Nanocomposites of poly(acrylonitrile-butadiene-styrene) and montmorillonite clay: dispersion and mechanical properties
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Polymer/montmorillonite clay (MMT) nanocomposites have produced significant commercial interest due to the excellent balance of properties, but the issues controlling proper clay dispersion are poorly understood. Current studies examine the effects of polymer and organoclay structure on properties of melt-processed poly(styrene-coacrylonitrile) (SAN)/MMT, where SAN models the more complex ABS/MMT composites used in computer housings. Initially we examined the effects of organoclay surfactant structure on filler dispersion and composite mechanical properties. The composite which exhibited the highest modulus and greatest particle viii aspect ratio (~50) was produced from an organoclay with the lowest molecular weight surfactant. Swelling of the MMT particles, measured by x-ray diffraction, was more strongly related to reduced surfactant molecular weight than surfactant functionality. The composite moduli were compared to Halpin-Tsai theoretical predictions from TEM-based aspect ratios. Given a range of surfactant structures, we then explored the appropriateness of the SAN matrix as a model for ABS. Electron microscopy showed that clay particles in ABS/MMT composites reside in the SAN matrix phase, accumulating at rubber particle surface. Modulus enhancement patterns were the same for a given organoclay, but reinforcement in ABS was lower due to poor orientation of particles at the rubber surface. Interactions between the polymer and silicate surface were probed by varying the SAN copolymer composition, accounting for variations in matrix modulus and melt viscosity. TEM-based image analysis coupled with Mori-Tanaka composite theory gave predictions which fit experimental moduli better than Halpin-Tsai. Higher acrylonitrile content lead to increased reinforcement in the 0-58 weight % acrylonitrile range. TEM-based specific particle densities reached ~8 particles/μm2 compared to well-exfoliated nylon 6 composites at 100 particles/μm2. Improvements in exfoliation were also noted for higher screw rpm. ix Based on enhancement in exfoliation for polyolefin-g-maleic anydride composites, the effect of maleic anhydride in SMA-based nanocomposites was studied. These materials produced the same properties on a weight percent basis as SAN-based nanocomposites, but particle densities remained lower than for polyolefin-g-MA mixtures. This behavior is explained by repulsive interactions between styrene and the alkyl tail of the surfactant, suggesting that polar surfactant tails could lead to improved exfoliation in styrene copolymer-based/montmorillonite nanocomposites.