Effect of Structure and Modification on the Properties of Hydrogels from N, N'-Dimethylacrylamide, Maleic Acid, and Its Application as a Urea Carrier

This study focused on the synthesis and characterization of Conventional Hydrogels (CH) and semi-interpenetrating network (Semi-IPN) Hydrogels (SH) derived from N, N'-Dimethylacrylamide (DMA) and Maleic Acid (MA), with an emphasis on their physicochemical properties and potential as urea carriers. The hydrogels were synthesized via free-radical polymerization of DMA and MA under varying monomer ratios, followed by post-modification of SH in Strong Acid (SA) and Strong Base (SB). The effects of these modifications were systematically investigated, revealing significant changes in pore structure, thermal stability, and swelling behavior. Scanning electron microscope (SEM) analysis confirmed a homogeneous porous structure, with pore sizes varying based on the hydrogel structure and modification environments. While the linear polymer only slightly decreased the glass transition temperature (Tg), the base environment decreased, and the acid environment significantly increased the Tg of the semi-IPN hydrogel. Modification in SB also enhanced the thermal stability of the hydrogel. The semi-IPN structure and SB modification significantly improved the swelling properties, with Swelling Ratios (SR) of 2974.6, 2508.9, 2152.4, and 5439.0% for SH, CH, SA, and SB, respectively. The application of SB as a urea carrier demonstrated a maximum urea absorption capacity of 152.9 mg/g, with adsorption kinetics following the Langmuir isotherm model. Urea release adhered to the Korsmeyer–Peppas model and exhibited pH-dependent release rates. Finally, the Urea/SB hydrogel positively supported the growth of mustard greens, highlighting its potential in agriculture. This study provided valuable insights into the design of hydrogels for nutrient carrier applications, offering a sustainable solution aligned with the requirements of modern farming techniques.