ON THE INTERMOLECULAR CROSS-LINKING OF PVA CHAINS IN AN AQUEOUS-SOLUTION BY GAMMA-RAY IRRADIATION

We studied how the polymer chains of poly(vinyl alcohol) (PVA) are cro sslinked in its O-2-free aqueous solution by gamma-rays. The static-dy namic laser light scattering method was employed for determining the m olecular weight (M-w), hydrodynamic radius (R-h) and radius of gyratio n (R-g) for the original PVA (M-w = 1.0 x 10(5)) and its radiation pro ducts. The gamma-ray irradiation from Co-60 to the PVA solutions with different concentrations was performed at different dose levels. It wa s found that both molar mass and size (i.e., R-g and R-h) of the radia tion products increased with a dose increase. The magnitude of these i ncreases was enhanced by decreasing the polymer concentration (C-p). I n particular, at C-p < the critical polymer concentration leading to m icrogel formation, very rapid increases in mass and size were observed even at a low dose level. In order to obtain more detailed informatio n about the irradiation-induced crosslinking of PVA, we studied the do se dependence of the molar concentration of intermolecular crosslinks in the system as a function of C-p using the dose curves of M-w. It be came apparent that, regardless of C-p, the intermolecular crosslinking reactions were dominant in the initial stage of irradiation. The radi ation yield of intermolecular crosslinking was found to increase linea rly when increasing C-p, and at C-p > the overlap concentration it asy mptotically approached a limiting value estimated from the dimerizatio n of pentane-2,4-diol as a model compound of PVA. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.