Photochromic and Fluorescent Probe Studies in Glassy Polymer Matrices. 5. Effects of Physical Aging on Bisphenol-A Polycarbonate and Poly(vinyl Acetate) As Sensed by a Size Distribution of Photochromic Probes

Multiple photochromic probes requiring different volumes to isomerize were used to monitor the effects of physical aging on Bisphenol-A polycarbonate (PC) glass. PC was found to possess a local free volume distribution that is more broad than those of polystyrene (PS) and poly(methyl methacrylate) (PMMA). Contrary to aging results in PS and PMMA, physical aging of PC affected the isomerization abilities of the smaller probes as well as the larger probes, and all probes are affected nearly equally. Physical aging in dry and wet poly (vinyl acetate) (PVAc) showed that absorbed water increases the fraction of local free volume sampled by azobenzene as would be expected in a plasticized system. Additionally, it was found that a local free volume distribution is difficult to determine with the photochromic technique in polymers such as PVAc that have glass transition temperatures near the experimental temperature. At temperatures near T_g, the polymer has enough mobility to allow structural reorganization on the time scales required to reach the photostationary state, generally, 10-30 min. Therefore, the environments surrounding the probes change before the photostationary state is established, thereby making interpretation of the data difficult.