Cellulose Nanocrystal/Poly(ethylene glycol) Composite as an Iridescent Coating on Polymer Substrates: Structure-Color and Interface Adhesion

Abstract

The broad utility as an environmentally friendly and colorful coating of cellulose nanocrystal (CNC) was limited by its instability of coloration, brittleness, and lack of adhesion to a hydrophobic surface. In the present work, a neutral polymer, poly(ethylene glycol) (PEG) was introduced into CNC coatings through evaporation-induced self-assembly (EISA) on polymer matrices. The structure-color and mechanical properties of the composite coating or coating film were characterized by UV-vis spectroscopy, polarized light microscopy (PLM), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WXRD), and tensile tests. Results showed that the reflective wavelength of the iridescent CNCs could be finely tuned by incorporation of PEG with varied loadings from 2.5 to 50 wt %, although the high loading content of PEG would produce some side effects because of the severe microphase separation. Second, PEG played an effective plasticizer to improve the ductility or flexibility of the CNC coating or coating film. Furthermore, as a compatibilizer, PEG could effectively and tremendously enhance the adhesion strength between CNCs and neutral polymer matrices without destroying the chiral nematic mesophases of CNCs. Environmentally friendly CNC/PEG composites with tunable iridescence, good flexibility, and high bonding strength to hydrophobic polymer matrices are expected to be promising candidates in the modern green paint industry.

Binary Cellulose Nanocrystal Blends for Bioinspired Damage Tolerant Photonic Films

“…These results are rare for synthetic systems, which are characteristically limited by trade‐offs. The properties measured here, while rivalling those of the gold‐standard natural materials like bone and nacre, are also the highest reported thus far for the pristine chiral nematic CNC films . The enhancements achieved upon introducing a minor fraction of t‐CNCs are attributed to a) an increased overlap length that enhances shear transfer and presents bigger integrated energetic barriers to deformation through stronger nonbonded interactions and b) superior contribution of t‐CNCs to structural toughening through crack deflection and bridging.…”

“…In contrast, the addition of t‐CNC not only moves E and ε f in the desired direction, it helps photonic films attain properties that rival some of the best natural helicoidal materials, as shown in Figure b. The moduli and strength measured here are the highest measured yet in the helicoidal CNC films and their composites . The strain to failure and toughness values are some of the best reported for pristine helicoidal CNC films.…”

“…a) Schematic of model helicoidal structure formed by the assembly of cellulose nanocrystals. b) Comparative Ashby plot of the elastic modulus (GPa) versus the strain to failure (%) from helicoidal CNC films and composites reported in literature . Error bars represent reported uncertainties.…”

“…This gap is attributable to the following observations. All studies on cellulose‐rich helicoidal materials utilize w‐CNCs as building blocks and the mechanical properties of the pristine w‐CNC films (100% w‐CNC) leave much to be desired. The pristine w‐CNC films are excessively brittle (strain to failure <0.5%).…”

The Self‐Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance

“…The co‐assembly of CNCs with weakly interacting additives significantly reduces the brittleness of the produced films while preserving the formation of the helicoidal nanostructure and causing a red‐shift of the optical bandgap (Figure e). This has been demonstrated with zwitterionic surfactants, neutral or anionic polymers, amino resins or sol–gel precursors, and crosslinked latex nanoparticles . The cholesteric arrangement of CNCs can also accommodate small nanoparticles without disruption of the phase, e.g., plasmonic gold nanorods, allowing complex chiroptical effects …”