The effect of cationic and anionic blocks on temperature-induced micelle formation

The influence of electrostatic interactions on micelle formation has been investigated in two series of charged temperature-sensitive triblock copolymers using small-angle X-ray scattering [Behrens, Lopez, Kjøniksen, Zhu, Nyström & Pedersen (2011). Langmuir, 28, 1105-1114; Behrens, Kjøniksen, Zhu, Nyström & Pedersen (2011). Macromolecules, 45, 246-255]. The results of these studies are compared to further elucidate the effect of an anionic and a cationic block in the triblock copolymer species. The two series of block copolymers have common water-soluble and temperature-sensitive blocks, methoxypoly(ethylene glycol) and poly(N-isopropylacrylamide), respectively, whereas the charged block differs in the two series: one has a cationic block, poly[(3-acrylamidopropyl) trimethyl ammonium chloride], and the other copolymer has an anionic block, poly(4-styrenesulfonic acid sodium). From the small-angle X-ray scattering study, performed in a temperature range between 293 and 343 K, all copolymers can be described as molecularly dispersed copolymers at ambient temperature, displaying inter-chain repulsion in aqueous solution, and these can be screened by addition of 30 mM NaCl. Both copolymer series display a transition temperature at elevated temperature, where the molecularly dispersed copolymers self-assemble into larger structures, which can be described as either spherical or cylindrical micelles. These structures are affected by the polymer composition, both with respect to the length of the temperature-sensitive block and the specific character of the charged block.