Synthesis and characterization of dual-responsive poly(N-vinylcaprolactam-co-N-methylolacrylamide) nanogels

This article reports the synthesis of poly(N-vinylcaprolactam-co-N-methylolacrylamide) (P(NVCL-co-NMA)) nanogels and investigates their thermo-/pH-responsive behavior. The formation of nanogels was synthesized using free radical emulsion polymerization by varying the monomer composition of NVCL:NMA, and their molecular structure was characterized by ¹H-NMR and FTIR. It was found that the nanogels were successfully prepared, and the nanogels exhibited LCST-type phase transition behavior. Cloud point transition temperature (T_c) was studied as a function of copolymer composition, MBA concentration, and pH of the solution by exploring their changes in turbidity using UV-vis spectrophotometer. Our studies reveal that T_c nanogels increased with increasing concentration of NMA, which is due to the hydrophilicity of NMA. Our research also demonstrated that the increase in MBA percentage could decrease the T_c of the synthesized nanogels. Interestingly, P(NVCL-co-NMA) nanogels showed not only a thermoresponsive behavior but also a pH response with increasing T_c in a strong acidic environment owing to the H-bonds within the polymer chains. The results show that nanogels with initial monomer composition of NVCL and NMA of 75% and 25%, respectively, and using 4% of MBA showed T_c around 35°C at pH 7.4. In addition, DLS studies also confirmed this result since the particle sizes became much larger after surpassing the temperature of 35°C. Due to this founding, such nanogels might have potential application in controlled release. Nevertheless, further studies regarding the adjustment of T_c are still needed.