The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 161

one can extract information about the average nematic order parameter and its
dispersion:

ð Sw S ðSÞ

M S ; 1 d S; (11a)

and:

ð S M S ; 1

2 w S ðSÞ

M S ; 2 d S: (11b)

The degree of the domain misalignment in (10) is hidden in the coefficients S 2
and S 4 (see Appendix for details).

4 Smeared Paranematic-to-Nematic Phase Transition in LCEs

In this section, the nature of the PN-N phase transition in LCEs will be presented, as
revealed by 2 H-NMR and supported by ac calorimetry. It will be demonstrated that
the smooth phase transition in LCEs is a manifestation of both the field conjugate to
the local order parameter (which in LCEs takes up close-to critical values) and the
pronounced heterogeneity of LCEs, in the sense of distributed random fields. Partic-
ular attention will be paid to the description of a simple model based on the LdG
approach, which considers these two features of LCEs. This model predicts the
temperature profiles of the first and the second moment of the 2 H-NMR spectral
lines. The accordance of this model with the experimental results will be discussed.
Finally, the idea of a “smeared criticality” in LCEs will be presented.

4.1 LCEs: Heterogeneous, Supercritical or Both?

Figure 12 shows three 2 H-NMR spectra of a typical nematic LCE that were
recorded at temperatures close to the PN-N transition. Three observations about
these spectra should be noted:

1. The temperature evolution of the average resonant frequency of a particular
spectral line is continuous, which implies a continuous temperature dependence
of the local order parameter
2. Even at high temperatures there is a nonzero splitting of the spectral doublet,
which indicates a residual nonzero paranematic order
3. Spectral lines are pronouncedly broadened at temperatures close to the phase
transition