CHAIN FLATTENING OF SPIN-CAST PMMA ON ALUMINUM MIRRORS - INFLUENCE OFPOLYMER TACTICITY

Polymer tacticity has already been shown to be an influential paramete r governing the behavior of polymer solutions and polymer blends, but not for polymer adsorption. However, using PMMA having a racemic fract ion (f(r)) ranging from 0.1 to 0.9, spin-cast on aluminum mirror surfa ces, makes it possible to work out configuration differences in the ad sorbed layer. Reflectance infra-red spectroscopy has been used to eval uate the fraction of surface bonded carbonyl groups (f(C=0)(ab)) and l ocal conformations of both main and side chain. The f(C=0)(ab) value i s 0.1 for highly syndiotactic PMMA and reaches 0.7 for mainly isotacti c PMMA. This latter very high value of f(C=0)(ab) can only be explaine d by chain flattening resulting from a distortion from its isotropic s tate. This distortion may be due to both strong interactions that may exist between PMMA and the aluminium substrate, and also to the spin-c ast process that may lead to chain straightening and flattening. The a ssumption of conformational energy changes of both backbone and side c hain is confirmed by changes in the FT-IR A upsilon(2)(1245 cm(-1))/A upsilon(1)(1265-1275 cm(-1)) ratio. Ellipsometric spectroscopy has pro vided information on the global conformation of adsorbed chains by pre cise thickness measurements. Thus, thickness (h) increases for diluted solutions linearly from 10 to 70 Angstrom as the average syndiotactic sequence length (n(s)) increases. However, IR and ellipsometric resul ts tend to prove that isotactic PMMA may adsorb on the aluminum surfac e in a more flattened way with longer chains than atactic and syndiota ctic chains. This can be explained by the ability of chain segmental r earrangements (meso tg state) that may allow isotactic PMMA to favor t he establishment of interfacial acid-base bonds while the syndiotactic PMMA local stiffness of the tt state does not. The influence of the c hain entropy in solution is also discussed as an important parameter. (C) 1997 Elsevier Science Ltd. All rights reserved.