Preparation and characterization of water-absorbent poly(acrylamide-co-acrylic acid salt)'s

Poly(acrylamide-co-acrylic acid salt) gel beads prepared by inverse suspension polymerization were used as model polymers for studying the swelling mechanism of water-absorbent polymers and the effects of polymerization parameters on swelling properties. It was found that the free swelling of single beads in water could be described as anomalous diffusion. However, the transport of fluid through a polymer bead bed (defined as penetration absorption) is a combination of diffusion absorption and capillary transport. Capillary transport was found to be the rate-controlling step and was defined as the controlling mechanism for the penetration absorption. The particle size of the beads and the nature of the liquid and the polymer matrix, are the three major parameters determining the swelling properties.; The kinetics of the penetration absorption, i.e., the water uptake (Q{dollar}sb{lcub}rm t{rcub}{dollar}) at time t, can be described by two parameters: the equilibrium water uptake (Q{dollar}sb{lcub}infty{rcub}{dollar}) and a characteristic time constant ({dollar}tausb{lcub}rm d{rcub}{dollar}). High values of Q{dollar}sb{lcub}infty{rcub}{dollar} and low values of {dollar}tausb{lcub}rm d{rcub}{dollar} are required to achieve a high rate of water absorption. Both values are primarily dependent on the chemical structure of the polymer matrix. In addition, {dollar}tausb{lcub}rm d{rcub}{dollar} is also affected by the capillary transport.; The effects of the polymerization parameters on the water-absorbing properties (i.e., Q{dollar}sb{lcub}infty{rcub}{dollar} and Q{dollar}sb{lcub}rm t{rcub}{dollar}) were examined by varying the polymerization stabilizer, crosslinking monomer and initiator concentrations, the feed monomer ratio and the type of base used to neutralize the acrylic acid. The values of Q{dollar}sb{lcub}rm t{rcub}{dollar} decreased with increasing stabilizer concentration owing to the decrease in particle size. Other polymerization parameters led to changes in chemical structure. The values of Q{dollar}sb{lcub}infty{rcub}{dollar} increased with decreasing crosslink density of the gel beads. While the crosslinking monomer formed crosslinks between the polymer chains through their multifunctional groups, the initiator and the feed monomer ratio affected the degree of crosslinking caused by chain transfer to polymer (defined as the self-crosslinking). The degree of crosslinking was enhanced by increasing the crosslinking monomer and initiator concentration. The highest degree of self-crosslinking was observed at the 50:50 molar ratio of acrylamide to acrylic acid salt.; The value of {dollar}tausb{lcub}rm d{rcub}{dollar} can be reduced by optimizing the crosslinking monomer concentration, increasing the initiator concentration and decreasing the interactions between polyions and counterions. To improve the kinetics of water absorption, the following are suggested: (1) prepare polymer beads with a narrow size distribution in the size range of 200 {dollar}mu{dollar}m; (2) prevent the formation of water-soluble polymers; (3) decrease the crosslinking monomer concentration as much as possible; and (4) increase the initiator concentration as much as possible.