The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 151

temperature profiles for the average physical quantities. Therefore, the question
remains as to whether the critical point exists for these systems, and if so, are the
LCEs confined to only one part of the temperature - field ( T-G ) phase diagram or
can they explore all possible states within it by changing some controlling
physical or chemical parameters? In order to answer this question one should
employ a method that is able to precisely determine the small quantities of latent
heat that are expected to be observed in the vicinity of the critical point.
In addition, it is essential to probe the local order parameter rather than the
average one. High-resolution calorimetry and 2 H-NMR spectroscopy, which are
introduced in the following two sections, represent a natural choice for such
methods.
2 Energy Fluctuations Near the Paranematic-to-Nematic
Conversion
2.1 High-Resolution Calorimetry
In this section, the basic principles of high-resolution calorimetry and its modes of
operation are presented. Moreover, the use of the method in order to study phase
diagrams is explained. The theoretical background of various high-resolution
calorimetric techniques was developed in the 1960s by independent groups
[ 26 - 28 ]. In the years that followed these techniques were used in numerous studies
of soft and solid materials, revealing many subtle features of phase transitions and
critical phenomena [ 29 , 30 ].
Here, we will focus on a specific high-resolution calorimetric technique, i.e.
nonadiabatic scanning calorimetry. This technique implies a calorimeter apparatus
capable of operating in the ac as well as in the relaxation mode. The combination of
these modes makes it possible to distinguish between the continuous and discontin-
uous transitions.
2.2 The ac Mode and the Relaxation Mode of Operation
In the principal ac mode of operation, an oscillating power is applied to the
investigated sample. The frequency is appropriately chosen to eliminate any tem-
perature gradients within the sample as well as to achieve good thermal equilibrium
between the sample and the bath. The heat capacity C p is derived as a function of
the applied power, the oscillating frequency and the temperature oscillations around
its mean value. The ac mode of operation is in fact sensitive only to the continuous