Pitch Gradation by Ion-Dragging Effect in Polymer-Stabilized Cholesteric Liquid Crystal Reflector Device

Abstract

An IR reflector based on polymer-stabilized cholesteric liquid crystal (PSCLC) can selectively tune IR light reflection for smart window application. Broadening the reflection bandwidth to block more IR heat radiation requires the expansion of the pitch distribution in the PSCLC. Traditional attempts using ex situ direct current (DC) bias upon an already polymerized PSCLC reflector usually require a sustaining potential difference holding the pitch gradient of the reflector. Removing the DC bias will lead to a reflect bandwidth comeback. Here, we have developed an in situ DC curing strategy to realize an irreversible reflect bandwidth broadening. Briefly, a DC bias was used to drive the redistribution of impurity cations, which can be captured by the ester group of oligomers, during the photopolymerization. During the slow polymerization process, such trapped cations will drag the oligomers towards the cathode and compress the pitch length near the cathode before the oligomers form the long polymer chain. Consequently, a frozen pitch gradient by such an in-situ-electric-field-assisted dynamic ion-dragging effect leads to the formation of a pitch gradient along the electrical field direction. After removing the DC bias, the as-cured polymer is observed to have frozen such a gradient pitch feature without recoverable change. As a result, the PSCLC reflector exhibits steady bandwidth broadening of 480 nm in the IR region, which provides the potential for saving energy as a smart window.

Enhanced Bandwidth Broadening of Infrared Reflector Based on Polymer Stabilized Cholesteric Liquid Crystals with Poly(N-vinylcarbazole) Used as Alignment Layer

“…When a DC voltage was applied, the reflection band is extending simultaneously towards both the red and blue sides compared to the original reflection notch wavelength ( Figure 5 a,b), resulting in bandwidth broadening. This can be explained by a non-uniform pitch distribution due to the ion-dragging effect in PSCLC systems under the applied electric field [ 20 , 27 ]. Moreover, on application of the same voltage, for the PVA–PVK cell, the bandwidth broadening is larger than that of the PVA–PVA cell ( Figure 5 c).…”

“…In buildings, energy savings of 20% can be made through smart windows due to reduced lighting, heating and cooling costs [ 2 ]. Currently, various technologies are known to fabricate such windows including electrochromic (EC) [ 3 , 4 , 5 ], suspended particle electrophoresis [ 6 , 7 ], polymer dispersed liquid crystals (PDLC) [ 8 , 9 , 10 ], polymer stabilized liquid crystals (PSLC) [ 11 , 12 , 13 , 14 , 15 , 16 ] and polymer stabilized cholesteric liquid crystals (PSCLC) [ 17 , 18 , 19 , 20 , 21 , 22 ]. Among them, electrically tunable infrared (IR) reflecting/transmitting windows based on PSCLC, which can selectively reflect different fractions of infrared radiation on demand without affecting the transmittance in the visible region, appear to be very attractive [ 21 , 22 ].…”

Research Progress of Cholesteric Liquid Crystals with Broadband Reflection

“…Thus, oligomer drift and further polymerization occur simultaneously, resulting in a final nonuniform network that modulates the pitch length of the ChLC over a larger range. [82] Based on the ion capture of oligomers, Hu developed an in-situ DC curing strategy, which involves UV irradiation while loading DC voltage (as shown in Figure 14). During the slow polymerization process, the oligomers formed first are able to trap impurity cations.…”

“…Under in-situ DC bias, these trapped cations drag the oligomers toward the cathode, where they form short pitches and are further immobilized under UV light irradiation. Thus, oligomer drift and further polymerization occur simultaneously, resulting in a final nonuniform network that modulates the pitch length of the ChLC over a larger range [82]. Based on the ion capture of oligomers, Hu developed an in-situ DC curing strategy, which involves UV irradiation while loading DC voltage (as shown in Figure 14).…”

“…Thus, oligomer drift and further polymerization occur simultaneously, resulting in a final nonuniform network that modulates the pitch length of the ChLC over a larger range. [82] Figure 14. Schematic illustration of pitch-length modulation in ChLC cell [82].…”