Transformations of radiation-generated macroradicals of poly (acrylic acid) in deoxygenated aqueous solutions.
Part I. Reaction pathways and the kinetics of macroradical decay
Polimery 1997, No 3, 168
The mechanism of transformations of a most simple polyelectrolyte, poly(acrylic acid)(PAA), was studied in dilute, deoxygenated aqueous PAA solutions submitted toionizing radiation. Of the intermediate water radiolyzates, hydroxyl radicals were found to be most reactive towards PAA (к = 2.2 •108 dm3 • mol-1 • s1 at pH = 9.6 and к = 9.0•107 dm3 • mol-1 • s1 at pH = 3.0); hydrogen atoms were much less reactive and solvated electrons were practically non reactive towards PAA under the experimental conditions. The •OH radicals reacted with the polymer only slightly selectively: the reaction involved abstraction of a hydrogen atom and produced macroradicals. The major reaction pathways of PAA macroradicals were found to include disproportionation, recombination, transformation (byH-shift) of the radicals in position β into ones in position ato the carboxylic group, and chain-breaking degradation. The decay rate of PAA macroradicals was found to vary considerably with the pH of aqueous PAA solutions, i.e., with the degree of dissociation of PAA's carboxylic groups. In acidic solutions the rate was high, but in an aqueous solution of apH of 9—10 containing the macroradicals of fully dissociated PAA, the half-lifeperiod was ca. 80 min at room temperature and at a dose of 100 Gy (Figs.1—6). This half-life value is several orders of magnitude higher than typical half-lives of non stabilized alkyl radicals under similar conditions. The effect is explicable in terms of repulsive electrostatic forces between the negatively charged chains carrying the radicals. Under thepulse-irradiation conditions, when numerous radical sites are simultaneously generated on a single (uncharged) polymer chain, the decay rate of nonionized macroradicals failed to follow the classical homogeneous second order reaction kinetics (Fig. 7). A detailed study involving PEO as a model polymer showed theradical decay to be describable by the nonhomogeneous kinetics laws involvingthe time-dependent rate constant, k(t) = βta-1,with a-values ranging from 0.4 to 0.7 (Fig. 9). The macroradicals in position β were found to become transformed into ones in position awith respect to the carboxylic group (Figs. 12, 13) via abstraction of a hydrogen atom from another segment of the chain and, this time, too, by apH-dependent process (Fig. 10). As a result, within a few or several seconds, neutral and akaline solutions contained the a-type radicals only.