In-Situ Observation of Thermal Amplification of Photogenerated Optical Anisotropy of Films of Azobenzene Polymers
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Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
EXPERIMENTS Materials Homopolymers with p-cyanoazobenzene side chains (1, 2, 3 in Figure 1, Table 1) were synthesized by radical polymerization of corresponding methacrylate monomers and purified by precipitation of THF solution into methanol. The preparation of a copolymer 4 was previously reported [101. Their physical properties are summarized in Table I. Thin films were prepared by spin-coating cyclohexanone solutions of these polymers on fused silica substrates for UV-visible absorption spectra measurements. For transmission ellipsometry, a BK-7 substrate having ground plane on one side was used to avoid baseline fluctuation due to multiple-reflection. The film thickness measured by a DEKTAK3ST (ULVAC Co.) was in the 30 - 100 nm range as. Physical measurements Films were illuminated with polarized 436 nm light from a Hg-Xe lamp (Supercure203S, San-ei Electric MFG. Co.) passed through glass filters (Y-43 and V-44, Toshiba) and a polarizer at room temperature. Light intensity was monitored by an optical power meter (Advantest TQ8210). Polarized UV-visible absorption spectra were measured by a HewlettPackard 8452A diode array spectrophotometer equipped with a polarizer. The change in birefringence during irradiation and heating was measured by an ellipsometer (BFA-150 JASCO) equipped with a hot stage (Mettler FP800) to control temperatures of samples. The wavelength of a probing light was 633 nm. The films were irradiated with s-polarized 436 nm light with incident angle of 30 0 at the intensity of ca. 0.2 mW cm- . Photoirradiated films were heated at the rate of 6 'C min-1 .
x
o0
o
0 N
o
0N
Table I. Physical properties of azobenzene polymers.
ON
Tg(°C) Ts1 (°C)
.0 NN
CN
CN
x 1
2 3
--
-O(CH -O(CH
2)2 --
4
2)6 --
Fig. 1. Structures of azobenzene polymers.
CN
1
185
2
109
3
61 73
4
Mwx10-4 Mw/Mn 4.3
3.3
160a
2.6
2.9
168
3.4
3.6
-
-
-
-
Ts, : Smectic-isotropic phase transition temperature. a A narrow nematic phase was observed below the clearing temperature [11].
142
RESULTS
Photodichroism Irradiation of thin films with LPL irradiation induces changes in the reorientation of azobenzenes as a result of E/Z photoisomerization to give optical anisotropy. Figure 2 shows photoinduced dichroism expressed as dichroic ratio (DR) upon irradiation with linearly polarized 436 nm light at an ambient temperature. DR values were obtained with polarized absorbances and defined as the following equation : DR = (A4i- - All) / (,4-_ + All) where Ai and A,, denote absorbances at kmax perpendicular to and in parallel with the electric vector of the light, respectively. DR values of homopolymers 1, 2 and 3 gradually increased during light irradiation. The longer the spacer of the polymers, the higher was DR values. On the other hand, DR of copolymer 4 leveled off at an exposure energy of 3 J cm 2 . It was reported that 2 shows a smectic and a narrow nematic phases while 3 has a smectic A phase [11]. Consequently, they displa
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