A Non-Standard Temperature Dependence of the Order Parameter of the 5CB Liquid Crystal Doped with an Azo-Derivative

Abstract

The effects of the cis and trans form of the 4-OMe-phenylazobenzene at 1% and 7% mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal were studied using the ESR spin probe technique. The presence of the trans azo-derivative induced a downshift of the nematic-isotropic transition temperature T _NI which increased with the concentration but did not change the dependence of the order parameter on the reduced temperature. The cis isomer caused even larger T_NI shifts, determining a marked decrease of the order parameter with a peculiar dependence on the reduced temperature, quite different from the standard Haller-type behavior. The spin probe dynamics appeared instead to be essentially unchanged by the presence of the azo-derivative.

Keywords:

• biphasic systems
• complex fluids
• ESR spin probe
• nematics
• phase transitions
• photoisomerization

PACS Numbers:

• 42.70.Df
• 42.79.Kt
• 61.30.Gd
• 64.70.Md
• 76.30.-v
• 82.70.-y

Acknowledgments

We thank MIUR (PRIN “Modelling and characterisation of liquid crystals for nano-organised structures” and FIRB “Molecular and organic/inorganic hybrid nanostructures for photonics”), INSTM and the University of Bologna for support.