Computer experiments on the internal dynamics of crystalline polyethylene: Mechanistic details of conformational disorder

The atomistic details of the internal dynamics of a polyethylene-like crystal are studied using molecular dynamics. Crystals with up to 6100 chain atoms have been studied for up to 30 ps. A microscopic description of the atomic motion has been examined and a link to available experimental data on the macroscopic and microscopic motion is provided. The results show that the onset of a significant population of rotational isomers is strongly altered by the intermolecular forces. Typical rates for the formation of isomers are 10 ¹⁰ to 10¹² s^-1 at 350 K (depending on the size of the simulated crystal, which changes the overall nature of the intermolecular forces) and increase exponentially with temperature. The large number of created defects causes a continuous decrease in the end-to-end distance. Specific defects, however, have extremely limited lifetime (i.e., those suggested by molecular mechanics calculations). These results suggest that at the temperatures where annealing or deformation of metastable crystals is possible, only randomly generated defects cause the macroscopically observed changes. The defects should move under the free enthalpy gradient set up within the crystal toward a more stable location. The activation energy required for motion which ultimately results in mass transport or lamellar thickening can be shown to be temperature and chain-length dependent. The highly uncorrelated behavior of the creation and annealing of defects reveals the underlying chaotic nature of the "transition" from an ordered crystal to a conformationally disordered crystal (CONDIS crystal). In the simulated case, the transition to the conformationally disordered state occurs gradually, involving little or no cooperative motion. This continuous transition to the condis state was suggested earlier on the basis of experimental evidence and is expected to occur in many other polymers in addition to and at lower temperature than possible additional first-order transitions to the condis state. Thennodynamic and kinetic parameters of the simulations have been determined and compared to the available experimental data with good agreement.