| Summary: | 碩士 === 輔仁大學 === 化學系 === 88 === Abstract
Photochromic dye molecules (Such as azobenzenes) change their molecular shape upon photo irradiation, and this property has been used extensively to control the orientation of liquid crystals by light. Besides, azobenzene liquid crystals have been developed, in which the azobenzene moiety plays roles as both mesogens and photosensitive chromophores. The trans form of azobenzene derivative is rod-like, which stabilizes the LC phase, whereas the cis form is bent and destabilizes the LC phase when it is present. Therefore, the trans-cis photoisomerization of azobenzene in the LC phase can cause disorganization of the phase structure. In addition, the cis-trans back-isomerization can take place either by a thermal process or a photochemical process. All these properties enable the comb-shaped side-chain LC polymer (SCLCP) containing an azobenzene moiety a promising material for optical switch and image storage.
This study synthesized six SCLCPs of this type in which the azobenzene moiety was attached with different electron-withdrawing group (i.e.,-NO2,-CN,-COCH3) on the para position, and the soft spacer used was either (-CH2-)3 or (-CH2-)9 segment. All their structure were characterized by GPC, NMR, IR, UV, DSC, TGA and POM. All there SCLCPs were stable up to 250℃, and exhibited some stable mesophases (mostly Smectic phase), as confirmed by POM and DSC observations.
Photoirradiation was performed with a 100W high-pressure Hg lamp. The change in transmittance of 365 nm absorbance band (corresponding to π-π* transition of trans-azobenzene) was monitored as a function of irradiation time. These results showed that the spacer length and electron-withdrawing capability were the two factors which enhancing the backward isomerization processes, but not for the forward trans-cis isomerization process, Dielectric relaxation studies exhibited a more clear-cut picture for the mesophase transitions of these SCLCP than that of DSC and POM showed. The two factors mentioned above also affects the location of occurrence of β-relaxation.
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