The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 178

a

T (K)

b

T (K)

x = 0.08 x = 0.28

352.4 345.4

351.0

344.1

PN

N

350.0

343.2

348.4

341.0

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

n (kHz)

n (kHz)

Fig. 24 Selection of 2 H-NMR spectra of side-chain LCEs doped with (a) 8 and (b) 28 wt% of a D 2 -
8CB. Arrows denote spectral features attributed to paranematic (PN) and nematic (N) phases. Data
taken from [ 3 ]

Sect. 3.1 with various weight concentrations of a D 2 -8CB, i.e. x LC = 0.28, 0.08 and
0.006. The last of the listed samples represented a control sample. Although it
contained practically no 8CB molecules it underwent the swelling with solvents,
which resulted in the extraction of the nonreacted mesogens, the crosslinkers and
the short polymer segments, and was thus free of all the additives that might have
induced the plasticizing effect.
In Fig. 24 , we compare 2 H-NMR spectra of the above-mentioned LCEs with
x LC ¼ 0.08 and x LC = 0.28 in the vicinity of their phase transition. It is obvious from

the comparison that the behaviour of a highly doped sample appears much closer to
below-critical than the behaviour of the LCE with a smaller amount of dopant
molecules, i.e. the spectra of the former exhibit significantly broader spectral lines
as well as feature coexisting paranematic and nematic peaks at T PN-N . Also, the
high-temperature splitting (corresponding to the paranematic order parameter) is
much smaller for the x LC ¼

0.08 sample. These
observations are in accordance with the outcomes of the quantitative M 1, n ( T ) and
M 2, n ( T ) data analysis, which results in a below-critical hGi / G C ¼ 0.95 0.2 for
x LC ¼

0.28 sample than for the x LC ¼

0.2 for x LC = 0.08.
A confirmation of the above finding also follows from the calorimetric measure-
ments. Figure 25 compares the C p ( T ) responses of the highly doped x LC ¼

0.28 and a supercritical value hGi / G C ¼ 1.5

0.28

LCE and the “extracted” control sample x LC ¼ 0.006. While the latter is clearly

supercritical, as zero latent heat is released at its phase transition, a mismatch
between the ac and relaxation runs is detected for x LC ¼ 0.28 LCE. The measured

0.05 J g 1
calculated by (14) from the parameters obtained in the fit of the M 1, n ( T ) and M 2, n ( T )
experimental data.

0.05 J g 1 is in good agreement with the value 0.30 latent heat 0.24