Surface characterization and adhesive properties of poly(imidesiloxane) copolymers containing multiple siloxane segment lengths

A series of poly(imidesiloxane) (SIM) copolymers have been synthesized from alpha,omega'-aminopropylpoly(dimethylsiloxane) (PDMS) of varying lengths, 2,2-bis(4-[4-aminophenoxyl]phenyl)propane (BAPP), and 4,4'-oxydiphthalic anhydride (ODPA). In this series, the total composition of PDMS was maintained at 10 wt % with multiple PDMS segment lengths of different relative compositions (e.g., 5% PDMS containing 1 repeat unit, designated G-1, and 5% PDMS containing an average repeat unit of 9, designated G-9, incorporated into the same polymer vs 1% G-1 and 9% G-9 in the same polymer). Two main polymer series were synthesized: one containing G-1 and G-9 in varying ratios and the other containing G-5 and G-9. Both of these series have been analyzed using angle-dependent X-ray photoelectron spectroscopy (XPS). The Si/N ratios as measured by XPS from all members of the G-9/G-1 series were statistically equivalent to that of the polymer containing pure 10% G-9, while that of the polymer containing 10% G-1 had a significantly lower value. This suggests that there is preferential segregation of the longer G-9 siloxane segment lengths to the surface. These same studies were repeated for the series containing both G-5 and G-9, and similar results were found, although the extent of segregation was less. The angle-dependent data were then used to obtain an in-depth profile by using a numerical algorithm, which was designed for samples with compositional gradients as is the case with this system. From the profiles, it was determined that the thickness of the surface PDMS layer of all polymers containing both G-9 and G-1 was the same for all compositions studied, while that of the pure 10% G-1 was much thinner. The adhesion strengths of these polymers were measured using peel strength tests, and the adhesion values were correlated to the XPS results. It was found that the adhesion of the pure 10% G-1 was much higher than that of any other polymer in the series. The remainder of the polymers in the series all had similar adhesion values. These results are consistent with a model of the surface, which has longer segment lengths preferentially segregating and dominating the adhesive properties.