Ionic arrest of segmental motion and emergence of spatio-temporal heterogeneity: a fluorescence investigation of (polyethylene glycol + electrolyte) composites.

Temperature dependent steady state and time resolved fluorescence measurements have been performed to explore the interaction and dynamics in polymer-electrolyte composite of the following general formula: [0.85 PEG + 0.15{f KNO3+ (1-f) LiNO3}], with f denoting fraction of potassium ion in the 0.15 mol electrolyte present in the medium. Poly(ethylene glycol) with number-averaged molecular weight of 300 (PEG300) has been employed as polymer and C153 as the fluorescent probe. Substantial excitation wavelength dependence of probe fluorescence emission in presence of electrolyte suggests presence of spatial heterogeneity which vanishes either upon raising temperature or removing the electrolyte. This has been interpreted as arising from the cation-induced arrest of polymer segmental motion. Temporal heterogeneity in these composites is manifested via fractional viscosity dependence of average solvation and rotation rates of the dissolved probe. Viscosity decoupling of these rates in composites is found to depend on cation identity and is also reflected via the corresponding activation energies. The degree of decoupling differs between solvation and rotation, inducing an analogy to the observations made in deeply supercooled liquids. In addition, conformity to hydrodynamic predictions is recovered by measuring f dependent solute rotation at higher temperatures. Several complimentary but different experiments are suggested to re-examine the mechanism proposed here, based on the fluorescence results, for the emergence of spatio-temporal heterogeneity in these composites and its disappearance either in the absence of any electrolyte or at higher temperatures.