Mass Dependence of the Activation Enthalpy and Entropy of Unentangled Linear Alkane Chains
Jack F. Douglas, Cheol Jeong
The mass scaling of the self-diffusion coefficient of polymer liquids, D , is one of the most basic characteristics of these complex fluids. Although traditional theories such as the Rouse model and reptation models of unentangled and entangled polymer melts, respectively, predict that is a universal constant, this scaling exponent has been reported to vary continuously upon cooling from -1.8 to -2.7 in alkanes. Significantly, this change with temperature occurs under conditions where the chains should not be entangled and this trend has been rationalized by the dependence of monomeric friction factor, based on free volume description of the dynamics of glass-forming liquids. Yet this variable mass scaling is observed at high temperatures where is Arrhenius and the applicability of free volume model is questionable. Using atomistic molecular dynamics simulation on unentangled linear alkanes in the melt, we find that the variation of with derives from the dependence of the enthalpy and entropy of activation on chain length. In addition, we find a sharp change in the melt dynamics when the number of carbon atoms n is near 17, as found also in viscosity measurements. A close examination of this phenomenon indicates that it arises from buckling transition in the alkane chains from rod-like to coiled configurations. Distinct entropy-enthalpy compensation relations are observed on either side of this conformational transition. These observations are important because the activation free energy parameters exert a strong influence on the dynamics of polymer melts that is not anticipated by either the Rouse and reptation models. We further expect these free energy parameters to be crucial for understanding the dynamics of nanocomposites and confined polymers because of the effect of interfacial interactions on the activation parameters.