Effect of the length of the carbochain on the phase behavior of side-chain cholesteric liquid-crystalline elastomers

In this study, we prepared two series of new side-chain cholesteric liquid-crystalline elastomers (PI and PII) derived from the same chiral bisolefinic crosslinking units and different nematic liquid-crystalline monomers. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared and H-1-NMR spectroscopy. The phase behavior properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. The effect of the length of the carbochain on the phase behavior of the elastomers was investigated. The polymers of the PI and PII series showed similar properties. Polymer P, showed a nematic phase, P-2-P-7 showed a cholesteric phase, and P-4-P-7, with more than 6 mol % of the chiral crosslinking agent, exhibited a selective reflection of light. The elastomers containing less than 15 mol % of the crosslinking units displayed elasticity, a reversible phase transition with wide mesophase temperature ranges, and a high thermal stability. The experimental results demonstrate that the glass-transition temperatures first decreased and then increased; the isotropization temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking agent. However, because of the different lengths of the carbochain, the glass-transition temperatures and phasetransition temperatures of the PI series were higher than those of the PII series, and the PI and PII elastomers had their own special optical properties. (c) 2005 Wiley Periodicals, Inc.