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  • 學位論文

雙親性ABA三段鏈共聚高分子薄膜的結構、機械、輸送性質之探討

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

指導教授 : 諶玉真

摘要


ABA雙親性三段鏈共聚高分子在醫藥工程領域是相當有潛力的材料,在本實驗中,我們利用耗散粒子動力學(DPD)模擬的方式,針對A3B14A3三段鏈共聚高分子在NPγT系統下於選擇性溶劑中形成的薄膜結構進行探討。U型和I型高分子共同存在於此薄膜中,與雙層膜不同。 當A和B段鏈間的不相容性較弱時,U和I構型間的轉換是很頻繁的。我們利用exponential decay來描述構型的轉換行為,我們發現鬆弛時間隨著A和B段鏈間的不相容性上升而上升,並且與薄膜的初始構型比例有關。但是當薄膜達到平衡時,薄膜的構型比例卻與A和B段鏈間的不相容性以及薄膜的初始構型比例無關,並且因為U型高分子構型亂度較高,使得平衡時U型高分子較多,所占的比例超過0.5。我們也研究了薄膜的結構、機械、輸送性質隨著A和B段鏈間不相容性不同之變化。當A和B段鏈間不相容性變強時,薄膜的厚度(hm)以及疏水厚度(h)都會上升,因為分子間排列地更加緊密。拉伸係數(KA)和彎曲係數(KB)都隨著A和B段鏈間不相容性上升也呈現上升的趨勢,並且KA、KB、h間算出KB/KAh2大約是2×10-3,符合薄膜間這三者性質應是常數的關係式。雖然A和B段鏈間不相容性對擴散係數的影響不明顯,但隨著A和B段鏈之間作用力的上升,滲透度有相當明顯地下降。 接著我們建構各種不同構型比例的薄膜,並將A和B間設定為固定的強相斥作用力,使得U和I間的構型轉換頻率幾乎可以忽略,這讓我們能夠得到與初始結構比例一樣的平衡薄膜。我們分析薄膜的各種性質是否會隨著環境中U型高分子數量的不同而有所變化,薄膜厚度(hm)與疏水厚度(h)隨著環境中U型高分子的增多而上升;但是,高分子的面積卻隨之下降,而高分子的體積則是上升。此外,隨著U型比例的上升,拉伸係數(KA)和彎曲係數(KB)都會減小,但仍滿足KB/KAh2大約是2×10-3的關係式。我們也發現高分子在周圍環境有較多U型高分子的情況下,較容易移動,並且有更多的物質可以穿越薄膜。這些結果顯示可以藉由調控U型高分子在薄膜中的比例,改變薄膜的特性。 由於我們很難在現實中利用初始結構控制薄膜的比例,因此,我們利用兩種方法來控制薄膜的平衡比例。我們先利用不同長度的B段鏈組成薄膜,結果發現當B段鏈的長度越長,薄膜中的U型高分子就越多。接著我們將B段鏈的硬度加大,但造成薄膜中的U型高分子減少,此時,薄膜結構中出現了孔洞;而當硬度太高時,高分子會呈現液晶的緊密排列方式,這使得薄膜中的孔洞反而會消失。

並列摘要


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.

參考文獻


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