Moisture-Triggered 71000-Fold Stiffness Change Materials Via Crystallization and Hydrogen Bonding

Stiffness change materials have been widely used in soft robots, intelligent adhesives and biomedical minimally invasive devices which can respond to specific stimuli. Compared with stimuli such as light, electricity, magnetism, temperature, and pressure, moisture is a non-toxic, readily available, and inexhaustible triggered source in the atmosphere. However, it is challenging to achieve a stiffness variable material with a large stiffness change using moisture. Traditional sensitive materials usually swell to expand the volume or dissolve to a liquid-like state when absorbing moisture, which is unfavorable for real applications. Herein, we demonstrate a polyvinylamine (PVAm)/polyethylenimine (PEI) composite film with variable stiffness change by moisture absorption and release. PVAm is a semicrystalline polymer with high stiffness and PEI forms intermolecular hydrogen bonding with primary amine groups on PVAm. The synergistic effect of hydrogen bonding and crystallization was maximized when 15 wt % PEI was added to the composite system, resulting in a large stiffness change of up to 7.1 × 10⁴ (0.022 MPa versus 1560.85 MPa) of PVAm/PEI composite film under 25 °C, 95% RH. The crystallinity structure and hydrogen bonding can be broken and reformed by adjusting humidity. As promising variable stiffness polymers, the developed PVAm/PEI composite film is demonstrated as a reconfigurable multitool for shape memory and locking on demand.