Atomistic Simulation of the αc-Relaxation in Crystalline Polyethylene
The existence and mobility of conformational defects that could account for the αc-relaxation in crystalline polyethylene are studied by atomistic simulation. Candidate defects are identified by an exhaustive search of conformation space. Their mobility for translation along the chain in the crystal is estimated using the Transition State Theory. Among point defects, we find a large number of candidates for the αc-relaxation. This solution space is simplified by grouping solutions into families that yield a common minimum energy defect conformation. Dynamic pathways for defect propagation from one unit cell to the next vary substantially in energy barrier, ranging from 4 to 15 kcal/mol, in accord with experimental values of 5?22 kcal/mol. The highest mobilities (lowest energy barriers) were obtained for pathways that visit different defect conformations as they pass from one unit cell to the next. The most facile dynamic pathway for the αc-relaxation was found to involve the cooperative rotation of two torsions within the defect while five intervening torsions remained unchanged. Finally, we propose a method for computing atomistically the activation volume associated with elastic deformation of the lattice due to defect insertion and motion. For the low barrier αc-relaxation pathway, we obtain a contribution to the activation volume of 0.9 to 6.4 Å3 at 0 K associated with reversible lattice distortion and a contribution of 11.1 Å3 due to chain translation, in substantial accord with values of 19?29 Å3 obtained at 343?424 K from dielectric α-relaxation experiments. These results indicate that relatively small, local defects in polyethylene can play a role in the dielectric α-relaxation.