The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 146

Contents
1 The Isotropic-to-Nematic Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
1.1 The Isotropic-to-Nematic Transition in LCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
1.2 The Paranematic-to-Nematic Transition in Liquid-Crystalline Elastomers . . . . . . . . . 152
2 Energy Fluctuations Near the Paranematic-to-Nematic Conversion . . . . . . . . . . . . . . . . . . . . . . 153
2.1 High-Resolution Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
2.2 The ac Mode and the Relaxation Mode of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
2.3 Heat-Capacity Response Near the Nematic Transition
in LCs and LCEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
2.4 Temperature-Mechanical Field Phase Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
3 Distribution of the Local Order Parameter and the Domain Director Alignment in LCEs 156
3.1 NMR of Deuterium-Labelled Mesogens in Liquid-Crystalline Materials . . . . . . . . . . . 156
3.2 2 H-NMR Spectra of Nematic LCs and LCEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
3.3 Inhomogeneity of the Director Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
3.4 Inhomogeneity of the Local Order Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
4 Smeared Paranematic-to-Nematic Phase Transition in LCEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
4.1 LCEs: Heterogeneous, Supercritical or Both? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
4.2 Distribution of Landau-de Gennes Expansion Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 165
4.3 Smeared Criticality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
5 Tailoring the Thermomechanical Response of LCEs by Influencing the Critical Behaviour 170
5.1 Impact of Chemical Composition: Crosslinking Density . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
5.2 Impact of Synthesis Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
5.3 Impact of Processing: Swelling with Low-Molar-Mass Nematogen . . . . . . . . . . . . . . . . 179
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Appendix: Derivation of 2 H-NMR Spectral Moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Abbreviations

2 H-NMR Deuteron nuclear magnetic resonance
I

Isotropic

LC Liquid crystal

LCE Liquid-crystalline elastomer

LdG Landau-de Gennes

N Nematic

OP Order parameter

PN Paranematic

TM Thermomechanical

1 The Isotropic-to-Nematic Transition

A recently revived topic in the field of liquid-crystalline elastomers (LCEs) is the
nature of the isotropic-to-nematic (I-N) transition in connection with the type of
elasticity in these materials, being either soft, semisoft or nonsoft [ 1 - 5 ] . The type