Post coarsening effects on membrane microstructure
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The goal of this research was to determine relationships between post-coarsening processing conditions and the microporous morphology of membranes. Specifically, the processes of matrix solidification in liquid -- liquid thermally induced phase separation (L−L TIPS), the drying of the microporous structure, and the uni-axial elongation of a simple microporous structure were examined. Additionally, the effect uni-axial elongation has on pore shape was included in a sieve filtration model to look at the impact on performance. A deterministic approach was taken to predict membrane morphologies resulting from the matrix solidification step that occurs in L–L TIPS. Many studies have examined the growth rate of droplets in the coarsening stage of membrane formation, but few have attempted to extend this information into the subsequent processing steps of matrix solidification, diluent extraction/exchange, and drying. The modeling of matrix solidification utilized Monte-Carlo routines to provide quantitative information on cell size and cell size distribution for a representative polymer -- diluent system. The predicted structures were in agreement with experimentally formed membranes. The information gained from matrix solidification modeling was used to make finite element (FE) simulations in ABAQUS CAE to model the drying of the microporous morphology, with capillary forces being the dominant force driving shrinkage and collapse of the structure. These FE simulations predicted no permanent deformation arising from only capillary forces, which was confirmed through experimental evidence showing no correlation to surface tension. For polar polymers an additional heuristic was proposed: use extractants that are more alkane-like, regardless of surface tension, to reduce the collapse of the structure. FE simulations were used to model the uni-axial elongation of track-etch membranes in an effort to change performance characteristics. The FE simulations accurately predicted pore shape changes comparable to experimental values. The pore shape change information was used to modify standard sieve filtration models. The modified sieve filtration models show that a relatively modest strain of 35% can double the initial flux of track-etch membranes.