Abstract
Solvent ingress into polymeric materials is of widespread industrial importance in the context of polymer durability. Evidence suggests that the synergetic effects of two or more mixed solvents acting together and of residual solvent left over in the polymer from manufacture can be particularly important. Nuclear magnetic resonance (NMR) imaging has been explored for many years as a method which can provide quantitative data on the diffusion dynamics of solvent ingress into polymer materials. Until recently, however, the possibility of imaging the polymer component has been either impossible with conventional techniques or limited when using specially adapted and technically demanding methodologies. The work presented here describes the first studies using the stray-field nuclear magnetic resonance imaging (STRAFI) method to investigate fundamental solvent transport processes in methacrylate polymer materials. Firstly, the ingress of methanol and methanol-acetone vapour mixtures into polymethylmethacrylate (PMMA) was investigated. Using deuterated solvents, spatially resolved NMR polymer signal was obtained from exposed and unexposed polymer. Analysis of the polymer signal revealed that in the case of pure methanol ingress, the polymer concentration and 1H spin-spin relaxation time is constant across the swollen region. The relaxation time is, however, greater by a factor of 1.1 compared to the unexposed rigid polymer. For mixed solvents, the polymer chain spin-spin relaxation time is greater by a factor ranging from 1.1 to 1.4 across the swollen region in samples exposed to solvent for up to a week, providing evidence for a spatial gradient in polymer chain dynamics. Small polymer concentration gradients are also seen in sample exposed to methanol-acetone vapour mixtures. This study produced the first spatially resolved quantitative data on both polymer and solvent concentrations in a glassy polymer system using NMR. The ingress of pure methanol vapour and methanol-acetone vapour mixtures containing up to 50 weight% acetone into PMMA exhibits Case II diffusion dynamics. The stray field method was combined with gradient spin echo diffusion techniques for the first time in order to perform high spatial resolution maps of the solvent self-diffusion coefficient. These experiments yield complimentary spatial gradients in the diffusion coefficient which typically varies by a factor of 2 across the swollen region. The magnitude of the diffusion coefficients vary from 0.15 to 0.30 x 10-6 cm2s-1 over 0.8mm of swollen polymer for pure methanol vapour in PMMA to 1.8 to 3.2 x 10 -6 cm2s-1 over 1.5mm of swollen polymer for a 70 weight% methanol-30 weight% acetone vapour mixture. A series of follow-up experiments were performed to study the effect of small residual amounts of acetone incorporated into PMMA samples during manufacture on the subsequent ingress of liquid methanol. A comprehensive study of the effects of both temperature and acetone pre-swelling on methanol transport in PMMA is presented. It is shown that the incorporation of small fractions of acetone enhances the rate of uptake of liquid methanol. A transition from Case II to Fickian diffusion dynamics with increasing acetone pre-swelling is observed. This transition can be reversed by lowering the temperature of the sample. In addition, the ingress of pure methanol liquid was studied in the temperature range 25 - 65 C. In contrast to previous studies, the diffusion dynamics remained Case II over this temperature range. Finally, an analysis of the STRAFI data is presented using current models of solvent transport in glassy polymers. This represents the first substantive tests of these models for macroscopic samples. The model predictions for polymer viscosity and solvent diffusivity are successfully correlated with independent analyses.