Cite this: J. Mater. Chem. A, 2018, 6, 828
引用： J. Mater.化学。一， 2018， 6， 828

Received 15th October 2017
收稿日期： 2017年10月15日

Accepted 14th December 2017
录用日期：2017年12月14日

DOI:  DOI：

We utilized electrospun polyimide nanofibers as building blocks to construct a hierarchically porous architecture through freeze-drying. Superelasticity, recoverable ultimate strain of , has been obtained by thermally induced intermolecular condensation. Aerogels also possess ultralow density, high-temperature stability, low thermal conductivity and excellent performance in filtration.
我们利用静电纺丝聚酰亚胺纳米纤维作为构建单元，通过冷冻干燥构建多级多孔结构。超弹性、可恢复的极限应变 是通过热诱导的分子间缩合获得的。气凝胶还具有超低密度、高温稳定性、低导热性和优异的 过滤性能。

An aerogel is a typical porous material with low density, large porosity, a high specific surface area and low thermal conductivity. Aerogels constructed through the traditional sol-gel process always show pearl-necklace-like three-dimensional (3D) architectures. The mechanical properties are restricted by the necks between the nanoparticles. Recently, aerogels employing nanofibers as building blocks have been developed to construct physically entangled and/or chemically cross-linked 3D networks. Compared with the sol-gel made aerogels, the nanofiber-derived aerogels avoid the stress concentration of necks and exhibit flexible behavior. However, cross-linkers are commonly required to strengthen the fibrous networks. facile and efficient way to synthesize superelastic nanofiber aerogels without cross-linkers is urgently needed.
气凝胶是一种典型的多孔材料，具有低密度、大孔隙率、高比表面积和低导热系数。 通过传统的溶胶-凝胶工艺构建的气凝胶总是呈现出珍珠项链般的三维（3D）结构。 机械性能受到纳米颗粒之间的颈部的限制。 最近，使用纳米纤维作为构建块的气凝胶已被开发出来，以构建物理纠缠和/或化学交联的3D网络。 与溶胶-凝胶制成的气凝胶相比，纳米纤维衍生的气凝胶避免了颈部的应力集中，并表现出柔韧的行为。然而，通常需要交联剂来加强纤维网络。 迫切需要一种简单有效的方法来合成不含交联剂的超弹性纳米纤维气凝胶。

Owing to the excellent thermal-oxidative stability, high mechanical strength, and good radiation and solvent resistance of polyimide (PI), PI aerogels are expected to be ideal lightweight functional materials for applications in severe environments. However, the long-term use temperature of PI aerogels reported is far below that of a PI film, and volume shrinkage
由于聚酰亚胺（PI）具有优异的热氧化稳定性、高机械强度以及良好的耐辐射性和耐溶剂性， PI气凝胶有望成为恶劣环境应用的理想轻质功能材料。然而，据报道，PI气凝胶的长期使用温度远低于PI薄膜，并且体积收缩

occurs at Attempts to increase the thermal stability of PI aerogels, such as introduction of bulky groups in PI chains and addition of cellulose nanocrystals, have been reported, while the effects are very limited. Very recently, PI fiber assembled sponges have been demonstrated to be stable up to
发生于 已经报道了增加PI气凝胶热稳定性的尝试，例如在PI链中引入大块基团 和添加纤维素纳米晶体 ，但效果非常有限。最近，PI纤维组装的海绵已被证明可以稳定地达到

Here, we developed a simple thermally induced intermolecular condensation method to cross-link the skeleton of PI nanofiber aerogels without using cross-linkers. The as-prepared polyimide nanofibers aerogels (PINFAs) have extraordinary flexibility and elasticity, ultralow density, large porosity, and especially excellent high-temperature stability in dimensions. Based on the combination of the structural features of the aerogels and the specific physicochemical property of PI, potential applications including filtration for have been demonstrated. This thermal cross-linking method might be extended to many other polymeric porous materials to improve their mechanical properties.
在这里，我们开发了一种简单的热诱导分子间缩合方法，在不使用交联剂的情况下交联PI纳米纤维气凝胶的骨架。所制备的聚酰亚胺纳米纤维气凝胶（PINFAs）具有非凡的柔韧性和弹性、超低密度、大孔隙率，在尺寸上具有特别优异的高温稳定性。基于气凝胶的结构特征和PI的特定物理化学性质的结合，已经证明了包括过滤在内的 潜在应用。这种热交联方法可以扩展到许多其他聚合物多孔材料，以改善其机械性能。

As shown in Scheme 1, we used 4,4'-oxydianiline (ODA) and pyromellitic dianhydride (PMDA) to synthesize precursor pol (amic acid) (PAA), followed by preparation of PAA nanofiber membranes through electrospinning. The PAA nanofiber membranes obtained were imidized through thermal treatment
如方案1所示，我们使用4,4'-氧二苯胺（ODA）和焦溴二苯酐（PMDA）合成前体pol （酰胺酸）（PAA），然后通过静电纺丝制备PAA纳米纤维膜。所得的PAA纳米纤维膜经热处理酰胺化

Scheme 1 Description of the fabrication process of PINFAs. Yellow fibers represent polyimide nanofibers. A plausible chemical structure after intermolecular condensation in PINFAs is presented.
方案 1 PINFA 的制造工艺描述。黄色纤维代表聚酰亚胺纳米纤维。提出了PINFAs分子间缩合后的合理化学结构。
to form PI nanofiber membranes (PINFMs). The average diameter of the PI nanofibers is (Fig. S1†). The PINFMs were homogenized in dioxane to obtain a welldispersed nanofiber dispersion (Fig. S2 ). Then the dispersion was frozen. During the freezing process, the solvent in the dispersion nucleated, grew and repelled the nanofibers to the moving frozen solvent front. After being completely frozen, the PI nanofibers were randomly distributed around the solvent crystals, leaving a cellular architecture interconnected by fibrous cell walls. The fibrous cellular architecture was maintained with little volume shrinkage during the sublimation of the solvent crystals in the freeze-drying process (Fig. S ). The obtained monolith was then heated at for to cross-link the networks by intermolecular condensation.
形成PI纳米纤维膜（PINFM）。PI纳米纤维的平均直径为 （图S1†）。将PINFM在二恶烷中均质化，以获得分散良好的纳米纤维分散体（图S2 ）。然后分散体被冻结。在冷冻过程中，分散体中的溶剂成核、生长并将纳米纤维排斥到移动的冷冻溶剂前沿。 完全冷冻后，PI纳米纤维随机分布在溶剂晶体周围，留下由纤维细胞壁相互连接的细胞结构。在冷冻干燥过程中，溶剂晶体升华过程中，纤维细胞结构得以维持，体积收缩很小（图S ）。然后对获得的单体进行加热 ，以便通过分子间缩合将网络交联。

Although the thermal cross-linking resulted in a weight loss of , the volume shrinkage was negligible ( ). According to ATR-FTIR spectroscopy (Fig. S4a ), the main functional groups of PI were largely maintained after the thermal treatment. XPS spectra (Fig. S4b ) show that the intensities of C1s of the carbonyl and O1s decreased obviously, in agreement with the generation of carbon dioxide and carbon monoxide during the thermal cross-linking. The broadening peak of N1s demonstrated the diverse chemical state of nitrogen. Because of the 3D cross-linked network, the asprepared PINFAs kept intact in dioxane under sonication. In contrast, the unheated monolith could re-disperse in dioxane (Fig. S5†).
虽然热交联导致了重量损失， 但体积收缩可以忽略不计（ ）。根据ATR-FTIR光谱（图S4a ），PI的主要官能团在热处理后基本保持不变。XPS光谱（图S4b ）表明，羰基和O1s的C1s强度明显降低，与热交联过程中二氧化碳和一氧化碳的生成一致。N1s的拓宽峰表明了氮的多样化化学状态。由于三维交联网络，制备的PINFA在超声处理下在二氧六环中保持完整。相反，未加热的巨石可以重新分散在二恶烷中（图S5†）。

By using different molds, PINFA products with various shapes can be readily obtained (Fig. 1a and b). Due to the flexibility of the sample, the PINFA film prepared can be tailored into a pre-designed pattern, folded and stuck together to form a hollow cube (Fig. 1a). By controlling the concentration of nanofibers in the dispersion, we produced PINFAs with a density in the range of and a porosity of 99.0-99.6% (Table S1†). SEM images show that the PINFAs obtained have hierarchically porous architectures with major pores of about and minor pores of hundreds of nanometers (Fig. 1c-e and S6 ). From a single cell wall, physically entangled nanofibers can be observed (Fig. 1d and e). The aerogels show a BET specific surface area of , which agrees well with the calculated value of based on an assumption of cylindrical nanofibers with a diameter of observed in SEM.
通过使用不同的模具，可以很容易地获得各种形状的PINFA产品（图1a和b）。由于样品的柔韧性，制备的PINFA薄膜可以定制成预先设计的图案，折叠并粘在一起形成一个空心立方体（图1a）。通过控制分散体中纳米纤维的浓度，我们生产了密度 在99.0-99.6%范围内的PINFAs（表S1†）。SEM图像显示，所获得的PINFA具有分层多孔结构，主要孔隙约为 数百纳米，小孔隙约为数百纳米（图1c-e和S6 ）。从单个细胞壁上，可以观察到物理纠缠的纳米纤维（图1d和e）。气凝胶的BET比表面积为， 这与基于在SEM中观察到的直径为圆 柱形纳米纤维的假设的 计算值非常吻合。

Fig. 1 Macro- and micro-morphology of PINFAs. (a) Photographs showing a hollow cube manufactured through tailoring and folding a PINFA film. (b) Photograph of PINFA products prepared from different molds. (c-e) SEM images of PINFAs with a density of at different magnifications.
图1 PINFA的宏观和微观形态。（a） 显示通过裁剪和折叠PINFA薄膜制成的空心立方体的照片。（b） 用不同模具制备的PINFA产品的照片。（C-E）不同放大倍率下密度为 PINFA 的 SEM 图像。
Because of the existence of physically entangled nanofibers and chemical cross-linking between the nanofibers in the skeletons, the PINFAs exhibited extraordinary flexibility and toughness. A monolith of PINFAs with a density of could recover to its initial dimensions immediately without fracture even when crushed by a vehicle (Movie S1, ESI ). A uniaxial compression test was conducted under a compressive strain of (Fig. 2a and b, and Movie S2 in the ESI ). The PINFAs could survive in a stress of and recovered the initial size after unloading. As shown in Fig. 2c, PINFAs exhibit much better mechanical performance in recoverable compressive strain and stress compared with other nanofiberconstructed aerogels.
由于物理纠缠纳米纤维的存在以及骨架中纳米纤维之间的化学交联，PINFA表现出非凡的柔韧性和韧性。密度为 的 PINFA 整体可以立即恢复到其初始尺寸，即使被车辆碾压也不会断裂（视频 S1，ESI ）。在压缩应变下 进行单轴压缩试验（图2a和b，以及ESI 中的电影S2）。PINFA可以在应力 下生存，并在卸载后恢复初始尺寸。如图2c所示，与其他纳米纤维构造的气凝胶相比，PINFA在可恢复的压缩应变和应力方面表现出更好的机械性能。

To understand the superelasticity of PINFAs, the uncrosslinked PINFAs (un-PINFAs) and the polybenzoxazine crosslinked PINFAs (pbo-PINFAs) were also fabricated (see methods
为了了解PINFAs的超弹性，还制备了未交联的PINFAs（un-PINFAs）和聚苯并噁嗪交联的PINFAs（pbo-PINFAs）（见方法

Fig. 2 Mechanical properties of PINFAs. (a) Photographs of PINFAs recovered from a compressive strain. (b) Compressive curve of PINFAs ( ). The inset shows the magnified curve in a blue box ( from to ). (c) The recoverable maximum strain and stress of PINFAs compared with other nanofiber aerogels. (d) Compressive curves of loading-unloading fatigue cycles at an of (several selected cycles are shown). (e) Tensile curves of PINFAs. (f) Tensile and recovery test for PINFAs at an of . (g) Photograph showing a tied PINFA belt. Unless otherwise specified, the densities of all PINFAs are .
图2 PINFA的力学性能。（a） 从 压缩应变中回收的PINFA的照片。（b） PINFA的压缩 曲线 （ ）.插图在蓝色框（ 从 到 ）中显示放大的曲线。（c） 与其他纳米纤维气凝胶相比，PINFAs的可恢复最大应变和应力。（d） 装卸疲劳循环的 压缩 曲线 （显示了几个选定的循环）。（e） PINFA的拉伸 曲线。（f） PINFAs的 拉伸和恢复试验 。（g） 显示系着PINFA腰带的照片。除非另有说明，否则所有 PINFA 的密度均为 。
in the ESI ). For un-PINFAs, a plastic deformation was left after a compression, and irreversible movement of nanofibers occurred accompanied with the flattening of the original cell structure (Fig. S7 and S8 ). For pbo-PINFAs, the mechanical properties were improved and a smaller plastic deformation of was observed after a compression owing to the immobilization of the fibrous networks by polybenzoxazine (Table ). However, the introduction of additional crosslinkers to improve the mechanical properties has a few obstacles, such as the uneven distribution of the cross-linkers, the weak interfacial interactions between cross-linkers and nanofibers and so on. Besides, polybenzoxazine (decomposition temperature of ) reduced the thermal stability of the samples ( , Fig. S9 ). Herein, PINFAs show much better mechanical properties and thermal stability than pboPINFAs. This is more likely because thermally induced intermolecular condensations occurred uniformly in the entire networks and formed strong covalent bonding at the interfaces between nanofibers.
在 ESI 中）。对于非PINFAs， 压缩后会留下 塑性变形，并且纳米纤维发生了不可逆的运动，并伴随着原始细胞结构的扁平化（图S7和S8 ）。对于pbo-PINFAs，由于聚苯并噁嗪固定了纤维网络， 因此在压缩后观察到较小的 塑性变形（表 ）。然而，引入额外的交联剂来改善机械性能存在一些障碍，如交联剂分布不均匀、交联剂与纳米纤维之间的界面相互作用较弱等。此外，聚苯并恶嗪（分解温度） 降低了样品的热稳定性（ 图S9 ）。在这里，PINFAs显示出比pboPINFAs更好的机械性能和热稳定性。这更有可能是因为热诱导的分子间缩合在整个网络中均匀发生，并在纳米纤维之间的界面上形成强共价键。

In the stress-strain ( curve (Fig. 2b), we can observe three characteristic regions in the loading process, which contains a linear elastic region for with a Young's modulus of 6.1 , a plateau region for , and a densification region for with a steeply increased stress. Fig. 2d presents the recyclable compressibility of PINFAs. Plastic deformation does not appear during 1000 loading-unloading fatigue cycles at an of , which is much better than their other counterparts. SEM images also indicate the robust structure of the PINFAs, since the morphology does not show a detectable change after the 1000 fatigue cycles (Fig. S10†). Although Young's modulus and stress at an of decline in the initial 100 cycles, they keep nearly constant in the remaining cycles (Fig. S11†). Due to the energy dissipation of the fibrous framework, loading-unloading curves exhibit obvious hysteresis. The energy loss coefficient, defined as a ratio of the loop area to the area under the loading curve, shows a constant value of about 0.35 after 100 cycles. Importantly, PINFAs can even be stretched. As shown in Fig. 2e, the elongation and stress at breakage are as high as and , respectively, higher than those of other nanofiber aerogels. The loadingunloading tensile test for PINFAs at a strain of indicates that the plastic deformation gradually increases to and the maximum tensile strength keeps constant at during the 500 cycles, suggesting negligible breakage in and between nanofibers (Fig. 2f, SEM images in Fig. S12 ). Moreover, a tied PINFA belt is shown in Fig. 2g, demonstrating the tolerance of a large deformation of bending and torsion. The excellent mechanical performance can be ascribed to the rigid polyimide chain and the physical entanglements and chemical crosslinkings between the nanofibers.
在应力-应变 曲线（图2b）中，我们可以观察到加载过程中的三个特征区域，其中杨氏模量为6.1的线弹性区域 ，以及 应力急剧增加的平台区域和致密化区域 。图2d显示了PINFA的可回收压缩性。在 的 1000 次装卸疲劳循环中不会出现塑性变形，这比其他同类产品要好得多。 SEM图像还表明了PINFA的坚固结构，因为在1000次疲劳循环后，其形态没有显示出可检测的变化（图S10†）。尽管杨氏模量和应力在最初的 100 个周期中 呈 下降趋势，但它们在剩余的周期中几乎保持不变（图 S11†）。由于纤维框架的能量耗散，上下载曲线表现出明显的滞后性。能量损失系数定义为环路面积与负载曲线下面积的比值，在 100 次循环后显示约 0.35 的恒定值。重要的是，PINFA 甚至可以被拉伸。如图2e所示，伸长率和断裂 应力分别高于其他纳米纤维气凝胶。 PINFAs在应变下的加载卸载拉伸试验 表明， 在500次循环中，塑性变形逐渐增加 ，最大拉伸强度保持不变，表明纳米纤维内部和之间的断裂可以忽略不计（图2f，图S12中的SEM图像 ）。此外，系带的PINFA皮带如图所示。 2g，表明了弯曲和扭转的大变形的公差。优异的机械性能可归因于刚性聚酰亚胺链以及纳米纤维之间的物理纠缠和化学交联。

The PINFAs also exhibit excellent thermal stability. The temperatures of weight loss are about 570 and under a nitrogen and an air atmosphere, respectively (Fig. 3a), higher than the of pbo-PINFAs. When PINFAs were heated at for in air, the volume shrinkage developed slowly to , and the weight loss reached only (Fig. 3b). The heated PINFAs still could tolerate a large strain of
PINFA还具有优异的热稳定性。在氮气和空气气氛 下， 失重温度分别约为570°C（图3a），高于 pbo-PINFAs。当PINFA在空气中 加热时 ，体积收缩缓慢发展到 ，并且仅 达到重量损失（图3b）。加热的 PINFA 仍然可以承受大的

Fig. 3 Thermal performance of PINFAs. (a) TGA curves for PINFAs under a nitrogen and an air atmosphere, respectively. (b) The percentages of volume shrinkage and weight loss of PINFAs being heated at for . The densities of the samples are .
图3 PINFA的热性能。（a） PINFAs在氮气和空气气氛下的TGA曲线。（b） PINFAs在加热时 体积收缩和失重的百分比。样品的密度为 。

and completely recovered after unloading (Fig. S13a ). No obvious changes in the morphologies of the nanofibers and the hierarchical architecture of the skeleton could be found from the SEM images (Fig. S13b and ). In contrast, the nanoparticles in the sol-gel made PI aerogels melted and aggregated severely at A severe shrinkage of or more is common for sol-gel made PI aerogels at
卸货后完全恢复（图S13a ）。从SEM图像中可以发现纳米纤维的形貌和骨架的层次结构没有明显的变化（图S13b和 ）。相比之下，溶胶-凝胶制备的PI气凝胶中的纳米颗粒在 溶胶-凝胶制备的PI气凝胶 的严重收缩率 或更高时严重熔化和聚集

The ultralow density, large porosity, extraordinary flexibility and excellent high-temperature stability of PINFAs promise many fascinating applications. The acoustic absorption coefficient of PINFAs is larger than 0.6 in a broad range (Fig. S14a ). The thermal conductivity of PINFAs is between 29.7 to with increasing density (Fig. S14b ), comparable to that of other fibrous aerogels. Polyimide is also a good carbon precursor. Here, un-PINFAs were carbonized to form carbon aerogels, which exhibited good conductivity, extraordinary flexibility, and potential application in pressure-responsive sensors (Fig. S15-17†).
PINFA的超低密度、大孔隙率、非凡的柔韧性和出色的高温稳定性有望实现许多引人入胜的应用。PINFAs的吸声系数在很宽的范围内大于0.6（图S14a ）。PINFA的导热系数在29.7至 29.7之间，密度不断增加（图S14b ），与其他纤维气凝胶的热导率相当。 聚酰亚胺也是一种良好的碳前体。 在这里，非PINFA被碳化形成碳气凝胶，表现出良好的导电性、非凡的柔韧性，并在压力响应传感器中具有潜在的应用（图S15-17†）。

Particulate matter (PM) pollution causes severe harm to human health, and how to remove PM effectively is an important issue nowadays. The large specific surface area of the porous architecture and the high dipole moment of make PINFAs a good candidate for the removal of .
颗粒物（PM）污染对人体健康造成严重危害，如何有效去除颗粒物是当今的重要课题。多孔结构 的大比表面积和高 偶极矩使 PINFA 成为去除 .

The filtration efficiencies of PINFAs, a 3M 9001 particulate respirator and PINFMs with the same area and mass are , and , respectively (Fig. 4 a and b). Both PINFAs and PINFMs show ultra-high filtration efficiencies, but the pressure drop of PINFAs is only , far below the for the PINFMs. This is due to the large difference in the porosities of the materials used. The pressure drop of PINFAs is only of that of a filter, ensuring the comfort of wearing it as a mask. The long-term filtration efficiency of PINFAs was demonstrated by runs under the mass concentration of above in a hazy day for at a velocity of . The filtration efficiency was still above . The calculated sorption dynamics is about . A large number of particles attached on the nanofibers in PINFAs, and the hierarchically porous framework is preserved, promising a high
具有相同面积和质量的 PINFA、3M 9001 颗粒物呼吸器和 PINFM 的过滤效率分别为 和 和 （图 4 a 和 b）。PINFA和PINFM都显示出超高的过滤效率，但PINFA的压降仅 远低于 PINFMs。这是由于所用材料的孔隙率差异很大。PINFA的压降仅 为 过滤器的压降，确保了将其作为口罩佩戴的舒适性。PINFAs的长期过滤效率通过 在朦胧的一天中以以上的质量浓度 运行，以。 过滤效率仍在以上 。计算出的吸附动力学约为 。在PINFAs中，大量的颗粒附着在纳米纤维上，并且保留了分层多孔框架，有望实现高

Fig. 4 Removing by PINFAs. (a) Filtration efficiency and pressure drop of PINFAs, and PINFMs. (b) Number concentrations of before and after filtration. It should be noted that the number concentration of after filtration by PINFMs was too low to be presented. (c and d) SEM images of PINFAs after long-term filtration of PM 2.5 .
图4 PINFA的去除 。（a） PINFA 和PINFM的过滤效率和压降。（b） 过滤 前后的浓度数。应该注意的是，PINFM过滤 后的数浓度太低，无法呈现。（C 和 D）PM2.5长期过滤后PINFAs的SEM图像。

filtration efficiency and a low pressure drop for long term use (Fig. and d). Due to the high thermal stability of PINFAs, they are suitable for filtration of high-temperature industrial gas and automobile exhaust.
过滤效率高，长期使用压力低（图1）。 和 d）。由于PINFA的热稳定性高，适用于高温工业气体和汽车尾气的过滤。

In conclusion, we have demonstrated the preparation of PI aerogels using PI nanofibers as building blocks via freeze-drying and thermally induced cross-linking. The aerogels prepared exhibit ultralow density, outstanding dimensional stability at high temperature, and excellent mechanical flexibility and toughness. Many promising applications can be expected due to the comprehensive properties of the aerogels obtained, especially in severe circumstances. By combining the distinct characters of high performance polymers and the structural features of the aerogels, polymeric aerogels aroused great research interest recently. Our strategy of thermally induced crosslinking of the architecture without using additional crosslinkers may be extended to other systems to improve the mechanical properties of the polymeric aerogels.
总之，我们已经证明了使用PI纳米纤维作为构建单元通过冷冻干燥和热诱导交联制备PI气凝胶。所制备的气凝胶具有超低密度、高温下出色的尺寸稳定性以及优异的机械柔韧性和韧性。由于所获得的气凝胶的综合性能，特别是在恶劣的情况下，可以预期许多有前途的应用。聚合物气凝胶结合高性能聚合物的独特特性和气凝胶的结构特征，引起了近期的极大研究兴趣。我们在不使用额外交联剂的情况下热诱导交联结构的策略可以扩展到其他系统，以改善聚合物气凝胶的机械性能。

There are no conflicts to declare.
没有要声明的冲突。

The authors are grateful for the financial support from the Ministry of Science and Technology (2014CB643600) and the National Natural Science Foundation of China (51373185, 51522308, 21421061) and we thank Prof. Lin Fan and Guodong Zhang for their helpful discussion.
感谢中国科学技术部（2014CB643600）和国家自然科学基金（51373185、51522308、21421061）的资助，感谢林凡教授和张国栋教授的有益讨论。

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