Regardless of the method of inducing phase separation (solvent–nonsolvent exchange or thermal quench) or the mechanism by which phase separation is initiated (spinodal decomposition or nucleation and growth), the initially homogeneous solution evolves into inviscid droplets of polymer-lean phase dispersed in a viscous polymer-rich fluid. As time passes, the droplets grow. Since these droplets ultimately become the cells within the membrane, a thorough understanding of the kinetics of droplet growth is essential for control of the final membrane morphology. Therefore, we have developed a coalescence-induced coalescence (CIC) theory that describes droplet growth and the fluid dynamics associated with droplet growth. The CIC studies delivered a three-dimensional representation of inviscid droplets within a viscous liquid matrix phase as a function of time. As such, the CIC model provides us with a starting point for the current research, which looks at the solidification, exchange, and drying steps of the typical membrane formation process. Solidification, exchange, and drying all can influence the microporous structure of the membrane.
The objective of this research is to relate final membrane structure to the suspension structure at the start of solidification, the physical properties of the polymer and diluent, and the processing conditions used in the final stages of the membrane formation process.
For the purpose of this study, we use the TIPS process and the formation of microfiltration / ultrafiltration membranes as representative of typical membrane formation processes. Specifically, two polymer–diluent systems are being investigated: poly(ethylene-co-vinyl alcohol) (EVAL) – glycerol and isotactic polypropylene (iPP) – diphenyl ether (DPE).
This project will deliver a simulation package that is capable of predicting membrane cell size, cell size distribution, pore size, pore size distribution, and connectivity. It will account for the membrane shrinkage, increased connectivity between cells, and reduced porosity observed in TIPS and phase inversion membrane production. The simulation package will provide manufacturers with a comprehensive tool that will allow them to minimize the costly and time consuming experiments needed in the development of new and improved membranes. The simulations will also allow manufacturers to tailor the final pore structure of membranes.
Solidification: Experimental evidence indicates that the droplet size in the liquid emulsion and the cell size in the final membrane are directly related, but not normally equal. Furthermore, experimental evidence indicates that the connectivity of cells via pores is significantly greater than one would predict from the CIC simulations and one would estimate based on the number of merging drops experimentally observed at any given time. These discrepancies have been determined to be largely the result of diluent expulsion from the matrix phase during solidification. Assuming a random trajectory of the expelled diluent allows for calculation of droplet growth using Monte Carlo methods. Coupled with the expulsion of diluent from the matrix is the fact that drops continue to merge during solidification. Current model results for an initial droplet volume fraction of 0.32 are shown in Figure 1 with good quantitative agreement.
Figure 1. Starting droplet size and final cell size (micrometers) as a function of growth time (seconds) for a starting droplet volume fraction of 0.32.
Exchange: The results from the solidification simulation are used as a starting point to determine the effect of diluent extraction on membrane structure. For the two main systems under investigation, EVAL–glycerol and iPP–DPE, simulations and experiments have shown the effects of diluent extraction are negligible. For these particular systems, the negligible impact of exchange is attributed to the crystalline structure of the polymers. Consequently, exchange is not included in the post-coarsening simulations.
Drying: Shrinkage of the membrane cells upon evaporation of the extractant has been reported in the literature. Figure 2 shows a micrograph of the typical cellular structure resulting from the TIPS process. Notice that the cells are distorted rather than spherical. This distortion, which is a result of the drying process, is incorporated in the model by using finite element methods to determine the effects of capillary pressures on the membrane microstructure. Figure 3 shows a representative segment of membrane with cells and connecting pores after the extractant has been evaporated and the structure has distorted due to the imposed stresses. Further work, including experimental studies, will be conducted in 2007 to verify the changes to pore and cell size distribution caused by the deformation during drying.
Figure 2. An EVAL–glycerol TIPS membrane extracted with isopropyl alcohol and dried under room temperature and pressure.
Figure
3. Microstructure deformation caused by capillary pressures within the
membrane. Colors represent the concentration of Mises Stresses within the
membrane.
Publications from this project: