(698e) In-Situ Observation of Phase Separation and Morphology of Polymeric Membrane Dopes in Shear and Elongational Flow

In hollow fiber membrane spinning, the effects of shear and elongational stresses on membrane morphology are often convoluted due to the complexity of the spinning process. In this study, we systematically investigate the effect of shear and elongational stresses on phase separation and membrane morphology utilizing microfluidics, video-microscopy and electron microscopy (SEM) of the formed fibers. Polymer solutions were extruded through microfluidic channels into non-solvent, and the in situ processes of phase separation and defect formation were observed through video-microscopy and compared with the resulting cross-sectional images of the fibers. The rheological properties of dope solutions and the resulting fiber morphology as a function of shear rate/elongational rate were combined to construct flow regime maps with clearly identifiable characteristic features. Each of the observed regimes was shown to have limiting physical process that determines fiber formation and morphology: at low extrusions rates mass transfer is the dominant process, the inherent thermodynamics of the polymer solution plays a key role at intermediate extrusion rates, and shear and elongational stresses are critical factors at high extrusion rates. Defect-free regimes were found in particular regimes and most importantly, high elongational stresses can eliminate the defect formation in membrane fabrication.