Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 62

phase transition, such samples show different types of deformation [ 86 , 87 ] .
The orientation pattern can be controlled by experimental parameters in larger
particles prepared by polymerization in a microfluidic system. Using different
amounts of shear during polymerization, Ohm et al. succeeded in preparing two
types of particle samples with a concentric and a bipolar orientation, respectively
[ 87 ]. Upon heating through the NI phase transition, the concentric alignment
produces an expansion perpendicular to the symmetry axis while the bipolar
samples contract.

2.2 Introduction of Actuator Systems

We will discuss several strategies to tether the properties of LCE actuators to
certain specifications, and then present examples of actuator systems of different
shapes and different domain sizes. We shall differentiate between macroscopic and
microscopic LCE actuators. Films (Sect. 2.2.1 ) and fibers (Sect. 2.2.2 ) typically
have at least one macroscopic dimension. Microscopic actuation systems from
LCEs have received much interest lately and have recently been the subject of
a specialized review [ 88 ] . In Sect. 2.2.3, we will discuss micrometer-sized actuators
that are fixed on a solid substrate to yield stimuli-responsive surfaces. In Sect. 2.2.4,
we will review several methods for preparation of colloid-like actuators that are
freely suspended in air or in a surrounding liquid.

2.2.1 Film Samples from LC Elastomers

The concept of actuation in aligned LCEs was first demonstrated in film samples.
Uniform orientation was achieved by mechanical stretching [ 89 ] of pre-crosslinked
films and subsequent full crosslinking under a load [ 18 , 90 , 91 ] . The shape change
of such a film is displayed in Fig. 7a . Subsequently, techniques using magnetic
fields [ 33 ] and uniform substrate layers [ 40 , 59 , 80 , 92 , 93 ] were utilized to achieve
alignment without the need for the two-step crosslinking procedure. In these cases,
the films were usually prepared on solid substrates and released after polymeriza-
tion by a sacrificial layer. An example of that kind of specimen is given in Fig. 7b .
Due to the flat geometry, films allow a defect-free director pattern leading to an LC
monodomain with a high value of the order parameter. This results in strong
deformations during the NI phase transition.
LCE films typically have lateral dimensions of several centimeters and a thick-
ness of hundreds of micrometers. This large size makes the samples ideally suited
for mechanical testing as well as for X-ray investigations. Accordingly, most
fundamental studies have been carried out on such films. One disadvantage of
film samples is the large amount of LC material often needed. This has been
overcome by a technique introduced by Ikeda and coworkers [ 94 ] . By laminating
a thin layer of aligned LCE material onto a nonresponsive plastic film, a composite