The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 158

a b

dn ® dq

n max µ S n PN µ S PN

n N µ S N

dn µ dS

n PN n N

n max

-40 -20 0 20 40 -40 -20 0 20 40

ν (kHz) ν (kHz)

2

Fig. 9 H-NMR spectrum of a conventional side-chain LCE doped with an a D 2 -8CB liquid crystal

recorded deep in the nematic phase (a) and in the vicinity of the PN-N phase transition (b). The
spectrum (a) reflects the distribution of the domain-director alignment, and (b) reflects the
distribution of the local order parameter

B 0

Fig. 10 Typical LCE sample
geometry and the preparation
of the sandwich sample for
angular-dependent 2 H-NMR
measurements

ϑ

n

n

NMR coil

An analysis of the domain-director misalignment in LCEs is most easily conducted
with spectra that are recorded deep in the nematic phase at various relative orientations
of the LCE's principal aligning direction with respect to the external magnetic field.
Figure 10 shows a typical setup for the 2 H-NMR of LCEs allowing this. An LCE strip
with the average director orientation (denoted by ) along the strip's long dimen-

sion is carefully cut into smaller, identically sized rectangular pieces. These are
stacked together into a block, while great care is taken to ensure that the principal
aligning orientation of the LCE perfectly matches for all the stacked pieces. Such a
specimen is fitted into the coil of the NMR probehead equipped with a goniometer that
makes it possible to set up an arbitrary orientation of the sample with respect to the
external magnetic field B 0 . The orientation is specified by the tilt angle

< n >

#ffð< n >; B 0 Þ:

2 H-NMR spectra recorded at different angles

are plotted in
Fig. 11 together with the fitted theoretical spectra. The latter were calculated by
assuming that the domain-director directions are distributed uniaxially according to
the probability distribution w n,0 (

Examples of #

y ), which is given by: