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

Abstract

In the early stages of radiolysis of 100% hydrolyzed poly(vinyl alcohol), PVA, ionizing radiation produces trapped electrons whose lifetimes depend on temperature and the water content. Pulse radiolysis of films containing pyrene show that the pyrene anion, Py −, in PVA is formed within the electron pulse, while in PVAC it takes 0.5 μs for the Py − to reach its maximum concentration. Similar observations of the pyrene cations show that the positive 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 radiolysis experiments. This is rationalized as the effect of very fast deprotonation producing radicals with less oxidizing ability. Significant yields of P + are produced in PVAC, due to a more stable positive hole in this polymer. 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/pyrene films gives rise to pyrene fluorescence. The decay of the fluorescence is not a single exponential as in photo-excitation, but exhibits an initial rapid decay. This is attributed to spur effects in the radiolysis. Following γ-irradiation at 77 K there is no thermally stimulated emission associated with the pyrene cation and anion decay process in PVAC due to the 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 degree of hydrolysis of the film. Contrary to the situation in fluids, the 1-hydro-1-pyrenyl radical is quite stable in PVA and PVAC as long as it is kept below the glass transition temperature(s) thus enabling a study of its physico-chemical properties.