Application of the entropy theory of glass formation to poly(alpha-olefins)

The entropy theory of glass formation, which has previously been developed to describe general classes of polymeric glass-forming liquids, is extended here to model the thermodynamic and dynamic properties of poly(alpha-olefins). By combining this thermodynamic theory with the Adam-Gibbs model (which relates the configurational entropy to the rate of structural relaxation), we provide systematic computations for all four characteristic temperatures (T(A), T(c), T(g), T(0)), governing the position and breadth of the glass transition, and the fragility parameters (D,m) describing the strength of the temperature dependence of the structural relaxation time, where T(A) is the temperature below which the relaxation is non-Arrhenius, T(c) is the crossover or empirical mode-coupling temperature, T(g) is the glass transition temperature, and T(0) is the temperature at which the extrapolated relaxation time diverges. These temperatures and fragility parameters are evaluated as a function of molar mass, pressure, and the length n of the alpha-olefin side chains. The nearest neighbor interaction energy and local chain rigidities are found to strongly influence the four characteristic temperatures and the low temperature fragility. We also observe an "internal plasticization" of the poly(alpha-olefins) wherein the fragility decreases as the number n of "flexible" side group units increases. Our computations provide solid support for a pressure counterpart of the Vogel-Fulcher-Tammann relation. The entropy theory of glass formation predicts systematic changes in fragility with chain stiffness, cohesive energy, polymerization index, and side chain length, and qualitative trends in these parameters are discussed.