Dynamic Light Scattering Study of Concentrated Triblock Copolymer Micellar Solutions under Pressure

Abstract

The results from a dynamic light scattering study of concentrated aqueous systems of polyethylene oxide−polypropylene oxide−polyethylene oxide triblock copolymer (PEO−PPO−PEO, Pluronic F108) are reported at pressures to 2500 bar and temperatures from 0 to 110 °C. The pressure−temperature (P−T) phase behavior for this micellar system exhibits several phase transformations at high concentrations including a transition from a liquid to a gel phase and a subsequent re-entrant transition from the gel phase to a liquid phase with increasing temperature. The gel phase region is bounded at low temperatures by a liquid-to-gel phase transition curve that increases in pressure with increasing temperature, eventually exhibits a maximum in pressure, and then at high temperatures rapidly decreases as the temperature is further increased. The high temperature branch of this curve represents a gel-melting transition. Aqueous F108 samples are doped with latex probe nanoparticles to facilitate the characterization of the local viscosity and also to serve as a reference for the measurement of the light intensity scattered by the Pluronic systems. Both the intrinsic collective dynamics of the Pluronic systems and the diffusive dynamics of the nanoparticle probes are characterized. The diffusive dynamics of the nanoparticle probes reveal the nonmonotonic character of viscosity that results from phase transformations dependent on pressure and temperature. The micellar system scattering intensity, obtained from DLS data through comparison with the light intensity scattered by the probe nanoparticles, increases with heating indicating critical pretransitional behavior of concentration fluctuations on approach to the cloud-point boundary over a broad temperature interval that spans the gel phase temperature range.