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Three polystyrene (PS)/clay hybrid systems have been prepared via in situ polymerization of styrene in the presence of unmodified sodium montmorillonite (Na‐MMT) clay, MMT modified with zwitterionic cationic surfactant octadecyldimethyl betaine (C18DMB) and MMT modified with polymerizable cationic surfactant vinylbenzyldimethyldodecylammonium chloride (VDAC). X‐ray diffraction and TEM were used to probe mineral layer organization and to expose the morphology of these systems. The PS/Na‐MMT composite was found to exhibit a conventional composite structure consisting of unintercalated micro and nanoclay particles homogeneously dispersed in the PS matrix. The PS/C18DMB‐MMT system exhibited an intercalated layered silicate nanocomposite structure consisting of intercalated tactoids dispersed in the PS matrix. Finally, the PS/VDAC‐MMT system exhibited features of both intercalated and exfoliated nanocomposites. Systematic statistical analysis of aggregate orientation, characteristic width, length, aspect ratio, and number of layers using multiple TEM micrographs enabled the development of representative morphological models for each of the nanocomposite structures. Oxygen barrier properties of all three PS/clay hybrid systems were measured as a function of mineral composition and analyzed in terms of traditional Nielsen and Cussler approaches. A modification of the Nielsen model has been proposed, which considers the effect of layer aggregation (layer stacking) on gas barrier. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1733–1753, 2007

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The synthesis of nanocomposites via emulsion polymerization was investigated using methyl methacrylate (MMA) monomer, 10 wt % montmorillonite (MMT) clay, and a zwitterionic surfactant octadecyl dimethyl betaine (C18DMB). The particle size of the diluted polymer emulsion was about 550 nm, as determined by light scattering, while the sample without clay had a diameter of about 350 nm. The increase in the droplet size suggests that clay was present in the emulsion droplets. X-ray diffraction indicated no peak in the nanocomposites. Transmission electron microscopy showed that emulsion polymerization of MMA in the presence of C18DMB and MMT formed partially exfoliated nanocomposites. Differential scanning calorimetry showed an increase of 18 °C in the glass transition temperature (T g) of the nanocomposites. A dynamic mechanical thermal analyzer also verified a similar T g increase, 16 °C, for the partially exfoliated nanocomposites over poly(methyl methacrylate) (PMMA). Thermogravimetric analysis indicated a 37 °C increase in the decomposition temperature for a 20 wt % loss. A PMMA nanocomposite with 10 wt % C18DMB−MMT was also synthesized via in situ polymerization. This nanocomposite was intercalated and had a T g 10° lower than the emulsion nanocomposite. The storage modulus of the partially exfoliated emulsion nanocomposite was superior to the intercalated structure at higher temperatures and to the pure polymer. The rubbery plateau modulus was over 30 times higher for the emulsion product versus pure PMMA. The emulsion technique produced nanocomposites of the highest molecular weight with a bimodal distribution. This reinstates that exfoliated structures have enhanced thermal and mechanical properties over intercalated hybrids.

Five styrene-butadiene rubber (SBR)/clay nanocomposite or hybrid systems were synthesized via mechanical mixing of SBR using a Brabender mixer and a 2-roll mill in the presence of unmodified sodium montmorillonite (Na-MMT) clay, MMT modified with octadecylamine (C18amine), MMT modified with a zwitterionic surfactant, octadecyldimethyl betaine (C18DMB), and MMT modified with a polymerizable cationic surfactant, vinylbenzyl octadecyldimethyl ammonium chloride (VODAC) or vinylbenzyl dodecyldimethyl ammonium chloride (VDAC). The surfactant chain length and functional groups affected the dispersion of clay nanolayers in the matrix and the overall properties of the nanocomposites. X-ray diffraction (XRD) revealed peaks corresponding to intercalated structures; transmission electron microscopy (TEM) observations agreed well with XRD assessment of the composites. SBR/VODAC-MMT system exhibited the best dispersion among the nanocomposites studied. VODAC-MMT was partially exfoliated in SBR matrix and the average aspect ratio of the nanolayer stacks or aggregates was high (20). Depending on the amount of clay, considerable mechanical reinforcement and gas barrier enhancement were achieved in nanocomposites over pure rubber. Tensile strength in excess of 18MPa was observed in SBR nanocomposites with 30 phr C18 organoclays. The storage modulus at 25C increased by a factor of four by incorporating 10 phr VODAC-MMT in SBR. The most pronounced oxygen barrier enhancement was again observed in SBR/VODAC-MMT nanocomposite with the reduction of permeability by 60% at silicate volume fraction of 0.06. The superior performance of nanocomposites containing VODAC-MMT is attributed to the presence of the vinyl-benzyl group and 18 carbon-atom tail in the surfactant leading to high compatibility with SBR and nano-scale dispersion in the SBR matrix.

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