Aggregation dynamics of the block copolymer L64 in aqueous solution: Copolymer-sodium dodecyl sulfate interactions studied by laser T-jump

The aggregation dynamics of aqueous solutions of the triblock copolymer L64 (from BASF, trade name "Pluronic", EO13PO30EO13) were investigated using i odine laser temperature jump. Three relaxation processes were detected. The first and fastest relaxation (tau (1)) occurred in the microsecond time ra nge and is associated with unimer insertion into micelles. The second and t hird relaxations (tau (2) and tau (3)) were on a longer time scale, 1-100 m s, and are associated with the rearrangement of the micelle size distributi on and micelle clustering, respectively. As the temperature was increased 1 /tau (1) and 1/tau (2) increased, reflecting faster unimer insertion and si ze redistribution. By contrast, 1/tau (3) decreased with temperature as a c onsequence of "critical slowing down" close to a critical point, here the c loud point. Progressive addition of sodium dodecyl sulfate (SDS) from 5 x 1 0(-5) to 2.5 x 10(-4) mol dm(-3) to L64 0.625 wt % solutions caused a sl;if t of the differential scanning calorimetry (DSC) maxima of the heat capacit y C-p toward lower temperatures (decrease of the critical micelle forming t emperature, cmt), which could be reversed at higher SDS concentrations abov e 1 mmol. In the kinetic experiments the progressive addition of SDS caused an increase in 1/tau (1) possibly because "looser" and smaller micelles ar e formed which present a reduced energetic barrier for unimer insertion. In contrast, increasing concentrations of SDS caused a decrease in 1/tau (2), and there is some evidence for a shift from fission/fusion toward a stepwi se mechanism more usually associated with ionic surfactant systems, because 0.25 mol dm(-3) NaCl addition reversed this effect. There was also a decre ase in 1/tau (3) as the SDS concentration increased. However, the trend cou ld be reversed by the addition of electrolyte which screened the electrosta tic interaction; the latter is in agreement with a clustering mechanism for this newly found third relaxation process.