General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 109

to discontinuous swelling. This immediately suggests a way to construct practical
devices in which the so-called volume phase transition can be induced, say by changes
in temperature (or voltage, pressure etc.). The kinetic aspects can be overcome by
combining micro-gel particles, and a number of potential applications of so-called
' gels can be formulated.

A thermodynamic system capable of transforming chemical energy directly into
mechanical work is referred to as a chemomechanical system. The history of such
materials goes back to such distinguished names in polymer science as Kuhn and
Katchalsky in the early 1950s. All living organisms move by the isothermal conversion
of chemical energy into mechanical work, as exempli

smart ' or ' intelligent '

ed by muscle, flagellar and ciliary

movements. Such highly ef cient energy conversion systems can be realized, at least

potentially, in synthetic polymer gels, which expand and contract upon changing their
solubility and/or degree of ionization in response to an external stimulus in the form of
thermal, chemical or electrical energy (Kaneko et al., 2002 ).
As mentioned, hydrogels prepared from poly(NIPAm) PAm which show so-called

lower critical solution temperature (LCST) behaviour (see Chapter 10 ) in aqueous solution
are thermoresponsive, and expand/contract below/above the phase transition temperature.
Another type of gel containing ionic groups can be actuated isothermally by an electric

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field. When the gel is negatively charged, it swells near the anode and contracts near the
cathode. The contraction rate is proportional to the external electric current, and changes in
volume of, say, ±50% are quite usual. Such a polyelectrolyte gel can be made to bend
backwards and forwards by the application of an alternating electric

field. Here water and
ions migrate towards the electrode, bearing a charge opposite in sign to the net charge in the
gel, and this coupling of electro-osmosis and electrophoresis is thought to be responsible
for the observed chemomechanical behaviour (Osada and Ross-Murphy, 1993 ). For a
long time development of such devices seemed not to progress beyond lab ,but

work by, for example, Siegel and co-workers (Siegel et al., 2004 ) has led to novel swelling/
de-swelling of glucose-binding hydrogels (Kataoka et al., 1998), for use in oscillatory
feedback insulin delivery systems.

' toys '

4.4 Transient networks

The dynamical mechanical moduli of covalent networks have been extensively studied in
relation to the mechanical properties of rubbers and elastomers; the chemistry has been
outlined above. A second type of network, entangled networks, has also been extensively
studied in relation to the flow characteristics of polymer melts (Graessley, 1974 ; Doi and

Edwards, 1986 ). The various mechanisms for stress relaxation and

flow of melts and
semi-dilute polymer solutions are now well understood. In the widely accepted

' repta-

'

tion model, each chain acts as if it is surrounded by a restrictive virtual tube formed by all

the other chains in the matrix. This modi

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a number of transport properties.
The effect of entanglements on the relaxation of polymer chains is seen in the spectrum of
shear modulus versus frequency, which shows a plateau which grows with the molecular
mass (more commonly referred to in this area as molecular weight). The plateau value is

es restricts