利用非溶劑誘導式相分離法製備雙連續結構且富含β-form 結晶型態之PVDF 多孔膜

Abstract

本研究是以非溶劑誘導式相分離法製備同時具有雙連續結構且富含β-form 結晶型態的聚偏二氟乙烯(Poly(vinylidene)fluoride, PVDF)薄膜，根據以往的經驗當製備出富含β-form 結晶型態的PVDF 薄膜其形成結構為機械強度不足的顆粒結構，但當製備出富含α-form 結晶型態的PVDF 薄膜其形成結構為機械強度較佳的雙連續結構。因此，為了提PVDF 薄膜的可應用價值並且充足利用具備壓電性質的特性，我們希望將β-form 結晶型態與雙連續結構做結合。 本研究分成兩個部分，第一個部分為利用溶劑特性、高分子分子量、鑄膜液濃度及鑄膜液溶解溫度提升鑄膜液entanglement(鏈段糾結程度)，這部分所使用的製程方法主要是液液相分離所主導的濕式法(Wet-immersed method, LIPS) 及結晶所主導的蒸氣誘導式相分離法(Vapor-induced phase separation, VIPS)，待找到最高entanglement 的條件後，第二部分為利用改變製膜方法調整非溶劑特性控制溶劑與非溶劑之間的相互質傳速率，希望使高分子富相不易合併而達到目標結構的目的，並且同時探討參數對於結晶型態的影響。結晶型態與結晶度由全反射式紅外線光譜儀與微差掃瞄熱差分析儀進行分析，結構則由掃描式電子顯微鏡進行分析。 在結構的部分，濕式法及蒸氣式誘導相分離法在磷酸三乙酯(Triethyl phosphate,TEP)系統形成的是雙連續構成的多孔顆粒結構及雙連續結構，在N-甲基吡咯烷酮(N-Methyl-2-Pyrrolidone, NMP)系統形成的是巨型孔洞及顆粒結構，然而，隨著entanglement的提升，巨型孔洞變小，顆粒的domain size(高分子合併尺度)變小。結晶型態的部分，我們相信PVDF的溶劑特性不同對於結晶型態形成種類是個關鍵，使用具有強極性的溶劑NMP容易誘導出β-form結晶型態，使用弱極性的溶劑TEP容易誘導出α-form結晶型態，因此選用NMP作為主要溶劑，推論極性的環境誘使鑄膜液內的鏈段偏向TTT conformation，當以液液相分離所主導時，先相分離而後結晶，鑄膜液entanglement會影響結晶環境誘導出不同的結晶型態，當鑄膜液的entanglement較低時，高分子鏈段容易轉動導致結晶排列成TGTG’ conformation形成較多的α-form結晶型態，反之，當鑄膜液的entanglement較高時，高分子鏈段不容易轉動導致結晶按照原本的TTT conformation進行結晶而誘導出較多的β-form結晶型態，當以結晶區所主導時，先結晶而後相分離，由於先形成結晶才產生相分離形成高分子富相，因此不論鑄膜液entanglement高低，其結晶都照著原本的TTT conformation進行排列形成較多的β-form結晶型態。最終使用高分子量1000kDa PVDF/NMP系統以低溫進行溶解(T_D=35℃)作為目前entanglement最高的條件，隨後改變非溶劑特性以低碳醇的雙槽法得到雙連續構成的多孔顆粒結構同時縮短相組成進入液液相分離的時間，得到以非溶劑誘導式相分離法製備出雙連續構成的多孔顆粒結構且富含70% β-form結晶型態之PVDF薄膜。 於未來的發展上面可望將此種富含β-form結晶型態的PVDF薄膜以極化的方式提升壓電特性，搭配機械強度較佳的雙連續結構解決薄膜過濾中結垢的問題。

In this work, the aim of preparing PVDF membranes with bi-continuous structure and β-form crystals via non-solvent induced phase separation method (NIPS) was investigated. Based on previous experience, PVDF membranes which were dominated by β − form crystals were composed of weak mechanical nodules structure, and α -form crystals could be favorably formed with strong mechanical bi-continous structure. Therefore, in order to improve the value of practical application of PVDF membranes and use piezoelectric properties sufficiently, hoping that we can combine β-form crystals with bi-continuous structure. There is two parts in this research, the first part is using many parameter to increase entanglement and the parameter including solvent characteristic, molecular weight, polymer concentration、dissolution temperature. We used LIPS which was dominated by L-L demixing and VIPS which was dominated by crystallization as preparation ways in this part. When we found the highest entanglement condition, we got on the second part. The second part is using different preparation ways to control non-solvent characteristic and the mass transfer between solvent and non-solvent. Hoped that polymer-rich phase can merge barely and obtain the target structure. In the meanwhile, we can obverse the effect of crystal polymorphism. The crystal polymorphism and crystallinity which were testified in FTIR-ATR spectra and DSC .The morphology structure were testified in SEM.In the part of structure ,the porous nodules composed of bi-continuous structure and the bi-continuous structure were formed with LIPS and VIPS in TEP system. The marco-voids structure and the nodules structure were formed with LIPS and VIPS in NMP system. However, with the increase of entanglement, the decrease of marco-voids and the domain size for nodules. In the part of crystal polymorphism, we believed that the solvent characteristic is a critical condition to the forming of crystal polymorphism. It is easy to induce β-form crystals using polar solvent like NMP and induce alpha-form using non-polar solvent like TEP. Therefore, we choose NMP as the solvent. The polar environment induce the TTT conformation of the polymer chain in casting solution. If the L-L demixing dominated, solution started phase separation and then crystallized. Different crystal polymorphism were induced by crystallize environment with entanglement in casting solution. With low entanglement in casting solution, polymer chains can move easily and cause TGTG' conformation forming α-form crystals.Conversely, with high entanglement in casting solution, polymer chains can move hardly and crystallize in TTT conformation originally forming β-form crystals. If the crystallization dominated, solution crystallized first and then started phase separation. Thus, no matter entanglement is low or high, the crystals crystallize in TTT conformation originally forming β-form crystals. Finally, our research used highmolecular weight 1000kDa PVDF/NMP system dissolved in low temperature resultingin the highest entanglement of casting solution , and prepared by the dual bath in low C alcohol. Reducing the time of phase composition staying in L-L demixing area caused porous structure with bi-continuous. The porous PVDF membrane with bi-continuous structure and 70% ratio of β-form crystals with non-solvent induced phase separation was fabricated. The future of development, we purpose that PVDF membrane composed of β-form crystals can be enhanced piezoelectric properties by polarization. Polar crystals combine with better mechanical bi-continuous structure which can dissolve the fouling of membrane filtration.