Order and Disorder in Liquid-Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 226

a b c repeat unit

O

Si Si

Si Si

O O

O O

O

9 9 2

Si Si

O

repeat unit

TR5-el: Sm-A 52 °C I

TR5: siloxane spacer: crosslinker:

Fig. 21 (a) Molecular structure of the main-chain smectic system TR5 with pentaphenyl trans-
verse rods. (b) X-ray picture of the smectic-A structure of the elastomer stretched at room
temperature. (c) Structural model of the elastomer [ 160 ]

which the sample still remains transparent, a transition to a highly disordered
nematic-like state occurs. At the highest strain, the X-ray peak reveals a correlation
length x 10 nm corresponding to only four smectic layers.

Finally, we come to an example corresponding to the symmetry of Fig. 1d : the
side-on main-chain system. Again only one case has been studied in some detail
[ 155 , 160 ]. The structure of this compound (abbreviated as TR5-el) is shown in
Fig. 21a . No X-ray data are available for the homopolymer. The X-ray picture of a
stretched elastomer sample shows a smectic-A phase with an appreciable number of
harmonics (Fig. 21b ) . However, the combination of the rigid pentaphenyl rod and
the flexible siloxane main-chain leads to packing constraints that make the siloxane
chains align parallel to the rods (see Fig. 21c ). As a result, the appropriate scheme
corresponding Fig. 1d (polymer chain perpendicular to smectic layer normal) does
not apply. In agreement with the model of Fig. 21c , the high-resolution X-ray
lineshape data fit into the general trends described earlier in this section for end-on
main-chain elastomer systems.

6 Conclusions and Outlook

In this review we have discussed ordering and frustration in LC polymer networks.
In the first part, we treated the dominant role of the polymer backbone anisotropy in
shaping the specific properties of nematic polymers and elastomers. Using results of
neutron and X-ray measurements and applying some theoretical models, we have
demonstrated how orientational order induces chain anisotropy in nematic poly-
mers, which, in turn, determines their macroscopic shape. In spite of these results
there is still need for more extensive information on the anisotropic shape of LC
polymers. Such results could provide clues for the application of a greater variety of
polymers for crosslinking. Up to now, most elastomer systems use flexible silox-
anes as the polymer backbone.