Electrically Tunable Hysteretic Photonic Gels for Nonvolatile Display Pixels

Abstract

Materials that display hysteresis, such as ferromagnetic or ferroelectric materials, have been extensively investigated for their potential engineering applications towards electromagnetic memory devices and switches. While similar hysteretic volume phase transitions are observed in many soft organic materials, including hydrogels [1] and biomaterials, [2] it has not been well exploited but is often considered as problematic because it complicates the calibration procedure and affects signal reproducibility in many conventional applications of hydrogels in sensors and drug delivery. [3] However, such hysteretic volume phase transitions of hydrogels can be utilized in a positive way when they are combined with other functional materials and carefully tuned to exhibit strong bistability. For example, Kim et al. recently reported wet photonic gel memory pixels by combining the hysteresis of hydrogels with photonic crystals. [4] Block copolymer photonic gels consisting of alternating glassy polystyrene (PS) and swellable poly(2-vinyl pyridine) (P2VP) gel layers showed pH-dependent photonic stop bands. The optical responses of the photonic gels varied with the direction of pH changes. This hysteretic optical response of photonic gels was able to be tuned and further optimized by controlling ion-pairing affinity between the protonated pyridine groups and their counteranions.Extending the previous work, herein we report electrochemically controllable nonvolatile photonic pixels and their applications towards e-papers. Our approach is clearly different from the nonvolatile photonic ink demonstrated by Ozin and co-workers [5] in the sense of using hydrogel hysteresis instead of using redox reaction of metallopolymers. Electroactive photonic pixels were achieved by coupling the hysteretic optical properties of PS-b-P2VP block copolymer photonic gels to the electrochemical cells, where the pH gradient can be tuned. Similar to other conventional hydrogels, [6] our photonic gels show volume expansion and contraction in response to the applied electric field, and accordingly exhibit various photonic colors. [7] We assumed that electroactive nonvolatile photonic gels can be achieved when the volume transition of gel layers was tuned to exhibit strong hysteresis with changes of electrochemically driven pH gradient.The electroactive photonic pixel comprises a simple electrochemical cell consisting of two transparent electrodes separated by 1 mm thick spacer and electrolytes (Figure 1 a). The block copolymer photonic gel films were coated on the working electrode. The block copolymer photonic gel films were prepared as previously described. [4,[7][8] Briefly, in-plane oriented lamellar films were first prepared by spin casting Figure 1. Schematic structure of photonic pixel and proposed mechanisms for gel swelling and deswelling by an electrically induced pH gradient.

Photonic Crystals for Chemical Sensing and Biosensing

“…There are two types of electrically responsive PhCs: those that can be stimulated by an external electrical signal,111 and those that respond to an electrochemical stimulus. The first type is mainly based on a combination of PhCs with liquid crystals (LCs).…”

“…Other approaches are based on polyelectrolyte hydrogels. Examples of electrochemically responsive PhCs include copolymers prepared from N ‐isopropylacrylamide and methacrylic acid,117 from 2‐hydroxyethyl methacrylate and 3‐(methacryloylamino)propyltrimethylammonium hexafluorophosphate,118 or from styrene and 2‐vinylpyridine in a 1D PhC 111…”

Photonische Kristalle für die Chemo‐ und Biosensorik

“…Es gibt zwei Arten elektrisch responsiver PhCs: Die einen können über ein externes elektrisches Signal adressiert werden,111 die anderen mithilfe elektrochemischer Reize. Beim ersten Typ wird hauptsächlich eine Kombination von PhCs mit Flüssigkristallen (LCs) eingesetzt.…”

Bio-Inspired Photonic Structures: Prototypes, Fabrications and Devices

“…By applying tunable voltage, macroscopic swelling and shrinking of the polymer matrix and microscopic Bravais lattice change responsible for the reverse coloration occur [59]. Stimulated by electrical forces, quite a few switchable coloration devices or sensors based on other materials or circumstances can be found elsewhere [60][61][62].…”