Temperature/pH Sensitive ABA Type Hist/NIPAM Triblock Copolymers

Temperature/pH-sensitive ABA amphiphilic triblock copolymers , poly (N-Acryloyl-L-Histidine)-b-poly (N-isopropylacrylamide)-b- poly(N-Acryloyl-L-Histidine) were successfully synthesized via two-step atom transfer radical polymerization (ATRP). The structure of the copolymers was characterized; temperature/pH-sensitive behaviors and the micellization in aqueous solutions were studied, and the mechanism of phase transition was also elucidated. The main contents include five parts as follows:(1) An ampholytic monomer N-acryloyl-L-histidine was obtained following the conventional acylation reaction of acryloyl chloride with the L-histidine in alkaline solution. The triblock copolymer was synthesized via two-step atom transfer radical polymerization (ATRP), and the formation of copolymers were confirmed by FT-IR and 1H NMR.(2)The CPs and LCSTs determined by turbidimetry and differential scanning calorimetry (DSC) decreased upon increasing solution concentration or NIPAM contents in the copolymer. The copolymer solution showed a fast thermo-responsive behavior, and the sol-gel time was in the range between 8 and 16 second.(3) The copolymer was capable of forming micelles at lower concentration, and pH was found to have no evident effect on CMC. Transmission electronic microscope (TEM) showed that the copolymer solutions could self-assembled into core-shell micelles whose diameters were uniform at pH2 and pH7. At pH5, their size became larger and non-uniform due to the fusion and aggregation of micelles.(4) A three-stage evolution of micelles at pH7 was proposed based on the results of dynamic light scattering (DLS) combined with the variable-temperature 1H NMR: (1)separately dispersed micelles stabilized by like-charged repulsion at shell layer during 20～25oC (2) water was expelled into the interior of micelle caused by the collapse of PNIPAM in core, resulting water-impregnated vesicles near 30oC; (3)complete phase transition of PNIPAM accompanied with micellar aggregationin the range of 35～40oC.(5) The copolymer solutions demonstrated a typical polyampholytic behavior. At pH5, there appeared the lowest transition temperature and fastest sol-gel transition speed. The size of micelles was dependent on pH of solutions. At pH=2 and pH=7, the micelles of copolymers were smaller and uniform; while at pH5, micelles was increased in size, which was originated from the aggregation of copolymer induced by electrovalent bonds between positive and negative ion pairs.