Tunable Dual-Thermoresponsive Phase Behavior of Zwitterionic Polysulfobetaine Copolymers Containing Poly(N,N-dimethylaminoethyl methacrylate)-Grafted Silica Nanoparticles in Aqueous Solution

Novel thermoresponsive copolymers of zwitterionic sulfobetaine methacrylate (SBMA)- and N,N-dimethylaminoethyl methacrylate (DMAEMA)-grafted silica nanoparticles are prepared via surface-initiated atom transfer radical polymerization. The phase behavior of these nanoparticles is investigated. The hybrid nanoparticles exhibit tunable phase transition temperature between the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST) in aqueous solution. A high PSBMA content in P(SBMA-co-DMAEMA) copolymer in the hybrid nanoparticles leads to aggregation at low temperatures because of dominant electrostatic interactions of ionic pairs, whereas a relatively low PSBMA content in the hybrid nanoparticles results in a phase transition at high temperatures as a result of predominant hydrophobic interactions of the PDMAEMA. Interestingly, hybrid nanoparticles with a suitable molar ratio of SBMA/DMAEMA exhibit both UCST and LCST in aqueous solution, where the water-insoluble microdomains of hybrid nanoparticles are generated by the PSBMA or PDMAEMA region depending on the temperature. The above dual-thermoresponsive phase transition is also controllable in salt solution. A facile change in the molar content of copolymer grafted on silica nanoparticles will tune the UCST and LCST readily.