以非溶劑誘導式相分離法製備聚偏二氟乙烯薄膜及其成膜機制之探討

Abstract

本研究以蒸氣誘導式及液態誘導式相分離法(vapor-induced and liquid-iuduced phase separation, VIPS & LIPS)製備聚偏二氟乙烯薄膜(poly(vinylidene fluoride), PVDF)，探討溶解PVDF高分子鑄膜液的溶解溫度(dissolution temperature, Tdis)如何影響PVDF薄膜表面形態(morphology)及結晶構型(polymorphism)，並探討薄膜形態及構型的成形機制，期望能建立一個結構控制的標準。研究結果顯示，當Tdis通過某一特定溫度後，薄膜表面形態及結晶構型會戲劇性地轉變，在此定義此轉折溫度為臨界溶解溫度(critical dissolution temperature, Tcri)。當溶解時的Tdis高於Tcri，成膜後所得到的薄膜表面形態為顆粒(nodules)狀結構，且高分子(顆粒)尺度大小隨溶解溫度增加而成長，我們稱這樣高分子富相在相分離後合併的行為為「自由自在的合併(free coarsening)」”；當溶解PVDF鑄膜液時的Tdis低於Tcri，在相分離時高分子富相的合併行為被抑制，難以合併成較大的尺度，所以所製備的薄膜為雙連續(bi-continuous)結構，我們稱這樣的合併行為為「受限制的合併(hindered coarsening)」。因此依照薄膜表面形態，可藉由Tcri將高分子在相分離的合併行為分為兩類，free coarsening 及hindered coarsening，所以Tcri為判斷結構合併的指標。薄膜表面形態在Tcri前後的轉變不僅僅是以N-甲基吡咯酮(N-methyl-2-pyrrolidone, NMP)為溶劑才會有的現象，同樣的轉折現象可以在以N,N-二甲基乙醯胺(N,N-dimethylacetamide, DMAc)及 N-二甲基甲醯胺(N,M-dimethylformamide, DMF)為溶劑，或是以水蒸氣或一系列的醇類為非溶劑時發現。 我們推測溶解高分子溶液時的溶解溫度(Tdis)改變溶液中結晶晶核的密度，鑄膜液中晶核密度會隨著Tdis的上升而減少，影響後續相分離時的膠化及結晶行為，而有不同的薄膜表面形態。本研究藉由鋼珠球在高分子溶液的沉降實驗探討晶核密度對溶液膠化行為的影響，並進一步藉由結晶型態的特徵峰的觀測來探討晶核密度對膠化及結晶的影響，在此提出兩種膠化機制，非結晶起始的膠化行為(Non-crystallization-initiation gelling, NC-gelling)及結晶起始的膠化行為(Crystallization-initiation gelling, C-gelling)，此兩種膠化行為以Tcri為分水嶺。在以VIPS成膜時，由於溶劑與非溶劑交換速度相對於LIPS慢，有機會使溶液中結晶現象優於液液相分離先發生，對於Tdis低於Tcri的系統，溶液會在結晶構形產生前即膠化，高分子鏈的運動強烈地受到影響，無法自由自在的合併為顆粒，因此後續的結晶不會造成尺度的成長；對於Tdis高於Tcri的系統，由於溶液內的晶核密度較低且膠化隨結晶而發展，使得高分子鏈受到結晶的驅動力較容易成長成較大的尺度。 本研究所使用的非溶劑誘導式相分離法主要分為兩種，蒸氣誘導式相分離法(VIPS)及液體誘導式相分離法(LIPS)，此兩種方式在於成膜過程中溶劑跟非溶劑的交換速度有著極大的差異，造成液液相分離(liquid-liquid demixing, L-L demixing)及結晶(crystallization)機制上的競爭差異。當以LIPS法搭配異丙醇(iso-propanol, IPA)為非溶劑製備薄膜時，由於溶劑與非溶劑快速交換，組成在相圖上會快速由均勻相進入液液相分離區，所以成膜機制主要是先由液液相分離所主導，儘管如此，結晶隨後亦會跟著發生，其中對於晶核密度少的溶液，高分鏈會先受到液液相分離的影響而形成典型的雙連續結構，再合併成顆粒狀結構，由於合併行為發生在雙連續結構出現之後，故僅僅合併成表面多孔的顆粒狀結構；反之，對於晶核密度多的溶液，若溶液運動受到快速膠化行為的影響，溶液快速變成半透明膠狀物質並維持雙連續架構。當將IPA置換成正丁醇，在相圖上，液液相分離線會往非溶劑的方向移動，使得組成需要較多的時間進行溶劑與非溶劑的交換來進入液液相分離區，因此組成有機會在結晶區停留較長的時間，所以結晶有機會優先或同時與液液相分離發生，使得成膜的行為及薄膜表面型態與以VIPS法所製備的薄膜相似。 此外，溶解溫度所造成晶核密度差異及兩種相分離的競爭，除了影響高分子鏈微米尺度(morphology)的排列行為，亦會影響其奈米尺度(polymorphism)的排列行為，此既為高分子結晶的特殊構型。在溶解溫度對VIPS成膜過程的影響，晶核密度高的情況下，高分子結晶傾向alpha晶型；晶核密度低的情況下，高分子結晶傾向beta晶型。若以LIPS法成膜搭配IPA成膜時，即以液液相分離主導的情況下，無論溶解溫度為何，高分子皆傾向排列成alpha晶型，因此依照其結晶的排列行為，在此亦將其分為受限制的結晶(hindered crystallization)及自由自在的結晶(free crystallization)。受限制的結晶代表高分子鏈在嚴苛的環境下排列形成結晶，就像結晶時會受到膠化的影響或是液液相分離的干擾，所以結晶會傾向最容易的排列方式，alpha晶型，反之，若高分子鏈可以自由自在的移動，結晶會受到溶劑極性的影響，beta晶型。 上述研究結果清楚地指出溶解溫度及結晶與液液相分離的競爭如何影響薄膜的形成過程，在此我們提出晶核密度會影響溶液的膠化行為，此膠化行為會在後續的相分離扮演極重要的角色，因此藉由膠化行為及相分離機制的競爭，可達到薄膜表面形態與結晶構型的控制，因而製備出富含alpha晶型的雙連續PVDF薄膜、富含alpha晶型且表面為多孔且粗糙顆粒的PVDF薄膜以及富含beta晶型緻密顆粒膜。

This dissertation shows how the morphology and polymorphism of poly(vinylidene fluoride) (PVDF) membranes prepared by using vapor-induced phase separation (VIPS) and liquid-induced phase separation (LIPS) were tuned by varying the dissolution temperature at which PVDF was dissolved (Tdis) to form the casting solution, without changing any other membrane preparation conditions. We also investigated the formation mechanism in the morphology and polymorphism and set up an index of controlling the membrane structure. The results showed that there existed an important transition dissolution temperature (Tdis), being referred to as the “critical dissolution temperature, Tcri”, across which the morphology and polymorphism of membranes drastically changed. With a Tdis higher than the Tcri, the prepared membranes were composed of nodules and the size of polymer domain (nodules) decreased as the Tdis decreased. Because the polymer rich phase could freely coarsen to a large domain during the phase separation, we called the system “free coarsening”. With a Tdis lower than the Tcri, the coarsening was suppressed so the membranes with lacy (bi-continuous) structure were obtained. Because the polymer chains hardly coarsened during the phase separation, we called the system “hindered coarsening”. Therefore, the morphologies were classified into hindered and free coarsening by Tcri. The Tcri was an indicator that the domain grew or not. The phenomenon was considered as general, as a Tcri was observed for all the three solvents, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc) and N,M-dimethylformamide (DMF), and the non-solvents, water and a series of alcohols, used in the present study. All the results indicated that the phenomena were general and not limited to a specific solvent and nonsolvent. We proposed that the Tdis affect the membrane morphology by changing the density of the nuclei for the crystallization contained in the casting solution. Lower Tdis resulted in higher nuclei density. The difference of nuclei density in the casting solution caused the different behaviours of gelation and crystallization which were testified in the falling-ball experiment, the observation of IR spectra. And two gelling processes were proposed, Non-crystallization-initiation gelling (NC-gelling) and Crystallization-initiation gelling (C-gelling). The NC-gelling and C-gelling could be determined by Tcri. During VIPS process, the crystallization happened prior to L-L demixing. With Tdis<Tcri, solution had already gelled to a certain degree when ordered conformations were detected, so the crystal domain hardly grew. With Tdis>Tcri, solution started to gel with formation of ordered conformations, so the crystal domain grew more freely. The domain formed nodules structure with dense surface by the driving force of the crystallization. When membranes prepared by LIPS with employing iso-propanol (iPA) as nonsolvent, the solvent exchanged with nonsolvent quickly, which caused the composition route enter the spinodal decomposition. Therefore, the membrane with bi-continuous structure was obtained. With Tdis>Tcri, because the crystallization happened sequentially, the polymer rich phase coarsened to nodules from bi-continuous structure for a solution with low gelation degree. Because of the competition between the coarsening and solidification, the polymer rich phase formed nodules with porous surface. On the contrary, for a solution with high nuclei density, the solution still could gel quickly. There the coarsening was suppressed. The solution formed translucent gel and revealed the bi-continuous structure after the L-L demixing. When IPA was replaced by normal-octanol (nOct), the L-L demixing curve moved toward to nonsolvent. Accordingly, the composition route on theoretical phase diagram could stay in the crystallization region for a longer time. Therefore, the similar result as membrane prepared by VIPS was obtained. In addition, the polymorphism of a PVDF membrane was not only affected by Tdis but also controlled by competition between the crystallization and the L-L demixing. For the polymorph evolution with the Tdis, when the crystallization was prior to the L-L demixing, alpha form can be favorably formed in solution with high nuclei density. The beta form prevailed as the Tdis increased to be higher than the Tcri. If the L-L demixing was prior to the crystallization, alpha form was mainly observed in membrane no matter which Tdis we used. Therefore, the crystallization was classified into hindered crystallization and free crystallization. The hindered crystallization is indicative of polymer chains ordered in a harsh environment, like NC-gelling process or the L-L demixing, and then polymer chains preferred to form the alpha form. If the polymer chains moved easily, the free crystallization can happen. The free crystallization resulted in beta form due to the solvent polarity. Finally, our works give a clear view about the competition between the crystallization and the L-L demixing, and point out the effect of nuclei density in membrane preparation. We propose that the competition between the two gelling processes play an important role in determining the PVDF membrane morphology and crystalline polymorphs, and the dissolution temperature can change the competition. The critical dissolution temperature can be interpreted as a dissolution temperature across which the dominant gelling process switched. The explanation shows a window how to prepare a bi-continuous membrane with alpha form, porous nodules with alpha form and dense nodules with beta form.