Novel superabsorbent copolymers of partially neutralized methacrylic acid and acrylonitrile: synthesis, characterization and swelling characteristics

Abstract

Novel superabsorbent copolymers (SAP) based on random copolymers of sodium methacrylate, methacrylic acid, and acrylonitrile (ACN) of varying compositions and cross-link densities were synthesized by free radical polymerization and were characterized. ACN content in the copolymers varied from 0 to 15 mol%. The SAPs were capable of absorbing 350–990 times their weight of water and 60–130 times their weight of saline (0.3 wt. % solution of sodium chloride in deionized water) depending on the composition and cross-link density of the copolymer. Swelling of the copolymers followed first-order kinetics with non-Fickian swelling behavior in deionized water. Water absorbency (Sw) was found to decrease with pressure on the gel side and a pressure coefficient (dSw/dp) of −0.6%/Pa was estimated for a given composition of the copolymer. Strength of the hydrogels increased with increasing cross-link density. Shear modulus of the hydrogels was found to vary from 1000 to 6000 Pa depending on the composition and cross-link density. Higher cross-link density of the SAP and higher salt concentration of the swelling medium reduced the swelling ratio, while higher extent of neutralization and lower salt concentration of the medium enhanced it. Different copolymer hydrogels were able to retain 50–78% of absorbed water at 37 °C after a period of 20 h, with the systems having lower ACN content retaining higher amount of water. The copolymers exhibited absorbency in the range of 1500–2000% by weight under a pressure of 6200 Pa when swollen in 0.9% aqueous solution of sodium chloride. Increase in ionic strength of the medium, pressure on the hydrogel, and temperature (of the medium) were proven to decrease the swelling commensurate with the thermodynamics of the system.