Nontranslational Diffusive Motion on Two Different Time Scales in

High-energy-resolution quasielastic neutron scattering on three complementary spectrometers has been used to investigate molecular diffusive motion in solid nanoparticles of the alkane n-C32H66. Nontranslational molecular diffusive motion in the plastic phase occurs on two different timescales: 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. 2 It has long been known that molecular crystals can undergo one or more solid-solid phase transitions in close proximity to their melting point [1]. In the case of normal alkane molecules [n-CnH2n+2 hereafter denoted Cn], Müller studied their precursor solid phases by x-ray diffraction nearly 80 years ago. He suggested that they might be characterized by quasi-free rotation of the molecular chains about their long axis but retain long-range positional order of the molecules in three dimensions [2]. The structure [3] and associated dynamics [4−6] of these so-calle