Molecular simulation for the effect of chain stiffness on polymer crystallization from the melts

The chain stiffness that arises from structural constraints and hindrances to internal rotation is intimately related to the macroscopic properties of polymeric materials. The main goal of this study is to examine the role chain stiffness plays in controlling polymer crystallization. Molecular simulations of the coarse-grained (CG) polymer model were used to determine the effect of chain stiffness on the structural formation upon stepwise cooling from the melts. The polyethylene (PE)-like model with different chain stiffness parameter (k) was proposed to modify the statistical weights in the rotational isomeric state (RIS) chains. The PE-like models with 0.5 < k < 1.0 and 1.0 < k < 1.5, can be regarded as more flexible to stiffer chains than normal PE (k = 1.0). Simulation results indicate that the rate of structural formation and the degree of crystallinity are increased better for normal PE chains (k = 1.0). Highly flexible chains (the case of k = 0.5) adopt larger amount of gauche conformation and have much difficulty to form the ordered structure. On the other hand, much stiffer chains (the case of k = 1.5), can still have some gauche conformation which cannot be converted to the trans state upon cooling from the melts. Our study implies that only polymers with the appropriate chain stiffness can exhibit clear evidence for crystallization.