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, 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.

Volumes of aqueous block copolymers based on poly(propylene oxides) and poly(ethylene oxides) in a large temperature range: A quantitative description

“…The V U of EO 13 PO 30 EO 13 (L64) [22,23] at 293 K, 298 K and 301 K and EO 76 PO 28 EO 76 [28] at 308 K and 318 K were determined. Camiré et al [29] measured the V U of EO 12 PO 67 EO 12 at 283 K, 295 K and 308 K whereas Meilleur et al [27] determined the v sp of different EO a PO b EO a at 278 K, 298 K and 318 K.…”

“…The standard enthalpy [15,16,24,25] of micellization is usually evaluated from the cmc dependence on temperature [36] by assuming that the heat capacity of micellization is null. This is not a general rule since a negative change of the heat capacity is obtained for these copolymers [27,29,32,33,35] in agreement with the hydrophobic desolvation which accompanies the micelles formation [37][38][39]. Scarce are the investigations of the thermodynamic properties (enthalpy [17,18], density [18][19][20][21][22][23]26,27,30], compressibility [20,21,26], expansibility [30] and heat capacity [26,27,29]) obtained from direct measurements.…”

Evaluation of the Physical‐Chemical Properties of Poly(ethylene oxide)‐block‐Poly(propylene oxide) by Different Characterization Techniques

“…As shown in the literature, [50] a value of K H < 0.5 is expected for molecules that do not interact, and higher values for the processes of micellization and phase separation.…”

“…Above 20 8C, the intrinsic viscosity decreases, suggesting a more compact arrangement of the micelles as the temperature increases. Others [50] have attributed this compaction to the intermolecular interactions. For the copolymer C 12 -(EO) 6 -(PO) 5 -OH the intrinsic viscosity begins falling off at the first increase in temperature from 10 to 15 8C.…”

Effect of drying on the mesoporous structure of sol–gel derived silica with PPO–PEO–PPO template block copolymer

“…The triblock copolymers form spherical or cylindrical micelles in aqueous solution above the critical micellization temperature (CMT) and critical micellization concentration (CMC) [45][46][47]. In general, the copolymer in an aqueous solution undergoes two relaxation processes: a fast process attributed to the micellization of the surfactant molecule (µs-ms time range), and a slower process due to rearrangement of the micelles and the formation of liquid crystalline phase (ms-s time range) [48][49][50].…”