Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 327

Schematic representation of a slide-ring gel with freely moveable figure-of-eight cross-links
functioning as ' pulleys ' (arrows). Free cyclic α -cyclodextrin molecules are threaded onto the linear
polymer, forming a polyrotaxane (PR) chain. Reproduced from Ito ( 2012 ) with permission from
Nature Publishing Group.

Figure 11.1

2001 ), only more recently has a fuller physical characterization involving rheology and
scattering been carried out.
In his recent review, Ito has gone beyond the synthesis to describe the mechanical
properties of his slide-ring system (Ito, 2012 ). The modulus does not increase propor-
tionally to the amount of

' cross-linker ' but shows a weaker dependence, although this

may be because of wastage cult to estimate the fraction of elastically

effective slide rings. That said, given enough of them, there is evidence that the system
does have an equilibrium Young

- it is obviously dif

s modulus, as if the cross-links were permanent. The
overall modulus is quite low, but Ito has suggested that this re

'

ects a new type of elastic

response, which he terms . Here it is assumed that the pulley effect

relaxes stress in a way similar to the reptation processes in an entangled polymer system.
He also suggests that free rings would tend to cluster along the chain, giving rise to a
heterogeneous density distribution, which then results in substantial entropy loss.

' sliding elasticity '

11.1.2 Polyelectrolyte complexes

A polyelectrolyte complex (PEC) involves a mixture of two polymeric components, one of
which is positively charged (cationic) and the other negatively charged (anionic). As we
have already noted ( Chapter 10 ), when two polymers are mixed, the normal tendency is for
phase separation. However, if the two systems are oppositely charged, it is enthalpically
favourable for them to

' '

so that the two macromolecules are bound together by
electrostatic interaction ( Figure 11.2 ). Although this method can be used to prepare small
complexes, there is a greater interest in either building up multi-layer structures by adding

neutralize