Correlations in Polymer Melts and Solutions As Investigated by Fluorescence Nonradiative Energy Transfer: A Novel Comparison of Theory to Experiment by Fluorescence Intensity Decay Measurements

A theoretical analysis of donor fluorescence intensity decay for a system of donor and trap (acceptor) end-labeled polymers has been developed to account for the effects of Förster nonradiative energy transfer (NRET) as a function of experimentally important parameters in a monodisperse polymer melt: polymer molecular weight, ratio of trap to donor concentration, and fraction of unlabeled polymer in the mix. The model distinguishes between donor- and trap-labeled material and employs intermolecular correlation functions which have been calculated to describe the chain statistics of a given polymer system. The model has been extended to describe the effects of NRET in semidilute solutions of end-labeled polymer by accounting for the swelling of the polymer by solvent and incorporating different chain statistics in the analysis. The donor fluorescence decay profiles calculated for perfectly labeled polymers employing phenanthrene as donor and anthracene as trap exhibit dramatic changes as a function of polymer concentration in solution or polymer molecular weight in bulk polymer. Comparisons of experimental decay profiles for solutions of terminally phenanthrene- and anthracene-labeled polyisoprene in toluene (with imperfect labeling) were made to theoretical predictions for this specific system. The good quantitative agreement between theory and experiment even in this system of low chromophore content suggests that fluorescence techniques may prove important in the study of correlations and chain statistics in more complicated systems, including blends of bimodally distributed homopolymers as well as blends of unlike polymers.