Photo and Thermal Degradation of a Cationic Superabsorbent Polymer

Abstract

A cationic superabsorbent polymer (SAP) was synthesized by carrying out the polymerization of [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride) with N,N 0 -methylenebisacrylamide as the cross-linking agent. The SAP was subjected to degradation in dry and the equilibrium swollen state by thermo gravimetric analysis and exposure to ultraviolet radiation, respectively. The photodegradation was monitored by measuring changes in the swelling capacity and the dry weight of the SAP. The thermal degradation of the SAP occurred in three stages after the initial removal of moisture and the activation energies of the decomposition were determined. INTRODUCTIONThree-dimensional cross-linked networks of hydrophilic polymers that are able to absorb and retain liquids more than 100 times of their own weight are known as superabsorbent polymers (SAPs) [1] . The SAPs contain ionic repeat units, and could classified be as anionic, cationic, amphoteric, or zwitterionic [2] . They find applications in personal hygiene products [2] , food packaging [2] , wound healing devices [3] and water treatment [4] . Although, most of the SAPs commercially produced are anionic (based on acrylic acid and its derivatives) [2] , a wide variety of other superabsorbent polymers have been synthesized [5][6][7][8][9] . The cationic monomers have also been polymerized, either independently [10] or with anionic SAPs to obtain amphoteric SAPs [11][12][13][14][15] .The synthesis routes of the monomer-based SAPs are well established, but there are only a few studies reporting their degradation [16][17] .The SAPs constitute a significant part of domestic waste, making the degradation studies of the synthetic SAPs important. Li and Cui [16] subjected acrylic acid and acrylamide-based SAP to ultravioletinduced decomposition and quantified the degradation by measuring the weight of the SAP at the end of the degradation. Their study, although the first report of its kind, did not provide information about the changes in the swelling capacity of the SAPs during the degradation. In our previous study, we had adopted an improved technique for the photodegradation of poly (acrylic acid-co-sodium acrylate-co-acrylamide) SAPs [17] .The changes in the swelling capacity as well as the residual weights of the SAPs were measured during the degradation and a possible mechanism of the degradation of SAPs was proposed. The SAPs were found to undergo degradation in two stages. In the first stage, the network became sparse and the swelling capacity increased but the residual weight fraction was constant. The rupture of the three-dimensional network took place in the second stage, and thus both the swelling capacity and the residual weights reduced drastically.In the present study, we report the photodegradation of a cationic SAP by the earlier developed method [18] . Here we extend the study to investigate the effect of cross-link density on the photodegradation of the SAP. The cationic SAP poly([2-(methacryloyloxy)ethyl]trimethyl ammonium chloride) (PMALETMAC) was synth...