Positron annihilation study of density fluctuations in amorphous poly(ethylene terephthalate) films in terms of quasispinodal decomposition

Positron annihilation lifetime spectroscopy was applied to study poly(ethylene terephthalate). Changes in the positronium yield as a function of the temperature and as a function of the elapsed experiment time at predetermined constant temperatures were measured. The long-lived component was due to ortho-positronium ($\mathrm{o}\text{\ensuremath{-}}\mathrm{Ps}$) pick-off annihilation, and the temperature dependences of the $\mathrm{o}\text{\ensuremath{-}}\mathrm{Ps}$ lifetime $({\ensuremath{\tau}}_{3})$ and the corresponding intensity $({I}_{3})$ were in good agreement with the molecular mobility of the polymer chain segments. The peak positions of the $\ensuremath{\beta}$ $(74\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and $\ensuremath{\gamma}$ $(\ensuremath{-}66\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ dispersions obtained by dynamic mechanical measurements were in good correlation with the first and second transitions of ${\ensuremath{\tau}}_{3}$. The mechanical dispersion and the first transition appeared at around $70\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, while the dispersion and the second transition appeared around $\ensuremath{-}66\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The curves of ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ as a function of the elapsed experiment time can be divided into three stages: stage I, where ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ were maintained constant; stage II, where ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ tended to decrease; and stage III, where ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ tended to keep constant again. The higher was the annealing temperature, the shorter became the periods of stages I and II. This result was similar to that obtained by using time-resolved light scattering. Furthermore, time-resolved x-ray diffraction indicated no existence of crystallites in stage II. Thus, it turned out that the decreases of ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ with elapsed time in stage II corresponded to the molecular ordering arising due to the density fluctuations of the amorphous phase associated with quasispinodal decomposition.