Stimulus-responsive hydrogels based on associative polymers

An important group of water soluble polymers are associative ones in which hydrophobic parts of the polymer molecules interact, self-assemble and enhance the viscosity of aqueous solutions even at low polymer concentrations. For many applications it would be beneficial to be able to combine the associative behaviour with stimuli-responsiveness.

Suitable stimuli include for example temperature, pH, ionic strength or variation of polymer or additive concentration. Developments in the controlled radical polymerization methods has enabled versatile modification of polymer structures, which in tum enables design of novel associating polymers. Two different stimuli-responsive hydrogel systems will be discussed.

Poly(N-isopropylacrylamide) (PNIPAM) has attracted attention due to its sharp and reversible transition behavior and well-defined demixing temperature in aqueous medium. This however only applies to atactic PNIPAM that is water-soluble at room temperature, as isotactic PNIPAM is insoluble in water and syndiotactic PNIPAM only barely soluble.

By utilising reversible addition-fragmentation transfer polymerization (RAFT) ABA stereoblock copolymers of PNIPAM with atactic and isotactic blocks have been synthesized. l The properties of aqueous solutions and hydrogels of these stereoblock copolymers were studied with respect to the molecular characteristics, ego order ofthe blocks, block lengths and molecular weight.2 Atom transfer radical polymerization (ATRP) was used in the synthesis of 4-armed star block copolymers having arms of poly(acrylic acid) (PAA) as the inner block and polystyrene (PS) as the outer block.

By making the PS block sufficiently short compared with the water soluble polyelectrolyte, PAA, block, water-soluble polymers with associative PS groups were realized. The resulting hydrogels were studied with respect to the polymer concentration, temperature and ionic strength.3 REFERENCES 1. Nuopponen M.; Kalliomaki K.; Laukkanen A.; Hietala S.; Tenhu H. 1.

Polym. Sci. Polym. Chern. 2008, 46, 38-46. 2. Hietala S.; Nuopponen M.; Kalliomaki K.; Tenhu H. Macromolecules, in press. 3. S. Hietala, P. Mononen, S. Strandman, P. Jarvi, M. Torkkeli, K. Jankova, S. Hvilsted, H. Tenhu Polymer, 48 (2007) 4087-4096.