Simultaneous Polymer Chain Growth With The Coexistence Of Bulk And Surface Initiators:Insight From Computer Simulations

Surface modification by means of polymer brushes has become a prevailing approach to tailor the chemical and physical properties of interfaces.Polymer brushes are widely used in both scientific research and industrial manufacture in the areas of stimuli-responsive materials,anti-biofouling,lubrication,colloidal stabilization,etc.The realization of live polymerization makes it possible to prepare a polymer material with a specific structure and molecular weight,which is also known as the molecular design of macromolecules.The ATRP(atom transfer radical polymerization)method is widely used in synthetic graft polymer production,due to the existence of a wide range of candidate monomers that makes it easy to obtain raw materials.On the other hand,the implementation conditions of ATRP are mild and the synthesis process is quite simple.In practice,experimentalists devote most energy towards studying the Mn(number-average molecular weight),the D(dispersity)and the grafting density of polymer chains derived from Mn and D.In the experiment,in order to measure Mn and D,it is necessary to cleave the polymers from the surface and probe them with size-exclusion chromatography(SEC).There is huge uncertainty about the SEC data because the amount of polymer cleaved from planar substrates is low.Due to this disadvantage of SEC,another method is practically applied in the experiment.In this method,bulk initiators are fed in addition to surface initiators,so that both of them can undergo polymerization reactions simultaneously,and the Mn and D of the surface polymer chains could be conjectured by those of the free polymer chains.This is based on an initial assumption that the grown surface chains have the same values of Mn and D as their bulky counterparts.Motivated by this,using molecular dynamics(Molecular Dynamics)simulations,in this paper we study the simultaneous chain growth of polymerization in a system where both surface and bulk initiators coexist.The influences of the density of surface initiators,the ratio of bulk to surface initiators,and the initial monomer concentration are analyzed in detail.In order to characterize the polymerization progress,we calculate the monomer conversion Xm,which is defined as the proportion of monomers that have already participated in the polymerization.For further characterizing the growing polymer chain lengths and dispersity,we calculate both the dispersity D and chain length distributions.From the monomer conversion rate Xm?polydispersity D(or monomer conversion rate Xm?time step t)map,mass distribution map and normalized mass distribution map,we have obtained some results.We find that when the surface initiator density is low enough,the practical experimental way to estimate the dispersity(D)of surface-initiated chains on the basis of the dispersity of bulk-initiated chains remains valid as long as the conformations of grafted chains remain within the mushroom regime(i.e.,the grafted chains are sparsely distributed).On the other hand,although the average chain lengths of surface and bulk polymers could be equivalent when certain conditions are met,their mass distributions are still different.We also find that increasing the fraction of surface initiators leads to an enlarged disparity in D and average length between surface and bulk chains,which is inconsistent with previous studies.This study helps in better understanding the cooperative competition and suppressing effect of bulk chains on surface grown chains,as well as the cause of the dispersity of the surface grown chains as compared to their bulk counterparts with the coexistence of bulk and surface initiators.