MOLECULAR-DYNAMICS SIMULATION OF THERMAL MOTION AND ELASTICITY OF A POLYMER-CHAIN

A molecular dynamics simulation was done to analyze the glass transiti on temperature and the elasticity of polybutadiene using a single poly mer chain. The radius of gyration of a free polymer chain without cons traint was calculated at various temperatures to decide the glass tran sition temperature. When the end-to-end distance of a single polymer c hain is constrained, a shrinkage force exerts on both ends. This const rained force was calculated for various end-to-end distances at 300 K, by using POLYGRAF. The constrained force was estimated in two differe nt ways for an average during 200 ps calculations for each distance. O ne estimate came by calculating directly the force exerted between the end atoms, and the other came by estimating from the free energy expr ession of the system. The constrained force was compared with that of the Gaussian chain model. For the free polymer chain, a bend was found in the line relating radius of gyration and temperature, and its temp erature was almost the same the an experimental glass transition tempe rature. The simulated force was in agreement with that of the Gaussian chain model before reaching a straight conformation, however, it incr eased rapidly when the polymer chain was further extended to a large e nd-to-end distance. An energetic force due to the van der Waals intera ction among non-bonding atoms affected the elasticity of the polymer c hain when it was in the random-coiling conformation at a small end-to- end distance.