Anisotropic polyimide (PI)/graphene composite aerogels were fabricated by unidirectional freezing. A poly­(amic acid) (PAA) ammonium salt/graphene dispersion was first synthesized by mixing together PAA, H2O, triethylamine (TEA), and graphene and then was successively subjected to one-way freezing, freeze-drying, and thermal imidization. The one-way growth of ice crystals endowed the composite aerogels with highly arranged tubular pores. The PI/graphene composite aerogels possessed anisotropic conductivity, electromagnetic interference (EMI) shielding, heat transfer, and compression performance. Moreover, the composite aerogels with low density (0.076 g·cm–3) exhibited high EMI shielding effectiveness (SE) of 26.1–28.8 dB, and its specific EMI SE value achieved 1373–1518 dB·cm2·g–1 when the graphene content was 13 wt %. The main electromagnetic interference shielding mechanism of these composite aerogels was microwave absorption. The composite aerogels had excellent thermal stability, and their 5% weight loss temperature was higher than 546 °C in nitrogen. This research provided an easy and environmentally friendly approach to prepare lightweight and anisotropic PI-based composite aerogels.

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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.

In this work, gradient polyimide (PI)/graphene composite aerogels were prepared with poly(amic acid) ammonium salt/graphene aqueous solution through layer-bylayer assembly, unidirectional freezing, freezing drying, and thermal imidization process. Each layer of gradient PI aerogels was consisted of oriented channel-like pores along the freezing direction. The gradient PI/graphene composite aerogels exhibited anisotropic conductivity and heat transfer property. The conductivity of composite aerogels in the perpendicular direction of oriented channel-like pores was higher than that along the direction of oriented pores. The heat transfer from the high-density end to the low-density end of gradient density composite aerogels was faster. Compared with those of homogeneous composite aerogel with same density, the compression yield stress of gradient density composite aerogels obviously decreased, and their compression platform region also obviously shortened. Moreover, when the compressive strain exceeded 35%, the compressive strength of gradient composite aerogel with more layers was much higher. K E Y W O R D S mechanical properties, microscopy, polyimides, porous materials, thermal properties 1 | INTRODUCTION Functional gradient materials (FGMs) are a new type of nonhomogeneous materials, their structure or composition varies gradually in spatial gradient. Although the term of FGM originates from Japan in 1980's, the gradient structure is not uncommon in nature, for example, bamboo and straw, animal skin, bones, teeth, and spider webs have gradient structure. 1-4 Because the structure or composition of FGMs varies gradually along one direction, the problems such as weak interfacial bonding or sudden change of properties, are effectively solved, which are caused by great physical or chemical differences between two type of different compositions. With the development of technology and the demand for high performance materials, gradient porous materials, whose

A kind of four‐functional epoxy resin containing tertiary ester groups (FETE) was designed and synthesized by two‐step reaction: first, the nucleophilic substitution reaction between linalool and isophthaloyl chloride, and then epoxidation. The chemical structure of obtained product was determined by infrared (IR) spectroscopy and nuclear magnetic resonance spectroscopy. Unlike traditional thermosetting epoxy resins, the cured FETE could degrade rapidly at low temperature (200–300°C). Dynamic mechanical analysis analysis showed that through copolymerization of FETE with (4,5‐epoxycyclohexane‐1,2‐dicarboxylic acid diglycidyl ester) epoxy resin (TDE‐85), the glass transition temperature (Tg) of copolymer system could be adjusted in the range of 136–179°C; FETE had good miscibility with TDE‐85. The thermal degradation behavior of the copolymer system was studied by theoretical calculation, thermogravimetric (TG), and TG coupled with FTIR analysis, the results showed the thermal decomposition performance of the copolymer system could be adjusted in the range of 200–400°C. The tertiary ester groups in FETE were easy to thermally degrade and caused the network skeleton of the copolymer system to collapse. The FETE would have good prospects in reworkable electronic packaging, recycling or degradable epoxy resin area.