Sun Qi, Faller Roland
Department of Chemical Engineering & Material Science, University of California at Davis, One Shields Avenue, Davis, California 95616, USA.
J Phys Chem B. 2005 Aug 25;109(33):15714-23. doi: 10.1021/jp045400d.
Local structural and dynamic properties of atactic polystyrene in a mixed solvent of cyclohexane (CH) and N,N-dimethylformamide (DMF) have been investigated using molecular dynamics simulations. We measure local conformations in the polymer and classify them by distance and angle distribution histograms. End-to-end distances and structure factors are employed to describe the static structure of polystyrene chains. Polystyrene concentration, including 1.6%, 4.8%, and 14% (by weight), and solution temperatures of 300, 330, or 360 K are used to elucidate the concentration and temperature dependencies of the solvation by the two solvents. Both solvent molecules preferentially approach the phenyl rings. At lower temperatures, polystyrene dissolves more favorably in cyclohexane. With rising temperature DMF molecules approach more closely with the oxygen oriented toward the phenyl rings. Additionally, the global and segmental relaxation times of the chains decrease and the system becomes more homogeneous. The segmental and global dynamics of polystyrene show different concentration behaviors: the reorientation times of solvent molecules and segments of polystyrene increase with concentration while the global dynamics of polystyrene decelerates as the concentration is changed from 1.6% to 4.8% but accelerates when the concentration rises to 14%. We conclude that the change of concentration from 4.8% to 14% qualitatively marks the change from a dilute to a semidilute solution. The diffusion constants of the small molecules and corresponding activation energies have also been measured. Our simulation data are compared with available experimental results and we find a satisfactory agreement.
利用分子动力学模拟研究了无规聚苯乙烯在环己烷（CH）和N，N-二甲基甲酰胺（DMF）混合溶剂中的局部结构和动力学性质。我们测量了聚合物中的局部构象，并通过距离和角度分布直方图对其进行分类。采用端到端距离和结构因子来描述聚苯乙烯链的静态结构。使用1.6%、4.8%和14%（重量）的聚苯乙烯浓度以及300、330或360 K的溶液温度来阐明两种溶剂溶剂化作用的浓度和温度依赖性。两种溶剂分子都优先靠近苯环。在较低温度下，聚苯乙烯在环己烷中溶解得更有利。随着温度升高，DMF分子更紧密地靠近，且氧原子朝向苯环。此外，链的整体和链段弛豫时间缩短，体系变得更加均匀。聚苯乙烯的链段动力学和整体动力学表现出不同的浓度行为：溶剂分子和聚苯乙烯链段的重排时间随浓度增加，而聚苯乙烯的整体动力学在浓度从1.6%变化到4.8%时减速，但在浓度升至14%时加速。我们得出结论，浓度从4.8%变化到14%定性地标志着从稀溶液到半稀溶液的转变。还测量了小分子的扩散常数和相应的活化能。我们将模拟数据与现有的实验结果进行了比较，发现吻合度令人满意。
