Study On The Conformational Behaviors Of Temperature- And PH-sensitive Graft Copolymers P(MAA-g-NIPAM)

The unique and novel properties of intelligent polymeric materials, which exhibit large property changes in response to small changes in external conditions, such as temperature, pH, chemicals, and ion strength, offer an unlimited amount of potential applications relevant to many fields. These applications include drug delivery systems, chemical separation, smart catalysts and so on. So these polymer materials have attracted much attention.PNIPAM is a kind of temperature sensitive polymer, containing hydrophilic amide group and hydrophobic vinyl backbone and isopropyl side groups. This character leads to the reversible temperature solubility behavior of PNIPAM aqueous solution. At low temperature, the strong hydrogen bonding between the hydrophilic groups and water outweighs the hydrophobic interactions of hydrophobic groups and water, leading to good solubility of the polymer in water; the higher temperature, the weaker hydrogen bonding, while hydrophobic interactions between hydrophobic side groups have been strengthened. Above LCST (at 33℃), interactions between hydrophobic groups become dominant, leading to an entropy-driven polymer collapse and phase separation; poly (methacrylic acid) (PMAA) is a pH sensitive polymer. Unlike other polyacids, it adopts hypercoiled conformation at low pH because of the hydrophobic interactions introduced by the methyl groups along the polymer backbone. However, on addition of base to solution, the carboxyl groups ionize and acquire negative charges. The increase in coulombic repulsive forces results in a non-uniform sudden conformational transition from the hypercoiled to expanded form. This conformational change is reversible. So, the temperature-/pH- sensitive graft copolymers can be obtained by copolymerizing PNIPAM with MAA in a certain manner.Based on the ideas above, we synthesized a series of temperature-/pH- sensitive graft copolymers, and investigated their conformation as external conditions changed for the graft copolymer solution in the present study. The experiment of this article includes three parts.(1) Five different composition temperature- and pH- sensitive poly(methacrylic acid-g-N-isopropylacrylamide)(P(MAA-g-NIPAM)) graft copolymers were synthesized by free-radical copolymerization utilizing macromonomer technique. The conformational behavior and sensitivities of P(MAA-g-NIPAM) in dilute aqueous solutions were investigated by UV-vis transmittance measurements and fluorescence probe technique. The results show that the P(MAA-g-NIPAM) graft copolymers had temperature- and pH- sensitivities, and the double sensitivities were independent each other. The conformational change for the graft copolymers in the process of temperature or pH cycle change is incomplete reversible. The different composition graft copolymers have the basically identical LCST. The more content of PNIPAM graft chains, the fast for the graft copolymers response to the temperature. So, we can adjust the graft copolymer conformational behaviors by change temperature or the graft copolymer solution pH, and achieve the micelle aim for the P(MAA-g-NIPAM) graft copolymers.(2) The complexation between temperature- and pH- sensitive poly (methacrylic acid-g-N-isopropylacrylamide) (P(MAA-g-NIPAM)) and poly (N-isopropylacrylamide) (PNIPAM) in dilute aqueous solution was studied by UV-vis, fluorescence probe technique and fluorescence anisotropy. The results show that that the complexation between P(MAA-g-NIPAM) and PNIPAM in aqueous solution depends on the pH of the solution and the composition of graft copolymer. Furthermore, the interaction force is hydrogen bonding. For the graft copolymer with lower grafting rate, the complexation between P(MAA-g-NIPAM) and PNIPAM occurs within a pH range of pH＜6, and this complexation changes the conformational behaviors of the P(MAA-g-NIPAM). But for the graft copolymer with higher grafting rate, there is almost no complexation between P(MAA-g-NIPAM) and PNIPAM within a wide pH range from 3 to10.(3) Pyrene and naphthalene labeled P(MAA-g-NIPAM) graft copolymers were prepared. Their conformational behavior was studied by Nonradiative Energy Transfer (FRET) in molecule level. The results show that the grafting rate has influence on the conformation of graft copolymer. As grafting rate increased at pH＜7, the conformation of graft copolymers changed from hyper-coiled to coiled. At the range of pH＞7, graft copolymers exist as an expanded coil conformation.