Molecular dynamics simulation of structure formation of short chain molecules

Molecular dynamics simulations are carried out to study the structure formation of 100 short chain molecules, each of which consists of 20 ${\mathrm{CH}}_2$ groups. Our simulations show that the orientationally ordered structure is formed from a random configuration by quenching. The global orientational order starts to increase suddenly after a certain duration and grow in a stepwise fashion afterwards. This behavior is also found in the growth process of the local orientationally-ordered domains. It is found from the microscopic analysis of the structure formation process, that parallel ordering of chain molecules starts to occur after the chain molecules stretch to some extent. From the analysis of the obtained orientationally ordered structure and the molecular mobility, we also find the following characteristic features: (i) The chain molecules are packed hexagonally at 400 K and the transition from the hexagonal phase toward the orthorhombic phase takes place as the temperature decreases. (ii) The gauche bonds in the same chain molecule tend to form gauche pairs. The gauche pairs with the same sign form the double gauche defects and those with the opposite sign form the kink defects. (iii) In the hexagonal phase, the chain molecules become longitudinally mobile. This result, which is obtained by the microscopic analysis of the chain motion, is the microscopic evidence to confirm the existence of the chain sliding diffusion in the hexagonal phase which underlies the sliding diffusion theory of polymer crystallization proposed by Hikosaka [Polymer 28, 1257 (1987); 31, 458 (1990)].