Phase Transitions in Aqueous Tri-block Co-polymers: NMR Relaxation Studies.

Longitudinal NMR relaxation times were used to investigate the thermal transitions occurring in an aqueous triblock copolymer of the poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) family. In such a system molecule-micelle and micelle-liquid crystal transitions are observed, depending on temperature and composition. The longitudinal relaxation time, R-1, significantly changes when the aforementioned phase transitions take place. In the case of molecule-micelle equilibrium, changes in R-1 values of the methyl (and methynic) group signal, located in the lipophilic portion of block copolymers, are observed. The effect is ascribed to a significant dehydration of the poly(oxypropylene) chains, as a consequence of micelle formation. Conversely, the thermal transitions from micelles to liquid-crystalline phases are associated with significant changes in the relaxation time of poly(oxyethylene) units. The latter effect is tentatively ascribed to a partial dehydration and/or interdigitation of the poly(oxyethylene) moieties in the block copolymer.