Hydroxyl-radical-induced reactions of poly(acrylic acid); a pulse radiolysis, EPR and product study. Part I. Deoxygenated aqueous solutions

Abstract

Hydroxyl radicals were generated radiolytically in N₂O-saturated dilute aqueous solutions of poly(acrylic acid), PAA. They abstract H-atoms from PAA both at the α- and β-position to the carboxy group. Using the pulse radiolysis technique the UV spectra of the PAA radicals were recorded. While the main absorption (λ_max < 250 nm) is due to both α- and β-type radicals, the α-radicals show long-wavelength shoulders at 290 and 330 nm at pH 3.5 and 10, respectively. At pH 10, where the lifetime of the radicals is very long (e.g. 20 min at pH 10 and a dose of 300 Gy), a 3.5-fold increase in the 330 nm absorbance was observed (k= 0.7 s^–1). This is attributed to a conversion of β- into α-radicals, and from this result it is concluded that the primary yield of α-radicals is ca. 30%. The lifetime of the radicals strongly depends on pH. At low pH, where the polymer has a random-coil conformation, the radical decay is fast, whereas in neutral and basic solutions the repulsive electrostatic forces of the negative charges cause the polymer to assume a rod-like structure and prevent the radicals from ready approach. Their decay then becomes slow. This allows the radicals to fragment (k= 0.025 s^–1 as determined by pulse conductometry at pH 8). As a consequence, scission dominates at high pH (determined as a decrease in the molecular weight using low-angle laser light-scattering). In acid solution crosslinking is the dominating process. However, the intermolecular crosslinking yield is low (G≈ 0.1 × 10^–7 mol J^–1), disproportionation of radicals, scission and intramolecular crosslinking (loop formation) being effective competing reactions. Cysteamine (10^–6 mol dm^–3) prevents chain scission by H-donation and equally low concentrations of Fe^II ions exert a similar effect by an as yet unknown mechanism. EPR data support the long lifetime of the PAA radicals and the β- to α-radical conversion.