Introduction - Physical Gels from Biological and Synthetic Polymers - page 11

3. The hydrophobic effect occurring with amphiphilic polymers. The insertion of hydro-
phobic functional groups into a water-soluble (hydrophilic) polymer generates this
effect, in which the hydrophobic entities tend to self-associate into micelle-like
domains. This invariably gives rise to a variety of structures depending on the
architecture of the copolymer: telechelic ( Figure 1.2f ), random-block ( Figure 1.2g ),
triblock copolymers ( Figure 1.2i ), linear or branched, hydrophobically modi ed

synthetic and bio-polymers.
4. Synergy between two different polymers ( Figure 1.2h ). This occurs in some mixtures
of polysaccharides, and some speci c synthetic polymers, which do not gel as single

components. Such a mechanism has features of both category 1 (conformational
change) and category 5 (immiscibility). Mixtures of similar triblock copolymers
with the same block length leads to immiscibility ( Figure 1.2j ).
5. Immiscibility or phase separation. This occurs mainly in binary polymer mixtures or in
single-polymer solutions when the solvent becomes poor, either at lower temperatures
or with selected organic solvents. Sometimes liquid

liquid phase separation is inhibited
by the high viscosity of the mixtures, but this is not the only mechanism, since either a
glass transition or crystallization may interfere with the phase separation. Under these
circumstances the overall morphology of the mixture can be changed from intercon-
nected paths ( Figure 1.3a and b ) to droplet-like inclusions ( Figure 1.3c ).

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As mentioned initially, the major function of gels in applications is often related to their
unique mechanical (rheological) properties. The aim in formulating gels is to try and tune
these properties from a molecular starting point, and this is the motive for instituting
chemical modi

cations, new syntheses or new biotechnological processes. Unfortunately,
even now the relation between molecular composition (in protein terms, the primary
structure), the secondary and tertiary structure (at a larger scale) and rheological properties
is far from straightforward. As mentioned before, a priori it is dif cult to predict the elastic

properties for chemical networks, where the junctions are only point-like assemblies. In
physical gels, understanding is only at its beginning and the reader may feel disappointed in
not

nding here the key to controlling rheological properties. However, some such cases can
be illuminated and we hope that this will become apparent after reading this topic.
In this volume we analyse the theoretical bases and the subtle experimental procedures
that allow us to explore the processes that lead to network formation and to determine this
special state called the . Here again, the debate is not yet closed, but we report

on some substantial improvements in analysing sol

' gel point '

gel transitions in a fundamental way
without disturbing the underlying processes, so helping to establish crucial

-

' critical '

parameters.
Innovative applications are mentioned right at the end of this topic, and make up the
last chapter. This is a rapidly growing area and there is a broad range of disciplines where
gels have found new applications. The growing areas particularly include the

biomedical area. We try to clarify not only how these new applications work, but also
how they can be better controlled or improved. Here, it is fair to point out that the
developers of such devices do not always display a comprehensive understanding of the
properties of their systems.

' smart '