Photocatalyzed degradation of polymers in aqueous semiconductor suspensions V. Photomineralization of lactam ring-pendant polyvinylpyrrolidone at titania/water interfaces

The mechanism of photo-oxidation of the water-soluble polyvinylpyrrolidone (PVP) having a pendant five-membered lactam ring is complicated by the manner by which the macromolecule adsorbs on the TiO2 particle surface in a heterogeneous dispersion. Experimental results and computer simulation of the initial process(es) infer three major steps for the photodegradation of the PVP structure. The first step is adsorption (or coagulation) of PVP on TiO2 particles as evidenced by the size distribution of TiO2 particles by dynamic light-scattering and by the electric charge on the TiO2 particle surface assayed by zeta -potential measurements. Molecular orbital simulations of initial processes were calculated at the AMI level using the MOPAC system available in the CAChe package. The second step, namely attack of PVP by (OH)-O-. and/or (OOH)-O-. radicals, involves cleavage of the PVP main chain and opening of the lactam ring in the PVP structure probed by temporal UV spectral changes and by a decrease of the molecular weight using gel permeation chromatographic methods. Additional details of the photo-oxidation mode of the lactam ring was examined by a detailed examination of the photo-oxidation of the model compound 2-pyrrolidone to ascertain formation of (OH)-O-. radical adducts, opening of the lactam ring, and identification of intermediates by HPLC, and C-13- and H-1-NMR methods. The fnal major step in the mechanism involves generation and subsequent conversion of the primary amine (methylamine from opening of the lactam ring) to yield ultimately NH4+ and NO3- ions, and conversion of the propanoic acid to acetic and formic acids and then to CO2. The effects of the extent of polymerization and variation in light intensity were examined using PVP samples having different hardness factors (hf) of 15 and 30, and different light intensities (namely, 1, 2, 3 and 4 mW cm(-2)), respectively. (C) 2001 Elsevier Science B.V. All rights reserved.