Static and dynamic properties of a (PEOPPOPEO) block copolymer in aqueous solution

Abstract

Aqueous solutions of a triblock copolymer of the type polyethyleneoxide- block-polypropyleneoxide- block-polyoxyleneoxide (Pluronic L-64, average composition 26 EO-units and 30 PO-units), were examined with static and dynamic light scattering (DLS), pulsed-gradient spin-echo (PGSE) NMR and fluorescence spectroscopy over a range of concentrations (0.2–25 wt%) and temperatures (15–60°C). Relaxation time distributions from DLS show L-64 to be molecularly dissolved at 21°C, and to form micelles at higher temperatures, which remain at high concentrations (25%) without formation of gel or liquid crystalline phases. The temperature where micelle formation starts is strongly concentration dependent, in contrast to the cloudpoint with remains fairly constant at 60°C. The hydrodynamic radii of the micelles, as obtained from DLS and PGSE NMR, are in reasonable agreement (60–80 Å), and also agree with an aggregation number reported from fluorescence-quenching studies, whereas static light scattering, evaluated according to normal practice, indicates much smaller aggregates. This is due to the presence in the solution of a mixture of monomers, micelles, and at low temperatures, also some strongly scattering larger aggregates, possibly emanating from a small percentage of a diblock impurity in the preparation. The diblock impurity aggregates are dissolved by the proper micelles at higher temperatures. Their presence is indicated also by the anomalous excimer formation at low temperatures, caused by pyrene becoming concentrated in the premicellar aggregates.