Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 179

10 4

G '(68)
G ”(68)
G '(60)
G ”(60)
G '(62)
G ”(62)
G '(64)
G ”(64)
G '(66)
G ”(66)

10 3

10 2

10 1

10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6

Frequency (Hz)

Figure 6.16 Master curve of the viscoelastic spectrum of MC solution (c = 4% w/w) at high temperature (60°
C<T < 68°C). Adapted with permission from Desbrieres et al.( 2000 ) © 2000 Elsevier.

The plateau value of G 0 corresponds to a small number of junctions per chain (3 or 4) if
the rubber-like elasticity model applies. Evidently thermogelation for MC corre-
sponds mainly to a step-like shift (by three orders of magnitude) of the characteristic
terminal relaxation time of the solution, which coincides with the incipient enthalpic
transition.
In order to

find an analogue of the phase separation of MC in synthetic polymers, we
can examine the temperature dependence of the other types of HM polymers.
Hydrophobically end-capped molecules such as PEO end-functionalized with ali-
phatic groups (Renou et al., 2009 ), PEO end-functionalized with alkyl groups (Winnik
and Yekta, 1997 ) or PEO end-capped with fluorinated groups (Berret et al., 2003 ) tend to

phase separate with increasing temperature. According to La

èche and co-workers

(La èche et al., 2003 ), end-capped PEO molecules with alkyl groups behave like

adhesive spheres. When solutions with concentrations between 1% and 10% w/w are
heated, phase separation takes place, characterized by the appearance of a clear top phase
and a turbid bottom phase whose turbidity decreases with time. The bottom phase is very
viscous. At large enough concentrations (20% w/w),

' soft ' or ' hard ' gels are observed at

room temperature (in the authors ow

when they are tilted) and solutions remain clear upon heating, but they lose their elasticity
and become

' classi cation, hard gels are those which do not

fluid (40°C). Phase separation with these molecules has quite the opposite
effect of the other thermogelling systems.
Thermo-associated copolymers were obtained by grafting randomly distributed poly
(NIPAm) ( Chapter 4 ) side chains on to PAA backbones (Durand et al., 2000 ). It was shown
that the viscosity of semi-dilute solutions measured at a xed shear rate (100 s − 1 )increased

by one to two orders of magnitude around 40°C, owing to a collapse transition of the
NIPAm side chains as detected by 1 H NMR. To interpret the important loss of mobility of
the side chains, the authors suggested that the association temperature, where viscosity
increases, is correlated with the formation of NIPAmmicrodomains, with characteristics of
a glassy state. The same backbone grafted with PEO chains does not show such an effect.