Variation of free volumes of poly(alkyl methacrylate) derivatives probed by positron annihilation lifetime technique

Positron annihilation lifetimes of PMMA, PEMA, PiPMA, PnPMA, and PnBMA were measured in the temperature range between 15 and 300 K, where delta, gamma, and beta relaxational transitions occur for these poly(alkyl methacrylate)s. The free volume is bigger for the longer side ester chain and the temperature dependence of free volume size is weaker for longer ester chains up to the beta transition. The variations of free volume size, content, apparent free volume fraction, and size distribution of the poly(alkyl methacrylate)s with temperature, calculated from the longest lifetime component due to the pick-off reaction of o-positronium, are discussed in terms of the rotational transitions of side chains obtained by the dielectric, viscoelastic and dynamic mechanical methods and good correlations are found. The free volume, in which positronium annihilates, is estimated to exist in the vicinity of alkyl groups bound to oxygen atom of side ester chains. Examination of the free volume size distribution reveals that FWHM increases just below the beta transition and that the minimum free volume size increases with increasing of chain length of n-alkyl group conjugated to esters.