Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 131

(a) (b)

Figure 5.3

Schematic drawings of possible mechanisms of network formation in helical gels: gelation on the
helical (a) and superhelical (b) levels. In (b), no branching from helical molecules occurs. Reprinted
with permission from Viebke et al.( 1994 ) © 1994 American Chemical Society.

chemical and so do

not allow 100% of the units to be involved in a double helix. Consequently, it seems
unlikely that a perfect superhelix can be formed. Here, advances are urgently required to
improve the technique so that a

' rogue residues ' (Morris et al., 1980a ), which act as ' helix stoppers '

gel can be examined without depositing a polymer
solution on to the mica. Small-angle X-ray scattering (SAXS) data is useful for discus-
sing the structure of gels, and will be described later.
Helix formation does not always lead to gel formation, but sometimes, because the
polyelectrolyte in solution contains helices, the overall pro

' raw '

le becomes stiffened (even
rod-like, in the limit of stiffness of polymer chains) and so can form either an isotropic or
anisotropic solution, or even precipitates. Rigid (or semi- exible) polymer chains in

solution behave differently than flexible chains in many respects. Because of their rod-

like structure, there is a possibility of formation of liquid crystalline ordered phases and
gels ( Chapter 3 ). It should also be pointed out that a helix - coil transition can occur even

in a high modulus gel, as discussed below.
Figure 5.4 shows the temperature dependence of the complex Young

s modulus and of
the mechanical loss tangent from observation of the longitudinal vibration of a cylin-
drically moulded gellan gel (Nitta et al., 2001 ). This method is free from slippage which,
for these systems, sometimes causes misleading interpretations (see Chapter 2 ). On
heating, in the temperature range where the loss tangent showed a step-like change, the
ellipticity showed a steep change and an endothermic peak appeared, originating from
the helix

'

coil transition. All these changes were also observed on cooling, although with
a slight thermal hysteresis. Since the cylindrical gel is of high modulus, this indicates that
the helix

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coil transition can still occur within such a gel, and suggests that such pertinent
molecular motion is not inhibited.

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