Influence of tacticity on the conformation and development of acid-base adducts of PMMA homopolymers adsorbed on aluminium mirrors

The influence of tacticity on (1) the conformation of adsorption at a polymer/substrate interface and (2) the magnitude and density of acid-base interactions developed at the interface, has been studied. Films of poly(methyl methacrylate) (PMMA) homopolymers of various tacticities were spin-coated on aluminium mirror substrates. Ellipsometric measurements show that the PMMA film thickness increases linearly with the syndiotactic sequence length (as determined by C-13 nuclear magnetic resonance spectroscopy.) These results tend to prove that the backbone conformation, the spatial extension and the segmental rigidity are affected when the density of racemic diads increases. Fourier transform infra-red reflection-absorption spectroscopy (IRAS) reveals orbital overlapping between the sp bonding orbital of the PMMA carbonyl oxygen atom and the sigma* antibonding hydroxyl orbital and allows us to calculate an enthalpy of interaction of -10.5 kJ mol(-1) Moreover, it is shown that for low amount of adsorbed polymer (10(-2)M), the density of acid-base interfacial adducts decreases as the average syndiotactic sequence length of the PMMA increases. This tends to confirm that the conformations of adsorbed isotactic PMMA chains are more extended than those for syndiotactic chains. IRAS analysis shows that the development of interfacial acid-base adducts induces backbone trans-gauche conformations changes in trans-trans conformations, leading then to an increase in the conformational energy of the main chain. This effect is magnified as the isotacticity of the adsorbed PMMA homopolymer increases. (C) 1997 Elsevier Science Ltd.