Water sorption behaviour of polyimide thin films with various internal linkages in the dianhydride component

Polyimide films have been prepared from reaction of 1,4-phenylenediamine (PDA) with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 4,4′-oxydiphthalic dianhydride (ODPA), diphenylsulfone tetracarboxylic dianhydride (DSDA) or (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The water sorption behaviour of thin films of the polyimides was strongly dependent on the chemical structure of the internal linkage and the morphological structure of the films. The in-plane orientation and molecular chain order decreased with increasing bulkiness of the internal linkage, leading to relatively low diffusion coefficient in DSDA-PDA and 6FDA-PDA films. The water diffusion coefficient varied in the range of 1.7×10^-10 to 10.5×10^-10cm²/s, and was in the increasing order: BPDA-PDA <ODPA-PDA <BTDA-PDA <DSDA-PDA <6FDA-PDA. However, BTDA-PDA and DSDA-PDA films having hydrophilic groups and poor packing order induced relatively high water uptake. 6FDA-PDA film showed relatively fast diffusion rate and high water uptake due to the amorphous structure and poor packing order, irrespective of hydrophobic hexafluoroisopropylidene linkage.