INFLUENCE OF TACTICITY ON THE CONFORMATION AND DEVELOPMENT OF ACID-BASE ADDUCTS OF PMMA HOMOPOLYMERS ADSORBED ON ALUMINUM 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 tacticitie s were spin-coated on aluminium mirror substrates. Ellipsometric measu rements show that the PMMA film thickness increases linearly with the syndiotactic sequence length (as determined by C-13 nuclear magnetic r esonance spectroscopy.) These results tend to prove that the backbone conformation, the spatial extension and the segmental rigidity are aff ected when the density of racemic diads increases. Fourier transform i nfra-red reflection-absorption spectroscopy (IRAS) reveals orbital ove rlapping between the sp bonding orbital of the PMMA carbonyl oxygen at om and the sigma antibonding hydroxyl orbital and allows us to calcul ate an enthalpy of interaction of -10.5 kJ mol(-1) Moreover, it is sho wn that for low amount of adsorbed polymer (10(-2)M), the density of a cid-base interfacial adducts decreases as the average syndiotactic seq uence length of the PMMA increases. This tends to confirm that the con formations of adsorbed isotactic PMMA chains are more extended than th ose for syndiotactic chains. IRAS analysis shows that the development of interfacial acid-base adducts induces backbone trans-gauche conform ations changes in trans-trans conformations, leading then to an increa se in the conformational energy of the main chain. This effect is magn ified as the isotacticity of the adsorbed PMMA homopolymer increases. (C) 1997 Elsevier Science Ltd.