高分子相分离的分子动力学模拟

Molecular Dynamics Simulation on Phase Separation of Polymer Blend Systems

用粗粒化分子动力学(MD)模拟方法从分子层次研究两组分聚合物共混体系相分离过程中的动力学.在相分离初期,相区尺寸不随时间增加而变化;在相分离中期,相区尺寸与时间有很好的标度关系,标度指数(α=1/3)符合L ifsh iz-Slyozov提出的以扩散为主导的蒸发-凝聚机理的标度预测;在相分离后期,体系实现宏观相分离,相区尺寸不再随时间改变而变化.体积分数小的高分子链尺寸在相分离过程中先收缩再扩张,在实现宏观相分离后,高分子链尺寸又回到本体状态尺寸.

A molecular dynamics simulation（MD） is performed to study the phase separation of binary symmetric polymer blend systems. The blend systems comprise of two kind of polymers with equal chain length of 20 and total chain number of 1 000. By adjusting chain numbers of polymer A, a series of volume fractions φ from 0.1 to 0.5 were obtained. Polymer chains were simulated by a coarse-grained bead-spring model. The interaction potential between monomers is purely repulsive Lennard-Jones 12-6 potential and the bond potential between neighbor monomers along the sequence of the chain is FENE potential. The collective structure factor evolving with time, S（ q,t）, is obtained by the fast Fourier transform（FFT） of the concentration fluctuation function. As the phase separation proceeds, the maximum of the collective structure factor increase with the increase of time. The average phase domain size R （t） keeps constant at the early stage. At the intermediate stage, R（t） increase with the increase of time and a scaling relation of R（t） vs. t^1/3 is observed. At the last stage of the phase separation, R （t） is time independent again. The time evolution of polymer chain size, as well as the coordination number of monomer is also investigated during the phase separation. All these indicate that MD simulation is an effective and advantageous method for studying the dynamics of phase separation of polymer blend systems.