The physics and mechanics governing the shape memory effect in shape memory polymers is not fully understood at the nanoscale. A very recent publication highligths a possible link between the free volume of the polymer and its ability to recover the original shape in shape memory cycles. In this study, it is demonstrated that there is a reorganisation of the monomeric unit of polymer chains via molecular sized free volume that makes the shape recovery possible. For that purpose, the free volume properties are measured in situ during the shape recovery process of a crosslinked polycyclooctene shape memory polymer by employing positron annihilation lifetime spectroscopy. By this technique, the size and distribution of the dynamic subnanoscale free volume holes are obtained by measuring the lifetime of the positrons inside the polymer at different stages of the shape recovery. (C) 2016 Elsevier Ltd. All rights reserved.