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

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 αvs.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_mα 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.