关键词: 聚苯乙烯, 分子动态学计算, 凝聚缠结, 构象, NMR

Abstract: The NMR relaxation measurement provides an experimental evidence for the existence of cohesional entanglement among side group, phenyl rings, of polystyrene (PS)in concentrated cyclohexane solution.Conformation of polystyrene ([CH₂CH]_n, n=6, 18, 21 and 60) in the condensed state was calculated by means of the molecular mechanics and molecular modeling in vacuo using the molecular graphics package HYP ERCHEM. The following results were obtained:1)for head to end connected polystyrene when n=6, different simulation run times was done at 240℃(melting point) and gradually cooled to ambient temperature. The equilibrium energy of the system decreases as the running time is prolong ed, the stacking of the benzene rings becomes more obvious. When the running time reached 20ps, the system approaches to the state of equilibrium, where a part of adjacent phenyl rings stack as pairs of parallel rings of different orientations.The distance between the two parallel phenyl rings is about 0.3nm. Prolonging the run time to 60-100ps, we obtained more ordered conformations with lower energies.Three or four adjacent phenyl rings became parallel. Similar results were obtained when the simulation temperature was increased to 1727℃, with a gain of shortening the time consumed. 2)In order to get information that is more approximate to the real polymer, the value n should be increased. However, simulation of this polystyrene with values, n, of 18, 21 up to 60 were conducted, due to the limitation of computation time with our personal computer. The same results were obtained.3)Simulations of head to head or end to end connected polystyrene (n=6)at the same conditions do not give similar results. Parallel phenyl rings with distance longer than 0.6nm were found.The above results suggest that there exist ensembles of 2, 3 and even 4 stacked adjacent phenyl rings being parallel with distances of ~0.3nm one to another with different orientations in condensed polystyrene. This gives the theoretical basis for the existence of the short-range order in polystyrene.

Key words: NMR, Polystyrene, Molecular mechanics, Cohesional entanglement, Conformation