The rheology and phase separation kinetics of mixed-matrix membrane dopes
Olanrewaju, Kayode Olaseni
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Mixed-matrix hollow fiber membranes are being developed to offer more efficient gas separations applications than what the current technologies allow. Mixed-matrix membranes (MMMs) are membranes in which molecular sieves incorporated in a polymer matrix do separation between gas mixtures based on the molecular size difference and/or adsorption properties of the component gases vis-à-vis the porous structure and the nature of adsorption sites in the molecular sieve. The development of MMMs to deliver on its promises has however been slow. The major challenges encountered in the efficient development of MMMs are associated with some of the paradigm shifts involved in their processing. For instance, mixed-matrix hollow fiber membranes are prepared by a dry-wet jet spinning method. For an efficient large scale processing of hollow fibers the rheology and kinetics of phase separation of the MMM dopes are important control variables in the process design. Therefore, this research thesis aims to study the rheology and phase separation kinetics of mixed-matrix membrane dopes. In research efforts to develop predictive models for the shear rheology of suspensions of zeolite particles in polymer solutions it was found that MFI zeolite suspensions have relative viscosities that dramatically exceed the Krieger-Dougherty predictions for hard sphere suspensions. Our investigations show that the major origin of this discrepancy is the selective absorption of solvent molecules from the suspending polymer solution into the zeolite pores. Consequently, both the viscosity of the polymer solution and the particle contribution to the suspension viscosity are greatly increased. A predictive model for the viscosity of porous zeolite suspensions incorporating a solvent absorption parameter, α, into the Krieger-Dougherty model was developed. We experimentally determined the solvent absorption parameter and our results are in good agreement with the theoretical pore volume of MFI particles. In addition, fundamental studies were conducted with spherical nonporous silica suspensions to elucidate the role of colloidal and hydrodynamic forces on the rheology of mixed-matrix membrane dopes. Also in this thesis, details of a novel microfluidic device that enables measurements of the phase separation kinetics via video-microscopy are presented. Our device provides a well-defined sample geometry and controlled atmosphere for in situ tracking of the phase separation process. We have used this technique to quantify the phase separation kinetics (PSK) of polymer solutions and MMM dopes upon contact with an array of relevant nonsolvent. For the polymer solution, we found that PSK is governed by the micro-rheological and thermodynamic properties of the polymer solution and nonsolvent. For the MMM dopes, we found that the PSK is increased by increased particles surface area as a result of surface diffusion enhancement. In addition, it was found that the dispersed particles alter the thermodynamic quality of the dope based on the hydrophilic and porous nature of suspended particles.