关键词: PNIPAM, 微凝胶, 无皂乳液聚合, 交联剂, 温敏性

Abstract: "Poly(N-isopropylacrylamide) (PNIPAM), as a typical temperature-responsive material, demonstrates good solubility in water at temperatures below its lower critical solution temperature (LCST, typically around 32 °C). However, when the temperature exceeds the LCST, the PNIPAM molecular chain will undergo a transition from coil to compact globosity, resulting in increased hydrophobicity and rapid precipitation from the water. Microgels based on PNIPAM can quickly respond to external temperature changes, achieving reversible hydrophilic-to-hydrophobic transformations. This is accompanied by changes in volume swelling and contraction, as well as adsorption and release of substances. Therefore, they show a wide range of application potential in intelligent textile materials, printing and dyeing wastewater treatment, cell scaffolds and drug controlled release. Among the preparation methods of PNIPAM microgels, emulsion polymerization plays an important role in large-scale production because of its advantages such as mild conditions, easy control and simple operation. Traditional surfactant-based emulsion polymerization usually relies on emulsifiers such as surfactants to ensure the stable dispersion of microgels in the emulsion. However, it is often challenging to fully remove emulsifier residues from the microgels synthesized via this method, which not only alters the material properties but also poses potential risks of biological toxicity and environmental contamination, limiting its wide application in food, medicine and bioengineering. The selection and optimization of the crosslinking agent are particularly crucial in the preparation of PNIPAM microgels. It not only determines the network structure and mechanical properties of the polymer, but also has a profound effect on the morphology, swelling properties and phase transition behavior of the microgels. Therefore, it is very important to select a suitable crosslinker and optimize its dosage to obtain PNIPAM microgels with excellent performance and wide application. To address the issue of surfactant residue in the conventional emulsion polymerization with surfactants for preparing poly(N-isopropylacrylamide) (PNIPAM) microgels, a soap-free emulsion polymerization was employed to synthesize PNIPAM microgels. This study thoroughly investigated the influence of crosslinker content on the microstructure, swelling properties, temperature-responsive characteristics, and thermodynamic behavior of PNIPAM microgels. By adjusting the crosslinker content, successful control over the morphology and properties of PNIPAM microgels was achieved. The results demonstrated that when the molar concentration of N-isopropylacrylamide (NIPAM) was fixed at 100 mmol/L, independent and uniform spherical microgels were formed when the molar concentration of BIS reached 5 mmol/L. Conversely, when the molar concentration of BIS was below 1 mmol/L, irregularly structured microgels were produced. As the molar concentration of BIS increased from 5 mmol/L to 15 mmol/L, the hydrated particle size of PNIPAM microgels decreased from 600 nm to 459 nm. Furthermore, with the increase in BIS molar concentration, both the swelling ratio and thermos-sensitivity of the microgels gradually decreased. These findings provide valuable insights for the design and functional optimization of PNIPAM microgels."

Key words: pnipam, microgels, soap-free emulsion polymerization, crosslinker, thermo-sensitivity