Effect of PEGDMA Molecular Weight on the Physicochemical Properties of Thermoresponsive N-Vinylcaprolactam-Based Polymers

Thermosensitive polymers P1–P5 of N-vinylcaprolactam (NVCL) and poly(ethylene glycol) dimethacrylates (PEGDMAs), av. Mn 750–20,000, were synthesized via surfactant-free precipitation polymerization (SFPP) using the cationic initiator 2,2′-azobis[2-methylpropionamidine] dihydrochloride (AMPA) at 70 °C. The polymerization course was monitored by conductivity. The physicochemical properties and thermoresponsive behavior of the polymers were investigated over the temperature range of 18–45 °C using dynamic light scattering, electrophoretic mobility measurements, surface tension, osmolality, attenuated total reflectance–Fourier transform infrared spectroscopy, and transmission electron microscopy. At 18 °C, all polymers formed nanoscale dispersed structures, with hydrodynamic diameters of approximately 23–41 nm. Heating induced pronounced changes in hydrodynamic diameter, with the magnitude of the response and transition temperature varying with PEGDMA chain length. The polymers differed in the extent of temperature-induced aggregation and in thermal hysteresis, reflecting differences in the reversibility of their temperature-induced changes in hydrodynamic diameter. Zeta potential and surface tension measurements revealed variations in interfacial properties among the polymer systems, whereas osmolality remained low and comparable between samples. Overall, variations in PEGDMA molecular weight were associated with differences in the physicochemical and thermoresponsive behavior of the NVCL-based polymer networks; however, no simple monotonic relationship between cross-linker molecular weight and the investigated properties was observed.