Facile Fabrication of Mechanically Strong and Thermal Resistant Polyimide Aerogels with an Excess of Cross-Linker

In this study, we report the effect of the number of formulated repeat units (n), the type of diamine and the cross-linker content on pore morphology, structure and mechanical properties of vacuum-dried polyimide aerogels. Morphology observations demonstrate that an excess of cross-linker is conducive to increase the pore size, strengthen and stiffen the network to avoid the collapse and suppress the irreversible shrinkage during the vacuum drying procedure. For 2,2'-dimethylbenzidine derived aerogels, the pore diameter increases with increasing cross-linker content or decreasing n. The shrinkage is higher in aerogels derived from 4,4'-oxidianiline due to the chain rigidity and mobility difference compared to rigid diamine-based aerogels, resulting in lower porosity and higher density. Compared with those previously reported aerogels with the same backbone chemistries, vacuum dried polyimide aerogels herein show higher compressive strength and Young's modulus resulting from higher density and coarser skeleton networks. More importantly, isothermal aging results reveal very low increasement of shrinkage and density occurring over the aging period, which even can be ignored. The facile, low-cost vacuum drying procedure, dramatically improved mechanical properties, and superior thermal resistance will broaden the practical applications of polyimide aerogels. (C) 2020 The Authors. Published by Elsevier B.V.