Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

Figures & data

Figure 1 Chemical structures of compounds used in this work. Phase transitions and corresponding temperatures are given at the bottom of panel (a).

Compositions of LC mixtures.

LC mixture	LC (wt%)			Acrylic prepolymer (wt%)			Photoinitiator (wt%)
	Pure NLC	Pure SmA∗		AC10^a	B1	B2	C
NLC/B1^b	97.80	–		1.8	0.30	–	0.1
NLC/B2^b	97.78	–		1.8	–	0.32	0.1
NSLC/1^b	97.56	0.222^c		1.8	–	0.32	0.1
NSLC/2	97.34	0.444		1.8	–	0.32	0.1
NSLC/3	97.11	0.666		1.8	–	0.32	0.1

^a The alkyl carbon number of the acrylic prepolymer A is 10.

^b LC devices with the best electrooptical properties and highest contrast ratios were obtained when the B1 content of the NLC/B1 mixture was 0.30 wt% and the B2 content of the NLC/B2 mixture was 0.32 wt% [40, 41]. Thus, we used NLC/B2 as the basis in studying the new NSLC mixture.

^c In the NSLC mixture, the component ratio was SmA∗: A_C10+B2+C=1:10. Contrast ratio, threshold voltage, driving voltage, gray-scale switching, response time and other parameters depended on SmA∗ concentration.

Figure 2 Schematics of multicomponent LC mixture cells after UV irradiation, showing formation of double VACOF layers due to phase separation.

Figure 3 Tilt angle θ of LC molecules deduced by comparing projection length d of LC molecules (obtained from XRD data) with theoretical length L.

Figure 4 Electrooptical effects related to photoinduced vertical alignment. (a) Dark state before application of voltage. (b) Bright state after application of voltage.

Figure 5 Definitions of rise time (τ_on) and fall time (τ_off) of LC molecule orientation.

Figure 6 UV conversion percentages of NLC/B1, NLC/B2, NSLC/1, NSLC/2 and NSLC/3 mixtures versus UV irradiation time (inset: time required to reach 89% conversion).

Phase-transition temperatures deduced from DSC thermograms, enthalpies of transitions, and optical textures observed with POM during second cooling (cell thickness 4 μm). The phase-transition temperatures were taken at the maxima of endothermic and exothermic DSC peaks during the second heating and cooling cycles at 5 °C min^-1. For the heating and cooling processes, we used positive (negative) values for endothermic (exothermic) variations in enthalpy. Cr=crystalline, N=nematic and I=isotropic liquid phase; •=phase exists.

LC mixture	Phase-transition temperature (°C)/enthalpy (J g^-1)
	Cr	Heating	N	Heating	I	POM image
		Cooling		Cooling
Pure NLC	•	−20.0 / +27.2	•	91.8 / +2.2	•
	•	−20.2 / −27.1	•	92.1 / −2.1	•
NSLC/1	•	−20.3 / +34.8	•	91.6 / +2.1	•
	•	−20.5 / −34.5	•	91.7 / −2.1	•
NSLC/2	•	−20.7 / +35.6	•	91.2 / +2.1	•
	•	−20.8 / −35.3	•	91.4 / −2.0	•
NSLC/3	•	−21.3 / −35.5	•	91.0 / −2.1	•
	•	−21.3 / −35.5	•	91.0 / −2.1	•

Figure 7 Spacing between SmA∗ LC layers as function of temperature during cooling from isotropic liquid phase. Top row: photographs and conoscopic patterns of SmA∗ LC compound. The inset in the main figure shows the structure and length of the SmA∗ LC molecule simulated with the CS Chem3D Ultra software. The right part shows the XRD patterns of the SmA∗ compound. I=isotropic liquid phase; Cr1,2=crystalline phase 1, 2.

Figure 8 Transmittance versus applied voltage for (a) NLC/B1, (b) NLC/B2, (c) NSLC/1, (d) NSLC/2 and (e) NSLC/3 mixtures. Inset: POM images before and after a voltage was applied to the LC cells.

Threshold voltages (V_th), driving voltages (V_on) and contrast ratios of LC mixtures.

LC mixture	Vth (V)	Von (V)	Vsat (V)	Contrast ratio
NLC/B1	1.64	2.27	6	1300:1
NLC/B2	1.80	2.37	6	2028:1
NSLC/1	1.81	3.92	7	2255:1
NSLC/2	1.93	4.78	7	1854:1
NSLC/3	2.23	4.96	7	1700:1

Rise times (τ_on), fall times (τ_off) and total response times (τ^tot) of LC mixture systems.

LC mixture	τon (ms)	τoff (ms)	τtot (ms)
NLC/B1	8.21	8.50	16.71
NLC/B2	2.03	17.88	19.91
NSLC/1	9.08	5.10	14.18
NSLC/2	11.17	5.32	16.49
NSLC/3	13.81	5.94	19.75

HoC YLeeJ Y 2010 Liq. Cryst. 37 997 http://dx.doi.org/10.1080/02678291003746247

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HoC YTsaiP SLinH GLiF CLinF HLeeJ Y 2011 Polym. Adv. Technol. at press

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