The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 147

of elasticity is directly linked to the giant thermomechanical (TM) response that has
been observed in monodomain LCEs (henceforth LCEs stands for monodomain
liquid-crystalline elastomers, unless otherwise stated). The giant TM response and
its control is of particular interest for applications. The TM response approximately
follows the temperature evolution of the nematic order parameter in LCEs [ 6 , 7 ], as
illustrated in Fig. 1 , and most of it occurs in the vicinity of the I-N phase transition,
i.e. where most of the enthalpy change is observed. As a consequence, the sharpness
of the I-N transition is simply related to the temperature range in which most of the
elongation or contraction occurs. Hence, by controlling the nature of the I-N
transition it is, for example, possible to change the response from on to off
(sharp, narrow temperature range) to a continuous (gradual, broad temperature
range) response. In order to achieve this, it is important to understand the underlying
mechanisms of the I-N transition and to define all the essential physical and chemical
parameters that allow us to change its nature.
In this chapter we will give a review of recent advances in understanding the
nature of the nematic phase transition in LCEs [ 3 - 5 ] . In addition, this chapter
explores the possibilities of controlling the critical behaviour of the nematic
transition and, thus, the type of thermomechanical response. In Sect. 1, the nature
of the nematic transition will be briefly discussed for pure liquid crystals (LCs) and
LCEs. Sections 2 and 3 focus on the application of two essential experimental
techniques, high-resolution ac calorimetry and deuteron nuclear magnetic reso-
nance ( 2 H-NMR) spectroscopy, to these systems. Section 4 explains the nature of
the nematic transition in LCEs as revealed by the two techniques. Section 5 is
devoted to experimental studies that systematically explored the possibilities of
tailoring the TM response of LCEs by a variation of the chemical composition and
other parameters during the synthesis.

"continuous"

"on-off"

0 0

T I-N T I-N

T

Fig. 1 Evolution of the thermal expansion ( Dl / l ) and the order parameter ( S ) for a discontinuous
phase transition ( left ) and for a supercritical conversion ( right ), corresponding to an “on-off” and a
“continuous” response, respectively