改質方法對複合膜微結構及滲透蒸發 效能之影響

Correlating Microstructure with Pervaporation Performance of Modified Composite Membranes

杜曼諾(Manuel-De Guzman)

中原大學/工學院/化學工程研究所/博士(2011年)

https://doi.org/10.6840/cycu201101118

摘要

本研究結合多種製膜與改質技術，製備不同表面與整體結構型態的複合膜：（1）微量空氣電漿誘導接枝聚合，（2）添加相容劑之高分子共混系統，（3）顆粒填充與高分子混摻系統，（4）結合旋轉塗佈與臭氧誘導接枝聚合技術，探討聚碳酸酯（PC）與聚偏二氟乙烯（PVDF）複合膜之結構特性與滲透蒸發效能間的關聯性。傳統上常對薄膜進行物理結構型態與化學組成的鑑定，而在本研究中，進一步結合新穎的正電子湮滅技術（PAS）探討複合膜中原子層級自由體積的變化。
實驗規劃首先，利用微量空氣電漿誘導接枝聚合技術，將甲基丙烯酸羥基乙酯（HEMA）單體接枝於多孔的聚碳酸酯（PC）基材膜表面，製備一親水選擇層複合膜；結果指出，由於PHEMA-g-PC複合膜具有親水的PHEMA選擇層，因此在25 ℃下分離進料90 wt%的乙醇水溶液具有高選擇性（分離係數為441或透過端水濃度為98 wt%），而多孔的PC基材則提供高透過量（透過量為410 g/m2h）。
接著探討PC高分子分別混摻（A） 無添加相容劑的低分子量PHEMA，與（B） 添加相容劑之高分子量PHEMA兩系統，對薄膜之影響。結果指出在（A）系統下，PHEMA對PC的添加量在30 wt%時為薄膜微結構之轉折點；當添加量為10-30 wt%時，隨著PHEMA添加量的增加，薄膜的自由體積尺寸會隨之減小且自由體積分布變窄；但過量的PHEMA添加（40-50 wt%）則得到相反的趨勢。進一步觀察，發現自由體積與滲透蒸發效能具有良好的關聯性；在操作溫度為25 ℃，以70 wt%的異丙醇水溶液為進料時，隨著PHEMA添加量增加（10-30 wt%），透過量（269-526 g/m2h）與選擇性（91.1-99.7 wt%）皆隨之上升；而過量的PHEMA添加量（40-50 wt%）則導致滲透蒸發效能大幅下降（透過量為1164-42418 g/m2h，透過端水濃度為65.1-36.3 wt%）。另外，在PHEMA添加相容劑的（B）系統中，發現相容劑的添加導致自由體積半徑減小、自由體積分率降低與自由體積分佈變窄，因此，在25 ℃下分離90 wt%乙醇水溶液，相較於未添加相容劑之PHEMA/PC薄膜，添加相容劑可使PHEMA/PC薄膜的透過量提升（由原先的1250 gm/m2提升至1675 gm/m2），選擇性增加（由原先的90 wt%增加至96.6 wt%）。
第三部分，探討以聚苯乙烯（PS）奈米球分散於PC高分子內所製備的混和基底薄膜（mixed matrix membranes, MMMs）微結構與薄膜效能之間的關聯性。由正電子湮滅光譜分析結果指出，此薄膜具有兩個正電子湮滅壽命3與4（相對應於R3與R4），並且隨著PS奈米球的含量增加而有所提升；並且發現薄膜滲透蒸發效能與自由體積有良好關聯性。當操作溫度為25 ℃，以90 wt%的乙醇水溶液為進料時，相較於相同厚度之PC薄膜，當添加量為10-30wt%之PS/PC薄膜的透過量可提升100 %，而添加量為40wt%之PS/PC薄膜的透過量則可提升高達120 %，且透過端水濃度皆維持在99.0 wt%。
最後，則探討S-PHEMA/PHEMA-g-PVDF複合膜之微結構與滲透蒸發效能之間的關聯性；實驗結果發現，在較高的旋轉塗佈轉速條件之下，可得到一較薄且較緻密之S-PHEMA選擇層，且自由體積較小；當轉速由1000 rpm增加至6000 rpm時，其分離98 wt%乙酸乙酯水溶液的透過量可由350 g/m2h提昇至420 g/m2h，而透過端水濃度皆高於96.7 wt%，表示以此方式製備之複合膜可提升滲透蒸發分離程序之透過量，並保有一定的選擇性。
本研究詳細的闡述利用不同方式所製備的複合膜之微結構與滲透蒸發效能之間的關聯性，並深入探討滲透蒸發在不同微結構內傳輸機制，有利於將來薄膜結構之設計及預測滲透蒸發之效能。

關鍵字

微結構 ； 滲透蒸發 ； 複合薄膜

並列摘要

We designed various composite membranes by different means of surface and bulk structure modifications: (1) residual air plasma-induced graft polymerization, (2) homogeneous polymer blending without compatibilization, (3) homogeneous polymer blending with compatibilization, (4) heterogeneous polymer blending in the form of particle filling or incorporation into polymer matrix, and (5) spin coating integrated with ozone-induced graft polymerization. The purpose was to understand the correlation between microstructural properties and the pervaporation (PV) performance of composite membranes of polycarbonate (PC) and poly(vinylidene fluoride) (PVDF). Conventional membrane properties such as morphological features and chemical structures can be used to interpret the membrane performance; however, it can be elucidated by probing the membrane characteristics on the atomic level with a powerful state-of-the-art technology called positron annihilation spectroscopy (PAS).
In part (1), a porous structure of PC membrane with macrovoids near the surface was fabricated by hydrophilizing with a hydroxyethyl methacrylate (HEMA) solution through residual air plasma-induced graft polymerization. The high separation factor of 441 or permeation water concentration of 98 wt% of the resulting PHEMA-g-PC composite membrane for separating a 90 wt% aqueous ethanol mixture at 25C could be attributed to the PHEMA graft layer, and the high permeation flux of 410 g/m2h could be correlated to the porous substructure of the modified PC membrane.
Part (2) is about PC blending with low-molecular-weight PHEMA without compatibilization. Results indicated that probing the microstructure of the blend membranes revealed a limiting PHEMA:PC ratio of 30 wt%. From 10-30 wt% content of PHEMA relative to PC, the trend of the free volume sizes was decreasing, and the free volume distribution was shown to be narrow. From 40-50 wt% PHEMA:PC, the opposite behavior was observed. As to the blend membrane morphologies, the surface remained dense and the cross-sections tended to get more porous with further addition of PHEMA. These morphological properties and free volume data correlated with the pervaporation performance for separating a 70 wt% aqueous isopropanol mixture at 25C as follows: from 10-30 wt% PHEMA:PC, flux and permeate water content increased (269-526 g/m2h and 91.1-99.7 wt% H2O); from 40-50 wt% PHEMA:PC, the pervaporation performance drastically dropped (1164-42418 g/m2h and 65.1-36.3 wt% H2O).
For part (3), PC was blended with high-molecular-weight PHEMA in the presence of a compatibilizer. Findings showed that the effect of adding a compatibilizer was to densify the compatibilized membrane cross-section and to make the average free volume radius smaller, the fractional free volume lower, and the free volume distribution narrower. This morphological and free volume behavior correlated with better pervaporation performance for separating a 90 wt% aqueous methanol mixture at 25C: for the case of 5 wt% PHEMA/PC, the pervaporation performance improved from 1250 gm/m2h (corresponding to 90.0 wt% water in permeate) without adding a compatibilizer to 1675 gm/m2h (96.6 wt% water) with the introduction of a compatibilizer.
From part (4), PALS data revealed that heterogeneous blend membranes consisting of crosslinked highly porous polystyrene (PPS) spheres dispersed in the PC matrix had two positron annihilation lifetimes 3 and 4 (corresponding to R3 and R4), both of which increased with higher loadings of PPS spheres. These results, along with the morphological structure of defect-free surface and well-dispered fillers in the polymer matrix, correlated with improvements in the permeation fluxes for separating a 90 wt% aqueous ethanol mixture at 25C, without affecting much the permeate water concentration of approximately 98.0 wt%. Compared to a pristine PC membrane with the same thickness as the PPS/PC blend membrane, the percentage increased in the permeation flux was roughly 100%.
Results from part (5) illustrated the correlation between the microstructure and the pervaporation performance of a multilayered composite membrane of S-PHEMA/PHEMA-g-PVDF (spin-coated layer of PHEMA over PHEMA graft deposited onto a PVDF substrate). Both the SEM micrographs and the depth profile showed that the higher the spin-coating rate, the thinner and the denser the S-PHEMA layer. This ultrathin highly dense layer, associated with very small free volume, correlated with increased permeation rate without sacrificing the concentration of water in permeatethe pervaporation performance for separating a 98 wt% aqueous ethyl acetate mixture at 25C improved from a flux of 350 g/m2h at 1000 rpm to 420 g/m2h at 6000 rpm, the permeate water content in both cases remained unchanged at about 96.7 wt%.
This research would elucidate the correlation between the microstructure and the pervaporation performance of the different composite membranes. The results obtained would then provide insights into the principle of the pervaporation membrane transport, thereby leading to an understanding of the areas of membrane structure design and performance prediction.
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並列關鍵字

microstructure ； pervaporation ； composite membrane

參考文獻

[112] Y. Jean, J. Zhang, H. Chen, Y. Li, G. Liu, Spectrochim. Acta, Part A 61 (2005) 1683.

[15] A. Qureshi, S. Shah, S. Pelagade, N. L. Singh, S. Mukherjee, A. Tripathi, U. P. Despande, T. Shripathi, Surface modification of polycarbonate by plasma treatment, 23rd National Symmposium on Plasma Science & Technology (PLASMA-2008), Journal of Physics: Conference Series 208 (2010) 1.

[100] W. S. Hung, C. H. Lo, M. L. Cheng, H. Chen, G. Liu, L. C. D. Nanda, K. L. Tung, S. H. Huang, K. R. Lee, J. Y. Lai, Y. M. Sun, C. C. Yu, R. Zhang, Y. C. Jean, Polymeric membrane studied using slow positron beam, Appl. Surf. Sci. 255 (2008) 201.

[2] S. Loeb, L. Titelman, E. Korngold, J. Freiman, Effect of porous fabric on osmosis through a Loeb-Sourirajan type asymmetric membrane, J. Membr. Sci. 129 (1997) 243.

[3] Y. B. Guo, F. C. N. Hong, Adhesion improvements for diamond-like carbon films on polycarbonate and polymethylmethacrylate substrates by ion plating with inductively coupled plasma, Diam. Relat. 12 (2003) 946.