藉由雙面揮發高分子液膜製備巨孔穿透薄膜

Abstract

呼吸圖法 (Breath Figure, BF)是一種在固體基材上製備有序等孔薄膜的方法，然而一般BF法的薄膜其孔洞僅存在於表面，無法形成穿孔，要依賴特殊的基材才能達成穿孔，例如水或冰，來增強基材的親水性，但這也增加了製備的難度，且薄膜的厚度至多為 1~2 μm，缺乏足夠的機械性質，需要將薄膜轉移至另外的多孔支撐物方可進行過濾應用，額外轉移步驟對薄膜難免會造成破損。 本研究引入新的相分離機制-水溶性溶劑輔助呼吸圖法 (Water Miscible Solvent-assistant Breath Figure, WMSBF)，使用苯乙烯-丙烯腈共聚物 (SAN)，混合氯苯 (CB)、氯仿 (CF)與二甲基亞碸 (DMSO)為高分子溶液，將無基板線圈浸泡至溶液後拉出形成液膜，透過WMSBF機制，雙面揮發的過程中孔洞會在液膜兩表面成核並深入成長，最終合併形成巨孔穿透薄膜。我們有系統地研究影響成膜的條件，也探討實驗變因，例如：高分子濃度、溶劑比例、環境濕度、氣流速度等，如何影響薄膜的孔徑、孔隙率、厚度及穿孔性等，最後成功製備平均孔徑約 2.2 μm之高分子穿孔薄膜，厚度可以達到 4 μm，具優異的機械強度，並可輕易地從線圈取下成為獨立膜，能直接進行過濾應用。透水性實驗中該薄膜在重力條件的壓力下，水通量為120 L m-2 h-1，在酵母過濾實驗中，就算在高壓力下過濾薄膜也不會破損，酵母攔截率幾乎為100 %。

Breath Figure (BF) is a method for preparing ordered isoporous membranes on solid substrates. However, by the BF method, pores generally only exist on the surface and cannot penetrate. For the perforation of the pores, it requires a special substrate, such as water or ice, to greatly enhance the hydrophilicity of the substrate, which adds the fabrication complication and difficulty. Furthermore, the thickness of the resulting membranes is at most 1~2 μm, lacking of sufficient mechanical properties. Therefore, the thin membranes are usually necessary to be transferred to another porous supports for filtration application. Additional transfer steps inevitably cause damage to the membrane. In this study, we introduce a new phase separation mechanism - water miscible solvent-assistant Breath Figure (WMSBF) for preparing styrene-acrylonitrile copolymer (SAN) porous membranes using chlorobenzene (CB), chloroform (CF) and dimethyl sulfoxide (DMSO) as solvents. The membranes are cast through double-sided evaporation of polymer liquid films formed on a wire loop that is dipped up from the polymer solutions. Through the WMSBF mechanism, the pores are nucleated on the two surfaces of the liquid films during evaporation, then grow into the films, and eventually merge to form the perforated macroporous membrane. We systematically studied the conditions that affect membrane formation and investigated how the experimental parameters, such as polymer concentration, solvent ratio, relative humidity, and air flow rate, affect the pore size, porosity, thickness, and perforation of the film. Polymer perforated membranes with an average pore diameter of approximately 2.2 μm and a thickness above 4 μm were successfully prepared. The membranes are mechanically robust and can be easily removed from the wire loop in a free-standing form, capable of direct usage for filtration applications. In the water permeability experiment, the membrane has a water flux of 120 L m-2 h-1 in a pressure under gravity conditions. In the yeast removal tests, the filtration can be conducted at elevated pressure without damaging the membrane and the yeast rejection rate is almost 100%.