Measurement and analysis of bath-side interfacial concentration gradients during membrane quenching by phase inversion
Gaides, Gary Edward
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https://hdl.handle.net/2142/21789
Description
Title
Measurement and analysis of bath-side interfacial concentration gradients during membrane quenching by phase inversion
Author(s)
Gaides, Gary Edward
Issue Date
1992
Doctoral Committee Chair(s)
McHugh, Anthony J.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Physics, Optics
Language
eng
Abstract
A technique based on Fourier transform optical theory has been developed which allows for transient measurements of bath-side interfacial refractive index gradients during the quench step of membrane formation by phase inversion. From examination of individual Fourier components in a diffraction pattern associated with this one-dimensional mass transport process one is able to overcome the temporal and spatial resolution limitations imposed by many of the more traditional optical techniques. A first order expression relating the local refractive index gradient in the quench cell as a function of its measured spatial offset in the Fourier plane was derived for the purpose of determining the interfacial gradient which may be readily converted to individual species (solvent/nonsolvent) concentration gradients. Real-time interfacial gradients were able to be measured anywhere from 2 to 5 seconds after initiation of the quench using this technique. The r.m.s. errors associated with the reported interfacial species concentration gradient values were determined to range between 3% and 7% of the reported values for quench times less than 60 seconds. However, the reproducability of the experiment was shown to be well within these error limits.
The bath-side interfacial refractive index gradient was measured as a function of time for both delayed (cellulose acetate in N,N-dimethylformamide with added nonsolvent ethanol) and instantaneous (cellulose acetate or polysulfone in N,N-dimethylformamide with added nonsolvent water) phase separation conditions with allowed variability in the following processing conditions: initial cast film thickness, initial solvent composition in the coagulation bath, initial nonsolvent composition in the casting solution, and initial polymer concentration in the casting solution. The directionality of the change associated with experimentally observed phenomena (bath-side interfacial solvent gradient and the time at which phase separation occurs in the film as determined by light transmission) for both rapid and delayed precipitating systems with appropriate changes in a given processing variable was consistent with qualitative mass transfer suggestions and existing model predictions for all conditions examined in this study. In addition, the experimental gradient data for all rapid precipitation cases were universally quantified through a non-dimensionalization procedure using a 2-parameter phenomenological model.
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