Interfacial conformation energies of stereoregular poly(methyl methacrylate) by infra-red reflection absorption spectroscopy

Infra-red reflection absorption spectroscopy (i.r.a.s.) has been performed on isotactic (i-) syndiotactic (s-) and atactic (a-) poly(methyl methacrylate) (PMMA) which have been spin-cast on aluminum mirrors. This spectroscopic investigation of the interface has highlighted differences in chain conformation for the stereoisomers at the surface. Indeed, the fraction of carbonyl bonded to the aluminum surface is higher for i-PMMA than for a-and s-PMMA. Furthermore, spectral subtraction combined to the van't Hoff relation, already used by O'Reilly and coworkers(1,2) for bulk samples, has provided experimental values of the interfacial conformational energy Delta E. These values, calculated from the absorption peaks in the infra-red range 1300-1100 cm(-1), are for the highest ones 7.8 kcal mol(-1) for s-, 5.0 kcal mol(-1) for a- and 2.1 kcal mol(-1) for the i-PMMA layer on the aluminum surface. The results show a large increase of the conformational energy of the adsorbed sample as compared to the bulk. Furthermore, the glass transition, which can be deduced from the infra-red spectral subtraction, exhibit a large increase in the case of i-PMMA, while it remains constant for a- and s-PMMA. These results are interpreted in terms of interfacial gauche to trans conformational changes and restrictions in molecular motions due to strong specific interactions of PMMA on aluminum. (C) 1997 Elsevier Science Ltd.