Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 56

cases, the transition from the LC phase to the isotropic phase triggers the actuation.
UV-driven systems contain photosensitive dyes that allow isothermal isotropization
by UV irradiation. Additionally, temperature-driven systems have been coupled
with other materials that allow the production of heat, either directly inside the
sample by an electric current, by a magnetic field, or by infrared irradiation. The
whole concept triggering the actuation will be discussed in Sect. 2.1.2 . Finally,
the direction in which a deformation occurs depends on the orientation pattern of
the LC material. This topic will be addressed in Sect. 2.1.3 .

2.1.1 LC Elastomer Structure and Strength of Actuation Properties

LCEs gain their actuation properties from the coupling between the elastomeric
network and the LC units. It causes an anisotropic chain conformation of the
network polymers if the mesogens exhibit an LC phase. Accordingly, the chains
adopt different radii of gyration parallel ( R || ) and perpendicular ( R ⊥ ) to the director.
Depending on the geometrical relation between the director and the long axis of
the polymer coil, we distinguish a prolate (parallel, R || > R ⊥ )) and an oblate
(perpendicular, R || < R ⊥ )) conformation (see Fig. 4 ) [ 10 , 48 - 50 ].
At the phase transition to the isotropic state, the polymer chain changes into
a spherical conformation, which leads to a reversible shape change. The strength of
this effect depends directly on the magnitude of chain anisotropy in the LC phase
and is therefore a function of the coupling between polymer chains and mesogens.
The architecture of the LC polymer (main-chain, side-on, end-on) has a strong
impact on this coupling behavior.
Two ways of coupling are possible, namely “through bond” due to the chemical
linkage between mesogens and polymer chain, and “through space” [ 10 , 28 , 48 , 49 ]

a

b

R

R

R R

R <R R >R

Fig. 4 (a) Oblate chain conformation: the long axis of the polymer chain is perpendicular to the
director. (b) Prolate chain conformation: the long axis of the polymer chain is parallel to the
director [ 5 ]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission