Abstract
Fluorescence of atactic monodisperse and isotactic polystyrenes has been measured in liquid mixtures showing cosolvent behaviour, as a function of the solvent composition. Polymer intrinsic viscosities and solvent liquid viscosities have also been measured as a function of the solvent mixture composition. The ratio of excimer to monomer intensities changes slowly with solvent viscosity and is almost independent of the hydrodynamic volume of the coil. Three alternative mechanisms for intramolecular excimer formation were considered to explain the observed behaviour: (i) energy trapping by preformed dimers, the concentration of which is configuration-dependent; (ii) long-range excimer formation by chain recoiling, whose contribution depends on segment density; and (iii) segmental rotation to the sandwich conformation during the monomer excited-state lifetime, whose probability depends on solvent viscosity. It is concluded that, for atactic polystyrene, the largest contribution comes from the first mechanism, whilst for isotactic polystyrene, segmental rotation appears to be the predominant mechanism. Long-range excimers play a negligible role in the excimer formation process for either tactic form.
Published Version
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