Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 76

a

photo-initiator, h

b

P S

d

Fig. 16 (a) Crosslinking of an FLC polymer in the polar state. (b) FLCE in bookshelf configura-
tion. The polar axis is perpendicular to the film. D d indicates the direction of the thickness change
induced by a mechanical field [ 36 ]

two orientations of the polar axis are still possible due to different tilt directions of
the smectic layers: the sample maintains macroscopically an apolar structure. A
second shear deformation applied parallel to the smectic layer planes (see Fig. 17 )
finally leads to a polar sample [ 166 , 167 ] . The influence of the second shear process
on the polarity of the sample can be determined from the evolution of the piezo-
electric constant d 33 perpendicular to the tilt direction. Initially, areas with opposite
polarity exist and the film does not show any signal, but if the mesogens become
reoriented by the shear, a signal will be measured [ 168 ] .
Starting materials for the preparation of FLCEs are depicted in Scheme 1 .
They include ferroelectric LC polymers with polymerizable groups (FLCE 1.1
and FLCE 2.2), which can be crosslinked as depicted in Fig. 3c ; reactive ferroelec-
tric monomer mixtures (FLCE 3.3, see Fig. 3d ); and mesogens with vinyl groups
that react with silanes to form either side-chain (FLCE 3.1 and BCFLCE) or main-
chain polymers (FLCE 1.2) according to Fig. 3a . Although the first three examples
(FLCE 1.1, FLCE 2.2, and FLCE 3.3) allow photocrosslinking in the neat s C *
phase, FLCE 3.1 and BCFLCE require crosslinking in solution and thus a two-stage
deformation process.