用Simha-Somcynsky状态方程改进高分子中溶剂扩散系数模型

A modified model of diffusion coefficient in polymer-solvent systems based on Simha-Somcynsky equation-of-state

将SimhaSomcynsky状态方程和VrentasDuda的自由体积理论模型相结合,提出改进的高分子中溶剂扩散系数模型.改进的模型利用状态方程和WLF理论定义的自由体积分数确定扩散过程的有效自由体积,避免原模型中繁琐的黏弹性实验测定.对苯、甲苯、乙苯、氯仿在聚苯乙烯、聚异丁烯和聚醋酸乙烯酯中的扩散系数的计算结果表明,改进模型的预测值与实验值取得较好一致,且自扩散系数的预测精度比原模型高.同时,改进的模型能够反映压力对扩散系数的影响.溶剂扩散系数随外界压强增加而减小,当溶剂浓度较低时,压强的影响非常显著,扩散系数可相差几个数量级.随着溶剂浓度升高,扩散系数接近常压时的扩散系数,压强影响可以忽略.

A modified diffusion model for polymer-solvent systems was proposed by introducing the SimhaSomcynsky （SS） equation-of-state （EOS） into the Vrentas-Duda free-volume model. The complex process of viscoelastic measurements in the Vrentas-Duda model was avoided successfully, since the effective free volume of a polymer can be estimated from the SS EOS together with the Williams-Landel- Ferry （WLF） fractional free volume. Calculated diffusion coefficients of benzene, toluene, ethylbenzene and chloroform in polystyrene, polyisobutylene and poly （vinyl acetate） indicated that predictions from the moditied model were consistent with experimental values, and the accuracy was better than those from the Vrentas-Duda model for predicting solvent self-diffusion coefficients. Moreover, the modified model can reflect the effect of pressure on diffusion coefficients. Solvent diffusion coefficients in polymers decrease with increasing pressure. Especially in dilute polymer-solvent systems, diffusion coefficients change by several orders of magnitude with increasing pressure. With the increase of solvent concentration,diffusion coefficients gradually approach those values under atmospheric pressure, thus the effect of pressure can be neglected reasonably.