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 (T-clpt) of about 33 degrees C, which were not significantly affected by x/y ratio or the value for n. Thermoassociative gel formation occurred above T-clpt at copolymer concentrations (C-copol) greater than or equal to 4 wt.%. This is a reasonably low C-copol value and is a consequence of the graft copolymer architecture employed. We investigated the effect of temperature, C-copol and copolymer structure on gelation and gel elasticity using variable - temperature dynamic rheology. For PDMA(30)(+)-g-(PNIPAm210)(14) solutions at 39 degrees C it was found that G' (elastic modulus) scales with C-copol according to G' similar to C-copol(3.85). 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). (C) 2009 Elsevier Ltd. All rights reserved.