凝聚态下聚苯乙烯的构象

由NMR弛豫测量提供了在环已烷溶剂中聚苯乙烯侧基苯环凝聚缠结的证据.文中用HyperChem程序中的分子力学和分子动态学的分子模型计算对真空状态中凝聚态下聚苯乙烯(CH₂CH]_n,n=6,18,21和60)的构象进行了计算.结果如下:1)聚苯乙烯(头-尾相接,当n=6在240℃(熔点)下进行不同时间的分子动态学拟合后,冷却到室温(25℃).随着拟合时间的增长,体系的平衡能量逐渐降低,结构中苯环的重叠更加明显.在拟合时间不太长(小于20ps)的情况下,体系即达一稳定构象状态.此时,聚苯乙烯中部份相邻的苯环以一对一对的形式基本上重叠,两苯环间相距~0.3nm;但各对苯环的方向各异.延长拟合时间(60-100ps)体系能量更低,出现相邻三个和四个苯环相互重叠的现象;升高拟合起始温度到1727℃,可以缩短拟合时间得到同样结果.2)为使计算结果更能符合真实高聚物,必须增大n值.由于计算速度所限,只能进行不太长时间的计算,取n=18,21和60的结果与n=6时完全一致.3)头-头或尾-尾相接的聚苯乙烯(n=6)在相同的条件下不能得到类似结果,相互平行的苯环均相距在0.6nm以上.计算结果充分说明凝聚态聚苯乙烯的构象存在有相邻侧基苯环(2,3,4个)呈相距~0.3nm的重叠状态,但重叠在一起的聚集体取向不尽相同.它为聚苯乙烯中存在有短程有序的凝聚缠结提供了理论依据.

CONFORMATION OF POLYSTYRENE IN THE CONDENSED STATE

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 polystyreneCH2CH](◇)(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 HYPERCHEM.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 prolonged, 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.