Molecular Dynamics of Physical Aging Processes in Polymer Glasses (Experiment andTheory)

摘要

We have studied free volume and aging in PVAc (Polyvinyl Acetate), PMMA(Polymethyl Methacrylate), PEMA (Polyethylene Methacrylate), and polycarbonates by fluorescence spectroscopy, Electron Spin Resonance (ESR), and Positron Annihilation Spectroscopy (PAS). The product of the ortho-positronium (o-Ps) lifetime Tau 3 and its formation probability I3 has a temperature dependence above T(g) that follows that of the total free volume. The product I3 Tau 3 shows a significant decrease as a function of time at temperatures below T(g). Some specimens with the same composition and the same free volume yielded different values for the individual factors I3 and Tau 3. This indicates that PAS can distinguish between samples that are indistinguishable by conventional methods. We also obtained preliminary results on simultaneous measurement of the positron lifetime and the momentum of the annihilated positron-electron pair. There is a clear difference between high-momentum and low-momentum lifetime distributions, with a much larger value of I3 for the high momentum curve. This is what one expects from the higher momentum involved in o-Ps annihilation. Keywords: Polymer glasses, Physical aging, Free volume, ESR, Electron spin resonance, Fluorescence spectroscopy, Positronium, PAS, Position lifetime, Annihilation, Molecular dynamics. (jg)