Sticky Multicolor Mechanochromic Labels

Abstract

Sticky-colored labels are an efficient way to communicate visual information. However, most labels are static. Here, we propose a new category of dynamic sticky labels that change structural colors when stretched. The sticky mechanochromic labels can be pasted on flexible surfaces such as fabric and rubber or even on brittle materials. To enhance their applicability, we demonstrate a simple method for imprinting structural color patterns that are either always visible or reversibly revealed or concealed upon mechanical deformation. The mechanochromic patterns are imprinted with a photomask during the ultraviolet (UV) cross-linking of acrylate-terminated cholesteric liquid crystal oligomers in a single step at room temperature. The photomask locally controls the cross-linking degree and volumetric response of the cholesteric liquid crystal elastomers (CLCEs). A nonuniform thickness change induced by the Poisson's ratio contrast between the pattern and the surrounding background might lead to a color-separation effect. Our sticky multicolor mechanochromic labels may be utilized in stress-strain sensing, building environments, smart clothing, security labels, and decoration.

Keywords: cholesteric liquid crystal elastomers; labels; mechanochromic devices; stickers; wearable devices.

Figure 1

(a) Chemical structure of the acrylate-terminated CLC oligomer and (b) preparation of the sticky multicolor mechanochromic labels from the CLC oligomer ink.

Figure 2

Optical properties of the mechanochromic label UV cross-linked with 4 mJ/cm². (a) Photographs of the labeled fabric (upper left) in the relaxed state; (upper right) under nonuniform strain; (lower left) bent; and (lower right) twisted (scale bars are 1 cm). (b) Reflectance spectra of the label under uniaxial strain and (c) their corresponding optical micrographs (scale bars are 25 μm). (d) Wavelength shift of the label obtained from the reflectance curves. (e) Reversible color-changing behavior of the label submitted to stretching-releasing cycles (ε_xx = 30%). S1 represents the first stretched state, and R1 represents the first relaxed state.

Figure 3

Examples of applications of sticky mechanochromic labels. (a) Electricity cable. The label can indicate if the cable is excessively deformed (scale bar is 1 cm). (b) Evaluation of structural integrity and crack propagation of brittle materials. The stress-concentrator effect locally blue shifts the red label when a stable crack (left) becomes unstable and starts to propagate (right), indicating the crack path in a strong green color (scale bars are 1 cm). (c) Strain sensor for monitoring human mobility. The red label was applied on a latex glove and reversibly blueshifts during bending of the finger (scale bars are 2 cm).

Figure 4

Color-separation mechanism of the label UV cross-linked with 6 and 4 mJ/cm². (a) UV cross-linking of the label. A photomask was prepared with a rectangular pattern divided into two halves that exhibit ca. 50% of transmittance difference. The UV dose was adjusted to 6 and 4 mJ/cm² in the clear and dark regions, respectively. The photomask was placed between the light source and the sample during UV cross-linking which was performed in a single step. (b) Schematic illustration of the uniaxial strain applied on the samples. (c) Reflectance spectra of the 6 and 4 mJ/cm² halves recorded under uniaxial strain. (d) Mismatch of the spectral shift between the 6 and 4 mJ/cm² halves. (e) Reversible color-changing behavior of the mechanochromic label submitted to stretching-releasing cycles. S1 represents the first stretched state and R1 represents the first relaxed state.

Figure 5

Color-separation mechanism of the label UV cross-linked with 6 and 3 mJ/cm². (a) Reflectance spectra of the 6 and 3 mJ/cm² halves recorded under uniaxial strain. (b) Mismatch of the spectral shift between the 6 and 3 mJ/cm² halves. The inset illustrates the uniaxial deformation applied in the iso-stress condition. (c) Reversible color-changing behavior of the label submitted to stretching-releasing cycles. S1 represents the first stretched state and R1 represents the first relaxed state.

Figure 6

Macroscopic response of the color-separating patterns. (a) Atom icon. The atom icon is revealed upon deformation and concealed in the relaxed state. (b) Emoji icon. The emoji icon is always visible and becomes clearer upon deformation (scale bars are 1 cm).