Cooperative Competition Between Copolymerization And Polymer Aggregation In The Polymer Solution

Copolymerization is one of the important ways to improve the performance and the use of polymers.It can be applied to control the composition,sequence and structure.For the incompatible components,due to the delicate balance between the interface energy and the chain conformational entropy,the block copolymers can undergo microphase separation to form nanoscale microphase structures.These microphase structures are thermo-dynamically stable and have quite regular spatial symmetry on the mesoscopic scale,so that these materials have wide applications in industry.In the recent decades,polymerization-induced phase separation during the copolymerization has attracted wide interest.The amphiphilic copolymer undergoes phase separation in a selective solvent to form micellar structure with a hydrophobic block chain as a core and a hydrophilic block as a shell.Besides,some scientists studied the bulk polymerization between immiscible monomers.By using a modified reaction procedure,they found that the incompatibility between polymerizing monomers can be used as a tool to influence the resulting copolymers' composition and sequence.What is the effect of the insolubility of monomers on the copolymer? Does the rate matter? These problems are crucial for the experimental design and improvement of the macroscopic properties of copolymer materials,and the core of solving these problems is to better understand the cooperative competition between polymer aggregation and copolymerization from a microscopic perspective.In this study,using dissipative particle dynamics simulations coupled with the stochastic reaction model,we investigate the polymerization-induced polymer aggregation process and the polymer aggregation enhanced polymerization process in a binary solution(A/B)by simply tuning the solubility of the solvent to one species of copolymerization.Our simulations indicate that it is a complicated interplay of the copolymerization on the formation of aggregates,namely,on one hand the polymerization may induce the aggregation of one species,it supplies the driving force of aggregation;on the other hand it has an effect of mixing the two species together.We also find that the polymerization process basically follows the first order reaction kinetics.With the increase of insolubility of B species in the solution,it continuously deviates from the first order reaction kinetics.In the symmetric copolymerization system,we find that the dispersity of copolymers monotonically decreases with the increase of reaction probability.This counterintuitive result can be understood via the comparison of diffusion-controlled kinetics and reaction-controlled kinetics.For the fast reaction,with the diffusion controlled process the B species has not got enough time to largely aggregate together before they are polymerized.As a result,the copolymerization still takes place in a homogeneous solution condition,which is beneficial to the growth of monodispersed chains.However,for obviously slow reaction,the aggregation of B species becomes complete at the early stage of copolymerization.As a consequence,the polymerization is splitted into two parts,the homopolymerization of A species in solution and another bulky homopolymerization of B species in the B aggregates.Such a difference of polymerization condition inevitably leads to the enhanced dispersity of polymer chains.In the asymmetric system,for systems with preferential copolymerization,the mass distribution shapes are Gaussian-like with certain peaks.For comparison,for systems with preferential homopolymerization,the mass distribution shape shows an obviously bimodal form.This study helps to better understand the cooperative competition between the reaction dynamics and the diffusion dynamics during the preparation of copolymer materials,and could act as a guide to better design and improve the copolymerization technologies in laboratories and in industry.