A study of thermoassociative gelation of aqueous cationic poly(N-isopropyl acrylamide) graft copolymer solutions

In this work thermoassociative gel formation of a new family of aqueous temperature-responsive copolymer solutions has been investigated. This was achieved using a cationic poly(N-isopropyl acrylamide) (PNIPAm) graft copolymer recently prepared [Liu R, De Leonardis P, Cellesi F, Tirelli N, Saunders BR. Langmuir 2008;24:7099]. The PDMA+x-g-(PNIPAmn)y copolymers have x and y values that originate from the macroinitiator; the value for n corresponds to the PNIPAm arm length. DMA+ is quarternarized N,N-dimethylaminoethyl methacrylate. The copolymer solutions exhibited cloud point temperatures (Tclpt) of about 33 °C, which were not significantly affected by x/y ratio or the value for n. Thermoassociative gel formation occurred above Tclpt at copolymer concentrations (Ccopol) greater than or equal to 4 wt.%. This is a reasonably low Ccopol value and is a consequence of the graft copolymer architecture employed. We investigated the effect of temperature, Ccopol and copolymer structure on gelation and gel elasticity using variable - temperature dynamic rheology. For PDMA+30-g-(PNIPAm210)14 solutions at 39 °C it was found that G′ (elastic modulus) scales with Ccopol according to G′ ∼ Ccopol3.85. The data suggested that a significant proportion of PNIPAm units is not directly involved in network formation. Thermoassociative gel formation and the gel properties for these systems appear to be governed by a balance between electrostatic repulsion involving the DMA+ units (favouring spatial extension of the copolymer backbones) and attractive hydrophobic interactions between PNIPAm side chains (favouring associative crosslink formation).