Elastic Photonic Microbeads as Building Blocks for Mechanochromic Materials

Abstract

Colloidal assemblies have been used to make mechanochromic materials. As interparticle spacing of colloids in the elastomeric polymer matrix can be changed by mechanical strain, the structural colors are also dynamically tunable. In common, such mechanochromic colloidal structures have been prepared in a film format. Here, we design elastic colloidal structures with a microbead format to provide high reconfigurability over a macroscopic shape and optical property. As the assembly of microbeads can fit into any macroscopic templates, we can utilize them as building blocks for secondary assembly to produce mechanochromic materials with any formats. To produce the elastic photonic microbeads, we use microfluidic technology to prepare oil-in-water emulsion droplets, where the dispersion of silica particles in an elastomer-forming photopolymerizable resin with acrylate group is used as a dispersed phase. The silica particles spontaneously form a non-close-packed colloidal array in each droplet due to repulsive interparticle potential, which is stabilized by in situ photopolymerization. The resulting microbeads display structural color depending on the size of silica particles. The mechanochromic microbeads are further assembled to form 2D and 3D structures, where the interstitial voids of the microbeads are filled with another elastomeric silicone-based polymer with higher modulus than the microbeads. These macroscale assemblies show rotation-independent structural colors and low angle dependency. More importantly, the assemblies show the mechanochromic property as individual microbeads dynamically deform under macroscopic strain.

Mechanochromism in Structurally Colored Polymeric Materials

“…It can be argued that these advances will be particularly impactful in the context of mechanosensing: such beads could be dispersed in different polymeric matrices, which obviates the restrictive requirements for fine nanoscale ordering on the matrix and fabrication approach, potentially widening the scope of these materials as mechanosensors. [85,89,179] Increasingly, researchers are introducing other functionalities to photonic or plasmonic polymers, drawing from diverse areas of material science: examples include supramolecular selfhealing, [256] shape memory, [245,[250][251][252]257] and tunable moduli. [178] These designer materials could be particularly useful in areas such as soft robotics or electronic skins, [63,258,259] where polymeric components with bright, tunable color and the ability to self-repair for an improved product lifetime are highly desirable.…”

“…Adapted with permission. [85] Copyright 2020, American Chemical Society. Examples of plasmonic mechanochromic polymeric materials.…”

Mechanochromism of Structural‐Colored Materials

“…Therefore, mechanochromic elastomers with higher elasticity and sensitivity could be achieved, as a result of the nonclose packed structure. [ 98–103 ] For instance, Ge and the co‐workers proposed an efficient and straightforward approach to fabricate 3D photonic crystals gels via the in situ self‐assembly of metastable silica colloidal crystals in a mixture of EG and poly(ethylene glycol) methacrylate(PEGMA) followed by photopolymerization. [ 98,99 ] Chen et al.…”

Photonic Plasticines with Uniform Structural Colors, High Processability, and Self‐Healing Properties

“…[ 12,13 ] In spite of their widespread applicability throughout soft photonics, how to balance their optical microstructures with material processability is still challenging. Traditionally, constructing various CPC architectures relies on the solution‐processing routes of colloidal suspensions, such as vertical deposition, [ 14 ] dip‐coating, [ 15 ] spray printing, [ 16 ] inkjet printing, [ 17,18 ] microfluidics, [ 19,20 ] etc. These processes are highly dependent upon the usage of the convective flow and require delicate evaporative conditions to obtain uniformly assembled structures over large areas.…”