- 學位論文
鑄膜液晶核密度對PVDF薄膜結構之影響
The influence of the density of crystalline nuclei in casting solution on PVDF membrane morphology
摘要
本研究係以非溶劑誘導式相分離法製備結晶性高分子PVDF薄膜，探討其結晶行為。文獻中多認為，結晶性高分子之成膜過程主要掌控於固-液相分離(結晶)與液-液相分離兩機制之競爭。近年來發現預先存在於鑄膜液內的晶核密度，能夠改變固-液相分離所產生的結構。本研究中藉著操控晶核密度去控制薄膜結構，操控的變數有溶解溫度、溶劑之選擇以及添加劑，使用的溶劑為NMP以及TEP，添加劑為水，利用蒸氣誘導式相分離法，以及乙醇槽濕式法成膜，降低溶劑與非溶劑之間質傳速度，增加鑄膜液位於固-液相分離的時間，以生成結晶。 結果顯示，較高的溶解溫度下，因高分子分散程度較佳，鑄膜液的晶核密度較低。使用NMP作為溶劑的系統，經由蒸氣誘導式相分離(VIPS)成膜，因蒸氣進入鑄膜液較慢，濃度組成會位在固-液相分離區較久，能夠成長結晶。若鑄膜液的晶核密度較低時，晶核在成長的階段能夠較不互相影響，因此生成結晶顆粒。當鑄膜液的晶核密度較高時，於固-液相分離區內晶核相互拉扯，造成結晶無法成長，隨著吸水量增加，當鑄膜液組成進入液-液相分離區後，受液-液相分離影響而形成了雙連續結構。而經由乙醇槽濕式法成膜，鑄膜液因為溶劑與非溶劑質傳較快，濃度組成較快進入液-液相分離區與固-液相分離區。在晶核密度較低時，液-液相分離先造成了雙連續相的出現，然而結晶的影響能夠讓高分子富相產生結晶繼續成長的現象，最後可觀察到雙連續聚集的顆粒結構。當晶核密度較高時，因晶核相互拉扯的影響而抑制了結晶的成長，最終薄膜依然為雙連續結構。 溶劑之溶解力亦會造成晶核密度的改變，使用溶解度較低的TEP作為溶劑，晶核密度會被提高，因此相同的溶解溫度下會得到較小的顆粒或雙連續的結構。添加劑亦有可能改變晶核密度，使用與高分子親合性佳的添加劑會讓晶核密度降低，顆粒結構增大，反之則得到較小的顆粒或雙連續結構。 由結晶形態的觀察可發現，以固-液相分離主導所生成的顆粒結構為β態。而先液-液相分離後才進行固-液相分離所形成的顆粒結構則是以α態為主。 實務的應用上，顆粒結構之薄膜其孔洞之間具有高度連通性，應用在分離程序上有很大的潛力。但尺度越大的顆粒，顆粒之間的連接點較少，因此薄膜強度會降低，雙連續結構薄膜其蕾絲的連接點較多，具有較佳的薄膜強度。在本研究中，使用NMP及TEP作為溶劑，皆可利用增加晶核密度去製備出雙連續結構薄膜，其孔隙度為75%~85%，孔徑範圍為0.11~0.48μm。
並列摘要
In this work, the crystallization behavior for poly(vinylidene fluorides) (PVDF) membranes via non-solvent induced phase separation (NIPS) was investigated. Formation mechanism of crystallizable polymer membranes is governed primarily by the competition between two types of phase separation, the solid-liquid demixing, or so called crystallization, and liquid-liquid demixing. Recent studies have shown that the nuclei density for initiation of crystallization also plays an important role in determining membrane morphology. The effect of nuclei density on the morphology of membranes was investigated by changing different conditions of dissolution temperature, solvent, and additives. Membranes were fabricated from PVDF/N-methyl-2-pyrrolidinone (NMP) and PVDF/Triethyl phosphate (TEP) solutions using vapor induced phase separation (VIPS) or wet immersion in EtOH bath. In those processes the composition path stayed in the crystallization longer because of low mass transfer rate between solvent and non-solvent. Result shows that at high dissolution temperatures, the nuclei density was low because the polymer chains were well-dispersed. For PVDF/NMP membranes prepared by VIPS, water moisture entered the casting solution slowly, and the composition path stayed in S-L demixing region longer so that crystallization occurred. In the case of low nuclei density, nuclei were not affected by each other in the growing stage and finally large particulate structures were formed. As for the case of high nuclei density, crystallization was suppressed because of high degree of polymer chain entanglement. As the composition entered the L-L demixing region, lacy structures were formed via L-L demixing. For membranes prepared by wet immersion in EtOH, the composition path entered the L-L demixing and S-L demixing region quickly because of fast mass transfer rate between solvent and non-solvent. In the case of low nuclei density, lacy structures formed first and then followed by crystallization. Particulate structures with bi-continuous surface were formed. As for the case of high nuclei density, crystallization was suppressed and lacy structures were obtained. Solvent and the additives also dominated the nuclei density, which was decreased in a solvent with higher solubility or by the addition of additives with better affinity for PVDF. Besides, the difference in the form of crystal from different preparation process was discovered. Membranes by liquid-liquid demixing followed by crystallization were of α-type crystal structure. However, membranes of β-type crystal were obtained by the domination of crystallization. In practical application, membranes with interconnected globule structures can be applied in some process because of high inter-connectivity and porosity. However, with the increase of globule diameter, the mechanism strength decreased. In this work, bi-continuous membranes were fabricated by increasing the initial nuclei density. The porosity was about 75 to 85%, and the pore size was in the range of 0.11 to 0.48μm.