synthesis and properties of poly(isobutyl methacrylate-co-butanediol dimethacrylate-co-methacryl...

Synthesis and Properties of Poly(isobutyl methacrylate-co-butanediol dimethacrylate-co-methacryl polyhedraloligomeric silsesquioxane) NanocompositesGUI ZHI LI,1 HOSOUK CHO,1 LICHANG WANG,1 HOSSEIN TOGHIANI,2 CHARLES U. PITTMAN, JR.1

1Department of Chemistry, Mississippi State University, Mississippi 39762

2Department of Chemical Engineering, Mississippi State University, Mississippi 39762

Received 12 August 2004; accepted 7 September 2004DOI: 10.1002/pola.20503Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Poly[isobutyl methacrylate-co-butanediol dimethacrylate-co-3-methacrylyl-propylheptaisobutyl-T8-polyhedral oligomeric silsesquioxane] [P(iBMA-co-BDMA-co-MA-POSS)] nanocomposites with different crosslink densities and different polyhedral oligo-meric silsesquioxane (MA-POSS) percentages (5, 10, 15, 20, and 30 wt %) were synthesizedby radical-initiated terpolymerization. Linear [P(iBMA-co-MA-POSS)] copolymers werealso prepared. The viscoelastic properties and morphologies were studied by dynamicmechanical thermal analysis, confocal microscopy, and transmission electron microscopy(TEM). The viscoelastic properties depended on the crosslink density. The dependence ofviscoelastic properties on MA-POSS content at a low BDMA loading (1 wt %) was similarto that of linear P(iBMA-co-MA-POSS) copolymers. P(iBMA-co-1 wt % BDMA-co-10 wt %MA-POSS) exhibited the highest dynamic storage modulus (E�) values in the rubberyregion of this series. The 30 wt % MA-POSS nanocomposites with 1 wt % BDMA exhibitedthe lowest E�. However, the E� values in the rubbery region for P(iBMA-co-3 wt % BDMA-co-MA-POSS) nanocomposites with 15 and 30 wt % MA-POSS were higher than those ofthe parent P(iBMA-co-3 wt % BDMA) resin. MA-POSS raised the E� values of all P(iBMA-co- 5 wt % BDMA-co-MA-POSS) nanocomposites in the rubbery region above those ofP(iBMA-co-5 wt % BDMA), but MA-POSS loadings � 15 wt % had little influence onglass-transition temperatures (Tg’s) and slightly reduced Tg values with 20 or 30 wt %POSS. Heating history had little influence on viscoelastic properties. No POSS aggregateswere observed for the P(iBMA-co-1 wt % BDMA-co-MA-POSS) nanocomposites by TEM.POSS-rich particles with diameters of several micrometers were present in the nanocom-posites with 3 or 5 wt % BDMA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem43: 355–372, 2005Keywords: morphology; nanocomposites; poly(isobutyl methacrylate-co-butanedioldimethacrylate-co-methacryl polyhedral oligomeric silsesquioxane) [P(iBMA-co-BDMA-co-MA-POSS)]; synthesis; viscoelastic properties

INTRODUCTION

Nanocomposites containing one or more phaseswith ultrafine (1–100 nm) dimensions have at-tracted considerable interest as researchersstrive to enhance material properties via nano-

scale reinforcement in contrast to conventionalparticulate-filled microcomposites.1,2 Polyhedraloligomeric silsesquioxane (POSS) modified nano-composites have attracted special attention.3–18

POSS compounds are 1–3 nm in diameter and areunique inorganic–organic hybrid reagents. Theirinorganic cage framework is made up of siliconand oxygen (SiO1.5)n (n � 8, 10, or 12). This inor-ganic cage is externally covered by organic sub-stituents, which can enhance their solubility inorganic media. One or more of these substituents

Correspondence to: C. U. Pittman, Jr. (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 355–372 (2005)© 2004 Wiley Periodicals, Inc.

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may contain polymerizable functional groups.Many POSS monomers have been copolymerizedwith other common monomers.3 This provides ameans of manipulating many properties of estab-lished polymer systems.4–14

The synthesis and properties of many mono-functional POSS copolymers have been reported.3

Examples include styrene-co-styryl–POSS,4 4-methylstyrene-co-styryl–POSS,5,6 methacrylate–POSS-b-n-butyl acrylate-b-methacrylate–POSS,7

norbornene-co-norbornyl–POSS,8,9 ethylene-co-norbornyl–POSS,10,11 propylene-co-norbornyl–POSS,10 ethylene-co-vinyl–POSS,12 propene-co-vinyl–POSS,12 and POSS–siloxane copoly-mers.13,14

Both monofunctional and multifunctional POSSmonomers have been used to modify thermoset net-work systems. Monofunctional epoxy-substitutedPOSS [(c-C6H11)7Si8O12(CH2CHCH2O)] was incor-porated into an epoxy resin.15 This resin’s glass-transition region was broadened with an increase inthe POSS content (�10 wt %), but there was nochange in the onset temperature of the glass tran-sition.15 We incorporated multifunctional POSS[(C6H5CHCHO)4(Si8O12)(CHACHC6H5)4] witheight reactive functional groups, including four�-substituted styrenes and four epoxidized sty-renes, into both epoxy17 and vinyl ester resins.18

This POSS unit dispersed extremely well in theepoxy network (on the molecular scale) according totransmission electron microscopy (TEM) observa-tions at a high POSS content (25 wt %).17 The glass-transition temperature (Tg) range was broadenedupon the cocuring of this POSS macromer into theepoxy resin at 120 and 150 °C, but the Tg values[the loss factor (tan �) peak temperature from dy-namic mechanical thermal analysis (DMTA)curves] remained unchanged. Incorporating smallamounts of this same multifunctional POSS(�10 wt %) into vinyl ester networks had almost noinfluence on Tg or the glass-transition region.18

However, both epoxy/POSS and vinyl ester (VE)/POSS composites exhibited higher storage moduli[when the temperature (T) was greater than Tg]than those of the neat epoxy and vinyl ester resins,respectively.

In this study, monofunctional 3-methacrylyl-propylheptaisobutyl-T8-polyhedral oligomericsilsesquioxane (MA-POSS) was incorporated intopoly(isobutyl methacrylate-co-butanediol di-methacrylate) [P(iBMA-co-BDMA)] crosslinkednetworks by radical-initiated terpolymerization.The crosslinking density of these networks wascontrolled through changes in the amount of di-

functional butanediol dimethacrylate (BDMA).The viscoelastic properties and morphologieswere measured by DMTA and TEM, respectively.The changes in the viscoelastic behavior and mor-phology upon the incorporation of MA-POSSinto P(iBMA-co-BDMA) networks at differentcrosslinking densities were investigated. The vis-coelastic properties of linear poly[isobutyl metha-crylate-co-3-methacrylylpropylheptaisobutyl-T8-polyhedral oligomeric silsesquioxane] [P(iBMA-co-MA-POSS)] copolymer nanocomposites werealso studied and compared with those of poly-(isobutyl methacrylate) (PiBMA) and crosslinkedpoly[isobutylmethacrylate-co-butanedioldimetha-crylate-co-3-methacrylylpropylheptaisobutyl-T8-polyhedral oligomeric silsesquioxane] [P(iBMA-co-BDMA-co-MA-POSS)] nanocomposites.

EXPERIMENTAL

Materials

MA-POSS (molecular weight � 942), in the formof a white powder, was a T8 POSS with sevenisobutyl groups and one 3-methacrylylpropylfunction on the eight-corner silicone cage atoms.It was purchased from Hybrid Plastics Co. andused as received. Isobutyl methacrylate (iBMA;F.W.142.2; bp � 155°) and BDMA (F.W.226.28; bp� 290°) were purchased from Aldrich ChemicalCo., Inc., and used without further purification. Amethyl ethyl ketone peroxide (MEKP) solution(�35 wt %) in 2,2,4-trimethyl-1,3-pentanediol di-isobutyrate was also purchased from AldrichChemical. Cobalt naphthanate (CoNap) wasbought from Dow Chemical Co. MEKP andCoNap were employed in all cases as the free-radical initiator/catalyst system.

Preparation of the Copolymer andTerpolymer Nanocomposites

MA-POSS portions (0.5, 1, 1.5, 2, and 3 g) wereadded to iBMA (9.5, 9, 8.5, 8, and 7 g, re-

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spectively). At low MA-POSS concentrations(�15 wt %), the white MA-POSS powder com-pletely dissolved in iBMA, yielding transparentsolutions at room temperature. However, at highMA-POSS concentrations (20 or 30 wt %), theMA-POSS/iBMA mixture. This was translucentat room temperature. Then, a 0.5 wt % MEKPsolution and 0.1 wt % CoNap were added to eachof these monomer mixtures, and this was followedby curing in air (88 °C for 72 h) and postcuring(100 °C for 24 h) to produce iBMA-co-MA-POSSlinear copolymers. After the curing, all these co-polymers (5, 10, 15, 20, and 30 wt % MA-POSS)were transparent. Heating at 88 °C during curingimproved MA-POSS solubility in iBMA and led togood transparency for the 20 and 30 wt % MA-POSS copolymers. PiBMA was prepared withidentical conditions to serve as a control samplewith which to compare the copolymers.

BDMA and iBMA mixtures with 99/1, 97/3, and95/5 weight ratios were prepared. Then, a0.5 wt % MEKP solution and 0.1 wt % CoNapwere added to each of these monomer mixtures.

Transparent mixtures were obtained upon shak-ing. After curing in air at 88 °C for 72 h, P(iBMA-co-BDMA) crosslinked network solids were made.All P(iBMA-co-BDMA) samples were postcured at100 °C for 24 h. P(iBMA-co-BDMA) samples with3 or 5 wt % BDMA were bent after the curingprocess at 88 °C for 72 h and at 100 °C for 24 h.Therefore, the samples containing 3 or 5 wt %BDMA were also postcured at 120 °C for 2 h andat 130 °C for 2 h under a low pressure (1.54 psi).

MA-POSS was weighed into various iBMA andBDMA mixtures (the iBMA/BDMA/MA-POSSweight ratios are shown in Table 1). MA-POSSdissolved in the iBMA and BDMA mixtures andyielded transparent solutions for 5, 10, or 15 wt %MA-POSS. The iBMA/BDMA/MA-POSS mixturesappeared translucent when 20 or 30 wt % MA-POSS was present. Then, a 0.5 wt % MEKP solu-tion and 0.1 wt % CoNap were added, and allsamples were cured at 88 °C for 72 h followed bypostcuring at 100 °C for 24 h. The nanocompositeswith BDMA concentrations of 3 wt % or morewere bent after curing at 88 °C for 72 h and at

Table 1. Properties of P(iBMA-co-MA-POSS) Copolymers and P(iBMA-co-BDMA-co-MA-POSS) Nanocomposites

Sample

iBMA/BDMA/MA-POSS

Density(g/cm3)

Tg

(°C)

E� at35 °C(GPa)

E� at145 °C(MPa)

SwellingVolumeRatio in

THFw/w mol/mol

1 100/0/0 100/0/0 1.047 85.4 0.87 0.33 —2 95/0/5 99.21/0/0.79 1.052 85.5 0.92 0.37 —3 90/0/10 98.35/0/1.65 1.057 83.0 1.00 0.41 —4 85/0/15 97.41/0/2.59 1.061 84.7 0.89 0.33 —5 80/0/20 96.36/0/3.64 1.065 80.8 0.86 0.30 —6 70/0/30 93.93/0/6.07 1.073 80.2 0.67 0.25 —7 99/1/0 99.37/0.63/0 1.051 87.3 1.00 0.42 4.468 94/1/5 98.55/0.66/0.79 1.054 84.8 1.01 0.40 4.329 89/1/10 97.65/0.69/1.66 1.059 87.6 1.01 0.40 3.31

10 84/1/15 96.67/0.72/2.61 1.063 85.2 0.93 0.40 3.8211 79/1/20 95.59/0.76/3.65 1.067 84.2 0.78 0.46 4.2512 69/1/30 93.05/0.85/6.10 1.076 82.7 0.88 0.28 3.5713 97/3/0 98.09/1.91/0 1.052 90.1 1.04 0.72 2.6014 92/3/5 97.21/1.99/0.80 1.053 92.1 0.82 0.62 Fractured15 87/3/10 96.24/2.09/1.67 1.060 91.7 0.98 0.64 Fractured16 82/3/15 95.18/2.19/2.63 1.046 91.9 0.84 0.73 Fractured17 77/3/20 94.01/2.30/3.69 1.057 87.5 0.79 0.67 Fractured18 67/3/30 91.26/2.57/6.17 1.076 85.9 0.76 1.23 Fractured19 95/5/0 96.80/3.20/0 1.057 92.8 0.94 0.78 Fractured20 90/5/5 95.85/3.35/0.80 1.034 92.3 0.75 1.48 Fractured21 85/5/10 94.81/3.51/1.68 1.040 93.0 0.87 1.54 Fractured22 80/5/15 93.67/3.68/2.65 1.062 91.0 0.87 1.73 Fractured23 75/5/20 92.41/3.87/3.72 1.070 89.7 0.96 1.62 Fractured24 65/5/30 89.45/4.32/6.23 1.085 89.9 0.64 1.64 Fractured

SYNTHESIS AND PROPERTIES OF NANOCOMPOSITES 357

100 °C for 24 h; this was similar to what wasobserved with the P(iBMA-co-BDMA) resin.Therefore, these samples were also postcured at120 °C for 2 h and at 130 °C for 2 h under apressure of 1.54 psi, respectively, to producestraight samples. P(iBMA-co-1 wt % BDMA-co-MA-POSS) nanocomposites with 5, 10, 15, 20, or30 wt % MA-POSS were transparent like theP(iBMA-co-MA-POSS) copolymers. However,phase separation occurred at high BDMA load-ings (3 or 5 wt %). Some visible white particles,from several micrometers to approximately0.5 mm in size, were formed during curing.

P(iBMA-co-3 wt % BDMA-co-MA-POSS) nano-composites with 5, 10, 15, or 20 wt % MA-POSSwere translucent, whereas the 30 wt % POSScomposite was opaque. The number of white par-ticles (POSS-rich phase) increased with an in-crease in the POSS content. Two layers wereformed during the curing of the P(iBMA-co-5 wt %BDMA-co-MA-POSS) composites containing 5,10, 15, 20, or 30 wt % MA-POSS. The upper layerwas slightly translucent, whereas the bottomlayer was opaque with many white particles.Thus, phase separation became more pronouncedas the crosslinking density increased.

Measurements

The bending dynamic storage modulus (E�) andtan � values were measured with a Polymer Lab-oratories DMTA MK3 instrument. Small-ampli-tude bending oscillations (both 1 and 10 Hz) wereused at a gap setting of 8.00 mm. The measure-ments were carried out from 30–32 to 150–155 °Cat a rate of 2 °C/min. All samples were approxi-mately 2.7–3.2 mm thick, 6.0–7.1 mm wide, and40 mm long.

The densities of the P(iBMA-co-MA-POSS)copolymers and P(iBMA-co-BDMA-co-MA-POSS)nanocomposites were measured with an elec-tronic densimeter (ED-120T) at 25.5 °C.

Each of the P(iBMA-co-BDMA-co-MA-POSS)nanocomposites (�0.5 cm3) was immersed in tet-rahydrofuran (THF) at room temperature. Themaximum swelling volumes for the 1 wt % BDMAnanocomposites with 5, 10, 15, 20, or 30 wt %MA-POSS and for the P(iBMA-co-BDMA) resinswith 1 or 3 wt % BDMA were measured afterswelling equilibrium was reached. The swellingvolume ratio was defined as this equilibrium(maximum) volume divided by the initial volumebefore swelling. The sample with the highcrosslink density exhibited a lower swelling vol-

ume ratio. However, the P(iBMA-co-BDMA-co-MA-POSS) composites with 3 or 5 wt % BDMAand the P(iBMA-co-5 wt % BDMA) resin fracturedinto many small pieces during THF swelling, sothese equilibrium swelling volumes could not bemeasured. The THF solutions, containing smallamounts of extracted linear copolymers, werecoated on KBr plates to obtain their Fouriertransform infrared (FTIR) spectra on an FTIRinstrument (Midac Corp.).

Samples with particles were chosen for confo-cal microscopy measurements. These sampleswere sectioned into films approximately 0.5 �mthick. Transmitted, differential interference con-trast images for these samples were acquiredwith a Leica TCS NT confocal laser scanning mi-croscope with a Leica DMIRBE inverted micro-scope with a PL APO 20� objective lens. A trans-mitted filter set, with double dichroic 488/568 andlong-pass 590 filter sets, was used with an exci-tation wavelength of 488 nm.

A JEM-100 CXII transmission electron micro-scope (JEOL USA, Inc.) operating at 60 kV wasused to characterize the phase morphology ofsome selected P(iBMA-co-BDMA-co-MA-POSS)nanocomposites. The specimens for this measure-ment were microtomed to an approximate thick-ness of 75 nm and set on a copper grid.

RESULTS AND DISCUSSION

Synthesis of the Nanocomposites

P(iBMA-co-BDMA) networks and P(iBMA-co-BDMA-co-MA-POSS) nanocomposites were pre-pared by free-radical initiation in bulk monomermixtures. The formation of the MA-POSS nano-composites is illustrated in Scheme 1. LinearP(iBMA-co-MA-POSS) copolymer nanocompos-ites were prepared for comparison. The viscoelas-tic properties (E�, Tg, and tan �) and the swellingbehavior were studied and compared.

Viscoelastic Properties of theP(iBMA-co-BDMA) Resins

The bending E� and tan � values versus the tem-perature at 1 Hz for pure PiBMA and P(iBMA-co-BDMA) resins (Fig. 1) show that the Tg and E�values increase as the BDMA content increasesfrom 1 to 3 or 5 wt %. The Tg values (tan � peaktemperature) are 85.4 °C (PiBMA), 87.3 °C(1 wt % BDMA), 90.1 °C (3 wt % BDMA), and

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92.8 °C (5 wt % BDMA). PiBMA exhibits aweaker bending tan � peak (1 Hz) than P(iBMA-co-1 wt % BDMA). The intensities decrease grad-ually as the BDMA content increases from 1 to 3or 5 wt %. E� values at 35 °C (T � Tg) of purePiBMA and its BDMA resins with 1, 3, or 5 wt %BDMA are 0.87, 1.00, 1.04, and 0.94 GPa, respec-tively. In the rubbery region (T � Tg), the E�values of all the resins are higher than those ofpure PiBMA, and they increase with the BDMAcontent, as expected. The E� values of PiBMA andits BDMA resins with 1, 3, or 5 wt % BDMA at 145

°C (T � Tg) are 0.33, 0.42, 0.72, and 0.78 MPa,respectively.

The swelling volume ratios (in THF) ofP(iBMA-co-BDMA) resins with 1 or 3 wt % BDMA(shown in Table 1) are 4.46 and 2.60, respectively,but the swelling ratio for P(iBMA-co-5 wt %BDMA) was not measured because of fracturing.The swelling ratios decrease because of highercrosslinking as the BDMA content increases from1 to 3 wt %. The crosslink density for the P(iBMA-co-BDMA) resins increases with an increase inthe BDMA loading. This is the reason that the Tg

Scheme 1. Synthesis of P(iBMA-co-BDMA-co-MA-POSS) nanocomposites.


and E� values in the rubbery region for thesecrosslinked networks increase with the BDMAcontent.

Viscoelastic Properties of the P(iBMA-co-MA-POSS) Linear Copolymers

P(iBMA-co-MA-POSS) linear copolymers werealso synthesized for later comparison withP(iBMA-co-BDMA-co-MA-POSS) crosslinked net-works with various crosslink densities. The bend-ing E� and tan � curves of these linear copolymersare shown in Figure 2. The 5 wt % (0.79 mol %),10 wt % (1.65 mol %), and 15 wt % (2.59 mol %)MA-POSS copolymers have slightly higher E� val-ues than PiBMA at both T � Tg and T � Tg.However, the copolymers with 20 or 30 wt % (3.64or 6.07 mol %) MA-POSS exhibit lower E� valuesthan PiBMA. The values of E� at 35 °C (T � Tg)for PiBMA and its copolymers with 5, 10, 15, 20,or 30 wt % MA-POSS are 0.87, 0.92, 1.00, 0.89,0.86, and 0.67 GPa, respectively. The correspond-ing E� values at 145 °C (T � Tg) are 0.33, 0.37,0.41, 0.33, 0.30, and 0.25 MPa. The 10 wt % MA-POSS copolymer exhibits the highest E� valuesover the entire temperature range. In both theglassy region and rubbery regions, the E� values

decrease with an increase in the POSS content(10, 15, 20, and 30 wt %).

PiBMA displays a weaker tan � peak in theglass-transition region than the P(iBMA-co-MA-POSS) copolymers with 5, 10, 15, or 20 wt %POSS. The tan � peak intensities drop steadily forMA-POSS loadings of 5, 10, 15, and 30 wt %.However, the 20 wt % MA-POSS copolymer’s tan� peak is more intense than those of the 10 and15 wt % POSS copolymers. Their Tg values are85.4 °C (PiBMA), 85.5 °C (5% MA-POSS), 83.0 °C(10% MA-POSS), 84.7 °C (15% MA-POSS),80.9 °C (20% MA-POSS), and 80.2 °C (30% MA-POSS). The tendency toward a drop in Tg athigher MA-POSS contents is illustrated by the 20and 30 wt % MA-POSS samples.

Viscoelastic Properties of the P(iBMA-co-BDMA-co-MA-POSS) Nanocomposites

P(iBMA-co-BDMA-co-MA-POSS) terpolymernanocomposites (Scheme 1) contain pendant MA-POSS units along the segments between crosslinksites in the P(iBMA-co-BDMA) networks. As theBDMA content increases, the mean segmentlengths decrease. The E� values at 35 and 145 °C

Figure 1. Bending E� and tan � values versus the temperature at 1 Hz for P(iBMA-co-BDMA) with 0, 1, 3, and 5% BDMA loadings.

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and the Tg values of all the linear copolymers andthe crosslinked resins are shown in Table 1.

DMTA of P(iBMA-co-1 wt % BDMA-co-MA-POSS) nanocomposites (Fig. 3) found only minorchanges in E� at T � Tg and T � Tg with 5, 10, or15 wt % MA-POSS (Table 1). The 20 wt % POSSnanocomposite has lower E� values than P(iBMA-co-1 wt % BDMA) at T � Tg but slightly higher E�values at T � Tg. The 30 wt % POSS nanocom-posite exhibits lower E� values over the entiretemperature range. At 35 °C (T � Tg), P(iBMA-co-1 wt % BDMA) and its 5, 10, 15, 20, and30 wt % MA-POSS nanocomposites exhibit E� val-ues of 1.00, 1.01, 1.01, 0.93, 0.78, and 0.88 GPa,respectively. At 145 °C (T � Tg), the correspond-ing values are 0.42, 0.40, 0.40, 0.40, 0.46, and0.28 MPa. The Tg values of P(iBMA-co-1 wt %BDMA) and its MA-POSS nanocomposites do notvary substantially until high MA-POSS levels,which induce a drop in Tg: 87.3 °C (0% MA-POSS), 84.8 °C (5% MA-POSS), 87.6 °C (10% MA-POSS), 85.2 °C (15% MA-POSS), 84.2 °C (20%MA-POSS), and 82.7 °C (30% MA-POSS). The 5wt % POSS nanocomposite exhibits a slightlymore intense tan � peak than that of P(iBMA-co-1wt % BDMA), whereas the 10, 15, 20, and 30 wt %

MA-POSS samples have less intense peaks thatdecrease with an increase in MA-POSS (see Fig.3). The P(iBMA-co-1 wt % BDMA-co-20 wt % MA-POSS) nanocomposite exhibits a more intense tan� peak than those of the 10 and 15 wt % MA-POSSnanocomposites; this trend is similar to that ob-served for the linear P(iBMA-co-MA-POSS) copol-ymers.

Increasing the crosslink density with 3 wt %BDMA raises the E� values at T � Tg above thoseof all the 1 wt % BDMA samples (Table 1 andFig. 4). In contrast to the 1 wt % BDMA series,the 30 wt % MA-POSS/3 wt % BDMA sample hasa high-temperature modulus (145 °C, 1.23 MPa)that is much higher than those of the other sam-ples in this group (0.72–0.62 MPa). In the glassyregion, however, this same sample exhibits thelowest E� values (i.e., 0.76 GPa at 35 °C) of this3 wt % BDMA series (1.04–0.79 GPa). At 35 °C (T� Tg), the E� values for P(iBMA-co-3 wt % BDMA)and its 5, 10, 15, 20, and 30 wt % MA-POSSnanocomposites are 1.04, 0.82, 0.98, 0.84, 0.79,and 0.76 GPa, respectively. At 145 °C (T � Tg),the corresponding E� values are 0.72 MPa (0%MA-POSS), 0.62 MPa (5% MA-POSS), 0.64 MPa(10% MA-POSS), 0.73 MPa (15% MA-POSS),

Figure 2. Bending E� and tan � values versus the temperature at 1 Hz for P(iBMA-co-MA-POSS) copolymers.


Figure 4. Bending E� and tan � values versus the temperature at 1 Hz for P(iBMA-co-3 wt % BDMA-co-MA-POSS) nanocomposites.


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0.67 MPa (20% MA-POSS), and 1.23 MPa (30%MA-POSS).

The Tg values of the P(iBMA-co-3 wt % BDMA-co-MA-POSS) nanocomposites with 5, 10, or15 wt % POSS remain almost constant (92.1,91.7, and 91.9 °C, respectively) and close to that ofthe P(iBMA-co-3 wt % BDMA) resin (90.1 °C).However, at higher MA-POSS loadings, the Tgvalue drops in this 3 wt % BDMA series: 87.5 °C(20% MA-POSS) and 85.9 °C (30% MA-POSS).Figure 4 illustrates the drop in the tan � peakintensities for the P(iBMA-co-3 wt % BDMA-co-MA-POSS) nanocomposites with an increase inthe MA-POSS contents. This result contrastswith the behavior of the P(iBMA-co-MA-POSS)copolymers and the P(iBMA-co-1 wt % BDMA-co-MA-POSS) nanocomposites. In addition, a minordrop in the bending E� values occurs between 61and 69 °C (T � Tg; Fig. 4) for each of the 5, 15, 20,and 30 wt % MA-POSS nanocomposites contain-ing 3 wt % BDMA. Very weak shoulders in thebending tan � curves can also be observed corre-sponding to these E� drops. This phenomenon dis-appeared completely in the second heating. Thesesub-Tg transitions might be related to MA-POSS

aggregation state changes occurring in these net-works.

The DMTA curves of the P(iBMA-co-5 wt %BDMA-co-MA-POSS) series (0–30 wt % MA-POSS) are given in Figure 5. Only the 20 wt %MA-POSS sample has higher E� values than theparent P(iBMA-co-5 wt % BDMA) resin at both T� Tg and T � Tg. The other nanocomposites ex-hibit lower E� values at T � Tg. (However, aboveTg, all the E� values are higher than those of thereference resin. Adding 5–30 wt % MA-POSS ap-proximately doubles the bending moduli in therubbery region.) Tg values are very similar for the5, 10, and 15 wt % POSS nanocomposites (92.3,93.0, and 91.0 °C) and close to that of P(iBMA-co-5 wt % BDMA) (92.8 °C). Tg values decreaseslightly for the 20 and 30 wt % POSS nanocom-posites (89.7 and 89.9 °C). The tan � peak inten-sities drop steadily as the MA-POSS content goesup in this 5 wt % BDMA series. This behavior hasalso been found in the 3 wt % BDMA series. The5 and 10 wt % MA-POSS nanocomposites (5 wt %BDMA) exhibit a small but abrupt drop in E� inthe 67–77 °C temperature range and very weakshoulders in their tan � curves (Fig. 5). This phe-



nomenon disappeared on the second heating, asobserved for the 3 wt % BDMA nanocomposites.

In summary, incorporating MA-POSS intoP(iBMA-co-BDMA) crosslinked resins affects theviscoelastic properties in different ways at differ-ent crosslink densities. At a low BDMA loading(1 wt %), the nanocomposites with 5, 10, and15 wt % MA-POSS exhibit E� values similar tothose of the P(iBMA-co-1 wt % BDMA) resin bothbelow and above Tg, whereas the 20 and 30 wt %POSS nanocomposites’ E� values are slightlylower. This effect is similar to that observed forlinear P(iBMA-co-MA-POSS) copolymers. At3 wt % BDMA, all nanocomposites show lower E�values at T � Tg than those of the parent P(iBMA-co-3 wt % BDMA) resin, and E� decreases with anincrease in MA-POSS from 10 to 15, 20, and30 wt %. However, in the rubbery region (T � Tg),little difference occurs in E� between P(iBMA-co-3 wt % BDMA) and its MA-POSS nanocompositesuntil the MA-POSS content reaches 30 wt %, atwhich point E� increases substantially. Ascrosslinking increases (5 wt % BDMA), higher E�values are found in the rubbery region when MA-POSS is present versus those of the P(iBMA-co-5 wt % BDMA) resin. However, all these nano-composites show slightly lower E� values thanthose of the P(iBMA-co-5 wt % BDMA) resin in

the glassy region (except the 20 wt % MA-POSSsample). The Tg values of all nanocomposites with5, 10, or 15 wt % MA-POSS are similar to those oftheir parent P(iBMA-co-BDMA) resins (havingequivalent weight percentages of BDMA),whereas the 20 and 30 wt % MA-POSS nanocom-posites exhibit slightly lower Tg values than thoseof their corresponding parent resins.

Effect of the Heating History on theViscoelastic Properties

The heating history has little influence on eitherP(iBMA-co-MA-POSS) copolymers or P(iBMA-co-BDMA-co-MA-POSS) nanocomposites. Represen-tative examples are displayed in Figures 6 and 7.P(iBMA-co-1 wt % BDMA-co-15 wt % MA-POSS)exhibits slightly lower E� values and a slightlyless intense tan � peak in the second heating(Fig. 6). The drop in E� values and a weak shoul-der in the tan � peak at 61.5–67.5 °C during thefirst heating disappeared completely during thesecond heating for P(iBMA-co-3 wt % BDMA-co-30 wt % MA-POSS) (Fig. 7). Furthermore, the tan� peak intensity in the second heating of thisnanocomposite is slightly smaller than those inthe first heating. The E� values of P(iBMA-co-3 wt % BDMA-co-30 wt % MA-POSS) are slightly

Figure 6. DMTA curves at 1 Hz for P(iBMA-co-1 wt % BDMA-co-15 wt % MA-POSS)nanocomposites during the first and second heating.

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higher at T � Tg and slightly lower at T � Tg inthe second heating than they are in the firstheating.

Properties of the Linear P(iBMA-co-MA-POSS)Copolymers and P(iBMA-co-BDMA-co-MA-POSS)Nanocomposites

Table 1 summarizes DMTA data (Tg and E� val-ues at 35 and 145 °C), densities, and volumeswelling ratios in THF of the linear copolymersand the P(iBMA-co-BDMA-co-MA-POSS) nano-composites. P(iBMA-co-MA-POSS) and P(iBMA-co-BDMA-co-MA-POSS) nanocomposites, at eachlevel of BDMA (1, 3, and 5 wt %) with POSSconcentrations of 15 wt % or less, exhibit Tg val-ues similar to those of PiBMA and the parentP(iBMA-co-BDMA) resins at equivalent BDMAloadings. However, copolymers and nanocompos-ites with 20 or 30 wt % POSS have slightly lowerTg values than those of PiBMA and their corre-sponding P(iBMA-co-BDMA) resins. It is clearthat large Tg increases are not achieved by at-taching these massive, anchorlike, pendant POSSfunctions to a fraction of the segmental chainsbetween crosslinks or to the linear polymerchains.

Haddad et al.4 reported the Tg values of threetypes of linear poly(styrene-co-monostyryl–POSS)copolymers; the other seven substituents on thecage silicon atoms of monostyryl–POSS wereisobutyl, cyclopentyl, or cyclohexyl groups. Thestyryl–POSS molar percentages were 4.6 (isobu-tyl), 4.3 (cyclopentyl), and 3.9% (cyclohexyl) inthese copolymers. The Tg values (tan � peak tem-peratures) of polystyrene and these three POSScopolymers were 129, 120, 131, and 138 °C, re-spectively. The E� values for these three poly(sty-rene-co-styryl–POSS) copolymers were all lowerthan those of pure polystyrene in the glassy re-gion (T � Tg). Furthermore, the E� values of thestyrene/isobutyl styryl–POSS copolymers werealso slightly lower that those of polystyrene in therubbery region (T � Tg).4 In contrast, the cyclo-pentyl and cyclohexyl styryl–POSS/styrene copol-ymers exhibited modestly higher E� values thanthose of polystyrene in the rubbery region (T� Tg). Clearly, the alkyl group has a major effecton the bulk properties of these styryl–POSS co-polymers.4 MA-POSS contains one 3-methacrylyl-propyl functional group and seven isobutylgroups. This structure is similar to that of isobu-tyl styryl–POSS. By analogy, it appears reason-able that incorporating MA-POSS into PiBMA

Figure 7. DMTA curves at 1 Hz for P(iBMA-co-3 wt % BDMA-co-30 wt % MA-POSS)nanocomposites during the first and second heating.


copolymers and the P(iBMA-co-BDMA) resins haslittle influence on the Tg values when the MA-POSS concentrations are 15 wt % or less(�4 mol %) and slightly reduces Tg values at 20and 30 wt % POSS.

The densities of all the P(iBMA-co-MA-POSS)copolymers and P(iBMA-co-1 wt % BDMA-co-MA-POSS) nanocomposites are higher than thoseof PiBMA and P(iBMA-co-1 wt % BDMA) (Table1). The densities increase with an increase in thePOSS contents. In contrast, the density does notincrease with higher MA-POSS loadings in thesystems with 3 or 5 wt % BDMA. For example,P(iBMA-co-3 wt % BDMA-co-15 wt % MA-POSS)and P(iBMA-co-5 wt % BDMA-co-MA-POSS) with5 or 10 wt % POSS have lower densities thanthose of the corresponding P(iBMA-co-BDMA)resins. Visible phase separation occurs in theP(iBMA-co-BDMA-co-MA-POSS) nanocompositeswith 3 or 5 wt % BDMA loadings. POSS aggrega-

tion might influence the densities. The free-vol-ume content is likely to vary with MA-POSS ad-dition. All the P(iBMA-co-1 wt % BDMA-co-5 to30 wt % MA-POSS) systems exhibit slightly lowerswelling ratios than that of the parent P(iBMA-co-1 wt % BDMA) resin. However, this does notseem to influence their E� or Tg values.

IR spectra of the residues isolated from theTHF extractions of the P(iBMA-co-BDMA-co-MA-POSS) nanocomposites exhibit OSiOOOabsorptions around 1111–1120 cm�1, which arecharacteristic of the T8 POSS cage. Thus, asmall fraction of the MA-POSS was incorpo-rated into the linear iBMA copolymers that arenot crosslinked into the network structure dur-ing the cure. The relative intensities ofOSiOOO absorptions for the extracted linearcopolymers increase with an increase in MA-POSS in the polymerization.

Scheme 2. Nanocomposite networks at low and high crosslink densities.

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At higher crosslink densities, the average seg-ment length between two BDMA crosslinks isshorter. Also, slightly more MA-POSS moietiesare pendant along these shorter segments than inthe nanocomposites, which have equal weightpercentages of MA-POSS but a lower crosslinkingdensity (lower BDMA content). This is due to thelower molar percentage of iBMA. This composi-tion difference is illustrated in two dimensions inScheme 2, in which the MA-POSS centers arerepresented as an inner open circle (the Si8O12core) and a gray outer region (the soft isobutylorganic surface volume).

High crosslink densities act to restrict the mo-tion of MA-POSS cores. Thus, pendant POSSunits, in turn, might more effectively restrictsome modes of iBMA backbone segmental motionat higher BDMA loadings. This might explainwhy the P(iBMA-co-5 wt % BDMA-co-MA-POSS)nanocomposites exhibit slightly higher E� values

at T � Tg than those of pure P(iBMA-co-5 wt %BDMA), whereas the reverse is found at lowerBDMA contents (1 or 3 wt %). Pendant MA-POSSgroups have a high molecular weight (901) andoccupy a large volume. Thus, MA-POSS functionshinder the segmental motion of the segments towhich they are attached at both low and highcrosslink densities. They are analogous to an an-chor attached to a jump rope; the jump-rope han-dles represent the segment ends at whichcrosslinks are present. However, one must recog-nize that the MA-POSS molar percentages inthese nanocomposites are low in all cases. In the1 wt % BDMA series, the MA-POSS molar per-centages range from 0.79 to 6.07% (5–30 wt %).Similarly, in the 5 wt % BDMA series, MA-POSSmolar contents range from 0.80 to 6.23% (5–30 wt%). Therefore, many chain segments can existwithout a POSS moiety appended. Their segmen-tal motions would not be retarded by POSS an-

Figure 8. Confocal laser optical micrographs of (a,b) P(iBMA-co-3 wt % BDMA-co-15% MA-POSS) and (c,d) P(BDMA-co-3 wt % BDMA-co-30% MA-POSS).


chors. The bulky heptaisobutyl-T8-POSS systemsmay also disrupt some chain-packing features ofthe parent P(iBMA-co-BDMA) resins, thereby in-troducing more free volume. This would lead tolower Tg values.

Morphology of the P(iBMA-co-BDMA-co-MA-POSS) Nanocomposites

POSS aggregation is another phenomena knownto occur in thermoplastic polymers10,11 and ther-mosetting resins18 containing pendant POSSgroups. Coughlin and coworkers10,11 showed thatpendant POSS moieties, randomly incorporatedinto polyethylene, could aggregate during the so-lidification of copolymer melts or upon the precip-itation of the copolymer from solutions. Highlyordered POSS domains formed in sizes and

shapes that were restricted by the attached poly-ethylene chains and the aggregation kinetics. InP(iBMA-co-BDMA-co-MA-POSS) systems with 3or 5 wt % BDMA, microsized POSS aggregationcould be observed by the naked eye. Variations inMA-POSS aggregation could also have unex-pected effects on the viscoelastic behavior.

P(BDMA-co-1 wt % BDMA-co-MA-POSS) nano-composites are completely transparent, and thissuggests molecular MA-POSS dispersion, as indi-cated in Scheme 2. However, some MA-POSSnanocomposites containing 3 or 5 wt % BDMAhave visible white particles. Confocal microscopywas used to study selected samples with visibleparticles. The optical micrographs are shownin Figures 8 and 9. Irregular areas as largeas a hundred micrometers can be observed[Figs. 8(a,c) and 9(a,c)]. Some visible irregular

Figure 9. Confocal laser optical micrographs of (a,b) P(iBMA-co-5 wt % BDMA-co-5%MA-POSS) and (c,d) P(BDMA-co-5 wt % BDMA-co-15% MA-POSS).

368 LI ET AL.

and dark POSS-rich phases with a crosslinkedresin phase interspersed can be seen [Figs. 8(b,d)and 9(b,d)]. At 5% BDMA, the composite with 15wt % MA-POSS [Fig. 9(c)] exhibits more of theseirregular areas than that containing 5 wt % MA-POSS.

TEM was also employed. No particles could beobserved for the transparent P(iBMA-co-1 wt %BDMA-co-MA-POSS) nanocomposites with 5, 15,or 30 wt % MA-POSS with TEM at magnificationsas high as 20,000�. In contrast, POSS-rich par-ticles in the 3 and 5 wt % BDMA composites were

observed by TEM (shown in Figs. 10 and 11). Alow density of particles are scattered withinP(iBMA-co-3 wt % BDMA-co-5 wt % MA-POSS)[Fig. 10(a)]. The largest particle has a diameter ofapproximately 0.8 �m. These features appear tohave darker and denser particles (or regions) in-side their boundaries. P(iBMA-co-3 wt % BDMA-co-MA-POSS) samples with 15 or 30 wt % MA-POSS exhibit irregular POSS-rich particles[Fig. 10(b–d)]. The particles in the 15 wt % MA-POSS sample [Fig. 10(b)] have diameters of ap-proximately 0.9–1.1 �m. Many POSS-rich parti-

Figure 10. TEM micrographs of (a) P(iBMA-co-3% BDMA-co-5% MA-POSS), (b,c)P(iBMA-co-3% BDMA-co-15% MA-POSS), and (d) P(iBMA-co-3% BDMA-co-30% MA-POSS).


cles are formed in the irregular phase-separatedregions of this 15 wt % POSS sample; the biggestirregular dark phase is about 2 �m [Fig. 10(c)].The sample with 30 wt % POSS [Fig. 10(d)] showsa large dark phase (ca. 4 �m). The size of thePOSS-rich particles or phases (dark features) inthe P(iBMA-co-3 wt % BDMA-co-MA-POSS) com-posites increases with the weight percentage ofPOSS.

When the crosslink density is raised with5 wt % BDMA, more rounded particles of thePOSS-rich phase were observed by TEM (Fig. 11)in samples with 5, 15, or 30 wt % MA-POSS. The

particle in Figure 11(a) within the 5 wt % POSSsample is about 1.3 �m in diameter, and there isa small particle as well (diameter � 0.06 �m). Thebig particle in Figure 11(b) is 1.6 �m, bigger thanthat in Figure 11(a). The diameters of the largerparticles in Figure 11(c,d) for the 15 wt % POSSsample are 1.1 and 1.9 �m, respectively. As thePOSS loading rises to 30 wt % while the BDMAconcentration is held at 5 wt %, increasingly big-ger particles [1.3-�m particle in Fig. 11(e) and2.9-�m particle in Fig. 11(f)] appear. The averagesize of the POSS-rich particles increases with anincrease in the POSS contents in the composites

Figure 11. TEM micrographs of (a,b) P(iBMA-co-5% BDMA-co-5% MA-POSS), (c,d)P(iBMA-co-5% BDMA-co-15% MA-POSS), and (e,f) P(iBMA-co-5% BDMA-co-30% MA-POSS).

370 LI ET AL.

with 5 wt % BDMA; this is similar to the trendfound with 3 wt % BDMA.

The observed particles occur when thecrosslink density is increased (3 and 5% BDMA),and their size and frequency increase when theMA-POSS loading is raised. (This is consistentwith MA-POSS phase separation occurring dur-ing polymerization as the entropy-of-mixing termdrops. As the BDMA concentration goes up, gela-tion also occurs earlier. Any phase-separated MA-POSS will homopolymerize and also incorporateiBMA and BDMA when radical centers polymer-ize near the periphery of the phase-separated re-gions. Thus, variations in the MA-POSS concen-tration may occur within the particles that form.These particles are chemically bonded to the con-tinuous matrix phase and are polymeric in na-ture.) A highly idealized aggregate is depicted inScheme 3; the number of MA-POSS moieties de-picted in the picture is far smaller than thatwhich is actually in the particles shown by TEM.

CONCLUSIONS

Chemically incorporating MA-POSS into theP(iBMA-co-BDMA) crosslinked resin networkinfluences the viscoelastic properties of the re-sulting nanocomposites. The effects depend on

the crosslink density, which is determined bythe BDMA loading. At a 1 wt % BDMA loading,the P(iBMA-co-BDMA-co-MA-POSS) nanocom-posites exhibit a variation of viscoelastic prop-erties versus the MA-POSS content similar tothose of linear P(iBMA-co-MA-POSS) copoly-mers. The P(iBMA-co-1% BDMA-co-10 wt %MA-POSS) nanocomposite exhibits the highestE� values of all the systems examined [higherthan those of the P(iBMA-co-1 wt % BDMA)resin]. These E� values decrease with an in-crease in the MA-POSS concentration from 10to 30 wt %. The isobutyl groups on MA-POSScan plasticize the lightly crosslinked network.This effect is more pronounced as the MA-POSSloading increases. Some compositions of the co-polymers and nanocomposites have higher E�values than the pure PiBMA and P(iBMA-co-1wt % BDMA) resin. However, at higher BDMAloadings (3 and 5 wt %), almost all the nano-composites exhibit lower E� values in the glassyregion than those of the corresponding P(iBMA-co-BDMA) resins. The behavior is complex.

The P(iBMA-co-3 wt % BDMA-co-MA-POSS)nanocomposites with 15 or 30 wt % MA-POSShave higher E� values than the parent P(iBMA-co-3 wt % BDMA) resin in the rubbery region. TheE� values of all P(iBMA-co-5 wt % BDMA-co-MA-POSS) nanocomposites in the rubbery region (T� Tg) are higher than those of the P(iBMA-co-5 wt % BDMA) resin and increase with an in-crease in the POSS concentration. The MA-POSSmoieties begin to dominate, perhaps because ofthe retardation of segmental motions by pendantMA-POSS groups. These restrictions becomemore serious at high crosslink densities (shorteraverage segment lengths).

The linear P(iBMA-co-MA-POSS) copolymersand all P(iBMA-co-BDMA-co-MA-POSS) nano-composites (1, 3, and 5 wt % BDMA) with 15 wt %or less MA-POSS exhibit Tg values similar tothose of the corresponding P(iBMA-co-BDMA)resins. The 20 and 30 wt % MA-POSS copolymersand nanocomposites exhibit slightly lower Tg val-ues than those of PiBMA or the correspondingP(iBMA-co-BDMA) resins. The heating historyhas little influence on the viscoelastic propertiesof P(iBMA-co-BDMA) linear copolymers andP(iBMA-co-BDMA-co-MA-POSS) nanocompos-ites.

A referee suggested that the interaction be-tween the soft volume of the isobutyl peripheryand iBMA may dominate the majority of the prop-erty changes at lower MA-POSS loadings, but

Scheme 3. Idealized MA-POSS aggregate particlewith a dense, mostly MA-POSS inner region formed byMA-POSS polymerization after phase separation. Thesurrounding regions contain more iBMA and BDMA,but the MA-POSS concentration is still higher thenthat in the continuous phase.


when a sufficient volume percentage MA-POSSloading is reached, then sphere–sphere (POSS)interactions dominate. This hypothesis is reason-able. However, at present, the situation seemsmore complex. As the MA-POSS loading in-creases, phase separation during polymerizationalso occurs (at 3 and 5% BDMA). This generateshigh MA-POSS content polymerized particlesthat are bonded to the surrounding matrix. Thesemust influence the properties.

POSS aggregates are not observed in theP(iBMA-co-1 wt % BDMA-co-MA-POSS) nano-composites by TEM. However, POSS-rich parti-cles are formed in the POSS nanocompositeswith 3 or 5 wt % BDMA. POSS-rich particlesseveral micrometers in diameter are present inthe 3 and 5 wt % BDMA composites (TEM andconfocal microscopy), and their size increases asthe POSS contents increase. MA-POSS is solu-ble in solutions of iBMA and BDMA. As poly-merization and crosslinking proceed, the MA-POSS solubility decreases, in part because ofentropy-of-mixing factors. Thus, a competitionbetween MA-POSS incorporation into the devel-oping polymer matrix and phase separation (fol-lowed by polymerization within the separatedphases) occurs. The phase-separated regionscan have various amounts of iBMA and BDMA,and the composition can vary within given par-ticles. A complex particle formation dependenceon the MA-POSS loading and the crosslinking(BDMA loading) exists.

This work was supported by the Air Force Office ofScientific Research (F496200210260) and by the Na-tional Science Foundation (EPSO132618).

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