Probing the Structure of Pluronic PEO−PPO−PEO Block Copolymer Solutions with Their Apparent Volume and Heat Capacity

Abstract

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, vp,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 (vmono and cpmono). If vp,φ ≈ vmono and cpp,φ ≈ cpmono, then all the copolymer molecules are in the monomeric state. If vp,φ ≳ vmono and cpp,φ > cpmono, then the copolymer molecules are in equilibriu...