Meta-terphenyl Reactive Mesogens for PSA LCD Alignment
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Solution Overview
Problem
Current liquid-crystal display (LCD) technologies, particularly in the polymer sustained alignment (PSA) type, face issues such as inadequate pretilt angle generation, high melting points of reactive mesogens (RMs), limited solubility, spontaneous crystallization, image sticking, and high viscosities leading to increased switching times and reduced reliability, especially under UV exposure.
Innovation Solution
The use of polymerisable compounds with a meta-terphenyl mesogenic core and reactive groups attached via spacer groups, which facilitate quick and complete UV-photopolymerisation at longer wavelengths without photoinitiators, reducing residual unpolymerised RMs and improving pretilt angle stability and solubility, thus enhancing display performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional reactive mesogens are used in PSA displays, then pretilt angle can be generated, but the melting points are high and solubility is limited leading to spontaneous crystallization
Solution Approach 1:
The patent modifies the molecular structure of reactive mesogens by introducing meta-terphenyl cores with specific spacer groups and terminal groups, changing physical parameters such as melting point and solubility while maintaining polymerisation functionality. This structural parameter change allows the compounds to remain soluble at lower temperatures and prevent spontaneous crystallization.
Solution Approach 2:
The patent creates composite liquid crystal media by combining meta-terphenyl-based reactive mesogens with host liquid crystal mixtures. This composite approach ensures compatibility and solubility while maintaining the desired pretilt angle generation and polymerisation properties, preventing phase separation and crystallization.
2Reliability
If conventional RMs are used, then polymerisation can occur, but high viscosities lead to increased switching times
Solution Approach 1:
The patent optimizes the viscosity parameter by designing meta-terphenyl-based compounds with specific molecular weights and structural flexibility through spacer groups. These parameter changes reduce the viscosity of the liquid crystal medium, enabling faster molecular reorientation and shorter switching times while maintaining adequate voltage holding ratio after polymerisation.
3Stability of the object's composition
If conventional RMs are used, then alignment can be sustained, but image sticking and mura occur under UV exposure
Solution Approach 1:
The patent employs meta-terphenyl-based reactive mesogens that are designed to polymerise completely and uniformly under UV exposure, creating a stable polymer network that sustains alignment without causing image sticking. The compounds are optimized to minimize residual unpolymerised material that could cause harmful effects, effectively replacing problematic conventional RMs.
Solution Approach 2:
The patent changes the photochemical parameters of the reactive mesogens by incorporating meta-terphenyl structures with specific absorptivity characteristics. These parameter changes enable complete and uniform polymerisation under standard UV curing conditions, preventing the formation of residual monomers that cause image sticking and mura defects while maintaining alignment stability.
4Reliability
If conventional RMs are used, then pretilt angle can be generated, but inadequate pretilt angle and high threshold voltage occur
Solution Approach 1:
The patent optimizes the pretilt angle parameter by designing meta-terphenyl-based compounds with specific spacer group lengths and terminal group configurations. These parameter changes enable the generation of adequate pretilt angles (typically 5-25 degrees) that reduce the threshold voltage required for switching while maintaining high voltage holding ratio after polymerisation, improving overall display efficiency.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
These compounds enable high specific resistance, low threshold voltage, fast response times, reduced image sticking and mura, and improved reliability with high voltage holding ratio (VHR) values, even after UV exposure, while maintaining low melting points and solubility.
Implementation Method 1
facilitate quick and complete UV-photopolymerisation at longer wavelengths without photoinitiators
Implementation Method 2
In the switched-off state, the molecules of the LC layer are aligned perpendicular to the electrode surfaces (homeotropically)
Implementation Method 3
On application of an electrical voltage to the two electrodes, a realignment of the LC molecules parallel to the electrode surfaces takes place
Implementation Method 4
OCB displays normally contain one or more birefringent optical retardation films in order to prevent undesired transparency to light of the bend cell in the dark state
Data Source
AI summary
The present invention relates to polymerisable compounds, to processes and intermediates for the preparation thereof, to liquid-crystal (LC) media comprising them, and to the use of the polymerisable compounds and LC media for optical, electro-optical and electronic purposes, in particular in LC displays, especially in LC displays of the polymer sustained alignment type.