Phase behavior and slow molecular dynamics in the glassy state and in the glass transformation of a nematic liquid crystal: 4CFPB

Abstract

ABSTRACTThe molecular mobility in 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB), a nematic liquid crystal, was studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). By TSDC a complex behavior was observed in which the main relaxation, with a wide distribution of activation energies, coalesces at high temperature with a poorly distributed mobility; this observation is in line with what was found by dielectric spectroscopy where a ‘double relaxation’ was observed. The step signal of the positional glass transition in DSC is a single and clean signal, which shows no evidence of the complexity revealed by the dielectric techniques; this signal seems to correspond to the usual main relaxation. The dynamic fragility values obtained by DSC and by TSDC indicate that 4CFPB is a moderately fragile glass former, in reasonable agreement with the values obtained by dielectric spectroscopy.Among the secondary relaxations in 4CFPB there are modes sensitive to physical aging, which makes them candidates for Johari-Goldstein’s relaxation status. It is emphasized, however, that this sensitivity to aging may not be a sufficient criterion for the attribution of this quality.