Preparation and Characterization of Thermosensitive Polyampholyte Nanogels

Abstract

Nanosized polyampholyte gel particles were synthesized by aqueous redox polymerization in the presence of sodium dodecylbenzene sulfonate as a surfactant. Acrylic acid (AAc) and 1-vinylimidazole (VI) were respectively used as an anionic and a cationic monomer, both of which were incorporated into the network of N -isopropylacrylamide (NIPA) cross-linked with N,N ‘ -methylenebisacrylamide. Potentiometric titration (0.05−0.5 M KCl solutions as a solvent) gave the following contents (in mol %) of anions ( A ) and cations ( C ) in the nanogels: A = 3 and C = 13 for nanogel G(1/4); A = 14 and C = 12 for nanogel G(1/1). It is worthy of notice that G(1/1) is insoluble in pure water but soluble in KCl solutions over the concentrations ( C S ) > 0.01 M; this phenomenon is the so-called “antipolyelectrolyte” behavior. Both nanogels were characterized in detail by a combination of static and dynamic light scattering, electrophoretic light scattering, and turbidity measurements. The pH dependence of the gel size exhibited a characteristic pattern, from which an isoelectric point (pI) was observed: pH 6.5 for G(1/4) and pH 5.3 for G(1/1). At pI, at which the negative and positive charges are fully balanced, the electrophoretic mobilities became zero. From the light scattering of nanogels in 0.1 M KCl solution at pH = pI, the following data were obtained: radius of gyration ( R g ) = 107 nm for G(1/4) and 81 nm for G(1/1); hydrodynamic radius ( R h ) = 132 nm for G(1/4) and 87 nm for G(1/1). Thus, the R g / R h ratio was within 0.85 ± 0.05, suggesting a spherical conformation of the nanogel particles. From the study of the effects of C S and temperature, the following characteristics were obtained: (i) R h of G(1/4) varies little depending on KCl concentration, while G(1/1) shows a slight increase in R h with C S from 0.01 to 0.5 M and a marked decrease at C S ∼ 2 M without accompanying aggregation. (ii) G(1/1) undergoes a shape phase separation at C S ∼ 0.008 when decreasing C S, whereas G(1/4) is dispersible both in pure water and in KCl solutions at C S < 1 M, the concentration at which aggregation takes place, as observed in the pure NIPA nanogel system. (iii) Both G(1/4) and G(1/1) shrink up to 41 and 51 °C, respectively, and around these temperatures the gel suspensions undergo a shape phase separation. The results obtained were compared with those of terpolymers as a control sample and discussed in terms of the intra- and interparticle interactions, in which hydrogen bonding and/or hydrophobic association, other than the usual electrostatic attraction, were found to play an important role.