Chapter 9
Dielectric and Electro-Optical Properties of a Ferroelectric Side-Chain Liquid Crystalline Polysiloxane Containing Azobenzene Dyes as Guest Molecules 1
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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 3, 2016 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch009
Rong-Ho Lee , Ging-Ho Hsiue , and Ru-Jong Jeng 1
Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan 400, Republic of China
2
Guest-host systems of a ferroelectric side-chain liquid crystalline polysiloxane (PS121A) containing azobenzene dyes have been investigated. Disperse orange 3 (DO3) with a strong dipole moment, and a liquid crystal 4'-(5-hexenyloxy)-4-methoxyazobenzene (HMAB) with a relatively weak dipole moment were used as the guest molecules. The intensity and frequency of the Goldstone mode were increased remarkably for the PS121A/DO3, due to the doping of the DO3 (5 wt. %). The guest molecule with a strong dipole moment results in larger fluctuation of the spontaneous polarization vector in each smectic layer under an applied electric field. As a result, large spontaneous polarization and short response time were obtained for this system. On the other hand, the doping effect of HMAB on dielectric and electro-optical properties was not significant for this system compared to that of the PS121A/DO3. The intensity and frequency of Goldstone mode were slightly increased even with a higher doping level (15 wt. %) of HMAB. PS121A/HMAB exhibited a smaller spontaneous polarization and longer response time than PS121A/DO3 sample. The doping of a suitable amount of the azobenzene dye in the LC phase of the FLCP was helpful for the improvement of the electro-optical properties of such guest-host system. Ferroelectric liquid crystals (FLC) have attracted attention because of their high speed response and memory effect (1-3). The characteristics of fast response and memory effect make them suitable in electro-optical device applications, such as display, light valve and memory devices. Ferroelectric side chain liquid crystalline polymers (FLCPs) exhibit desirable mechanical properties of polymers and electro-optical properties of low molecular weight FLC, which have been investigated extensively Corresponding author.
© 1999 American Chemical Society In Field Responsive Polymers; Khan, Ishrat M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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130 (4-5). The excellent film formability and electro-optical properties of FLCP is desirable for large area displays (6). FLCPs exhibiting a chiral smectic C (Sc*) phase over a broad temperature range (about 200 °C including room temperature) have been synthesized recently in our laboratory (7-8). FLCPs with such mesomorphic behavior have promising potential for electro-optical applications (9). However, high viscosity reduces the response ability of the mesogenic group in the Sc* phase toward an applied electric field for this type of FLCPs. This is due to the fact that the long and rigid mesogenic core has a lower mobility, thereby resulting in slow response toward the electric field. Therefore, the improvement of the thermal behaviors and electro-optical properties become a pertinent topic for this type of FLCPs. One of the approaches is the guesthost system consisting of an FLCP and an azobenzene dye (10). In this approach, it is found that the doping of the low molecular weight azobenzene dye into the Sc* phase would enhance the thickness of the smectic layer, and result in the high mobility of the mesogenic group for the FLCP (10). When a FLCP exhibits high molecular mobility in the fluctuation of the director of the tilt angle (Goldstone mode), the threshold field for switching can be lowered. As a result, the helical structure of the FLCP in the Sc phase can be unwound completely. Consequently, high spontaneous polarization (P ) and short response time (x) can be obtained. In this study, we describe here guest-host systems of an FLCP (PS121A) containing azobenzene dyes. Disperse orange 3 (D03) with a strong dipole moment, and a liquid crystal 4'-(5-hexenyloxy)-4-methoxyazobenzene (HMAB) with a relatively weak dipole moment were used as the guest molecules, respectively. The phase transitions and mesophase of the guest-host FLC polymeric materials were studied using differential scanning calorimeter and optical polarizing microscopy. Dielectric measurements were taken from a low temperature range over the liquid crystal phase. The relaxation behavior of the molecular and collective relaxations will be discussed. Moreover, temperature dependence of the P and x were also taken from the electro-optical measurements. The doping effect of two azobenzene dyes on the dielectric and electro-optical properties of the FLCP was investigated. s
s
Experimental Chemical structures of PS 121 A , D03 and H M A B are shown in Figure 1. The mixtures of the FLCP and azobenzene dyes (FLCP/D03 or FLCP/HMAB) with different weight ratios (sample DO305: 95/5, DO310: 90/10, DO330:70/30 and HMAB05: 95/5, HMAB10: 90/10, HMAB15: 85/15) were obtained by dissolving the compounds in chloroform. The solution of the mixture was kept in vacuum oven at room temperature to completely dry out chloroform. The thermal transitions of azobenzene dyes and guest-host FLC polymeric materials were determined by a differential scanning calorimeter (Seiko SSC/5200 DSC). The thermal transitions were read at the maximum of their endothermic or exothermic peaks. Glass transition temperature (Tg) was read at the middle of the change in heat capacity. Heating and cooling rates were 10 °C/min in all of these cases. The transitions were collected from the second heating and cooling scans. A
In Field Responsive Polymers; Khan, Ishrat M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 3, 2016 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch009
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