Moisture-Triggered\n71000-Fold Stiffness Change Materials\nVia Crystallization and Hydrogen Bonding

Stiffness\nchange materials have been widely used in soft robots,\nintelligent adhesives and biomedical minimally invasive devices which\ncan respond to specific stimuli. Compared with stimuli such as light,\nelectricity, magnetism, temperature, and pressure, moisture is a non-toxic,\nreadily available, and inexhaustible triggered source in the atmosphere.\nHowever, it is challenging to achieve a stiffness variable material\nwith a large stiffness change using moisture. Traditional sensitive\nmaterials usually swell to expand the volume or dissolve to a liquid-like\nstate when absorbing moisture, which is unfavorable for real applications.\nHerein, we demonstrate a polyvinylamine (PVAm)/polyethylenimine (PEI)\ncomposite film with variable stiffness change by moisture absorption\nand release. PVAm is a semicrystalline polymer with high stiffness\nand PEI forms intermolecular hydrogen bonding with primary amine groups\non PVAm. The synergistic effect of hydrogen bonding and crystallization\nwas maximized when 15 wt % PEI was added to the composite system,\nresulting in a large stiffness change of up to 7.1 × 10<sup>4</sup> (0.022 MPa versus 1560.85 MPa) of PVAm/PEI composite film under\n25 °C, 95% RH. The crystallinity structure and hydrogen bonding\ncan be broken and reformed by adjusting humidity. As promising variable\nstiffness polymers, the developed PVAm/PEI composite film is demonstrated\nas a reconfigurable multitool for shape memory and locking on demand.