N-isopropylacrylamide/2-hydroxyethyl methacrylate star diblock copolymers: Synthesis and thermoresponsive behavior

Tri-arm star diblock copolymers, poly(2-hydroxyethyl methacrylate)-block-poly(N-isopropylacrylamide) [P-(HEMA-b-NIPAAm)] with PHEMA and PNIPAAm as separate inner and outer blocks were synthesized via a two-step ATRP at room temperature. The formation, molecular weight and distribution of polymers were examined, and the kinetics of the reaction was monitered. The PDI of PHEMA was shown to be lower, indicator well-controlled polymerization of trifunctional macro-initiator and resultant star copolymers.The thermoresponsive behavior of diblock copolymer aqueous solution were studied by DSC, phase diagrams, temperature-variables H-1 NMR, TEM and DLS. The results revealed that introducing a higher ratio of HEMA into copolymers could facilitate the formation of micelles and the occurrence of phase transition at lower temperatures. TEM images showed that I-(HEMA(40)-NIPAAm(320))(3) solutions developed into core-shell micelles with diameters of approximately 100 nm. I-(HEMA(40)-NIPAA(320))(3) was used as a representative example to elucidate the mechanism underlying temperature-induced phase transition of copolymer solution. In thus study we proposed a three-stage transition process: (1) separately dispersed micelles state at approximate to 17-22 degrees C; (2) aggregation and fusion of micelles at approximate to 22-29 degrees C; (3) sol-gel transition of PNIPAAm segments at approximate to 29-35 degrees C, and serious syneresis of shell layers.