Water sorption and diffusion behaviours in thin films of photosensitive polyimides

Abstract

Several photosensitive polyimide (PSPI) precursors were synthesised by the acid/base complexations of conventional poly(amic acid) precursors with photochemically cross-linkable 2-(dimethylamino)ethyl methacrylate. PSPIs in films were prepared from the photosensitive precursors by thermal imidisation, whereas the corresponding PIs in films were prepared from the conventional poly(amic acid)s: rod-like poly(p-phenylene pyromellitimide) (PMDA-PDA), rigid (poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semi-flexible poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA), semi-flexible poly(4,4′-oxydiphenylene pyromellit-imide) (PMDA-ODA), and flexible poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). Water sorption and diffusion behaviours in the PSPIs were gravimetrically measured at 25°C in 100% relative humidity and compared with those of the corresponding PIs. For the PSPIs as well as the PIs, the water sorption and diffusion behaviours were nearly Fickian, regardless of the backbone chemistry. However, those were strongly dependent upon the polyimide backbone chemistry and precursor origin. In addition, those in polyimides were affected by the bulky photosenistive groups, even though they were temporarily linked to the precursor polymers and then debonded from the backbones and ultimately outgassed during the thermal imidisation process. For PMDA-PDA, BPDA-PDA and BPDA-ODA, the PSPIs absorbed water more quickly than the corresponding PIs, whereas for PMDA-ODA and BTDA-ODA, the PSPIs absorbed water less quickly than the corresponding PIs. In contrast to the water diffusion, all the PSPIs absorbed slightly more or a great deal more water than the corresponding PIs, depending on the backbone chemistry. All the measured water sorption and diffusion behaviours in the PSPIs were understood by considering changes in the morphological structure (namely, chain order and orientation), voids, and residues possibly induced by the bulky photosensitive groups, in addition to the Tg as well as the chemical affinity to water.