Dynamical Properties of Chain-Like Fluids: Computer Simulation and Experiment

This research has focused on furthering our knowledge about the dynamic behavior of polymeric fluids in the melt or liquid state. In our approach, we apply computer simulation in order to elucidate the microscopic dynamics of model polymers. A new algorithm for discontinuous molecular dynamics is presented for hardchain models. This algorithm permits the first multi-billion time step simulations of chain-like fluids and provides new information on the transport properties and entanglement dynamics of the hard-chain fluids. This technique is used to determine the self-diffusion coefficient, shear and longitudinal viscosities, and thermal conductivity for short chain fluids of length from 2 to 16 at volume fractions ranging from 0.1 to 0.5. Chain fluid results are compared to hard-sphere fluid results and to the corresponding Enskog theory. The new algorithm is used to study dynamical properties over a much broader range of chain lengths. Analysis of the mean-square displacement (MSD), Rouse modes, dynamic structure factors, and end -to-end vector correlations on chains of length 8 to 192 provide information about chain entanglement. The inner segment MSD of the 192-mers is consistent with predictions of the tube model reproducing the three scaling regimes that are postulated. In addition, anomalous diffusive behavior is observed in the inner segment MSD as the chains cross over into the free diffusion limit. Definitive plateau -like behaviors are observed in the density-density correlations, normal coordinate decay, and end-to-end vector relaxation of the 192-mer fluids at the highest density. These cumulative results suggest the presence of knot-like formations whose lifetime exceeds the tube decay time. A high-pressure cell capable of operating at 4000 bar is designed and used in photon correlation spectroscopy (PCS) studies of the structural relaxation in poly(propylene oxide). For the time domain of the PCS measurements, structural relaxation is described by a stretched exponential decay followed by a long-time tail. The mean relaxation time exhibits an exponential dependence on pressure. The measured relaxation shows the presence of two distinct processes. The fast relaxation is associated with local segmental motions while the slow relaxation is related to polymer diffusion.