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Electronic excitation transfer as a probe of phase behavior in polymer composites

We use time-resolved fluorescence anisotropy decay measurements to watch the complete process of phase- separation of molecularly mixed polymer blends. Fluorescent chromophores are covalently incorporated into one of the polymers in a blend, and electronic excitation transfer (EET) between these chromophores contributes to the rate of fluorescence anisotropy decay, which is measured using time- correlated single photon counting. Since the EET is highly sensitive to the chromophore distribution, this technique can reveal the single-chain structure and spatial distribution of polymer chains in the blend on the angstroms distance scale. Blend samples are studied both by rapidly quenching the phase-separating material below its glass- transition temperature and analyzing nanoscopic aggregates trapped in the sample, as well as by watching the process evolve in situ as the sample is heated above the glass transition and through the cloud-point. We find that not only the annealing temperature, but also the variable rate of heating can have a dramatic effect on the aggregation state of the blend at the nanoscopic level. Therefore, we may be able to influence those physical properties of the blend which depend on the aggregation state in specific ways. We examine the onset of phase separation while varying the molecular weight and chromophore content of one of the blend components and the blend composition.