Aggregation Dynamics of the Block Copolymer L64 in Aqueous Solution: Copolymer−Sodium Dodecyl Sulfate Interactions Studied by Laser T-Jump

Abstract

The aggregation dynamics of aqueous solutions of the triblock copolymer L64 (from BASF, trade name “Pluronic”, EO13PO30EO13) were investigated using iodine laser temperature jump. Three relaxation processes were detected. The first and fastest relaxation (τ1) occurred in the microsecond time range and is associated with unimer insertion into micelles. The second and third relaxations (τ2 and τ3) were on a longer time scale, 1−100 ms, and are associated with the rearrangement of the micelle size distribution and micelle clustering, respectively. As the temperature was increased 1/τ1 and 1/τ2 increased, reflecting faster unimer insertion and size redistribution. By contrast, 1/τ3 decreased with temperature as a consequence of “critical slowing down” close to a critical point, here the cloud point. Progressive addition of sodium dodecyl sulfate (SDS) from 5 × 10-5 to 2.5 × 10-4 mol dm-3 to L64 0.625 wt % solutions caused a shift of the differential scanning calorimetry (DSC) maxima of the heat capacity Cp toward lower temperatures (decrease of the critical micelle forming temperature, cmt), which could be reversed at higher SDS concentrations above 1 mmol. In the kinetic experiments the progressive addition of SDS caused an increase in 1/τ1 possibly because “looser” and smaller micelles are formed which present a reduced energetic barrier for unimer insertion. In contrast, increasing concentrations of SDS caused a decrease in 1/τ2, and there is some evidence for a shift from fission/fusion toward a stepwise mechanism more usually associated with ionic surfactant systems, because 0.25 mol dm-3 NaCl addition reversed this effect. There was also a decrease in 1/τ3 as the SDS concentration increased. However, the trend could be reversed by the addition of electrolyte which screened the electrostatic interaction; the latter is in agreement with a clustering mechanism for this newly found third relaxation process.