Abstract The exceptional static and dynamic physical properties of poly(urethane-urea) (PUU) elastomers make them prime candidates for impulsive loading structural applications, such as blast protection coatings. Since the theoretical physical properties of carbon nanotubes (CNTs) are among the best for any currently known material, a number of previous studies explored the use of CNTs as nanoscale fillers to enhance the properties of PUU nanocomposites. However, due to the challenges inherent in dispersing CNTs in a PUU matrix and the resulting random orientation of the CNTs, these previous works observed marginal improvements in physical properties, and were unable to establish clear structure-property relations. Here, we report the synthesis of aligned-CNT (A-CNT) reinforced PUU polymer nanocomposites (A-PNCs) by infusing A-CNT forests with a stoveable PUU, and establish process-structure-property relations that quantify the contribution of CNT confinement on the PUU mechanical response. This stoveable process was achieved using blocked isocyanate which prevented polymerization until the blocks were removed with heat. PUUs of two distinct compositions were explored: one with 40 wt% hard-segment content (PUU211) and the other with 66 wt% hard-segment content (PUU541). Thermogravimetric analysis indicates that A-CNTs enhance the thermal stability of the hard-segment phase in PUU A-PNCs at 340 °C by up to 45% over the baseline PUUs. Atomic force microscopy reveals that the elongated nanophase hard-segment formations along the CNT axis observed only in the nanocomposites were of similar characteristic size to the average inter-A-CNT spacing (∼70 nm), indicating a strong influence of A-CNTs on the size and orientation of hard-segment nanophases, as corroborated via small angle X-ray scattering. Nanoindentation testing reveals that PUU A-PNCs possess significant elastic anisotropy, and exhibit enhanced longitudinal effective indentation moduli of ∼460 MPa (>3 × that of the PUU211 baseline) and ∼1350 MPa (∼1.5 × that of the PUU541 baseline) for PUU211 and PUU541 nanocomposites, respectively. This difference in magnitude of CNT reinforcement efficacy indicates that CNT confinement leads to significant hard-segment re-organization in the PUU211 A-PNCs, whereas the interconnected network of hard-segments in the PUU541 is affected by CNT templating to a lesser extent. Dynamic nanoindentation testing results are consistent with these interpretations, where longitudinally-loaded PUU211 A-PNCs are found to exhibit a >3 × enhancement in storage modulus at 1 Hz of ∼730 MPa, whereas the longitudinally-loaded PUU541 A-PNCs exhibit a slightly enhanced storage modulus enhancement at 1 Hz of 2190 MPa (∼1.5 × that of the PUU541 baseline). Reinforcement of PUUs with A-CNTs is a promising way to tune the physical properties of the PNCs; higher A-CNT packing densities, where the inter-CNT spacing could approach the nanophase characteristic diameter, could further enhance the PUU performance in ballistic protection applications.

中文翻译：

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可加热聚（氨基甲酸乙酯-尿素）排列的碳纳米管纳米复合材料中的强过程-结构相互作用

摘要 聚（氨基甲酸酯 - 脲）（PUU）弹性体卓越的静态和动态物理性能使其成为冲击载荷结构应用（如防爆涂层）的主要候选者。由于碳纳米管 (CNT) 的理论物理性能是目前已知材料中最好的，因此之前的许多研究探索了使用 CNT 作为纳米级填料来增强 PUU 纳米复合材料的性能。然而，由于将 CNT 分散在 PUU 基质中所固有的挑战以及由此导致的 CNT 随机取向，这些先前的工作观察到物理性能的边际改善，并且无法建立明确的结构-性能关系。这里，我们报告了通过向 A-CNT 森林注入可加热的 PUU 来合成对齐的 CNT (A-CNT) 增强的 PUU 聚合物纳米复合材料 (A-PNC)，并建立量化 CNT 限制对PUU 机械响应。这种可烘烤的过程是使用封闭的异氰酸酯实现的，它阻止聚合，直到用热去除嵌段为止。研究了两种不同成分的 PUU：一种具有 40 wt% 的硬链段含量 (PUU211)，另一种具有 66 wt% 的硬链段含量 (PUU541)。热重分析表明，与基线 PUU 相比，A-CNT 将 PUU A-PNC 中硬段相在 340°C 下的热稳定性提高了 45%。原子力显微镜显示，仅在纳米复合材料中观察到的沿 CNT 轴的细长纳米相硬段形成与平均 A-CNT 间距（~70 nm）具有相似的特征尺寸，表明 A-CNT 对硬段纳米相的尺寸和取向，通过小角度 X 射线散射得到证实。纳米压痕测试表明，PUU A-PNCs 具有显着的弹性各向异性，PUU211 的纵向有效压痕模量提高了~460 MPa（> 3 × PUU211 基线）和~1350 MPa（~ 1.5 × PUU541 基线）和 PUU541 纳米复合材料。这种 CNT 增强功效的大小差异表明 CNT 限制导致 PUU211 A-PNC 中显着的硬段重组，而 PUU541 中硬段的互连网络受 CNT 模板的影响较小。动态纳米压痕测试结果与这些解释一致，其中发现纵向加载的 PUU211 A-PNC 在 1 Hz 时表现出大于 3 倍的储能模量增强，约为 730 MPa，而纵向加载的 PUU541 A-PNC 表现出轻微的在 1 Hz 时增强了 2190 MPa 的储能模量增强（~1.5 × PUU541 基线）。用 A-CNTs 增强 PUUs 是调节 PNCs 物理特性的一种很有前景的方法；更高的 A-CNT 堆积密度，其中 CNT 间距可以接近纳米相特征直径，可以进一步提高弹道保护应用中的 PUU 性能。