Segmental Mobility of Polyolefin Melts

Processes on different length scales affect the dynamics of chain molecules. Small-scale effects determine the mobility of individual chain segments and, with it, much of the temperature and density dependence of the transport properties of polymers.They are conveniently described by a local friction coefficient, which is inversely proportional to the probability for segmental motion. In this work, an exact enumeration scheme for the simulation of iInteractions and relative motion of two short chain sections on a lattice is combined with an equation of state to predict the variation of the friction coefficient with temperature, pressure, and small-scale chain structure.The method is applied here to polyolefins, i.e., hydrocarbon chains that differ mostly in small-scale architecture, and the theoretical results are compared with experimental data.For temperatures well above the glass transition temperature, the approach gives a good qualitative representation of the variation of the friction coefficient with chain structure and temperature.Furthermore, there is excellent agreement between calculated and experimental pressure variation of the viscosity at low to moderate pressures.A first estimate of the effects of chain flexibility on segmental mobility is also included.