Study on the Relationship Between Molecular Structures of Chiral Liquid Crystals Derived from Optically Active Lactic Acid and the Formation of Blue Phases

Autor: Wen-Ling Yang, 楊文菱
Rok vydání: 2012
Druh dokumentu: 學位論文 ; thesis
Popis: 100
A few of liquid crystal materials exhibiting blue phases have been reported in a single compound, moreover, the blue phases in the materials possess a very small temperature range (cal. 0.5-2℃), mainly because of the blue phases are frustration phases. Thus, in order to find more of blue phase materials and shed light on the relationship between molecule structure and the formation of blue phases, in this work, we designed and synthesized five series of compounds derive from optically active lactic acid, according to the previous results from our laboratory, for the investigation. The results of the works are divided into two part for discussion, and the results for discussion are in term of the effect of (i) achiral terminal alkyl chain length ‘m’ of the materials, (ii) chiral terminal alkyl chain length 'n' of the materials and (iii) the achiral terminal alkyl chain directly connected to the benzene ring with ether (-O-) linking group, and achiral terminal alkyl chain connected directly to the benzene ring of the materials, on the generation of blue phase liquid crystals. Five series of the materials for the study with the general formulas are depicted below. The first part of compounds for second series of the materials: Ia(m=6-9, n=0, X=O) Ib(m=6-9, n=1, X=O) The second part of compounds for third series of the materials: IIa(m=6-8, n=0, X= –) IIb(m=6-8, n=1, X= –) IIc(m=6-8, n=2, X= –) The third part of the works was to investigate whether the binary mixture of the blue phase materials could enhance the thermal stability of the blue phase. Thus, binary mixtures (BM-I and BM-II) with the weight percentage ratios of 100/0, 75/25, 50/50, 25/75, 0/100 of the compounds were prepared for the investigation. The corresponding molecular structures of the target compounds are depicted below: The first pairs of compounds for the binary mixture, BM-I The second pair of compounds for the binary mixture, BM-II The results from the study of five new series of novel chiral materials derived from optically active (S)-lactic acid show that, chiral compounds Ia(m=6-9, n=0, X=O) and Ib(m=6-9, n=1, X=O), depending on the molecular structure, various mesomorphic phases: BP, N*, TGBA* and SmA* phases can be found. The appearance of frustrated phases (BP and TGB* phases) indicate that all compounds possess high chirality. The results of chiral compounds IIa(m=6-8, n=0, X= –) display only BP and N* phases, indicating that the alkyl group directly attaches to the phenyl ring without ether (-O-) linking group could depress the formation of higher order smectic phases. Similar results also present in the compounds IIb(m=6-8, n=1, X= –) and IIc(m=6-8, n=2, X= –). In general, compounds derived from (S)-lactic acid with oxygen linking group are easier to generate the stable blue phases as compared to that without oxygen linking group. It is also interested to found that compounds consist of branched chain in the chiral tail favor the formation of the blue phase as compared the corresponding compounds Ia(m=6-9, n=0, X=O), Ib(m=6-9, n=1, X=O) and IIa(m=6-8, n=0, X= –), but it is not feasible to the formation of blue phase as compared the corresponding compounds IIb(m=6-8, n=1, X= –) and IIc(m=6-8, n=2, X= –). The investigation of the binary mixtures on the formation blue phase demonstrates that the mixtures have no any significant enhancement on the thermal stability of the blue phase. In conclusion, our results show that the blue phase materials be designed and synthesized, however, the temperature range of blue phase in this work is only reached as large as 5.2℃. The results of binary mixtures of the blue phase materials also indicate that the enhancement of temperature range of blue phase by the mixing method is not feasible.
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