Effect of molecular orientation on polymer free volume distribution: An atomistic approach

Many mechanical and rheological properties of polymers are related to the amount and the distribution of its free volume. In this work, the evolution of free volume and its distribution in a linear model polymer resembling polyethylene under extensional strain are studied using a combination of molecular dynamics (MD) and Voronoi tessellation. When the molecular orientation due to stretch increases, the total number of voids in the sample decreases along with the number-average void size, while the number of larger unoccupied regions in the polymer increases, which become more elongated due to stretch. The hand-sphere probe shows that the overall free volume is decreasing during stretching; however, the reduction is not distributed evenly in the region. Free volume associated with atoms located away from the ends of molecular chains decreases while the free volume associated with atoms located at the molecular ends increases with stretch. Results from this computational work are in good agreement with several experimental observations reported in the literature.