| Abstract: | The molecular orientation of adsorbed polyimides and model compounds with respect to highly oriented pyrolytic graphite (HOPG) surfaces was determined by reflection-absorption infrared (RAIR) spectroscopy. As the thickness of the pyromellitic diimide (PMDI) films decreased, bands near 766 and 729 cm-1, which were assigned to the CNC out-of-plane bending modes, were relatively strong in the spectra, demonstrating that PMDI was adsorbed flat-down with the imide rings parallel to the HOPG surface. As the thickness of the N,N'-diphenylpyromellitic diimide (DPPMDI) films was decreased, bands near 1785 and 1722 cm-1 which were assigned to the C=O stretching modes, were relatively strong in the RAIR spectra. Moreover, bands near 869, 841, 742, and 686 cm-1 attributed to the out-of-plane bending modes of monosubstituted benzene rings became considerably stronger. It was suggested that the DPPMDI molecules nearest to the HOPG surface were adsorbed edge-on with the carbonyl groups perpendicular to the surface and with the monosubstituted benzene rings parallel to the surface. There were significant differences in relative band intensity observed between the transmission and RAIR spectra of polyimides derived from pyromellitic dianhydride (PMDA) and oxydianiline (ODA). The band near 1725 cm-1 due to the C=O stretching mode increased its relative intensity in the RAIR spectra of PMDA/ODA polyimides, while the bands near 1499 and 1237 cm-1 due to the ODA moieties decreased their relative intensities, implying that PMDA moieties were oriented edge-on with the carbonyl groups perpendicular to the surface and that the ODA moieties were oriented mostly parallel to the surface. However, the RAIR spectra obtained from PMDA and 2,2-bis4-(4-aminophenoxy)-phenyl]-hexafluoropropane (4-BDAF) polyimides were similar to the transmission spectra of the polyimide, indicating that PMDA/4-BDAF was randomly oriented on the HOPG surface. |
