THE VISUALIZATION OF SPATIAL GRADIENTS IN POLYMER AND SOLVENT DYNAMICS FOR MIXED-SOLVENTS INGRESSING POLY(METHYL METHACRYLATE) USING STRAY FIELD MAGNETIC-RESONANCE-IMAGING

Stray-held magnetic resonance imaging (m.r.i.) has been used to study the ingress of methanol and methanol-acetone vapour mixtures into poly (methyl methacrylate), PMMA. For pure methanol, the polymer concentrat ion and H-1 spin-spin relaxation time is found to be constant across t he swollen region. The relaxation time is, however, greater by a facto r of 1.1 compared to the rigid polymer. For mixed solvents, the polyme r chain spin-spin relaxation time is greater by factors ranging from 1 .1 to 1.4 across the swollen region in samples exposed to solvent for up to 1 week, providing evidence for a spatial gradient in the polymer chain dynamics. Small polymer concentration gradients are also seen i n samples exposed to methanol-acetone mixtures. In further experiments , stray field imaging and gradient spin echo diffusion techniques have been combined for the first time in order to make high spatial resolu tion measurements of the solvent self-diffusion coefficient. These exp eriments yield complementary spatial gradients in the diffusion coeffi cient such that the diffusion coefficient typically varies by a factor of 2 across any one sample, ranging from 0.15 to 0.30 x 10(-6) cm(2) s(-1) over 0.8 mm for pure methanol in PMMA and front 1.8 to 3.2 x 10( -6) cm(2) s(-1) over 1.5 mm for 70% methanol-30% acetone. These result s are discussed in the context of previous liquid-state m.r.i. investi gations of the solvent alone in similar systems. (C) 1997 Elsevier Sci ence Ltd.