EFFECTS OF PHYSICAL AGING ON POLARIZATION-INDUCED PHOTODIMERIZATION IN THE GLASSY REGION OF POLY(METHYL METHACRYLATE)

Intramolecular photodimerization of the photochromic molecule 9-hydrox ymethyl-10-[(1-naphthyl methoxy)-methyl]anthracene (HNMA) was investig ated by ultraviolet-visible (UV-VIS) spectroscopy in the glassy region of poly(methyl methacrylate) (PMMA) under different aging conditions. The PMMA matrix was annealed for different time intervals in the glas sy region after quenching from 140 degrees C (37.8 degrees C above the glass transition temperature (T-g)). By following the photodimerizati on upon irradiation with linearly polarized light, it was found that t he reaction kinetics of HNMA reflects the rearrangements of local free -volumes of the matrix. The maximum efficiency (eta(max)) of the react ion induced by linearly polarized light decreases with increasing agin g time (t(a)). Under aging above 60 degrees C, eta(max) decreased mono tonously with aging time, whereas it did not change systematically for a PMMA aged at 40 degrees C. The secondary relaxation of the PMMA mat rix may thus play a key role in the aging process at low temperature. On the other hand, the reaction kinetics of HNMA can be well expressed by the sum of two exponential functions of irradiation rime and a bas eline. The two fractions of these decay processes are significantly af fected by aging time whereas the influence on the two reaction rates i s less remarkable. The baseline increases with increasing aging time, reflecting shrinkage of samples under aging. These experimental result s are discussed in relation to the aging data obtained by other techni ques such as dielectric relaxation and dynamic mechanical measurements .