Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 57

caused by the anisotropic environment created by the alignment of the mesogens.
For LC polymer architectures in which the long axes of the mesogens are parallel to
the polymer backbone (main-chain, side-on), both effects act in the same direction
and cause a strong elongation of the chains parallel to the director (prolate confor-
mation). Comparing the two systems, we find that main-chain polymers show
stronger chain anisotropy than side-on systems. The reason is that the “through
bond” coupling in the main-chain architecture is stronger, because the mesogens are
directly incorporated into the polymer backbone. Decreasing the spacer length of
a side-on system (giving it more main-chain character) increases the coupling
between mesogen and backbone and thereby increases the chain anisotropy [ 28 , 51 ] .
In the case of end-on systems, the “through bond” coupling induces an elonga-
tion of the polymer chains perpendicular to the director, while the “through space”
coupling prefers a parallel extension of the chains. This conflict concerning orien-
tation leads to weaker chain anisotropy in end-on systems than in the two other
architectures. The direction in which the backbone elongates depends on many
factors, like the nature of the LC phase (nematic or smectic), the spacer length, and
the chemical structure of the backbone polymer [ 28 ] . Concluding, we expect the
strongest backbone anisotropy for main-chain polymers, followed by side-on and
then end-on systems. Using D-NMR and small-angle neutron scattering (SANS)
[ 50 - 58 ], it was indeed shown that the degree of backbone anisotropy decreases for
the three architectures in the order main-chain end-on.

Because the shape-changing effect has its origin in the deformed polymer chain
conformation, we expect the strongest shape change in main-chain systems, and the
weakest one in end-on systems. This prediction has been confirmed by comparing
the shape-changing capabilities of three prominent LCE systems with different
architectures (see Fig. 5 ) . The main-chain elastomer by Ahir et al. [ 47 ] changes its

side-on

> >

mainchain

side on

end on

Fig. 5 Comparison of the contraction in the direction of the director for different polymer
architectures. Data were taken from [ 18 , 47 , 59 ]