The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 162

Fig. 12 Temperature-
dependent 2 H-NMR spectra
of a typical side-chain LCE.
Data taken from [ 3 ]

T PN-N + 2.5 K

T PN-N

T PN-N -4 K

-40 -20 0 20 40

n (kHz)

From these observations, the calorimetric measurements (Fig. 5b ) and the
observations from other studies in the search for an appropriate description of the
PN-N phase transition in LCEs, it seems that we start from two radically different,
but not mutually exclusive, assumptions:

1. LCEs are close-to critical or even supercritical . The behaviour of S ( T ) in LCEs
can be attributed to the supercritical character of the PN-N transition [ 48 ] . It was
shown in Sect. 1 (3) that a linear coupling of the nematic order parameter with a
conjugate internal or external field G (accounted for by the free-energy
term

GS ) can drive the transition into the supercritical regime, characterized
by zero latent heat and a smooth, continuous S ( T ) profile (Fig. 13a ). This occurs
whenever G exceeds the critical value G C . The effect of this field may be
regarded as similar to, e.g. the effects of a high magnetic field, which recently
has been successfully used to suppress the first-order character of the I-N
transition in polymer LCs [ 11 ] . Microscopically, this field may be regarded as
the sum of the local direct and indirect interactions between a mesogen and its
neighbouring mesogens, the polymer-backbone segments and the crosslinkers.
Such an assumption is supported by the fact that in many conventional LCE
networks with a broad transition region, nonzero values of S have been experi-
mentally determined at temperatures that are more than 20 or 30 K above the
nominal T PN-N . On the other hand, this scenario is questioned by the fact that in
most of the investigated LCEs, the latent heat has been detected at the phase
transition (see Fig. 5b and also Sect. 5 ).
2. LCEs are heterogeneous . LCEs are inherently heterogeneous materials due to the
composite nature of their structure at the nanoscopic level. Any local variations
in the concentrations of their constituents, the polymer backbone, the cross-
linkers and the mesogen, can result in a distribution of the intermolecular
coupling coefficients. This introduces a certain degree of glass-like behaviour
to the thermodynamics of the system [ 25 ].
The description of this heterogeneity may be very complicated when starting
from a microscopic model. A much more accessible picture for introducing the