In this work, we first synthesized modified cellulose nanocrystals (MCNC) with 2,4-toluene diisocyanate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide via a two-step in-situ polymerization method, and then incorporated it with shape memory polyurethane (SMPU) to fabricate composite. It was found that the urethane chains and phosphorus-containing flame retardant were covalently grafted onto the cellulose nanocrystals surface, and the MCNC exhibited good dispersion level in the SMPU matrix. Due to the crystallization-induced and flame retardant effects of the prepared MCNC, it could be acted as a multifunctional reinforcer in SMPU composites. Noticeably, the most balanced improvements in crystallization, shape memory, mechanical, thermal stability, and flame retardancy properties of composites were observed with the incorporation of 3 wt% MCNC. Shape memory experiment demonstrated that the SMPU-MCNC3 composite exhibited admirable thermal induced shape memory behavior, and its shape fixity and shape recovery ratios were maintained above 90% after five cycles. More importantly, the SMPU-MCNC3 composite showed well balanced mechanical (σ_b = 13.1 MPa, ε_b = 189.3%), limiting oxygen index (23.8%), and UL-94 rating (V-2) among the prepared polyurethane films. In addition, the representative SMPU-MCNC3 showed obviously reductions in heat and smoke production during combustion as compared to the neat SMPU. The present work offers a promising route to fabricate shape memory polyurethane composites with versatile functions.

中文翻译：

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综合性能均衡的形状记忆聚氨酯/改性纤维素纳米晶复合材料的制备

在这项工作中，我们首先通过两步原位聚合法合成了含有 2,4-甲苯二异氰酸酯和 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide 的改性纤维素纳米晶体 (MCNC)，以及然后将其与形状记忆聚氨酯 (SMPU) 结合以制造复合材料。发现氨基甲酸酯链和含磷阻燃剂共价接枝到纤维素纳米晶体表面，MCNC 在 SMPU 基体中表现出良好的分散水平。由于制备的 MCNC 具有结晶诱导和阻燃作用，它可以作为 SMPU 复合材料中的多功能增强剂。值得注意的是，在结晶、形状记忆、机械、热稳定性、掺入 3 wt% MCNC 观察到复合材料的阻燃性能。形状记忆实验表明，SMPU-MCNC3复合材料表现出令人钦佩的热致形状记忆行为，五次循环后其形状固定率和形状恢复率保持在90%以上。更重要的是，SMPU-MCNC3 复合材料表现出良好的机械平衡（σ_b  = 13.1 MPa, ε _b  = 189.3%)、极限氧指数 (23.8%) 和 UL-94 等级 (V-2) 在制备的聚氨酯薄膜中。此外，与纯 SMPU 相比，具有代表性的 SMPU-MCNC3 显示出在燃烧过程中产生的热量和烟雾明显减少。目前的工作为制造具有多功能的形状记忆聚氨酯复合材料提供了一条有前途的途径。