PHOTOCHROMIC AND FLUORESCENT-PROBE STUDIES IN GLASSY POLYMER MATRICES .4. EFFECTS OF PHYSICAL AGING ON POLY(METHYL METHACRYLATE) AS SENSED BY A SIZE DISTRIBUTION OF PHOTOCHROMIC PROBES

The effects of physical aging on poly(methyl methacrylate) (PMMA) have been investigated by observing the isomerization behavior of photochromic probes requiring various volumes to isomerize. Physical aging in PMMA decreases the isomerization ability of probes requiring larger volumes to isomerize more than the probes that require smaller volumes, similar to results found previously in polystyrene (PS). These results indicate that, upon physical aging, larger pockets of local free volume decrease in number more than the smaller pockets, causing the size distribution of local free volume to narrow upon aging. The degree of the decrease in a local free volume fraction was found to depend on the aging time and aging temperature. From the isomerization behavior of the larger probes, unannealed PMMA was also found to have a broader size distribution than unannealed PS. The fraction of local free volume large enough to allow diphenylstilbene to isomerize in PMMA is almost completely lost upon physical aging at 60-degrees-C for 100 h, suggesting that through physical aging it may be possible to remove almost completely regions of local free volume exceeding certain sizes.