(241b) The Importance of Dope Stability in Mixed Matrix Hollow Fiber Membrane Formation
The so-called, mixed matrix hybrid membrane approach relies upon the combination of polymer and molecular sieves to achieve improved selectivity and processing ease. The molecular sieves are chosen for their high separation efficiencies; while the polymer provides ease of processibility to the composite. Prevention of sieve aggregation and dope phase stability must be maintained prior to precipitation in a hollow fiber spinning process. Moreover, the environment of the sieve-polymer interface during phase separation of the nascent membrane must be controlled.
The first stage of dope formulation involves creating a stable sieve-solvent dispersion. Surface modification of a hydrophilic sieve surface was pursued using two quite different approaches. The first approach uses polymer grafted to the sieve surface. The second method uses a Grignard reagent to modify the surface. Both types of modified sieves were used to formulate dopes with similar polymer addition and mixing steps. The polymer grafted sieves form a more stable dispersion in solvent compared to the Grignard treated samples. Surprisingly, however, the Grignard treated sieve dope shows much better long term stability than the polymer grafted sieve dope. In spite of a low sieve loading in the dopes, both suspensions display unexpectedly higher viscosities than their polymer-only dope counterparts. Shear studies indicate structure formation and elasticity in the polymer grafted samples that was not apparent in the Grignard treated sample. On the other hand, selectivity enhancement and excellent sieve-polymer adhesion was observed for the membranes spun with the Grignard treated sieves but not those formed from the polymer grafted sieve dope. These early results suggest the utility of independent rheological tests to screen dopes for unstability in the shearing environment which are encountered in fiber spinnerets.