Localized diffusive motion on two different time scales in solid alkane nanoparticles

¹ Department of Physics and Astronomy and University of Missouri Research Reactor, University of Missouri Columbia, MO 65211, USA
² Spallation Neutron Source, Oak Ridge National Laboratory - Oak Ridge, TN 37831, USA
³ Center for Neutron Research, National Institute of Standards and Technology - Gaithersburg, MD 20899-6102, USA
⁴ Department of Materials Science and Engineering, University of Maryland - College Park, MD 20742, USA
⁵ Facultad de Física, Pontificia Universidad Católica de Chile - Santiago, Chile

^a taubh@missouri.edu

Received: 26 July 2010
Accepted: 3 September 2010

Abstract

High-energy-resolution quasielastic neutron scattering on three complementary spectrometers has been used to investigate molecular diffusive motion in solid nano- to bulk-sized particles of the alkane n-C₃₂H₆₆. The crystalline-to-plastic and plastic-to-fluid phase transition temperatures are observed to decrease as the particle size decreases. In all samples, localized molecular diffusive motion in the plastic phase occurs on two different time scales: a “fast” motion corresponding to uniaxial rotation about the long molecular axis; and a “slow” motion attributed to conformational changes of the molecule. Contrary to the conventional interpretation in bulk alkanes, the fast uniaxial rotation begins in the low-temperature crystalline phase.