(530g) Structure and Dynamics of Star Polymer Melts Above Glass-Transition Temperature

Structural and dynamical properties of star-shaped polymer melts were investigated by performing molecular dynamics simulations based on a bead-spring model. Our model is based on an extension of Dobkowski's chain-length dependence correlation function [Eur. Polym. J. , 1982, 18 , 563].  It accounts for the fact that f chains of specific lengths are tethered to a branch point and it describes the properties under melt conditions and above the glass transition temperature, which, to date, are poorly understood. Although, simulations of the densities and isokinetic temperatures, based on the canonical definition of the laboratory glass-transition (T_g), are well described by the correlation function, a subtle behavior is manifested as the architecture becomes more complex. High functionality stars exhibit the opposite behavior from that of linear chain polymers and low functionality stars that is with increasing the arm length the density decreases rather than increasing. The difference between values of T_g and densities of low and high functionality stars is more pronounced for short arm lengths; for sufficiently long arm lengths, the behavior of stars of all functionalities is similar. Moreover, we find that increasing the arm length and the functionality leads results in a weaker dependence of the relaxations on temperature, i.e.: less fragile glass-forming behavior. Complementary results such as the specific volume and number of neighbors in contact provide further insights on the subtle relation between the structure and dynamics. The findings provide fundamentally important insights toward understanding the processing of polymers, colloids and nanocomposites.