L. Zeman et T. Fraser, FORMATION OF AIR-CAST CELLULOSE-ACETATE MEMBRANES .2. KINETICS OF DEMIXING AND MICROVOID GROWTH, Journal of membrane science, 87(3), 1994, pp. 267-279
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.