PHENOMENOLOGY AND MECHANISMS OF UNIDIMENSIONAL STRESS-DEPENDENT MICROMOLECULAR TRANSPORT IN A STIFF-CHAIN POLYMER .3. EFFECT OF MODIFICATION OF THE POLYMER

A detailed study of the kinetics and mechanism of micromolecular trans port in cellulose acetate films containing 2.0 acetate groups per gluc ose unit (CA-2.0) is reported. The polymer was prepared by controlled hydrolysis of CA-2.45 films studied in preceding articles. The same se ries of simple liquid penetrants varying from weak swelling agent to g ood solvent of the polymer was used. As before, measurement of rates o f penetration along the polymer film confined between glass plates was supplemented with information on penetrant distribution profiles in t he polymer film and on the corresponding deformation and structural re laxation of the swelling polymer, deduced from refractive index and bi refringence profiles, respectively. Transport was studied in (a) unori ented CA-2.0 films and (b) uniaxially oriented films with penetration normal and parallel to the orientation axis. This was equivalent to va rying the viscoelastic polymer properties affecting transport, under o therwise identical experimental conditions. The results complemented a nd extended those previously obtained with CA-2.45 in interesting ways and were successfully interpreted on the basis of a previously develo ped theoretical model designed to represent the influence of(a) the st ress generated by the constraints imposed on the swelling polymer, and (b) the viscoelastic response of the latter thereto, on the transport mechanism. It was shown that the observed differences in transport me chanism in CA-2.45 and CA-2.0 are primarily related to the correspondi ng changes in the sorptive capacity of the polymer for the relevant pe netrant rather than the chemical constitution of the latter. The most striking result in this respect was that the remarkable kinetic patter n (which involved a drastic change from Case I kinetics for penetratio n across, to Case II kinetics for penetration along, the axis of orien tation) exhibited by oriented CA-2.45 film penetrated by the strong sw elling agent of the series of penetrants used, namely methylene chlori de, was reproduced here for the penetration of acetone, which occupies the slot of strong swelling agent in the case of CA-2.0. (C) 1997 Joh n Wiley & Sons, Inc.