Bret Mayo

Research Specialist
Center for Nanoscale Science and Engineering
North Dakota State University

Molecular free volume is commonly defined as the total space occupied by a material minus the space occupied by its constituent atoms. The concept has been known to polymer scientists for decades and has served as a valuable tool to model behavior of some materials and offer a means to simply visualize complex phenomena. However, it is immediately obvious upon consideration of this simple definition that the concept can be extended to any material, not just polymers.

The concept of molecular free volume has been used to help describe viscosity, diffusion of solvents in polymers, polymer blend compatibility, and glass transitions. Most of the molecular free volume based models employ empirical parameters. Consequently, there are numerous reports of systems that do not follow the various molecular free volume models. However, most models seem to focus solely on the total molecular free volume in a system, ignoring the distribution or redistribution of molecular free volume within a material.

Based on some early work done at Battelle Memorial Institute and the University of Missouri (KC) a few years ago, it seems clear that positron annihilation spectroscopy (PAS) is one tool which can lead to understanding some of the relationships between molecular free volume and material performance. Physicists had used various materials to study positron behavior as early as the late 1950's. Early pioneering work at Battelle changed this perspective by 180 o when positrons were used to study materials. This work showed convincingly that PAS could be used to study molecular free volume directly. Perhaps more importantly, the positron's ability to seek out atomic-scale voids and provide data on both the size and distribution of the voids (and sometimes even the shape of the voids) makes PAS a unique tool for characterizing materials of all kinds.