Search for Ordering in a Glass Forming Liquid. Robert L. Leheny, The University of Chicago, Chicago, IL 60637.

As liquids supercool, the time scales which characterize their response to an external perturbation increase dramatically. We have performed neutron diffraction measurements [1] on supercooled D-propylene glycol, an organic molecular liquid, in a search for structural changes which we might associate with this slowing of the dynamics. We combine results from the SAD and GLAD spectrometers at IPNS, Argonne to cover a broad range of wave vector (0.01 < Q < 30 Å^-1). Our measurements span the range of relevant temperatures from room temperature, where propylene glycol's dynamics approach those of a normal liquid, to 90 K, where extrapolations predict that relaxation times have diverged. For all temperatures, the scattering intensity at small wave vector is consistent with that of a homogeneous liquid. We contrast this result with the predictions of theoretical models which introduce clustering to explain the slowing dynamics. At larger wave vectors we detect subtle but systematic temperature dependencies in the measured spectra. Using molecular dynamics simulations to model the structure factor, we identify these changes with the increasing density of the liquid and increasing orientational ordering of neighboring molecules on cooling. This orientational ordering depends strongly on the relative positions of the molecules. We discuss the possible consequences of these structural changes for the slowing dynamics.

[1] R. L. Leheny, N. Menon, S. R. Nagel, D. L. Price, K. Suzuya, and P. Thiyagarajan, J. Chem. Phys. 105, 7783 (1996).