surface study of ladderlike polyepoxysiloxanes

Surface Study of Ladderlike Polyepoxysiloxanes

WEI-YU CHEN,1 YUHUI LIN,2 KUMARI P. PRAMODA,2 K. X. MA,2 T. S. CHUNG1,2

1 Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

2 Institute of Materials Research and Engineering, 10 Kent Ridge Crescent, Singapore 119260

Received 24 May 1999; revised 17 September 1999; accepted 20 September 1999

ABSTRACT: By using the two-liquid geometric method and the three-liquid acid-basemethod, we are the first to determine the surface tensions of ladderlike polyepoxysi-loxanes by the measurement of contact angles on thin films. Three kinds of ladderlikepolymers have been synthesized: AOC (which has the alkyl group and the epoxy groupgraft to the ladderlike polysilsesquioxane chain), AOCOP (which has the alkyl group,phenyl group, and epoxy group graft to the ladderlike chain), and AOP (which has thephenyl group and epoxy group in the ladderlike side chain). The results showed thatwhen different liquids and different theories are chosen to determine the surfaceenergies, there are some minor differences in the values but a similar trend is stillexhibited. The surface energies of these three polymers are in the following order ofgSAOC , gSAOCOP , gSAOP. Interestingly, the surface energy increases for thesepolymers are mainly from the nonpolar part of the polyepoxysiloxanes. XPS surfaceanalysis indicated that the Si and O ratios of these polymers at the air-polymerinterface were in the order of AOC . AOCOP . AOP, suggesting Si atoms were morelikely to migrate to the polymer surface and the bulky effect of the phenyl groups couldalso interfere with the migration of the Si atoms. As a result, Si and O ratio at theinterface determines the order of apparent surface energy for these three polymers.Experimental data also reflect that there are differences between the ladderlike poly-epoxysiloxanes and the commercially available linear polysiloxanes. © 2000 John Wiley &Sons, Inc. J Polym Sci B: Polym Phys 38: 138–147, 2000Keywords: surface tension; surface energy; contact angle; ladderlike polymers; poly-silsesquioxane; polyepoxysiloxane

INTRODUCTION

In 1960, Brown and co-workers first reported thesynthesis of high Mw polyphenylsilsesquioxane.They proposed that this polymer possess a stereo-regular double-chain structure with cis-syndio-tactic configuration known as “ladderlike” linearnetwork structure. From then on, scientists be-gan to enlarge the family of ladderlike polymers.A series of highly ordered ladderlike polysilses-quioxanes (LPS) has been synthesized in recent

years.1–3 There is an obvious difference in theappearance and properties between the ladder-like polysilsesquioxane and the usual linear, sin-gle-chain polysiloxane. In general, single-chainpolysiloxane is a viscous liquid or gum, such asmethyl–silicone oil or phenyl–silicone oil used asa thermal transfer medium and reactive methyl–hydrogen–silicone oil. The latter is a very impor-tant reactive polymer from which many func-tional silicone polymers can be derived. Ladder-like polysilsesquioxane can be coated as atransparent, glasslike film on glass or metal sub-strates from its solution with a common organicsolvent. These kinds of polymers provide a newseries of promising polymers with excellent elec-tric and optical properties, high thermostability,

Correspondence to: T. S. Chung (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 38, 138–147 (2000)© 2000 John Wiley & Sons, Inc.

138

environmental resistance, laser-damaged resis-tance, and fine film-forming ability.

As widely known, polysiloxane is one of thebest candidates to be used to modify the mechan-ical properties of epoxy resin because of its un-usual flexibility and thermal stability.4 However,the thermodynamical immiscibility between ep-oxy and polysiloxanes restricts this. A way toimprove the fracture toughness is to use modifiedpolysiloxanes such as functional group-cappedpolysiloxane and polysiloxane block copolymer.

A series of ladderlike silicon epoxies were suc-cessfully synthesized in our laboratory recently.Unlike common silicone polymers and epoxies,this kind of polymer has special properties andwill have many potential applications, especiallyfor the electronic materials. Among all the prop-erties, the surface property is the most importantbecause of its relationship with the adhesion, pro-tective coatings, thin film technology, and so on.5

Although polymer scientists are aware of the im-portance of surface properties, few articles havebeen published on the subject of epoxysiloxanes.To the best of our knowledge, almost none of themare related to the ladderlike polymers and arecommercially available.

There are several techniques available for themeasurement of the surface energy of a polymer.6

Although a pendant-drop method is suitable forpolymeric melts, the indirect sessile-drop (contactangle) method is for solid polymeric surfaces, be-cause the energy of polymer solids cannot be mea-sured directly. Basically, this method consists ofmeasuring contact angles of various liquids on asolid polymer surface and the use of an appropri-ate theory (refer to Theory section) to estimatethe solid surface energy. This method has beenemployed extensively.

Ko et al.7 studied the effect of radiation-in-duced graft on the contact angle of polymeric sur-faces and utilized both the two-liquid geometricmethod and the two-liquid harmonic method todetermine the surface energies from contact an-gles. They found that the values of dispersioncontribution (gS

b) and polar contribution (gSp) to

the surface energy are strongly dependent on thepair of liquids chosen for the contact angle mea-surement.

Grundke et al.8 investigated surface propertiesof flame and oxygen plasma-pretreated polymerblends and copolymers by contact angle. Theycalculated the components (dispersion, polar com-ponents, acid, and base) of the surface energy not

only from the two-liquid method, but also fromthe three-liquid method (refer to Theory section).

Patwardhan et al.9 studied the surface proper-ties of fluorinated phenylmethylsiloxane poly-mers and some related polymers. They used wa-ter and methylene iodide to measure the contactangles of their polymer films, then calculated sur-face energies by the two-liquid geometric method.

X-ray photoelectron spectroscopy (XPS), whichwas developed by Siegbahn and his co-workers,5

has been extensively used in the study of polymersurface. Chan5 has summarized the applicationsof XPS including surface modification, polymerchain mobility, contamination, degradation, andadhesion failure, and so on. Recently, Kato andhis co-workers studied styrene and fluoroalkylfu-marate copolymers by both contact angle mea-surement and XPS.10

In this article, we report the surface character-ization of a new kind of ladderlike polyepoxysilox-anes. A sessile drop method was employed tomeasure the contact angles of ladderlike polyep-oxysiloxanes and different methods were used tocalculate the surface energy. We also analyzedthe surface chemistry by means of XPS to inves-tigate the relationship between surface energyand surface composition of polyepoxysiloxanefilms.

EXPERIMENTAL

Materials

All the materials, which are used in the presentstudy, were synthesized in our laboratory. Thestructures of the three polymers are shown inFigure 1. They are AOC (which has the long alkylgroups and the epoxy groups graft to the polysil-sesquioxane), AOCOP (which has the long alkylgroup, phenyl groups, and epoxy groups graft tothe ladderlike chain), and AOP (which has phe-nyl groups and epoxy groups in the ladderlikeside chain). Table I shows the molar ratios in theladderlike chains.

Differential Scanning Calorimetry

A Perkin–Elmer Pyris 1 DSC was utilized. Theinstrument was calibrated with high purity melt-ing standards (indium and zinc). All the measure-ments were run under dried nitrogen. Sampleswere first heated to 250 °C at 10 °C/min and heldat that temperature for 5 min. Then the samples

SURFACE STUDY OF LADDERLIKE POLYEPOXYSILOXANES 139

were cooled to 50 °C at 10 °C/min and held at 50°C for 5 min. After that, the samples were heatedagain from 50 °C to 250 °C at 10 °C/min. Figure 2illustrates one of the heating traces of the ladder-like polymer.

Fourier Transform Infrared

Before curing and after curing in DSC (firstheated to 250 °C at 10 °C/min and held at thattemperature for 5 min, and then cooled down), theFTIR spectra of ladderlike polyepoxysiloxaneswere scanned using a Perkin–Elmer Spectrum2000 FTIR spectrometer.

Samples Preparation

Before the contact angle measurement, the lad-derlike polyepoxysiloxanes were dissolved in xy-lene (commercially available), then the solutionwas coated on a clean glass slide by a spin-coatingmethod.6 The spin coating was done at 2000 RPMfor 40 s using a Laurell™ Single Wafer Spin Pro-cessor (Model WS-200-4NPP). After coating, thefreshly spun film sample was immediately placedinto an oven and heated to 200 °C for 2 h in orderto remove the solvent and to cure the epoxies. Theinterior of the oven was cleaned before heating.Spin coating has many advantages; one of them isto get very smooth surface that is important to thecontact angle measurement.

Contact Angle Measurement

Deionized water, glycerol, formamide, diiodo-methane, and 1-bromonaphthalene (ABN) werechosen as the test liquids because there are sig-nificant data available for these liquids. All werereagent grade and used as received. Table II tab-ulates their basic surface tension parameters (inmJ/cm2).11,12

Since structures and chemical properties ofpolymer surfaces are not necessarily invariablebut may depend on time and environment,5,13

Figure 1. Chemical structure of ladderlike polyep-oxysiloxanes.

Table I. Molar Ratio of LadderlikePolyepoxysiloxanes

Epoxy(mol %)

Alkyl(mol %)

Phenyl(mol %)

1. AOC 30 70 02. AOCOP 30 35 353. AOP 30 0 70

Figure 2. DSC of ladderlike polyepoxysiloxane.

140 CHEN ET AL.

functional groups of homopolymers and block co-polymers might restructure or reorient in re-sponse to different environments. In order to min-imize this effect and to get a reliable data, allsample preparation and contact angle measure-ments were carried out in the same environment,using the same droplet size14 for the same refer-ence liquids. At least 10 droplets were measuredfor each liquid.

The contact angle measurements were per-formed on a Rame–Hart contact angle goniometer(model 100-22). Liquid droplets were introducedby a Gilmont microsyringe onto the polymer sur-faces. Advancing contact angles were recorded.

X-ray Photoelectron Spectroscopy

The surface of thin ladderlike polyepoxysiloxanefilms was characterized using X-ray photoelec-tron spectroscopy (XPS). The XPS measurementswere performed on a VG ESCA LAB 220I-XLspectrometer with a magnesium anode sourceproducing MgKa (125.3 nm) X-ray at 12 KeV anda pass energy of 20 eV. The XPS data analysiswas carried out using software from VG ESCALab. The shift of the binding energy due to sur-face charging effect was corrected by assumingthe binding energy of C 1s to be 285.0 eV. Atomicconcentrations were determined from the peakareas using the sensitivity factors provided by themanufacturer.

Theory

When a liquid drop is in contact with an ideallysmooth, homogeneous solid, it exhibits an equilib-rium contact angle, which can be expressed byYoung’s equation:11–12

gLVcos u 5 gSV 2 gSL (1A)

where gLV is the surface tension of the liquid inequilibrium with its own vapor, gSL the interfa-cial tension between liquid and solid, gSV the sur-face tension of the solid in equilibrium with thesaturated liquid vapor, and u the contact angle. Interms of gS (surface tension of the solid), Young’sequation is rewritten as:

gLVcos u 5 gS 2 gSL 2 pe (1B)

where pe [ (gS 2 gSV), equilibrium pressure. It isgenerally believed that, if the contact angle isgreater than zero, pe is negligible. Therefore, eq 1can be rewritten as:

gLVcos u 5 gS 2 gSL (2)

Two-Liquid Geometric Method

Owens and Wendt15 and Kaelble16 extendedFowkes’ hypothesis17,18 and gave the followingequation to express gSL:

gSL 5 gS 1 gLV 2 2~gSdgLV

d !1/2 2 2~gSpgLV

p !1/2 (3)

Combining equations 2 and 3 yields:

gLV~1 1 cos u! 5 2~gSdgLV

d !1/2 1 2~gSpgLV

p !1/2 (4)

where superscript d refers to the dispersion (non-polar) component, and p refers to the polar (non-dispersion) component, including all the interac-tions established between the solid and liquid,such as dipole–dipole, dipole-induced dipole andhydrogen bonding, and so on.

Table II. Surface Tension Parameters (in mJ/m2) of Testing Liquids11,12,22

3-Liquid Method Water Glycerol Formamide CH2I2 ABN

g1 25.5 3.92 2.28 0.0 0.0g2 25.5 57.4 39.6 0.0 0.0gAB 51.0 30.0 19.0 0.0 0.0gLW 21.8 34.0 39.0 50.8 43.5g 72.8 64.0 58.0 50.8 44.4

2-Liquid Methodg p 51.0 26.4 18.7 2.3gd 21.8 37.0 39.5 48.5g 72.8 63.4 58.2 50.8


Because gS is the sum of surface tension com-ponents contributed from dispersion and polarparts:

gS 5 gSd 1 gS

p (5)

Equations 4 and 5 provide a method to estimatesurface tension of solids. Using two liquids withknown gL

d and gLp for contact angle measurements,

one could easily determine gSd and gS

p by solvingthe following two equations:

gLV1~1 1 cos u1! 5 2~gSdgLV1

d !1/2 1 2~gSpgLV1

p !1/2

gLV2~1 1 cos u2! 5 2~gSdgLV2

d !1/2 1 2~gSpgLV2

p !1/2 (6)

The values of gLd and gL

p of reference liquids havebeen provided by Kaelble.11

Lifshitz-van der Waals-Acid-Base Theory(Three-Liquid Acid-Base Method)

Van Oss et al.19,20 has proposed a methodologythat introduces a new meaning of the concepts of“apolar” and “polar,” the later cannot be repre-sented by a single parameter such as gp used inthe two-liquid geometric method.

Because the surface energy can be separatedinto several components as:

g 5 gd 1 gdip 1 gind 1 gh 1 . . . (7)

where the superscripts, d, dip, ind, and h refer to(London) dispersion, (Keesom) dipole–dipole, (De-bye) induction, and hydrogen bonding forces, re-spectively. Van Oss and Good re-expressed theabove equation as:

g 5 gLW 1 gAB (8A)

gLW 5 gd 1 gdip 1 gind (8B)

where LW stands for Lifshitz–van der Waals andthe superscript AB refers to the acid-base inter-action. Since a hydrogen bond is a proton-sharinginteraction between an electronegative moleculeor group and an electropositive hydrogen, a hy-drogen bonding is an example of Lewis acid (elec-tron acceptor) and Lewis base (electron donor).Van Oss et al.20–22 therefore treated hydrogenbonding as Lewis acid-base interactions and inaddition, they created two parameters to describe

the strength of Lewis acid and base interactions:g1[ (Lewis) acid parameter of surface free en-ergy; g2 [ (Lewis) base parameter of surface freeenergy.

giAB 5 2Îgi

1gi2 (9)

Based on these definitions, a material is clas-sified as a bipolar substance if both its g1 and itsg2 are greater than 0 (gi

AB Þ 0). In other words, ithas both nonvanishing g1 and g2. A monopolarmaterial is one having either an acid or a basecharacters, which means either g1 5 0 and g2 . 0or g1 . 0 and g2 5 0. An apolar material isneither an acid nor a base (both its g1 and its g2

are 0). For both monopolar and apolar materials,their gi

AB 5 0 . Therefore, the criterion, gAB 5 0,for a substance to be apolar is not sufficient.

How do we calculate these surface energy pa-rameters? van Oss, Good, and their co-workershave developed a “three-liquid procedure” (eq 10)to determine gS by using contact angles tech-niques and a traditional matrix scheme.

gLV1~1 1 cos u1! 5 2~ÎgSLWgLV1

LW

1 ÎgS1gLV1

2 1 ÎgS2gLV1

1 !


LW

1 ÎgS1gLV2

2 1 ÎgS2gLV2

1 !


LW

1 ÎgS1gLV3

2 1 ÎgS2gLV3

1 ! (10)

In short, to determine the components of gS ofa polymer solid, it was recommended6,19–22 to se-lect three or more liquids from the reference liq-uid table, with two of them being polar, the otherone being apolar. Moreover, the polar pairs, waterand ethylene glycol, and water and formamidewere recommended to give good results, whereasapolar liquids are either diiodomethane or a-bro-monaphthalene. Since the LW, Lewis acid, andLewis base parameters of gLV1, gLV2, and gLV3 areavailable,11,22 one can determine gS using its pa-rameters LW, Lewis acid, and base by solvingthese three equations simultaneously.

RESULTS AND DISCUSSION

Differential Scanning Calorimetry

The DSC gave the heat evolution of these ladder-like polymers during polymerization. Figure 2

142 CHEN ET AL.

displays the DSC traces for one of the ladderlikepolyepoxysiloxanes. The wide peak during thefirst heating is corresponding to the polymeriza-tion of the AOP, the cooling trace and the secondheating trace do not show any noticeable peak,that means, during the first heating, most of theepoxy groups in the polymer chain had reacted.

Fourier Transform Infrared

Figure 3 shows the FTIR spectra of one of theladderlike polyepoxysiloxanes. There are twospectra in the figure, one is the spectrum of thepolymer before curing, and the other is the spec-trum of the polymer after curing. The wide peakaround 1130 cm21 represents the vibration of theSiOOOSi. The 832 and 925 cm21 bands existingbefore curing are associated with the epoxygroups. After curing, these peaks disappeared,suggesting that the epoxy groups fully reactedduring curing.

Surface Energies of Polyepoxysiloxanes

Using the spin-coated transparent thin films forthe measurement of the contact angles, we gotwell-repeated data. The average contact anglesare tabulated in Table III along with the standarddeviations. The standard deviations are verysmall, which means that the surfaces of thesefilms are smooth and uniform. There are differ-ences between the ladderlike polyepoxysiloxanesand the commercial epoxies. For example, it isdifficult for the most commercial epoxies to usethe spin-coating method to yield a smooth film.This is because it is not easy to degas the com-mercial epoxies during the curing process, andthe gas trapped inside in the epoxies will causethe surface roughness.

During the contact angle measurement, water,glycerol, and formamide were utilized as polarliquids, whereas diiodomethane and ABN wereutilized as apolar liquids. Using the two-liquidgeometric method and three-liquid acid-basemethod with different liquid pairs or sets, wecalculate the surface energies and Tables IV andV summarize the outcome.

Table IV provides the surface energies calcu-lated by using the two-liquid geometric method.Three different liquid pairs were used for themeasurements and calculations. Each pair hasone polar liquid and one apolar liquid.6 By com-paring these results, one can see that for all thethree polymers, surface energies calculated bydifferent pairs show a similar trend of gS, which isgSAOC , gSAOCOP , gSAOP. The increasing ofsurface energy mainly comes from gS

d.On the other hand, we could observe that for

one polymer, the surface energies calculated bydifferent pairs have some differences. This againimplies that different liquids chosen in determin-

Figure 3. FTIR spectra of ladderlike polyepoxysilox-ane.

Table III. Contact Angle Determination (in degree) on Ladderlike Polyepoxysiloxanes

Liquid Water Glycerol Formamide CH2I2 ABN

AOCContact angle 99.23 92.67 85.98 59.54 49.83Standard deviation 0.7440 0.7418 0.6233 1.3275 0.6181

AOCOPContact angle 97.86 87.85 85.29 54.46 41.61Standard deviation 0.8276 0.7714 0.7318 1.2772 0.5533

AOPContact angle 90.00 79.74 74.38 40.46 26.15Standard deviation 1.9232 0.7122 0.5381 0.7877 0.9680


ing contact angle give different outcome.6 Onereason is that actually the parameters of the test-ing liquids are only semiexperimental values (theparameters in some other references sometimeshave a little bit of difference), and of course, whenwe did the experiments, there were experimenterrors. Another reason is that either for the two-liquid method or three-liquid method, we postu-late that the presence or absence of the liquid

does not affect the molecular configuration of thepolymer surface, and the spreading pressure isneglected. But in fact, there are some effectsthere. In this article, we only consider there isdifference when the differences of the surface en-ergies between two polymers are more than 2mJ/m2 by using the same method and the samepairs or sets of liquids.

From the results we know that all the threeladderlike polymers are nonpolar polymers. On anonpolar surface, gS

p may be zero, and gSd is al-

ways greater than zero because of the universal-ity of dispersion forces.11 When calculating thesurface energies, we found that for some pairs,there is a little bit negative in gS

p. We also ran intothis problem when we calculated gS

1 using thethree-liquid acid-base method. Since the negativeenergies are physically meaningless, we suggestit as zero. Experimental error, spreading pressureeffect, and so on may be the reasons that result ina negative gS

p or gS1. Robert J. Good and Carel J.

van Oss also discussed this and gave some possi-ble explanations in their review paper.12

The surface energy values calculated by usingthe three-liquid acid-base method are given inTable V. Four liquid sets were used for the calcu-lation. Each set has two polar liquids and oneapolar liquid.6 For each polymer, the gS obtainedby using different sets still have some minor dif-ferences. The average gS is about 29.75 mJ/cm2,33.19 mJ/cm2 and 40.09 mJ/cm2 for AOC,

Table IV. Surface Energies Using the Two-LiquidGeometric Method

gSd

(mJ/m2)gS

p

(mJ/m2)gS

(mJ/m2)

AOCWater–CH2I2 28.32 0.64 28.96Glycerol–CH2I2 32.25 0 32.25Formamide–CH2I2 33.03 0 33.03Mean 31.20 0.21 31.41

AOCOPWater–CH2I2 31.44 0.54 31.98Glycerol–CH2I2 34.72 0 34.72Formamide–CH2I2 37.43 0 37.43Mean 34.53 0.18 34.71

AOPWater–CH2I2 38.37 1.10 39.47Glycerol–CH2I2 42.52 0 42.52Formamide–CH2I2 44.55 0 44.55Mean 41.81 0.37 42.18

Table V. Surface Energies Using the Three-Liquid Acid-Base Method

gS1 gS

2 gSAB gS

LW gS

AOCWater–glycerol–CH2I2 0.00 2.61 0.00 28.84 28.84Water–glycerol–ABN 0.00 2.50 0.00 30.66 30.66Water–formamide–CH2I2 0.00 3.81 0.00 28.84 28.84Water–formamide–ABN 0.00 3.87 0.00 30.66 30.66Mean 0.00 3.20 0.00 29.75 29.75

AOCOPWater–glycerol–CH2I2 0.00 1.71 0.00 31.76 31.76Water–glycerol–ABN 0.00 1.58 0.00 34.61 34.61Water–formamide–CH2I2 0.00 4.92 0.00 31.76 31.76Water–formamide–ABN 0.00 4.53 0.00 34.61 34.61Mean 0.00 3.19 0.00 33.19 33.19

AOPWater–glycerol–CH2I2 0.00 2.91 0.00 39.38 39.38Water–glycerol–ABN 0.00 2.83 0.00 40.80 40.80Water–formamide–CH2I2 0.00 5.51 0.00 39.38 39.38Water–formamide–ABN 0.00 5.56 0.00 40.80 40.80Mean 0.00 4.20 0.00 40.09 40.09

144 CHEN ET AL.

AOCOP and AOP, respectively. By the three-liquid acid-base method, one can get the sametrend as obtained from the two-liquid geometricmethod; gS increases mainly because of the gS

LW.Comparing the two-liquid geometric method tothe three-liquid acid-base method, we found thatthe increase in surface energy mainly comes fromthe nonpolar part, both methods give reliableresults.

The increase in surface energy mainly from thenonpolar part may be explainable if one considersthe chemical structures of these three polymers.The AOC, which only has alkyl groups and nophenyl groups, has the lowest surface energy. TheAOP, which only has the phenyl groups, has thehighest surface energy, and the surface energy ofthe AOCOP, which consists of both the alkylgroups and the phenyl groups, has the value inbetween those of AOC and AOP. In general, ifone of the pendant groups on the silicon is a bulkgroup such as phenyl, the surface energy is rela-tively high,23,24 our results agree with it. On theother hand, it has been known that the incorpo-ration of nonpolar OCH2O moieties into hydro-carbon structures leads to increased surface ten-sions, so as to siloxanes.9 Thus both the alkylgroups and the phenyl groups increase the sur-face energy. In our polymers, it is apparently thatthe phenyl groups are more effective in view ofincreasing the surface energy.

Among the three ladderlike polymers, theAOC has the lowest surface energy. When com-paring its surface energy to that of polydimethyl-siloxane (23.5 mJ/m2, using water and CH2I2 asthe testing liquid and calculated by the two-liquidgeometric method),9 we found that ours is quitehigh. This may be due to that fact that the AOChas long alkyl substituents and the epoxygroups,9 as well as unique ladderlike chain struc-ture. As we know, the linear siloxane has theunusually flexible SiOOOSi bonds (bond anglesbetween 105 and 180°)24 in the siloxanyl moiety.This backbone flexibility and its incompatibilitywith other organic polymers make SiOOOSibonds easily moving to the surface and thus pro-duce low energy surface. Because the low-energycomponent tends to migrate to the polymer-airinterface as a result of the thermodynamic drivein order to minimize the surface energy,5 there-fore when polydimethylsiloxane or siloxane con-taining copolymers are blended with various or-ganic polymers, the air-polymer surfaces of theresulting systems are dominated by the low sur-face energy siloxane.25 Because the ladderlike

structure in our polyepoxysiloxanes makes thechains less flexible than the linear siloxane, thebond angle of SiOOOSi in our polyepoxysilox-anes should be different from that of the linearsiloxane. As a result, the ladderlike backbonesare not as easy as the linear siloxane to migrate tothe surface during the film formation.

The situation becomes more complicated whenwe compare the surface energy of AOCOP withpolyphenylmethylsiloxane (33.2 mJ/m2, testingcondition same as the above),9 no significant dif-ference in surface energy is noticeable. What isthe reason for it? This interesting result mainlyarises from the steric effect on surface energy.The steric hinder effect of the phenyl group makesitself more likely to appear on the surface. Thus,the AOCOP has a higher surface energy than theAOC. This steric hinder effect can also be used toexplain the reason that the AOP has the highestsurface energy among the three ladderlike poly-mers.

X-ray Photoelectron Spectroscopy Study

Figure 4 shows a wide scan XPS spectrum of athin ladderlike polyepoxysiloxane film. From thespectrum it is clear that there are no impuritiespresent in the samples, except the accepted C, O,Si, and the Auger peaks for the C and O.

The calculated atomic concentrations of C, O,Si, and the ratio of Si and O are shown in TableVI. Theoretical ratio of Si and O is about 0.55,which can be calculated by counting their respec-tive number of atoms from the monomer struc-ture. However, this ratio was found to be 0.7013,0.6668, and 0.5728 for AOC, AOCOP, and AOP,respectively from the experimental data. Compar-ing the theoretical ratio of Si and O, one can see

Figure 4. XPS spectra of ladderlike polyepoxysilox-ane.


all the ratios of the three polymers measured byXPS are higher than the theoretical ratio, whichmeans Si is more likely to move to the surface.25

On the other hand, AOC has the highest Si/O,and AOP has the lowest Si/O, which is quite closeto the theoretical ratio. According to the chemicalstructures of the polymers, we could find thatboth phenyl groups and alkyl groups are grafteddirectly to the Si atoms and therefore they arecloser to Si atoms than to O atoms. This meansthat both the phenyl and alkyl groups are morelikely to shield the Si atoms than the O atoms.Considering the bulky effect of the phenyl groups,one can see that phenyl groups are more effectivethan alkyl group in shielding the Si atoms. Theabove explanation is in agreement with experi-mental data from XPS, which have also good cor-relation with surface energy data obtained fromthe contact angle measurements.

The atomic percentage of carbon for AOCOP isthe highest among the three polymers. This ismaybe due to the long alkyl chain, the intramo-lecular and intermolecular interaction betweenthe alkyl chain and phenyl groups, and molecularorientation of the ladderlike polymers. In theAOC case, since the long alkyl chain is flexible, itmay be oriented in many directions. In the AOPcase, the phenyl group is rigid, therefore its Catom percentage is higher than that in AOC poly-mer. It is possible the molecular interaction ofphenyl and alkyl groups as well as the molecularorientation of ladderlike polymers makes the Catom percentage of AOCOP the highest amongthe three polymers, since the molecular interac-tion may make the alkyl oriented in certain direc-tions, but not others. However, we do not haveevidence at this moment and we prefer to leavethe question open.

CONCLUSIONS

We have determined the surface tensions of threeladderlike polyepoxysiloxanes using contact angle

measurement and the two-liquid geometric andthree-liquid acid-base methods. No matter whichset of liquids and which method we chose, all thesurface energies of three types of ladderlike poly-epoxysiloxanes showed a similar trend as follows:gSAOC , gSAOCOP , gSAOP.

In other words, polyepoxysiloxanes (AOC) hav-ing alkyl and epoxy groups in the ladderlike sidechain have the lowest surface energy, whereaspolyepoxysiloxanes (AOP) having phenyl and ep-oxy groups grafted to the ladderlike polysilsesqui-oxane chain have the highest surface energy.Polyepoxysiloxanes (AOCOP), which consist ofboth the alkyl groups and the phenyl groups, havethe value in between those of AOC and AOP.Clearly, the phenyl groups are more effective inview of increasing the surface energy than alkylgroup. XPS data suggests that the steric hindereffect of the phenyl group makes itself more likelyto appear on the surface.

The authors would like to express their gratitude to theNational University of Singapore (NUS) for their finan-cial support. Thanks are also due to Ms. S. X. Cheng,Ms. W. H. Lin, and Mdm. L. K. Leong for their kindhelp and useful comments. Special thanks are due tothe Institute of Materials Research and Engineering(IMRE) of Singapore for the equipment and financialassistance for Ms. W. Y. Chen.

REFERENCES AND NOTES

1. Xie, Z. T.; He, Z. Q.; Dai, D. R.; Zhang, R. B. Studyon the Synthesis and Characterization of Soluble,High Molecular Weight and Ladderlike Polymeth-ylsilsesquioxane; Chin J Polym Sci 1989, 7, 183.

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Table VI. Atomic Concentrations of C, O, Si(by XPS)

C O Si Si/O

AOC 0.3297 0.3940 0.2763 0.7013AOCOP 0.6498 0.2101 0.1401 0.6668AOP 0.5755 0.2699 0.1546 0.5728

146 CHEN ET AL.

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surface study of ladderlike polyepoxysiloxanes

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