Preparation and Characterization of Semi-Alicyclic Polyimide Resins and the Derived Alignment Layers for Liquid Crystal Display Technology

Abstract

Uniform alignment of rigid-rod liquid crystal (LC) molecules under applied voltage is critical for achievement of high-quality display for thin-film transistor-driven liquid crystal display devices (TFT-LCDs). The polymeric components that can induce the alignment of randomly aligned LC molecules are called alignment layers (ALs). In the current work, a series of organo-soluble polyimide (SPI) ALs were designed and prepared from an alicyclic dianhydride, hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA), and various aromatic diamines, including 4,4′-methylenedianiline (MDA) for SPI-1, 4,4′-aminodianiline (NDA) for SPI-2, 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenylmethane (TMMDA) for SPI-3, and 3,3′-diethyl-5,5′-dimethyl-4,4′-diaminodiphenylmethane (DMDEDA) for SPI-4. The derived SPI resins were all soluble in N-methyl-2-pyrrolidone (NMP). Four SPI alignment agents with the solid content of 6 wt.% were prepared by dissolving the SPI resins in the mixed solvent of NMP and butyl cellulose (BC) (NMP/BC = 80:20, weight ratio). Liquid crystal minicells were successfully fabricated using the developed SPI varnishes as the LC molecule alignment components. The SPI ALs showed good alignment ability for the LC molecules with the pretilt angles in the range of 1.58°–1.97°. The LC minicells exhibited good optoelectronic characteristics with voltage holding ratio (VHR) values higher than 96%. The good alignment ability of the SPI ALs is mainly attributed to the good comprehensive properties of the SPI layers, including high volume resistivity, high degree of imidization at the processing temperature (230 °C), good rubbing resistance, good thermal stability with glass transition temperatures (Tgs) higher than 260 °C, and excellent optical transparency with the transmittance higher than 97% at the wavelength of 550 nm.

Synthetic strategies for highly transparent and colorless polyimide film

“…In recent years, transparent and colorless polymers with good thermal and mechanical properties have been successfully applied in the fields of display devices, optical films, flexible solar cells, flexible printed circuit boards, touch panels and so on, 1–16 making these materials a new hotspot in polymer materials research. With the increasing demands for lighter, thinner, more portable, and foldable display devices, replacing the fragile inorganic glass substrates with transparent and colorless polymer plastic substrates is a predictable trend.…”

Transparent polyimide films with ultra-low coefficient of thermal expansion

“…14 On the premise of the good heat resistance, mechanical properties and low dielectric constant of traditional polyimide films, the formation of charge transfer complex (CTC) could be reduced through the design of molecular structure, and thus the absorption of visible light is inhibited, resulting in the improvement of the optical properties. [15][16][17][18][19][20][21] The formation of intermolecular CTC is usually destroyed by introducing non-conjugated structures (ether bond, alicyclic structure) 2,7,22 and high electronegative groups (fluorine-containing groups) 2,14,[23][24][25] into the molecular structure. However, the improvement of optical properties by reducing the conjugation of electron clouds is at the expense of the loss of excellent heat resistance.…”

Fabrication of TiO2-Embedded Polyimide Layer with High Transmittance and Improved Reliability for Liquid Crystal Displays

“…The ITO electrode is used not only in LCDs but also in many displays such as organic light emitting diodes (OLED) and quantum dot light emitting diodes (QLED) [ 6 , 7 , 8 , 9 ]. In addition, the alignment layer that determines the initial orientation of the LC is also important, and the most commonly used material for this layer is polyimide (PI) [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. The LC can be uniformly aligned along a desired direction through the interactions of the LC molecules at the molecular level.…”