A Study Of Thermosensitive Diblock Star Copolymers

Thermoresponsive diblock star copolymers, poly(2-hydroxyethyl methacrylate -b-N-isopropylacrylamide) (P(I-(HEMA-b-NIPAAm)3), with well-defined architecture were synthesized via atom transfer radical polymerization(ATRP). The copolymers were characterized and their thermosensitive behaviors were studied; the mechanism of phase transition was also elucidated. The main work was summarized as follows:(1) A tris-arm initiator(Tri-E) was synthesized by reacting triethanolamine with bromoisobutyryl bromide; P(I-(HEMA-b-NIPAAm)3) copolymers were prepared by a two-step method in methanol via ATRP.(2) The formation and composition of P(I-(HEMA-b-NIPAAm)3) were confirmed by FTIR and 1H NMR. GPC analysis demonstrated that the polydispersity index of PHEMA was lower. The cloud point (CP) and low critical solution temperature (LCST) were determined by turbidimetry and differential scanning calorimetry (DSC), respectively; the thermoresponsive behavior of copolymer solutions was analyzed by variable-temperature 1H NMR. The results indicated that the region of the transition temperature of the copolymer solutions was down sifted and widened upon increasing PHEMA contents in the copolymers.(3) The critical micellar concentration (CMC) of P(I-(HEMA-b-NIPAAm)3) solutions was estimated by measuring surface tension. The results showed that the CMC values of 1:4 and 1:8 copolymer solutions were 0.0082 mg/mL and 0.0102mg/mL, respectively. Transmission electronic microscope (TEM) revealed that P(I-(HEMA-b-NIPAAm)3) solutions self-assembled into clear core-shell micelles whose diameter approximated to 100 nm.(4) The responsiveness of copolymer solutions was investigated by variable dynamic light scattering(DLS). P(I-(HEMA-b-NIPAAm)3) solutions were supposed to undergo three-stage transitions during heating: (1) separately dispersed micellar state during 17~22℃; (2) the aggregation and fusion of small size micelles between 22 and 29℃; (3) occurrence of PNIPAAm phase transition, serious dehydration of segments in shell layer, and inter-micellar connection, resulting in the sol-gel transition of P(I-(HEMA-b-NIPAAm)3) solutions in the range of 29~35℃.(5) The viscosity of the polymer solution was measured with a rotary rheometer.