Diffusion of solvent from a cast cellulose acetate solution during the formation of skinned membranes

Abstract

The transport of solvent out of a cast cellulose acetate (CA) solution into the coagulation bath during membrane formation is treated as a diffusion process. From the increase of solvent concentration in the bath with time (solvent leaching experiments) an overall solvent diffusion coefficient has been calculated. In size these coefficients compare well to mutual pseudo-binary solvent-non-solvent diffusion coefficients determined by means of a classical boundary broadening method applied to ternary solutions with fixed CA concentration, but with a gradient in solvent-nonsolvent composition. Since binary polymer-solvent interdiffusion coefficients are at least one order of magnitude lower, it is concluded that the diffusion of solvent into the coagulation bath is essentially a pseudo-binary solvent-non-solvent diffusion process. Combination of experimental results with model calculations for the effect of a thin dense skin on the diffusion of solvent out of the sublayer shows that the casting-leaching diffusion coefficient can be used to describe the out-diffusion of solvent from the layer under the skin provided that the relative skin resistance is not too high, or that the skin thickness is small.

Triggering Degradation of Cellulose Acetate by Embedded Enzymes: Accelerated Enzymatic Degradation and Biodegradation under Simulated Composting Conditions

“…There was no steric hindrance in the active sites of IL, as evidenced by the higher V max for IL compared to free lipase (Table ). CA has been shown to have very little diffusional resistance, , which might explain the nonaffected enzyme–substrate reactions after immobilization of lipase. Also, a higher affinity was noticed toward the substrate in IL.…”

Review on the development of defect‐free and ultrathin‐skinned asymmetric membranes for gas separation through manipulation of phase inversion and rheological factors

“…Nevertheless, the nascent skin layer, which is formed during dry‐phase inversion, acts as a resistive barrier between the coagulation bath and the interior region of the membrane. Strathmann et al have determined that the diffusion coefficients of skin layer are much lower (<100 times) than the bulk structure of sublayer 44. The top layer directly influences the subsequent layers by controlling the counterdiffusion rate of solvents and nonsolvents through membrane during wet‐phase inversion.…”

Investigation into the rheological properties of PES/NMP/nonsolvent membrane‐forming systems

“…The thermodynamics of the dope can be described satisfactorily with a triangle phase diagram 4–6. The NS–solvent exchange and the composition path after immersion can be measured with various techniques7–11 or simulated by mass‐transfer models 12–15…”