Order and Disorder in Liquid-Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 191

2.3 Chain Conformation of Main-Chain Polymers

During the last two decades, main-chain LC polymers have been intensively studied
by means of SANS, in particular materials based on polyesters and polyethers [ 21 ]
with relatively long flexible spacers (8-11 carbon atoms). SANS measurements in
the isotropic phase of a series of polyesters of different molecular mass [ 33 ]
indicate hR g i to be proportional to the degree of polymerization. This provides

proof of the Gaussian character of the main chain in the isotropic phase, with a
persistence length, l , of 1.6 nm, which is close to that of well-known flexible
polymers (0.8-1 nm). In spite of the large fraction of rod-like mesogenic fragments,
the main chain remains rather flexible.
In the nematic phase, SANS patterns of oriented samples show extremely
anisotropic chain conformations, the chain size parallel to n being about an order
of magnitude larger than in the perpendicular direction [ 34 - 36 ]. For example,
D'Allest et al. [ 34 ] report a ratio of gyration radii as large as R g// / R g ⊥

8, giving

for the ratio of step lengths, l // / l ⊥ 60. Under these conditions, whole chains are

forced into an elongated shape: short chains unfold and become nearly rod-like
while longer chains can show rapid reversals of chain direction - so-called hairpin
defects (Fig. 3c ) . The formation of hairpins recovers part of the entropy initially lost
during the chains straightening, due to their random placements along the chain
contour length. Upon decreasing the temperature, hairpin defects become exponen-
tially unlikely and their increasing separation causes the effective step length l // to
grow with the nematic order [ 35 , 37 , 38 ].
The number of hairpins in a nematic main-chain polymer is given by L /2 H ,
where L is the average contour length of the chain and 2 H its dimension parallel
to n. SANS of both polyesters [ 35 ] and polyethers [ 21 ] gives similar results: the
polymer chains are confined in very long (2 H

20-35 nm), thin ( R 0.8-

1.8 nm) and well-oriented (order parameter P 2 0.8-0.9) cylinders (see

Fig. 3c ) . The number of hairpins for such a cylinder varies from one to two. We
conclude that, in contrast to the situation in the isotropic phase, in the nematic
phase the chain organization of main-chain polymers is very different from that of
conventional flexible polymers. The chain conformation appears to be effectively
fully extended.
Apart from hairpins, other types of defect can be present in main-chain polymers
(see Fig. 4 ) . First, we note that chain ends represent a source of the local distortion
of the director field [ 39 ]. Furthermore, a certain number of hairpins could become
entangled. In contrast to standard hairpins, these kinds of defect cannot be removed
by applying mechanical stress. Such entangled hairpins can easily suppress chain
reptation and thus represent a source of (physical) crosslinking in the polymer
matrix. Although not being quenched, as crosslinks in elastomer networks they
introduce local sources of random orientational disorder in the director field.
Main-chain polymers seem to have little tendency to smectic phases. Only
relative recently has the synthesis been reported of some main-chain systems with
a direct transition from the isotropic to either a smectic-A [ 40 , 41 ] or a smectic-C