Abstract
We studied the surface and microstructure of cellulose acetate (CA) films to tailor their barrier and mechanical properties for application in electrochromic devices (ECDs). Cross-linking of CA was carried out with pyromellitic dianhydride to enhance the properties relative to unmodified CA: solvent resistance (by 43% in acetone and 37% in DMSO), strength (by 91% for tensile at break), and barrier (by 65% to oxygen and 92% to water vapor). Surface modification via tetraethyl orthosilicate and octyltrichlorosilane endowed the films with hydrophobicity, stiffness, and further enhanced solvent resistance. A detailed comparison of structural, chemical, surface, and thermal properties was performed by using X-ray diffraction, dynamic mechanical analyses, Fourier-transform infrared spectroscopy, and atomic force microscopy. Coplanar ECDs were synthesized by incorporating a hydrogel electrolyte comprising TEMPO-oxidized cellulose nanofibrils and an ionic liquid. When applied as the top layer in the ECDs, cross-linked and hydrophobized CA films extended the functionality of the assembled displays. The results indicate excellent prospects for CA films in achieving environmental-friendly ECDs that can replace poly(ethylene terephthalate)-based counterparts.
Highlights
The current environmental pressure, exacerbated by the utilization of fossil-based resources, has led to increased interest in bio-based materials.[1,2] Those prepared from cellulose derivatives are of particular interest, given their relatively low cost and facile processing compared to unmodified cellulose, coupled with excellent physicomechanical properties.[3,4] The most studied cellulose derivative, cellulose acetate (CA),[5] is obtained from either heterogeneous[6−8] or homogeneous[9] reactions
(4) A hydrogel-based electrolyte containing cellulose nanofibers and ILs was incorporated in electrochromic devices (ECDs). (5) While CA has been previously applied as a gel electrolyte in ECDs,[24] here we report for the first time the use of CA films (CAF) as a top cover to seal a coplanar ECD
The free hydroxyl groups of CA attack the carbonyl carbon of the activated intermediate, resulting in the release of the catalyst back to the reaction medium, followed by a proton transfer and product formation. This product reacts with another CA molecule, following the same mechanism, which leads to cross-linking between adjacent CA chains
Summary
The current environmental pressure, exacerbated by the utilization of fossil-based resources, has led to increased interest in bio-based materials.[1,2] Those prepared from cellulose derivatives are of particular interest, given their relatively low cost and facile processing compared to unmodified cellulose, coupled with excellent physicomechanical properties.[3,4] The most studied cellulose derivative, cellulose acetate (CA),[5] is obtained from either heterogeneous[6−8] or homogeneous[9] reactions. The cross-linked films produced from the 8% CA solution and 40 mL casting volume (labeled as CAF-C) presented the best mechanical performance and were considered for further hydrophobization and application in electrochromic displays. The effect of cross-linking and hydrophobization on tensile properties of the films is shown in Figure 3a and Table 1. The CAF-C film swelled (10% increase) in water, whereas it dissolved (18%) in acetone after 24 h, indicating that a small fraction of polymer was not cross-linked. Our observations indicate that the hydrophobization of cross-linked CA film improved the barrier properties, providing better isolation and sealing of the device, increasing the shelf life and functionality of the assembled displays
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