Synthesis and swelling behavior of environmentally friendly starch-based superabsorbent hydrogels reinforced with natural char nano/micro particles

Abstract Herein, the proficiency of natural char (NC) as cost-effective and biocompatible filler for the synthesis of high-performance starch-based superabsorbent composites was evaluated. To this goal, NC powder, ball-milled natural chars (BMNCs) and chemically modified NC nanoparticles (NCNPs) were synthesized and then utilized as fillers for preparation of starch-g-poly(acrylic acid-co-acrylamide) superabsorbent composites. The morphology and surface functionalities of the as-synthesized fillers and their composites were scrutinized using FTIR, FESEM, elemental mapping, AFM, Boehm titration and TGA analyses. The results proved that the composite performance was controlled by the filler’s properties (i.e., morphology, size, surface chemistry and functionalities) and NCNPs with the smallest particle sizes (14.8 ± 3.0 nm) and the highest oxygen functionalities was selected as the best filler candidates. Coexistence of the physical and covalent cross-linkage in the NCNPs/Hydrogel nanocomposite could be proposed as a reason for its two-fold water absorbency (389.9 g/g) in comparison with the neat hydrogel (202.1 g/g). However, the addition of NC and BMNC in the polymer matrix could not significantly improve the water absorption performance of the composite. Absorbency under load (AUL) results showed that the gel strength of samples displayed the following descending order of NCNPs/Hydrogel > BMNC/Hydrogel > NC/Hydrogel > neat hydrogel > commercial hydrogel. All synthesized starch-based superabsorbent (nano)composites showed a Fickian water diffusion mechanism and pseudo-second-order swelling kinetic model. Moreover, after 14 days, NCNPs/Hydrogel nanocomposite showed three-fold water-retention (%) capability compared with the neat hydrogel (23.1% vs. 7.1%, respectively).