Multiscale Modeling Of Polymeric Systems: From The Atomistic To The Mesoscopic Scales And Bridging The Gap

Polymers can be theoretically and computationally described by models pertaining to different length scales and corresponding time scales.These models have traditionally been used independently of each other.Recently,considerable progress has been made in systematically linking models of different scales.In this thesis,computational methods are described as well as applied to study how to develop coarse-grained(CG) force fields for polymers from the atomistic molecular dynamics(MD) simulation.We have developed CG force fields for cis-poly(1,4-butadiene) homopolymer and poly(styrene-b-butadiene)(PS-b -PB) diblock copolymer.We describe the computational methods and discuss how they are applied to develop CG force fields for these polymers.We successfully tested these CG model against the chain properties derived from the atomistic MD simulation;the results suggest that the CG force fields are effective models.We have developed CG force field for the dissipative particle dynamics(DPD) simulation of the poly(styrene-b-isoprene)(PS-b-PI) diblock copolymer.The DPD model is constructed to match the physical description and structural properties of the PS-b-PI diblock copolymer.The CG force field for the diblock copolymer system includes bonded and non-bonded interactions,which are,respectively,derived from atomistic MD simulations and determined through fitting the experimental interfacial tension values.The morphologies obtained from our simulations for the diblock copolymer systems can be compared qualitatively with other simulations based on particle models.We have studied the formation of toroidal micelles of amphiphilic triblock copolymers in dilute solution using the DPD approach.Some microstructures of complex morphology having toroidal micelles have been observed in the simulations; the toroidal micelle formation is in accordance with previous theoretical prediction of the toroidal structure in cylindrical micelle suspensions.These findings are very interesting,and these complex morphologies enrich our knowledge of the potential products obtained from the self-assembly of block copolymers. We have studied shape transformations of vesicles using the DPD approach. The amphiphilic molecule is built from two different hydrophilic blocks on the sides and a hydrophobic block in the middle.A plethora of complex vesicle shapes is revealed by the DPD simulations.These simulated vesicles agree with theoretically derived vesicle shapes based on the spontaneous curvature model and also with experimental observations.Besides,DPD simulations have also been employed to study the fusion and fission dynamics of polymeric vesicles formed from amphiphilic triblock copolymers.Two different pathways for both fusion and fission processes of two-component vesicles have been found in the simulations.Moreover, the fission process of single-component vesicles has also been studied in the simulations,and the daughter vesicles have the same composition as the parent vesicle.The innovations of this dissertation are summarized as followed:1.We have extended the coarse-graining strategy to diblock copolymer system. The new force fields have been considered to distinguish between different monomers when calculating distribution functions.Besides,We have developed CG force field for the DPD simulation of the PS-b-PI diblock copolymer.2.The dynamic assembly of toroidal micelle structures of amphiphilic triblock copolymers in dilute solution has been investigated using DPD simulations. Direct observation of these dynamic processes has been used to find out their possible formation mechanism.3.A plethora of complex vesicle shapes,including some that have not been reported in previous simulation studies on vesicles,such as starfish-shaped, toroidal,long rod-like and inverted vesicles,are obtained in the DPD simulations; Moreover,two different pathways for both fusion and fission processes of polymeric vesicles have been found in our DPD simulations.