Prof. Tao XIE's Group

Mechanical stresses are ubiquitous in materials. Although they are often detrimental to mechanical properties, well controlled stresses can lead to unusual opportunities in various technological areas including flexible electronics, 4D printing, microfabrication, and actuators. Full cycle manipulation of stresses including its generation, storage, and release can therefore have far-reaching consequences. Here, we report a digital photothermal mechanism enabled by laser printing that allows unparalleled freedom in stress manipulation in a dynamic covalent shape memory polymer network, critically without altering its free-standing geometric shape. The digital photo-thermal effect permits spatiotemporal stress control via plasticity enabled by dynamic covalent bond exchange whereas the elasticity based shape memory mechanism ensures its geometric stability regardless of the stress. This leads to a two-dimensional film of any arbitrarily distributed stress which is invisible under regular light but can be visualized under polarized light due to the birefringence. Owing to the rich achievable birefringent colors and their actively controllable nature, we call them invisible mechanical colors which are useful for encoding colored hidden information. Our approach of digital stress manipulation in a free-standing polymer expands the technological potential in areas for which stresses are relevant.

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Figure 1. Stress induced mechanically colored images. a, b, Flower and butterfly painting in bright (upper) and dark field (lower). c, Horse painting in Chinese traditional style. d, Images of zebra created via laser direct writing in bright (upper) and dark field (lower). e, Invisible quick response code. f, Erasing and rewriting letters. g, Controlled formation of 3D shapes via heating induced release of digital stress patterns in 2D free-standing films. All the scale bars are 5 mm.