Solvent penetration into polymers studied by 1HNMR microscopy

1H NMR microscopy has become established as an essential tool for the study of the diffusion of small molecules through solid polymers. Since the initial experiments over 30 years ago, numerous examples of the application of 1H NMR microscopy to penetrant diffusion have been presented, aimed at elucidating important technological processes, such as solvent ingress into cross‐linked films as well as tablet dissolution and hence drug delivery. The profile of penetrant concentration can provide insights into the mechanism of diffusion, distinguishing between Fickian diffusion driven by a concentration gradient and non‐Fickian diffusion in which either the diffusion coefficient depends on penetrant concentration or on the relaxation of the polymer chains under swelling stress. Examination of the literature indicates that quantitative interpretation of imaging intensities must be treated with care. A number of examples have been reported in which the relaxation times of the penetrant protons depend on the local concentration of the penetrant, indicative of strong interactions with the polymer chains