FORMATION OF AIR-CAST CELLULOSE-ACETATE MEMBRANES .2. KINETICS OF DEMIXING AND MICROVOID GROWTH

In Part I of this work, we described results concerning formation of m acrovoids in air-cast cellulose acetate (CA) membranes. In Part II, we report on the kinetics of membrane formation in one of the systems st udied in Part I. The system consisted of 2-methyl-2,4-pentanediol (MPD ), acetone and CA 394-60S (with a weight-average molar mass, M(w) = 2. 39.10(5) g/mol). The evaporation rate of MPD was found to be negligibl e at room temperature and gravimetric monitoring of acetone evaporatio n allowed determination of the average mixture composition during memb rane formation. Acetone evaporation rates were measured gravimetricall y for MPD/acetone mixtures and for two ternary membrane-forming soluti ons. Composition change of the ternary mixture at the plate (belt) sid e during 20 min of membrane formation was monitored by infrared spectr oscopy. Microvoid (pore) growth kinetics was studied by quenching part ially formed air-cast membranes (precursors) in water after predetermi ned intervals of air exposure. The microvoids on precursor surfaces we re then visualized by scanning electron microscopy (SEM). The image an alysis of electron micrographs yielded modal (most frequent) values of microvoid diameters, D, for the two surfaces of each precursor. Plots of D2 versus time were found to be linear with slope values around 1. 10(-10) cm2/s. We suggest that the microvoid growth rates may be relat ed to the ''slow'' reptational diffusion of entangled polymer chains. Polymer reptation is about five orders of magnitude slower than diffus ion of solvent or nonsolvent and may be, therefore, rate-controlling.