以非溶劑誘導式相分離法製備聚偏二氟乙烯薄膜及利用傅立葉轉換紅外線光譜儀分析其成膜機制之探討

Abstract

本研究觀察並分析溶劑、溶解溫度、醇類非溶劑對非溶劑誘導式相分離法製備的聚偏二氟乙烯（poly(vinylidene fluoride)，PVDF）薄膜的結構和晶型的影響。透過成膜過程中的傅立葉轉換紅外線光譜變化和鑄膜液的流變性質，有助於了解成膜機制。 在溶劑方面，在非溶劑吸入速率低的情況下，鑄膜在液-液相分離之前更容易發生固-液相分離，而將鑄膜暴露於水蒸氣中是降低非溶劑吸入速率的有效方法。當固-液相分離領先液-液相分離，PVDF在液-液相分離之前結晶，則晶核形成了顆粒結構；當液-液相分離先固-液相分離時，PVDF在液-液相分離之後結晶，液-液相分離成為主要的結構形成機制。此外，結果表明，鑄膜中的溶劑移除速率在晶型形成方面發揮關鍵作用。在溶劑移除速率較低的情況下，鑄膜中仍有足夠的溶劑存在，足以影響PVDF的結晶，溶劑極性主導了結晶晶型。高極性的溶劑，如甲基磷醯胺（Hexamethylphosphoramide，HMPA），誘導形成極性β相結晶；而極性較低的溶劑，如磷酸三乙酯（Triethyl phosphate，TEP），則誘導非極性α相結晶。在溶劑移除速率較高的情況下，在成膜過程中，鑄膜液中的溶劑濃度很快變得太低，無法影響晶型的形成。在這種情況下，溶劑極性對晶型的形成幾乎沒有影響，主要的晶型是動力學上更有利的α相。觀察到鑄膜液的黏度對於凝聚槽中溶劑移除速率方面發揮關鍵作用。高溶液黏度阻礙溶劑在溶液中的擴散，從而導致較低的溶劑移除速率，使溶劑極性能夠主導晶型的形成。 製備薄膜時，使用了具有不同偶極矩的溶劑，包括HMPA、N-甲基吡咯烷酮（N-Methyl-2-pyrrolidone，NMP）、N,N-二甲基乙醯胺（N,N-Dimethyl Acetamide，DMAc）和TEP。隨著溶劑偶極矩的增加，薄膜中極性結晶相的含量和水滲透率均一致地增加。結果表明，使用HMPA、NMP或DMAc溶解PVDF時，溶劑的偶極矩越高，鑄膜中的溶劑移除速率越低，因為鑄膜液的黏度較高，使得鑄膜表面的溶劑濃度較高，導致相分離後表面形成更多孔洞，並延長了溶劑主導的結晶過程。由於TEP的極性較低，它誘導非極性結晶並對水的親和力較低，這解釋了使用TEP作為溶劑時，水滲透率和極性結晶含量較低的原因。這些結果顯示了在成膜過程中，溶劑極性及其移除速率如何與薄膜結構在分子尺度（與結晶相關）和奈米尺度（與水滲透率相關）相關並相互影響。 在溶解溫度方面，隨著溶解溫度的增加，鑄膜液中的晶核密度變低，結晶的顆粒結構尺寸會逐漸變大。鑄膜液的黏彈測試顯示，鑄膜液的膠化現象會隨著溶解溫度和溶劑而改變。低溶解溫度和弱溶劑會使得鑄膜容易膠化。膠化會抑制結晶而使得溶劑極性對晶型的影響降低，並傾向於形成動力學上較有利的α相。 在醇類非溶劑方面，醇類非溶劑會影響溶劑與非溶劑之間的質傳交換速率。在成膜過程中，醇類非溶劑的吸入速率低，在發生液-液相分離之前，鑄膜液有更多的時間可以結晶。隨著非溶劑醇類碳數的增加，高分子鏈能有充足的時間進行結晶來完成固-液相分離的結構。當使用高碳醇非溶劑時，溶劑移除速率下降，使得溶劑極性對結晶環境的影響時間較長。

In this study, we observed and analyzed the effects of solvent, dissolution temperature, and alcohol non-solvent on the structure and the crystalline phases of poly(vinylidene fluoride) (PVDF) membranes prepared by nonsolvent-induced phase separation (NIPS) method. By detecting the Fourier transform infrared (FTIR) spectrum changes during the membrane formation process and testing the rheological properties of the casting solution, it helps to understand the membrane formation mechanism. Regarding the solvent, with low nonsolvent intake rate, the casting film had high tendency to perform solid-liquid (S-L) demixing before liquid-liquid (L-L) demixing, and exposure of casting films to water vapor was an efficient way to lower the nonsolvent intake rate. Due to the dominance of S-L demixing over L-L demixing, PVDF crystallized before L-L demixing and the crystalline nuclei grew into nodular (spherulitic) structure. When L-L demixing precedes S-L demixing, PVDF crystallizes after L-L demixing, which was the main mechanism for structure formation. Furthermore, the results indicate that the solvent removal rate from casting solutions played a crucial role in determining the crystalline phases. With a low solvent removal rate, enough solvent remained in the casting solution to influence PVDF crystallization, and the solvent polarity governed the formation of crystalline phases. A solvent with high polarity, such as hexamethyl phosphoramide (HMPA), induced the formation of polar β-phase crystals and a solvent with lower polarity, like triethyl phosphate (TEP), induced nonpolar α-phase crystals. With a high solvent removal rate, the solvent concentration in the casting solution quickly became too low to influence the formation of crystalline phases during the membrane formation. In this case, the solvent polarity had little effect on the formation of crystalline phases and the dominant crystalline phase was the kinetically favorable α-phase. It was also observed that the viscosity of casting solutions played a critical role in determining the solvent removal rate in coagulation bath. High solution viscosity hindered solvent diffusion in the solution, leading to a low solvent removal rate, allowing the solvent polarity to dominate the formation of crystalline phases. Solvents with different dipole moments, including HMPA, N-Methyl-2-pyrrolidone (NMP), N,N-Dimethyl Acetamide (DMAc), and TEP, were used during the membrane preparation. Both the fraction of polar crystalline phase and the water permeability of the prepared membrane consistently increased with an increasing solvent dipole moment. The results indicate that, with HMPA, NMP or DMAc being used to dissolve PVDF, a solvent with a higher dipole moment resulted in a lower solvent removal rate from the cast film due to the higher viscosity of the casting solution. The lower solvent removal rate allowed a higher solvent concentration on the surface of the cast film, leading to a more porous surface and longer solvent-governed crystallization. Because of its low polarity, TEP induced non-polar crystals and had a low affinity for water, accounting for the low water permeability and the low fraction of polar crystals with TEP as the solvent. The results provide insight into how the membrane structure on a molecular scale (related to the crystalline phase) and nanoscale (related to water permeability) was related to and influenced by solvent polarity and its removal rate during membrane formation. Regarding the dissolution temperature, as the temperature increased, the nuclei density in the casting solutions decreased, and the nodule particle size of the crystalline structure gradually increased. Rheological tests of the casting solution demonstrated that gelation phenomena in the casting solution changed with the dissolution temperature and the solvent. A lower dissolution temperature and a weaker solvent made the casting solution more prone to gelation. Gelation would inhibit crystallization and reduced the influence of solvent polarity on crystalline phases, promoting the formation of the kinetically favorable α-phase. Regarding the alcohol nonsolvent, they affected the mass transfer rate between the solvent and nonsolvent in the casting solution. During the membrane formation, alcohol nonsolvent with lower intake rates provided more time for the casting solution to crystallize before the liquid-liquid demixing occurred. With long-chain alcohol nonsolvent, the polymer chains have sufficient time for crystallization to complete the structure of the solid-liquid demixing. With long-chain alcohols nonsolvent, the solvent removal rate decreased, prolonging the time for the solvent polarity to affect the crystalline environment of the casting film.