Adsorption of stereoregular poly(methyl methacrylates) on γ-alumina:: Spectroscopic analysis

Three poly(methyl methacrylates) (PMMA) with a racemic fraction ranging from 0.25 to 0.91 have been adsorbed from a chloroform solution on gamma-alumina and studied by diffuse reflectance infrared spectroscopy (DRIFT) and solid-state C-13 NMR spectroscopy. From DRIFT spectra, it is seen that the fraction of carbonyl groups bonded to the surface goes from 0.29 for s-PMMA to 0.41 for i-PMNIA, but there is a larger amount of s-PMMA retained on the surface (at any given solution concentration). NMR spectroscopy indicates, from shifts of the methylene and alpha-methyl carbon peaks (due to the gamma-gauche effect), that the adsorption is accompanied by changes in conformation, with an increase in the number of trans conformers, particularly with i-PMMA. These results indicate that s-PMMA adsorbs on gamma-alumina in a brush-like configuration, with a relatively small number of groups attached to the surface, and tails and loops sticking out of the surface, whereas i-PMMA adsorbs in a sheet-like configuration, with a greater number of interacting groups. In both cases, the adsorption is accompanied with an increase in the number of trans conformers as compared to the bulk conformation. (C) 1999 John Wiley & Sons, Inc.