Measurement Instruments and Sample Fabrication
4.4 Polar anchoring energy
The physical properties at the boundary between alignment layer and liquid crystal are of great impact to the performances of LC devices. Polar anchoring energy is one of the significant factors directly influences the electro-optic characteristics of TN cells.
Patterned ITO glasses are used as substrates for polar anchoring energy measurement. The area of electrode is 1 cm2. The alignment directions of two substrates are anti-parallel and the cell gap is around 10μ m. Nematic LC (Merck MJ 051989) is injected into the empty anti-parallel cells. Liquid crystal analysis system is used to measure the polar anchoring energy based on the capacitance method and Eq.
3-14 in the section 3.5 is used for the calculation of the polar anchoring energy. We determine the polar anchoring energy at different rubbing strength conditions for various alignment materials and elucidate the electro-optical properties of TN cells affected by the anchoring energy.
We measure the polar anchoring energy for polyimide at three different rubbing strength conditions, as described in Table 4.4. N is the number of rubbings, M is the pile impression (mm), n is the rotation speed of the roller (s-1), V is the advancing speed of the substrate (mm/s). The magnitude of rubbing strength is referred in the
section 2.1.1.
Strong rubbing Medium rubbing Weak rubbing
N 5 1 1
M 0.2 mm 0.2 mm 0.2 mm
n 8.3rpm/s 8.3rpm/s 0.83rpm/s
V 7.3mm/s 7.3mm/s 10.95mm/s
Rubbing
Strength 159.66mm 31.93mm 1.94mm
0
Table 4.4 Rubbing parameters at three different rubbing conditions
The values of C , C// and Cinf can be obtained from Fig. 4-3. In this experiment, the electrode area S is 0.0001 m2. The parameters for determining the polar anchoring energy of polyimide from Eq. 3-14 are listed in detail in Table 4-5.
The values of polar anchoring energy for polyimide at strong, medium, and weak rubbing are 1.07x10-3 J/m2, 3.15x10-4 J/m2, 1.19x10-4 J/m2, respectively. There is a considerable difference of an order in the magnitude of polar anchoring energy for polyimide at strong and medium rubbing conditions. The larger value of polar anchoring energy can enhance the electro-optical performance. The electro-optical
Vth(V) C(pF) C//(pF) Cinf (pF) W (J/m2) Strong PI 1.1 299.3 1082.7 1087.8 1.07×10-3 Medium PI 1.1 348.6 1262.4 1286.4 3.15×10-4 Weak PI 1.1 339.7 1228.1 1290.1 1.19×10-4 Table 4-5 Necessary parameters for determining polar anchoring
energy of polyimide at different rubbing strength conditions
Fig. 4-3 Capacitance dependence on applied voltage for polyimide at strong, medium, and weak rubbing conditions, respectively
y = -1958.4x + 1290.1
property of polyimide at strong rubbing strength shows lower threshold and saturation voltages of 0.21 V and 0.36 V compared to those at medium rubbing strength, as illustrated in Fig. 4-4. Weak rubbing strength causes poor performance of V-T curve of polyimide. In general, rubbed PI is one kind of strong polar anchoring energy, as known in literature [22, 35, 75].
Fig. 4-5 indicates the specific values of C, C// and Cinf for rubbed PSSA at
Fig. 4-4 Electro-optical properties of TN cells using rubbed PI at weak, medium, and strong rubbing strengths. The inset shows normalized V-T curves
0 1 2 3 4 5 6 7 8
The comparison of V-T curve for rubbed PSSA at strong and medium rubbing strengths is described in Fig. 4-6. There is a slight shift of 0.12 V and 0.25 V in the values of threshold and saturation voltages of rubbed PSSA at these two rubbing conditions. Generally, we can suppose that PSSA induced by rubbing treatment is one kind of medium polar anchoring energy (on the order of 10-4).
Vth(V) C(pF) C//(pF) Cinf (pF) W (J/m2) Strong PSSA 1.1 366.1 1291.4 1087.8 2.65×10-4 Medium PSSA 1.1 340.3 1149.8 1202.2 1.08×10-4 Table 4-6 Necessary parameters for determining polar anchoring
energy of PSSA at different rubbing strength conditions
1/V
Fig. 4-5 Capacitance dependence on applied voltage for PSSA at strong (top) and medium (bottom) rubbing conditions
0
Capacitance dependence as a function of applied voltage and the linear fit curve supply the values of C, C// and Cinf for rubbed PVP, as observed in Fig. 4-7. In the case of rubbed PVP, the values of polar anchoring energy at strong and medium rubbing strengths are 5.46x10-4 J/m2 and 1.99x10-4 J/m2 in turn, as observed in Table 4-7. Despite of being rubbed at strong rubbing condition, there is only a small increase in the value of polar anchoring energy at strong rubbing condition.
0 1 2 3 4 5 6
Fig. 4-6 Electro-optical properties of TN cells using rubbed PSSA at medium and strong rubbing strengths. The inset shows the normalized V-T curves
0 1 2 3 4 5 6
The increase of polar anchoring energy can result in a better performance of electro-optical properties of rubbed PVP at strong rubbing strength, as observed in Fig.
4-8. The threshold and saturation voltages of rubbed PVP at strong rubbing strength are lower in the magnitude of 0.26 V and 0.29 V, respectively, than those at medium
Vth(V) C(pF) C//(pF) Cinf (pF) W (J/m2)
Fig. 4-7 Capacitance dependence on applied voltage for PVP at strong (top) and medium (bottom) rubbing conditions
Table 4-7 Necessary parameters for determining polar anchoring energy of PVP at different rubbing strength conditions
rubbing strength. Although the V-T characteristics are improved, the values of polar anchoring energy at these two rubbing conditions are on the order of 10-4 in general.
Therefore, it can be suggested that rubbed PVP has medium polar anchoring energy
Fig. 4-8 Electro-optical properties of TN cells using rubbed PVP at medium and strong rubbing strengths. The inset shows the normalized V-T curves
0 1 2 3 4 5 6
Fig. 4-9 Electro-optical properties of TN cells using rubbed PI, PSSA, and PVP at strong rubbing strength. The inset shows the normalized V-T curves
Fig. 4-9 illustrates V-T curves made from rubbed PI, PSSA, and PVP at strong rubbing strength. Weaker polar anchoring energy of rubbed PSSA and PVP can result in their electro-optical characteristics are not really as perfect as those of rubbed PI.
Besides, the transmittance property and order parameter (as observed in Table 4-8) are almost identical for both rubbed PSSA and PVP layers.
Strong PI Strong PSSA Strong PVP
Order parameter 0.6 0.55 0.57
Fig. 4-10 Capacitance dependence on applied voltage for PVPD at strong (top) and medium (bottom) rubbing conditions
Table 4-8 Order parameters of rubbed PI, PSSA, and PVP at strong rubbing strength
Similarly, the values of polar anchoring energy for PVPD at strong and medium rubbing strengths are 8.96x10-5 J/m2 and 5.82×10-5 J/m2, as seen in Fig. 4-10 and Table.
4-9. Therefore, rubbed PVPD is considered as one type of weak polar anchoring energy. Besides, rubbed PS is also known as one kind of weak polar anchoring energy [35, 80].
Overall, the polar anchoring energy affects V-T properties. A better performance in terms of electro-optical characteristics is found at larger values of polar anchoring energy. In general, rubbed PI has strong polar anchoring energy (on the order of 10-3).
Rubbed PSSA and PVP are kinds of medium polar anchoring energy (on the order of 10-4). Rubbed PVPD and PS are types of weak polar anchoring energy (on the order of 10-5).
Vth(V) C(pF) C//(pF) Cinf (pF) W (J/m2) Strong PVPD 1.1 352.4 1285.5 1378.8 8.96×10-5 Medium PVPD 1.1 391.9 1385.6 1550.2 5.82×10-5 Table 4-9 Necessary parameters for determining polar anchoring
energy of PVPD at different rubbing strength conditions