|Abstract: ||Self-assembled polymer membranes have attracted a growing attention due to their multifunctionality and stability. Compared to lipid membranes, polymer membranes have enhanced mechanical and transport properties with high molecular weight. Numerous experimental techniques have been developed to explore membrane characteristics; however, experimental microscopic observations and knowledge of vesicles are limited. Mesoscale simulations can complement experimental studies of the membrane features at the microscopic level and thus provide a feasible method to better understand the relationship between the fundamental structures and physicochemical properties of a membrane. Moreover, the predictive ability of the simulation approaches may greatly assist developments and future applications of biomimetic membranes.
This dissertation uses dissipative particle dynamics (DPD) to explore the self- assembly of three polymeric systems. We have paid particular attention to the fundamental properties of polymer membranes and their biological behaviors. There are three parts in this dissertation. In the first part (Chapter 3), the formation and physical properties of solid-supported polymer bilayer (SPB) on an adhesive substrate have been explored. SPB is developed by the adsorption of vesicles formed by diblock copolymers in a selective solvent. The adsorbed vesicle can remain intact or become ruptured into SPB, depending on the interaction between solvophobic block and solvent and the interaction between solvophilic block and substrate. The morphological phase diagram of adsorbed vesicles is acquired. The influences of polymer adhesion strength and solvophobicity on the geometrical and mechanical properties of SPB are systematically studied as well. It is found that vesicular disruption is easily triggered for strong adhesion strength Moreover, for strong adhesion strength and weak solvophobicity, the fluctuation of membrane height is impeded while the area fluctuation is enhanced.
In the second part (Chapter 4), Instead of forming typical bilayer or monolayer membrane, both the bridge (I-shape) and loop (U-shape) conformations are coexistent in the planar membranes formed by ABA triblock copolymers in a selective solvent. The non-equilibrium and equilibrium relaxation dynamics of polymer conformations are monitored. The non-equilibrium relaxation time depends on the initial composition and grows (increases) with (an increase in) the immiscibility between A and B blocks. The equilibrium composition of the loop-shape polymer is independent of the initial composition and A-B immisibility. However, the extent of equilibrium composition fluctuations subsides as A and B blocks become highly incompatible. The influences of the A-B immiscibility on the geometrical, mechanical, and transport properties of the membrane are also investigated. As immiscibility increases, the overall membrane thickness and the B block layer thickness (h) rise (increase) because of the increment of (in) the molecular packing (density). As a result, both the stretching (K_A) and bending (K_B) moduli grow significantly with increasing A-B immiscibility. Consistent with typical membranes, the ratio K_B/K_A h^2=2×〖10〗^(-3) is a constant. Although the lateral diffusivity of polymers is insensitive to the immiscibility, the membrane permeability decreases substantially as A-B immiscibility is increased.
In the third part (Chapter 5), The influences of the branching patterns on the membrane properties of Janus dendrimers in water have been investigated. The hydrophobic fluorinated dendron (RF) contains three types of branching patterns, including 3,4-, 3,5-, and 3,4,5-RF. Consistent with experimental results, the hydrophobic layer thickness (H_B) follows the order: 3,5-RF < 3,4-RF < 3,4,5-RF, which can be explained by the extent of interdigitation (∆h) : 3,5-RF > 3,4-RF > 3,4,5-RF. Moreover, the 3,4,5-RF membrane shows the highest stretching modulus (KA) and the lowest lateral diffusivity (D). The 3,5-RF membrane is similar to the 3,4-RF membrane but exhibits higher KA and smaller D. For the nano-sized dendrimersome, its bilayer thickness is less than that of the planar membrane due to its larger extent of interdigitation. The coassembly of dendrimersomes with lipids has been studied as well. The thickness and the extent of interdigitation of the lipid-rich domain for the hybrid membrane is significantly affected by the lipid concentrations (∅_l) and the branching patterns. As ∅_l increases, the thickness of the lipid-rich domain grows corresponding to the decrease of interdigitation of the lipid-rich domain.