Spatial regimes in the dynamics of polyolefins: self-motion

Molecular dynamics simulations are used to investigate the spatial dependence of dynamics in a series of polyolefins. The dynamic indicator used is the self-intermediate scattering function, which parallels the observable in an incoherent quasielastic neutron scattering experiment such as time of flight or backscattering. As with neutron time of flight experiments, two processes are evident. The fast process is a single exponential, and has relaxation times that scale as q(-2), where q is the momentum transfer. The slow process is the stretched exponential decay usually associated with the motion underlying the glass transition. The stretching exponent is a function of spatial scale, with the minimum values occurring near the spatial scale of interchain packing. Relaxation times for the slow process scale as q(-2/beta) for all materials investigated. The relative contribution of the two processes is a function of spatial scale, with the crossover from fast to slow dynamics at the location of closest possible interchain contacts, which is approximately three times the cage size. These observations apply equally well to the four materials considered. We consider the relative ordering of relaxation times of the series in light of their local chain architecture. This ordering varies depending on the observable calculated..