Nanovoid relaxation in a series of copolyester glasses under cyclic loading using synchronous PALS

This study examines if correlation between a specific molecular motion and the growth and relaxation of nanovoids, which are precursors to crazes, can be estab- lished. A novel technique, positronium annihilation lifetime spectroscopy (PALS) synchronized with cyclic stress, is reported. In this technique the positronium annihilation signal was accumulated in sixteen channels corresponding to sixteen phases of the sinu- soidal load, which allowed statistically significant data for nanovoid generation and relax- ation due to the fluctuating stress to be accumulated. This technique was applied to a series of copolymers of poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedi- methylene terephthalate) (PCT). Previous studies have shown that the cyclohexylene rings in the main chain of PCT actively undergo chair-boat-chair conformational transi- tions in the glassy PCT at around room temperature. The PET-co-PCTseries was also cho- sen for this study because the crazing stress increases systematically with the cyclohexy- lene content. The synchronized PALS data are consistent with the hypothesis that tran- sient nanovoids generated by the cyclic stress in polymers containing more cyclohexylene rings relax more readily than those with fewer rings. The results also correlate well with the crazing stress in this series of copolymers. V C 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1410-1417, 2007