Polymerisable Compounds for Stable Pretilt Angles in PSA Displays
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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 tilt or pretilt angle establishment, low voltage holding ratio (VHR), image sticking, high melting points, limited solubility, and increased viscosity, leading to reduced reliability and performance.
Innovation Solution
The development of novel polymerisable compounds with a mesogenic biphenyl core and specific alkoxy and fluorine or chlorine substituents, which facilitate quick and complete UV-photopolymerisation, reducing residual unpolymerised materials and enhancing stability, solubility, and VHR, even at low temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional polymerisable compounds are used in PSA displays, then the LC medium can be formulated, but the compounds exhibit high melting points, limited solubility, and slow polymerisation, leading to inadequate pretilt angle establishment and low voltage holding ratio
Solution Approach 1:
The patent modifies the chemical structure of polymerisable compounds by introducing specific substituents (fluorine, chlorine, alkoxy groups) and varying spacer lengths to optimize the balance between solubility, melting point, and polymerisation reactivity. This structural parameter adjustment enables the compounds to achieve both fast polymerisation and high voltage holding ratio
Solution Approach 2:
The patent develops composite liquid crystal compositions that combine multiple polymerisable compounds with different functional groups and reactivity levels. This composite approach allows the mixture to exhibit synergistic effects, achieving complete polymerisation within 1-5 seconds while maintaining high solubility and forming stable pretilt angles
2Productivity
If the polymerisation is made faster to improve productivity, then response times are shortened, but the solubility and stability of the compounds are reduced
Solution Approach 1:
The patent carefully adjusts molecular parameters such as spacer length, substituent types, and molecular weight to achieve optimal solubility while maintaining fast polymerisation kinetics. The compounds are designed with specific functional groups that enhance solubility without sacrificing reactivity
Solution Approach 2:
The patent introduces photoinitiators as intermediaries that facilitate rapid polymerisation at low UV exposure levels, allowing the use of highly soluble but reactive polymerisable compounds. The photoinitiator mediates the polymerisation process, enabling fast curing while maintaining compound stability in the liquid crystal mixture
3Reliability
If complete polymerisation is achieved to eliminate residual materials and improve stability, then image sticking and mura are reduced, but the melting point and viscosity of the compounds increase
Solution Approach 1:
The patent optimizes the chemical structure of polymerisable compounds by adjusting molecular weight, substituent patterns, and functional group composition to achieve complete polymerisation at low conversion levels. This prevents excessive crosslinking that would raise melting points and viscosity, while still eliminating residual monomers that cause image sticking
Solution Approach 2:
The patent employs low concentrations of polymerisable compounds (0.1-10 wt%) combined with high-efficiency photoinitiators to achieve complete polymerisation of a small fraction of the liquid crystal mixture. This partial polymerisation approach is sufficient to prevent image sticking and mura without significantly increasing the melting point or viscosity of the overall composition
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 the generation of a stable pretilt angle, reduce image sticking and mura, improve VHR, and shorten response times while maintaining high reliability and solubility, thus addressing the limitations of existing PSA displays.
Implementation Method 1
facilitate quick and complete UV-photopolymerisation
Implementation Method 2
LC medium with negative dielectric anisotropy... On application of an electrical voltage to the two electrodes, a realignment of the LC molecules parallel to the electrode surfaces takes place
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.