A study of thermoassociative gelation of aqueous cationic poly(N-isopropyl acrylamide) graft copolymer solutions R. Liu a, b , F. Cellesi c , N. Tirelli c , B.R. Saunders a, * a Polymer Science and Technology Group, The School of Materials, The University of Manchester, Grosvenor Street, Manchester M1 7HS, UK b School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, P.R. China c Laboratory of Polymers and Biomaterials, School of Pharmacy, The University of Manchester, Oxford Road, Manchester M13 9PT, UK article info Article history: Received 9 November 2008 Received in revised form 14 January 2009 Accepted 15 January 2009 Available online 23 January 2009 Keywords: Thermoassociative gel formation Cationic PNIPAm graft copolymer Dynamic rheology abstract 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 acryl- amide) (PNIPAm) graft copolymer recently prepared [Liu R, De Leonardis P, Cellesi F, Tirelli N, Saunders BR. Langmuir 2008;24:7099]. The PDMA þ x -g-(PNIPAm n ) y copolymers have x and y values that originate from the macroinitiator; the value for n corresponds to the PNIPAm arm length. DMA þ is quarternarized N,N-dimethylaminoethyl methacrylate. The copolymer solutions exhibited cloud point temperatures (T clpt ) of about 33  C, which were not significantly affected by x/y ratio or the value for n. Thermoasso- ciative 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 þ 30 -g-(PNIPAm 210 ) 14 solutions at 39  C it was found that G 0 (elastic modulus) scales with C copol according to G 0 w C copol 3.85 . The data suggested that a significant proportion of PNIPAm units is not directly involved in network formation. Ther- moassociative gel formation and the gel properties for these systems appear to be governed by a balance between electrostatic repulsion involving the DMA þ units (favouring spatial extension of the copolymer backbones) and attractive hydrophobic interactions between PNIPAm side chains (favouring associative crosslink formation). Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction In previous work [1] we introduced a new temperature-respon- sive family of cationic graft copolymers, PDMA þ x -g-(PNIPAm n ) y . DMA þ and NIPAm are quarternarized N,N-dimethylaminoethyl methacrylate and N-isopropylacrylamide, respectively. That study involved a comprehensive investigation of dilute dispersion behaviour. These copolymers are potentially of significant interest because they can adsorb to anionic dispersions and confer respon- sive behaviour to them [2]. Our copolymers are architecturally unique compared to other thermoassociative copolymers [3–6] because they are cationic graft copolymers. They were constructed using atom transfer radical polymerization (ATRP). Thermoassocia- tion of the PDMA þ x -g-(PNIPAm n ) y chains occurs [1] at or above the cloud point temperature (T clpt ). However, the gelation temperatures have not yet been determined in the absence of shear and the factors governing the elasticity of the gels have not been investigated. In this study we extend our previous work to sensitively probe gel forma- tion and properties using dynamic rheology. The aims of this study were to gain insights into thermoassociative gel formation and to improve the understanding of the relationships between copolymer structure and gel elasticity. Temperature-responsive copolymers undergo temperature- triggered chain contraction [7–9] when the solution temperature reaches the lower critical solution temperature (LCST). They can, through thermoassociation, form macromolecular micelles. The seminal work in the context of concentrated solutions of ther- moassociative copolymers was conducted by Hourdet et al. [4,5,10–14]. Their copolymers were prepared using conventional free-radical polymerization and often contained NIPAm. Structural variations studied by those workers included anionic copolymers [5,11,15] as well as cationic copolymers [4]. The anionic copolymers contained a negatively charged backbone and PNIPAm side chains [5]. The cationic copolymer consisted of a linear copolymer with * Corresponding author. E-mail address: brian.saunders@manchester.ac.uk (B.R. Saunders). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.01.035 Polymer 50 (2009) 1456–1462