SORPTION AND DIFFUSION OF WATER-VAPOR IN POLYIMIDE FILMS

Sorption and diffusion of water vapor are investigated gravimetrically for polyimide films. The activity dependence of the solubility and diffusion coefficients, S and D, respectively, is classified under four types: (1) constant S and D type, (2) dual-mode sorption and transport type, (3) dual-mode type followed by a deviation due to a plasticization effect at high vapor activity, and (4) constant S and D type followed by a deviation due to water cluster formation at high activity. For the dual-mode type, the Henry's law component is much larger than the Langmuir component except at low activity, and therefore deviation in behavior from the first type is small. S is larger for polyimides with higher content of polar groups such as carbonyl, carboxyl, and sulfonyl. D is larger for polyimides with a higher fraction of free space, with some exceptions. The polyimide from 3,3',4,4'-biphenyltetracarboxylic dianhydride and dimethyl-3,7-diaminodibenzothiophene-5,5-dioxide belongs to the third type and displays both large S and large D. The polyimide from 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 4,4'-oxydianiline belongs to the fourth type, and has the largest D but rather small S because of the hydrophobic C(CF3)2 groups.