Structures And Coil-globule Transition Of Regularly Distributed Multiblock Copolymers

Coarse-grained models for β-cyclodextrin (β-CD) and adamantane (ADA) areproposed by fitting to their experimental host guest complex equilibrium constantin solution. By using Brownian dynamics simulations, we suggest a simplesupramolecular route for synthesizing multiblock copolymers (MBCs) via formingcomplexes between β-CD and ADA groups terminated at the chain ends of diblockcopolymers (DBCs). The chain length distribution of the resulted MBC is found tofollow the statistics of Flory formula for typical linear condensation polymerizationprocess. Therefore, the proposed supramolecular route can be viewed as a novellinear condensation polymerization process with DBCs as reactive monomers. Dueto the complex formations between head and tail (β-CD and ADA), ring-shapedMBCs are also observed in our simulations, which will reduce the yield of theMBC. Because we are using a generic model for DBC, the proposed route ofbuilding MBCs are applicable for all synthetic DBCs with two ends terminated byeither β-CD or ADA groups.We study the coil-to-globule transitions of both homopolymers and multiblockcopolymers using integrated tempering sampling method, which is a newlyproposed enhanced sampling method that can efficiently sample the energy spacewith low computational costs. For homopolymers, the coil-to-globule structuretransition temperatures (Ttr) are identified by the radius of gyration of the chain.The transition temperature shows a primary scaling dependence on the chain length(N) withTtr～N1/2. For multiblock copolymers, the coil-to-globule transition can beidentified as first order, depending on the block size and the difference in attractiveinteractions of blocks. The influence of mutating a small portion of strongly attractive blocks to weakly attractive blocks on the coil-to-globule transition isfound to be related to the position of the mutation.We study the coil-globule transitions of homopolymers and multiblockcopolymers with different topology and stiffness by using molecular dynamics withintegrated tempering sampling method, which is a novel enhanced samplingmethod that can efficiently sample the energy space with low computational costs.The method is proved to be efficient and precise to study the structural transitionsof polymer chains with complex topological constraint, which may not be easilydone by using conventional Monte Carlo method. The topological constraint affectsthe globule shape of polymer chain, thus further influences the coil-globuletransition. We find that increasing topological constraint generally decreasescoil-globule transition temperature for homopolymers. For semiflexible chains, anadditional first-order like symmetry-broken transition emerges. For blockcopolymers, the topological constraint does not obviously change the transitiontemperature, but greatly reduces the energy signal of the coil-globule transition.