Functionalized Cellulose-Reinforced CMC-Based Hydrogels: NaOH-Urea Catalyzed Oxidation and Mechanical Pretreatments, Crosslinking with Citric Acid and Epichlorohydrin, and Characterization

To improve the performance of carboxymethylcellulose (CMC)-based copolymer hydrogels with low gel strength, additives like natural cellulose are extensively utilized but its structural limitations restrict successful integration into CMC-based hydrogels without modification. In this study, it was examined the effects of wood pulp with and without treatments as additives on the performance of hydrogels. Pretreatment of cellulose was carried out using hydrogen peroxide oxidation catalyzed by NaOH-urea, as well as mechanical pretreatment using PFI or colloid mills. In the cross-linking of these semi-synthetic polymers, either epichlorohydrin (ECH) or citric acid (CIT) were used. FTIR spectroscopy, DSC, and electron microscopy techniques, as well as swelling and water absorption tests, were used to analyze the chemical, physical, and morphological properties of hydrogels. The successful cross-linking by both agents was verified by FTIR spectra. For all hydrogel groups, regardless of the cross-linking agent used, the swelling ratio in alkaline solutions was higher than that in neutral and acidic environments. CIT-crosslinked hydrogels displayed a swelling rate 16 times greater at pH 7 than the control group. Enhanced swelling performance was observed in solutions with Na⁺ and K⁺, but hydrogels demonstrated a 50–70% reduction in performance in the presence of Mg²⁺ and NH₄⁺. DSC curves demonstrate that crystallinity diminished due to alkaline oxidation pretreatments, and the thermal stability of ECH-crosslinked hydrogels exceeds that of CIT by 40%. SEM images show different patterns and fractures in ECH samples, while CIT samples reveal smoother hydrogel surfaces and the presence of fibers.