Recently, transparent flexible displays have attracted considerable attention for a range of applications, such as smart windows, public displays and digital signage. Many of them must be composed of the main transparent display and light shutter, where a light shutter is needed to block the rear li...
Recently, transparent flexible displays have attracted considerable attention for a range of applications, such as smart windows, public displays and digital signage. Many of them must be composed of the main transparent display and light shutter, where a light shutter is needed to block the rear light when a high contrast image must be shown in the main display. The essential properties of the light shutter are high transmittance in transparent-mode and high blocking of light in shading-mode. As a candidate for the light shutter, the PDLC or PNLC (polymer dispersed or polymer network liquid crystal) has attracted considerable research attention. Normal PDLC or PNLC shows a white background due to light scattering in shading-mode, which has a problem to decrease the contrast ratio of a transparent flexible display. Black dye-doped PDLCs were developed to provide a black-background. The normally transparent property of a PDLC or PNLC, which is transparent at the voltage-off state, is often required for power saving when transparent-mode is used as the main display-mode. Few studies of both black-background and reverse-mode PDLC or PNLC have been carried out. As a first study, black background and reverse-mode PNLC is discussed. The PNLC was composed of a negative liquid crystal, black dye and RM (reactive-mesogen) materials. A normally transparent state was realized by the vertical alignment of a liquid crystal and black dye, and the black-background was achieved using a randomly oriented liquid crystal and black dye caused by the contribution of the RM polymer network at the voltage-on state. This study examined the electro-optical properties of black-background reverse-mode polymer network liquid crystal cells suitable for light shutters applicable to transparent flexible displays. The black dye-doped vertically aligned liquid crystal mode was used for high transparency at off-state and black-background at on-state. A reactive-mesogen material was used for the formation of a polymer network providing a random orientation of liquid crystal, which resulted in high scattering at the on-state. Through a parametric study, the E-O properties were strongly dependent on the RM concentration and UV exposure intensity. 12 wt. % was found to be the optimal RM concentration for high E-O performance. The E-O performance increased with increasing UV intensity in the range, of 2mW/cm2 to 20mW/cm2. High transmittance, TH=73.8%, a high transmittance difference, ΔT (TH-TL)=66.7%, and a good black-background were obtained in the case of LC:RM:dye concentration ratio of 87:12:1wt.% and a UV exposure intensity and time of 20 mW/cm2 and 1 min, respectively. Using the optimal parameters, the long-term stability of the polymer network was secured and no significant black dye degradation by UV exposure occurred. As a second study, reverse-mode PNLC with mechanical stability was discussed. The polymer concentration in reverse-mode PNLC should be optimized to achieve both high transmittance and high opacity. To enhance the mechanical stability, polymer wall was introduced, and it was formed in the LC layer by using a polymerization-induced phase separation (PIPS) method. The phase separation is controlled deliberately by UV irradiation in two regions of nearly pure LC and solid polymer walls. In this study, a two-step UV exposure method was introduced: the first UV exposure being for polymer wall formation, and the second exposure for the polymer network. We achieved a flexible reverse-mode PNLC with good mechanical stability and polymer network structure as well as high electro-optical properties using a two-step UV–exposure method based on a polymerization-induced phase-separation technique, where bisphenol A dimethacrylate was used as a polymerizable monomer and reactive mesogen was used: the first UV exposure to form polymer walls using photo mask for high mechanical stability, and the second UV exposure to get polymer networks for high scattering. The reverse-mode PNLC cells showed good mechanical stability without degradation in electro-optical properties. The best performance of the reverse-mode PNLC was achieved with the following parameters within meaningful parameter variations for monomer and RM concentration in the LC mixture: 8wt%, 12wt%, 16wt%, and 24wt%. The high performance was achieved in the following parameter conditions: (1) concentration ratio of LC : monomer : RM concentration = 84wt% : 8wt% : 8wt%, (2) UV intensity and time = 5mW/cm2 and 1min for the first exposure and 5mW/cm2 and 3min for the second exposure. Highly transparent off-state and low light leakage on-state were achieved, that is a maximum transmittance difference between transparent and opaque states of ∆T=60.7% was obtained. The high performance reverse-mode PNLC made by the two-step UV–exposure method in this study would be applicable to various transparent flexible displays.
Recently, transparent flexible displays have attracted considerable attention for a range of applications, such as smart windows, public displays and digital signage. Many of them must be composed of the main transparent display and light shutter, where a light shutter is needed to block the rear light when a high contrast image must be shown in the main display. The essential properties of the light shutter are high transmittance in transparent-mode and high blocking of light in shading-mode. As a candidate for the light shutter, the PDLC or PNLC (polymer dispersed or polymer network liquid crystal) has attracted considerable research attention. Normal PDLC or PNLC shows a white background due to light scattering in shading-mode, which has a problem to decrease the contrast ratio of a transparent flexible display. Black dye-doped PDLCs were developed to provide a black-background. The normally transparent property of a PDLC or PNLC, which is transparent at the voltage-off state, is often required for power saving when transparent-mode is used as the main display-mode. Few studies of both black-background and reverse-mode PDLC or PNLC have been carried out. As a first study, black background and reverse-mode PNLC is discussed. The PNLC was composed of a negative liquid crystal, black dye and RM (reactive-mesogen) materials. A normally transparent state was realized by the vertical alignment of a liquid crystal and black dye, and the black-background was achieved using a randomly oriented liquid crystal and black dye caused by the contribution of the RM polymer network at the voltage-on state. This study examined the electro-optical properties of black-background reverse-mode polymer network liquid crystal cells suitable for light shutters applicable to transparent flexible displays. The black dye-doped vertically aligned liquid crystal mode was used for high transparency at off-state and black-background at on-state. A reactive-mesogen material was used for the formation of a polymer network providing a random orientation of liquid crystal, which resulted in high scattering at the on-state. Through a parametric study, the E-O properties were strongly dependent on the RM concentration and UV exposure intensity. 12 wt. % was found to be the optimal RM concentration for high E-O performance. The E-O performance increased with increasing UV intensity in the range, of 2mW/cm2 to 20mW/cm2. High transmittance, TH=73.8%, a high transmittance difference, ΔT (TH-TL)=66.7%, and a good black-background were obtained in the case of LC:RM:dye concentration ratio of 87:12:1wt.% and a UV exposure intensity and time of 20 mW/cm2 and 1 min, respectively. Using the optimal parameters, the long-term stability of the polymer network was secured and no significant black dye degradation by UV exposure occurred. As a second study, reverse-mode PNLC with mechanical stability was discussed. The polymer concentration in reverse-mode PNLC should be optimized to achieve both high transmittance and high opacity. To enhance the mechanical stability, polymer wall was introduced, and it was formed in the LC layer by using a polymerization-induced phase separation (PIPS) method. The phase separation is controlled deliberately by UV irradiation in two regions of nearly pure LC and solid polymer walls. In this study, a two-step UV exposure method was introduced: the first UV exposure being for polymer wall formation, and the second exposure for the polymer network. We achieved a flexible reverse-mode PNLC with good mechanical stability and polymer network structure as well as high electro-optical properties using a two-step UV–exposure method based on a polymerization-induced phase-separation technique, where bisphenol A dimethacrylate was used as a polymerizable monomer and reactive mesogen was used: the first UV exposure to form polymer walls using photo mask for high mechanical stability, and the second UV exposure to get polymer networks for high scattering. The reverse-mode PNLC cells showed good mechanical stability without degradation in electro-optical properties. The best performance of the reverse-mode PNLC was achieved with the following parameters within meaningful parameter variations for monomer and RM concentration in the LC mixture: 8wt%, 12wt%, 16wt%, and 24wt%. The high performance was achieved in the following parameter conditions: (1) concentration ratio of LC : monomer : RM concentration = 84wt% : 8wt% : 8wt%, (2) UV intensity and time = 5mW/cm2 and 1min for the first exposure and 5mW/cm2 and 3min for the second exposure. Highly transparent off-state and low light leakage on-state were achieved, that is a maximum transmittance difference between transparent and opaque states of ∆T=60.7% was obtained. The high performance reverse-mode PNLC made by the two-step UV–exposure method in this study would be applicable to various transparent flexible displays.
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