As the temperature of Pluronic copolymer solutions increases, the copolymer molecules evolve from a region with only monomers, monomer only, to a region where monomers are in equilibrium with micelles, monomer − micelle, to a region where only micelles are found, micelle only. In this article, we analyze the concentration dependence (0.1−10% w/w) of the reduced viscosity, ηred, the apparent specific volume, v p, φ, and the apparent specific heat capacity, cpp, φ, in terms of these regions at 5, 25, and 45 °C. Six Pluronics (F38, P85, P104, P103, L122, and L101) have a molecular weight around 5000 g·mol-1 and two (F108 and P105) have a higher molecular weight. The method compares the apparent specific volume and heat capacity of the copolymer in solution with its additive values corresponding to the monomeric state (v mono and cpmono). If v p, φ ≈ v mono and cpp, φ ≈ cpmono, then all the copolymer molecules are in the monomeric state. If v p, φ ≳ v mono and cpp, φ > cpmono, then the copolymer molecules are in equilibrium with micelles. Finally, if v p, φ > v mono and cpp, φ ≈ cpmono, then the copolymer molecules are all in the micellar state. At 5 °C, all the Pluronics studied are in the monomeric state between 0.1 and 10 g/mL. At 25 °C, the monomer only, monomer − micelle, and micelle only regions are all encountered depending on the molecular weight and the polyoxypropylene content. At 45 °C, F38 is still in the monomeric state and P85 molecules are in equilibrium with monomer only at low concentrations. All the other copolymers investigated form only micelles. The investigation of the apparent specific volume and heat capacity has proven quite sensitive, in particular for the identification of the concentration range where monomers are in equilibrium with micelles.
The phase diagram of aqueous oxyethylene(12)-oxypropylene(67)-oxyethylene(12>, was determined from-10 to +40 °C and up to 50% (w/w). Isotropic solution, liquid crystal, and gel phases are observed as well as an anomalous region consisting of slightly turbid blueish solutions. Two endothermic transitions border a region of high viscosity that leads to the gel phase. The apparent volume and heat capacity of Pluronic L-122 and of 2-butoxyethanol in the aqueous mixtures of Pluronic L-122 were determined at 10, 22, and 35 °C. At 10 °C, the isotropic solutions of Pluronic L-122 go through phase separation with a continuous increase in the apparent volume and a sharp increase in the apparent heat capacity. At 22 °C, a transition occurs at low concentrations into the biphasic region, with a sharp increase in the apparent volume and a large maximum in the apparent heat capacity. It is suggested that, from 22 to 35 °C, the transition is displaced to a concentration lower than that of the range covered in this investigation so that the trends of the thermodynamic properties correspond to the tail of the transition. This is consistent with an extrapolated volume higher than the one calculated by additivity of the homopolymers at infinite dilution at 35 °C.
Pluronics are block copolymers composed of a central block of polypropylene oxide and two side chains of polyethylene oxide. They are used in water to generate aggregates and gels or added to phospholipid suspensions to prepare microparticles for drug delivery applications. The structure of these systems has been widely investigated. However, little is known about the mechanisms leading to these structures. This investigation compares the apparent molar volumes and heat capacities of Pluronics F38, F108, F127, P85, P104, and P103 at 25 °C in water and in the presence of lecithin liposomes. The changes in molar volumes, heat capacities, and enthalpies generated by a mass-action model are in good agreement with the loss of hydrophobic hydration of the polypropylene oxide central block of the Pluronics. However, the molecularity of the endothermic transitions is much smaller than the aggregation numbers reported in the literature for the same systems. It is suggested that Pluronics go through dehydration of their central block to form unimolecular or small entities having a hydrophobic polypropylene oxide core. In water, these entities would assemble athermally to form larger aggregates. In the presence of liposomes, they would be transferred into the hydrophobic lecithin bilayers of the liposomes. Light transmission experiments suggest that the liposome suspensions are significantly altered only when the added Pluronics are in the dehydrated state.
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