Preparation of thermosensitive nanogels by photo-cross-linking

 A novel method to prepare thermosensitive nanogels from photocross-linkable copolymers of N-isopropylacrylamide and dimethyl maleinimido acrylamide (DMIAAm) was developed. The colloidal nanogels were formed by UV irradiation of solutions of thermosensitive polymers in water at 45 °C. The compositions of the photopolymer solutions were varied by changing the amount of DMIAAm in the photopolymer chains (2–10 mol%) or by varying the sodium dodecyl sulfate (SDS) concentration. The resultant nanogel particles were rather spherical and showed large changes in hydrodynamic diameters in the vicinity of the phase transition temperature of the corresponding linear photopolymers. The particle sizes of the nanogels and their swellability could be controlled through the UV irradiation time, the chromophore content and the SDS concentration. An increase in the chromophore content and the SDS concentration resulted in nanogels with smaller dimensions. The hydrodynamic diameters of the nanogels decreased significantly from 2 to 10 min UV irradiation time but not significantly after that. The phase transition of the photopolymer solutions and the respective nanogels could be adjusted by the chromophore content or the SDS concentration. An increase in the chromophore content leads to lower phase-transition temperatures, whilst an increase in the SDS concentration elevated them. Pulsed-field-gradient NMR proved a useful tool to investigate the network formation in the nanogels by determining changes in the diffusion coefficients.