Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 180

6.7 Conclusions

In this chapter, we report on a mechanism of formation of non-permanent networks
through hydrophobic interactions. Most of the examples presented are obtained by
modi

cation of existing polymers or synthesis of amphiphilic polymers with various
architectures. According to the degree of hydrophobicity (alkyl or

fluorinated chains are
the most hydrophobic ones) and the blockiness (random or block hydrophobic domains),
the HM polymers self-associate at room temperature, producing varying degrees of
modi

cation of the Newtonian viscosity.
When the viscosifying effect is not suf ciently large, addition of ionic surfactants, which

act to strengthen the hydrophobic associations, is an ef

cient way to enhance them. The
concentration of surfactant has an optimum value, above which the whole solution phase
separates. In most cases, when the temperature is increased, solutions containing HM
polymers phase separate macroscopically. Consequently this thermogelation effect requires
a subtle balance between the association of the hydrophobic groups and the solubility of the
whole polymer. The Pluronic polymers exhibit this property and are recognized as good
candidates for thermally viscosifying solutions, particularly at clinically relevant temper-
atures, and for subsequent pharmaceutical applications. Such Pluronic solutions are homo-
geneous and show no phase separation in the temperature range where micelles are created.
Methyl cellulose is the other well-known example. Here the heterogeneity of commercially
available molecules is a limitation in exploring detailed mechanisms, but the system
appears to show competition between global phase separation and local association. For
concentrated solutions of methyl cellulose, a different mechanism, related to the formation
of the liquid crystalline state, has been suggested (Yin et al., 2006 ). To be fully understood,
this type of material needs more study, particularly based on well-synthesized samples.
Thermogelling systems based on NIPAm and polyelectrolytes may suggest other, more
complex microstructures and mechanisms.

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