外力驱动作用下高分子链在表面吸附性质的计算机模拟

Computer simulation of adsorption properties of polymer on surface under external driving force

采用退火法模拟研究受外力F驱动的高分子链在吸引表面的吸附特性.通过高分子链的平均表面接触数〈M〉与温度T之间的关系计算临界吸附温度T_c,并发现T_c随着F的增加而减小;进而通过高分子链的均方回转半径分析外力驱动作用对高分子链构象的影响,并从回转半径极小值或者垂直外力方向的y和z分量的变化交叉校验临界吸附点T_c.模拟计算了处于吸附状态的高分子链随着外力F的增加是否会发生吸附状态到脱附状态的相变以及发生相变所需施加的外力是否由温度所决定.模拟结果表明:两种不同温度下高分子链的吸附性质和构象性质受外力驱动作用而产生不同现象,在温度区间T*_c<T<T_c时会发生脱附现象,而在T<T*_c时不会发生脱附现象.

Monte Carlo simulation is performed to study the adsorption properties of polymers on an attractive surface.Annealing method is adopted to simulate the adsorption characteristics and conformational changes of polymer chains driven by an external driving force F. In simulations using cooperative motion algorithm, the ensembles of monomers located at lattice sites are connected by non-breakable bonds. When the external force is F = 0, the finite-size scale method can be used to determine the critical adsorption temperature（Tc） of the polymer chain on the attractive surface,but when the external force is F 0, the dependence of the average number of surface contacts （M） on the chain length N is unrelated to temperature T. Therefore, Tc cannot be obtained by the finite-size scale method. However,the pseudo-critical adsorption temperature Tc can be estimated by a function of the average number of surface contacts〈M〉 and the temperature T for the chain length N = 200. And then Tc decreases with external force F increasing. The phase diagram is obtained for the polymer chain between the desorbed state and the adsorbed state under temperature T and external driving force F. Furthermore, the influence of the external driving force on the conformation of the polymer chain is analyzed by the mean square radius of gyration of polymer chains. The critical adsorption point Tc can be checked roughly by the minimum location of the mean square radius of gyration or by the variation of its components in the Y and Z direction perpendicular to the external force. With the increase of the external force F for adsorbed polymer, the temperature T can determine whether polymer is changed from the adsorption state to the desorption state and where the force is located at the transformation. There are two different cases, that is, the polymer can be desorbed at the temperature Tc〈T〈Tc and the polymer cannot be desorbed at T〈Tc. In this paper, we discuss these two cases for the adsorption of polymer on the attractive surface： weak and strong adsorption. In the first case,the adsorption is strongly influenced by the external driving force. By contrast, in the strong adsorption, the adsorption is weakly influenced by the external force. Our results unravel the dependence of adsorption of polymer on external driving force, which is also consistent with the phase diagram of adsorption and desorption of polymer chains.