The interaction of water vapor with the surface and bulk of poly(imidesiloxane) copolymers has been characterized in an attempt to determine the important factors in the copolymer's resistance to water ingression. The multiblock copolymers were synthesized from benzophenone tetracarboxylic dianhydride, bisaniline P and pre-formed amine-terminated poly(dimethylsiloxane) oligomers, with phthalic anhydride as an end-capping agent. Similar copolymers had been previously shown to have reduced water sorption, increased surface hydrophobicity, and increased adhesive durability in hot, wet environments.
Inverse gas chromatography was used to conduct surface energetics studies on copolymers of different siloxane concentration and a polyirnide homopolymer. Free energies of specific interaction of water vapor, AGsp°, with the polymer surfaces were found to decrease with the incorporation of siloxane into the polyirnide. The dispersive components of the solid surface free energy of the siloxane- containing copolymers were equal within error to that of pure poly(dimethylsiloxane), indicating a PDMS-rich, hydrophobic surface. The AGsp° of the copolymers were not significantly different, suggesting that the copolymer surfaces were very similar. This indicated a minimum weight percent of siloxane incorporation required to maximize the copolymer's surface water resistance. The minimum amount for the studied system was at most ten percent.
Diffusion coefficients of water vapor in the polyimide and copolymers were determined from gravimetric sorption experiments. Higher levels of siloxane incorporation caused a definite increase in the diffusion coefficient, indicating a decreased resistance to water ingression. The increase in diffusion was found to be influenced by siloxane block length and was interpreted in terms of morphological and free volume theories. The diffusion coefficient of a 10 weight percent PDMS copolymer, however, was found to be the same within error as the polyimide diffusion oefficient.
The incorporation of siloxane into polyimides has been shown to increase water resistance due to the hydrophobicity of the siloxane-rich surface. However, high siloxane contents also increased the rate of water ingression in the bulk of the polymer. Increased water resistance of the surface may be achieved at lower siloxane concentrations without increasing diffusive (or decreasing mechanical) properties to undesirable levels.
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