The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 176

ac
relaxation

0.6

σ = 24 kPa

0.4

PN

0.2

N

σ = 91 kPa

0.0

320 330 340 350

T (K)

Fig. 22 The ac ( solid symbols ) and relaxation ( open symbols ) run heat-capacity temperature
profiles DC p and DC p,eff for the nematic main-chain LCEs stretched with different mechanical
loads (24 kPa and 91 kPa) during the second crosslinking step. Data taken from [ 7 ]

5.2.2 Temperature in the Second Crosslinking Step

In order to investigate the impact of the temperature at which the second cross-
linking step takes place, again two main-chain LCE monodomain samples were
compared [ 51 ] . Both of the samples had identical compositions and both were
prepared in the same manner as in Sect. 5.2.1. During the second crosslinking step
an identical aligning stress, exceeding the threshold aligning value, was imposed on
both samples. The only difference between the two synthetic procedures was the
temperature at which the second crosslinking step was made. This temperature was
373 K (i.e. well above the T PN-N of about 335 K) for one sample and 323 K ( i.e.
corresponding to the nematic phase) for the other sample.
The heat-capacity anomalies obtained for the two samples by ac and relaxation
runs are shown in Fig. 23 . The results show a striking effect on the phase-transition
behaviour. The LCE crosslinked in the PN phase exhibits a smeared and suppressed
C p anomaly, with only a slight mismatch between the ac and relaxation data,
yielding a latent heat of the order of the estimated experimental error
(0.05 J g 1 ). In contrast, the LCE crosslinked in the nematic phase shows a much
narrower and steeper anomaly, reminiscent of the I-N transition in LCs, with a
significant amount of latent heat ( L ¼ 0.05 J g 1 ). The temperature range in

which half of the enthalpy change occurs is three times broader for the LCE
crosslinked in the isotropic phase. This indicates that LCEs of the same composi-
tion crosslinked in the nematic phase will exhibit a TM response that is a rather on-
off type of response, in contrast to the LCEs crosslinked in the paranematic phase,
which would have a more gradual thermomechanical response. When compared to

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