Nanostructure and Properties of Polysiloxane-Layered Silicate Nanocomposites

SHELLY D. BURNSIDE,* EMMANUEL P. GIANNELIS Department of Materials Science and Engineering, Bard Hall, Ithaca, New York 14850

Received 21 June 1999; revised 28 March 2000; accepted 28 March 2000

ABSTRACT: The relationship between nanostructure and properties in polysiloxane layered silicate nanocomposites is presented. Solvent uptake (swelling) in dispersed nanocomposites was dramatically decreased as compared to conventional composites, though intercalated nanocomposites and immiscible hybrids exhibited more conventional behavior. The swelling behavior is correlated to the amount of bound polymer (bound rubber) in the nanocomposites. Thermal analysis of the bound polymer chains showed an increase and broadening of the glass-transition temperature and loss of the crystallization transition. Both modulus and solvent uptake could be related to the amount of bound polymer formed in the system. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1595–1604, 2000 Keywords: nanocomposites; elastomers; swelling; bound polymer

INTRODUCTION not observed in samples reinforced with fumed silica. Mark and coworkers have also noted sig- Recent interest in composites with nanoscale or nificant enhancements in ultimate strength and molecular dimensions has grown as researchers rupture energy in poly(dimethyl siloxane) strive to extend the utility of these materials.1–8 (PDMS) reinforced with nanosize, in situ sol-gel Polymer matrix nanocomposites in particular derived silica when compared to PDMS compos- have been studied extensively as workers dis- ites filled with fumed silica.1–3 cover improvements in physical properties both Of particular interest are layer inorganic ma- through optimizing molecular interactions be- terials which can act as nanofillers. Conducting tween reinforcer and matrix and through reduc- polymers have been combined with layered inor- tion in size of the reinforcing agent. For example, ganics to form anisotropically conducting nano- poly(vinyl acetate) nanocomposites reinforced composites as potential battery materials.9–12 with in situ sol-gel derived silica demonstrate re- Polymer layered silicate nanocomposites also ex- markably higher tensile properties above the hibit dramatic property enhancements in perme- glass-transition temperature (Tg) when compared ability, modulus, thermal stability, and flame re- to samples reinforced with conventional fumed tardancy.13–18 These enhancements have been at- silica.4 The enhancements were attributed to ex- tributed to the large aspect ratios provided by tensive hydrogen bonding between the polymer layered materials with thicknesses on the order of and the in situ derived silica samples, which was a nanometer and widths and lengths on the order of 100–1000 nm. Elastomeric nanocomposites represent an in- *Present address: Laboratory of Photonics and Interfaces, 17–22 Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, teresting subgroup. There is a rich history of Switzerland elastomers filled with traditional fillers such as Correspondence to: E. P. Giannelis (E-mail: epg2@cornell. carbon black, fumed silica, or minerals.23 The edu) polymer/reinforcer interface has been found to Journal of Polymer Science: Part B: Polymer Physics, Vol. 38, 1595–1604 (2000) © 2000 John Wiley & Sons, Inc. play an overwhelmingly crucial role in determin- 1595 1596 BURNSIDE AND GIANNELIS

Table I. Relative Amounts of Reactans Used in the Synthesis of Siloxane Nanocomposites

Mass Mass Weight % Volume Fraction Water TEOS Tin(II) Polymer (g) Silicate (g) Silicate Silicate (␾) (␮L) (␮L) Ethylhexanoae (␮L)

10 00 0305 0.995 0.005 0.5 0.0019 1 30 5 0.99 0.01 1 0.0037 1 30 5 0.97 0.03 3 0.011 3 29 5 0.95 0.05 5 0.022 5 28 5 0.93 0.07 7 0.028 7 28 5 0.9 0.1 10 0.040 10 27 5

ing reinforcement levels and concomitant prop- preparations 1–10 ␮L of deionized water or abso- erty enhancements.24 The level of reinforcement lute ethanol per gram of silicate were also added depends on the interaction between the inorganic to aid dispersion. Tetraethylorthosilicate (TEOS, and polymer and the surface area available for Aldrich) was used as a crosslinking additive, and interaction, and is typically reflected in properties tin (II) ethylhexanoate (Gelest) served as a cata- such as modulus, swelling, abrasion resistance, lyst. For every gram of polymer, 30 ␮L of TEOS and strength. Models have been developed to pre- and 5 ␮L of tin ethylhexanoate were used. Sam- dict interfacial strength from swelling behavior.25 ples were crosslinked at room temperature under In previous work, poly(dimethylsiloxane) vacuum to remove both gases generated during (PDMS) nanocomposites reinforced with organi- crosslinking and air pockets introduced into the cally modified layered silicates were synthesized mixture during sonication. Products were allowed and their thermal stability investigated.17 In this to cure for a minimum of 12 h. A copolymer of paper, we present the mechanical properties and poly(dimethylsiloxane) with poly(diphenylsilox- ϭ ϭ solvent uptake of these novel materials and dis- ane) (Mw 25,000, Mw/Mn 1.5, PDPS–14–18 cuss them in terms of their nanostructure. mol %, United Chemicals) was used as an alter- nate polymer matrix. Sodium montmorillonite (CEC ϭ 0.9 meq/g, Southern Clay Products) was EXPERIMENTAL substituted for the organically modified montmorillonite in some mixtures with PDMS. Nanocomposite Synthesis ␣ ␻ ϭ , -silanol terminated PDMS (Mw 19,000, ϭ Characterization Mw/Mn 1.5 from United Chemical Technologies) was used as received, and was combined neat X-ray diffraction patterns were obtained on a with the proper amount of dimethylditallow-ex- Scintag X-ray diffractometer using Cu K⅐ radia- changed montmorillonite (Southern Clay Prod- tion (⅐ ϭ 1.5406 Å). ucts) by sonicating the mixture for 2 min with an Thermal transitions were probed using a Seiko ultrasound probe (Sonics and Materials Vibra- 5200 differential scanning calorimeter (DSC) Cell). The dimethylditallow montmorillonite is a cooled with liquid nitrogen in an atmosphere of commercial product, which was synthesized by flowing nitrogen. Samples were scanned at a rate ion-exchanging Naϩ-montmorillonite with a cat- of 10 °C/min in a nitrogen atmosphere over a ion exchange capacity of 0.9 meq/g with dimethyl temperature range of Ϫ150 °C to room tempera- ditallow ammonium bromide containing 70, 25, 4, ture. Samples were initially cooled to Ϫ90 °C at a and1mol%ofC18,C16,C14, and C12 carbon rate of 10 °C/min and left to equilibrate for 5 min chains, respectively. The concentration of the or- to allow for crystallization. After cooling to Ϫ150 ganosilicate in the nanocomposites ranged from °C, the samples were allowed an additional 5 min 0.5–10 wt % . Table I lists the relative amounts of for equilibration. A PerkinElmer System 7e dy- reactants per gram of nanocomposite. Volumes namic mechanical analyzer (DMA) equipped with were carefully measured using a Fisher Microtip a liquid nitrogen dewar in a helium atmosphere Pipette. When larger samples were required, the was used to characterize mechanical behavior. amounts were multiplied appropriately. In some The modulus of crosslinked samples was mea- POLYSILOXANE-LAYERED SILICATE NANOCOMPOSITES 1597

sured as a function of temperature while increas- nanocomposites, as suggested by the lack of dis- ing the temperature at a rate of 15 °C/min. The tinct reflections in the XRD pattern shown in instrument was operated in the tensile mode at a Figure 1. The original (001) peak of the layered constant frequency of 1 Hz and at a strain of 0.7%. silicate before mixing with PDMS is shown for Solvent uptake was measured on crosslinked comparison with a d-spacing of 25Å. The lack of samples using previously published techniques.26 layer registry and order evidenced by the lack of A sample was weighed and then immersed in X-ray diffraction pattern in the hybrid is sugges- toluene. After three days, the swollen sample was tive of layer delamination and the formation of rapidly blotted and reweighed, minimizing evap- dispersed nanocomposites. Hybrids prepared oration of the toluene. The sample was reim- from a PDMS-poly(diphenyl siloxane) random co- mersed in toluene. Reweighing continued for suc- polymer containing 14–18 mol % diphenylsilox- cessive days and the final weight of the swollen ane and a dimethylditallow-exchanged layered sample was recorded when three weighings silicate show a distinct (001) peak at 39Å, sug- agreed with each other, indicating the establish- gesting that the copolymer chains have interca- ment of an equilibrium. lated into the layers increasing the d-spacing but The amount of bound polymer was determined have not caused layer delamination as in the thermogravimetrically using established proce- PDMS case. (Hybrids that maintain the layer reg- dures.27 Uncrosslinked samples were put in an istry of the silicate particles are termed interca- excess of toluene, a good solvent for PDMS (10–20 lated.) In contrast, no intercalation or dispersion ϫ the sample weight), and stirred rapidly at tem- takes place when pristine (sodium-exchanged) peratures between 30 and 40 °C to remove the montmorillonite is combined with either PDMS or unbound PDMS chains. The toluene was ex- the PDMS-PDPS copolymer leading to immiscible changed daily for 5 days. The remaining gel was hybrids. Recent experimental and theoretical put in a vacuum oven at 100 °C overnight to work has shown that the interactions between the remove excess toluene. TGA analysis of the gel polymer matrix and the silicate can be tailored to was performed from room temperature to 800 °C produce a range of microstructures, from immis- in a nitrogen atmosphere, degrading the polymer cible to dispersed.28 This and the work cited above chains and alkyl ammonium chains but leaving underscore the importance of tailoring the poly- the silicate layers intact. A PerkinElmer TGA 7 mer–silicate interactions in order to optimize sil- was used for the thermogravimetric analysis. The icate delamination and dispersion. amount of bound polymer was calculated from the TGA data after correcting for the mass of the Solvent Uptake and Determination of Bound alkyl ammonium cations in the organosilicate. Rubber Equilibrium swelling data in toluene for different RESULTS AND DISCUSSION siloxane nanocomposites as a function of silicate loading are shown in Figure 2. Vro/V rf , where the subscripts o and f refer to the unfilled network Synthesis and Structure Determination and nanocomposite respectively, is the relative Siloxane nanocomposites are prepared by sus- solvent uptake calculated using pending the organosilicate in a free-flowing siloxane at room temperature and sonicating. Previ- volume of unswollen polymer matrix V ϭ ously, we showed that the nanostructure of r volume of swollen polymer matrix PDMS-silicate hybrids is determined by the type Ϫ 17 Ϫ ␳ 1 of organic cation in the silicate. The degree of ms m PDMS ϭ ͫ1 ϩ ͩ ͪͩ ͪͬ (1) silicate dispersion is usually obtained using X-ray m ␳Toluene diffraction (XRD) measurements. Intense reflec- ␪ ϭ tions in the range 2 3–9° indicate a well- ms and m are the weight of the swollen and un- ␳ ordered intercalated hybrid with alternating swollen samples, respectively, and Toluene and ␳ polymer–silicate layers. In contrast, dispersed or network are the corresponding densities. As nor- exfoliated hybrids exhibit no distinct features in mal in elastomer literature, the swelling values this 2␪ range due to the lack of layer registry. are plotted as a function of ␾/(1 Ϫ ␾), where ␾ is Organically modified silicates containing dimeth- the volume fraction of silicate in the hybrid to ylditallow cations lead to dispersed or exfoliated correct for the inability of the inorganic material 1598 BURNSIDE AND GIANNELIS

Figure 1. X-ray diffraction patterns of intercalated and disordered or exfoliated nanocomposites. The layered silicate is a dimethylditallow ammonium-exchanged montmorillonite shown also for comparison. The intercalated nanocomposite contains 10 wt % silicate and a copolymer of poly(dimethylsiloxane) with poly(diphenylsiloxane) [PDPS-14-18% (mol), remainder PDMS]. The disordered nanocomposites contains 10% (wt) silicate and poly(dimethylsiloxane). to swell. For the small volume fractions used in physisorption, or in extreme cases, chemisorption this study, this correction is minimal. The dis- of the polymer on the filler surface.25 persed nanocomposites exhibit greatly reduced Table II quantifies the bound rubber formed in solvent uptake when compared to intercalated or the uncrosslinked dispersed hybrid with varying immiscible hybrids. In the dispersed hybrids, the silicate loading, from TGA analysis of the gel solvent uptake initially decreases dramatically, (bound polymer and silicate) remaining after ex- then begins to level off as silicate loadings ap- traction of an uncrosslinked nanocomposite with proach 2%. Similar plateaus in solvent uptake toluene, a good solvent for PDMS. The initial have been previously noted in other nanocompos- dramatic increase with final plateau mirrors the ites.20,21 In contrast to the disprersed nanocom- swelling behavior for this system, suggesting an posites, the intercalated and immiscible systems inverse relation between solvent uptake and decrease quasilinearly over the same loading bound rubber formation in the nanocomposite. range. This behavior mimics that of conventional Interestingly, beyond an initial plateau, the elastomeric composites such as styrene (buta- amount of bound rubber per silicate layer de- diene) rubber (SBR) filled with carbon black, creases with increased silicate content (Table II where the solvent uptake expressed as Vro/Vrf is a and Fig. 3). We suggest this decrease arises from linear function of ␾/(1 Ϫ ␾) (Fig. 2). In that possible overlap of bound polymer regions as the case the slope corresponds to the interfacial amount of silicate content increases. strength.25 Conventional composites with carbon black In conventional elastomeric composites the re- display roughly 30–40% bound rubber at filler duction in solvent uptake with increasing filler concentrations of 20–30% (vol).24 In dispersed content has been correlated to the formation of nanocomposites comparable amounts of bound bound rubber, viz., polymer chains at the inter- polymer can be achieved at roughly one twentieth face, which are affected by the filler. The forma- of the inorganic loading. We attribute the large tion of bound rubber has been attributed to either amount of bound rubber to the large surface area POLYSILOXANE-LAYERED SILICATE NANOCOMPOSITES 1599

Figure 2. Toluene uptake in crosslinked siloxane systems. The disordered and immiscible nanocomposites contain dimethylditallow ammonium-exchanged montmorillonite and sodium montmorillonite, respectively, in a PDMS matrix. The intercalated nanocomposites contain dimethylditallow ammonium-exchanged montmorillonite in a PDMS/PDPS copolymer matrix. For comparison the solvent uptake of styrene–butadiene rubber (SBR) ﬁlled with carbon black is also shown.

available in the layered silicate system (750 m2/g tem. Since the swelling behavior practically mir- for montmorillonite29) as compared to that of con- rors the amount of bound polymer in the nano- ventional ﬁllers, such as unaggregated carbon composite we suggest that the swelling behavior black (13–100 m2/g24). This large surface area of arises from the nanostructure rather than from the silicates can only be exploited in the dispersed increased crosslinks in the network. Other ﬁllers, or exfoliated nanocomposites. notably carbon black, have been shown to in- We note that the measurement of bound rub- crease the activity of crosslinking additives in the ber has been performed on an uncrosslinked sys- network-forming system and result in increased crosslink density.30 As enhanced moduli and decreased solvent uptake can also be due to in- Table II. Bound Polymer in PDMS Disordered creased crosslink density, it is important to dif- Nanocomposites ferentiate between the two causes when attempt- ing to understand the behavior of a system. Bound Bound Rubber Fraction Rubber per Layera ␾ Fraction (1 ϫ 106 nm3/layer) Thermal Transitions

00 0 Differential scanning calorimetry (DSC) was used 0.002 0.08 40 to probe the crystalline and glass transitions in 0.004 0.16 40 the nanocomposites (Table III ). Below room tem- 0.01 0.54 27 perature, the pure siloxane exhibits a glass tran- 0.03 0.57 19 ϭϪ sition (Tg 125 °C), a “cold” crystallization (Tc 0.05 0.60 14 ϭϪ 84 °C), and two melting transitions (Tm1 ϭϪ ϭϪ aCalculated using an estimated volume per silicate layer of 47, Tm2 35 °C), in good agreement with 1.0 ϫ 10Ϫ21 m3 and assuming a homogeneous composite. literature data for PDMS.31 The dual melting 1600 BURNSIDE AND GIANNELIS

Figure 3. Bound rubber fraction (triangles) and volume of bound polymer per silicate layer (circles) in PDMS nanocomposites as a function of silicate loading. transitions are thought to reflect differences in transitions remain virtually unchanged but the crystallite size or perfection due to defects from heat of fusion decreased. PDMS filled with fumed 32 chain ends. The nanocomposites display slightly silica exhibit lower enthalpies and higher Tm,as 34,35 higher Tg and broader transitions than the pure measured using calorimetry. Crosslinked net- PDMS. PDMS filled with in situ generated SiO2 works containing in situ generated PDMS exhibit 32 and TiO2 also show slight increases in glass-tran- no cold crystallization transition. It has been sition temperature as previously noted.33 suggested that fillers accelerate the rate of crys- In contrast, the crystallization process is af- tallization due to either orientation of the ad- fected more in the nanocomposites. The cold crys- sorbed layer (bound polymer) or lower activation tallization temperature remains fairly constant energy of nucleation due to the silica parti- but the enthalpy of crystallization is dramatically cles.34,35 Decreases in the enthalpy of fusion have decreased (Table III). In addition, the melting been shown to only qualitatively relate to the

Table III. Thermal Transitions in PDMS Disordered Nanocomposites by DSC

⌬ ⌬ ⌬ Tg Tc Hc Tm1 Hm1 Tm2 Hm2 ␾ (°C) (°C) (mJ ⅐ mgϪ1) (°C) (mJ ⅐ mgϪ1) (°C) (mJ ⅐ mgϪ1)

Uncrosslinked Nanocomposites 0 Ϫ124.7 Ϫ84.4 Ϫ2.5 Ϫ47.3 6.0 Ϫ35.2 11.7 0.011 Ϫ123.2 Ϫ84.4 Ϫ1.7 Ϫ47.4 5.5 Ϫ35.3 11.5 0.028 Ϫ123.0 Ϫ84.6 Ϫ1.8 Ϫ47.6 5.3 Ϫ35.6 11.6 0.04 Ϫ122.8 Ϫ84.5 Ϫ1.6 Ϫ47.7 5.2 Ϫ35.7 11.2 Bound Polymer 0.011 Ϫ114.3 ——Ϫ48.1 2.9 Ϫ35.6 4.9 (broad) 0.028 Ϫ107.8 ——Ϫ48.2 3.5 Ϫ35.7 4.8 (broad) 0.04 — — — Ϫ48.3 3.2 Ϫ36.1 4.4 POLYSILOXANE-LAYERED SILICATE NANOCOMPOSITES 1601

Table IV. Storage Modulus and Thermal the cold crystallization transition and the melting Transitions in PDMS Disordered Nanocomposites by temperatures are slightly lowered, with de- DMA creased enthalpies of fusion. The bound polymer formed in fumed silica/PDMS composites simi- T T Youngs Modulus alpha crystalline larly exhibit neither a glass transition nor a crys- ␾ (°C) (°C) (MPa) tallization temperature.36 With a maximum 0 Ϫ110 Ϫ73 1.0 bound polymer fraction of 0.27, these fumed silica 0.002 N/Aa N/A 1.3 composites have enthalpies of fusion, which de- 0.004 N/A N/A 1.6 crease by roughly one third as compared to pure 0.022 Ϫ107 —b 2.5 PDMS. The decrease in enthalpy of fusion is 0.028 Ϫ105 — 2.8 larger for the layered silicate nanocomposite as 0.040 Ϫ100 — 3.0 compared to the fumed silica containing hybrids qualitatively reﬂecting the greater amounts of aMeasurements not taken. bTransition temperature not observed or too broad to ac- bound polymer formed with layered silicate rein- curately determine. forcement.

Dynamic Mechanical Analysis amount of bound polymer, so a more detailed analysis is not presented here.35 Dynamic mechanical analysis (DMA) was used to To further characterize the nature of the bound measure viscoelastic properties as a function of polymer calorimetry data were also obtained from temperature in both unﬁlled and PDMS nano- hybrids were the noninteracting polymer (non- composites (Table IV). The thermal transitions bound polymer) has been removed (Table III). were calculated from plots of tan ␦. Figure 4 com- These samples exhibit a very broad glass-transi- pares the unﬁlled PDMS network and a dispersed tion temperature, at temperatures generally nanocomposite containing 4% silicate. The unre- higher than the pure PDMS. In the sample with inforced sample displays two transitions, one at the highest concentration of silicate (0.04) the Ϫ110 °C, corresponding to the ␣ or glass transi- transition was too broad to measure. Addition- tion, and one at Ϫ73 °C, corresponding to the ally, no enthalpy of crystallization is observed for crystalline transitions. These temperatures corre-

Figure 4. DMA data for a crosslinked PDMS disordered nanocomposite containing 4% silicate (dashed line) and a reference PDMS sample (solid line). 1602 BURNSIDE AND GIANNELIS

Figure 5. Bound rubber fraction (triangles) and relative modulus (circles) as a function of silicate loading for PDMS disordered nanocomposites. Inset shows the direct correlation of bound rubber with relative modulus. late well with those previously observed in pure ride. We note that the level of reinforcement nec- PDMS samples using DMA.37 In contrast, the essary to observe broadened transitions in either nanocomposites exhibit a less well-defined tran- of the previous studies is well above that in lay- sition at Ϫ107 to Ϫ100 °C. The crystalline tran- ered silicate nanocomposites, a reflection of the sitions have also been extremely broadened, such high amounts of bound polymer formed at low that separating the ␣ transition from the crystal- concentrations of layered silicate. line transition is extremely difficult and no tem- Figure 4 shows the DMA pattern for PDMS perature assignments were attempted. Siloxane and a dispersed PDMS nanocomposite containing systems filled with fumed silica also exhibit ex- 4% silicate. Samples were observed from Ϫ150 °C tensively broad tan ␦ peaks. Additionally, sam- to approximately Ϫ25 °C, and between room tem- ples at loadings at or in excess of 20 wt % fumed perature and 100 °C, to enable the analyzer pa- silica exhibit a broad second thermal transition rameters to be optimized for the three decade loss 40–50 °C above the glass transition, which ex- in modulus. As no thermal transitions are ob- tends for roughly 75 °C.38 This thermal transition served between room temperature and 100 °C, has been attributed to the polymer closely inter- tan ␦ data for this region is omitted for clarity. acting with the silica filler, what we have termed Below the glass transition, roughly a 40–50% in- the bound polymer. The siloxane polymer used in crease in modulus is observed in the nanocompos- the fumed silica study was a copolymer of PDMS ite as compared to the unfilled polymer. From with 7 mol % methylphenyl siloxane to inhibit room temperature to 100 °C, storage moduli were crystallization of the PDMS and allow observa- temperature independent, as expected for tion of the peak arising from the bound polymer. crosslinked elastomers. The nanocomposites dis- Other workers have studied pure PDMS filled played moduli higher than the unreinforced with fumed silica (14–31 vol %) or sodium chlo- PDMS, with the maximally reinforced system ex- ride (48 vol %), reporting that fumed silica filled hibiting a modulus roughly three times that of the samples exhibit broadened thermal transitions polymer (Table IV). while the sodium chloride filler did not broaden or Figure 5 compares the amount of bound poly- shift the transitions.37 The difference between the mer to the relative modulus as a function of sili- two systems was attributed to polymer-fumed sil- cate in the nanocomposites. Modulus increases ica interactions, which are absent in sodium chlo- seen in other filled polymer systems have been POLYSILOXANE-LAYERED SILICATE NANOCOMPOSITES 1603 attributed to entanglements, temporary restric- 2. McCarthy, D. W.; Mark, J. E.; Clarson, S. J.; tions, which prevent the easy movement of the Schaefer, D. W. J Polym Sci Part B: Polym Phys polymer chains past one another, between the 1998, 36, 1191. free and the bound polymer. For example, poly- 3. Sun, C.-C.; Mark, J. E. Polymer 1989, 30, 104. (ethylene) composites filled with silica do not ex- 4. Landry, C. J. T.; Coltrain, B. K.; Landry, M. R.; hibit any reinforcement, as measured using tear Fitzgerald, J. J.; Long, V. K. Macromolecules 1993, 26, 3702–3712. strength, until the bound polymer chains are long 39 5. Giannelis, E. P. Adv Mater 1996, 8, 29. enough to allow for entanglements. Addition- 6. Wang, Z.; Pinnavaia, T. J. Chem Mater, in press. ally, a mean-field-like tube model has been devel- 7. Jose-Yacaman, J.; Rendon, L.; Arenas, J.; Puche, oped to describe polymer-filler junctions in carbon M. C. S. Science 1996, 273, 223–225. black filled poly(butadiene) and SBR where mod- 8. See references in Chem Mater 1996, 8, 1569–1840. ulus increases are concluded to emanate from 9. Vaia, R. A.; Vasudevan, S.; Krawiec, W.; Scanlon, entanglements between the bound polymer and L. G.; Giannelis, E. P. Adv Mater 1995, 7, 154–156. the free polymer.40 Thus, we suggest that the 10. Hutchison, J. C.; Bissessur, R.; Shriver, D. F. Chem modulus enhancements observed herein also Mater 1996, 8, 1597–1600. arise from interactions between the bound and 11. Kerr, T. A.; Wu, H.; Nazar, L. F. Chem Mater 1996, the free polymer in the matrix. 8, 2005. 12. Kanatzidis, M. G.; Bissessur, R.; DeGroot, D. C.; Schindler, J. L.; Kannewurf, C. R. Chem Mater CONCLUSIONS 1993, 5, 595–96. 13. Messersmith, P. B.; Giannelis, E. P. Chem Mater The mechanical properties and swelling behavior 1994, 6, 1719–1725. of PDMS nanocomposites is discussed in terms of 14. Okada, A.; Kawasumi, M.; Usuki, A.; Koyima, Y.; their nanostrucure. Solvent uptake (swelling) in Kurauchi, T.; Kamigaito, O. Mater Res Soc Symp dispersed nanocomposites was dramatically de- Proc 1990, 171, 45. creased as compared to conventional composites, 15. Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. though intercalated nanocomposites and immis- Chem Mater 1990, 8, 1728. cible hybrids exhibited more conventional behav- 16. Lan, T.; Pinnavaia, T. J. Chem Mater 1994, 6, 2216. 17. Burnside, S. D.; Giannelis, E. P. Chem Mater 1995, ior. The swelling behavior is correlated to the 7, 1597. amount of bound polymer (bound rubber) in the 18. Lee, J. D., unpublished work, Cornell University, nanocomposites. Thermal analysis of the bound 1998. polymer chains showed an increase and broaden- 19. Wang, Z.; Lan, T.; Pinnavaia, T. J. Presented at the ing of the glass-transition temperature and loss of National Meeting of the Materials Research Soci- the crystallization transition. Both modulus and ety, Boston, MA, December 1996; Paper V5.20. solvent uptake could be related to the amount of 20. Wen, J.; Mark, J. E. Rubber Chem Technol 1995, bound polymer formed in the system, indicative of 67, 806. the large influence of bound polymer in these 21. Fukumori, K.; Usuki, A.; Sato, N.; Okada, A.; nanocomposite systems as compared to more con- Kurauchi, T. Advanced Materials for Future Indus- ventional composites. tries: Needs and Seeds. Proc. 2nd Japan Interna- tional SAMPE 89, Tokyo, Japan, 1991. 22. Wen, J.; Wilkes, G. Chem Mater 1996, 8, 1667. This work was sponsored by Office of Naval Research. The authors also acknowledge support from Xerox and 23. Kraus, G. Reinforcement of Elastomers; Inter- Dow-Corning. SDB acknowledges financial support science Publishers: New York, 1965. from a Department of Defense fellowship. They also 24. Kraus, G. In Science and Technology of Rubber; thank Prof. C. Cohen, Prof. J. Mark, Prof. R. Krish- Eirich, F., Ed.; Academic Press: New York, 1978. namoorti, Prof. E. Manias, and Dr. R. Vaia for many 25. Kraus, G. 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