Molecular origin of the shape memory properties of heat-shrink crosslinked polymers as revealed by solid-state NMR

Understanding the shape memory properties of heat-shrink polymers (HSPs) at the molecular level is crucial for the design and synthesis of advanced HSP materials. Herein, we employed a variety of in situ variable-temperature (VT) solid-state nuclear magnetic resonance (NMR) techniques, in combination with other methods, to investigate the evolution of the individual components of a poly(ethylene-covinyl acetate)-based HSP with mobility contrast and segmental orientation during the heat-shrink process. In situ VT H-1 T-2 relaxometry experiments clearly revealed the presence and evolution of rigid, semi-rigid and mobile components associated with stable crystallites and crosslinkage, less-stable crystallites and the amorphous phase in HSPs with increasing temperature, respectively. In particular, the reversible switching phase should be predominately attributed to the semi-rigid crystalline components, which dramatically decreased after the onset temperature and completely disappeared at the end temperature used in the heat-shrink process. The fixed phase associated with the rigid crosslinkage was observed at high temperatures. Furthermore, the activation energy (E-a.) of the mobile components decreased after the heat-shrink process, indicating the chain relaxation of deformed segments in the expanded sample. This was confirmed by Baum Pines H-1 double-quantum experiments, which also revealed an inflection point of the chain mobility at the onset temperature (similar to 330 K) of the heat-shrink process, at which the restricted mobile chains in the expanded sample are nearly completely relaxed. This imbues HSPs with the ability to shape change. In addition, two-dimensional wide-angle X-ray diffraction (WAXD) indicated that the weak orientation of crystalline domains in HSP disappears after the heat-shrink process. Based on the NMR and WAXD experimental results, a model was proposed to describe the molecular mechanism underlying HSPs' shape memory properties. Finally, proton T-2 relaxometry combined with multiple-quantum NMR was confirmed to be a powerful method to study HSPs shape memory properties. (C) 2016 Elsevier Ltd. All rights reserved.