The effect of mesogenic and non-mesogenic crosslinking units on the phase behaviour of side-chain smectic and cholesteric elastomers

Abstract

Chiral monomer (M₁ ), mesogenic and non-mesogenic crosslinking agents (C₁ and C₂ ), and the corresponding liquid crystalline elastomers (P₁ and P₂ series), have been synthesised. Their chemical structures have been characterised by Fourier transform infrared or ¹H nuclear magnetic resonance and their phase behaviour investigated by differential scanning calorimetry, polarising optical miscoscopy, thermo-gravimetric analysis (TGA) and X-ray diffraction. The effect of the crosslinking unit on the phase behaviour of the elastomers has been studied. M₁ showed a cholesteric oily streak and focal conic texture. C₂ exhibited a nematic enantiotropic thread-like and schlieren texture, and a monotropic fan-shaped texture in the S_A phase. Due to the introduction of the mesogenic crosslinking unit, elastomers, P_2-1 −P_2-5 , exhibited a cholesteric phase, while elastomers, P_1-1 −P_1-4 , derived from a non-mesogenic crosslinking unit, exhibit a S_A phase. As the content of the crosslinking unit increased, the T _g of the P₁ series initially decreased and then increased, and the T _i of the series decreased. In the P₂ series the T _g increased, but the T _i initially increased and then decreased. TGA confirmed that all the elastomers had improved thermal stability.

Keywords:

• liquid crystalline elastomers
• phase behaviour
• mesogenic crosslinking unit
• smectic phase
• cholesteric phase

Acknowledgements

The authors are grateful to the National Science Foundation of China (No. 50503005) and Fundamental Research Funds for the Central Universities (No. N090505002 and N090105001) for financial support during this study.