Mesoscale simulation of polymer reaction equilibrium:Combining dissipative particle dynamics with reactionensemble Monte Carlo. II. Supramolecular diblock copolymers

摘要

We present an alternative formulation of the reaction ensemble dissipative particle dynamics(RxDPD) method M. Lisal, J. K. Brennan, and W. R. Smith, J. Chem. Phys. 125, 16490 (2006),a mesoscale simulation technique for studying polymer systems in reaction equilibrium. TheRxDPD method combines elements of dissipative particle dynamics (DPD) and reaction ensembleMonte Carlo (RxMC), and is primarily targeted for the prediction of the system composition,thermodynamic properties, and phase behavior of reaction equilibrium polymer systems. Thealternative formulation of the RxDPD method is demonstrated by considering a supramoleculardiblock copolymer (SDC) melt in which two homopolymers, A_nandB_m,can reversibly bond atterminal binding sites to form a diblock copolymer,A_nB_m.We consider the effect of terminalbinding sites and the chemical incompatibility between A- and B-segments on the phase behavior.Both effects are found to strongly influence the resulting phase behavior. Due to the reversiblenature of the binding, the SDC melt can be treated as the reaction equilibrium system A_nTo simulate the A_n+B_m=A_nB_m melt, the system contains, in addition to full A_n,B_m,and A_nB_mpolymers, two fractional polymers: one polymer eitherfA_nor fB_m,and onefractional polymer fA_nB_m,which have fractional particles at the ends of polymer chains. Thesefractional particles are coupled to the system via a coupling parameter. The time evolution of thesystem is governed by the DPD equations of motion, accompanied by random changes in thecoupling parameter. Random changes in the coupling parameter mimic forward and reverse reactionsteps as in the RxMC approach, and they are accepted with a probability derived from the expandedensemble grand canonical partition function. Unlike the original RxDPD method that considerscoupling of entire fractional polymers to the system, the expanded ensemble framework allows astepwise coupling, thus greatly increasing the efficiency of the RxDPD approach. The RxDPDtechnique rigorously satisfies thermodynamic equilibrium, but not the hydrodynamic behavior.However, the approximate treatment of the hydrodynamics can be minimized by simulating a largenumber of particles.