以濕式法製備表面具有緻密層與圖案結構的抗阻塞薄膜

Abstract

薄膜過濾技術由於其操作簡單、節省能源等優勢而成為具有前瞻性的水循環再生技術之一。在使用薄膜過濾技術時，膜的結垢問題常限制了薄膜的壽命和分離效率，而在減少薄膜結垢之策略中，膜表面的圖案化技術在近年來受到廣泛關注。在膜表面製作圖案可增加薄膜過濾面積，而流體施加於圖案之剪切應力與圖案周圍之流場也減輕結垢問題。然而過去的研究中，膜表面圖案的形狀簡單且對稱，在本研究中，我們希望進一步研究圖案的特性如何影響膜的抗結垢效果，為達到這個目的，我們構思出三種考量到流場之圖案，分別為波浪形、箭頭形和反箭頭形:在波浪形圖案中引入平行水流且忽寬忽窄之渠道；箭頭圖案中，除了平行水流之渠道，我們預留了具有傾斜角度的通道使懸浮物往渠道集中；反箭頭圖案即反轉180度之箭頭圖案，因箭頭圖案不具對稱性，因此可在表面積相同的情況下比較不同圖案間對抗結垢效果之影響。 首先觀察旋轉塗布機轉速對膜厚與圖案高度之影響，觀察到轉速越高，膜厚越薄，但圖案高度基本不受轉速影響。接著觀察圖案製備於薄膜後之樣貌，發現圖案實際之尺寸因成膜過程中薄膜收縮而較原設計尺寸小。接著我們進行薄膜之性能測試，量測了具有不同圖案薄膜之純水通量後，觀察到所有圖案之純水通量皆因薄膜表面增加了表面積而較無圖案之薄膜大，但因圖案不同，因此使得在相同透膜壓力下，圖案之局部壓力和流場也不同，使得圖案之純水通量並不完全與表面積成正比。在純水中添加牛血清白蛋白(BSA)以量測圖案抗阻塞能力，我們觀察到箭頭圖案具有有效之抗阻塞能力，其他圖案則無。所有圖案因流場不同使得BSA在圖案底部分布不均，在波浪形圖案中，水流在窄處較快，BSA淤積較少，但在寬處流速慢，BSA淤積較多。在箭頭圖案中與反箭頭圖案中，在渠道和傾斜流道的交會處則淤積較多BSA，但箭頭圖案的傾斜流道中水流流向與渠道水流流向為順向，反箭頭則為逆向，因此箭頭圖案中傾斜流道處BSA淤積較少，反箭頭圖案淤積較多。透過比較箭頭與反箭頭圖案的結果，我們證明良好的圖案可有效的減少薄膜的阻塞，不良的圖案形狀不僅可能無法減少阻塞，甚至可能加劇薄膜的淤積而影響其使用壽命。 因為以濕式法製備之圖案化薄膜表面充滿了孔隙，而這些孔隙可能會加劇薄膜表面結垢，為了製作出具有緻密表面的圖案化薄膜，我們需要加速模具和鑄膜液之間的溶劑和非溶劑(水)交換，並以提高模具中的儲水量和透水率來實現此目的。嘗試過數種方法後，我們發現將具有高親水性之聚乙二醇二丙烯酸酯(PEGDA)以2wt%的濃度添加至聚二甲基矽氧烷(PDMS)，並在製膜之前，將模具浸泡在水中一整夜，即可成功地製備出在圖案側具有緻密結構的薄膜。然而，其抗阻塞效果並不如預期，推斷可能是因為薄膜的表面具有孔隙時，會造成局部流場之變化，導致薄膜表面剪切力較大，反而減少BSA淤積。

Membrane filtration is a popular approach for water regeneration due to its simple operation and energy-saving advantages. In this technology, membrane fouling is the main issue that determines the lifespan and the efficiency of the process. Among various strategies to mitigate membrane fouling, patterned membrane has garnered much attention in recent years. In addition to increasing the filtration area of the membrane, the surface pattern alters the local flow field and alleviates fouling. However, in previous studies, the shapes of the patterns were simple and symmetric. In this research, we proposed three pattern designs including wave, arrow, and reverse arrow. By comparing the antifouling effects between different patterns, we aimed to understand which characteristics of the surface pattern contributes to the antifouling ability of membranes. To start our study, we first determined the impact of rotation speed of spin coating during casting process on membrane thickness and pattern height. We found that higher rotation speed resulted in thinner membranes while the pattern height remained relatively constant. Next, we examined the fidelity of the patterns on the membrane and discovered that the actual dimension of the patterns was smaller than the original design due to shrinkage during phase inversion. Subsequently, we measured the pure water flux of the membranes and found that all patterned membranes performed better than flat membrane, attributed to the increased surface area of the patterned membranes. However, due to the variations in pattern shape, the local pressure and flow field differed even under the same transmembrane pressure, leading to a non-linear relationship between the pure water flux and membrane surface area. To evaluate the antifouling capability, we introduced bovine serum albumin (BSA) aqueous solution as the filtrate. We found that only the membrane with arrow pattern showed effective antifouling ability. By examining the distribution of BSA on membranes using confocal laser microscopy, we found BSA are more likely to deposit on the places where the shear stress is lower or where multiple flows converge. Moreover, by comparing the results of membranes with arrow and reverse arrow patterns, we concluded that surface pattern could reduce or enhance membrane fouling, depending on the local flow filed caused by the surface pattern. On the patterned membrane prepared by the wet immersion method, we observed that the surface is filled with pores and we believed that these pores may also promote membrane fouling. To avoid pore formation on the membrane surface, we need to accelerate solvent and non-solvent (water) exchange between the mold and the casting solution during the phase inversion process. After several attempts, we have successfully made patterned membranes with dense surface by incorporating highly hydrophilic poly(ethylene glycol) diacrylate (PEGDA) to polydimethylsiloxane (PDMS) mold. However, the membranes with dense patterned surface show no antifouling ability, even worse than those with pores. We speculated that pores on the patterned surface may change local flow field and induces greater shear stress, resulting in less deposition of BSA. To conclude, we have shown that only a well-designed surface pattern can improve the antifouling ability of membranes. However, a more detail study is still needed to give reliable design principles for real applications.