A Computer Simulation Study Of Structure And Dynamics Of Block Copolymers And Polymer Nanocomposites

For the computer simulation of polymer system,an approciate simulation model is very important to cover the large spatial and long time scales in the system.In recent years,a variety of simulated coarse grainning(CG)methods have been developed to solve this problem.Among these methods,the iterative Boltzmann inversion(IBI)method is one of the representative methods.However,the CG potential is usually developed at a specific temperature and can not be applied to other temperatures.In the work of my PhD thesis,after developing CG potentials at two different temperatures for both melt and glassy states,we used a two-points fitting method to solve the temperature transferability problem.The CG models developed by using this method are proved to have a good ability in reproducing structure properties in the system,such as density,radial distribution function,bond distribution,and angle distribution in the system.More importantly,our method provides a straight forward method to develop a temperature transferable CG model for a polymer system.With the development of nanotechnology,breakthroughs in technical bottlenecks are urgently needed to obtain small sized ordered structures.Block copolymers can self-assemble to form many microscopic ordered structures because of the chemical incompatibility between blocks.The ordered structures at 10 nm formed by short block copolymers have been achieved recently in experiments.On the other hand,one can also chang the topology of polymer chain to adjust the size of ordered structures.For example,the size of ordered structures formed by cyclic diblock copolymers usually is smaller than that linear diblock copolymers.In addition,recently experiments and theories have shown that polydispersity has important influences on the phase behvavior of block copolymers.We investigate the phase behavior of short polydisperse linear and cyclic diblock copolymers by dissipative partice dynamics(DPD)simulations.For the phase behavior of short linear diblock copolymers,our simulation has very good agreement with theoretical predictions and experimental observations for monodisperse and one-sided polydisperse systems.On the other hand,we find the formation of an irregular bicontinuous(BIC)structure in one-sided polydisperse system.In two-sided polydisperse system,the order-disorder transition point is increased due to the existence of highly asymmetric chains.The phase behavior of cyclic diblock copolymers has similar characteristics of self-assembly as found in linear diblock and triblock copolymers due to the similarily in the molecular topology.In monodisperse and one-sided polydisperse systems,the self-assembly behavior of cyclic diblock copolymer is almost the same as that of linear diblock copolymer,but cyclic system has a higher order-disorder transition point.In two-sided polydisperse system,the self-assembly behavior of cyclic diblock copolymer is more similar to the triblock copolymer.When the blocks forming bicontinuous structure has a fixed length,the specific surface area of BIC structure formed by cyclic diblock copolymer is larger than linear counterpart,and therefore might be more suitable for the applications where bicontinuous structures are desired,for instance,in solar cells.On the other hand,with the development of materials science and the improvement of material properties,nanocomposites have attracted more and more attention.In order to have a better application of nanocomposite materials,a deep understanding of the impacts introduced by the nanoparticles are highly desired.For a composit system composed by linear polymer chains and cross-linked single chain nanoparticles,we investigate the influence of nanoparticles on the system dynamics at different temperatures by using molecular dynamics(MD)simulation.Crosslinked single-chain nanoparticle is soft in nature,they have large deformability at high temperature at the nanoparticle surface area.Such deformability is found to be very important which can accelerate the dynamics of surrounding polymer chains.However,with the decrease of temperature,the deformability of nanopartilce decreases and it will cause an excess free energy barriar on the diffusion of polymer chains.As a consequence,the diffusion of melt polymer chains can be reduced.