**1. Introduction**

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In advanced thin film transistor liquid crystal display devices (TFT-LCDs), alignment layers (ALs) are playing an ever-increasing important role for achieving a high-quality optoelec‐ tronic display [1]. The main function of AL materials is to align the rod-like liquid crystal (LC) molecules at a constant angle (so-called pretilt angle) to the local surface. Thus, when the electrical field is applied, the LC molecules can respond rapidly so as to result in a dis‐ play effect. The characteristics of AL materials, including their abilities to achieve a proper pretilt angle for LC molecules, to achieve a high voltage holding ratio (VHR) and a low re‐ sidual direct circuit voltage (RDC) value for the LCD devices, high thermal stability, high mechanical strength to resist rubbing process and their planarization ability are often taken into deliberate consideration in developing new generations of TFT-LCDs [2].

Currently, polyimides (PIs) are one of the most important AL materials for TFT-LCDs due to their intrinsic high thermal resistance, good mechanical properties and unique LC alignment ability [3]. However, conventional wholly aromatic PIs suffer from their poor solubility in common solvents. Thus, in practice, they can only be used in the form of soluble precursors, poly(amic acid)s (PAAs). For example, in conventional twisted nematic LCD (TN-LCD) and super-twisted nematic LCD (STN-LCD) fabrications, wholly aromatic PAA solution is first spin-coated onto an indium tin oxide (ITO) substrate, and then the solution is imidized at elevated temperatures up to 300 o C to form the imidized PI alignment layer. However, the

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high curing temperature of PAAs often causes serious damage for the temperature-sensitive components in TFT-LCDs, such as the color filters which would be destroyed when the tem‐ peratures are higher than 230 o C [4]. Hence, PI alignment layers with low curing tempera‐ tures (<220 o C) have been developed in the past decades [5-7].

In this chapter, a series of novel semi-alicyclic PI alignment layers were designed and syn‐ thesized. The designed novel PI materials are expected to show good solubility in organic solvents (thus low curing temperatures) and exhibit high VHR and low RDC values for TFT-LCDs. For this purpose, a class of substituted-tetralin alicyclic dianhydrides was synthe‐ sized first via a low-cost route using maleic anhydride and substituted styrene compounds as the starting materials (Scheme 1). Then, a series of semi-alicyclic PIs were synthesized from the newly-developed dianhydrides and commercially available aromatic diamines. The effects of the structures of the semi-alicyclic PIs on their thermal stability and optical properties were investigated. At last, a series of test LCD cells with fringe field switching (FFS) mode were fabricated using the novel PIs as the alignment layers. The electrical char‐

Styrene, *p*-methylstyrene, *p*-*tert*-butylstyrene and *p*-fluorostyrene were purchased from To‐ kyo Chemical Industry Co., Ltd., Japan (TCI) and used as received. It is unnecessary to re‐ move the inhibitors in the chemicals. Maleic anhydride was obtained from Beijing Yili Fine Chemicals, China and used as received. 4,4'-Methylenedianiline (MDA, TCI, Japan) was re‐

(*n*-hexadecanoxy)benzene (16PDA) was synthesized in our laboratory and purified by continuous recrystallization from ethanol. Commercially available *N*-methyl-2-pyrrolidi‐ none (NMP), *N,N-*dimethylacetamide (DMAc), cyclopentanone (CPA), γ-butyrolactone (GBL) and ethylene glycol monobutyl ether (butyl cellosolve, BC) were purified by distillation pri‐ or to use. The other commercially available reagents were used without further purification.

Inherent viscosity was measured using an Ubbelohde viscometer with a 0.5 g/dL NMP solu‐

values of the PI films were measured using an X-rite color i7 spectrophotometer with PI film samples at a thickness of 30-40 μm in accordance with the procedure described in ASTM D1925 "Test method for yellowness index of plastics" and in ASTM D1003 "Standard test method for haze and luminous transmittance of transparent plastics", respectively. The col‐

or parameters were calculated according to a CIE Lab equation. *L*\*

means white and 0 implies black. A positive *a*\*

cates a green color. A positive *b*\*

C. Fourier transform infrared (FT IR) spectra were obtained with a Tensor 27 Fourier transform spectrometer. Ultraviolet-visible (UV-vis) spectra were recorded on a Hitachi U-3210 spectrophotometer at room temperature. The cutoff wavelength was defined as the point where the transmittance drops below 1% in the spectrum. Prior to test, PI samples

C. Absolute viscosity was measured using a Brookfield DV-II+ Pro viscometer at

C for 1 h to remove the absorbed moisture. Yellow index (YI) and haze

C overnight prior to use. 2,4-Diamino-

Organo-soluble Semi-alicyclic Polyimides http://dx.doi.org/10.5772/51182 271

is the lightness, where 100

means a red color, and a negative one indi‐

means a yellow color, and a negative one indicates a blue

acteristics (pretilt angle, VHR and RDC) of the cells were preliminarily studied.

crystallized from ethanol and dried in vacuum at 80 o

**2. Experiments**

**2.2. Measurements**

were dried at 100 o

tion at 25 o

25 o

**2.1. Materials**

Besides the curing temperature consideration, the high VHR and low RDC values of the de‐ vices are also highly desired for advanced TFT-LCD fabrication in order to achieve a highresolution display (high contrast, low image sticking, etc) [8]. VHR and RDC values of the TFT-LCD devices are influenced by many factors, including the characteristics of LC materi‐ als, the features of the PI alignment layers and the display modes of the devices. Among the factors, the effects of the chemical structures of the PI alignment layers are often critical. For instance, it has been well established that the highly conjugated molecular skeletons in wholly-aromatic PIs often lead to low VHR and high RDC values for the devices [9]. Thus, PI alignment layers with low conjugated structures have been paid increasing attentions.

Considering the above-mentioned structure-property relationship for PI alignment layers used for advanced TFT-LCDs, alicyclic or semi-alicyclic PIs have been confirmed to be the best candidates as AL materials up to now. Especially, semi-alicyclic PIs derived from alicy‐ clic dianhydrides and aromatic diamines possess the best combined properties, including good thermal stability, good solubility in organic solvents, acceptable mechanical properties, good optical transparency, high VHR and low RDC values. Thus, semi-alicyclic PIs have been widely investigated as AL materials for advanced TFT-LCDs [10-12]. Figure 1 illus‐ trates the developing trajectory of PI alignment layers with different kinds of display modes.

**Figure 1.** Developing trends of PI alignment layers for LCDs.

In this chapter, a series of novel semi-alicyclic PI alignment layers were designed and syn‐ thesized. The designed novel PI materials are expected to show good solubility in organic solvents (thus low curing temperatures) and exhibit high VHR and low RDC values for TFT-LCDs. For this purpose, a class of substituted-tetralin alicyclic dianhydrides was synthe‐ sized first via a low-cost route using maleic anhydride and substituted styrene compounds as the starting materials (Scheme 1). Then, a series of semi-alicyclic PIs were synthesized from the newly-developed dianhydrides and commercially available aromatic diamines. The effects of the structures of the semi-alicyclic PIs on their thermal stability and optical properties were investigated. At last, a series of test LCD cells with fringe field switching (FFS) mode were fabricated using the novel PIs as the alignment layers. The electrical char‐ acteristics (pretilt angle, VHR and RDC) of the cells were preliminarily studied.
