| Hollow fiber membranes are formed by an isothermal wet spinning process wherein the polymer (in this case Nylon 66) precipitates as a crystalline aggregate from the supersaturated membrane forming solution. The interrelationships between polymer solution formulations and the concentrations for the two coagulation baths (inner and outer), and the effects of these variables on the final hollow fiber membrane structure (i.e., application) are presented in this research work. Complete phase diagrams (binodal, spinodal and crystallization isotherms) for polyamide and poly(vinylidene fluoride) membrane forming systems at room temperature are experimentally determined and the thermodynamic states of the polymer solutions are given from Flory-Huggins theory with concentration dependent ternary interaction parameters. Hollow fiber parameters are predicted from flat plate membrane experiments. An analysis is presented which describes the local composition evolutions within the polymeric solution, prior to polymer precipitation, during the fiber spinning process wherein two sets of transports occur from inner and outer membrane-bath interfaces. The structures of the ultimate membrane are observed by SEM and found to be consistent with the predictions of this diffusion model. The results form a basis for predicting, a priori, conditions for microporous hollow fiber formation for polymers of very different structures. |
