EPR probe study of the molecular dynamics of the supercooled liquid, the glass state, and the glass transition in liquid-crystalline MBBA [n-(p-methoxybenzlidene)-p-butylaniline]

Computerized data-gathering techniques were employed to obtain electron-paramagnetic-resonance (EPR) probe molecule spectra reflecting the molecular dynamics of a typical nematic liquid crystal, MBBA, in the supercooled temperature region previously unaccessible to traditional analytical methods other than thermal analysis. Detailed line-shape simulations of the supercooled and glassy spectra recorded at 0° and 90° agree with the percolation model of glass formation, and confirm the validity of using extrapolated values of EPR and diffusion parameters from the nematic temperature region through the onset of the glass transition. Near this glass phase transition, from about 220 K and below, the spectra recorded at both 0° and 90° orientations are found to be the sums of liquid and rigid EPR spectra, providing further evidence for the coexistence of two different types of phase regions. Sudden changes in signal intensity observed at the glass transition are interpreted as resulting from EPR signal saturation effects associated with changes of $T_1$, the spin-lattice relaxation time, resulting from the change of a substantial amount of the sample from the supercooled liquid to the solidlike glass state. A faster molecular diffusion rate is observed at the glass transition for nematic liquid crystals than for isotropic liquids. This may be a result of the nematic order quenched into the glass.