Thermally Superstable Cellulosic-Nanorod-Reinforced Transparent Substrates Featuring Microscale Surface Patterns

Abstract

The recent rapid expansion of thin-film, bendable, and wearable consumer (opto)­electronics demands flexible and transparent substrates other than glass. Plastics are the traditional choice, but they require amelioration because of their thermal instability. Here, we report the successful conversion of a soft and thermally vulnerable polymer into a highly thermally stable transparent nanocomposite material. This is achieved by the meticulous choice of a polymer with a glass-transition temperature below 0 °C that gives stable mechanics above room temperature, reinforcing the polymer with a load-bearing hierarchical network of the incredibly strong and stable natural material: cellulose nanorods. Owing to the Pickering emulsification process, the nanocomposites inherit the self-assembled structural hierarchy from the cellulose nanorod-encapsulated resin droplets. The ameliorated nanocomposites have highly desirable high-temperature endurance (∼150–180 °C) in terms of the thermomechanical, thermodimensional, and thermo-optical performance. Any photonic nano- or microstructures can be directly molded on the surface of the nanocomposites in high precision for better light management in photonic and opto-electronic applications. The highlight of this work is the demonstration of a highly thermally stable microlens array on the ameliorated transparent nanocomposite.

Highly Thermal‐Resilient AgNW Transparent Electrode and Optical Device on Thermomechanically Superstable Cellulose Nanorod‐Reinforced Nanocomposites

“…Interestingly, the strain‐to‐failure of the acrylic plastic increased more than twofold from ≈7% up to ≈16% with the CN reinforcement. This result can be explained by a synergistic mechanism in our hierarchical nanocomposites, by which crack initiation in the plastic droplets by the tensile force was effectively restricted and deflected by their surrounding strong CN network and the load on the droplets was effectively dissipated to the stretchable bulk CN network . The high strain‐to‐failure of our transparent nanocomposite substrates and electrodes is a desirable feature for flexible optoelectronic applications.…”

“…Preparation of CNs : The CNs were prepared from wood powder following the process described in our previous report . Briefly, the wood powder was first extracted with an azeotrope of ethanol:toluene:acetone (1:2:1) in a Soxhlet apparatus for 10 h to remove extractives, and then treated with acidified‐NaClO 2 and KOH to remove the lignin and most of the hemicelluloses.…”

“…Specifically, cellulose nanocrystals or nanorods (CNs) exhibit incredible thermal stability with a coefficient of thermal expansion (CTE) of ≈0.1 ppm K −1 and an elastic modulus ( E ) of ≈130 GPa that remains stable from approximately −200 °C to >100 °C . In recent studies, we have demonstrated that the thermal dimensional stability of a plastic can be improved by approximately 60 times (from ≈190 to ≈3.4 ppm K −1 ) by incorporating nanocelluloses as a hierarchical reinforcement at a content as low as 10 wt% . Unlike other reinforcing nanomaterials, such as clays, silica, alumina, boron nitride, and zirconium tungstate, which must be incorporated at a high percentage (often ≈50% by weight), the low nanocellulose content also resulted in a high optical transmittance (≈90%) and great mechanical flexibility (fracture strain of up to ≈15%).…”

“…Unlike other reinforcing nanomaterials, such as clays, silica, alumina, boron nitride, and zirconium tungstate, which must be incorporated at a high percentage (often ≈50% by weight), the low nanocellulose content also resulted in a high optical transmittance (≈90%) and great mechanical flexibility (fracture strain of up to ≈15%). The reinforced plastics also retained a high optical clarity even at 180 °C and exhibited no birefringence compared with a thermally stable polyethylene terephthalate (PET) film …”

“…Microdroplets of an acrylic monomer (2,2‐bis[4‐(acryloxypolyethoxy)phenyl]propane) were first encapsulated into the CN network in a Pickering emulsion and then collected on a filter membrane, hot‐pressed, and polymerized to generate hierarchically reinforced plastic substrates followed by spin coating with AgNWs ( Figure ). This particular acrylic plastic is promising because of its high flexibility, similar refractive index to that of cellulose, and having no unwanted tint like polyimide as well as the high thermal stability of its optical transparency even at 180 °C compared with PET . The resulting transparent and flexible nanocomposite electrodes not only exhibit a low sheet resistance (12.4–15.6 Ω sq −1 ) but also survived repeated extreme heating and cooling at temperatures of 150 and −196 °C, respectively, because of the high thermal dimensional stability.…”