Thermal characterisation of thermotropic nematic liquid-crystalline elastomers

ABSTRACT

Nematic liquid-crystalline elastomers (LCEs) are weakly cross-linked polymeric networks that exhibit rubber elasticity and liquid-crystalline orientational order due to the presence of mesogenic groups. Three end-on side-chain nematic LCEs were investigated using real-time synchrotron wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and thermogravimetry (TG) to correlate the thermal behaviour with structural and chemical differences among them. The elastomers differed in cross-linking density and mesogen composition. Thermally reversible glass transition temperature, T_g, and nematic-to-isotropic transition temperature, T_ni, were observed upon heating and cooling. By varying the heating rate, T_g⁰ and T_ni⁰ were determined at zero heating rate. The temperature dependence of the orientational order parameter was determined from the anisotropic azimuthal angular distribution of equatorial reflections seen during real-time WAXS. Results show that the choice of cross-linking unit, its shape, density, and structure of co-monomers, all influence the temperature range over which the thermal transitions take place. Including multi-ring aromatic groups as cross-linkers increased the effective stiffness of the cross-linking, resulting in a higher glass transition temperature. The nematic-to-isotropic transition temperature increased in the presence of multi-ring aromatic structures, as either cross-linkers or mesogens, particularly when the multi-ring structures were larger than the low-molar-mass mesogen common to all three samples.

Graphical Abstract

KEYWORDS:

• Side-chain liquid-crystalline elastomer
• temperature-modulated differential scanning calorimetry
• thermogravimetric analysis
• wide-angle X-ray scattering
• orientational order

Acknowledgement

The authors thank Dr Lixia Rong and Dr Maya K. Endoh (Koga) for assistance with X-ray experiments.

Additional information

Funding

Support for this research was provided in part by the National Science Foundation, Polymers Program of the Division of Materials Research, under [grant number DMR-1206010], and through the MRI Program under [grant number DMR-0520655], which provided thermal analysis instrumentation. A portion of this research was conducted at the Brookhaven National Synchrotron Light Source, beam line X27C, supported by the Department of Energy.