Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 178

phases, thermal gelation of MC is associated with phase separation in solution, and with a
lower critical solution temperature (LCST). The hydrophobic association and entangle-
ment of these high molecular mass polymers contributes to the formation of junctions
which have a long lifetime. In the work of Li (MC sample with a degree of substitution of
1.8 and an average molecular mass M w = 310 000 g mol − 1 (SM 4000)), solutions with

concentrations of 3 nite

low-frequency storage modulus which increases with temperature. Although approaching
the solubility limit of MC at high temperature, the system shows no indication of a
signi

- 4% w/w exhibit a sol - gel transition versus temperature, with a

cant fall in the shear moduli, even at temperatures as high as 80°C. According to
the author, the Winter

Chambon criteria do not apply to this transition. The spectra
illustrated by Li resemble those of a chemical gelation with

-

cross-links, and
the transition was analysed as if it were a percolation transition versus temperature. The
solution is viscoelastic at room temperature, but exhibits a large increase in G 0 and G 00 as
temperature approaches 60°C, following the enthalpic transition.
It is known that all commercial MC samples are heterogeneous (Desbrieres et al.,
1998 ) and often poorly characterized. Li observed a structuring of the solution before the
enthalpic transition (60°C), where G 0 starts increasing, while G 00 decreased, possibly
related to local association of the hydrophobic groups. The incipient phase separation
induces a much larger organization of the entangled solution, whereby phase separation
creates large topological reorganizations which are very different from the previous
examples in this chapter.
A more detailed analysis was reported by Desbrieres et al.( 2000 ), who investigated
their specially synthesized and well-characterized MC samples. They used a similar
degree of substitution (1.7) and a viscosity average molecular mass of 140 000 g mol − 1
(Methocel A4). Rheological measurements were performed over a very large frequency
range and clearly showed two regimes: at low temperature (20

' permanent '

40°C) the viscoelastic
behaviour of an entangled solution was dominated by the non-associative part of the
macromolecular chains, with a distribution of relaxation times and a Newtonian viscosity
which decreases with temperature. During heating a discontinuous change of the terminal
relaxation time was seen in a turbid solution, corresponding to the incipient development
of strong hydrophobic interactions: the angular frequency at the crossing point (G 0 = G 00 )
varied from 1000 to 1 rad s − 1 around 60°C. This very important discontinuity was not
reported in other types of systems; it may be the signature of the phase separation of the
entangled solutions. A master curve at temperatures between 60°C and 68°C was
established, as shown in Figure 6.16 .
The master curve at the high-temperature limit clearly shows some relaxation at low
frequencies, as expected in non-permanent networks, whereas the experiments by Li do
not show this, which may in this case correspond either to unexplored lower frequencies
or to the nature of the commercial sample employed. According to Desbrieres et al.
( 2000 ), the gelled solution of MC is a non-permanent network, in agreement with the
hydrophobic association mechanisms reported in this chapter. The other interesting and
surprising feature of the viscoelastic spectrum of MC at high temperatures is that a single
relaxation mechanism can

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fit the low-frequency relaxation, as with telechelic polymers.