Molecularly-based numerical evaluation of free volume in amorphous polymers

A study of the microstructure of polymers is essential for the understanding of their anomalous diffusion properties. The concept of free volume may be utilized to study relaxation phenomena and diffusion processes in polymers. Since a quantitative definition of free volume is not possible, a new concept called available volume is introduced. A simple geometric definition of available volume, relevant for the diffusion of small molecules in polymers, is given, based on the molecular structure. An initial molecular structure is obtained using a modified RIS (rotational isomeric state) approach. Then, with the aid of a molecular mechanics-based minimization procedure, computationally generated molecular structures for some polymers are used to estimate the available volume, using the proposed geometric definition. Numerically evaluated available volume distributions are then compared for different types of amorphous polymers, e.g. polycarbonates and polyacrylates, stressing their relevance to the diffusion properties.