Dissipative Particle Dynamics Simulation Studies On The Self-assembly Of Stimuli-responsive Polymers

Self-assembly is the process in which building blocks form ordered or functional structures spontaneously.The self-assembly of stimuli-responsive polymers in bulk or solution systems forms a variety of complex nanostructures,which can further respond to external signal inputs.Scientists have paid much attention on it because of its wide use in microelectronics,nanomaterial science,medicine and biology.Nowadays,a large number of experimental studies have been carried out on the self-assembly of stimuli-responsive polymers.It is an important topic to study the formation mechanism,regulation method and their applications.Studying the self-assembly of stimuli-responsive polymers by using computer simulations overcomes some constraints of experimental technology effectively,and provides a systematic way to study impact factors on various self-assembled structures and analyze their formation mechanisms from a microscopic perspective.It is known to us that the self-assembly of polymer is not only controlled by thermodynamic conditions,but also affected by the dynamics of chains.In stimuli-responsive polymers systems,there are two timescales: One is the timescale characterizing external stimulus and the other is the timescale for the response of self-assembled structures.Controlling the competition of these two timescales will control the morphological behavior.In this dissertation,we perform extensively dissipative particle dynamics simulations and focus on the competition of these two timescales to study the following topics: We study the regulation of relatively slow chemical reaction on the surface pattern of polymer brush,where the self-assembly of polymer brush is a fast process.The regulation of fast light irradiation on the non-equilibrium self-assembled structures of light-responsive block copolymer in bulk is studied.At this time the self-assembly of block copolymer is a slow process.And we further investigate in the system of light-responsive block copolymer brush with fast reaction and fast self-assembly process,the possibility of using self-assembled structures to accelerate the cyclization of single polymer chain.The main contents of this dissertation are as follows:(1)We investigate the effect of solvophilic A and C blocks on the self-assembled structures of solvophobic B-blocks in both ABA and ABC polymer brush systems.If the solvophilic A-blocks is grafted to the substrate in AB brush system,B-blocks form spherical micelle structures.They are immersed in the layer of A-blocks.Then,tethering a very small solvophilic block A(C)to the free end of the AB brush may increase local density of B-blocks significantly.A-blocks pulls B-blocks toward the interface between polymer brush and solvents,which changes the self-assembled structure from spherical micelles to ripples.The increase of upper solvophilic blocks changes the lateral density distribution of B-blocks further,causing the domain size of the ripple structure first decrease and then increase.In ABC brush,the incompatibility between the A and C blocks reduce the vertical domain separation effectively.Then,we introduce extremely short free solvophilic blocks A(C)in dilute solution to tether to the free ends of self-assembled spherical micelles in AB brush system by using a reaction model.We find that the formation of ripples is mainly affected by the content of tethered reactive solvophilic blocks,and the effect of reversible reaction rate is small.We believe that modifying polymer brushes by tethering solvophilic blocks to the chain ends can help to fabricate stimuli-responsive surfaces and adjust their nanoscopic surface patterns.(2)We study the non-equilibrium morphological behavior of light stimuli-responsive azobenzene group contained diblock copolymers in time-oscillatory light irradiations.The effects of volume fractions of blocks,interactions between components,oscillation periods and contents of responsive chains on morphologies are studied.We find the formation of dissipative steady-state structures in the very fast oscillation.This oscillation period is much smaller than the diffusional timescale of block copolymer.When oscillation period increases a little,oscillatory dissipative structures are formed with periodic structural variation on the basis of the dissipative steady-state structure.When oscillation period increases further,no regular structures are formed.In addition,the dissipative steady structures can be finely tuned by adjusting the fraction of photo-responsive chains in block copolymer melt.Our results provide a guidance for designing azobenzene-contained block copolymers that are able to form dissipative structures in time-oscillatory irradiations and helps us to predict the self-assembled morphologies.(3)We investigate the possibility of self-assembled structures of light-responsive block copolymer brush to accelerate cyclization of linear polymer chain.Firstly,we adopt the porous templates and amphiphilic surface patterns as research models to study the accelerating effect of linear polymer chain(linear precursor)cyclization in dilute solution.In large-scaled preparation of cyclic polymers,the production of linear by-products has become the main restriction of the preparation of high-purity cyclic polymers.We believe that restricting the diffusion of polymer chain ends can accelerate cyclization and reduce linear by-products as well.In the porous template system,the effect of pore size and adsorption strength on the entry process,together with the effect of confinement shape,size and adsorption strengths on cyclization process inside the confinement are studied.And we redefine the total production time of cyclic polymer.We find that the total production time is governed by the interplay between the size effect caused by decreasing pore size and the adsorption of the pore.The strong size effect suppresses polymer entry but accelerates chain cyclization.The stronger adsorption promotes polymer entry but decelerates chain cyclization.Comparing to the total production time in free solution,the production time in the small spherical confinement with strong adsorption is smaller.If chain cyclization is permitted during it entering the confinement,the competition between steric hindrance and adsorption provides an additional ‘virtual' confinement at the boundary between confinement and free solution.There is an optimal cyclization time under small confinement.Then,in the amphiphilic surface pattern model,by controlling the interactions between solvophobic surface pattern and the linear precursor,the linear precursor can be confined near the pattern and chain cyclization will be accelerated.We study the effect of adsorption strengths,pattern size and pattern spacing on cyclization time in patterned substrate system.We find that when the chain size matches the pattern size,acceleration effect becomes the most obvious.If we increase the pattern spacing properly,chain cyclization can be accelerated further.Then,the effect of self-assembled structures of light-responsive block copolymer brush on the cyclization time is studied.By adsorbing linear precursor on the self-assembled structure of block copolymer,we can change the confinement shape and size for chain cyclization using light.Cyclization time can be controlled without additional modification.The above results show the possible applications of stimuli-responsive polymers under the condition of fast reaction and fast self-assembly process.Our results provide a guidance for the efficient preparation of cyclic polymers in experiments.