POSITRON-ANNIHILATION LIFETIME STUDY OF VAPOR SORPTION IN POLYPROPYLENE AND POLYTETRAFLUOROETHYLENE

Positron annihilation lifetime (PAL) spectra were measured for two dif ferent kinds of polymers: polypropylene (PP) and polytetrafluoroethyle ne (PTFE), during sorption of vapors of the following molecules: n-hex ane, cyclohexane, benzene, methyl methacrylate, acrylic acid, and wate r. The behavior of ortho-positronium (o-Ps) annihilation parameters: l ifetime, tau(3), and intensity, I-3, can be explained by considering d ifferent sorption mechanisms in rubbery and glassy polymers. The mean size of the intermolecular-space holes and the relative free-volume fr action of PP and PTFE were estimated before and during the sorption pr ocess from the o-Ps parameters obtained. Sorption of n-hexane, cyclohe xane, and benzene in both polymers affect the o-Ps component in a syst ematic way, for PP tau(3) and I-3 tend to decrease, while in PTFE, tau (3), stay almost constant and I-3 diminishes. These effects are interp reted in relation to the different states of the polymers studied: rub bery for PP and glassy for PTFE. In case of methyl methacrylate molecu les, the behavior of o-Ps parameters was more complicated. For acrylic acid and water, their associated polarity provokes a diminution in I- 3, which is explained from the viewpoint of the spur model of positron ium formation. Large changes in the mean size and the fraction of free volume, as well as their constant behavior in some cases, were interp reted to be due to positive or negative interactions, respectively, be tween vapor molecules and PP and PTFE, which could be correlated with the solvent-interaction parameters calculated. (C) 1995 John Wiley and Sons, Inc.