Molecular Origin of the Shape Memory Properties of Heat-shrink Crosslinked Polymers as Revealed by Solid-state NMR
作者：Fenfen Wang, Rongchun Zhang, Aixuan Lin, Rui Chen, Qiang Wu, Tiehong Chen, Pingchuan Sun*
關鍵字：shape memory, heat-shrink polymer, poly(ethylene-co-vinyl acetate),solid-state NMR
論文來源：期刊
具體來源：Polymer 2016, 107, 61-70
發表時間：2016年
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-co vinyl
acetate)-based HSP with mobility contrast and segmental orientation during the
heat-shrink process. In situ VT 1H T2 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 (Ea) 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 1H double-quantum experiments, which also revealed an inflection point of the chain mobility at the onset temperature
(~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 T2 relaxometry
combined with multiple-quantum NMR was confirmed to be a powerful method to
study HSPs shape memory properties.