| Summary: | 博士 === 中原大學 === 化學工程研究所 === 100 === 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 25C 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 25C 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 25C: for the case of 5 wt% PHEMA/PC, the pervaporation performance improved from 1250 gm/m2h (corresponding to 90.0 wt% water in permeate) without adding a compatibilizer to 1675 gm/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 25C, 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 permeatethe pervaporation performance for separating a 98 wt% aqueous ethyl acetate mixture at 25C 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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