仿生多重响应形状记忆聚氨酯结构构筑及性能调控的研究

Multi-stimuli responsive shape memory (MRSM) polyurethanes can change shapes and dimensions in real time in response to many types of extremal environmental stimuli. The more types of stimuli responded, the stronger the ability of the polymer can adapt environment and carry information. Nonetheless, the current developed polyurethanes (PU) can only response to three external stimuli through a hybrid of or grafting of functional groups into PU prepolymer system. Herein, the main research difficulties are how to enrich the stimuli types (at least four) of PU without change of its mechanical properties and existed shape memory ability. Our previous study has shown the evident MRSM feature of α-keratin fibers that consist of three key structural factors of crystal, hydrogen bond (HB) and disulfide bond (DB, can exchange roles of netpoint and switch in specific MRSM behaviors), where four stimuli responsive shape memory can be realized through copolymerizing such three factors. In this project, we propose to mimic the structure and four stimuli sensitive characteristics of α-keratin fiber, and develop two bio-inspired PU systems using 1,5-pentanediol-3-mercapto for constructing DB, N,N-bis(2-hydroxylethyl) isonicotinamine for constructing HB, polypropylene glycol (a trifunctional polymer) for chemical crosslinker and hexamethylene diisocyanate - 1,4-butanediol for physical crosslinker. The chemical and physical crosslinkers will be used for making PU membrane and filament samples, respectively. Referring to the characterization and measurement of keratin fiber’s MRSM behavior, this project intends to systematically study the MRSM of synthetic PUs under the stimuli of water, heat, ethanol, UV-light and reductant (NaHSO3) solution respectively. The relationship among components, structure and shape memory effects of synthetic PUs will be investigated, which will render the controllability of as-made PUs under each stimulus through changing the contents of DB, HB and crosslinkers, and finally optimizing the MRSM performance and establishing theoretical model of multi-stimuli responsive shape memory behaviors. The work being proposed in this project will be conducive to understand the role transition mechanism of reversible covalent bonds in MRSM and provide a novel approach for designing high intelligent materials, and will have high significance to promote the fundamental study of other natural intelligent materials and complex stimuli-responsive smart synthetic materials.

多重响应形状记忆聚氨酯（PU）可响应多种外部环境刺激而实时改变形状。响应刺激种类越多，则其适应环境及搭载信息的能力越强。然而，利用杂化和接枝功能因子的方式，目前仅实现到三重响应记忆效应。因此，如何增加PU响应种类而不影响其力学和已有的记忆能力已然成为难题。项目前期通过对角蛋白纤维多重响应形状记忆的研究，发现结晶、氢键和二硫键可组合成四重响应结构，原因是二硫键可在节点和开关之间转换。本项目拟模仿该四重响应结构，选取巯基单体构筑二硫键、吡啶基团构筑氢键、化学/物理交联点构筑节点，以此共聚合成仿生PU体系。然后，分别在水、热、乙醇、紫外线和还原剂下研究仿生PU样品的多重响应记忆效应，并表征开关和节点在记忆过程中的变化特点。通过调控开关和节点来探索其含量对应各类响应记忆的量化关系，以此构建仿生PU体系结构-记忆的理论模型。该模型对指导天然智能材料的研究和复杂刺激响应合成材料的研发都具有重要的意义。

近年来，多重响应形状记忆聚合物（MRSMPs）受到越来越多的关注。相比于传统记忆聚合物，MRSMPs可根据环境需要响应多种刺激以达到形状回复的目的。MRSMPs极大地提升了材料的行为控制度、选择自由度及复杂度，因此专家认为MRSMPs增强了传统智能材料的智能性和环境适应性，具有重要的研发意义和潜在价值。Nature和Science在不断追踪和报道MRSMPs的发展趋势，国家自然科学基金委也在强调研究环境刺激响应智能高分子的必要性。本项目呼应基金指南，研究了动物毛纤维的多重响应形状记忆性能（MRSME），明确了纤维内潜在刺激响应结构因子含量及产生MRSME的基础，即二硫键在UV光、还原剂下为开关，结晶为节点；氢键是水、还原剂和热刺激的开关，二硫键和结晶都为节点。各类开关在纤维中的摩尔含量>10%时，纤维临时形状的固定率>90%。实验结果表明，当节点种类越多、摩尔含量越高时，形状记忆过程中的形状回复率越高。参照毛纤维响应结构因子种类和含量，本项目制备出了仿生聚氨酯（PU）体系并将其加工成样品。在水、热、UV光和还原剂刺激下，仿生PU样品的临时形状固定率和形状回复率比动物毛纤维对应指标都高出20%以上，表明动物毛发的二级结构和低二硫键含量对MRSME有一定的影响。因此，项目明确了加入响应结构因子含量与对应MRSME的关系机制，特别是调控二硫键的含量如何保证反应过程的顺利实施，本项目都做了反复尝试，最终确定了制备PU高效MRSME的工艺流程方案。对PU 样品进行了各类表征来阐明结构因子体系含量对PU制品在力学和MRSME等方面的影响，实验表明化学交联点的PU样品力学指标高于同等参数下物理交联点PU样品25%，基于实验结果建立了双节点-双开关结构的形状记忆模型。更进一步，将仿生PU材料应用于柔性传感基底及传感介电层，发现MRSMPs促进了柔性传感器的耐使用性和可靠性，避免了传感器频繁校零或基线漂移，使用频次提升了80%~100%，这为MRSMPs柔性传感材料在可穿戴电子设备等系统的应用提供了广阔的前景。