近壁模型高分子链的计算机模拟

COMPUTER SIMULATIONS OF A MODEL POLYMER CHAIN NEAR A SURFACE

采用简立方格点上的普通无规行走 (NRW)和自避行走 (SAW )为模型链 ,研究了不可穿透的刚性壁附近高分子的构象性质 ,得出近壁链的构象熵和尺寸随一端点与壁之间距离z0 的变化 .所用的计算机模拟技术包括精确计数和MonteCarlo模拟 .数值结果表明 ,近壁链的构象熵降低遵循一种简单规律 ,即当链长愈长或一端愈接近于壁时 ,与长度相同的自由链相比 ,链因壁限制所致构象熵的降低愈大 .当链十分接近于壁时 ,其均方末端距〈R2 〉(尤其是与壁垂直的分量〈R2z〉)大于自由链的相应值 ;随z0 增大 ,〈R2 〉及〈R2z〉开始减小 ,通过某一极小值 ,然后上升 ;当z0 →∞时 ,趋于自由链的极限值 .换言之 。

By means of the normal random walk (NRW) and self-avoiding walk (SAW) as the models of a polymer chain, the conformational properties of a polymer chain near an impenetrable plane were investigated, which gave the conformational entropy and chain size as functions of z(0), the distance of one chain end to the plane. The computer simulation techniques used include the exact enumeration and Monte Carlo simulation, It was shown that the decrease in the conformational entropy of this confined polymer chain obeys a simple law. The reduction of a chain in conformational entropy due to the confining effect of a surface increased with increasing the chain length and decreasing the distance z(0) . The mean square end-to-end distance (especially its z-component) of the confined polymer chain near an impenetrable plane was larger than the corresponding value of the free chain, as the end distance z(0) was very small. When the end distance z(0) increased the ratio of <R-2> and <R-=(2)> decreased in the beginning, reached a minimum and then increased at last. As the end distance z(0) --> infinity, this ratio approached to a limit value equal to the corresponding value of a free chain. In other words, the size of the polymer chain was experienced a process of rapid contraction and then gradual extension.