Structure of poly(methyl methacrylate) chains adsorbed on sapphire probed using infrared-visible sum frequency generation spectroscopy

We have studied the orientation of the train segments of a poly(methyl methacrylate) (PMMA) adsorbed layer at the CCl4-sapphire interface using surface-sensitive IR-visible sum frequency generation (SFG) spectroscopy. The SFG spectra of PMMA chains adsorbed on sapphire indicate ordered ester methyl groups. In comparison, we did not observe any significant contributions from the backbone methylene and alpha methyl groups, suggesting that these groups are disordered. No change in the structure of the adsorbed layer is observed upon cooling the solvent below the theta temperature; this is consistent with the picture of flat adsorbed chains on the surface. Interestingly, the orientation of the ester methyl groups of a spin-coated PMMA film at the PMMA-sapphire interface is similar to that of the same groups in the chains adsorbed from solution.