拉力诱导下的紧密高分子链相变行为

采用相互作用自回避行走(interacting self-avoiding walks,ISAWS)模型研究了一端固定的紧密高分子链在拉伸过程中的低温相变行为,观察到在拉伸过程中当温度T<0.1时平均拉力会出现一个震荡,随着温度的升高这种震荡现象又渐渐消失,这是由于紧密高分子链在低温时类似于β折叠的"冻结构象"被拉开而引起的.比较吸附条件下和无吸附作用下平均拉力、自由能以及相变行为的差别,发现在吸附条件下在拉伸的初始阶段为了克服表面吸附的相互作用,拉力会出现一个峰.吸附作用也使得外界作用到高分子链上的实际有效拉力减小,造成崩塌相态(collapsed phase)区域面积减少.另外发现在吸附条件下平均拉力还受温度变化的影响.在拉伸的初期由于单体间存在体积排除效应,平均拉力是随着温度的升高而降低,随着拉伸的深入当末端距到达一定长度时平均拉力是随着温度的升高而增加.并同Kumar等人在不考虑吸附作用下拉伸紧密高分子链得到的结果进行了比较.这些研究对于进一步研究外力诱导下吸附紧密高分子的相变有一定的参考价值.

PULL-INDUCED PHASE TRANSITION IN COMPACT CHAINS

The study presents the low temperature phase transition of compact chains with one end fixed based on the interacting self-avoiding walks(ISAWS) on the square lattice.The simulation is realized by transfer matrix(TM) method.During the tensile process,the average pulling force has an obviously oscillations at T<0.1,and the oscillations vanishes with the temperature increasing.Since the compact chain has "frozen conformations" like β-sheets,and when about half a layer has been opened,the average pulling force decreases obviously.The results agree well with the conclusions by Sanjay Kumar et al.The average pulling force ,the average free energy and the phase transition for compact chains adsorbed on the neutral surface and the attractive surface are investigated respectively.During the initial tensile process,in order to overcome the adsorbed surface,the average pulling force has a peak value,and this phenomenon is also explained by the free energy.The adsorbed surface can make the available average pulling force decrease,so that the area of collapsed phase decreases.On the other hand,the average pulling force is also influenced by temperature.At the beginning,when the end-to-end distance x<12,with the influence of excluded volume effects,the average pulling force decreases with the temperature.When end-to-end distance 12≤x≤16,the average pulling force keeps a plateau of a constant force around 0.9 with different temperatures.But with the further extension of the compact chain,when the end-to-end distance x>16,the average pulling force increases with the temperature.This result has already been found by Kumar et al.for the biopolymers without considering the attractive surface.With considering the influence of excluded volume effects,when the end-to-end distance x is less than the average size of the coil(without force),the compact chain needs a compressing force instead of a pulling force,i.e.when T=1.0,the end-to-end distance x=8,the pull-induced force is compressing force,and comparing with the average compressing forces of compact chains adsorbed on the neutral surface and the attractive surface,the adsorbed surface makes the available average compressing force decreases.These investigations may provide some insights into the phase transition in the force-induced compact chains.