Transformations of radiation-generated macroradicals of poly (acrylic acid) in deoxygenated aqueous solutions.
Part II. Chain scission and crosslinking
Polimery 1997, No 4, 219
The PAA macroradicals generated in dilute deoxygenated solutions undergo intermolecular crosslinking and degradation (chain scission) reactions. The direction and the amplitude of changes in the average molecular weight of the irradiated polymer are the net effect of the above competitive antagonistic processes. Difference between the radiation-induced degradation yield and the intermolecular crosslinking yield was found to be strongly related to pH (Fig.2). Scission prevailed in alkaline and in neutral solutions; in acidic solutions (pH < 3), crosslinking predominated, presumably because crosslinking rates vary considerably with pH. Pulse radiolysis with conductivity detection was applied — for the first time for a synthetic polyelectrolyte — to study the kinetics of scission processes. For this reaction, the half-life value was found to be ca. 30 s (pH = 8—9; Fig. 5); a slow degradation process, promoted by secondary reactions, was found to accompany the main reaction. EPR spectra and analytical data obtained for the reaction products allowed to suggest a mechanism of transformations of terminal radicals resulting from scission. Irradiation of PAA in aqueous deoxygenated solutions was found to yield a PAA hydrogel. Directions were defined as regards optimum process conditions: an appropriate dose allows a gel product to be obtained with adesired crosslinking density that determines the equilibrium degree of gels welling (Fig. 9). The resulting PAA hydrogels were found to respond to changes in extraneous conditions (pH) by suitably read justing the property values (degree of swelling, dimensions), a feature typical of the so-called "intelligent hydrogels" (Fig. 10). The present results were used to discuss the previously reported  mechanism of PAA radiolysis.