Influence of writing temperature for methoxy azobenzene-containing homopolymer

4.7.1.2 Influence of writing temperature for methoxy azobenzene-containing homopolymer

Stumpe et al.^[204] investigated the temperature dependence of the laser induced birefringence in smectic polyacrylate based copolymers with mixtures of cyano azobenzene chromophores and cyanobiphenyl or N-(4-methoxybenzylidene)-4-butylaniline mesogens. In contrast to a holographic experiment in this kind of experiment only one laser beam is used to photoorient the chromophores in a sample thus, no overexposure is possible and the expected refractive index difference between irradiated and non-irradiated area is higher. An increase of photoorientation at writing temperatures above the glass transition temperature was found as well as an increase in photo-induced birefringence. The photo induced birefringence increased above T_g with a maximum at a temperature ~20 °C below the clearing temperature and decreased again until the clearing temperature was reached. Notably, for all polymers the orientation was not stable and decayed after the irradiation was stopped. Above the glass transition temperature full thermal relaxation occurred within 50 sec.

Wendorff et al. ^[205] presented a nematic (g 28 SmA 96-98 N 130-134 I) polyacrylate with an ethoxy azobenzene chromophores as well as the respective cholesteric (g 27 SmA 83-85 N* 127-128 I) copolymer with a mixture of the azobenzene chromophore and a cholesteryl-based mesogen. They studied the influence of the light intensity as well as the writing temperature in a holographic experiment. A significant increase in the diffraction

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efficiency with increasing temperature from 80 to 120 °C was found. Nevertheless in both polymers the inscribed gratings vanished in a matter of seconds due to thermal relaxation in the system although liquid crystalline phases were present. The rate of relaxation also increased with increasing temperature.

Ikeda et al.^[206] investigated the holographic behavior of nematic poly(meth)acrylate based copolymers with mixtures of nitro azobenzene chromophores with tolane or cyanobiphenyl mesogens. Higher diffraction efficiency and very low writing times were observed at writing temperatures above the glass transition temperatures. The thermal relaxation occurred on the same time scale thus no stable gratings were obtained in the liquid crystalline temperature range.

Smectic photoaddressable block copolymer systems based on a methyrylate segment with a cyano azobenzene chromophore and a PMMA matrix were investigated by Alcalá et al..^[188] As observed by the other groups for homopolymers and copolymers they found an increase of photo-induced birefringence with increasing temperature up to 70 °C, i.e.

~16 °C above Tg of the azobenzene-containing segment. At 90 °C no birefringence could be induced although the clearing temperatures were determined around 150 °C.

The low refractive index modulation obtained at room temperature indicates that the photo induced orientation of the chromophores is insufficient. The mobility of azobenzene units should increase with temperature and a faster photo-orientation should be achieved (faster birefringence growth rate) at higher temperatures.

To investigate the influence of the writing temperature on the holographic properties the temporal evolution of the refractive index modulation was recorded at different temperatures from 20 to 120 °C in steps of 20 °C (Figure 4.44). The maximum refractive index modulation increases with increasing temperature up to 100 °C from n1max = 0.01 (20 °C) to n1max = 0.03 (100 °C). From 100 °C to 120 °C n1max decreases significantly and resulting in a lower n1max than recorded at 20 °C. A radical change in the holographic behavior is evident in the temperature range around 80 °C. Below 80 °C the refractive index modulations drops slightly immediately after switching off the writing beam due to relaxation processes. At a temperature of 80 °C and higher, refractive index modulations exhibits postdevelopment (i.e. the refractive index modulation increases although the writing laser is switched off). In the temperature range up to 80 °C the writing process was stopped at 500 s without reaching the maximal value. For 100 °C a significant reduction of the writing time can be observed, whereby n1max is already reached after 40 s.

At 120 °C tmax lowered to 10 s but also the n1max is lower by the factor of six compared to writing at room temperature.

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These results indicate that a substantial orientation of the chromophores is achieved by writing at a temperature around 80 °C. The resulting postdevelopment indicates an increased order into mono-domain like structures (only liquid crystalline azobenzene-containing polymers exhibit postdevelopment, see chapter 4.1). The local heating resulting from the laser beams is estimated to be in the range of 10 to 15 °C.^[207] If this effect is taken into account the resulting temperature (i.e. 90-95 °C) is in good agreement with the determined glass transition temperature of III (Tg = 92 °C).

0 100 200 300 400 500 600 700

Figure 4.44: Temporal evolution of refractive index modulation at different temperatures for thin films (d = 0.69 µm) annealed at 120 °C for 2 h of the methoxy azobenzene-containing homopolymer III. Arrows indicate writing laser switch off.

In previous studies the chromophores of amorphous azobenzene-containing polymers could be oriented in the solid state and the stability of the inscribed gratings decreases when the glass transition temperature is reached.^[29] In contrast, liquid crystalline azobenzene-containing polymers show an increase of the chromophore orientation with increasing temperatures even above the glass transition temperature as described for literature discussed above. This trend is also observed in this thesis for investigations on homopolymer III. In contrast to most of the studies the holographic gratings inscribed in thin smectic films of homopolymer III exhibited a high stability and even a postdevelopment in the liquid crystalline temperature range.

Results obtained for the homopolymer III indicated that in this smecitc azobenzene-containing polymer system significant chromophore reorientation only occurs when the writing temperature is in the range of the glass transition temperature of the polymer. In a temperature range above the glass transition temperature and below the clearing

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temperature rapid formation of holographic gratings can be achieved that exhibit postdevelopment at the given temperature.

4.7.1.3 Influence of spacer lengths of methoxy azobenzene-containing block