Molecular origin of the shape memory properties of heat-shrink crosslinked polymers as revealed by solid-state NMR
Molecular origin of the shape memory properties of heat-shrink crosslinked polymers as revealed by solid-state NMR
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固态核磁共振揭示热收缩交联聚合物形状记忆特性的分子起源
DOI：
10.1016/j.polymer.2016.11.009
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发表时间：
2016
期刊：
影响因子：
4.6
通讯作者：
Sun Pingchuan
中科院分区：
文献类型：
--
作者：
Wang Fenfen;Zhang Rongchun;Lin Aixuan;Chen Rui;Wu Qiang;Chen Tiehong;Sun Pingchuan
在分子水平上了解热缩聚合物 (HSP) 的形状记忆特性对于先进 HSP 材料的设计和合成至关重要。在此，我们采用了各种原位变温（VT）固态核磁共振（NMR）技术，并结合其他方法，研究了热收缩过程中具有迁移率对比和片段取向的聚（乙烯-醋酸乙烯酯）基HSP的各个组分的演变。原位 VT1HT2 弛豫实验清楚地揭示了随着温度升高，HSP 中分别与稳定微晶和交联、不稳定微晶和非晶相相关的刚性、半刚性和移动组分的存在和演变。特别是，可逆转换相应主要归因于半刚性结晶成分，其在起始温度后急剧下降，并在热收缩过程中使用的最终温度完全消失。在高温下观察到与刚性交联相关的固定相。此外，热收缩过程后移动部件的活化能（Ea）下降，表明膨胀样品中变形片段的链松弛。 Baum−Pines1H 双量子实验证实了这一点，该实验还揭示了热收缩过程起始温度 (~330 K) 处链移动性的拐点，此时膨胀样品中受限的移动链几乎完全松弛。这赋予了 HSP 塑造变革的能力。此外，二维广角X射线衍射（WAXD）表明热收缩过程后HSP中晶域的弱取向消失。基于NMR和WAXD实验结果，提出了一个模型来描述HSP形状记忆特性的分子机制。最后，质子T2弛豫测量与多量子核磁共振相结合被证实是研究HSP形状记忆特性的有力方法。
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 VT1HT2relaxometry 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−Pines1H 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, protonT2relaxometry combined with multiple-quantum NMR was confirmed to be a powerful method to study HSPs shape memory properties.