Effect of additives on the micellization of PEO/PPO/PEO block copolymer F127 in aqueous solution

Abstract

The micellization of an ethylene oxide-propylene oxide (PEO-PPO-PEO) symmetrical triblock copolymer (Pluronic®) F127 (EO 99PO 65EO 99, mol. wt. PPO=1750, %PEO=70) in aqueous solution in the presence of various additives (i.e. sodium chloride, urea and sodium dodecyl sulfate (SDS)) is examined by cloud point, surface tension, dye spectral change, sound velocity, viscosity and dynamic light scattering measurements over the temperature range 25–50°C. The critical micelle concentration (CMC) of copolymer altered significantly in the presence of additives. While the addition of sodium chloride lowered the CMC, the addition of urea showed the reverse trend. The presence of added sodium chloride develops hydrophobicity in the PPO moiety and reduces hydrophilicity of PEO moieties, favoring micellization of the block copolymer at relatively lower concentrations than in water at ambient temperature. The increase in CMC of copolymer in presence of urea is interpreted in terms of enhanced solubility of semipolar PPO moiety and also PEO moiety. The critical micelle temperatures (CMTs) show a marked decrease in the presence of added sodium chloride. CMCs obtained by different methods are in good agreement. The addition of SDS to aqueous copolymer solutions leads to the formation of copolymer-SDS complex (or mixed micelle) showing polyelectrolyte nature. Surface tension/dye spectral change measurements reveal aggregation of SDS taking place at concentration much below its CMC, indicating clearly SDS-copolymer interaction. The addition of SDS suppresses the micellization of copolymer and beyond a particular SDS concentration only, SDS micelles with one or two copolymer molecules are present predominantly.