Polymerizable LC Medium for Stable PSA Display Alignment
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Solution Overview
Problem
Existing LC displays face challenges with incomplete polymerization of reactive mesogens (RMs) leading to residual unpolymerized RMs, which cause image sticking and tilt angle instability, especially under UV exposure, and require faster and more reliable polymerization at longer UV wavelengths.
Innovation Solution
The use of LC media comprising specific polymerizable compounds with formula I and II, which enable complete and controlled polymerization of RMs, even at longer UV wavelengths, ensuring high reliability and stability of the tilt angle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If shorter UV wavelengths are used to polymerize RMs, then polymerization effectiveness is improved, but reliability of the LC mixture decreases due to reaction with alignment layer polyimide
Solution Approach 1:
The patent changes the wavelength parameter of UV irradiation from shorter wavelengths (e.g., 300-320nm) to longer wavelengths (e.g., 365nm or 385nm). This parameter change allows effective polymerization of RMs while avoiding the harmful reaction with the polyimide alignment layer, thus resolving the contradiction between polymerization effectiveness and mixture reliability
Solution Approach 2:
The patent introduces a photosensitizer as an intermediary substance that enables polymerization at longer UV wavelengths. The photosensitizer absorbs the longer wavelength UV light and transfers energy to initiate RM polymerization, acting as a mediator that decouples the polymerization process from the harmful short-wavelength UV radiation that reacts with the alignment layer
2Reliability
If longer UV wavelengths are used to avoid alignment layer reaction, then reliability is improved, but polymerization effectiveness decreases
Solution Approach 1:
The photosensitizer serves as an intermediary that bridges the gap between longer UV wavelengths and RM polymerization. It absorbs the longer wavelength UV light (365nm or 385nm) that doesn't harm the alignment layer and converts this energy into the activation needed for RM polymerization, thus maintaining both reliability and productivity
Solution Approach 2:
The patent changes the irradiation wavelength parameter to longer values and compensates for the reduced direct polymerization effectiveness by introducing a photosensitizer that is specifically tuned to absorb at these wavelengths, thereby restoring polymerization effectiveness while maintaining the reliability benefits of longer wavelengths
3Loss of time
If residual unpolymerized RMs remain after polymerization, then polymerization time is reduced, but image sticking and tilt angle instability occur
Solution Approach 1:
The photosensitizer enables complete and uniform polymerization of RMs throughout the LC mixture by efficiently transferring energy from UV light to RMs. This results in thorough polymerization with minimal residual unpolymerized RMs, eliminating image sticking and tilt angle instability while maintaining reasonable polymerization times
Solution Approach 2:
The patent employs extended UV irradiation time with longer wavelengths and a photosensitizer to ensure continuous and complete polymerization throughout the entire LC mixture. This continuous action ensures that even RMs in difficult-to-reach locations are fully polymerized, eliminating residuals that would cause reliability problems
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
The solution provides LC media with high VHR, reliability, low image sticking, and stable tilt angles, suitable for PSA and SA displays, using UV-LED lamps for efficient polymerization.
Implementation Method 1
After filling the LC medium into the display, the RMs are then polymerized in situ by UV photopolymerization, while a voltage is applied to the electrodes of the display.
Implementation Method 2
UV-LED lamps have also been proposed for use in the PSA process, as they show less energy consumption, longer lifetime, and more effective optical energy transfer to the LC medium due to the narrower emission peak
Data Source
AI summary
The present invention relates to an LC medium (as a subcategory of liquid crystal material), comprising one or more polymerizable compounds, to its use for optical, electro-optical and electronic purposes, in particular in LC displays, especially in LC displays of the PSA (polymer sustained alignment) or SA (self-aligning) mode, to an LC display of the PSA or SA mode comprising the LC medium, and to a process of manufacturing the LC display using the LC medium, especially an energy-saving LC display and energy-saving LC display production process.