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.
