Compared with the irreversible shape-shifting feature of the traditional one-way shape-memory effect (1W-SME), two-way shape-memory materials (2W-SMMs) can exhibit programmable and reversible shape switching between two or more distinct shapes and show great potential in many areas such as artificial muscles and robots. The stress-free 2W-SME can be simply realized in semicrystalline polymer networks with a broad melting temperature (T_m) range; however, the working mechanism of these two-way shape-memory polymers has not been clearly understood from the view of crystalline behavior such as stress-induced crystallization and self-nucleation. Herein, we develop a series of PCL-based networks (NW-PCLDA-BA) with a broad melting temperature range (ΔT_m) via copolymerization of n-butyl acrylate (BA) and PCL-diacrylate (PCLDA) with gradient molecular weights. The stress-free 2W-SME with desirable reversible actuation was realized through the partial melting–recrystallization process of PCL crystalline domains by tuning the applied programming stress and actuating high temperature (T_high) in a rational range of ΔT_m. The influences of the self-nucleating effect and the changes in the microstructure of the crystalline domain on the stress-free 2W-SME were systematically investigated, and the mechanism of the actuation behavior was discussed. This work provides a facile strategy to achieve the tunable stress-free 2W-SME as well as the fast evaluation of the best 2W-SME performance in a semicrystalline network system.

中文翻译：

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自成核和应力诱导结晶对半晶网络可调谐双向形状记忆效应的影响

与传统单向形状记忆效应（1W-SME）的不可逆形状转换特性相比，双向形状记忆材料（2W-SMM）可以在两种或多种不同形状之间表现出可编程和可逆的形状转换，并且在人工肌肉、机器人等诸多领域显示出巨大潜力。无应力 2W-SME 可以简单地在具有宽熔化温度 (Tm) 范围的半结晶聚合物网络_中实现；然而，从应力诱导结晶和自成核等结晶行为的角度来看，这些双向形状记忆聚合物的工作机制尚不清楚。在此，我们开发了一系列基于 PCL 的网络 (NW-PCLDA-BA)，具有较宽的熔化温度范围 (Δ T _m) 通过具有梯度分子量的丙烯酸正丁酯 (BA) 和 PCL-二丙烯酸酯 (PCLDA) 的共聚。通过 PCL 晶畴的部分熔融-再结晶过程，通过调整施加的编程应力和在ΔT _m的合理范围内驱动高温 ( T _{high )，实现了具有理想可逆驱动的无应力 2W-SME}. 系统研究了自成核效应和晶域微观结构的变化对无应力2W-SME的影响，并讨论了驱动行为的机理。这项工作提供了一种简便的策略来实现可调谐的无应力 2W-SME 以及对半晶网络系统中最佳 2W-SME 性能的快速评估。