A Non-Equilibrium Dissipative Particle Dynamics Simulation Study On The Micelle And Thin Film Self-Assembly By Block Copolymers

Self-assembly of block copolymers is the process in which linear, graft, comb, and star block copolymers organize into ordered and/or functional structures without human intervention. These structures include thin films, polymer brushes, multilayered films, hybrid systems that combine polymers and particles, thin films of polymer networks, and membranes with channels/pores and nanoparticles (colloids, micelles, nanogels, capsules and vesicles, core-shell particles, hybrid particle-in-particle structures). In melt, the block copolymers via self-assembly can form ordered thin films on substrate. In solution, block copolymers via hydrophobic and hydrophilic interaction to form micelle and composite systems that homopolymer chains are adsorbed on the surface. These materials are possess potential applications and rapid developments in lots of aspects, such as photoelectricity apparatus, panel displays, lithography, high density data storage, as well as drug delivery, tissue engineering and biosensors. These materials are also related to soft matter so that a strong responsive signal can be induced by small stimuli. Stimuli-responsive polmer materials can change diffusion, wettability and adhesion of components on external field, as well as transform chemical and biochemical signals into mechanical, thermal, electrical and optical signals and so on. For example, the morphologies of block copolymer thin films are largely dependent on selective solvents, the homopolymer desorption and micelle fission induced by a weak shear. So far, the nature and driving force of self-assembly, especially in the non-equilibrium conditions (electric field, magnetic field, flow field), the stimuli-responsive mechanism of block copolymers self-assembly have not been fundamentally resolved.With the rapid development of stimuli-responsive and functional polmer materials, combining theoretical and simulation with experiment to study the self-assembly of block copolymers, becomes increasingly important in scientific area. With the invention of computer in the twentieth century fifties, the computer simulation method has made considerable progress. The precise quantum chemical method successfully resolves the structures of small molecules, reaction mechanisms, and optical, electromagnetic, magnetic properties. In the polymer field, there are also a number of successful simulation methods, including molecular dynamics, Monte Carlo, Brownian dynamics, dissipative particle dynamics, lattice Boltzmann method, self-consistent field theory, the dynamic density functional functional theory and so on. For the block copolymers, a. computer program can simulate the real dynamic process of self-assembly, and then test the theory and experiment, and make up the deficiencies. Using the effective dissipative particle dynamics and its derivative methods, we are able to study the block copolymers self-assembly in-depth. It is a good method to predict and interpret the results that experiment and theory can not obtain. In dissipative particle dynamics method, all the interactions among particles by three basic pairwise forces, including conservative force, dissipative force and random force. The spring force and the bond angle potential are also added to the DPD method, which used to control the equilibrium bond length and the flexibility of the polymer chain. These forces are softer and the integration time step is larger, so the time scale in DPD simulation can be at milliseconds, while the length scale can be at microns.In this paper, non-equilibrium dissipative particle dynamics and multi-body dissipative particle dynamics simulation methods are used to study on the thermodynamic and kinetic behavior of micelle formation and the adsorbed homopolymer under shear field, the structures and properties of thin films self-assembly by the linear diblock, linear triblock, and comb-type diblock, comb-type triblock copolymers with different evaporation, different surface, different components and different chain flexibilities, respectively. The main results are as follows:(1) We use equilibrium dissipative particle dynamics method to investigate the micelle formed by self-assembly of block copolymers, where a homopolymer chain is adsorbed on the micelle surface. Under shear flow, we use non-equilibrium dissipative particle dynamics method to investigate the processes of the homopolymer desorption and micelle fission. The results reveal the dependence of desorption time on the shear rate.(2) We use multibody dissipative particle dynamics method to investigate the thin film formed by self-assembly of linear diblock and linear triblock copolymers on attractive surface induced by evaporation, both in symmetric and asymmetric conditions. We consider the type and composition of block copolymers, the solvent effect, the surface effect and the flexibility of chains on the morphologise of linaer diblock and linean triblock copolymer thin films. We obtain two-layer and multi- layer lamellar morphology parallel to the surface, lamellar morphology perpendicular to the surface, spherical morphology and disorder morphology, and study the time evolution of the order parameter and the film thickness. The results show that the type and composition of block copolymers, the evaporation, the surface, and the flexibility of the chains determine the film morphology, the film thickness and the degree of ordering. The attractive solvent vapor increases the film thickness and impacts the film morphology at the interface. The attractive surface decreases the film thickness and impacts the film morphology near the surface. Solvent vapor and the surface strongly influence the interface and surface structures of the thin films, and then induce the formation of ordered thin films. When the solvent vapor and the surface interactions are constant, the composition of block copolymers has little effect on the film thickness, but has great influence on the degree of order. If one of the compositions is shorter, it is easy to form disordered morphology with small order parameter. On the other hand, if the two compositions are nearly equal, it is easy to form lamellar morphology with big order parameter. The chain's flexibility has a great influence on thin film morphology, too.(3) We use multibody dissipative particle dynamics method to investigate the thin film formed by self-assembly of comb-type diblock and comb-type triblock copolymers on surface induced by evaporation condition. We consider the type of block copolymers, the solvent effect, and the surface effect on the morphology of comb-type diblock and comb-type triblock copolymer thin films. We obtain order in order lamellar morphology, sandwich morphology, spherical morphology and disorder morphology, and study the time evolution of the order parameter and the film thickness. The results show that the type of block copolymers, the evaporation, and the surface determine the film morphology and the degree of order. The film thickness is also determined by solvent vapor and surface, where the attractive increases the film thickness and the attractive surface decreases the film thickness. Solvent vapor and the surface induce the interface and surface structure of the thin film, and then induce the formation of ordered thin films. However, the comb-type copolymers, compared with the linear block copolymers are not easy to form lamellar morphology parallel or perpendicular to the surface, more likely to form order in order lamellar morphology.In summary, our results may be helpful for the gene therapy, regulation of complex assembly, and the structure control of the mesoporous materials.