This study examines if correlation between a specific molecular motion and the growth and relaxation of nanovoids, which are precursors to crazes, can be established. 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 sinusoidal load, which allowed statistically significant data for nanovoid generation and relaxation 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-cyclohexylenedimethylene terephthalate) (PCT). Previous studies have shown that the cyclohexylene rings in the main chain of PCT actively undergo chair-boat-chair conformational transitions in the glassy PCT at around room temperature. The PET-co-PCT series was also chosen for this study because the crazing stress increases systematically with the cyclohexylene content. The synchronized PALS data are consistent with the hypothesis that transient 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. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1410–1417, 2007