore, the effect of additives on light

scattering by the molten polymer was investigated by melt-intrinsic haze measurements. For this

29

purpose, compression-molded molten specimen, sandwiched between 3 mm glass plates with 1 mm

spacers were placed onto the haze port of the vertically-oriented hazemeter (cf. Figure 4.a). As shown

in Figure 4.b, the sample of HDPE – 4, pellet containing additives exhibited significantly more

pronounced light scattering by the molten polymer when compared with that of the additive-free

polymer. SEM images of films of HDPE – 4, pellet reveal relatively large spherulitic solid-state

structures (cf. Figure 4.c), consistent with the higher haze value of the respective resin, and the absence

of those features in films of neat HDPE – 4, powder.

Figure 4.

Schematic of the design of the melt-intrinsic haze measurements (a); melt-intrinsic haze values of HDPE – 4, pellet

( ■ ) and neat HDPE – 4, powder ( □ ) at room and above the melting temperature (b); SEM images of films

produced with HDPE – 4, pellet (left) and neat HDPE – 4, powder (right) samples (c).

30

3) Thermo-mechanical Properties

Low haze, which can be readily obtained with very low-density PEs, as shown in Figure 2, is generally

preferable for the packaging industry. However, thermal and mechanical properties of PEs of such low-

density polyethylenes are compromised.

Figure 5.a shows a plot of the melting temperature versus density revealing the major penalty in terms

of melting – and, associated service temperature. A plot of the Young’s modulus as a function of the

density of compression-molded PE films is shown in Figure 5.b. Similar to the penalty in melting

temperature, the stiffness also dramatically diminishes for the lower density, more transparent PEs,

which is consistent with previous studies 3, 9. A compromise must, therefore, be found between optical

and thermo-mechanical properties. For this purpose, in the following chapters a range of

nucleating/clarifying agents will be investigated for the polyethylenes with the objective that they exhibit

superior thermal and mechanical properties, i.e. HDPE or LLDPE, in combination with advantageous

optical characteristics such as those that are typically observed for the low-density resins.

Figure 5.

Plot of melting temperature versus density of different polyethylenes (a) and Young’s modulus versus density of

compression-molded polyethylene films (b). Solid symbols denote experimental data obtained for the “base” PE

resins and open symbols refer to data obtained from the technical data sheets of the respective PE resins described

in the materials section. The solid line in (a) is a guide to the eye only, and extrapolates to equilibrium melting

temperature of PE at ~141.5 °C 10.

31

4 Conclusions

In this chapter, the ways in which optical, thermal and mechanical properties of polyethylenes can be

altered by tailoring the chain architecture of the polymer were investigated. The interdependence

between mechanical, thermal and optical properties is summarized in Figure 6. It can be seen that highly

transparent polyethylenes can readily be produced, but this desirable property is achieved at the expense

of a major reduction of their equally-relevant thermal (i.e. melting temperature) and mechanical (i.e.

stiffness) characteristics, which are of a paramount importance for many applications. Therefore,

developing a polyethylene-based material with optimal optical and thermo-mechanical properties

remains a major challenge.

Figure 6.

Plot of Young’s modulus (blue-square symbols) and melting temperature (red-triangle symbols) versus “overall

haze” for polyethylenes, showing the trade-off between optical and thermo-mechanical properties. Solid symbols

denote experimental data obtained for the “base” PE resins and open symbols refer to data obtained from Ref. 8

and the technical data sheets of the respective PE resins described in the materials section.

32

5 References

1. Popli, R.; Mandelkern, L. J. Polym. Sci. Pol. Phys. 1987, 25, (3), 441-483.

2. Kaplan, W. A. Ed. Modern Plastics Encylopedia '98. McGraw-Hill: New York, 1997; p 52.


Dekker: New York, 2000; p 2, 304.

4. Kaiser, W. Kunststoffchemie für Ingenieure: Von der Synthese bis zur Anwendung. Hanser:

Munich, 2011; p 236.



6. Wunderlich, B.; Czornyj, G. Macromolecules 1977, 10, (5) 906.

7. Peacock, A. J.; Mandelkern, L. J. Polym. Sci. Pol. Phys. 1990, 28, (11), 1917-1941.

8. Loiseau, E. Master Thesis ETH Zurich, 2010.

9. Saavedra, J. V.; Patel, R.; Ratta, V. US Patent 8,092,920, 2012.

10. Wunderlich, B. Thermal Analysis. Academic Press: San Diego, 1990; p 418.

33

Chapter 3

Efficiency of Commercial Additives for


35

1 Introduction

Sorbitol-based and 1,3,5-benzene trisamide-based additives currently are the most widely-used

commercial nucleating/clarifying agents for isotactic polypropylene (i-PP). Sorbitol derivatives such as

(1,3:2,4)-dibenzylidenesorbitol (DBS), 1,3:2,4-di-p-methylbenzylidenesorbitol (MDBS), 1,3:2,4-

bis(3,4-dimethylbenzylidene)sorbitol (DMDBS), 1,3:2,4-di-p-ethylbenzylidenesorbitol (EDBS) and

1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol (TBPMN) have been

comprehensively investigated as efficient i-PP nucleators/clarifiers 1-13. The remarkable performance

and the commercial success of sorbitol derivatives result from, among other things, their chemical

structure that allows them to easily dissolve in, and recrystallize from the polymer melt, yielding a three-

dimensional nanofibrillar network of a large surface area for subsequent nucleation of the polymer

chains 14, 15. Recently, a new group of substituted 1,3,5-benzene trisamides, i.e. 1,3,5-tris(2,2-

dimethylpropionylamino)benzene (TDMPAB), has been advanced, providing superior nucleation and

imparting drastically improved optical properties at ultra-low concentrations, accompanied by

outstanding thermal stability and excellent solubility in the polymer melt, thus facilitating and improving

homogeneous dispersion of the additive during processing 16-20.

Even though the aforementioned additives were presented as nucleating and clarifying agents for

polyolefins, there are few documented studies about their use for polyethylenes (PEs) 21-23, which are

capable of providing an increased toughness over a wider temperature range due to their lower glass

transition temperatures and allowing for the production of low-energy-consuming artifacts as a result of

their relatively low melting temperatures when compared with i-PP. In this chapter, in a first attempt,

different commercial additives from the 1,3,5-benzene trisamide families (i.e. TDMPAB) and sorbitol

derivatives (i.e. DMDBS and TBPMN) have been screened for potential use with the four “base” PE

resins employed in this thesis (previously introduced in Chapter 2). The chemical structures of the

additives are shown in Figure 1.

Figure 1.

Chemical structures of the additives used in this study: 1,3,5-tris(2,2-dimethylpropionylamino)benzene

(TDMPAB) (left), 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) (middle) and 1, 2, 3-trideoxy-4,6:5,7-

bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN) (right).

36

2 Experimental

1) Materials

The “base” polyethylene resins listed in Chapter 2 were supplied by The Dow Chemical Company and

used as received. The compounds, 1,3,5-tris(2,2-dimethylpropionylamino)benzene (TDMPAB,

Irgaclear XT386, CAS Registry Number: 745070-61-5) from Ciba Speciality Chemicals, now BASF

SE, Basel; 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS, Millad 3988, CAS Registry

Number: 135861-56-2) from Milliken Chemicals and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-

propylphenyl)methylene]-nonitol (TBPMN, NX 8000, CAS Registry Number: 882073-43-0) from

Milliken Chemicals, were used as received.

2) Processing

Injection-molding plaques

The same processing scheme as shown in Chapter 2-Figure 1 was followed to produce injection-molded

plaques. PE/additive blends were compounded in a laboratory co-rotating mini-twin-screw extruder

(Xplore (DSM), 15.0 ml) at 40 r.p.m. for 5 min at 220 °C. Series of PE with different additive

concentrations were prepared commencing with a PE masterbatch (12.7 g) comprising 2 or 5 % w/w

additive, which was subsequently diluted to lower concentrations. For the masterbatch preparation,

pellet grades (LLDPE – 8, 1MI and LLDPE – 8, 25MI) were compounded with the mini-twin-screw

extruder and powder grades (HDPE – 4 and LLDPE – 6) were dry blended. For each concentration, 8.1

g of the compounded mixture was extruded into a laboratory mini injector (Xplore (DSM), 12.0 ml).

Subsequently, the desired amount of PE/additive was added to the remaining 4.6 g masterbatch. By

repeating this procedure, blends of PE and the additives were prepared with decreasing additive

concentrations in the range of 2 or 5 % w/w to as low as 0.005 % w/w. Thereafter, the molten

polymer/additive blends were injected into a mold kept at room temperature to produce plaque samples

(thickness 1.0 mm, diameter 25.0 mm). The entire micro-scale polymer processing was conducted at

220 °C under a nitrogen blanket. Reference samples of the neat polymer were produced according to the

identical procedure.

37

3) Analysis



instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 24. A circular area on

the plaque samples, 18.0 mm in diameter, was illuminated by the light beam; the recorded haze values

hereafter are referred to as “overall-area haze”. In addition, haze values of circular area with 8.0 mm

diameter on samples (those free of apparent surface irregularities) were recorded and, hereafter, are

referred to as “small-area haze”. In order to eliminate the effect of surface scattering, “bulk haze”

measurements were conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner

GmbH, Germany) with non-drying immersion oil (Cargille Series A refractive index oil of n = 1.5150

± 0.0002) which has a similar refractive index as the polymer plaques. Haze values reported here

correspond to the average of values measured for five samples.

As a complementary method to haze measurements, photographs of the plaques possessing minimum

“overall-area haze” values of each PE/additive series were taken with a digital camera and

quantitatively analyzed by the method described below. As depicted in Figure 2, plaque samples were

placed onto a 0.1 mm thin patterned stainless steel background mask. The samples were illuminated

from the bottom with a light source (“Micron” Tavola Luminosa, Osram L 15W/10 daylight fluorescent

tube) and subsequently a digital image was taken with a Canon EOS 20D DSLR camera from the top.

The background mask pattern consists of a simple grid of 2 mm stripes separated by 2 mm wide gaps,

resulting in an alternating series of lit (bright) and unlit (dark) sample areas. Analysis of the

corresponding digital images were performed by quantifying the sharpness of the transition from bright

to dark areas along any perpendicular to the grid pattern. But before presenting the results obtained, it is

important to point out the definition of pixel intensity which quantifies the sharpness of these transitions.

Digital images are two-dimensional grids of pixel intensity values with the width and height of the image

being defined by the number of pixels in x (rows) and y (columns) direction. Thus, pixels are the smallest

single components of images, holding numeric values, referred as “pixel intensities” that range between

black and white. The characteristics of this range, i.e. the number of unique intensity (brightness) values

that can exist in the image are defined as the “bit” (depth of the image) and specify the level of precision

in which intensities are coded 25. In this study, 8 bit RGB (a widely used color space) files obtained by

the camera were converted to 8 bit gray-scale files which display 256 (28) gray levels (integers only).

Subsequently, images were quantitatively analyzed with ImageJ software. 5 pixel wide line was drawn

perpendicularly to the mask grids and its “gray value” profile was plotted (cf. Figure 2.c-left). The

sharpness of the transitions from bright (gray value > 100) to dark (gray value < 100) and vice versa was

quantified by plotting the “derivative of the gray value” profile (cf. Figure 2.c-right): inflection points

38

in the crossovers appear as sharp positive/negative peaks and taken as a complementary method to haze,

i.e. with higher absolute peak values indicating less haze/better transparency. In order to quantitatively

analyze the differences between the samples, values of the “gray value derivative” are compared for

different clarifying agents for each resin in Figure 4.b, 6.b, 8.b, 10.b.

Figure 2.

New method to determine qualitative and quantitative optical properties of injection-molded plaque samples to

complement haze measurements. Samples are placed onto a background mask and illuminated from the bottom

(a). Digital image (b) taken with a camera is analyzed by evaluating the pixel intensities along the arrow shown in

the photo graph: “Pixel intensities/gray values” along the “distance” are plotted (left) and the corresponding

“derivative” curve (right) is shown in (c).

39

Thermal analysis


Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at a standard


complete melting of the polymer and prevent self-nucleation, samples were kept for 5 min at the

maximum temperature prior to cooling. The reported crystallization temperatures correspond to the peak

temperatures in the DSC thermograms.



solutions (~1 % w/w) of neat PE and PE containing 2 % w/w of the additives in p-xylene, yielding thin

films after evaporation of the solvent; these were then molten at temperatures above the melting

temperatures of the additive in the blend and subsequently quenched to room temperature. The solidified

films were coated with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini

scanning electron microscope (LEO Elektronenmikroskopie GmbH, Germany).

40


Nucleation efficiency and optical properties of the solidified binaries comprising “base” polyethylenes

(i.e. HDPE – 4, LLDPE – 6, LLDPE – 8, 1MI and LLDPE – 8, 25MI) and the aforementioned additives

TDMPAB, DMDBS and TBPMN were investigated at additive concentrations ranging from 0 % w/w

up to 2 or 5 % w/w, and are presented in the following sections. In each section, peak crystallization

temperatures of the polyethylenes (Tc, PE) in PE/additive blends and increase in Tc, PE with respect to that

of the neat resins (ΔTc, PE) at concentrations where maximum crystallization peak temperature (Tc, max) is

obtained are presented (Figure 3, 5, 7 and 9). Additionally, optical properties of the blends are shown

by plotting “overall-area haze”, “small-area haze” and “bulk haze” versus additive content and their

decrease with respect to the neat resins (Δhaze) at concentrations where minimum haze (Hazemin) is

obtained are shown (Figure 4.a, 6.a, 8.a and 10.a).

1) HDPE – 4

In Figure 3, are presented the polymer peak crystallization temperatures (Tc, PE) at low additive content

mixtures in HDPE – 4. The data indicates no significant differences relative to the neat polymer. In other

words, introduction of the additives does not affect the nucleation efficiency, presumably due to the high

intrinsic nucleation- and crystallization growth rate of HDPE – 4.

Figure 3.

Peak crystallization temperatures of HDPE – 4 containing TDMPAB, DMDBS or TBPMN (from left to right).

Red, dashed lines indicate the Tc, PE of the neat resin.

On the other hand, haze measurements of HDPE – 4 plaques containing the sorbitol clarifying agents

(i.e. DMDBS and TBPMN) presented in Figure 4.a show improvement of clarification of the polymer,

whilst 1,3,5-benzene trisamide based TDMPAB induced no reduction in haze.

41

Figure 4.

Haze of HDPE – 4 containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze” (■), “bulk

haze” (●) and “small-area haze” (⋆) of injection-molded plaques, plotted as function of the additive content. Red,

dashed lines indicate the “overall-area haze” values of the neat resin. In the table below are listed haze values of

the neat HDPE – 4, and values of the decrease in haze with respect to the neat resin (Δhaze) at concentrations (%

w/w) where minimum haze (Hazemin) is observed: * = “overall-area haze”, ** = “small-area haze”, *** = “bulk

haze” (a). Quantitative analysis of the optical properties of the HDPE – 4 injection-molded plaques comprising

0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.5 % w/w TBPMN (from left to right): “gray value derivative”

plotted versus the “distance” along the arrow direction as shown in the corresponding plaque photos (b).

42

Most interestingly, reduced haze with TBPMN, for instance ~40 % “small-area haze” at concentrations

of 0.5–3 % w/w, is fairly successful for HDPE resins. Quantitative analysis of the digital images of the

HDPE – 4 plaques according to above described method in Figure 2 are shown in Figure 4.b. Plaques

comprising DMDBS and TBPMN have sharper transitions and a higher level of “gray value derivative”

in comparison with TDMPAB, which is consistent with the reported haze values. Finally, as revealed

by the data presented in Figures 3 and 4, there appears to be no consistent connection between nucleation

efficiency and the ability of compounds to reduce haze, which is in accord with previous reports 16, 19, 26.

2) LLDPE – 6

Corresponding data regarding the nucleation efficiency for LLDPE – 6 compositions are presented in

Figure 5. As can be seen, a minor increase in Tc, PE (i.e. ~2 °C) and an insignificant decrease in haze (i.e.

~5 %) (cf. Figure 6.a) for the binaries with TDMPAB reveal its poor nucleation and clarification ability

in LLDPE – 6. On the other hand, the sorbitol derivatives, DMDBS and TBPMN, induced a modest

increase in Tc, PE at fairly low concentrations (i.e. ~5 °C at 0.5 % w/w and ~6 °C at 0.25 % w/w,

respectively).

Figure 5.

Peak crystallization temperatures of LLDPE – 6 containing TDMPAB, DMDBS or TBPMN (from left to right).

Red, dashed lines indicate the Tc, PE of the neat resin. In the table below are listed the Tc, PE of the neat LLDPE – 6,

the values of the maximum increase in Tc, PE with respect to the neat resin (ΔTc, PE) at concentrations (% w/w)

where Tc, max is observed.

43

Regarding the effect of the addition of sorbitols on optical properties to this PE resin, from the data

presented in Figure 6.a, it can be concluded that DMDBS and TBPMN are poor clarifying agents for

the reduction of “overall-area haze” of LLDPE – 6, but are significantly effective in decreasing the

“bulk haze”. This observation points to the processing issues causing the imperfections on the surface

of injection-molded plaques which was previously discussed in Chapter 2-Figure 3. It should be noted

that a high degree of surface imperfections spread over the sample surface did not permit to measure

“small-area haze” for the mixtures with LLDPE – 6. The results observed with “bulk haze”

measurements of LLDPE – 6 with DMDBS and TBPMN are highly encouraging, manifest in significant

reductions, i.e. 32 % and 25 %, at concentrations of 0.25 % w/w and 0.5 % w/w, respectively.

Figure 6.

Haze of LLDPE – 6 containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze” (■) and

“bulk haze” (●) of injection-molded plaques, plotted as function of the additive content. Red, dashed lines indicate

the “overall-area haze” values of the neat resin. In the table below are listed haze values of the neat LLDPE – 6,

and values of the decrease in haze with respect to the neat resin (Δhaze) at concentrations (% w/w) where minimum

haze (Hazemin) is observed: * = “overall-area haze”, *** = “bulk haze” (a).

44

Figure 6.

Continued; quantitative analysis of the optical properties of the LLDPE – 6 injection-molded plaques comprising

0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.5 % w/w TBPMN (from left to right): “gray value derivative”

plotted versus the “distance” along the arrow direction as shown in the corresponding plaque photos (b).

As it was previously found for HDPE – 4, quantitative analysis obtained by the digital images of the

plaques in Figure 6.b reveal that DMDBS and TBPMN have higher “gray value derivative”, in accord

with haze measurements. Changes in the positive/negative peak values are likely due to surface

imperfections of the samples.

Clarification with TBPMN of LLDPE – 6 is observed over a relatively wide concentration range of the

additive (i.e. 0.5–3 % w/w), as it was previously observed with HDPE – 4. This result can possibly be

attributed to the high solubility of TBPMN in PE during processing over a large concentration regime.

Therefore, solubility limits of TBPMN were systematically investigated in a more detailed study of the

phase behavior of the binary system with LLDPE – 6 in Chapter 5.

45

3) LLDPE – 8, 1MI

From the data presented in Figure 7 and Figure 8, it can be concluded that, similar to HDPE – 4 and

LLDPE – 6, TDMPAB displays an insignificant enhancement of nucleation and clarification efficiency

(i.e. 3 °C increase in Tc, PE and 11 % decrease in haze) with LLDPE – 8, 1MI. By contrast, as shown in

Figure 7, DMDBS and TBPMN enhance the nucleation of LLDPE – 8, 1MI by a ~9 °C increase in Tc,

PE with respect to the neat polymer. Even though efficient nucleation occurs at fairly high concentrations

(i.e. respectively above 1.25 % w/w and above 1 % w/w), this value is an encouraging result for reducing

the processing cycles of polyethylenes.

Figure 7.

Peak crystallization temperatures of LLDPE – 8, 1MI containing TDMPAB, DMDBS or TBPMN (from left to

right). Red, dashed lines indicate the Tc, PE of the neat resin. In the table below are listed the Tc, PE of the neat

LLDPE – 8, 1MI, the values of the maximum increase in Tc, PE with respect to the neat resin (ΔTc, PE) at

concentrations (% w/w) where Tc, max is observed.

All three types of haze measurements in Figure 8.a: “overall-area haze”, “small-area haze” and “bulk

haze”, reveal a substantial decrease in haze for the compositions of DMDBS and TBPMN with LLDPE

– 8, 1MI. In a most impressive manner, “small-area haze” decreases to the level which is comparable

to the clarified i-PP (i.e. DMDBS: 14 % for LLDPE – 8, 1MI, ~15 % for i-PP 10 and TBPMN: 17 % for

LLDPE – 8, 1MI, 10 % for i-PP 13). In addition to the improved haze of LLDPE – 8, 1MI with DMDBS

and TBPMN, quantitative analysis obtained from the corresponding plaque images in Figure 8.b

consistently show higher “gray value derivative” values.

46

Figure 8.

Haze of LLDPE – 8, 1MI containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze”

(■), “bulk haze” (●) and “small-area haze” (⋆) of injection-molded plaques, plotted as function of the additive

content. Red, dashed lines indicate the “overall-area haze” values of the neat resin. In the table below are listed

haze values of the neat LLDPE – 8, 1MI, and values of the decrease in haze with respect to the neat resin (Δhaze)

at concentrations (% w/w) where minimum haze (Hazemin) is observed: * = “overall-area haze”, ** = “small-

area haze”, *** = “bulk haze” (a). Quantitative analysis of the optical properties of the LLDPE – 8, 1MI injection-

molded plaques comprising 0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.25 % w/w TBPMN (from left to

right): “gray value derivative” plotted as function of the “distance” along the arrow direction as shown in the

corresponding plaque photos (b).

47

For the two sorbitol-based additives, concentrations at which Hazemin values were found closely match

those for the LLDPE – 6 resin (i.e. 0.25 % w/w for DMDBS and 0.25–1.50 % w/w for TBPMN in

LLDPE – 8, 1MI). The change in haze with its strong dependence on additive concentration was also

investigated in a more detailed study of the phase behavior of LLDPE – 8, 1MI/TBPMN binaries in

Chapter 5.

4) LLDPE – 8, 25MI

As it was found for the previous PE resins, TDMPAB also does not cause an increase in Tc, PE of LLDPE

– 8, 25MI (cf. Figure 9) and decreases the haze of neat polymer only to a rather modest degree (i.e. 27

% decrease in haze, to 62 %) (cf. Figure 10). As can be seen in Figure 9, DMDBS and TBPMN also

improve nucleation of LLDPE – 8, 25MI (i.e. ~7 °C and ~8 °C increase in Tc, PE, respectively) – as in

the case of LLDPE – 6 and LLDPE – 8, 1MI – from additive concentrations exceeding ~0.5 % w/w.

Figure 9.

Peak crystallization temperatures of LLDPE – 8, 25MI containing TDMPAB, DMDBS or TBPMN (from left to

right). Red, dashed lines indicate the Tc, PE of the neat resin. In the table below are listed the Tc, PE of the neat

LLDPE – 8, 25MI, the values of the maximum increase in Tc, PE with respect to the neat resin (ΔTc, PE) at

concentrations (% w/w) where Tc, max is observed.

As it was previously observed for LLDPE – 8, 1MI, all three types of haze measurements reveal

substantial amounts of decrease for the compositions with sorbitols and LLDPE – 8, 25MI (see Figure

10.a). Outstanding clarification performance of the sorbitols was also found for LLDPE – 8, 25MI (i.e.

48

24 % and 15 % “small-area haze” for respectively DMDBS and TBPMN at concentrations as low as

0.25 % w/w) comparable to clarified i-PP 10, 13. Beneficially, and in contrast to results observed with the

previous resins, the relatively small differences between the “overall-area haze” values and the “small-

area haze” and “bulk haze” values, permits to use this particular resin for injection-molding, high-

thickness applications. This result indicates that rheology of the low molecular weight LLDPE – 8, 25MI

resin in injection-molding is more suitable for producing plaques and other artifacts with homogeneous

surfaces compared with the other resins. Again quantitative analysis of the plaque samples in Figure

10.b reveal higher “gray value derivative” values for DMDBS and TBPMN as in the case of previous

resins, which is also consistent with the above haze results.

As for the previous resins, in Chapter 5, the phase behavior of the LLDPE – 8, 25MI/TBPMN binary

was also studied in order to understand the concentration dependency of the additive on haze of the

samples in more detail.

Figure 10.

Haze of LLDPE – 8, 25MI containing TDMPAB, DMDBS or TBPMN (from left to right): “overall-area haze”

(■), “bulk haze” (●) and “small-area haze” (⋆) of injection-molded plaques, plotted as function of the additive

content. Red, dashed lines indicate the “overall-area haze” values of the neat resin. In the table below are listed

haze values of the neat LLDPE – 8, 25MI, and values of the decrease in haze with respect to the neat resin (Δhaze)

at concentrations (% w/w) where minimum haze (Hazemin) is observed: * = “overall-area haze”, ** = “small-

area haze”, *** = “bulk haze” (a).

49

Figure 10.

Continued; quantitative analysis of the optical properties of the LLDPE – 8, 25MI injection-molded plaques

comprising 0.05 % w/w TDMPAB, 0.25 % w/w DMDBS, 0.25 % w/w TBPMN (from left to right): “gray value

derivative” plotted as function of the “distance” along the arrow direction as shown in the corresponding plaque

photos (b).

5) Structure

In order to gain further understanding of the nucleation and clarification efficiency of the aforementioned

additives with different PEs, scanning electron microscopy (SEM) was conducted with a focus on

LLDPE – 8, 25MI, as optimum performance was observed with this particular grade. For this purpose,

solution-cast samples of neat LLDPE – 8, 25MI and polymer/additive blends were produced, dried,

melted and subsequently quenched. Representative results of this study are presented in Figure 11. The

SEM image of the neat polymer shows clear, classical spherulitic structures (upper-left), while the

polymer containing the additives feature significantly different microstructures. The self-assembly and

crystallization of the additives in PE varies in shape and size for different molecular compounds. The

polymer comprising 2 % w/w 1,3,5-benzene trisamide based TDMPAB reveals large crystal domains

of the additive (upper-right), which significantly scatter visible light. Besides, PE lamellae both grow

onto the additive and at the same time macromolecular organization arises similar to that in the neat

polymer. By contrast, structures obtained with the sorbitol derivatives, DMDBS and TBPMN

(respectively lower-left and lower-right), at the same additive content are seen to feature fine-fibrils of

the additive and, consequently, form a highly beneficial fibrillar, large topological surface-network for

PE to grow predominantly onto them. Finally, it should be noted that the sorbitol derivatives form fibrils

with dimensions of which are not of the order of the wavelength of visible light, thus reducing light

scattering further leading to enhanced clarification performance.

50

Figure 11.

SEM images of the neat LLDPE – 8, 25MI (upper-left) and the same resin containing 2 % w/w of the additives

TDMPAB (upper-right), DMDBS (lower-left) and TBPMN (lower-right).

4 Conclusions

In summary, industrially widely-used additives for i-PP were explored as potential nucleating and

clarifying agents for the “base” polyethylenes used in this thesis. In general, the 1,3,5-benzene trisamide

based TDMPAB exhibits poor nucleation and clarification for all PE resins. On the other hand sorbitol

derivatives, DMDBS and TBPMN, improve nucleation of the PEs, (i.e. an increase in crystallization

peak temperatures up to 9 °C), and induce a drastic enhancement of optical properties (i.e. reduction in

haze to values as low as 14 %), which are similar to that of the clarified i-PP.

Additionally, as shown in Appendix-Section 1, addition of DMDBS to LLDPE – 8, 1MI for thin 50 μm

thin blown-film applications, yields similar properties as observed for the 1 mm thick injection-molded

plaques in this chapter. For instance, “bulk haze” of LLDPE – 8, 1MI blown-films comprising 0.2 %

w/w DMDBS is 3.5 times less than that of the neat polymer (i.e. 1.9 % and 7 %, respectively); this ratio

is 2.5 for the injection-molded plaques comprising 0.25 % w/w DMDBS (i.e. 25 % and 62 %,

respectively).

51

SEM studies demonstrated that the size and shape of the aggregates of the additives have an important

influence on the level of light scattering by the final solid material. While large crystal domains of

TDMPAB do not cause clarification, the fine-fibrillar networks of DMDBS and TBPMN reduce haze

in a most beneficial manner. Additionally, investigating the crystal structure of polyethylene (i.e.

LLDPE – 8, 25MI) in Appendix-Section 3 revealed similar WAXD patterns for the neat polymer and

polymers comprising commercial additives.

However, it should be reemphasized that the macromolecular architecture of the PEs by itself also plays

an important role in nucleation and clarification performance of the additives. For instance, as can be

concluded from the results obtained with HDPE – 4, identical additives are not as effective as in other

resins in terms of nucleation and clarification. This is summarized more quantitatively in Table 1 in

which data are presented regarding minimum and maximum values of haze observed for the different

neat polymers and those to which TBPMN is added, as well as their Δhaze values. Remarkably, when

comparing haze of HDPE – 4 and LLDPE – 8, 1MI – which are of similar magnitude – addition of

TBPMN leads to a reduction of 20-25 % in the former case and 43-19 % in the latter. Equally striking

is a comparison between LLDPE – 8, 1MI and LLDPE – 8, 25MI for which Δhaze is observed to be 43-

19 % and 73-58 %, respectively. Therefore, the influence of the macromolecular structure of PE on the

efficiency of a particular additive will be explored in Chapter 6 in more detail.

Table 1.

Summary of the results presented in this chapter for four “base” PE resins: haze of neat PE, minimum haze value

observed for TBPMN comprising PEs (Hazemin) and corresponding value of the decrease in haze with respect to

the neat resin (Δhaze). The range of the values were recorded according to different haze measurements: * =

“overall-area haze”, ** = “small-area haze”, *** = “bulk haze” (a).

52

5 References

1. Garg, S. N.; Stein, R. S.; Su, T. K.; Tabar, R. J.; Misra, A., Kinetics of Aggregation and Gelation.

North-Holland: Amsterdam, 1984.

2. Fillon, B.; Lotz, B.; Thierry, A.; Wittmann, J. C. J. Polym. Sci. Pol. Phys. 1993, 31, (10), 1395-

1405.

3. Smith, T. L.; Masilamani, D.; Bui, L. K.; Khanna, Y. P.; Bray, R. G.; Hammond, W. B.; Curran,

S.; Belles, J. J.; Bindercastelli, S. Macromolecules 1994, 27, (12), 3147-3155.

4. Mathieu, C.; Thierry, A.; Wittmann, J. C.; Lotz, B. Polymer 2000, 41, (19), 7241-7253.

5. Nagarajan, K.; Myerson, A. S. Cryst. Growth Des. 2001, 1, (2), 131-142.

6. Hoffmann, K.; Huber, G.; Mader, D. Macromol. Symp. 2001, 176, 83-91.

7. Zhao, Y.; Vaughan, A. S.; Sutton, S. J.; Swingler, S. G. Polymer 2001, 42, (15), 6587-6597.

8. Marco, C.; Ellis, G.; Gomez, M. A.; Arribas, J. M. J. Appl. Polym. Sci. 2002, 84, (13), 2440-

2450.

9. Martin, C. P.; Vaughan, A. S.; Sutton, S. J.; Swingler, S. G. J. Polym. Sci. Pol. Phy. 2002, 40,

(19), 2178-2189.


Macromolecules 2003, 36, (14), 5150-5156.

11. Marco, C.; Ellis, G.; Gomez, M. A.; Arribas, J. M. J. Appl. Polym. Sci. 2003, 88, (9), 2261-

2274.

12. Mai, K. C.; Wang, K. F.; Han, Z. W.; Zeng, H. M. J. Appl. Polym. Sci. 2002, 83, (8), 1643-1650.

13. Bernland, K.; Tervoort, T.; Smith, P. Polymer 2009, 50, (11), 2460-2464.

14. Thierry, A.; Straupe, C.; Lotz, B.; Wittmann, J. C. Polym. Commun. 1990, 31, (8), 299-301.





17. Kristiansen, P. M.; Gress, A.; Smith, P.; Hanft, D.; Schmidt, H.-W. Polymer 2006, 47, (1), 249-

253.

18. Wang, J. D.; Dou, Q. Colloid Polym. Sci. 2008, 286, (6-7), 699-705.


2010, 211, (2), 171-181.

20. Abraham, F.; Kress, R.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys. 2013, 214, (1), 17-

24.

21. Schmidt, H.-W.; Blomenhofer, M.; Stoll, K.; Meier, H.-R. US Patent 0,149,663 A1, 2007.

22 Cheruvu, S.; Lo, F. Y.-K.; Ong, S. C.; Su, T.-K. World Intellectual Property Organization

95/13317, 1995.

53

23 Miley, J. W.; Carroll, C. C.; Lever, J. G.; Mehl, N. A.; Salley, J. M. US Patent 5,973,043, 1999.



25, Ferreira, T.; Rasband, W. S. ImageJ User Guide — IJ1.46, imagej.nih.gov/ij/docs/guide/,

2010–2012; p 10, 142.



55

Chapter 4

New “Designer” Nucleating/Clarifying Agents for

Polyethylene

57

1 Introduction

The use of nucleating/clarifying agents in processing of polyolefins is a most industrially relevant

method to efficiently convert these commodity polymers into consumer products 1. Addition of small

amounts of members of the most successful group of nucleating agents, i.e. certain sorbitol derivatives

such as 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-

propylphenyl)methylene]-nonitol (TBPMN), raises the peak crystallization temperature of isotactic

polypropylene (i-PP) 2, 3 and polyethylene (PE) as previously shown in Chapter 3, which leads to

reduction in the production cycle of melt-processed polymer artifacts. In addition, sorbitol family-

nucleated polymer has lower values of haze (i.e. <20 %) than neat i-PP 2 and neat PE (as shown in

Chapter 3) which makes these blends attractive for packaging applications. However, members of the

sorbitol family with their sugar-based chemical structure suffer from thermal instability during polymer

processing which may lead to discoloured, odorous end-products 4-10.

As pointed out in Chapter 3, a new class of additives based on 1,3,5-benzene trisamides recently was

presented as i-PP nucleating/clarifying agents that circumvents the above disadvantages. This family is

generally capable of raising the crystallization temperatures of i-PP, and some of them providing drastic

improvement to optical properties at ultra-low concentrations, accompanied by outstanding thermal

stability and excellent solubility in the polymer melt, thus facilitating and improving homogeneous

dispersion of the additive during processing 11-15. It has been also found that C3-symmetric

supramolecular entities, which can be synthesized by the reaction of primary amines with 1,3,5-benzene

tricarboxylic acid chloride, preferentially form 1-dimentional, columnar aggregation and the surface

repeating distance between the rod-shaped aggregates of the additive allows for possible epitaxial

interactions with i-PP 16.

In the presented chapter, a wide range of 1,3,5-benzene trisamide and 1,4-phenylene bisamide-based

additives were examined as nucleating/clarifying agent candidates for PE. An extensive library of

compounds was synthesized as described in the experimental section. Four different families of species

were designed to comprise three functional moieties (as illustrated in Figure 1.a, b, c, d) with the generic

structure of A—(X—R) 2, 3:

i. a central core, A – here phenyl;

ii. moieties capable of forming hydrogen bonds, X – here amides, which promote one-dimensional

growth with columnar self-assembly (a) or two-dimensional growth with lamellar self-assembly

(b, c, d); in the latter series the direction of the amide bond was systematically inversed;

iii. a peripheral group, R – here apolar substituents to enable dissolution of the (semi-polar)

compounds in the molten, hydrophobic polymer and possibly provide epitaxial interactions with

it.

58

Figure 1.

Generic structure of novel, potential nucleating/clarifying agents: 1,3,5-benzenetricarboxylic acid derivatives,

compounds 1.1-1.10 in Table 1 below (a); terephthalic acid derivatives, compounds 2.1-2.13 in Table 1 (b);

aminobenzoic acid derivatives, compounds 3.1-3.4 in Table 1 (c); 1,4-diaminobenzene derivatives, compounds

4.1-4.4 in Table 1 (d); and two compounds with a derivative of the central phenyl core, compounds 5.1and 5.2 in

Table 1 (e, f).

59

2 Experimental

1) Materials

Additives

All 1,3,5-benzene trisamide and 1,4-phenylene bisamide based additive materials were synthesized by

and obtained from the group of Prof. Hans-Werner Schmidt, University of Bayreuth, Germany and used

as received.

Synthesis of 1,3,5-benzenetricarboxylic acid derivatives

The compounds 1.1-1.10 shown in Table 1, were prepared from 1,3,5-benzenetricarboxylic acid

trichloride according to a general procedure described in Ref 12. 12.

Synthesis of terephthalic acid derivatives

The compounds 2.1-2.10 shown in Table 1, were prepared according to the general procedure as follows.

Terephthaloyl chloride was added to the solution of the corresponding amine, anhydrous LiCl,

triethyleneamine, which were previously dissolved in dry tetrahydrofuran (THF) under inert atmosphere

and cooled to 0 °C. The reaction mixture was refluxed for 12 h and subsequently was cooled to room

temperature. The resulting precipitate was filtered off, dried under vacuum and recrystallized.

The compounds 2.11-2.13 shown in Table 1 were prepared by mixing the corresponding amine and

dimethyl terephthalate in a reaction vessel. The reaction mixture was subjected to microwave irradiation

at 150 °C for 3 h. Isolation of the products and purification were performed in the same way as above.

Synthesis of aminobenzoic acid derivatives

For the preparation of compounds 3.1-3.4 shown in Table 1, different acid chlorides were reacted under

the above-described conditions (as explained for the compounds 2.1-2.10) with 4-amino-N-(R)-

benzamide derivatives, and purified as above.

Synthesis of 1,4-diaminobenzene derivatives

The procedure followed for the compounds 2.1-2.10 was also applied to synthesize the compounds 4.1-

4.4 shown in Table 1 by reacting 1,4-diaminobenzene with different acid chlorides.

60

In addition, the compounds 5.1-5.2 shown in Table 1 were prepared by mixing

cyclohexyanemethylamine and, 2-propyloxy dimethyl terephtalate and 2-propyl-terephthalic acid

dimethylester, respectively. The reaction mixture was kept at 120 °C for 1 day and 130 °C for 5 days,

respectively. Isolation of the products and purification were performed in the same way as above for the

compounds 2.1-2.10.

The reference compounds, 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS, Millad 3988, CAS

Registry Number: 135861-56-2) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol

(TBPMN, NX 8000, CAS Registry Number: 882073-43-0) from Milliken Chemicals, were used as

received.

Polyethylene

The selected “base” polyethylene resin was LLDPE – 8, 25MI as listed in Chapter 2 and used

throughout the present chapter. This particular grade was chosen as it featured the most promising

properties to achieve clarification, as demonstrated in Chapter 3. Some additional experiments were

carried out with LLDPE – 6 and LLDPE – 8, 1MI. All grades were supplied by The Dow Chemical

Company and used as received.

2) Processing

Injection molding plaques

The same processing scheme as shown in Chapter 2-Figure 1 was followed to produce injection-molded


(Xplore (DSM), 15.0 ml) at 40 r.p.m. for 5 min at 220 °C. Series of PE with different additive

concentrations were prepared commencing with a PE masterbatch (12.7 g) comprising 2 % w/w additive,

which was subsequently diluted to lower concentrations. For the masterbatch preparation, pellet grades

(LLDPE – 8, 1MI and LLDPE – 8, 25MI) were compounded with the mini-twin-screw extruder and

powder grade (LLDPE – 6) was dry blended. For each concentration, 8.1 g of the compounded mixture

was extruded into a laboratory mini injector (Xplore (DSM), 12.0 ml). Then the desired amount of

PE/additive was added to the remaining 4.6 g. By repeating this procedure, blends of PE and the additive

were prepared with decreasing additive concentrations in the range of 2 % w/w to as low as 0.005 %

w/w. Eventually, the molten polymer/additive blends were injected into a mold at room temperature to

produce plaque samples (thickness 1.0 mm, diameter 25.0 mm). The entire micro-scale polymer

processing was conducted at 220 °C under a nitrogen blanket. Reference samples of the neat polymers

were produced according to the corresponding procedure.

61

3) Analysis



instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 17. A circular area on

the plaque samples, 18.0 mm in diameter, was illuminated by light beam; the recorded haze values

hereafter are referred to as “overall-area haze” as before. In addition, haze values of circular area with

8.0 mm diameter on samples (those free of surface irregularities) were recorded and hereafter are

referred to as “small-area haze”. In order to eliminate the effect of surface scattering, “bulk haze”

measurements were conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner

GmbH, Germany) with non-drying immersion oil (Cargille Series A refractive index oil of n = 1.5150

± 0.0002) which has a similar refractive index as the polymer plaques. Haze values reported here

correspond to the average of values measured for five samples.

Thermal analysis


Switzerland) calibrated with Indium. DSC thermograms were recorded under nitrogen at a standard


complete melting of the polymer and prevent self-nucleation, samples were kept for 5 min at the

maximum temperature prior to cooling. The reported crystallization/melting temperatures of the

additives and polyethylenes correspond to the peak temperatures in the DSC thermograms.

Thermo-gravimetric analysis (TGA)

Thermal stability of the additives was analyzed using a Mettler Toledo TGA/SDTA851 instrument.

Samples were heated from 50 °C to 700 °C at a heating rate of 10 °C/min under nitrogen atmosphere.

Reported temperatures correspond to the temperature at which 5 % sample weight loss occurred.



solutions (~1 % w/w) of neat PE and PE containing 0.1 or 2 % w/w additive in p-xylene, yielding thin

films after evaporation of the solvent; these were subsequently molten at temperatures above the melting

temperatures of the additive in the blend and quenched to room temperature. The solidified films were

coated with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini scanning electron

microscope (LEO Elektronenmikroskopie GmbH, Germany).

62


1) Properties

In order to explore potential nucleating- and clarifying capabilities of the spectrum of compounds

synthesized, blends of LLDPE – 8, 25MI with additives over a wide concentration regime (i.e. 0.005-2

% w/w) were prepared and their nucleation/clarification efficiency was examined. Most prominent

results of this study are presented in Table 1. For convenient reference, results obtained with the

commercial additives, DMDBS and TBPMN, are reproduced here from Chapter 3.

Table 1.

Different properties of neat PE (I) and PE containing DMDBS (II) or TBPMN (III) (for this page): chemical

structures of compounds synthesized; their melting temperatures (Tm) and their crystallization temperature (Tc, a);

approximate temperatures of the onset of weight loss at 5 % w/w (Twl, 5 %); maximum obtained peak crystallization

temperature of LLDPE – 8, 25MI (Tc, PE), increase in the crystallization temperature relative to neat LLDPE – 8,

25MI (ΔTc, PE) and its corresponding additive concentration; values for “overall-area haze” versus “additive

concentration” (red, dashed line indicates the haze values of neat LLDPE – 8, 25MI); minimum obtained haze

values (Hazemin), decrease in haze relative to neat LLDPE – 8, 25MI (Δhaze) and its corresponding additive

concentration. n.a. = not applicable; dec. = decomposition; e. = evaporation.

63

Table 1.

continued; 1,3,5-benzene trisamides (1.1-1.5).

64

Table 1.

continued; 1,3,5-benzene trisamides (1.6-1.10).

65

Table 1.

continued; terephthalic acid based 1,4-phenylene bisamides (2.1-2.5).

66

Table 1.

continued; terephthalic acid based 1,4-phenylene bisamides (2.6-2.10).

67

Table 1.

continued; terephthalic acid based 1,4-phenylene bisamides (2.11-2.13)

68

Table 1.

continued; aminobenzoic acid based 1,4-phenylene bisamides (3.1-3.4).

69

Table 1.

continued; 1,4-diaminobenzene based 1,4-phenylene bisamides (4.1-4.4).

70

Table 1.

continued; N,N’-Bis-cyclohexylmethyl-2-propyloxy-terephthalamide (5.1) and N,N’-Bis-cyclohexylmethyl-2-

propyl-terephthalamide (5.2).

As can be seen from the data summarized in Table 1, the present family of 1,3,5-benzene trisamides

(1.1-1.10) generally featured limited nucleation efficiency, more specifically a 1-4 °C increase in

crystallization temperature only and unsatisfying clarification with minimum haze values above 75 %.

However, certain of compounds of the 1,4-phenylene bisamide species showed an increase in the

crystallization temperature of 6 °C (e.g. 2.2), which is fairly successful for PE and a moderate decrease

in the haze up to ∆haze = 40 % (i.e. 2.2, 2.3) already at beneficially low additive content. Encouragingly,

these results are comparable to those obtained with the widely commercially-used sorbitol product

DMDBS (II).

2) Structure

In order to further investigate our results regarding the nucleation and clarification efficiency of 1,3,5-

benzene trisamides and 1,4-phenylene bisamides in LLDPE – 8, 25MI, scanning electron microscopy

(SEM) was conducted. More specifically, the structure of the “aggregates” of the additives in LLDPE –

8, 25MI matrix was analyzed. For this purpose, solution-cast samples of LLDPE – 8, 25MI containing

2 % w/w of 1.7, 2.2 and, for comparison, sorbitol derivatives II, III were produced, dried, molten and

subsequently quenched. Representative results of this study are presented in Figure 2. The SEM images

reveal large crystal domains of the 1,3,5-benzene trisamide compound 1.7 (upper-left), which

71

significantly scatter visible light. In contrast, 1,4-phenylene bisamide compound 2.2 (upper-right) at the

same content featured a molecular organization as rod-like, shish-kebab-type structures of a very fine

appearance, which reduce scattering of light. LLDPE – 8, 25MI comprising the sorbitol derivative

compounds II (lower-left) or III (lower-right) is seen to crystallize in the form of shish-kebab-type

fibrils of the additive (also previously shown in Chapter 3-Figure 11) in the same manner as observed

with 2.2 consisting of a fibrillar network although with smaller widths of fibrils. The latter are not of the

order the wavelength of visible light, thus further reducing light scattering, as evident from the haze

measurements presented above in Table 1.

Figure 2.

SEM images of LLDPE – 8, 25MI containing 2 % w/w of 1.7 (upper-left), 2.2 (upper-right), DMDBS (II) (lower-

left) and TBPMN (III) (lower-right).

Most interestingly, compound 2.2 exhibits optimal nucleation efficiency at concentrations as low as 0.05

% w/w, which is in contrasts to DMDBS (II) for which concentrations around 0.5 % w/w are required.

Furthermore, the same compound 2.2 is an efficient clarifier already at concentrations of ~0.1 % w/w,

which is less than half the amount needed for DMDBS (II). The solubility limit of 2.2 was systematically

investigated in a study of the phase behavior of the binary system comprising LLDPE – 8, 25MI in

Chapter 5.

72

3) Side Groups

As is evident from the data collected in Table 1, the role of the side group (R) is of paramount importance

for controlling the optical properties by providing surfaces that promote (possible epitaxial) growth of

the polymer. For instance, unlike i-PP comprising trisamides with tert.-butyl side group 11, 1,4-

phenylene bisamide with the same side group (i.e. 2.7) did not show any clarification effect in LLDPE

– 8, 25MI, possibly due to a non-epitaxial match between polyethylene and this side group. Furthermore,

whilst compounds comprising phenyl-based side groups generally exhibit inefficient clarification (i.e.

2.11, 2.12), cyclohexyl-based moieties mostly feature a more promising decrease in haze values of the

PE (i.e. 2.2, 2.3, 4.2 and 4.3). This effect possibly can be attributed to a change in repeating distance

between and/or along the additive aggregates which is reduced when compared with the phenyl-based

side group species.

4) Core Structure

Introducing asymmetry by inversion of one of the amide bonds (X) for the derivatives of amino benzoic

acid (3.1-3.4) generally did not show a clarification effect in contrast to terephthalic acid derivatives

(2.1-2.4) and 1,4-diaminobenzene derivatives (4.1-4.4). As can be seen from Table 2, contrary to as

clarifying additive for i-PP as previously shown for 1,3,5-benzene trisamides 11, 1,4-phenylene

bisamides based on terephthalic acid, which is bonded to the core with carbonyl group (-C=O) (2.1-2.4),

featured improved clarification performance in comparison to the 1,4-phenylene bisamides based on

trans-1,4-diaminocyclohexane attached to the core with an amine group (-NH).

In more elaborate studies, the influence of inversion of amide bonds in cyclohexylmethyl-substituted

1,4-phenylene bisamides on the phase behavior, clarification performance and morphological structures

as additive in LLDPE – 8, 25MI is presented in Figure 3. As is evident from the data presented in Table

2 and Figure 3 (a), inversion of amide moieties in the core structure leads to pronounced differences in

clarification of PE. When comparing the haze values of the PE/additive series comprising compounds

2.2, 3.2, 4.2 (respectively 0, 1, 2 amide bonds reversed), additives 2.2 and 4.2 show improvement in

clarifying ability to concentrations as low as 0.1 % w/w. Figure 3 (c) reveals that fibrillar structures of

the additives 2.2 and 4.2 reduced scattering of light unlike compound 3.2 which did not prevent

formation of common spherulitic structures of the polymer, indeed consistent with the haze data results.

However, the phase behavior of the PE/additive systems shown in Figure 3 (b) was found to be little

affected by the change of direction of amide bonds, this in contrast to their clarifying performance. In

view of the above noted strong similarities in the phase behavior of the PE/additive mixtures, the change

in optical properties can be attributed to changes in the parameters of the crystal unit cell of the additive,

and, therewith, possibly its epitaxial matching ability with the crystallizing polymer.

73

Table 2.

Chemical structures of 1,3,5-benzene trisamides (left) and 1,4-phenylene bisamides (right) featuring inversion of

one or more of the amide bonds, and minimum “overall-area haze” values of i-PP (left) 11 and PE (right) at 0.15

% w/w additive content.

74

Figure 3.

Values of “overall-area haze” versus “additive concentration” of the compounds 2.2, 3.2, 4.2 (from left to right)

in LLDPE – 8, 25MI and red, dashed lines indicate the haze of neat LLDPE – 8, 25MI (a); respective

melting/dissolution (Tm,a, ▲) and crystallization temperatures (Tc,a, ▼) of the additive and melting/dissolution

(T m, PE, ●) and crystallization temperatures (T c, PE, ○) of LLDPE – 8, 25MI (b); SEM images of LLDPE – 8, 25MI

containing 0.1 % w/w of the respective additive compounds (c).

75

Finally, optical properties of one of these species, i.e. 2.2, N,N’-bis(cyclohexylmethyl)-1,4-phenylene

dicarboxamide (BCPCA), with “base” polyethylene resins LLDPE – 6; LLDPE – 8, 1MI; LLDPE – 8,

25 MI are presented in addition to some previously shown for LLDPE – 8, 25MI (cf. p. 65) in Figure 4.

For comparison, the results obtained for the binary systems of the corresponding resins with DMDBS

are also shown in Table 3. For the aforementioned resins comprising BCPCA and DMDBS, the decrease

in “overall-area haze”, “small-area haze” and “bulk haze” with respect to that of the neat resins

(Δhaze), at concentrations where minimum haze (Hazemin) is obtained are listed. Hazemin values of the

BCPCA binaries, especially for the “bulk haze” measurements (shown in bold, cf. Table 3), approach

those of the corresponding values obtained with DMDBS. Most interestingly, already at very low

BCPCA concentrations (e.g. 0.1 % w/w) the “overall-area haze”, “small-area haze” and “bulk haze”

values were found to sharply decrease for LLDPE – 8, 1MI and LLDPE – 8, 25MI, whereas the

corresponding concentration is higher (e.g. 0.25 % w/w) in the case of DMDBS (shown in bold, cf.

Table 3). These results also correspond with the blown-film applications of the same samples, which are

shown in Appendix-Section 1.

From the data presented in Table 3 it can be also concluded that BCPCA as well as DMDBS are equally

poor clarifying agents for reduction of the “overall-area haze” of LLDPE – 6, but equally effective in

enhancing the optical characteristic of “bulk haze” values. This observation distinctly points to

processing issues with the polymer itself. Highly encouraging are the results collected for experiments

with LLDPE – 8, 25MI where major reductions in all three values of haze were found. Here of particular

interest is the strong reduction of the additive concentration at which Hazemin is observed (i.e. by a factor

2.5 for BCPCA in comparison with the classical DMDBS).

Figure 4.

Haze of LLDPE – 6; LLDPE – 8, 1MI; LLDPE – 8, 25MI (from left to right) containing BCPCA: “overall-area

haze” ( ■ ), “small-area haze” ( ⋆ ) and “bulk haze” ( ● ) of injection-molded plaques, plotted as function of the

additive content. In the graphs, red, dashed lines indicate the “overall-area haze” of the corresponding neat resins.

76

Table 3.

In this table are listed the haze of the neat polyethylenes (shown in red) and, the values of the decrease in haze with

respect to the neat resins (Δhaze) for BCPCA and DMDBS at concentrations (% w/w) where minimum haze

(Hazemin) is observed: * = “overall-area haze”, ** = “small-area haze”, *** = “bulk haze”.

4 Conclusions

In summary, a family of novel additives were explored as potential nucleating and clarifying agents for

polyethylene in this chapter. Certain features of the presented additives can be tailored through a

judicious selection of the substituents for the side groups and by the inversion of amide bonds in the

core structure.

It is anticipated that certain compounds of the 1,4-phenylene bisamide family can be promising

candidates owing to their improved nucleation efficiency and clarification performance which are

comparable to industrially widely used DMDBS, but -in contrast to the latter- accompanied by thermal

stability at elevated processing temperatures.

77

5 References

1. Zweifel, H.; Amos, S. E. Plastics Additives Handbook. 5th ed.; Hanser Gardner Publications:

Cincinnati, OH, 2001; p 949.


Macromolecules 2003, 36, (14), 5150-5156.


4. Libster, D.; Aserin, A.; Garti, N. Polym. Advan. Technol. 2007, 18, (9), 685-695.

5. Libster, D.; Aserin, A.; Garti, N. J. Colloid Interface Sci. 2006, 302, (1), 322-329.


7. McDonald, J. G.; Cummins, C. L.; Barkley, R. M.; Thompson, B. M.; Lincoln, H. A. Anal.

Chem. 2008, 80, (14), 5532-5541.

8. Fujiyama, M.; Wakino, T. J. Appl. Polym. Sci. 1991, 42, (10), 2749-2760.

9. Rekers, J. W. US Patent 5,049,605, 1991.

10. Mannion, M. J. US Patent 5,198,484, 1993.




2010, 211, (2), 171-181.

13. Abraham, F.; Kress, R.; Smith, P.; Schmidt, H.-W. Macromol. Chem. Phys. 2013, 214, (1), 17-

24.


253.

15. Wang, J. D.; Dou, Q. Colloid Polym. Sci. 2008, 286, (6-7), 699-705.

16. Kristiansen, M.; Smith, P.; Chanzy, H.; Baerlocher, C.; Gramlich, V.; McCusker, L.; Weber, T.;

Pattison, P.; Blomenhofer, M.; Schmidt, H.-W. Cryst. Growth Des. 2009, 9, (6), 2556-2558.



79

Chapter 5

Phase Behavior of Polyethylene and Selected

Nucleating/Clarifying Agent Binary Systems

81

1 Introduction

The nucleation and clarification performance of sorbitol derivatives such as 1,3:2,4-bis(3,4-

dimethylbenzylidene)sorbitol (DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)

methylene]-nonitol (TBPMN) for certain polyolefins, most notably isotactic polypropylene (i-PP) 1, 2,

and polyethylenes (PEs) (cf. Chapter 3) are governed by the phase behavior of the polymer/additive

binary systems, and, therefore, strongly dependent on the additive concentration.

It has been shown earlier 3 that fine-fibrillar structures of the additives are generated in the hyper-eutectic

composition region of the monotectic phase diagrams of i-PP/sorbitol binaries. These fine-fibrillar

structures of the additives act as efficient nucleation sites and, therewith, prevent common spherulitic

crystal growth of the polymer. Instead, the resulting microstructure features a more random organization

of long-range structures – so-called rod-like “shish-kebab” type structures – that result in reduced

scattering of light by the material.

Aiming at a somewhat similar combination of features as the above-mentioned sorbitol additives which,

crucially, exhibit relatively poor thermal stability during processing, a library of tricarboxamides and

bisamides have been synthesized that do not feature this drawback and their potential use as

nucleating/clarifying agents has been investigated in Chapter 4. One of these species, i.e. N,N’-

bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide (BCPCA), was found to be an efficient nucleator

for PE and was able to reduce the haze of PE, as a matter of fact, at substantially lower additive

concentrations than in the case of sorbitol-based clarifiers.

In the present chapter, the phase behavior of binary systems of “base” PEs with TBPMN is reported,

accompanied by a comparative study of the binary system of LLDPE – 8, 25MI/BCPCA, by analogy

with previous studies on i-PP/sorbitols and i-PP/tricarboxamides 1-4. Furthermore, the solid-state

structure of the neat polyethylenes and those comprising the nucleating/clarifying agents was

investigated.

82

2 Experimental

1) Materials

The “base” polyethylene resins listed in Chapter 2 were supplied by The Dow Chemical Company and

used as received.

The compounds, 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS, Millad 3988, CAS Registry

Number: 135861-56-2) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]-nonitol

(TBPMN, NX 8000, CAS Registry Number: 882073-43-0) were used as received from Milliken

Chemicals. N,N’-bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide (BCPCA) was obtained from

the group of Prof. Hans-Werner Schmidt, University of Bayreuth, Germany as described in Chapter 4.

2) Processing

For thermal analysis, PE/additive blends were melt-compounded in a laboratory co-rotating mini-twin-

screw extruder (Eindhoven University of Technology, the Netherlands) at 100 r.p.m. for 5 minutes at

240 °C (compositions with TBPMN) or 260 °C (compositions with BCPCA) with under a nitrogen

blanket. Series of PE with different additive concentrations were prepared commencing with a neat PE

(5.0 g), which was subsequently blended with PE/additive masterbatch comprising up to 5 or 10 % w/w

of the additive, hereafter are referred to as the “concentration series”. For each concentration, 2.5 g of

the compounded mixture was extruded. Subsequently the desired amount of PE/additive mixture was

added to the remaining 2.5 g. By repeating this procedure, blends of PE and additive were prepared with

increasing additive concentrations in the range of 0.1 to 5 or 10 % w/w. Additional mixtures for thermal

analysis comprising higher contents of the additive were prepared by dry blending and melting in a

differential scanning calorimeter (DSC) crucible, hereafter are referred to as the “crucible blend series”.

3) Analysis


High-temperature optical characteristics were determined by first preparing compression-molded

polymer films between glass slides using 1 mm spacers, and subsequently heating them above their

melting temperatures, i.e. between 170 °C and 260 °C. Since it is difficult to place molten samples onto

the detection port with horizontal light beam orientation, the Haze-Gard Plus® instrument was turned

vertically as depicted in Chapter 2-Figure 4. Haze values according to ASTM standard D1003 5 thus

recorded are hereafter referred to as “melt-intrinsic haze”.

83

Thermal analysis




complete melting of the polymer and to prevent self-nucleation, samples of the “concentration series”

were kept for 5 min at the maximum temperature prior to cooling. On the other hand, samples of the

“crucible blend series” were heated and cooled three times with 30 min isothermal steps at the

maximum temperature to obtain satisfactory dispersion of the additive in the polymer melt. The reported

crystallization and melting temperatures correspond to the peak temperatures in the DSC thermograms.



solutions (~1 % w/w) of neat PE and PE containing 0.1 or 2 % w/w additive in p-xylene, yielding thin

films following evaporation of the solvent; these were subsequently molten at temperatures above the

melting temperatures of the additive in the blend and quenched to room temperature. The solidified films

were coated with a thin conductive layer of platinum and imaged using a LEO 1530 Gemini scanning

electron microscope (LEO Elektronenmikroskopie GmbH, Germany).


The melting and crystallization behavior of blends of “base” PEs with nucleating/clarifying agents

covering the entire composition range was investigated. Temperature/composition diagrams of the

binary systems of “base” PEs/TBPMN (cf. Figure 1, 3, 5, 7), accompanied by a comparative study of

LLDPE – 8, 25MI/BCPCA (Figure 9), were constructed from the data obtained by differential scanning

calorimetry (DSC) for cooling and heating (left and right graphs in the corresponding figures,

respectively).

1) HDPE – 4/TBPMN Binary System

The temperature/composition phase diagrams of HDPE – 4/TBPMN mixtures are presented in Figure

1. It can be seen that the system shows a similar, simple monotectic phase behavior as the binaries of i-

PP with DMDBS or TBPMN 1, 2. As in the case of i-PP/TBPMN binaries, TBPMN displays high

compatibility with HDPE – 4, which is evidenced by the onset of liquid-liquid phase separation regime

at a relatively high additive concentration of 5 % w/w for the present system.

84

Figure 1.

Crystallization (left) and melting (right) temperature/composition diagrams for the binary system HDPE – 4/

TBPMN. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive

crystallization or dissolution/melting ( ■, □ ) and crystallization or melting of the polymer ( ●, ○ ). Solid symbols

denote data obtained for the samples of the “concentration series” and open symbols refer to the “crucible blend

series”. The symbol N refers to TBPMN, PE to the respective resin, L to liquid, S to solid. The drawn lines are

guides for the eye only.

Liquid-liquid phase separation was also investigated by examining light scattering from the

polymer/additive molten blend between glass slides with 1 mm spacers as described in the experimental

section. Subsequently, haze values were recorded at different concentrations of the additive at elevated

temperatures (i.e. below and above the Tm of the additive), and finally these values are presented in the

melting/dissolution binary phase diagram of HDPE – 4/TBPMN in Figure 2. From this “melt-intrinsic

haze” data, it can be seen that there is a significant amount of light scattering by the molten binaries at

a concentration of 5 % w/w (i.e. 96 % haze at 230 °C), in contrast to those of lower concentrations (6,

8, 10 % haze at 230 °C for additive concentrations of 2, 3, 4 % w/w, respectively). This result can be

attributed to the onset of the liquid-liquid phase separation regime, which plausibly generates additive-

rich domains, that strongly scatters light, leading to even more pronounced light scattering just below

the melting/dissolution temperature of the additive (i.e. 100 % haze at 215 °C). Unlike the observations

at low concentrations, enhanced haze is observed at elevated temperatures relative to the room

temperature for the liquid-liquid phase separation regime (i.e. 90 %, 100 %, 96 % haze at 25 °C, 215 °C,

230 °C, respectively for the concentration of 5 % w/w). This result can be understood by a refractive

index difference between molten polymer and additive-rich domains that can induce enhanced light

scattering relative to the refractive index difference between solid polymer and additive in the

microstructure of the material.

85

Figure 2.

Expanded view of the melting temperature/composition diagram of the binary system HDPE – 4/TBPMN; the

numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature. Values

at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.

2) LLDPE – 6/TBPMN Binary System

Corresponding data for the phase behavior of LLDPE – 6/TBPMN compositions are presented in Figure

3. Similar to the HDPE – 4, simple monotectic phase behavior which comprises a broad liquid-liquid

phase separation concentration regime (i.e. starting from 5% w/w additive) was observed for the

present binary system. Below this concentration regime, TBPMN exhibits outstanding solubility in the

molten polymer and upon cooling this homogeneous liquid, the additive forms fine-structures in the

polymer-rich liquid for which light scattering is negligible. As is evident from Figure 4, “melt-intrinsic

haze” data reveals essentially no light scattering just below and above the melting/dissolution

temperature of the additive (i.e. 1 % haze) up to the concentration of 4 % w/w. However, at the

concentration of 5 % w/w, already in the fluid phase, a significant amount of light scattering is observed

(i.e. 85 % haze at 230 °C). Upon cooling, the additive assembles into domains below the additive

melting/dissoultion temperature which significantly scatter light (i.e. 99 % haze at 215 °C) and

ultimately leads to a final solid-state structure with high haze (i.e. 82 % at room temperature). Higher

haze in the molten state relative to the one at room temperature for the present concentration can be

attributed to the change in refractive index difference between polymer and additive for different

temperatures as it was previously found for HDPE – 4.

86

Figure 3.

Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 6/






Figure 4.

Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 6/TBPMN; the

numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature. Values

at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.

87

3) LLDPE – 8, 1MI/TBPMN Binary System

In Figure 5, temperature/composition diagrams of TBPMN in LLDPE – 8, 1MI resin are shown. As in

the case of HDPE – 4 and LLDPE – 6, simple monotectic phase behavior exhibiting liquid-liquid phase

separation starting from 5 % w/w additive was also observed for this binary system.

Figure 5.

Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 8, 1MI/






Again, TBPMN exhibits high solubility within the molten polymer below the onset concentration of the

liquid-liquid phase separation regime. As it can be seen from Figure 5, almost no light scattering is

observed up to 4 % w/w in the molten state (i.e. 1-5 %“melt-intrinsic haze” at 230 °C). However, in the

case of 4 and 5 % w/w, “melt-intrinsic haze” values increase to 67 % and 92 % at the corresponding

temperature, which stems from liquid-liquid phase separation. Further cooling the fluid phase of these

concentrations below the additive dissolution/melting temperature leads to large domains of the additive

in the polymer melt which significantly scatter light (i.e. 98 % and 100 % haze below the additive

dissolution/melting temperature for the concentrations of 4 % w/w and 5 % w/w, respectively). Similar

to results recorded for the previous resins, enhanced haze is observed at elevated temperatures in

comparison to that at room temperature at concentrations in the liquid-liquid phase separation regime

(i.e. 3-5 % w/w).

88

Figure 6.

Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 8, 1MI/TBPMN;

the numbers correspond to “melt-intrinsic haze” values at the corresponding concentration and temperature.

Values at 25 °C correspond to “bulk haze” measurements as described in Chapter 3.

4) LLDPE – 8, 25MI/TBPMN Binary System

As it was found for previous PE resins, simple monotectic phase behavior with the liquid-liquid phase

separation starting around 5 % w/w additive content was also observed for the LLDPE – 8,

25MI/TBPMN binary system, as demonstrated in Figure 7.

Again the solubility of TBPMN in the molten polymer was also examined with “melt-intrinsic haze”;

measurements are shown in Figure 8. Similar to the previous resins, starting at the 4 % w/w

concentration, a high degree of light scattering is observed above and below the additive

melting/dissolution temperatures (i.e. 99 % haze at 220 °C, 100 % haze at 210 °C), which leads to solid-

state microstructure that feature a high haze value (i.e. 88 % haze at room temperature). Again, at these

concentrations in the liquid-liquid phase separation regime, molten blends featured increased haze in

comparison with that at room temperature, as in the case of previous binary systems. However, below

the concentration of 3 % w/w, the homogeneous liquid of the additive and polymer barely scatters light

(i.e. 2-7 %“melt-intrinsic haze” at 230 °C). When these compositions are cooled down below the

additive/melting dissolution temperature, the structures formed by the additive in the polymer melt do

not significantly scatter light (i.e. 3 %, 7 %, 29 % haze below the additive dissolution/melting

temperature for the concentrations of 0.5 % w/w, 1 % w/w, 2 % w/w, respectively) and solid-state

material with low haze is obtained (i.e. 30 %, 35 %, 50 % haze at room temperature for additive

concentrations of 0.5 % w/w, 1 % w/w, 2 % w/w, respectively).

89

Figure 7.

Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 8,

25MI/TBPMN. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive





Figure 8.

Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 8, 25MI/TBPMN;



90

5) LLDPE – 8, 25MI / BCPCA Binary System

As can be seen in Figure 9, the study of phase behavior of the binary system LLDPE – 8, 25MI/BCPCA

also clearly indicates a similar, simple monotectic phase behavior as previously observed for the binaries

with TBPMN. However, unlike for the sorbitol binary systems, for additive concentrations exceeding 1

% w/w, unexpectedly three different thermal transitions of the additive were observed in both cooling

and heating temperature/composition diagrams. Therefore, these thermal transitions of neat BCPCA

were investigated by differential scanning calorimetry (DSC) and temperature-dependent wide-angle X-

ray diffraction (WAXD) as shown in Appendix-Section 2.

Figure 9.

Crystallization (left) and melting (right) temperature/composition diagrams for the binary system LLDPE – 8,

25MI/BCPCA. In the diagrams the symbols refer to the DSC experimental data for different transitions: additive

crystallization or dissolution/melting ( ■, □ ), solid-state transitions of the additive ( ▲, ▼, , ) and crystallization

or melting of the polymer ( ●, ○ ). Solid symbols denote data obtained for the samples of the “concentration

series” and open symbols refer to the “crucible blend series”. The symbol B refers to BCPCA, PE to the respective

resin, L to liquid, S to solid. The drawn lines are guides for the eye only.

Strikingly, and similar to the binary systems of polyethylenes with TBPMN studied in the previous

sections, a broad liquid-liquid phase separation regime was observed for the present blends, starting

from a concentration of around 2 % w/w up to virtually the BCPCA axis. In other words, BCPCA

binaries exhibit the monotectic composition close to the polymer axis, causing already at low

concentrations the additive to crystallize first, and therewith provide the active additive solid surface

suitable for polymer nucleation and clarification. Liquid-liquid phase separation was also investigated

by examining light scattering from the molten polymer/additive blends. From the “melt-intrinsic haze”

91

data, it can be seen that there is a significant amount of light scattering by the molten binary at additive

concentrations approaching 2 % w/w (i.e. 61 % haze at 250 °C) which can be attributed to the onset of

the liquid-liquid phase separation regime. Liquid-liquid phase separation leads to domains of high

additive content which significantly scatter light, leading to even more pronounced light scattering just

below the melting/dissolution temperature of the additive (i.e. 89 % haze at 240 °C) and high turbidity

materials at room temperature (i.e. 93 % “bulk haze” at 25 °C).

Figure 10.

Expanded view of the melting temperature/composition diagram of the binary system LLDPE – 8, 25MI/BCPCA;



6) Structure

In order to gain insight into the structure of the “aggregates” of the clarifying agents and its implications

for clarification of PE, scanning electron microscopy (SEM) was conducted. The SEM images in Figure

11 show clear, classical spherulitic structures for all “base” polyethylenes (left columns), whereas the

polymers containing 2 % w/w TBPMN feature a rod-like, shish-kebab-type morphology (right column)

in the hyper-eutectic composition region of the monotectic phase diagram, in accordance with earlier

observations for i-PP 3. At increased magnification, the radially-ordered lamellae constituting the

spherulites in the neat polyethylenes can be discerned. However, four different polyethylenes of different

macromolecular structures, nucleated with TBPMN, revealed dramatically different size of polymer and

fibrillar-additive crystalline entities. Therefore, in Chapter 6, the influence of the macromolecular

structure of PE on the solidification behavior of both the additive and the polymer, their resulting

structures and the haze of the nucleated blends will be investigated in more detail.

92

Figure 11.

SEM images of neat HDPE – 4; LLDPE – 6; LLDPE – 8, 1MI; LLDPE – 8, 25MI (a, b, c, d – left and middle

column) and the same polymers containing 2 % w/w TBPMN (right column). Scale bars 1 μm.

93

The structure of BCPCA crystallized in LLDPE – 8, 25MI was also analyzed by scanning electron

microscopy (SEM) and is shown in Figure 12. The samples of LLDPE – 8, 25MI containing 0.1 and 2

% w/w BCPCA reveal a very broad fibril-width distribution of the additive aggregates. This is different

when compared to the observations for TBPMN, and which is manifested in the modest clarification

performance of BCPCA. However, there are regions, which consist of fine-fibrillar molecular

organization of BCPCA (Figure 12 – left images), that show lower widths of fibrils in PE comprising

0.1 % w/w additive in comparison to that containing 2 % w/w additive, therewith reducing scattering of

light, indeed consistent with the aforementioned phase and haze behavior of the LLDPE – 8,

25MI/BCPCA binary system.

Figure 12.

SEM images of LLDPE – 8, 25MI containing 0.1 % w/w (top) and 2.0 % w/w (bottom) of BCPCA revealing the

variations in additive aggregation within the same sample.

94

4 Conclusions

Monotectic phase behavior was demonstrated for the binary systems of polyethylenes with the sorbitol

derivative, TBPMN and a new nucleating/clarifying agent, BCPCA – similar to that previously reported

for i-PP. Obviously, the existence of liquid-liquid phase separation that typifies both systems is

extremely significant, since it efficiently directs the position of the monotectic composition. For

instance, as shown in Figure 13, the BCPCA binary exhibits the monotectic composition close to the

polymer axis causing, already at low concentrations, the additive to first crystallize, and therewith

provide the active additive solid surface suitable for polymer nucleation and clarification.

Figure 13.

Crystallization (left) and melting (right) temperature/composition diagrams of the binary system LLDPE – 8, 25MI

with BCPCA (red symbols), DMDBS (black symbols) and TBPMN (blue symbols). Different symbols refer to

data for different transitions: additive crystallization or dissolution/melting ( ▲, ▲, ▲ ) and crystallization or

melting of the polymer ( ●, ●, ● ).

Finally, it should be noted that the above presented data clearly indicates that the prevention of the

formation of spherulitic polymer structures by nucleating/clarifying agent is not sufficient for achieving

clarification. Even though the 3D fine fibrillar-network of TBPMN exists in polyethylenes, the size of

the fibril-width and polymer lamellae should not be of the order of the wavelength of visible light.

Therefore, the macromolecular structure of PE and its interaction with additive will be examined further

in Chapter 6. Furthermore, the broad fibril-width distribution of BCPCA is still an issue that causes a

relatively high degree of light scattering and wide variations in values of haze in different parts of the

various samples.

95

5 References


Macromolecules 2003, 36, (14), 5150-5156.





253.



97

Chapter 6

Influence of Polyethylene Macromolecular Structure

on Crystallization onto 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-

propylphenyl)methylene]-nonitol

99

1 Introduction

The nucleating and clarifying abilities of sorbitol derivatives such as 1,3:2,4-bis(3,4-

dimethylbenzylidene)sorbitol (DMDBS) and 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)

methylene]-nonitol (TBPMN) are governed by the phase behavior of the polymer/additive binary

systems, and therefore, are strongly dependent on the additive concentration 1, 2. It has been shown in

Chapter 5 that fine-fibrillar structures of the additives are generated in the hyper-eutectic regime of the

monotectic temperature/composition diagrams of “base” polyethylenes (PEs)/TBPMN binaries (cf.

Figure 1 – Ns + L1 regime) as in the case of isotactic polypropylene (i-PP) 3. In that preliminary study, a

change in the hyper-eutectic composition and, alteration in the crystal size of the additive and the

polymer were observed for different macromolecular structures.

Independent increase of certain PE characteristics – such as comonomer content, side-chain length and

molecular weight – results in a decrease in the melting/crystallization temperatures of the neat polymer

which leads to change in the melting/crystallization temperatures of the additive in the blend, which in

turn shifts the liquidus line of hyper-eutectic composition 4-6. In Figure 1, the changes in the position of

the liquidus line due to the reduced melting/crystallization temperatures of the polymer are depicted.

Figure 1.

Schematic of the partial, low additive content section of the monotectic temperature/composition diagram of the

PE/TBPMN binary system with a focus of the regime of hyper-eutectic composition and its positioning with a

change in the melting temperature of neat PE. The symbol N refers to TBPMN; PE to the respective resin; L to

liquid; S to solid; Tc, a, 1 and Tc, a, 2 to the crystallization temperature of the additive for the corresponding liquidus

lines.

100

In the most simple case, when there is no change in enthalpy and entropy of mixing, the eutectic

composition shifts downward to the temperature-composition axes (cf. Figure 1-black, dashed curve) 6.

However, in reality, changes in enthalpy and entropy shift the liquidus lines – for example towards the

additive axis (cf. Figure 1-green curve) when the solvating power of the polymer for the additive

increases or towards the polymer axis (cf. Figure 1-blue curve) when the solvating power of the polymer

for the additive decreases 6. Therefore, the crystallization temperature of the additive at a specific

concentration in the hyper-eutectic composition can either increase (Tc, a, 1) or decrease (Tc, a, 2) dependent

on the change of the mixing enthalpy and entropy of the system.

Following classical crystallization arguments, an increase of Tc, a reduces the supercooling, which

generally leads to the formation of larger crystalline entities 7 and the opposite is commonly observed at

decreased values of Tc, a. These differences are expected to influence not only just the size of the additive

crystals, but therewith the area available for subsequent nucleation and growth of the polymer onto them,

with anticipated significant influence on haze of the solidified system.

In the present chapter, a wider range of PEs covering a broad spectrum of macromolecular structures

blended with TBPMN was examined in order to explore the influence on the nucleation/clarification

efficiency of the same additive on them. By changing the particular macromolecular intrinsic parameters

of the PEs (i.e. molecular weight, molecular weight distribution and comonomer content), alterations in

the additive fibril-width and other interactions of the polymers with TBPMN were investigated. That

specific additive was chosen as it was found to be the compound to optimally enhance the optical

properties of PE (cf. Chapter 3).

101

2 Experimental

1) Materials

The polyethylene resins used throughout this study are homopolymers and copolymers of ethylene-

octene; selected characteristic properties are listed in Table 1. The polymers were supplied by The Dow

Chemical Company and used as received. The nucleating/clarifying agent 1,2,3-trideoxy-4,6:5,7-bis-O-

[(4-propylphenyl)methylene]-nonitol (TBPMN, NX 8000, CAS Registry Number: 882073-43-0) from

Milliken Chemicals, also was used as received.

2) Processing

Blend preparation

Mixtures of the polyethylenes comprising 0.5 % w/w TBPMN were compounded in a laboratory co-

rotating mini-twin-screw extruder (Eindhoven University of Technology, the Netherlands) at 100 r.p.m.

for 5 minutes at 240 °C under a nitrogen blanket. Reference samples of the neat polymers were produced

according to the same procedure.

Injection molding plaques

The same processing scheme as shown in Chapter 2-Figure 1 was followed to produce injection-molding


(Xplore (DSM), 15.0 ml) at 40 r.p.m. for 5 min at 220 °C. Eventually, the molten polymer/additive

blends were injected into a mold at room temperature to produce plaque samples (thickness 1.0 mm,

diameter 25.0 mm). The entire micro-scale polymer processing was conducted at 220 °C under a

nitrogen blanket. Reference samples of the neat polymers were produced according to the corresponding

procedure.

3) Analysis



instrument (BYK Gardner GmbH, Germany) according to ASTM standard D1003 8. In addition to

“overall haze”, in order to eliminate the effect of surface scattering, “bulk haze” measurements were

also conducted by filling a 50.0 x 45.0 x 2.5 mm cuvette, (AT-6180 from BYK Gardner GmbH,

Germany) with non-drying immersion oil (Cargille Series A refractive index oil, n = 1.5150 ± 0.0002)

which has a refractive index similar to the polymer plaques. Haze values reported here correspond to the

average of measured for five samples.

102

Thermal analysis




complete melting of the polymer and to prevent self-nucleation, the samples were kept for 5 min at the

maximum temperature prior to cooling. The reported melting and crystallization temperatures

correspond to the peak temperatures in the DSC thermograms. The degree of crystallinity of the

polymers was calculated from the enthalpy of fusion, derived from the endothermic peak, adopting a

value of 293 J/g for 100 % crystalline polyethylene 9.

Mechanical properties

Uniaxial tensile testing of the samples was performed on an Instron 5864 tensile testing machine

equipped with pneumatic clamps. The instrument was set up with a ±100 N static load cell and was used

in constant rate of elongation mode (12 mm/min). All tests were carried out at room temperature on

dogbone-shaped specimens of ~100 µm thickness, 2 mm width and 12 mm gauge length. All reported

Young’s modulus (E) values of the samples correspond to an average of five measurements.


Samples for scanning electron microscopy (SEM) studies were prepared by hot-stage melting of

previously blended material on glass slide at 240 °C and; these were subsequently quenched to room

temperature. The solidified films were coated with a thin conductive layer of platinum and imaged using

a LEO 1530 Gemini scanning electron microscope (LEO Elektronenmikroskopie GmbH, Germany).

103


1) Properties

In order to investigate the effect of macromolecular structure of the various polyethylenes on the

aggregation behavior of the additive and subsequent crystallization of the PEs onto them, blends of the

different pilot plant resins comprising 0.5 % w/w TBPMN were prepared and their

nucleation/clarification efficiency were examined, as well as solid-state structure of the additive and the

polymers. This particular concentration of TBPMN was chosen as it yielded the most promising

clarification performance, as demonstrated in Chapter 3. For reference purposes, data obtained with one

of the “base” PEs, i.e. LLDPE – 8, 1MI, are also included.

Characteristics of the various polyethylenes used in this study, including the grades of homopolymers

and copolymers comprising ethylene-octene comonomers and their blends comprising 0.5 % w/w

TBPMN, are presented in Table 1: densities; melt index; number-average molecular weight (Mn),

weight-average molecular weight (Mw) and polydispersity index (PDI, Mw/Mn); octene (C8) comonomer

content; peak melting temperatures of the neat resins (Tm, p) and blends (Tm, b); Young’s modulus of the

neat resins (Ep) and blends (Eb); crystallinity of the neat resins (wp) and blends (wb); peak and onset

crystallization temperatures of the polymer (Tc, p, Tc, o) and increase in peak and onset crystallization

temperatures relative to the neat PE resins (ΔTc, p, ΔTc, o); and peak and onset crystallization

temperatures of the additive (Tc, a (peak) and Tc, a (onset)); “overall haze” and “bulk haze” of the blends

and decrease in “overall haze” (∆hazeO) and “bulk haze” (∆hazeB) relative to the neat PE resins.

2) Nucleation

The nucleation efficiency of TBPMN in different PE blends was examined by peak and onset

crystallization temperatures of the additive (Tc, a (peak) and Tc, a (onset)). From the data shown in Table

1, it can be concluded that there is only a few degrees of difference between Tc, a (peak) and Tc, a (onset).

In Figure 2, additive crystallization temperatures are shown by plotting DSC cooling thermograms of

PE/TBPMN blends and Tc, a (onset) values are plotted against the comonomer content of the resins. It

appears that additive crystallization temperature reveals a random trend with comonomer content of the

resins. This result can be possibly attributed to the other variables in the PE macromolecular structures

such as Mw, PDI, and distribution of the comonomer along the backbone chain which alter the system’s

mixing enthalpy and entropy to shift the liquidus either to the polymer axis or the additive axis as

demonstrated in Figure 1.

104

Ta

ble

1.

Su

mm

ary

of

pri

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arac

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stic

s o

f ex

per

imen

tal

PE

res

ins

and

th

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nd

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0.5

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BP

MN

in

co

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n w

ith

th

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ba

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res

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LD

PE

– 8

, 1

MI.

105

Figure 2.

DSC cooling thermograms showing exothermic peaks of the additive crystallization (left) and Tc, a (onset) of 0.5

% w/w PE/TBPMN blends versus comonomer content of the PE resins (right). The numbers and corresponding

symbols refer to the resins, which are listed in Table 1.

Subsequently, the nucleation efficiency of PE chains onto the solidified TBPMN was also examined by

crystallization peak and onset temperatures of the polymers in the blends (Tc, p and Tc, o), and their

increase relative to the neat PE resins (ΔTc, p and ΔTc, o). Systematic reduction in Tc, o values with increasing

comonomer content of PE resins can be seen in Figure 3 (left) as expected.

Figure 3.

Tc, o (left) and ∆Tc, o (right) of 0.5 % w/w PE/TBPMN blends versus comonomer content of the PE resins. Symbols

refer to the resins, which are listed in Table 1 and the dotted line is guide to the eye only.

106

As can be seen from Figure 3 (right), the effect of TBPMN on the nucleation efficiency of the polymers

is generally comparable for all the resins (i.e. ΔTc, o = ~4 °C for the samples 1-9). However, a much

higher ΔTc, o = ~8 °C is observed for the sample 10. This result can possibly be attributed to the

crystallization of low molecular weight chains onto the high molecular weight chains of the polymer by

itself due to the broad molecular weight distribution (i.e. PDI = 4.1) of the neat polymer, when compared

with the otherwise similar resin 9.

3) Optical Properties

In Figure 4, “bulk haze” of the blends and their decrease relative to the neat resins (∆hazeB) are plotted

against the comonomer content of the resins. Copolymers, which have similar molecular weight as their

homopolymers (samples 1-3), and comprising 0.3 mol % octene-comonomer (samples 4-6) exhibit

reduced haze and increased ∆haze relative to the homopolymers. Samples 7 and 8 possessing low

molecular weight at higher comonomer contents show higher haze with minor amount of ∆haze. Among

the experimental resins, not surprisingly lowest haze values were found for the blends comprising higher

comonomer contents (samples 9 and 10), but no obvious trend in ∆haze was detected.

Figure 4.

“Bulk haze” of 0.5 % w/w PE/TBPMN blends (left) and the decrease in “bulk haze” relative to that of the neat

resins (right) versus comonomer content of the PE resins. Symbols refer to the resins which are listed in Table 1.

4) Fibril-width and Fibril-width-distribution

In order to further investigate the effect of PE macromolecular structure to the final shish-kebab-type

structures, scanning electron microscopy (SEM) studies were conducted for the PE/TBPMN blends by

examining the additive fibril-widths and PE lamellar-widths. In Figure 5, SEM images of the PEs

107

comprising 0.5 % w/w TBPMN are presented according to Mw and comonomer content of the resins. In

the following, the most prominent results of this study are presented.

Change of comonomer content at the same Mw

The homopolymers (samples 1-3) and the copolymers comprising 0.3 mol % octene-comonomer

(samples 4-6) possessing the same Mw were compared. Samples were categorized as Group 1 (samples

1 and 4); Group 2 (samples 2 and 5); Group 3 (samples 3 and 6).

Figure 5 indicates that copolymers generally feature thinner fibril-widths of the additive accompanied

by reduced haze. Group 1 and Group 2 reveal reduced additive fibril-width for the copolymers indeed

associated with a reduced Tc, a (onsets), i.e. fibril width size: 1 > 4, respectively Tc, a (onsets): 149 °C,

144 °C and 2 > 5, respectively Tc, a (onsets): 144 °C, 142 °C. For the Group 3, the fibril-width of the

additive in copolymer is found to be larger than in the homopolymer, unlike in the former groups.

However, the size of the fibril-width in the present group was found to be also related to the Tc, a (onsets),

as in the case of former groups: i.e. fibril-width size: 6 > 3, respectively Tc, a (onsets): 149 °C, 148 °C.

In other words, additive crystallization occurs at lower temperatures, which means less time for the

additive to grow laterally resulting in a smaller fibril-width in the final solid-state structure.

Change of Mw at the same comonomer content

As can be seen in Figure 5, increasing molecular weight of the copolymers comprising 0.3 mol % octene-

comonomer (samples 4-6), appears to result in an uncorrelated change of the fibril-width. However, like

in the previous cases, the fibril-width of the additive reveals an identical dependence on Tc, a (onsets):

fibril-width size: 6 > 4 > 5, respectively Tc, a (onsets): 149 °C, 144 °C, 142 °C.

Effect of low molecular-weight fraction

Samples 7 and 8, which comprise low molecular weight PE fractions feature a large fibril-width-

distribution of the additive (cf. Figure 5). It can be concluded that low molecular-weights generally cause

an increase of the fibril-width-distribution, which significantly scatters light leading to an increase in

haze.

Effect of high comonomer content

Samples 9 and 10, which have higher comonomer content than all other resins exhibited a more

homogeneous distribution of the width of the additive network and a decrease in the size of fibril-width,

together with a substantial reduction of the polymer lamellar-width (cf. Figure 5). In accordance with

the haze results, a decrease in the widths of the structural features of both components in the final solid-

state reduces scattering of visible light and hence leads to lower haze as shown in the previous section.

108

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109

The haze results prove that preventing only the lateral-growth of the fibrils of the additive is not

sufficient to improve optical characteristics. Polymer lamellar-width is another important factor that

must be taken into account. For instance, even though the SEM image of sample 4 in Figure 5 reveals a

fine size of the additive fibrils, the size of the polymer lamellar-width is of the order of the wavelength

of visible light leading to high haze.

5) Mechanical and Thermal Properties

In Figures 6 and 7, the Young’s modulus, the melting temperature and crystallinity of the neat resins

together with the blends are plotted against comonomer content. Similar to the findings in Chapter 2-

Figure 5.b, the corresponding properties decrease with increasing comonomer content which is opposite

to the trend found for optical properties.

Beneficially, and in accord with previous studies 10-12, the Young’s modulus of blends comprising the

additive are generally ~1.5 times higher than that of the neat resins. This value can reach up to 2 for the

ones comprising higher comonomer contents (i.e. samples 9 and 10).

Figure 6.

Young’s modulus of neat resins and 0.5 % w/w PE/TBPMN blends versus comonomer content of the PE resins.

Symbols refer to the resins, which are listed in Table 1 and, the symbols including a cross represent blended resins.

The dotted lines are guides to the eye only.

Finally, the data presented in Figure 7 indicates that addition of the nucleating/clarifying agent TBPMN

results in essential insignificant change in the values of the melting temperature and degree of

crystallinity of the different PE resins.

110

Figure 7.

Melting temperature (left) and degree of crystallinity (right) of neat resins and of 0.5 % w/w PE/TBPMN blends

versus comonomer content of the PE resins. Symbols refer to the resins, which are listed in Table 1 and, the

symbols including a cross represent blended resins. The dotted lines are guides to the eye only.

4 Conclusions

In summary, a family of experimental PE resins with different macromolecular structures was explored

to examine the crystallization behavior of the nucleating/clarifying agent TBPMN and subsequent PE

lamellar overgrowth onto the additive fibrils. It was shown that the nucleation and clarification

performance of TBPMN in the blends can be tailored with different intrinsic characteristics of the PE

grades such as comonomer content, molecular weight or molecular weight distribution. It appears that

there is a noticeable dependency of the lateral-growth of TBPMN fibrils and Tc, a of the additive – for

example, early additive crystallization at high Tc, a leaves more time for the lateral-growth of fibrils to

form long-range structures. However, Tc, a and fibril-width did not exhibit a systematic trend dependent

on the comonomer content. Finally, it should be noted that not only the size of the additive fibrils

determines haze. When either the width of the polymer lamella or additive fibril are of the order of the

wavelength of visible light, samples significantly scatter visible light and hence exhibit a high level of

haze.

111

5 References


Macromolecules 2003, 36, (14), 5150-5156.





Dekker: New York, 2000; p 173.

5. Simpson, D. M.; Vaughan, G. A. Ethylene Polymers, LLDPE. Encyclopedia of Polymer Science

and Technology. 2001, (2), 441-482.

6. Koningsveld, R.; Stockmayer, W. H.; Nies, E. Polymer Phase Diagrams a Textbook. Oxford

University Press: Oxford, 2001; p 109, 176.

7. Young, R. J. Introduction to Polymers. University Press: Cambridge, 1986; p.175, 188.



9. Wunderlich, B. Thermal Analysis. Academic Press: San Diego, 1990; p 418.

10. Pukanszky, B.; Mudra, I.; Staniek, P. J. Vinyl Addit. Techn. 1997, 3, (1), 53-57.

11. Zhang, Y. F.; Xin, Z. J. Appl. Polym. Sci. 2006, 100, (6), 4868-4874.

12. Zhang, Y. F. J. Macromol. Sci. B 2008, 47, (6), 1188-1196.

113

Chapter 7

Conclusions and Outlook

115

1. Conclusions

This thesis describes a comprehensive study of modifying the crystallization of polyethylenes (PEs) with

the aim of drastically improving their optical transparency.

Investigations of optical, thermal and mechanical properties of PEs, possessing modified chain

architectures, demonstrate that transparency can be readily obtained for the PEs comprising a high

degree of branching, but with the penalty of a major reduction in their equally-relevant thermal (i.e.

melting temperature) and mechanical (i.e. stiffness) properties. As the most prominent conclusion of this

thesis, addition of a particular nucleating/clarifying agent to PE – intrinsically possessing superior

thermal and mechanical properties (i.e. linear low-density polyethylene, LLDPE) – exhibits

advantageous optical characteristics such as those observed for the low-density resins (cf. Figure 1). The

results of this work summarized in Figure 1 demonstrate that the Young’s modulus of the clarified PE

can be ~50 times higher than that for the low-density resin of comparable optical performance, and

features a melting temperature ~65 °C higher than that of the latter polyethylene.

Figure 1.

Plot of Young’s modulus (blue-square symbols) and melting temperature (red-triangle symbols) versus "overall

haze” for polyethylenes, showing the trade-off between optical and thermo-mechanical properties. Solid symbols

on the right correspond to the neat resin and solid symbols on the left which follow the dashed-arrows correspond

to the “bulk haze” of clarified resin (sample 9 from Chapter 6). Solid-arrows correspond to the increase in Young’s

modulus and melting temperature for the clarified resin with respect to the low-density resin possessing comparable

haze.

116

In an attempt to gain deeper understanding of what characteristics a clarifying agent should possess in

order to obtain improved optical performance of polyethylene, scanning electron microscopy (SEM)

was used as a tool to investigate the influence of such additives on the microstructure of the solid-state

material. The additives 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol, DMDBS; 1,2,3-trideoxy-4,6:5,7-

bis-O-[(4-propylphenyl)methylene]-nonitol, TBPMN; N,N’-bis(cyclohexylmethyl)-1,4-phenylene

dicarboxamide, BCPCA that imparted improved clarification of PE, were indeed found to prevent

spherulitic growth of the polymer during crystallization – instead, polymer crystals grew onto the

fibrillar surface of the additives and hence featured random rod-like shish-kebab-type structures as

observed in the case of clarified i-PP 1. However, it was found – and this is another main conclusion of

this thesis – that preventing spherulitic growth is not sufficient for obtaining improved clarification for

polyethylenes. The additive fibril and polymer lamellar-widths – which are of the order of the

wavelength of visible light (400-700 nm) – for different macromolecular structures of PE can already

induce to high degree of light scattering. Therefore, homogeneous microstructures of a fine-fibrillar

network of the additive and polymer lamellar-widths that do not exceed the wavelength of visible light

are pre-requisites to obtain clarified PE as found for the samples 9 and 10 in Chapter 6.

2. Outlook

Epitaxial interaction of additive with polyethylene

Even though it is demonstrated that polyethylenes can be clarified, the exact mechanism leading to the

advantageous microstructure is still not well-understood. It is clear that the surface provided by an

additive should strongly promote nucleation of the polymer and, of course, this can be provided by the

chemical structure of the additive fibrils which can feature matching crystallographic distances for

enabling epitaxial growth 2-6 or minimizing the energy barrier for crystal growth by featuring grooves on

the surface of those fibrils 7-9. Therefore, establishing a more detailed understanding of (meso-)epitaxial

interactions with PEs, particularly concerning the correlation between the chemical structure and surface

topology of additives, could represent an important objective for further studies. Moreover, additional

efforts on the design and development of the aramid-based compounds could be a salvation for thermal

instability of the currently employed sorbitols during polymer processing and their migration issues 10-

16.

117

Designing optimized polyethylene macromolecular structure

Whilst injection-molded plaques of the clarified PE – possessing haze comparable to clarified i-PP –

presented in this thesis, haze measurement represents only a relatively small area of the sample; i.e.

those that are free of surface imperfections due to polymer processing issues. Contribution of “surface

haze” to the “overall haze” reveals a major effect of the surface imperfections on the optical properties

of the specimen (i.e. 0.25 % w/w TBPMN in LLDPE – 8, 1MI: “overall haze” 54 %, “small area haze”

17 %, “bulk haze” 20 %). Therefore, designing an optimized PE molecular architecture which reduces

“surface haze” should be a study of interest for industrial scale-up and implementation.

Nucleation studies of TBPMN and subsequent polymer crystallization onto it described in Chapter 6 –

by investigating a wide range of PEs of different comonomer content, molecular weight and molecular

weight distribution – show that there is a noticeable correlation between the lateral growth of the additive

fibrils and the crystallization temperature of the additive (Tc, a). For instance, relatively early

crystallization of the additive at high Tc, a plausibly leads to lateral-growth of the fibrils for a longer

period of time until PE crystals start to grow, and hence the additive-fibrils feature courser structures in

the final solid-state material. However, no systematic trends were found between comonomer content,

molecular weight of the polymer and Tc, a in the polymer melt. In order to establish a more profound

understanding for the dependency of the lateral growth of additive fibrils in the polymer melt and Tc, a,

more detailed systematic studies should be performed by more precisely controlling the macromolecular

structure parameters (i.e. molecular weight, molecular weight distribution, sequence of the comonomers

in the backbone).

Last but not least, having demonstrated significant progress in understanding the microstructure of the

clarified PE, further studies of epitaxial interactions with additives accompanied by designing the

optimal macromolecular structure could open possibilities for obtaining highly-transparent high-density

polyethylene, which would feature the benefits of further improved thermal and mechanical properties.

118

References

1. Bernland, K. M. Nucleating and Clarifying Polymers. Ph.D. Dissertation, Swiss Federal

Institute of Technology Zurich, Nr 19388, Zurich, 2010.

2. Lotz, B.; Wittmann, J.-C. Macromol. Chem. Phys. 1984, 185, (9), 2043-2052.

3. Mathieu, C.; Thierry, A.; Wittmann, J.-C.; Lotz, B. Polymer 2000, 41, (19), 7241-7253.

4. Alcazar, D.; Ruan, J.; Thierry, A.; Lotz, B. Macromolecules 2006, 39, (8), 2832-2840.

5. Thierry, A.; Straupe, C.; Wittmann, J.-C.; Lotz, B. Macromol. Symp. 2006, 241, (1),103-110.

6. Kristiansen, M.; Smith, P.; Chanzy, H.; Baerlocher, C.; Gramlich, V.; McCusker, L.; Weber,

T.; Pattison, P.; Blomenhofer, M.; Schmidt, H.-W. Cryst. Growth Des. 2009, 9, (6), 2556-

2558.



8. Siripitayananon, J.; Wangsoub, S.; Olley, R. H.; Mitchell, G. R. Macromol. Rapid Comm.

2004, 25, (15), 1365-1370.

9. Vaughan, A. S.; Hosier, I. L. J. Mater. Sci. 2008, 43, (8), 2922-2928.

10. Libster, D.; Aserin, A.; Garti, N. Polym. Advan. Technol. 2007, 18, (9), 685-695.

11. Libster, D.; Aserin, A.; Garti, N. J. Colloid Interace Sci. 2006, 302, (1), 322-329.


13. McDonald, J. G.; Cummins, C. L.; Barkley, R. M.; Thompson, B. M.; Lincoln, H. A. Anal.

Chem. 2008, 80, (14), 5532-5541.

14. Fujiyama, M.; Wakino, T. J. Appl. Polym. Sci. 1991, 42, (10), 2749-2760.

15. Rekers, J. W. US Patent 5,049,605, 1991.

16. Mannion, M. J. US Patent 5,198,484, 1993.

119

Appendix

120

1. Blown-film Applications

Blown-films of neat LLDPE – 8, 1MI and its blends with 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol

(DMDBS) were obtained from Martin Hill, Dow Chemical Company, Tarragona, Spain and used as

received. Monolayer polyethylene films were produced on a single-screw extruder (Collin, Germany)

and fitted with a film-blowing die. Processing parameters are provided in Table 1, and haze results of

the films are listed in Table 2.

Processing parameters

Screw speed (rpm) 60

Melt temperature (°C) 240

Take-up speed (m/min) 4.3

Average thickness (μm) 50

Table 1.

Processing parameters for the production of blown-films comprising LLDPE – 8, 1MI and its mixtures with

DMDBS.

Table 2.

“Overall haze”, “bulk haze” and “contribution of the surface haze” to the overall haze for the blown-films of neat

LLDPE – 8, 1MI and its blends with DMDBS. In the bars, grid-line area represents “bulk haze” and the blank area

represents “surface haze” for the corresponding samples.

Blown-films of neat LLDPE – 8, 1MI and mixtures with N,N’-bis(cyclohexylmethyl)-1,4-phenylene

dicarboxamide (BCPCA) were prepared as follows. Respective amounts of the additive and the polymer

were inserted into a plastic bag and thoroughly mixed. The powder mixtures were compounded and

extruded with a twin screw extruder (Leistritz ZSE MAXX, screw diameter = 27 mm; screw l/d ratio =

121

48). Processing parameters: Cylinder temperature: 170 °C (all heating zones), screw speed: 300 – 400

U/min, output: 25-30 kg/h. The extruded strands were quenched in a water bath and cut with a pelletizer.

Monolayer polyethylene films were produced as above. Additional process parameters are provided in

Table 3, and haze results of the films are listed in Table 4. As a comparative study, blown films supplied

by the DOW Chemical Company was also shown in Table 4. The higher haze values for the films of the

same concentration can be possibly attributed to poor mixing and surface melt fracture.

Processing parameters

Screw speed (rpm) 60

Melt temperature (°C) 220

Take-up speed (m/min) 1.95

Average thickness (μm) 50

Table 3.

Processing parameters for the production of blown-films comprising LLDPE – 8, 1MI and its mixtures with

BCPCA.

Table 4.

“Overall haze”, “bulk haze” and “contribution of the surface haze” to the overall haze for blown-films of neat

“LLDPE – 8, 1MI” and its mixtures with BCPCA. In the bars, grid-line area represents “bulk haze” and the blank

area represents “surface haze” for the corresponding samples. ( * ) represents the results obtained by the blown-

films produced in the DOW Chemical Company.

122

Both additives reveal significant amount of “surface contribution to the overall haze” due to the surface

irregularities that come from the issues of free-surface flow processes as it was previously explained in

Chapter 1-Clarification Section. However, the decrease in “bulk haze” of these films, i.e. 1.3 % for

BCPCA and 1.9 % for DMDBS, points to the clarification efficiency of these additives for different

processing applications.

Young’s modulus of selected blown-film samples are listed in Table 5 according to the experimental

procedure to determine mechanical properties in Chapter 2 and Chapter 5. Films comprising additives

reveal improved mechanical performance relative to the neat polyethylene samples. Naturally,

orientation induced by the film-blowing process imparts improved stiffness in comparison to

compression-molded samples. The modulus in transverse direction (TD) is slightly higher than that

found in the machine direction (MD) for all samples, which is consistent with the previous records 1-3.

Krishnaswamy et al. explained this result by relative contributions of the crystalline and non-crystalline

parts to the deformation along the two directions; i.e., tensile deformation along TD involves a favorable

contribution from the polymer lamellae along their long axis, while the deformation in MD generally

involves the interlamellar non-crystalline phase.

Table 5.

Young’s modulus of selected blown-films in machine direction (MD), transverse direction (TD), and compression-

molded (CM) films of the same blends.

1. Krishnaswamy, R. K.; Lamborn, M. J. Polym. Eng. Sci. 2000, 40, (11), 2385-2396.

2. Simpson, D. M.; Harrison, I. R. J. Plast. Film. Sheet. 1994, 10, (4), 302-325.

3. Zhou, H.; Wilkes, G. L. J. Mater. Sci. 1998, 33, (2), 287-303.

123

2. Solid-state Transitions of N,N’-bis(cyclohexylmethyl)-1,4-phenylene

dicarboxamide (BCPCA)

Thermal transitions of neat N,N’-bis(cyclohexylmethyl)-1,4-phenylene dicarboxamide (BCPCA) were

briefly investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction

(WAXD). DSC analysis revealed three different endothermic peaks (i.e. ~132 °C, ~162 °C, ~273 °C) in

the heating experiments and four exothermic peaks (i.e. ~122 °C, ~152 °C, ~239 °C ~259 °C) in the

cooling cycle (cf. Figure 1).

Figure 1.

Differential scanning calorimetry (DSC) thermograms of neat BCPCA; cooling from the melt at 300 °C (blue

curve), subsequent heating (red curve) recorded at a cooling and heating rate of 10 °C/min.

In order to further investigate a possible temperature-dependent crystal structure of BCPCA, WAXD

measurements were performed at three different temperatures, which were specifically chosen to be

below the temperatures of DSC exothermic peaks (i.e. 25 °C, 150 °C, 190 °C). Wide-angle X-ray

diffraction (WAXD) was performed on an Oxford Instruments XCalibur PX diffractometer using MoKα

radiation (0.71 Å wavelength). BCPCA powder was sealed inside glass capillary tubes (Hilgenberg; 1.5

mm outer diameter). Sample temperature was controlled using the Cryojet accessory by streaming

temperature-stabilised (± 0.1 °C accuracy) nitrogen gas over the capillary tube. The samples were

equilibrated at each temperature for 15 min prior to each measurement, which were carried out with 10

min integration time. The two-dimensional diffraction patterns were radially integrated following

correction for background signal. Three different WAXD patterns were found at these corresponding

temperatures (cf. in Figure 2), indeed consistent with additive solid-state transitions observed in the

binary phase diagrams presented in Chapter 5-Figure 9 and in the above Figure 1. As these transitions

occurred above the crystallization temperature of the polymers employed in this work, this finding was

deemed to be of academic interest for the pure additive, and hence was not further explored.

124

Figure 2.

Wide-angle X-ray diffractograms (WAXDs) of neat BCPCA at 25 °C (black curve), 150 °C (blue curve), 190 °C

(red curve). a.u. = arbitrary unit.

3. Wide Angle X-Ray Diffraction (WAXD) of LLDPE – 8, 25MI and

Blends of LLDPE – 8, 25MI/additives

WAXD patterns of injection-molded plaques are obtained using a Panalytical Empyrean diffractometer with Cu

Kα source between 5-55° using 48 seconds per step acquisition time and a step size of 0.0066°.

Figure 3.

Wide-angle X-Ray diffractograms (WAXDs) of the blends of LLDPE – 8, 25 MI comprising 0.25 % w/w DMDBS

(red), 0.25 % w/w TBPMN (blue), 0.05 % w/w TDMPAB (green), 0.1 % w/w BCPCA (purple) showing a

similarity with the WAXD pattern of neat LLDPE – 8, 25 MI (black) .

125

Acknowledgements

During the time of my PhD, I realized that the answer is so simple to the question of “where is my

home?” which I was asking to myself in recent years. My working environment became my “home”

for the last three years. I spent much more time there than in my apartment. I saw the people, who

contributed to this work, more than my family and my friends. Huge “THANKS” to everyone, who

contributed to the mess on my desk, which is shown in Figure 1.a.

First and foremost, I would like to thank truly to Paul Smith for giving me an opportunity to breathe the

air of Polymer Technology and to “exploit” his broad scientific and life visions. When I first came to

this lab in 2011 as a master student, I was even not at the level of zero – I was standing in minus. At that

time, I never thought that this person will completely change my life and will become my “doctor father,

intellectual father, senior co-worker and, last but not least, second father”. Since I knew him, he never

let me to say “thank you” to him. He always said “keep your thanks to the end”. And now, it is time to

say “thank you Paul” for growing me up scientifically and personally to be ready for the real world. I

will remain forever grateful for his support, patience and trust in me even in times when I didn’t have it

myself. Furthermore, it was a great experience to participate in his research vision, and I feel immense

gratitude for teaching me the importance of self-criticism and intuition to succeed rather than practicing

techniques. Apart from all those, I would like to say special thanks for his creative presents from his

spectacular sense of humor, which we usually receive together with his “suggestions” for our scientific

drafts. For instance, as it can be seen in Figure 1.b, the cover page of Chapter 3 from this thesis was

redesigned by Paul Smith, after a glass of raki fell down on the papers of the chapter.

Furthermore, I’m especially indebted to “my second doctor father”, Hans-Werner Schmidt, for his

invaluable contributions to this work, teaching me the wonders of “trisamides & bisamides”, being a co-

examiner of this thesis, and, last but not least, always offering me a warm-welcome in his group in

Bayreuth.

I would like say special thanks to Jan Vermant for accepting to be a co-examiner of this thesis and his

interest in this work.

DOW Europe GmbH is gratefully acknowledged as an industrial partner of this work for their financial

and material support (cf. Figure 1.c). A special thanks goes to Martin Hill, Selim Bensason, Rudolf

Koopmans for their contributions to this work in our interesting meetings.

126

I would like to say my sincere thanks to the “Bayreuth crowd” for their significant amount of

contributions to this work and their spectacular hospitality. Petra Weiss, for organizing my stay in

Bayreuth by thinking about and arranging every small details. Klaus Kreger, for organizing the synthesis

and characterization of the compounds in Chapter 4 (cf. Figure 1.d). Sandra Ganzleben, Jutta Failner,

Doris Hanft and Rika Schneider, for their experimental support for the synthesis of the compounds and

their invaluable experimental assistance by waiting in front of the DSM machine with me to process

injection-molded plaque samples (cf. Figure 1.e) and at the same time speaking basic German with me

to improve my language skills. A special thanks goes to my lovely friend Julia Singer, for her bicycle

supply and making my time enjoyable outside of the lab in Bayreuth.

Furthermore, I am deeply indebted to the “mothers” of this work, Karin Bernland and Eve Loiseau, for

sharing their previous experiences with me. Especially, their theses were among the fixtures at my desk

during my PhD (cf. Figure 1.f).

There are many people who contributed experimentally to make this thesis possible. First, Kirill Feldman

is thankfully acknowledged for his help and advice for many experiments that I made since my master

studies. I would like to express my sincere gratitude to Stephan Busato for sharing his expertise in

photography for this work, thinking out of the box and, introducing a new photography-based

measurement which is alternative to haze measurements in Chapter 3, and, last but not least, for his

invaluable contributions to the cover of this thesis (cf. Figure 1. g). I am deeply indebted to Derya Erdem

for her assistance to improve my practical skills in SEM; without her it wouldn’t be possible to have

those beautiful pictures in this thesis (cf. Figure 1. h). I would like to thank Werner Schmidheiny for his

equipment support, especially for melt-intrinsic haze measurements in Chapter 5. Martin Hill (from

DOW, Tarragona), Markus Blomenhofer (from Lifocolor Farben GmbH), Thomas Schweizer, Werner

Schmidheiny, Raphael Schaller (from ETH Zurich) are gratefully acknowledged for the film-blowing

samples (cf. Figure 1.i). I would like to thank to Julia Dshemuchadse, Aleksandr Perevedentsev and

Derya Erdem for the WAXD measurements.

I had an immense luck to be a part of the “Chuchihästli and Geordies’ crowd” at the time before and

during my PhD. Aleksandr Perevedentsev (Alexito), thanks for his tireless scientific curiosity, being a

truly intellectual friend, his proof reading of this thesis and his arrogant jokes that come from his warmth,

which colored of our days (cf. Figure 1.j). Raphael Schaller (Raphi), thanks for his endless help, kindness

and generosity both at the work and outside of the work (cf. Figure 1.k). Vappu Hämmerli, thanks for

her endless support for the administrative work, her invaluable friendship, nice accompany on ski slopes,

lunches in Kerala and so many other things… (cf. Figure 1.l). Sebastian Radermacher (Noodle boy),

thanks for his experimental support and dog familiarization therapy with Cody to overcome my phobia

to the dogs (cf. Figure 1.m). Furthermore, I would in particular to express my thanks to Irene Bräunlich,

127

Andreas Brändle, Paola Orsolini, Gagik Ghazaryan, Felix Koch, Jan Giesbrecht, Harald Lehmann, Theo

Tervoort, Walter Remo Caseri, Ueli Suter, Han Meijer, Wolfgang Kaiser, Sylvie Smith, Tom Schenkels,

Coen Clarjis, Maike Quandt, Louis Schär, Beniamino Paú-Lessi, Alessandro Ofnär, Fabio Bargardi,

Victoria Blair, Ljiljana Palangetic, Martina Pepicelli and all other members of the “Soft Materials”.

And of course, a special thanks goes to my dear friends, “Altin Kizlar”: Huriye Erdogan, Pinar Senay

Özbay, Ece Öztürk, Gökce Yazgan. I feel blessed by having friends like you. Thanks for always being

ready to laugh with me and having a free shoulder for my tears. Güne baslamadan önce sizin

hediyelerinizi bilgisayar ekraninin önünde görmek, en büyük motivasyon kaynagimdi (cf. Figure 1.n).

Siz olmadan bir Zürih ve doktora nasil olurdu bilemiyorum…

This thesis is the culmination of my scholar and family education, which started 28 years ago on the

lands of Anatolia. I will always feel immense gratitude to my parents, Selma & Mehmet Aksel, and my

brother, Tansel Aksel, for doing their best to shape my personal growth since I was born. Canim ailem,

bugünlere gelmemde verdiginiz tüm maddi ve manevi destekleriniz icin cok tesekkürler…

Finally, I thank Sergio for the weekends he spent his time with me in my office and convincing me to

go climbing wall at the breaks (cf. Figure 1.o); reminding my capabilities when I forget them; calming

me down when I am stressed; stimulating me to think, question, criticize and discuss more about

everything and; last but not least, making my life meaningful over the last year.

128

Figure 1.

Photo-collage of working-desk of Seda Aksel in ETH Hönggerberg, HCI H 506 (a) and zoom-in photos of selected

items which evoke memories of the people who made this thesis possible.

129

Curriculum Vitae

Seda Aksel was born in Izmir, Turkey, on the 12th of April 1987. She attended high school in

Istanbul and graduated in 2005. Subsequently, she started her studies in Materials Science with a

minor degree in Chemistry at Sabanci University, Istanbul, Turkey. Following a three months

research stay at the University of Cambridge, in the Department of Materials in 2009, she

successfully completed her Bachelor degree. In 2010, she started Master of Science (M. Sc.) studies

in Materials Science at the Swiss Federal Institute of Technology (ETH) Zürich, which she

successfully completed with a master thesis in 2012 on the topic of “Decreasing the crystallinity of

polyethylene oxides for the applications of solid-state Li-battery electrolytes” in the Polymer

Technology Group, headed by Paul Smith. She then again joined the Polymer Technology group at

ETH Zürich where she conducted the doctoral studies on the topic of “Nucleation and Clarification

of Polyethylenes” under the supervision of Prof. Paul Smith in collaboration with the Dow Chemical

Company and Prof. Hans-Werner Schmidt, Bayreuth University.

130

“Every clarification breeds new questions.”

Arthur Bloch

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