Molecular dynamics studies of entangled polymer chains

The thesis presents three molecular dynamics studies of polymeric ensembles in which the chain entanglement plays the major role in the internal dynamics of the system. A coarse-grained model is used for representing the polymer chains as strings of beads connected by finite-extensible springs. In a dense ensemble of such chains, the strong bonds along the polymer backbone coupled with the repulsive Lennard-Jones interaction between unconnected beads prevent the chains from crossing each other. This results in an entangled system with motion restrictions for each chain imposed by the intertwining with the neighboring ones. Studying the chain dynamics inside entangled ensembles of identical chains is the main purpose of this thesis. Due to the entanglement, the polymers are restricted to a reptation motion. The chain movement is influenced significantly by chain length N, temperature T and chain stiffness. In order to analyze the effect of these parameters, three systems were investigated by computer simulation. The polymer melt: at relatively high temperatures, the ensemble of entangled polymers behaves like a highly viscous fluid. Above the entanglement length N > Ne, in the reptation regime, the chain self-diffusion decreases with the square of the chain length. This is in contrast with a linear dependence on N predicted by Einstein's relations and applicable only to short chains in Rouse regime. The polymer glass: as the temperature is lowered below the glass transition temperature (T < Tg), the diffusion process ceases. The system enters the glassy state characterized by structural arrest in which the local random bead motion is ineffective in inducing large-scale chain diffusion. The adhesion between two polymer bulks via connector chains: the adhesion strength is measured in debonding simulations. As the two polymer bulks are pulled apart the connector chains are forced to disentangle from the bulks following a reptation motion. Remarkably, the total work necessary to break the adhesion is again proportional to the square of the connector length. This confirms that the entanglement of the connectors inside the polymer bulks is the essential mechanism realizing the bondage. The prominent contribution of this thesis is the systematic study of how these phenomena are influenced by the chain stiffness, modeled by a bending potential and a novel form of torsion potential acting on three, respectively four, successive beads along the polymer backbone. By incorporating chain stiffness, our study shows that the effects of chain entanglement become more evident: as chain stiffness is enhanced, the entanglement length (Ne) increases, the glass transition occurs at higher temperatures (Tg) and the adhesion via connector chains becomes stronger. The results reported in this thesis are based on a massive computational effort amounting to about 10 CPU years.