Delayed Fertilizer Release via Shell Bursting Under Thermal Stimuli: Achieved by Polyamide Coated Cellulose Hydrogel Composites With UCST Behavior

ABSTRACT To address the premature release issue of traditional slow‐release fertilizers triggered by transient temperature fluctuations, a UCST cellulose‐based hydrogel coated with polyamide (PA) was developed for fertilizer controlled release via a delayed thermoresponsive mechanism that exists a period between stimulus and response. By regulating the acrylic acid (AAc)‐acrylamide (AM) monomer ratio during the synthesis process, the structure, swelling behavior, and thermoresponsive performance of the hydrogel were precisely tailored. The dense PA coating provided excellent sustained release capacity and limited the swelling ratio of the hydrogel under transient temperature fluctuations, thereby preventing premature release of fertilizer. Only under sustained high temperatures, hydrogel swelling with sufficient time induces rupture of the outer PA coating, shifting the release control from the rate‐limiting PA shell to the rapidly releasing hydrogel core. This composite material enables precise, staged nutrient release, effectively mitigating premature fertilizer release under abrupt temperature changes.