Micellization and gelation of a triblock copolymer of oxyethylene and oxypropylene, E93P44E93, 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 a triblock copolymer of oxyethylene (E) and oxypropylene (P), E₉₃P₄₄E₉₃, encompassing non-micellar, micellar and gel phases. At the critical micelle temperature (c.m.t.), there was a marked transition in the relaxation times of the hydrophobic P block attributed to a change from well solvated mobile chains below the c.m.t. to a more restricted, concentrated micelle-core environment above the c.m.t. However no transition in the properties of the hydrophilic E block was observed at the c.m.t., indicating the continuation of a solvated, mobile chain structure. At the transition from a micelle solution to a structured gel, transitions were observed in the relaxation times of both E and P blocks, consistent with gel formation by close packing of micelles at a spacing determined by the excluded volume rather than the hydrodynamic volume. Transient nuclear Overhauser effects (NOE) indicated that in the micelles there was considerable interpenetration of the E and P blocks at the 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 the phase transitions, whereas the slow correlation time representing larger-scale motions was much more affected. For the P block, both fast and slow correlation times increased significantly on micellization.