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 conven- tional 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 montmo- rillonite 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.