Gelation of aqueous solutions of diblock and triblock copolymers of ethylene oxide and 1,2-butylene oxide studied by 1H nuclear magnetic relaxation

Abstract

¹ H NMR longitudinal and transverse relaxation times have been measured as a function of temperature at 300 and 500 MHz for aqueous solutions of diblock and triblock copolymers of ethylene oxide (E) and 1,2-butylene oxide (B), encompassing micellar sol and gel phases. At the phase transitions from a micelle to a gel structure, transitions were observed in the relaxation times of the E block, consistent with gel formation by close packing of micelles at a spacing determined by the thermodynamic radius rather than the hydrodynamic radius. Transient nuclear Overhauser effects indicated that in the micelles there was considerable interpenetration of the E and B blocks at the core/fringe boundary. The relaxation times were analysed using a two-step correlation function. For the E block, the fast correlation time representing local segmental motions was little affected by gelation, whereas the slow correlation time representing larger-scale motions was much more affected. Towards the lowest temperatures studied (< 10 °C), the relaxation times of the B block indicated incipient micelle dissociation.