Chemistry Reference
In-Depth Information
2.9.
LIQUID CRYSTAL NETWORK GELS
The swelling of a nematogenic network by either an isotropic or nematic sol-
vent is more complex than that of a classical network. In the swelling case,
the volume conserving constraints need not be applied, as the volume frac-
tion of the network is able to change by solvent exchange. The change in the
degree of swelling thereby offers alternative routes to lower the free energy.
As we know, the unique feature of nematic polymers is that molecules
change their shape, in contrast to simple nematic rods. Shape changes of
the constituent polymers of a network mean that the network too changes its
shape. This is further reflected in its macroscopic properties. The nematic
behavior couples to volume change and makes swelling of these networks
more complex than the swelling of isotropic networks.
For simplicity, we restrict our consideration to nematic polymers of pro-
late shape. This includes (i) main chain nematic polymers and (ii) side
chain nematic polymers, but in such a way that they preferentially align
with the backbone, thus giving the features of (i). This has been called
the N
III
phase. The other prolate possibility in the N
II
phase is that the
backbone takes a prolate shape while the side chains are forced to have
oblate symmetry. We shall only discuss the nematic polymers of (i), but
the method and the conclusions are quite general.
There are three combinations of the two components of nematic network
gels: (1) an isotropic network swollen by a nematic solvent, reminiscent of
the polymer dispersed liquid crystal systems (PDLC). This case (1) was
discussed by Brochard (1979) and Ballauff (1991); (2) a nematic network
swollen by an isotropic solvent was actually studied experimentally by
Carudo
et al.
(1992) and theoretically by Warner and Wang (1992a);
(3) both components can order at a temperature above the glass transi-
tion. Actually the first two systems are special cases of the last one which
has been experimentally investigated (Zentel, 1986; Barnes
et al.,
1989;
Kishi
et al.,
1994) and theoretically studied (Wang & Warner, 1997).
We now start the case (2) to focus on the new effects of nematic gels,
and then discuss the general case (3).
2.9.1.
Nematogetic network swollen by isotropic solvent
The free energy per site of gels consists of three parts
F
=
F
mix
+
F
el
+
F
nem
,
(2.152)
