Thermochromic Cholesteric Liquid Crystal Microcapsules with Cellulose Nanocrystals and a Melamine Resin Hybrid Shell

Abstract

Thermochromic coatings that can change their color in response to variations in ambient temperature have various potential applications. Cholesteric liquid crystals (CLCs) are promising thermochromic materials due to their selective light reflection and wide regulation range. However, it remains a challenge to fabricate thermochromic coatings that combine good responsivity, mechanical strength, fabrication feasibility, and flexibility. In this study, CLC microcapsules containing cellulose nanocrystals (CNCs) and a melamine–formaldehyde (MF) resin hybrid shell were fabricated via in situ polymerization using CNC-stabilized Pickering emulsions as templates. The CNCs were employed as both Pickering emulsifiers and alignment agents of CLCs to prepare CLC Pickering emulsions. The CLC microcapsules were mixed with curable binders to obtain coating slurries, and thermochromic coatings were prepared by painting the slurries on substrates and drying. The thermochromic coatings could adjust their color in the visible wavelength range in a temperature range of 12 to 42 °C. Moreover, the obtained thermochromic coatings displayed a relatively high reflectance of up to 30–40% and can even be applied to flexible substrates. The CLC microcapsules with CNCs and an MF hybrid shell are promising in the field of smart decorative paints, anti-counterfeit labels, and artificial skins.

Synthesis, characterization, and functionalization of superadhesive melamine formaldehyde polyvinylpyrrolidone resins doped with metallic and graphene oxide nanoparticles for better industrial applications

“…The intensified phonons with MFPR emitted a higher energy of all the wavelengths. The aqueous MFPR solution has transmitted a sharper light like the liquid crystal that emits light in LCD displays, [64][65][66] similarly the MFPR could behave like a liquid crystal to emit a light. So, MFPR could replace the complicated liquid crystal systems in display applications.…”

Unclonable human-invisible machine vision markers leveraging the omnidirectional chiral Bragg diffraction of cholesteric spherical reflectors

“…To make CSRs that can be practically useful, one generally polymerizes and crosslinks the liquid crystal into a solid 58 – 65 , although it is also possible to keep it fluid by containing it inside a rubbery or solid polymeric shell 66 – 70 . A problem with droplets/beads is that topological defects cannot be avoided: because all possible orientations of m overlap at the core of a sphere with radially aligned helix, each CSR droplet has a point defect at its center 71 – 76 , depicted as a black circle in Fig.…”

“…While annealing may improve the situation, it can take many days and the alignment often gets even worse upon polymerization 58 . The annealing time is reduced and the overall alignment quality improved by using very small cholesteric droplets, about 5–10 µ m in diameter 59 , 70 . This comes at the cost, however, of an increased relative impact of the defects, which retain their scale regardless of droplet diameter.…”

Preparation of Thermochromic Microcapsules of Bisphenol A and Crystal Violet Lactone and Their Effect on Coating Properties

“…Ma et al [ 19 ] combined thermochromic materials with optimized disperse cationic dyes and designed a new type of thermochromic microcapsule to expand the color range of thermochromic materials. Yang et al [ 20 ] prepared cholesteric liquid crystals (CLC) microcapsules containing cellulose nanocrystals (CNC) and melamine–formaldehyde (MF) resin hybrid shell by means of in situ polymerization with CNC-stabilized Pickering emulsion as template. A thermochromic coating was prepared by mixing CLC microcapsules with a curable adhesive.…”