Conformation of stereoregular PMMA in solutions during the temperature induced conformational transition

The temperature induced conformational transition of stereoregular PMMA in diluted or semi-diluted solution was studied by viscometry, NMR and FT-IR, The conformational energies of polymer chains were calculated from infrared spectral subtraction by following the method described by O'Reilly and Mos her [O'Reilly JM, Mosher RA, Macromolecules, 14 (1981) 602; O'Reilly JM, Te egarden DM, Mosher RA. Macromolecules, 14 (1981) 1693]. The fraction of seq uences containing gauche conformation that can be deduced shows a large inc rease during the conformational transition. This increase in the probabilit y of gauche conformations accounts for an increase in the chain flexibility deduced from the decrease of the calculated characteristic ratio. The incr ement in the hydrodynamic volume of the chain in chloroform that occurs at the transition temperature can be correlated with an increase in the densit y of the polymer-solvent specific interactions. The range of temperatures o f the conformational transition, namely, 20-40 degrees C, was found to be i ndependent of the quality of the solvent (theta or athermic solvent). Howev er, the cooperativity of the conformational changes sigma is shown to depen d on the solvent for the i-PMMA, Further, the aggregating character of the solvent (o-dichlorobenzene) shifts the observed transition for the i-PMMA s olution towards higher temperatures which is ascribed to the disassociation of the aggregated structures. (C) 1999 Elsevier Science Ltd. All rights re served.