Structural, Mechanical, and Transport Properties of Membranes Formed by Amphiphilic ABA Triblock Copolymers

• Main Authors: Min-Yi Chen, 陳旻詣
• Other Authors: Yu-Jane Sheng
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/mtn5kj

碩士 === 國立臺灣大學 === 化學工程學研究所 === 105 === ABA amphiphilic triblock copolymers are potential materials that can be widely used in the medical science and engineering. In our study, planar membranes formed by A3B14A3 triblock copolymers in a selective solvent are explored by dissipative particle dynamics in a constant normal pressure and constant surface tension ensemble. Unlike bilayer membranes, both the bridge (I-shape) and loop (U-shape) conformations coexist. When hydrophilic (A) and hydrophobic (B) blocks are weakly immiscible, the dynamic change in shape between U- and I-shape is significant. The relaxation dynamics of polymer conformations is monitored and can be described by the exponential decay. The relaxation time depends on the initial composition and grows with the immiscibility between A and B blocks (aAB). However, the equilibrium composition of the membrane is independent of the initial composition and aAB and it is found that the equilibrium composition of the U-shape polymer (f_u^*) exceeds 0.5 due to the fact that U-shape polymer possesses larger conformation entropy. The influences of the immiscibility between A and B blocks (aAB) on the geometrical, mechanical, and transport properties of the membrane are also investigated. As aAB increases, the overall membrane thickness and the B block layer thickness (h) rise because of the increment of the molecular packing. As a result, both the stretching (KA) and bending (KB) moduli grow significantly with increasing aAB. Consistent with typical membranes, the ratio KB/KAh2~2×10-3 is a constant. Although the lateral diffusivity of polymers is insensitive to the immiscibility, the membrane permeability decreases substantially as aAB is increased. For strong immiscibility between hydrophilic (A) and hydrophobic (B) blocks, the transformation between U- and I-shape is negligible. Therefore, the equilibrium composition is the same as the initial composition we constructed. Structural, mechanical, and transport properties of the membrane are analyzed for membrane with various fraction of the U-shape copolymer. The thickness of the ABA membrane and the B block layer (h) increase as the fraction of U-shape copolymer (f_u) increases. However, the averaged cross-sectional area per polymer decreases while the averaged volume per polymer rises as f_u grows. Moreover, the stretching modulus (KA) and the bending modulus (KB) of the membrane are found to decline with increasing f_u while KB/KAh2~2×10-3. Based on the velocity autocorrelation, the ability of ABA copolymers to move within the membrane enhances when more U-shape copolymers are present. Additionally, more substances penetrate across the membrane as f_u raises. These results indicate that the characteristic properties of ABA membrane can be adjusted by altering the fraction of U-shape copolymers. In reality, it is difficult to construct the membrane with specific f_u; therefore, Two methods are proposed to manipulate the equilibrium conformation. First, various lengths of B block are adopted. It is found that more U-shape copolymers exist in the membrane as the length of B block increases. Next, the stiffness of the B block is enhanced to reduce the fraction of U-shape copolymer in the membrane. The result shows that some voids appear in the membrane when the stiffness of the B block is increased. Nevertheless, as the stiffness of the B block exceeds certain value, voids vanish because the copolymers behave as liquid crystals.