The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 168

a b

0.6 0.6

0.4 0.4

0.2 0.2

0.0 0.0

0 1 2 3 4 0 1 2 3 4

G / G C G / G C

Fig. 16 Smeared criticality in LCEs from the point of view of the distribution function w G ( G )of
the internal mechanical field G . The depicted distributions are calculated for hGi¼ 0.8 G C and s G

¼ 0.8

G C corresponding to an effectively supercritical LCE (b). The extent of the below-critical
component ( G < G C ) of each LCE is given by the surface of the shaded area

0.8 G C corresponding to an effectively subcritical LCE (a) and for hGi¼ 2.0 G C and s G ¼

5 Tailoring the Thermomechanical Response of LCEs
by Influencing the Critical Behaviour

In the previous sections, the basic principles of the PN-N transition in LCEs and the
experimental techniques were introduced to the reader. The issue of a “smeared
criticality” observed in LCEs was introduced in Sect. 4 . In this section, the experi-
mental results providing an insight for the understanding of the PN-N transition are
presented. These data were obtained by deuteron NMR and ac calorimetry on side-
chain and main-chain LCEs. The distinct role of each parameter that affects the
critical behaviour of the PN-N phase transition of LCEs will be demonstrated in
different subsections. These parameters influence the relative strength of the
locked-in mechanical field G and, as demonstrated in the previous sections, they
may alter the order of the PN-N transition.

5.1 Impact of Chemical Composition: Crosslinking Density

The idea that the critical behaviour of LCEs may be influenced by varying the
geometry and density of the crosslinkers was introduced in an early theoretical
work [ 23 ] . Indications that an increased crosslinking density leads to a more
gradual (more supercritical-like) thermoelastic response and birefringence temper-
ature dependence may be found in various publications [ 7 , 18 ] . Recently, the first
systematic and dedicated experimental investigation, by means of 2 H-NMR and
high-resolution calorimetry [ 4 , 5 ], demonstrated that the increase in the crosslinkers