469088 Molecular Alignment of Phase-Separated, Segmented Polyurethanes in Thin Films Using Intrinsic Fluorescence
Intrinsic fluorescence of aromatic rings typically results in two types of emission: monomer fluorescence and excimer fluorescence. Monomer emission occurs from the isolated single excited state of a phenyl ring whereas excimer emission occurs from two phenyl rings in a parallel, sandwich-like conformation with a separation distance of several Angstroms. Using a novel approach involving intrinsic fluorescence of a segmented polyurethane composed of a poly(propylene oxide) soft segment and 2,4-toluene diisocyanate hard segment, both monomer and excimer emission are observed. There are two pathways for excimer formation: dynamic or static. Using excitation spectra, we observe ground-state interactions of the hard segments in polyurethane thin films. In sufficiently thin (≤ 200 nm) polyurethane films, on both interacting (quartz; with a potential for surface hydroxyl groups to hydrogen bond with carbonyl groups in polyurethane hard segments) and non-interacting (methylated glass; with no surface hydroxyl groups) substrates, ground-state dimers are formed and produce static excimer fluorescence.
Using the ratio of excimer emission to monomer emission (Ie/Im) as a function of film thickness, the effect of confinement on molecular orientation was examined. Upon changing the interactions with the substrate, the bulk film ratios were unaffected. On the quartz substrate, there was little or no temperature dependence of excimer fluorescence. In contrast, with the non-interacting methylated substrate, excimer fluorescence exhibited a significant temperature dependence. As the film is confined to thicknesses below 200 nm, the films on both substrates begin to align effectively to produce more excimer fluorescence from ground-state dimers than monomer fluorescence. This effect is seen to a larger degree on the methylated substrates than on the quartz substrates. The excitation spectra of polyurethane thin films on the methylated substrate are indicative of both dynamic and static excimer formation whereas the films on a quartz substrate show only static excimer formation. This difference in excimer fluorescence is hypothesized to be the result of the suppression of dynamic excimer formation by hydrogen bonding interactions between the quartz substrate and the polymer. Due to these interactions, a more rigid environment is introduced from the substrate. In addition, the effects of this rigid substrate environment are observed to propagate up to 200 nm from the substrate into the film. This effect is seen both above and below the glass transition temperature of the hard segment.
Interfaces in thin films lead to alignment and formation of ground-state dimers not seen in bulk polyurethanes. With these techniques, we were able to discern the non-covalent interactions that impact the molecular alignment of thin film polyurethanes. In the absence of hydrogen-bonding at the substrate, the molecular alignment of the hard segments due to aromatic interactions is significant in thin films. In the presence of hydrogen bonding at the substrate, the molecular alignment of aromatic rings into ground-state dimers is still present but the dynamic formation of dimers is substantially reduced.