Liquid Crystal Polymer: Next-Generation Functional Display and Optical Film Material

There are currently various types of optical film products available on the market, with different functionalities. Based on material classification, they can be divided into two main types: isotropic polymer films and anisotropic liquid crystal films. This article will focus on anisotropic liquid crystal films or the raw materials for liquid crystal polymers - UV-reactive liquid crystal materials.

UV-reactive liquid crystal materials

As we know, liquid crystal refers to a state of matter that is between liquid and solid. Some people also refer to it as a mesogen because it exists between the liquid and solid states. Therefore, reactive liquid crystals are also known as Reactive Mesogens by the Merck company.

In simple terms, reactive liquid crystals have a molecular structure that includes a liquid crystal core (core group) and one or more reactive functional groups (reactive groups) at the ends of the molecule, in addition to regular photoelectric liquid crystal molecules. These reactive groups can undergo photopolymerization to form a polymer network, which is the liquid crystal polymer. Since the polymerization initiators used are mostly UV photosensitive (wavelength 254-365nm), they are called UV-reactive liquid crystals.

Advantages of liquid crystal polymer optical films

Traditional optical films are mainly made from polymers that are stretched in a uniaxial or biaxial manner. The originally disordered molecular axes of isotropic polymers become tilted or aligned in an anisotropic manner when stretched, resulting in a difference in the propagation speed of incident light in different directions. This phenomenon is called phase delay (Retardation, R), which can be used to adjust or compensate for the phase of light.

The phase delay can be calculated by multiplying the difference in biaxial refractive indices (Birefringence, △n) and the thickness (d) of the film, i.e., R = △nd. Whether it is rod-shaped or discotic liquid crystal molecules, although the overall anisotropy still depends on the arrangement rules, the birefringence of liquid crystals is generally around 0.1, which is ten times or even a hundred times higher than that of traditional polymer stretched films. Therefore, the optical film made from liquid crystal polymers can have a very thin thickness, making it ideal for roll-to-roll coating processes.

The aforementioned overall anisotropy of liquid crystals is related to their arrangement regularity. Due to the self-assembly property of liquid crystals, under specific conditions, a comprehensive and single regularity called a monodomain can be achieved.

By utilizing different surface treatments and material designs, small molecule liquid crystals can exhibit various alignment orientations, such as planar, vertical, tilted, or hybrid orientations, as shown in Figure 1. This makes them suitable for the design requirements of various optical films.

Therefore, the use of reactive liquid crystals for the production of optical films has advantages such as large areas, monodomains, and aligned morphologies, which cannot be achieved by traditional polymer optical films.

Applications of liquid crystal polymer optical films in the display field

Liquid crystals have various alignment orientations that can be used for the production of optical films with diverse requirements. Currently, this includes wide viewing angle films, brightness enhancement films, polarizing films, and other optical films used in display devices. The following describes the relationships between various optical films and reactive liquid crystals.

Wide viewing angle films

Optical compensation film materials developed for TFT-LCD wide viewing angle characteristics can be broadly divided into extensible polymer types such as PC and PS, reactive liquid crystal types such as discotic liquid crystals and rod-like liquid crystals, and recently used in-cell coating materials such as PI. In terms of compensation function, they can be classified into uniaxial and biaxial films based on the optical axis direction. Uniaxial films refer to compensation films where the refractive indices nx, ny, nz in the xyz directions satisfy the relationship nx=ny≠nz. Biaxial films, on the other hand, refer to compensation films that satisfy the relationship nx≠ny≠nz.

In simple terms, wide viewing angle films are phase compensation films based on the principle of "cutting long and supplementing short". Due to the leakage phenomenon caused by the constraint of liquid crystal display driver molecules under boundary conditions, three types of uniaxial structure compensation films, namely A-plate, O-plate, and C-plate, can be used.

Based on the differentiation of the optical axis of phase compensation films, films with the optical axis parallel to