Multireactive Monomers for PSA Liquid Crystal Alignment
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Solution Overview
Problem
Current liquid crystal (LC) displays, particularly PSA displays, face issues with inadequate tilt or pretilt angles, voltage holding ratio (VHR), solubility, and high viscosities, leading to problems like 'image sticking' and prolonged response times, which affect contrast, luminance, and viewing angles.
Innovation Solution
The use of multireactive polymerisable compounds with spacer groups in the LC medium, which facilitate rapid and complete polymerisation, reducing residual unpolymerised materials and improving electrical properties, such as VHR and solubility, while maintaining low pretilt angles and viscosity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional polymerisable compounds are used in PSA displays, then the polymerisation process occurs, but the polymerisation is incomplete and residual unpolymerised materials remain, causing image sticking and inadequate voltage holding ratio
Solution Approach 1:
The patent changes the chemical parameters of the polymerisable compound by introducing multifunctional monomers with multiple polymerisable groups (e.g., diacrylate, trimethacrylate) and adjusting molecular weight and functional group density. This enables complete polymerisation by providing more reaction sites, eliminating residual monomers that cause image sticking, and achieving adequate voltage holding ratio through complete conversion of polymerisable groups.
2Reliability
If multireactive polymerisable compounds are used to achieve complete polymerisation, then residual unpolymerised materials are reduced, but the viscosity of the LC medium increases and solubility decreases
Solution Approach 1:
The patent optimizes parameters by selecting multifunctional monomers with specific molecular weights and flexible spacer groups (e.g., propylene oxide, ethylene glycol chains) that maintain low viscosity and high solubility in the liquid crystal host mixture while providing multiple polymerisable groups for complete polymerisation. The balance between reactivity and fluidity is achieved through careful molecular design.
Solution Approach 2:
The patent creates a composite liquid crystal medium combining the host liquid crystal mixture with multifunctional polymerisable compounds that have complementary properties. The host mixture provides solubility and low viscosity, while the multifunctional monomers provide complete polymerisation capability. This composite approach allows the system to achieve both complete polymerisation and maintain desirable flow and solubility characteristics during the liquid state.
3Productivity
If the polymerisation is made rapid and complete, then the establishment of desired tilt angles is faster, but the response times of the display are prolonged
Solution Approach 1:
The patent segments the polymerisation process into controlled stages using photopolymerisation with specific wavelength UV light and photoinitiators. The polymerisation occurs during manufacturing to establish the alignment layer and tilt angles, while the final display operation uses electrical fields for rapid switching. This segmentation allows complete polymerisation for stability without affecting the rapid electrical response during display operation, as the polymer network provides structural stability while the liquid crystal molecules maintain mobility under electrical fields.
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
This approach enables faster establishment of desired tilt angles, higher VHR values, reduced 'image sticking', and improved display performance with shorter response times and broader viewing angles, enhancing the overall efficiency and reliability of LC displays.
Implementation Method 1
a polymerisable compound... wherein the polymerisable compound is polymerised
Implementation Method 2
the LC medium usually has a negative value of the dielectric (DC) anisotropy. In the switched-off state, the molecules of the LC layer are aligned perpendicular to the electrode surfaces... 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 3
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 application relates to a liquid crystal (LC) medium comprising a polymerisable compound and one or more mesogenic or liquid crystalline compounds comprising an alkenyl group, as well as to a process for its preparation, to its use for optical, electro-optical and electronic purposes, in particular in an LC display, especially in an LC display of the PSA ("polymer sustained alignment") type, and to an LC display comprising it.