Fluorescence sensor techniques to examine heterogeneity in polymer systems from the macroscale to the nanoscale.

摘要

Heterogeneity in polymers is studied using novel fluorescence techniques. Wide-field optical microscopy (WFOM) was used track the two-dimensional diffusion of rhodamine 6G dye molecules through a poly(n-butyl methacrylate) (PBMA) film at 8°C above its glass transition temperature, Tg. Mean square displacement data yielded a bulk translational diffusion coefficient (D) of 1 x 10-11 cm2/s. The distribution of diffusive displacements over a time interval of 0.2 s was non-Gaussian, direct evidence of spatially heterogeneous dynamics. This behavior was dictated by approximately 1% of the displacements with a value greater than 200 nm, showing that molecules in fast relaxing regions dominate D, supporting a proposed explanation of the translation-rotation paradox.; A model of probe diffusion through a medium with heterogeneous mobility was developed. Simulations showed that over sufficiently long time scales the displacement distribution becomes Gaussian, with the value of the root mean square displacement at the non-Gaussian/Gaussian transition providing a measure of the minimum size scale containing the bulk distribution of dynamics. The experimental distribution was extrapolated to longer times, yielding 150 nm for this size scale in PBMA at Tg + 8°C. The simulations, combined with previous experimental results, refined the picture of small molecule diffusion in polymers near Tg, showing that molecules undergo the bulk of their translation through extended regions of fast and medium relaxation dynamics, with a barrier to diffusion into slower relaxing regions.; Cure monitoring was demonstrated for an epoxy resin using intrinsic and extrinsic fluorescence. In both cases, a change in spectral shape was quantified by a ratio of fluorescence intensities, a more robust technique than typically used in the past. To translate the technique to an industrial setting a multi-sensor, laser-based fiber optic system was constructed to measure conversion as a function of position. Fluorescence probe techniques were introduced to monitor resin component mixing. By dissolving a different, directly excited fluorophore in each resin component, an intensity ratio was used to monitor overall stoichiometry within +/-1%. Using a single solvatochromatic fluorophore, nanoscale stoichiometry was monitored within +/-1%. Potential was shown to apply fluorescence nonradiative energy transfer to monitor nanoscale mixing.