Liquid Crystal Elastomers and Light - Liquid Crystal Elastomers: Materials and Applications - page 108

number density of mesogens, and, since T c ¼ rU= 5 k , it reduces the transition

temperature. As can be seen from ( 4 ) , this is equivalent to increasing the tempera-
ture. This point is discussed in detail in [ 66 ] . The effects of photoisomerization
alone are very similar to heating. In addition to photoisomerization, azo and similar
compounds also absorb a considerable amount of light, and the temperature conse-
quently rises. Both mechanisms - heating and conformational changes - are
necessarily both present. Determining the relative importance of these two
mechanisms is a challenging problem; measurements directed at addressing this
question are under way [ 67 ] .

3.1.4 Indirect Optical Torque

In 1990, Janossy showed that a small amount of dye added to a nematic liquid
crystal dramatically reduces the threshold intensity of the optical Freedericksz
transition [ 68 ] . Subsequently, it was demonstrated that the underlying process is
an optically driven Brownian ratchet mechanism [ 69 - 71 ]. Here, energy, but not
momentum, from the radiation field causes unidirectional continuous rotation of
dye molecules in the nematic, exerting a torque on the director that exceeds the
direct optical torque by orders of magnitude. Similar mechanisms could, in princi-
ple, be realized in LCEs. Whether such processes are viable in overcoming the
orienting effect of the network is not clear; the viability of such Brownian motor
processes in LCEs is an intriguing open problem.
Since direct optical forces and torques are too small to overcome the orienting
effects of the network, viable photoactuation processes must all involve effective
energy transfer from the radiation field to the material. Some illustrative examples
are given in Sect. 3.2.

3.2 Photoactuation

In a seminal work exploring in LCEs, photoactuation, Finkelmann and coworkers
incorporated azo-group containing crosslinkers into a siloxane-based side chain
nematic LCE. They found that a monodomain sample will contract along the
director on UV illumination [ 66 ], and will recover to its original shape in the
dark. The response of the system is shown in Fig. 8 ; the characteristic response
times are tens of minutes. One possible mechanism here, not discussed in [ 66 ] , is
the contraction of crosslinks formed by the azo compounds, which would induce
anisotropy (contraction along the polarization direction) even in isotropic rubbers.
Heating, which certainly takes place on illumination, reduces the orientational
order. Since the order parameter tensor plays the role of stress, a reduction of
order gives rise to contraction along the director. It is pointed out that the effects of
trans - cis photoisomerization are not readily distinguishable from heating through
absorption. This work introduced a new optomechanical effect in solids, and related