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Title: The effects of additives in liquid crystal display systems
Author: Cho, Y.-I.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2005
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Most of the previous attempts to make displays with weak anchoring conditions relied upon there being a weak interaction between the liquid crystal materials and the alignment layer. In this thesis, perfluorinated additives are used to make a slippery surface in order to induce weak anchoring conditions. This slippery surface approach via perfluorinated additives has the advantage that it can be utilized in current display systems without the need to change any of the mechanical components. The perfluorinated additives were designed to have an affinity with either the host LCs or the alignment layer in the LCD cells. The additives were tested in twisted nematic (TN) and super twisted nematic (STN) LCD cells and induced weak anchoring conditions depending on the chemical structure of the additives and the host liquid crystal mixture. In each experiment, 0.5% of additive was mixed with the chosen host and did not phase separate in the LCD cells. The response time of the host was reduced by 3.5% to 13% depending upon the host-additive combination used. Studying the relationship between the chemical structure of the additives and their effectiveness in reducing the response time of the host revealed that the structural requirements for an additive to decrease the response time of E7 (one of the nematic hosts used) are as follows: moderate polarity, medium or small molecular size, ester or ether linking groups or hydroxyl functional groups and a mixture of small and medium sized molecules. However, the reduction in the response time of the liquid crystal system was limited in some cases, because the reduction in the turn-on time was outweighed by the increase in the turn-off time of the host LC mixture. The effects of the perfluorinated additives on the flexo-electro optic switching of a chiral nematic liquid crystal were also examined. The perfluorinated additives reduced the response time of the host LC by 8% to 30%. We also found that the performance of the flexo-electro optic switching was mostly influenced by the effect of the additives on the bulk properties of the host LC (as opposed to the effect on the properties at the substrate). In conclusion the perfluorinated additives reduced the response time of the host by inducing a slippery surface in some limited cases and can be used for: providing the weak anchoring necessary for the manufacture of bistable displays; increasing the temperature range of blue phase materials and improving flexo-electro optic switching.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available