Melt-shear assembly process has gained significant attention for its ability to fabricate densely packed three-dimensional colloidal photonic films with a core-shell structure of colloidal nanoparticles, exhibiting distinct properties. These films hold great potential as sensor materials due to their mechanochromic characteristics, which enable wavelength changes in response to various stimuli by diversifying the core and shell materials.
<br> In this study, the melt-shear process was employed to produce four colloidal photonic films with varying mechanical properties, utilizing the differences in their mechanical characteristics to create heterogeneous bimorph structures. The core material, polystyrene, and interlayer material, poly(methyl methacrylate), were synthesized using multistep starved-feed polymerization. The shell materials, poly(methyl acrylate), poly(hexyl methacrylate), poly(ethyl acrylate), and poly(2-ethylhexylacrylate), with different glass transition temperatures, were used to form a core-interlayer-shell(CIS) structure. Scanning electron microscopy (SEM) was employed to confirm the synthesis of nanoparticles and the formation of photonic crystals with diverse reflective colors. The synthesized nanoparticles were then processed into colloidal photonic films using the melt-shear assembly method. The optical properties of the fabricated films were examined using optical microscopy (BX-43, Olympus) and spectrophotometry (USB4000 VIS-NIR, Ocean Optics). Additionally, the strain-dependent optical signals were observed, revealing the mechanochromic properties of the three-dimensional colloidal photonic films.
<br> To evaluate the mechanical properties of the fabricated colloidal photonic films, a universal testing machine (UTM) was used. The films exhibited behavior similar to the shell matrix, displaying trends resembling the glass transition temperatures. Higher glass transition temperatures resulted in enhanced mechanical properties, while lower temperatures exhibited liquid-like behavior at room temperature. Leveraging the mechanical properties of the colloidal photonic films, structures were designed to control the extent of mechanochromic response to external strains, thereby regulating sensitivity to applied tension. Furthermore, by combining two photonic films with differing mechanical properties, heterostructured bimorphs were created, demonstrating distinct mechanochromic properties along the axis of tension. These structures not only enable intuitive detection of fine strains without the need for external equipment or energy consumption but also suggest the potential for strain sensing in a two-dimensional context.
