Magneto-responsive hydrogels based on maghemite/triblock terpolymer hybrid micelles

We report on a new approach towards magneto-responsive hydrogels, showing a reversible gelation upon inductive heating via AC magnetic fields. An aqueous solution of a triblock terpolymer with a partially quaternized poly(2-vinylpyridine) (Pq2VP) outer block, a water soluble poly(ethylene oxide) (PEO) middle block and a thermo-sensitive poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P(GME-co-EGE)) end block (Pq2VP-b-PEO-b-P(GME-co-EGE)) was mixed with a solution of sodium citrate stabilized superparamagnetic maghemite nanoparticles. Due to electrostatic interactions between the oppositely charged particles, surface and the Pq2VP block of the triblock terpolymer, well-defined hybrid micelles with a superparamagnetic core were formed. The number of triblock terpolymer chains stabilizing the nanoparticles was found to be constant above a critical triblock terpolymer/maghemite ratio, i.e. for higher polymer contents hybrid micelles and free (non-bound) triblock terpolymers are present in solution. Thermo-reversible hydrogels are formed via an open association of hybrid micelles at temperatures above the cloud point of the P(GME-co-EGE) corona blocks, which form the network junctions. The superparamagnetic character of the maghemite nanoparticles enables contactless heating, and thus gelation, by applying AC magnetic fields, as demonstrated by high frequency magnetocalorimetry. The thermo-reversible gelation and the dynamic-mechanical properties of the hydrogels were studied by rheology.