Effects of the degree of hydrolysis on radiation induced reactions in the poly(vinyl alcohol)-poly(vinyl acetate) system

In the early stages of radiolysis of 100% hydrolyzed poly(vinyl alcohol), P VA, ionizing radiation produces trapped electrons whose lifetimes depend on temperature and the water content. Pulse radiolysis of films containing py rene show that the pyrene anion, Py-, in PVA is formed within the electron pulse, while in PVAC it takes 0.5 mus for the Py- to reach its maximum conc entration. Similar observations of the pyrene cations show that the positiv e holes produced in the initial act of ionization in PVA do not oxidize Py, as shown in both low temperature steady state gamma radiolysis and pulse r adiolysis experiments. This is rationalized as the effect of very fast depr otonation producing radicals with less oxidizing ability, Significant yield s of P+ are produced in PVAC, due to a more stable positive hole in this po lymer. Decreasing the degree of hydrolysis of PVA affects the extent of the cation process. The G value for the Py+ formation increases from 0 to 0.45 on decreasing the degree of hydrolysis to zero. Pulse radiolysis of PVAC/p yrene films. gives rise to pyrene fluorescence. The decay of the fluorescen ce is not a single exponential as in photo-excitation, but exhibits an init ial rapid decay. This is attributed to spur effects in the radiolysis. Foll owing gamma -irradiation at 77 K there is no thermally stimulated emission associated with the pyrene cation and anion decay process in PVAC due to th e polarity of the polymer matrix. Py- in PVA undergoes protonation forming the 1-hydro-1-pyrenyl radical as shown in both pulse radiolysis and steady state gamma radiolysis experiments. This process does not depend on the deg ree of hydrolysis of the film. Contrary to the situation in fluids, the 1-h ydro-1-pyrenyl radical is quite stable in PVA and PVAC as long as it is kep t below the glass transition temperature(s) thus enabling a study of its ph ysico-chemical properties. (C) 2001 Elsevier Science Ltd. All rights reserv ed.