高分子溶液的流變性質與膜結構之關聯

Abstract

先前的研究認為，高分子薄膜的結構型態與其鑄膜液在介穩區(meta-stable region)中的穩定程度有關，此穩定程度可以用溶液在介穩區的臨界滯留時間(tmc)來評估。tmc愈長，表示高分子溶液在介穩區較穩定，需停留較久才會進行成核成長(nucleation and growth)相分離，在此停留時間，溶劑與非溶劑交換會持續進行，溶液組成便有較高的機會進到非穩區(unstable region)，以spinodal decomposition的機制進行相分離，形成孔洞連通的結構。 本研究進一步對tmc與鑄膜液流變性質的關聯性作探討，結果顯示tmc與高分子鏈的鬆弛時間(relaxation time)有關。鑄膜液鬆弛時間愈長的成膜系統，對應的tmc值會愈高，容易製備出孔洞連通的高分子膜。 另外也觀察到高分子鏈的鬆弛時間長短與其分子間作用力的強弱有關，因此醋酸纖維素(cellulose acetate, CA)、三醋酸纖維素(cellulose triacetate, CTA)這類具分子間氫鍵作用力的成膜系統，會較容易形成孔洞連通的結構。除了分子間作用力外，高分子鏈的糾纏(entanglement)亦會影響鬆弛時間。對於不易糾纏的聚甲基丙烯酸甲酯(poly(methyl methacrylate), PMMA)溶液，要在高分子濃度較高時，鬆弛時間才能高到可以生成連通孔洞的程度。本研究亦將引用文獻上對於有高分子鏈糾纏與無糾纏狀態下分子間作用力對鏈鬆弛時間影響的理論分析，來解釋本研究所觀察到的現象。

According to some literatures, the structure of polymer membrane is related to the stability of its casting solution in meta-stable region, which can be evaluated by the critical residence time for the solution to stay in meta-stable region (tmc). Larger tmc means that the polymer solution is more stable in meta-stable region. As a result, it will undergo nucleation and growth only if the residence time is long enough. During this residence time, the solvent and nonsolvent will keep exchanging with each other, causing more opportunity to enter the unstable region and undergo spinodal decomposition to generate the bi-continuous lacy structure which has more connective pores. This research further focused on the relationship between tmc and the rheological properties of the casting solution. The results show that tmc is related to the relaxation time of polymer chains. The membrane formation systems with higher relaxation time will also have higher tmc, and thus more easily to obtain the pore-connective structure. The value of the relaxation time is associated with the intermolecular interaction. Therefore, polymers which can form intermolecular hydrogen bonding, such as cellulose acetate or cellulose triacetate, will have more chance to get the pore-connective structure. Except for the intermolecular interaction, the entanglement of the polymer chain will also affect the relaxation time. For the poly (methyl methacrylate) system, whose polymer chains are more difficult to entangle, higher polymer concentration is needed for the relaxation time to rise to an extent that connective pores can be generated. This research will also explain the above phenomenon using the literatures which analyze the effect of intermolecular interaction on the relaxation time of polymer chains with and without entanglement.