Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 174

each block. A number of triblock copolymers of this family have been shown to
aggregate in the form of micelles with a core dominated by PPO and a corona dominated
by hydrated PEO blocks (Alexandridis et al., 1994 ).
Numerous publications deal with the critical micellization temperature (CMT) for
Pluronics as a function of solution concentration. Pluronics with a high PPO content form
micelles at lower concentrations and lower temperatures. Pluronics with the same hydro-
phobic segment and increasing hydrophilic segments have a small increase in their CMC
and CMT, indicating that micelle formation becomes more dif

cult for the more hydro-
philic molecules, but the effect of PEO is less pronounced than that of PPO: the primary
factor in the micellization process of Pluronics is PPO. Such studies deal with rather
dilute solutions (c ~ 1%). Being concerned here with the gelation phenomenon, we must
consider solutions with larger polymer concentrations.
Gelation of this type of polymer is basically different from those described in
Chapter 3 . Pham Trong et al.( 2008 ) analysed the correlation between micelle formation
and rheological changes of solutions of the material known as F127, where n =98and
m= 67, with increasing temperature. Microcalorimetry is a very sensitive method for
detecting micellization; the signal is endothermic during heating and perfectly reversible
on cooling. The peak width, between the onset and peak end temperatures, is about 10°C.
The peak areas of the signals were found to be directly proportional to the polymer
concentration (between 0.025% w/w and 20% w/w), suggesting that, at the end of the
enthalpic transition, almost all single polymers are included in micelles, so that the
enthalpy of micellization per chain can be derived directly. The micellization temper-
atures vary with the concentration, between 30°C and 15°C (peak maximum).
As shown in Figure 6.13 , at the end of the enthalpic peak and for the most concentrated
solution a small endothermic peak is observed which corresponds to the crystallization
temperature of the micelles. Figure 6.13 also shows the superposition of the enthalpy
signal and the rheological measurements. Three domains appear, which correspond to
three microscopic structures. Initially, in region (1) the solution is Newtonian; the
polymers are well dispersed and overall viscosity and relative viscosity versus water
both decrease as temperature increases, meaning that the polymer is less swollen; and its
hydrodynamic volume decreases, so the solvent becomes less good. In region (2)
micelles start to assemble and the viscosity increases, with no measurable storage
modulus. The micelles repel each other, as grafted brushes on colloidal particles
( ) with a large number of arms, and the solution is expected to behave as

a suspension of hard spheres. Assuming a constant aggregation number and a micelle
radius from light scattering experiments (~11 nm), the variation of relative viscosity with
micelle volume fraction should follow the phenomenological models of Krieger and
Dougherty ( 1959 ). The solvent being a single-polymer solution with a decreasing
concentration, interpretation of viscosity measurements suggests that the radius of
these micelles steadily decreases with solvent quality, consistent with this poor solvent
quality as PEO becomes less hydrophilic. Finally, region (3) corresponds to a colloidal
crystal of spherical micelles (Artzner et al., 2007 ).
For these systems, then, gelation is the transition from a colloidal suspension of
micelles which behave as hard spheres to a colloidal crystal formed of these micelles.

' hairy grains '