Langmuir, Vol.12, No.20, 4697-4703, 1996
Probing the Structure of Pluronic Peo-PPO-Peo Block-Copolymer Solutions with Their Apparent Volume and Heat-Capacity
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, eta(red), the apparent specific volume, upsilon(p,phi), and the apparent specific heat capacity, cp,(p,phi), in terms of these regions at 5, 25, and 45 degrees 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 (upsilon(mono) and cp(mono)). If upsilon(p,phi) approximate to upsilon(mono) and cp(p,phi) approximate to cp(mono), then all the copolymer molecules are in the monomeric state. If upsilon(p/phi) greater than or similar to upsilon(mono) and cp(p,phi) > cp(mono), then the copolymer molecules are in equilibrium with micelles. Finally, if upsilon(p,phi) > upsilon(mono) and cp(p/phi) approximate to cp(mono), then the copolymer molecules are all in the micellar state. At 5 degrees C, all the Pluronics studied are in the monomeric state between 0.1 and 10 g/mL. At 25 degrees C, the monomer only, monomer-micelle, and micelle only regions are all encountered depending on the molecular weight and the polyoxypropylene content. At 45 degrees 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.
Keywords:OXIDE) TRIBLOCK COPOLYMERS;AQUEOUS-SOLUTION;MICELLE FORMATION;LIGHT-SCATTERING;MICELLIZATION;THERMODYNAMICS;WATER;MODEL