title: Effect of Bulky Substituents in the Polymer Backbone on the Properties of Polyimide Aerogels.
abstract: With unique advantages over inorganic aerogels including higher strengths and compressive moduli, greater toughness, and the ability to be fabricated as a flexible thin film, polymer aerogels have the potential to supplant inorganic aerogels in numerous applications. Among polymer aerogels, polyimide aerogels possess a high degree of high thermal stability as well as outstanding mechanical properties. However, while the onset of thermal decomposition for these materials is typically very high (greater than 500 °C), the polyimide aerogels undergo dramatic thermally induced shrinkage at temperatures well below their glass transition (Tg) or decomposition temperature, which limits their use. In this study, we show that shrinkage is reduced when a bulky moiety is incorporated in the polymer backbone. Twenty different formulations of polyimide aerogels were synthesized from 3,3,'4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-oxidianiline (ODA) or a combination of ODA and 9,9'-bis(4-aminophenyl)fluorene (BAPF) and cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC) in a statistically designed study. The polymer concentration, n-value, and molar concentration of ODA and BAPF were varied to demonstrate the effect of these variables on certain properties. Samples containing BAPF showed a reduction in shrinkage by as much as 50% after aging at elevated temperatures for 500 h compared to those made with ODA alone.
authors.affiliation: NASA Glenn Research Center , 21000 Brookpark Road, Cleveland, Ohio 44135, United States.
authors.forename: Rocco P
authors.identifier: http://orcid.org/0000-0001-7538-6700
authors.initials: RP
authors.lastname: Viggiano
authors.affiliation: NASA Glenn Research Center , 21000 Brookpark Road, Cleveland, Ohio 44135, United States.
authors.forename: Jarrod C
authors.initials: JC
authors.lastname: Williams
authors.affiliation: Case Western Reserve University , 2100 Adelbert Road, Cleveland, Ohio 44106, United States.
authors.forename: David A
authors.identifier: http://orcid.org/0000-0001-5111-0558
authors.initials: DA
authors.lastname: Schiraldi
authors.affiliation: NASA Glenn Research Center , 21000 Brookpark Road, Cleveland, Ohio 44135, United States.
authors.forename: Mary Ann B
authors.identifier: http://orcid.org/0000-0003-2513-7372
authors.initials: MA
authors.lastname: Meador
keywords: aerogels; mesoporous; polyimides; porous polymers; thermal shrinkage