利用改變高分子溶解溫度及添加含水無機鹽來調控聚偏二氟乙烯薄膜之結構及結晶型態

Abstract

近年來發現聚偏二氟乙烯（Poly(vinylidene) fluoride, PVDF）高分子的溶解溫度能夠影響鑄膜液內晶核密度進而改變薄膜最終的結構以及添加含水無機鹽至PVDF鑄膜液中能夠改變薄膜結晶型態。本研究係以蒸氣誘導式相分離法（Vapor-induced phase separarion, VIPS）來製備半結晶性高分子聚偏二氟乙烯（Poly(vinylidene) fluoride, PVDF）薄膜，使用的溶劑為磷酸三乙酯（Triethyl phosphate, TEP）。本研究分成兩個部分，第一部分探討PVDF的溶解溫度如何影響薄膜結構及結晶型態，為了探討溶解溫度的影響，利用鋼珠球沉降實驗來觀察鑄膜液如何形成膠化以及搭配傅立葉轉換紅外線光譜儀（Fourier-transform IR, FTIR）來分析溶液在膠化過程中是否出現特定的排列。另一部分為添加含水無機鹽 （Ca(NO3)2•4H2O） 對於PVDF薄膜結構及結晶型態之影響。 PVDF溶解溫度不同能夠影響薄膜最終的結構，60℃溶解可得到雙連續結構而120℃溶解可得到顆粒結構，兩者得到的薄膜結晶型態皆以alpha態為主，即具有TGTG’的排列。當添加非溶劑水於不同溶解溫度之PVDF/TEP溶液中皆會造成溶液膠化，在溶液膠化之前先觀察到TGTG’的排列可知膠化是結晶所造成的。我們相信TGTG’的排列與纖維狀結構其形成機制是來自於高分子與溶劑之間的作用力。除此之外，我們也推測鑄膜液內晶核密度與溶解溫度的關係是影響薄膜最終結構的可能原因。當晶核密度夠高結晶區域有足夠的連接性時，纖維狀的膠化結構能夠維持，不會隨著後續放入水槽而結構遭受破壞。 當添加2 g/L的含水無機鹽於PVDF/TEP溶液中對於薄膜結構及結晶型態皆有巨大的轉變，加入鹽類干擾了高分子與溶劑之間的作用力而抑制了高分子鏈TGTG’排列的生成，我們觀察到在溶液膠化之前已有TTTT的排列，而最終得到的薄膜結構為富含beta態晶型的顆粒結構。 添加鹽類不僅能夠有效增加beta態晶型的含量，且薄膜結構由原本的雙連續結構轉變為顆粒結構，因此我們可藉由改變鹽類濃度來調整薄膜結構及beta態晶型的比例。當鹽類濃度低於2 g/L時，我們觀察到TGTG’與TTTT排列之間的競爭，發現1 g/L是臨界鹽類濃度，鹽類濃度大於1 g/L時，薄膜結構由雙連續為主轉變成以顆粒為主，且結晶型態由alpha態轉成beta態。當鹽類濃度大於2 g/L時，TGTG’的排列幾乎可被抑制而得到90%的beta態晶型比例。隨著鹽類濃度增加，TTTT排列出現的時間提早並且得到的顆粒結構其尺寸較小。

Recent studies have shown that the poly(vinylidene) fluoride （PVDF） dissolution temperature used to prepare a casting solution can affect the nuclei density in the solution and thus plays an important role in determining the resulting membrane morphology. It has also been shown that adding hydrated salt in the PVDF casting solution dramatically affects the membrane crystalline polymorphism. In the present study, PVDF membranes were prepared by vapor-induced phase separation（VIPS）method and using triethyl phosphate （TEP） as the solvent. There are two main topics in the present work. One is about how the PVDF dissolution temperature affected the membrane morphology and polymorphism. To get insight into the effect of the dissolution temperature, falling-ball experiments were performed to study how the casting solution gelled and and Fourier-transform IR（FTIR）analysis on the solution was conducted to detect when ordered chain conformation was formed during the gelation. The other topic is abou the influence of adding hydrated salt （Ca(NO3)2•4H2O） on the morphology and polymorphism of PVDF membranes. The PVDF dissolution temperature dramatically influenced the morphology of the resulting membranes. A dissolution temperature of 60℃ resulted in membranes with bi-continuous structure, while 120℃ brought about nodular membranes. Both membranes mainly contained alpha crystalline form, which is characterized by TGTG’ conformation. For both cases, the PVDF/TEP solutions gelled after the addition of water in them. And the TGTG’ conformation was detected before the solution began to gel, indicating that the gelation was initiated by crystallization. We believe that the polymer-solvent interaction was the mechanism responsible for the formation of TGTG’ conformation and the bi-continuous （fibrillar） structure. Also, we propose that the dependence of the nuclei density in the solution on the dissolution temperature is a possible explanation for why the dissolution temperature can affect the membrane structure. The bi-continuous （fibrillar） gel structure could only be retained when the nuclei density was high enough, to result in enough connectivity among the crystalline domains in the fibrils that the fibrils would not break up when the gel was immersed in water. A dramatic change in membrane morphology and polymorphism was observed with adding 2 g/L of hydrated salt in the PVDF/TEP solution. The addition of salt interfered the polymer-solvent interaction and inhibited the formation TGTG’ conformation. Therefore, we detected TTTT conformation prior to solution gelation. The resulting membrane contained nodules with beta crystalline form that is composed of all trans conformation. The addition of hydrated salt not only increased the beta form content but also converted the membrane structure from bi-continuous to nodular. We thus can adjust the membrane morphology and the beta ratio by changing the concentration of the added salt. When the concentration was lower than 2 g/L, we observed competition between TGTG’ and TTTT conformations. We found 1 g/L was a critical salt concentration, higher than which the dominant membrane structure switched from bi-continuous to nodular and the dominant polymorph changed from alpha to beta. When the concentration was higher than 2 g/L, the TGTG’ conformation was almost fully inhibited and the ratio of beta-form reached 90%. With increasing salt concentration, TTTT conformation occurred earlier and the size of nodules became smaller.