Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 18

Fig. 3 Schematic representation of the polymer chain conformation: spherical with R x ¼ R y ¼ R z
(a), prolate with R z > R x , R y (b), and oblate with R z < R x , R y (c). R z and R x , R y are the radii of
gyration parallel and perpendicular to the axis of highest priority (nematic director, smectic layer
normal, or helix axis), respectively

as their position and linkage within the polymer chain play an important role. The
knowledge of the chemical constitution of the polymer chain and the corresponding
chain conformation is the key to the understanding of the orientation behavior of
LC elastomers. In the following we restrict the discussion to polymers having rod
like mesogens.
For main chain polymers in the LC state the coil is strongly extended along the
director (prolate chain conformation) showing an anisotropy of the radii of gyration
parallel and perpendicular to the director of up to R || / R ⊥ 5[ 46 - 53 ]. On a local

scale the polymer chain is fully extended which is in agreement with 2 H-NMR
experiments [ 54 ] as well as theoretical predictions of Yoon and Flory [ 55 ]. For
large flexible spacers separating the mesogenic groups the existence of hairpins,
i.e., local backfolding of the polymer main chain, has been proven by SANS
experiments [ 50 , 51 , 53 , 56 ] . Investigations on smectic-C fluctuations existing in
the nematic phase showed a strong orientational coupling of the mesogenic units
and the global chain conformation, so that the whole polymer chain becomes
inclined in the layers of the S C short-range order [ 53 ]. Prolate chain conformations
also exist for smectic main chain polymers. However, the tendency of backbone
backfolding might be stronger in lamellar mesophases [ 36 - 38 ] .
For side chain polymers with side-on attached mesogenic units (Fig. 1b ) very
similar results are obtained. For short flexible linkages between the polymer
backbone and the mesogenic units, prolate chain conformations comparable to
main chain polymers are found ( R || / R ⊥

5). However, with increasing spacer
length the chain anisotropy decreases. The limit is reached using a spacer length
of x ¼ 11 ( x is the number of spacer atoms), where a nearly spherical coil is

obtained ( R || / R ⊥ 1) [ 57 - 62 ] . This can be explained by the so-called “jacketed

effect”: as a consequence of the lateral attachment to the polymer chain, the rod-like
units and the polymer backbone are oriented parallel to each other on a local scale.
For short spacers lengths the polymer backbone is stretched significantly parallel to
the mesogenic rods. Although the polymer main chain, e.g., a polysiloxane or
polyacrylate, is intrinsically flexible, it forms a highly elongated polymer coil. As
the spacer length increases this effect becomes less pronounced until these steric