Introduction - Physical Gels from Biological and Synthetic Polymers - page 7

of chains in solution. In addition, so-called such as chain ends or

closed loops do not generally contribute to connectivity and so to macroscopic proper-
ties. Network topology is regarded as a major factor in

' network defects '

uencing the elasticity of net-
works, since the number of elastically effective junctions of the network is often
signi cantly different from the total number of junctions. The determination of the

number of cross-links active in the network relies on models, whereas the stoichiometric
number of cross-links can, in principle, be measured by analytical means (such as
spectroscopy or calorimetry). In physical gels, these dif

culties are even more pro-
nounced: such factors as the course of the thermal process, the non-equilibrium state and
the slow kinetics of reorganization of labile networks induce substantial changes in
macroscopic properties. For this reason, the basic properties of chemically cross-linked
networks are outlined in Chapter 4 since they provide some of the necessary background
for understanding physical gels.

1.3.2 Hybrid organic - inorganic materials

A new area has opened up with the inorganic materials. The

formation of chemical bonds between organic and inorganic components allows
molecular composites with novel properties to be produced. We take as an example
hybrid networks containing clay particles. Clays are layered aluminosilicates (typically
silica tetrahedra bonded to alumina octahedra) present in sheet-like structures with
charge compensating counterions (such as Li + ,Na + ,K + or Ca ++ ) located in the inter-
layer spaces. One important consequence of this charged nature is that clays are
generally hydrophilic. The compatibility between organic polymers and inorganic
hosts results in systems exhibiting so-called

eldofhybridorganic -

morpholo-
gies. In intercalated structures, the organic component is inserted between the layers of
the clay in such a way that the inter-layer spacing is expanded. In exfoliated structures,
the layers of the clay have been completely separated and are randomly distributed
throughout the organic matrix; the type of layer

' intercalated ' or ' exfoliated '

' delamination ' determines the proper-

ties of the clay nanocomposites.
For example, Haraguchi and Takehisa ( 2002 ) were able to prepare a new type of
hydrogel based on poly(N-isopropylacrylamide) (poly(NIPAm)), polymerized in situ in
the presence of exfoliated, uniformly dispersed clay particles. The free radical polymer-
ization was initiated from the clay surface, without the use of an organic cross-linker, so
the clay sheet itself acts as a large cross-linker. Here it was thought that initiator was
adsorbed on to the surface of the clay particles, and the monomer and catalyst were in the
surrounding liquid. The properties of these gels are very different from those of the
polymer cross-linked with a conventional organic cross-linker. Compared to the latter,
the change of mechanical properties of the hybrid gel is impressive. For example, in the
stress

-

strain curve, the hybrid gel can reach a maximum elongation of 1000% before
fracture, and its behaviour is almost completely reversible, whereas some unswollen
chemical gels are relatively brittle. Other hybrids have been developed by intercalation of
biopolymers in clays, providing new nanocomposite materials, although this topic again
falls outside the scope of this volume.