Compressibility and thermal expansion coefficients of nanocomposites with amorphous and crystalline polymer matrix

Abstract

Clay-containing polymeric nanocomposites (CPNC) with polystyrene (PS) or polyamide-6 (PA-6) matrix were studied within T = 300–600 K and P = 0.1–190 MPa. From the Pressure–Volume–Temperature ( PVT) data the derivatives: compressibility, κ, and thermal expansion coefficient, α, were computed as functions of T, P and clay content, w. Dependence of these coefficients on P and T were significantly different for the amorphous PS than for the semi-crystalline PA-6. In the PS plots of κ and α vs. T the presence of secondary transitions, T β/ T g ≈ 0.9 ± 0.1 and T c/ T g = 1.2 ± 0.1, were detected and the clay effect at low T was prominent, affecting the physical aging. The isobaric values of α = α( T) were characterized by nearly T-independent values in the glassy and molten phase, connected by a large transitory region stretching from the ambient pressure values of T g to T c; this region was even more prominent in κ = κ( T). The derivative properties of PA-6 based CPNC were distinctly different. Here, the isobaric κ = κ( T) followed the same dependence on both sides of the melting zone, while the isobaric α = α( T) dependencies were dramatically different for the solid and molten phase; at T < T m α linearly increased with T, after melting its value sharply decreased, and then at T > T m (depending on w and P) either increased or decreased with T. Interpretation of the behavior in the melt and glass is based on the Simha–Somcynsky (S–S) cell-hole theory while that of the semicrystalline state on the Midha–Nanda–Simha–Jain (MNSJ) cell theory. In spite of the nonequilibrium conditions below the main transition point, T g or T m, the theories well predict the observed dependencies.