Associative phase behaviour and disintegration of copolymer aggregates on adding poly(acrylic acid) to aqueous solutions of a PEO-PPO-PEOtriblock copolymer

Abstract

The influence of adding poly(acrylic acids) of two different chain lengths (PAA₂₅ and PAA₆₀₀₀) on the phase behaviour of the amphiphilic triblock copolymer Pluronic® P104 (P104 = (EO)₂₇(PO)₆₁(EO)₂₇, where EO = ethylene oxide, PO = propylene oxide) in H₂O was investigated. The resulting phase equilibria and structures were investigated by visual inspection, using crossed polarizers, and small angle X-ray scattering (SAXS). At low and intermediate P104 concentrations an associative phase separation was found, where a concentrated phase separated out from a dilute one on adding PAA. The corresponding phase separation region appeared in the phase diagram as a closed, droplet-shaped miscibility gap that was significantly larger for the longer PAA. Neither of the coexisting phases contained any long-range ordered structures. At higher concentrations of P104 (above ca. 25 wt%) no miscibility gap appeared but, remarkably, the various ordered liquid crystalline phases observed in binary P104/H₂O mixtures were eventually destroyed upon the replacement of H₂O by PAA. A similar effect was found when propionic acid (PrA), corresponding to the repeating unit of the PAA chain, was added to aqueous P104. A decrease in the PAA length, in the series PAA₆₀₀₀ - PAA₂₅ - PrA, increased the efficiency to destroy the structured phases. NMR self-diffusion measurements showed that the self-assembled P104 aggregates dissolved on replacing water with PrA. The same mechanism was found to be responsible for the effect of added PAA, that is, PAA (or PrA) acts as less selective “solvent” for the PEO and PPO blocks compared to water.