Measurement and analysis of bath-side interfacial concentration gradients during phase inversion

Abstract

An optical technique is presented for measurement of interfacial concentration gradients following quenching of polymer films into a liquid coagulation bath; the process commonly referred to as phase inversion. Refractive index gradients on the bath side of the film-bath interface are monitored through analysis of the far-field diffraction pattern of a light beam transmitted through the bath. These are converted to bath-side interfacial concentration gradients. Monitoring light intensity through the film side allows simultaneous determination of the phase separation time. Data for delayed precipitation of the cellulose acetate/N,N-dimethylformamide/ethanol system, and rapid precipitation (less than 1 sec) of the cellulose acetate or polysulphone/N,N-dimethylformamide/water systems are presented. Variations in the process conditions: initial cast film thickness, and initial coagulation bath and/or cast film composition were tested. Changes in the observed variables (interfacial solvent gradient, time for phase separation, and change in film thickness) associated with each of the process variables are consistent with qualitative and quantitative considerations of the mass transfer process. The lack of correlation of the data with diffusion-controlled modeling parameters indicates that the bath-side processes are convection domination. Gradient data for rapid precipitation conditions are quantified in terms of a two-parameter mass transfer model.