A series of density gradient polyimide (PI) aerogels were prepared using layer‐by‐layer assembly and radial freezing method. Briefly, a layer of poly(amic acid) (PAA) ammonium salt aqueous solution was first radially frozen. Then, the second layer of PAA ammonium salt (PAS) aqueous solution of different concentration was added on the top of the first PAS layer and radially frozen. A multilayer gradient PAS solid sample could therefore be fabricated by repeating this similar procedure. The density gradient PI aerogels were obtained after freezing drying and final thermal imidization treatment. Each layer of gradient PI aerogels had anisotropic pore structure, which consisted of tube‐like pores along the radial direction and toward the center axis of the cylindrical samples. The compressive strength of five‐layer gradient PI aerogel was higher than that of three‐layer gradient and single‐layer PI aerogels with the same density. The gradient PI aerogels exhibited anisotropic heat transfer behavior in the direction of density gradient, and heat transfer from the higher density side to the lower density side was faster.

中文翻译：

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通过逐层组装制备的梯度聚酰亚胺气凝胶的性能

使用逐层组装和径向冷冻方法制备了一系列密度梯度聚酰亚胺（PI）气凝胶。简而言之，首先将聚（酰胺酸）（PAA）铵盐水溶液层径向冷冻。然后，将不同浓度的第二层PAA铵盐（PAS）水溶液添加到第一PAS层的顶部并径向冷冻。因此，可以通过重复此类似步骤来制作多层梯度PAS固体样品。在冷冻干燥和最终的热酰亚胺化处理之后获得密度梯度PI气凝胶。每层梯度PI气凝胶均具有各向异性的孔结构，该结构由沿径向方向和圆柱样品中心轴的管状孔组成。具有相同密度的五层梯度PI气凝胶的抗压强度高于三层梯度和单层PI气凝胶的抗压强度。梯度PI气凝胶在密度梯度方向上表现出各向异性的传热行为，并且从较高密度侧向较低密度侧的传热更快。