Physicochemical Properties of Poly(N-isopropylacrylamide-co-4-vinylpyridine) Cationic Polyelectrolyte Colloidal Microgels V. T. Pinkrah, † M. J. Snowden,* ,†,‡ J. C. Mitchell, † J. Seidel, § B. Z. Chowdhry, †,‡ and G. R. Fern ‡ School of Chemical & Life Sciences, and Medway Sciences, University of Greenwich, Medway University Campus, Chatham Maritime, Kent ME4 4TB, UK, and Institute of Physical Chemistry, Freiberg University, D-09596 Freiberg, Germany Received July 23, 2002. In Final Form: November 13, 2002 Aqueous colloidal poly(N-isopropylacrylamide-co-4-vinylpyridine) [poly(NIPAM-co-4-VP)] copolymer microgels have been synthesized using different percentages of 4-vinylpyridine (4-VP). A surfactant-free emulsion polymerization reaction using N-isopropylacrylamide (NIPAM) and 4-vinylpyridine comonomers cross-linked with N,N′-methylenebisacrylamide was utilized. The reaction was initiated using the cationic initiator 2,2′-azobis(2-amidinopropane) dihydrochloride. Transmission electron micrograph data show the copolymer microgels to be monodisperse spheres. The pH and electrolyte sensitivity of the copolymer microgels have been studied, as well as temperature sensitivity, since microgels undergo a reversible vol- ume phase transition in response to heating and cooling. Changes in the hydrodynamic diameters in the system were monitored as a function of temperature (25-60 °C), pH (3-8), and ionic strength (10 -3 -10 -1 mol dm -3 NaCl or NaClO4) using photon correlation spectroscopy. The hydrodynamic diameter of poly- (NIPAM-co-4-VP) microgels increases with decreasing pH, as the vinylpyridine units become more protonated. However, the hydrodynamic diameter decreases with increasing ionic strength (over the pH range 3-8) and with increasing temperature (at pH 3 and pH 6). UV-visible spectrophotometry measurements showed a good correlation between the molar absorption and the percentage of vinylpyridine incorporated. Potentiometric titrations were used to determine the pKa values of the copolymer microgels. 1. Introduction Poly(N-isopropylacrylamide) [i.e. poly(NIPAM)] ho- mopolymer is a nonionic, linear, water-soluble polymer. Colloidal poly(NIPAM) microgels, on the other hand, are intramolecularly cross-linked polymeric particles usually dispersed in an aqueous solvent, displaying hydrodynamic diameters in the range of ∼100 nm to 1 µm. 1 Poly(NIPAM) microgels have attracted significant interest in the recent scientific literature, 2-10 in part because such particles undergo a thermoreversible volume phase transition (VPT) at 34 °C in water. 11 In fact, the physicochemical properties of microgels, based on poly(NIPAM) and various copolymer derivatives, have been widely investigated 1,2,4,11 in relation to the VPT changes that occur in response to a number of external stimuli such as pH, 12 temperature, 1,3 and ionic strength. 12,13 The swelling ratio and the temperature at which the VPT occurs for bulk poly(NIPAM) gels dramatically in- crease when ionizable groups are incorporated within the polymer network. 14 Microgels that are sensitive to pH can, on the other hand, be prepared via copolymerization by incorporating acidic or basic groups into the polymer network. Microgels based on homopoly(NIPAM) have the limita- tion that the thermally induced VPT occurs at a fixed temperature of ∼34 °C. 14 This limitation can be overcome by the introduction of comonomers into the backbone of the polymer chain. Homopoly(NIPAM) does not show a polyelectrolyte-like solution response to increasing elec- trolyte concentration. Like all water-soluble polymers, however, the polymer-solvent interaction parameter () will increase on going from water to electrolyte and the solvent quality will decrease. This will result in e.g. a lowering of the VPT value for poly(NIPAM). Other studies 12 have investigated the effect of electrolyte on the swelling and dispersion stability of poly(NIPAM) micro- gels. The study considered a range of different electrolytes. The effect of increased electrolyte concentration generally resulted in a decrease in the hydrodynamic diameter of the microgel. 12,13 Recently, thermosensitive copolymers of NIPAM and acrylic acid have also been prepared. 12 The VPT at which * Corresponding author address: Medway Sciences, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK. Tel: +44 (0) 208 331 9981. Fax: +44 (0) 1634 883 044. E-mail: m.j.snowden@ greenwich.ac.uk. † Medway Sciences, University of Greenwich. ‡ School of Chemical & Life Sciences, University of Greenwich. § Freiberg University. (1) Murray, M.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73. (2) Pelton, R. H.; Chibante, P. Colloid Surf. 1986, 20, 247. (3) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (4) Mcphee, W.; Tam, K. C.; Pelton, R. J. Colloid. Interface Sci. 1993, 24, 156. (5) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (6) Pelton, R. H.; Pelton, H. M.; Morphesis, A.; Rowell, R. L. 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