Mechanism of Hydration Induced Stiffening and Subsequent Plasticization of Polyamide Aerogel

Abstract

The mesoporous polyamide (PA) aerogel similar in chemical structure to DuPont's Kevlar is an advanced thermal insulation material tested in airspace applications. Unfortunately, the monolithic aerogel readily absorbs humidity (from moist air), which dramatically alters its mechanical properties. The compressive strength of the PA aerogel first increases when its water content increases, but subsequently decreases following additional hydration. To provide a coherent explanation for this non-monotonic change, the aerogel is hydrated stepwise and its hydration mechanism is elucidated by multiscale experimental characterization. The molecular structure is investigated by solid-state and liquid-state nuclear magnetic resonance (NMR) spectroscopy, and the morphology by small angle neutron scattering (SANS) at each equilibrium hydration state. The physico-chemical changes in the molecular level and in the nanoscale architecture are reconstructed. The first water molecules bind into the vacancies of the intermolecular H-bonding network of the PA macromolecules, which strengthens this network and causes concerted morphological changes, leading to the macroscopic stiffening of the monolith. Additional water disrupts the original H-bonding network of the macromolecules, which causes their increased segmental motion, marking the start of the partial dissolution of the nanosized fibers of the aerogel backbone. This eventually plasticizes the monolith.