Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 175

10 5

0.14

10 4

10 3

Flow (mW)

G ' (Pa)
G ” (Pa)

0.12

10 2

1 2 3

0.25

10 1

0.20

0.10

10 0

0.15
0.10

10 -1

0.05
0

0.08

10 -2

10 -3

20 21 22 23 24 25

T ( ° C)

0.06

10 -4

5 0 5 0 5 0 5 0 5 0 5 0 5 0

Temperature ( ° C)

Data for micellization of F127 versus temperature, as determined by two different experimental
methods: microcalorimetry (solid curve, showing endothermic peak) and rheological
measurements (G 0 ( □ ) and G 00 ( ● ) measured at 1 Hz) for c = 20% w/w. Adapted with permission
from Pham Trong et al.( 2008 ) © 2008 Elsevier.

Figure 6.13

For lower polymer concentrations, the crystallization peak is much smaller; possibly the
solution is partially structured into ordered domains and the rheological measurements
show a less steep temperature variation. In the latter case the transition corresponds to
that of a viscoelastic liquid. The shear moduli of the crystalline phase in Figure 6.13 show
little frequency dependence, but the loss modulus is high and close to the value of the
storage modulus. The origin of the elasticity in the colloidal crystal phase has been
discussed in the literature, and can be attributed either to the interfacial tension between
water and PPO, by analogy with an emulsion system (Hvidt et al., 1994 ), or to the
entanglement of the PEO chain (Lau et al., 2004 ), while the value of the loss modulus
could be related to friction between micelles. There is no quantitative estimate of these
contributions, and this is still an open field of investigation. During the reverse cooling

cycle, while the calorimetric signal is re

ected perfectly there is a large hysteresis in
solution rheology; much higher temperatures are needed in order to erase the thermal
history of the sample.
Another example of thermogelation is observed in solutions of poly[F127] such as
EG56® from PolymerExpert. These are copolymers composed of three linear molecules
with urethane and allophanate links (Pham Trong et al., 2008 ). Their precise architecture
is unknown; they may be branched or linear molecules. Formation of micelles during
heating is detected by microcalorimetry measurements in parallel with the increase of the
dynamic viscosity. For these associating polymers, the structure of the micelles formed
by the PPO blocks is probably more disordered (aggregation number, functionality), but
the enthalpy is approximately the same as for F127. The marked difference appearing in