Positronium Annihilation Lifetime and Dynamic Mechanical Studies of γ-Relaxation in BPA-PC and TMBPA-PC Plasticized by TOP

Abstract

Positronium annihilation lifetime spectroscopy (PALS) was employed to study the structure and dynamic properties of glassy BPA-PC and TMBPA-PC containing tris(2-ethylhexyl) phosphate (TOP) as diluent molecules. For BPA-PC/TOP blends, the results showed that the total hole volume fraction is insensitive to the addition of TOP up to 10 wt % but increases at higher TOP concentration. However, in the case of TMBPA-PC/TOP blends, a different trend was observed. The total hole volume fraction decreases monotonically with TOP concentration up to 15% and thereafter increases slightly from 15 to 20%. Dynamic mechanical relaxation was also used to monitor the dynamics of the blends. This part of the work was motivated by a prediction of the coupling model (made before the experimental measurement was carried out) that addition of TOP should have opposite effects on the γ-relaxation times of BPA-PC and TMBPA-PC. Indeed, it was found by dynamic mechanical measurement at constant frequency that, while the γ-relaxation peak temperature of BPA-PC was shifted to higher temperature with addition of TOP, the opposite effect was found in TMBPA-PC. The γ-relaxation peak temperature of TMBPA-PC was shifted to lower temperatures upon addition of TOP with little change in the width of the peak as predicted by the coupling model. The predictions of the coupling model and the dynamic mechanical results are supported by (1) dielectric relaxation measurements of the γ-relaxation performed on the same TMBPA-PC/TOP blends by Rizos and co-workers [accompanying paper] and also (2) the phosphorus-31 nuclear magnetic resonance data on diluent dynamics in the same BPA-PC/TOP and the TMBPA-PC/TOP blends obtained by Bergquist et al. [accompanying paper], which is shown by them to be also consistent with the lattice model for diluent dynamics.

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