Effect of Siloxane Segment Length on the Surface Composition of Polyimidesiloxane Copolymers and its Role in Adhesion by

A series of polyimidesiloxane (SIM) copolymers, based on a.co-aminopropylpoly(dimethylsiloxane)(PDMS),2,2-bis(4-[4-aminophenoxyl]phenyl)propane(BAPP)and 4,4'-oxydiphthalic-anhydride(ODPA) was synthesized in our laboratories. We investigated the effect of siloxane segment length on the surface composition of the SIM copolymers and its role in adhesion. The aim is to elucidate the correlation between polymer structure, surface composition, and adhesion strength. Using a deconvolution computer program, compositiondepth profiles of the near surface region to approximately 100Ä depth were simulated from the results of angle-dependent Electron Spectroscopy for Chemical Analysis (ESCA) experiments. The simulated composition-depth profiles show that the topmost surface of the air (free) surface of the 75um thick SIM copolymer film consists of a siloxane-rich layer, even with the shortest siloxane segment. For a given siloxane bulk content, a longer siloxane segment gives a surface richer in siloxane. The adhesion strength was determined by peel strength testing of SIM copolymers Fe/Ni Alloy-42 laminates. The peel strength decreases exponentially with increasing thickness of the surface siloxane-rich layer, which corresponds to longer siloxane segment lengths. However, all values of adhesion strength of SIM copolymers are higher than that of pure polyimide. This confirms our previous resultsthat both siloxane and polyimide are essential components to rendering the high adhesion strength. The siloxane component provides a good diffusive ability, while the polyimide component interacts with the substrate to give rise to adhesion strength. Longer siloxane segments in the SIM copolymers are found to be detrimental to the adhesion strength by way of interference with the polyimide-metal interaction.