Rapid and solvent-free synthesis of pH-responsive graft-copolymers based on wheat starch and their properties as potential ammonium sorbents

Abstract

A rapid and cost-effective reactive extrusion (REx) method was employed in this study as an alternative technique for the graft-copolymerization of non-food grade native wheat starch with acrylamide, and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) monomers, with a total starch/monomer ratio of 3:1, using twin-screw extrusion technology. The influence of AMPS content ratio on the REx process was monitored using specific mechanical energy, die pressure and torque values recorded during the extrusion. The as-prepared starch-copolymers were characterized using ATR-FTIR, NMR, TG-DSC, and elemental analysis. An average grafting efficiency and monomer conversion of ~61 and ~86%, respectively, was achieved within 5 min of extrusion at a high starch concentration (0.75:1 w/w starch-water). The copolymer with starch/acrylamide/AMPS weight ratio of 75:20:4 showed the highest swelling capacity in water, while behaving similarly to polyelectrolyte networks in the presence of free ions in both NaCl and NH4Cl solutions. A steady NH4+ adsorption capacity was also recorded for these starch-copolymers within the pH range of ~5.5–8.5, which exceeded those of the natural sorbents. These findings indicate the suitability of the starch-copolymers as potential precursors of effective sorbents, thus confirming the feasibility of using REx to produce pH-responsive hybrid copolymers based on wheat starch at low-cost.