Monte Carlo Simulations And Experimental Observations Of Soft Matter Composed Of Chain-like Molecules

In the past two decades, polymers, biomembranes, liquid crystals, colloids and gels etc. have been integrated into "soft matter". Many phenomena in these systems share a similar physical mechanism. Moreover, combinations of basic elements can exhibit new properties. Soft matter distinguishes itself as being very sensitive to external stimuli. Entropy or thermal fluctuation is dominant in these systems. Soft matter are very complex, usually containing mesoscopic ordered structures.To date, the study of soft matter is one of the "hottest" scientific topics, and meanwhile encounters many difficulties. In experiment, it is hard to simultaneously detect the properties of different length scales, i.e. the microscopic chain structure, mesoscopic aggregation morphology and macroscopic mechanical properties. In the theoretical aspect, the analytical treatment is hard to perform due to the complicated underlying interactions.Today, computer simulation has been regarded as the third research approach besides theory and experiment, and become a powerful tool in the study of soft matter. Monte Carlo (MC) simulation is a widely used stochastic simulation method, and lattice MC simulation is especially efficient as the computing cost is concerned. MC simulation can be applied to study different scale properties simultaneously and the dynamics process.This Ph. D thesis focuses on some soft matter such as block copolymer, polymer brush, and vesicle etc with MC simulations. As a continuation of the previous research of master thesis, a real-time observation of the wetting process of dry cylindrical hydrogel in water was also performed experimentally.The main achievements are summarized as follows:1.The rheological properties of concentrated block copolymer solutions under oscillatory shear have been studied by nonequilibrium MC simulation. M.icrophase separation of block copolymer has been examined. We examined the dynamic shear flow of multi-amphiphile systems by introducing the shear field in the formulism of nonequilibrium lattice MC simulation. Stress tensor has also been obtained based upon sampled configuration distribution functions. Phase transition and associated rheological behaviors of micellar solution have been investigated. The simulation outputs of frequency sweep and temperature sweep are consistent with the experimental observations in literature. Chain deformation during oscillatory shear flow has also been revealed. The highlight of our simulation is that it can, at small computing cost, investigate polymer chains simultaneously at different spatial scales, i.e., macroscopic rheological behaviors, mesoscopic self-assembled structures, and microscopic chain configurations.2. With the employment of above-mentioned nonequilibrium MC simulation of lattice chains, we reproduced the "abnormal" phenomenon of abrupt increase of normal stress of polymer brush under oscillatory shear flow reported by Klein et al. in Nature, and meanwhile argued their interpretation. The equilibrium and dynamic properties of polymer brush, especially the properties subjected to oscillatory shear have been investigated by us. The grafted chain under oscillatory shear flow exhibited a waggling behavior like a flower, and the segments were found to have different oscillatory phases along the chain contour. The simulation reproduced the abrupt increase of the first normal stress difference N\ when the flow velocity was over a critical value, as observed in the experiment. However, our simulation outputs did not reproduce the suspected brush thickening with shear velocity increased, which was used to account for the abrupt normal stress increase in the literature. The increase of N\ is, according to our simulation, an inherent behavior of polymer brush under oscillatory shear with large deformation. The simulations with different frequenciesfurther strengthened our viewpoint.3. The spontaneous formation of double-layered vesicles from pure and mixed amphiphiles has been simulated, and re-distribution of amphiphiles between inner and outer layers during vesicle fusion was predicted. Both pure and mixed amphiphiles were self-assembled into vesicles under appropriate conditions. When mixed amphiphiles were examined, the amphiphiles with longer hydrophilic blocks preferred to segregating into the outer monolayer of the resultant vesicles, which is consistent with the experimental observations in the recent literature by Eisenberg et al. The kinetic study reveals that the increase of vesicle size is mainly caused by the mechanism of vesicle fusion at the early stage, and the evaporation-condensation mechanism cannot be neglected at the late stage. The fusion of vesicles is accompanied with translocation of chains from the outer monolayer to the inner monolayer. Compared to the chains with shorter hydrophilic blocks, those with longer hydrophilic blocks exhibit stronger trends to translocate from the outer monolayer to the inner one in vesicle self-adjustment.4. Lattice MC simulation was used to examine the lateral phase separation in double-layered vesicles, and the budding processes were reproduced. Domain induced budding has been observed when the phase separation was triggered by weaken attraction between different kinds of amphiphiles, while only phase-separated domains were observed in the case of enhanced attraction between similar amphiphiles. Our results consist with the Lipowsky theory. The effect of the spontaneous curvature on the budding direction has been examined. It was found that the budding of domains towards the inner was difficult to be achieved in the simulation due to the restriction of the parent vesicle. The kinetic study suggests that the domain growth in our simulation follows the evaporation-condensation mechanism.5. By real-time experimental observations, we have found that the wetting process of dried hydrogels obeys the mechanism of case-II diffusion. A cylindricalpoly(acrylic acid-co-acrylamide) gel was prepared and the swelling of dried gels was observed in real time after the gels were re-immersed in water. Regular patterns have been found during this process. The wetting process was divided into three stages: first, swelling in the radial direction with cusp-like patterns evolving on the surface; second, shrinking in the radial direction while swelling in the axial direction; the last one, re-swelling in the radial direction. Our quantitative measurements also reveal that the movement of the "wetting front" at the first stage exhibits case-II like diffusion when water penetrates into polymeric network.