Gradient structure polyimide/graphene composite aerogels fabricated by layer‐by‐layer assembly and unidirectional freezing

Abstract In this work, gradient polyimide (PI)/graphene composite aerogels were prepared with poly(amic acid) ammonium salt/graphene aqueous solution through layer‐by‐layer 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.