Novel transition behavior in aqueous solutions of a charged thermoresponsive triblock copolymer

Turbidity, dynamic light scattering, and steady shear viscosity measurements have been performed under the influence of a temperature gradient (0.2 °C/min) on aqueous solutions of a newly prepared monodisperse low-molar-mass (Mw = 1.34 × 104) methoxy-poly(ethylene glycol)-block-poly(N-isopropylacrylamide)-block-poly(4-styrenesulfonic acid sodium) triblock copolymer (MPEG53-b-P(NIPAAM)54-b-P(SSS)4). The results on this charged triblock copolymer revealed a novel temperature-induced transition from micelles to intermicellar complexes and the formation of collapsed and charged structures at higher temperatures. These experimental methods gave similar transition peaks. Intensity light scattering experiments conducted at several fixed temperatures on solutions of the triblock copolymer disclosed similar transition peaks in the radius of gyration. Cryogenic transmission electron microscopy (Cryo-TEM) measurements on 1 wt% samples at different temperatures, disclosed the building up of association structures at the maximum of the intermediate transition peak, and the breaking down of interchain associations at high temperatures. In the presence of salt, the transition peak at intermediate temperatures was still visible but the formation of flocs at high temperatures was strengthened because of screening of the repulsive electrostatic interactions, and it is also possible that deteriorated solubility of the polymer plays a role. These findings show that even with a short-chain polymer the formed association structures can undergo a major temperature-induced transition from unimers–micelles–intermicellar complexes to collapsed structures. By comparing the heating and cooling cycles, hysteresis effects in the transition behavior were disclosed.