Glass transition and molecular motions in the glassy state of a nematic liquid

Abstract

The real and imaginary parts of the complex dielectric permittivity of the supercooled nematic phase of N(o-hydroxy-p-methoxy) benzylidene-p-butylaniline have been measured from 50 to 10⁵ Hz and 77 to ∼ 40 K above its glass transition temperature (= 204 K). The nematic phase has two relaxation regions, one above and the second below T _g, designated as α and β relaxations, respectively. The half-width of the α-relaxation is 2 decades and remains constant with temperature; the half-width of β relaxation is 4 decades, and increases on cooling. Annealing reduces the amplitude of β relaxation. The rate of α relaxation follows the Vogel-Fulcher-Tamman equation, but that of the β relaxation an Arrhenius equation with an activation energy of 20 kJ mole⁻¹. The static dielectric permittivity of the isotropic, nematic and crystalline phases has also been measured. Little change in the short-range order occurs at the nematic-isotropic transition.

In its kinetic and thermodynamic behaviour, the nematic phase glass has a striking resemblance to amorphous polymers, rigid molecular glasses and orientationally disordered crystals, suggesting that the molecular mechanism of relaxations is less dependent upon the complexity of molecules or state of their aggregation than is generally believed.