Computer simulation on self-assembly of complex block copolymer in selective solvent

Abstract

The self-assembly of block copolymers, as a fascinating bottom-up strategy to design nanostructured materials, attracts much attention in recent nanotechnologies. In dilute solution, the amphiphilic block copolymers can form various micelles in mesoscale, such as sphere, rod, toroid and vesicle. The various morphologies have potential applications in nanotechnology as templates, nanoreactors or carriers for medicine. In this thesis, computational methods are adopted to investigate the self-assemblies from complex block copolymer in selective solvent. Furthermore, the application of micelles such as core-shell micelles and segmented worm micelles on drug delivery system will be explored in our thesis. In chapter 1, we introduce the basic theory of self-consistent field theory and dissipative particle dynamic method. The coarse-grained (CG) method, Flory-Huggin interaction parameters which bridge the connection between real polymers and simulation particles are also introduced in our thesis. In chapter 2, microstructures self-assembled by amphiphilic ABC star and -shaped block copolymers in dilute solution have been investigated by self-consistent field theory. For the star copolymer, we find a core-shell-corona structure is more favored than layer-by-layer structure when the two hydrophobic chains are relatively asymmetric. This may ascribe to the fact that polymer with higher interfacial energy will try its best to avoid contact with the solvent to lower the system energy. The self-assembled morphologies could be significantly influenced by the confinement thickness due to the loss of conformation entropy. Only segmented worm like micelles could be entropic favorable with very small film thickness. For the -shaped block copolymers, the effects of architectural parameters and the interaction strength among the three blocks have been studied systematically. Our calculation results show that the distance of the two graft blocks has stronger effect than the length of graft blocks and the position of the first graft point on the phase behavior. The interaction strength among the three blocks is another important factor in controlling the resulting microstructures. Compound-core, multicompartment, and multicore micelles are observed in the case of pi-shaped ABC block copolymers with hydrophilic backbone block A and hydrophobic graft blocks B and C. Core–shell–corona, incomplete skin-layered and hamburger micelles are formed when graft block C is hydrophilic and blocks A and B are hydrophobic. The wormlike multicore micelles have drawn our attention. We find that the morphology of wormlike multicore micelle can be controlled by changing the distance of the two graft blocks of the pi-shaped block copolymers. In all of the wormlike multicore micelles, the streamline wormlike micelle is more stable than other wormlike micelles from the free energy analysis. In chapter 3, The laterally nanostructured vesicle by self-assembly of μ-[poly(ethylethylene)][poly(ethylene oxide)][poly(perfluoropropylene oxide)] (μ-EOF) star terpolymers in aqueous solution has been investigated by dissipative particle dynamics (DPD) simulation. The simulation results show that the laterally nanostructured vesicle forms when the length of the hydrophilic O block is relatively short. In the lateral nanostructure, the hexagonally packed domains formed by the hydrophobic F block are immersed in a two-dimensional hydrophobic E block matrix. The formation condition and microscopic structure of the nanostructured vesicles in simulations are agreed well with the reported experimental results. In the dynamic of the vesicle, it follows three stages: (1) spherical and short cylindrical raspberry-like micelles combine into medium polygonal sheet; (2) medium polygonal sheet grows to form large polygonal sheet with tail; and (3) large polygonal sheet with tail fold over and form vesicle. The closure of polygonal sheet in the third is ascribed to the asymmetric distribution of stress induced by small fluctuation in system. The effect of hydrophobic F blocks is also been studied too. The structural evolution from nanostructured bilayer vesicle to F-bump-E bilayer vesicle can be controlled by increasing the hydrophobic block E length. Both the vesicles are formed via a bending and closing mechanism, however, the inducing mechanism for bending are different. For the F-bump-E vesicle, the bending is caused by the dispersion of the hydrophilic molecule into the hydrophobic core. We also find that the cycle to form a vesicle via disperse-to-bend is much faster than fluctuate-to-bend pathway. The results gained through the simulations provide a new insight into the formation mechanism of the multicompartment micelles. In recent years, interest in micelles as potential delivery vehicles for pharmaceuticals, gene therapy agents, pesticides, personal care products, and even food formulations has grown enormously. In chapter 4, the drug release mechanism of polymeric delivery vehicle (polymeric microsphere) is investigated with Dissipative particle dynamics simulation. A core-shell spherical micelle with drug encapsulated in the core is obtained in our simulation. By changing the medium from neutral to acid, the drugs release via a diffuse mechanism. Both the formation mechanism of the encapsulant and the release mechanism for the drugs are studied in our work. For the formation process, it can be ascribed as the coalesce of the small clusters and the disperse of the drugs; while for the drug release behavior, the process can be divided for three stages: (1) swell of the polymeric carrier, (2) drug diffuse in the carrier and some acid molecules disperse into the carrier, (3) drug release towards the acid medium. Our results might provide a mesoscopic methodology for the evaluation and prediction for polymeric self-assemblies as a carrier for pharmaceutical interest. In many cases one might wish to deliver two or more active agents to the same place at the same time. As a result, whether the agents would undergo chemical reaction before reaching the site of interest should be considered. Although it is possible to prepare micelle aggregates of two distinct types, this does not solve the problem of guaranteeing both agents to arrive at the same place or at the same time. Multicompartment micelles could offer this possibility by dividing the core of a micelle into two or more distinct nanodomains, different agents might be transported simultaneously within one micelle while being kept separate within the various core compartments. In this chapter we also demonstrate the segmented worm micelles made by the self-assembly of mikto-arm star terpolymer, which could sequester two different small-molecule agents in separate domains. By adjusting the environment from neutral to acid, morphologies evolve from segmented worm micelles to raspberry-like micelles as well as the two agents releasing from the micelles.

Date of Award	14 Feb 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Yiu Robert LI (Supervisor)