Positron Annihilation Spectroscopy as a Probe of Microscopic Structure and Physical Aging in Polymer.
Abstract
Positron annihilation is studied as a characterization method for the properties of polymers. Previous studies indicate that the ortho-positronium lifetime tau _3 and intensity I_3 is correlated to the free volume "hole" size and number density of holes in a polymer. Positron annihilation lifetime (PAL) studies in polymers measure the change in free volume, and they are sensitive to different physical environments. PAL studies of the temperature dependence of a bisphenol-A polycarbonate shows that the free volume increases with increasing temperature, and it also obtains the transition temperatures T_{rm g} and T_beta^', from the tau_3 curve and the I_3 curve, respectively. The isothermal aging in polycarbonate shows that: I_3 decreases while tau_3 remains constant during a long-time annealing at a temperature far below T_{rm g}; and I_3 remains constant while tau_3 goes through a "over shooting" in the first few hours after quenching and annealing at a temperature just below T_{rm g}. The free volume in polycarbonate increases (as a result of an increase in tau_3 ) with applied tensile strain up to 4%, then levels off. Similarly, the free volume in polymethyl methacrylate (PMMA) decreases (as the result of tau_3 ) with applied compressional strain also up to -4% then levels off. A negative change in both tau_3 and I _3 has been observed when polycarbonate is under 3% tensile strain and after release of strain. A more advance technique of positron annihilation, PAL-momentum correlation which can give more detailed information about free volume structure in polymers, has also been studied and improved. Two 5-cm-diameter, 5-cm-long CsF scintillation detectors for lifetime measurement, and a 30-cm-diameter Anger camera whose y-analog pulse gives one-dimensional ACAR information, comprise a new experimental arrangement of PAL-momentum correlation system. Its triple -coincidence counting rate is about 2.5 per minute per microcurie of positron source and system time resolution is 260 ps and angular resolution is 3.5 mrad.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1992
- Bibcode:
- 1992PhDT........49Y
- Keywords:
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- Physics: Condensed Matter; Physics: Nuclear