Effect of SiO2 nanoparticle doping on electro-optical properties of polymer dispersed liquid crystal lens for smart electronic glasses
© Kim et al.; licensee Springer. 2015
Received: 12 October 2014
Accepted: 4 November 2014
Published: 18 March 2015
In this paper, SiO2 nanoparticle doped polymer dispersed liquid crystal (PDLC) lenses were made from a mixture of prepolymer, E7 liquid crystal and SiO2 nanoparticles by the polymerization induced phase separation (PIPS) process for smart electronic glasses with auto-shading and auto-focusing functions. Electro-optical properties of doped and undoped samples including transmittance, driving voltage, contrast ratio and slope of the linear region of the transmittance-voltage were measured, compared and analyzed. Driving voltage of SiO2 nanoparticle doped PDLC lenses moderately improved. But the slope of linear region, response time and contrast ratio deteriorated, especially the latter two. It can be assumed that these doping effects were due to the mechanistic change from liquid-gel separation to liquid-liquid separation by the fast heterogeneous nucleation rate caused by the increased nucleation at the surface of SiO2 nanoparticles. The marked deteriorations of falling response time and contrast ratio were due to well defined liquid crystal molecules in LC droplets, which induced slow and imperfect random rearrangement of LC molecules at the off state.
Recent studies of smart electronic glasses (E-glasses) with various functions have been reported [1-7]. E-glasses with auto-shading and auto-focusing functions have been developed for shortsighted people and applications. Auto-shading is the ability of the e-glasses to automatically increase or decrease the amount of external light, through automatic opening and closing of the aperture when the external light is strong and weak, respectively. Auto-focusing refers to the automatic modulation of the focal length with the distance from the subject. Using polymer dispersed liquid crystal (PDLC) lenses, we have studied smart electronic glasses with auto-shading and auto-focusing functions [1,2]. The electro-optical properties can be further improved. To enhance electro-optical properties of PDLC lenses, such as driving voltage, slope, response time and contrast ratio, one of many approaches is to induce the heterogeneous nucleation for the phase separation to solid polymer and LC droplets. Through the addition of very small sized particles, initiation of heterogeneous nucleation at the surface of nano-sized particles can be expected.
Presently, doping of SiO2 nanoparticles was done during the fabrication of PDLC lenses. These lenses were fabricated and evaluated for use in auto-shading and/or auto-focusing e-glasses. Electro-optical properties of the undoped and doped PDLC devices including transmittance, driving voltage, slope of linear region, contrast ratio, and rising and falling response time were measured to assess the potential application to auto-shading and/or auto-focusing e-glasses. Mechanisms of heterogeneous nucleation and phase separation are proposed to explain the behaviors of the electro-optical properties.
The fabrication process of PDLC lens process has been previously summarized [1-3]. The average size and purity of SiO2 nanoparticles obtained from Sigma Aldrich were 10–20 nm and 99.95%, respectively. The concentrations of the doped SiO2 nanoparticles varied from 0–2.0% in weight in 0.5% increments. For PDLC lens fabrication, NOA 65 (Norland Products) and nematic LC E7 were used as the pre-polymer and nematic LC mixture, respectively. NOA 65 is a mixture of trimethylopropane diallyl ether, trimethylolpropanetris thiol, isophorone diisocyanate ester and a benzophenone photoinitiator [8,9], which makes it an ultraviolet (UV) curable optical adhesive. Liquid crystal E7 consists of 4-pentyl-4’-cyanobiphenyl (5CB), 4-heptyl-4’-cyanobiphenyl (7CB), 4-octyloxy-4’-cyanobiphenyl (8OCB), and 4-pentyl-4’-cyanoterphenyl (5CT) [9,10]. E7 was synthesized by homogeneous mixing of 51 wt% of 5CB, 25 wt% of 7CB, 16 wt% of 8OCB and 8 wt% of 5CT at 75°C, which is much higher than the nematic-to-isotropic transition temperature of 61.0°C. The mixing ratio of NOA 65 and E7 was 40% NOA 65 to 60% E7 LC. PDLC devices were made by sandwiching the NOA 65 and E7 mixture between two 30 × 40 mm2 indium tin oxide-coated glass plates with the cell gap maintained at 30 μm using poly(methyl) methacrylate (PMMA) polymer ball spacers having a uniform diameter. For PDLC lenses, photopolymerization of the mixture of NOA 65 prepolymer and E7 LC was induced using an UV lamp. UV curing intensity was fixed as 580 μW/cm2 at 365 nm. The samples were not cooled on water cooling plate during UV curing .
Doped and undoped PDLC surfaces were observed by optical microscopy and scanning electron microscopy (SEM). The electro-optical property of the PDLC device was measured as described previously [1-3]. A He-Ne laser with a wavelength of 620 nm and a photodetector were used as the light source and detector, respectively. A digital oscilloscope, function generator, power amplifier and digital multimeter were used for the measurements. Transmittance changes with the voltage applied to the PDLC devices, falling and rising response times and contrast ratio were measured. Slopes of the characteristic transmittance-voltage curves with the applied voltage of the PDLC lens were obtained for versions of smart e-glasses including active shutter glasses for three-dimensional high definition television, and smart glasses with auto-shading and/or auto-focusing functions. Rising and falling response times were measured at an applied voltage of 40 V using a digital oscilloscope and function generator. Contrast ratios were derived by comparing the light transmittance of the on-state with that of the off-state of PDLC lenses.
3 Results and discussion
The optical transmittances-voltage curves of the PDLC polymer lens were well-defined S-shaped curves (Figure 1), except for 0.5% SiO2 nanoparticles. The driving voltage moderately shifted from 25.7 V to the lower voltage of 22.8 V with the increase of SiO2 nanoparticle concentration from 1.0% to 2.0%. The driving voltage for the device with 0.5% SiO2 nanoparticles displayed a voltage of 56.4 V. This sample was not good for the smart electronic glasses because it was unsaturated at the higher voltage, and thus yielded an incomplete optical transmittance-applied voltage curve. When no SiO2 nanoparticles were added, the driving voltage was 23.3 V. Doping of SiO2 nanoparticles into PDLC lenses moderately reduced the driving voltage to 22.8 V.
In the gel formation process, SiO2 nanoparticles may function as heterogeneous nucleation sites (i.e., heterogeneous nuclei). As the amount of SiO2 nanoparticles increased, the heterogeneous nucleation sites for the solidification of prepolymer would correspondingly increase. This increase of heterogeneous nuclei for gel formation of the prepolymer would accelerate polymerization. Faster polymerization seems to induce gel formation from liquid-liquid separation to liquid-gel separation, similar to the cooling that occurs during UV curing , resulting in the reduction of the driving voltage as the concentration of SiO2 nanoparticles is increased. However, the driving voltage of 22.8 V for samples with 2.0% SiO2 nanoparticles was slightly lower than the driving voltage of 23.3 V for PDLC lens without SiO2 nanoparticles. This indicates that the homogenous nucleation rate without SiO2 nanoparticles in PDLC lens is almost similar to heterogeneous nucleation rate of the sample doped with 2.0% SiO2 nanoparticles.
These results supported the view that PDLC lenses undoped and doped with SiO2 nanoparticles differ in phase separation mechanisms. While homogeneous nucleation is the dominant mechanism for phase separation to the solid polymer and LC droplets for the undoped device, heterogeneous nucleation is dominant for doped samples.
For PDLC device with 0.5% SiO2 nanoparticles, the amount of SiO2 nanoparticles was too small to form prepolymer gel by heterogeneous nucleation at the surface of SiO2 nanoparticles. Thus, its optical transmittance-applied voltage curve was immature and exhibited no saturation region at higher voltage.
Since the gel formation of pre-polymer was sufficiently rapid, one can assume that the gel point of NOA 65 prepolymer was reached faster and this early polymerization resulted in intensive liquid-gel separation. Thus, droplets of the heavily doped PDLC device have better molecular arrangement than lightly doped devices, which can be aligned at low voltage . Also, the difference in the initial separation process may explain the improvement of the driving voltage. The initial phase separation from the mixture of NOA 65 and E7 LC into LC droplets surrounded by solid polymer wall changed from a liquid-gel separation to a liquid-liquid separation by heterogeneous nucleation at the surface of SiO2 nanoparticles. The heavier the PDLC lenses were doped with SiO2 nanoparticles, the faster was the heterogeneous nucleation rate.
Deterioration of the slope seemed to be caused by well-aligned molecules in the LC droplets. For this alignment to occur, more opposite force and more time are needed to randomly rearrange at the off state (i.e., slow and incomplete return to the original and random arrangement of molecules in LC droplets). Slow and imperfect rearrangement of LC arrangement induced a mild slope (Figure 2).
These results also can be explained on the basis of a mechanism change from liquid-gel separation to liquid-liquid separation. Deterioration of falling response time seems to have been caused by well-aligned molecules in LC droplets, as discussed earlier. Well aligned LC molecules require time for random rearrangement at the off state (i.e., slow falling response time). Like the driving voltage and the slope of the linear region, the new mechanism for phase separation – liquid-gel separation –caused a slow response time.
In this study, electro-optical properties of polymer dispersed liquid crystal lens were enhanced by doping of SiO2 nanoparticle. Driving voltage of 22.8 V for the sample with 2.0% SiO2 nanoparticles was moderately lower than the driving voltage of 23.3 V for PDLC lenses without SiO2 nanoparticles. The slope of the linear region deteriorated from 9.24%/V for the undoped device to 5.85%/V for the samples of 1.5% SiO2 nanoparticles, which was the minimum value. If conditions that improve the response time and contras ratio are optimized, it is thought to be applied to smart electronic glasses.
This work was supported by the Technology Innovation Program (10041957, Design and Development of fiber-based flexible display) funded by the Ministry of Trade, industry & Energy (MI, Korea) and Civil-Military Technology Cooperation Center.
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