Chain flattening of spin-cast PMMMA on aluminum mirrors: Influence of polymer tacticity

Abstract

Polymer tacticity has already been shown to be an influential parameter governing the behavior of polymer solutions and polymer blends, but not for polymer adsorption. However, using PMMA having a racemic fraction (ƒ r) ranging from 0.1 to 0.9, spin-cast on aluminum mirror surfaces, makes it possible to work out configuration differences in the adsorbed layer. Reflectance infra-red spectroscopy has been used to evaluate the fraction of surface bonded carbonyl groups (ƒ ab CO) and local conformations of both main and side chain. The ƒ ab CO value is 0.1 for highly syndiotactic PMMA and reaches 0.7 for mainly isotactic PMMA. This latter very high value of ƒ ab CO can only be explained by chain flattening resulting from a distortion from its Isotropic state. This distortion may be due to both strong interactions that may exist between PMMA and the aluminium substrate, and also to the spin-cast process that may lead to chain straightening and flattening. The assumption of conformational energy changes of both backbone and side chain is confirmed by changes in the FT-IR Av 2(1245 cm −1) Av 1(1265–1275 cm −1 ) ratio. Ellipsometric spectroscopy has provided information on the global conformation of adsorbed chains by precise thickness measurements. Thus, thickness ( h) increases for diluted solutions linearly from 10 to 70 A as the average syndiotactic sequence length ( n s ) increases. However, IR and ellipsometric results tend to prove that isotactic PMMA may adsorb on the aluminum surface in a more flattened way with longer chains than atactic and syndiotactic chains. This can be explained by the ability of chain segmental rearrangements ( meso tg state) that may allow isotactic PMMA to favor the establishment of interfacial acid-base bonds while the syndiotactic PMMA local stiffness of the tt state does not. The influence of the chain entropy in solution is also discussed as an important parameter.