Influence of spacer lengths of methoxy azobenzene-containing block copolymers

The influence of the spacer connecting the azobenzene chromophore to the polymer backbone was studied by holographic experiments. Thin films were prepared via spin coating followed by annealing the samples for 2 h at 10 °C below the clearing temperature of the respective polymer. In order to obtain more reliable results, the samples were measured several times (depending on the variance three to eleven times) and the average of the experiments was calculated (see Table 4.8).

The high order in the smectic mesophase is attributed to be responsible for the longer writing times to reach the maximum of the temporal evolution of the refractive index modulation and the stability of the inscribed gratings. Therefore, the holographic properties of annealed smectic thin films and thin films quenched to an amorphous state were compared. The same procedures were used that are described before. Initially amorphouse polymer films were prepared by heating the samples above the clearing temperature (Tcl(Azo)) and subsequent rapid cooling (quenching) below the glass transition temperature (Tg(Azo)) on a copper block standing in liquid nitrogen. By holographic illumination with two polarized light beams, it might be possible to induce a liquid crystalline mesophase in the initially amorphous samples,^[173] similar to results reported for the low-molecular-weight compounds.^[199] For the annealed diblock copolymers the maxima of the ππ*-transitions were located in the range of 346 nm to 346 nm as listed in Table 4.8, whereas the quenched samples show a small red shift. This can

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be explained by the reduced formation of the molecular aggregates in the liquid-crystalline phase. Compared to the results obtained for thin films of III described above this also confirms an amorphous phase in the quenched sample.^[19]

All samples of the investigated block copolymers exhibited good stability of the inscribed holographic gratings, i.e. the refractive index modulation did not decrease significantly after the initial relaxation. The respective holographic results as well as the thickness of each film are given in Table 4.8.

Table 4.8: Results of the holographic experiments on thin films of methoxy azobenzene-containing diblock copolymer series 6 and 7 at room temperature

block

For easier comparison in the Figure 4.45 the writing time to reach 90% of the maximum refractive index modulation (t90%) for all diblock copolymers are plotted. The writing time t90% in this series of diblock copolymers increases from 6a to 6c. With increasing spacer length, the tendency of the azobenzene chromophores to move independently from the back bone rises and the anisotropy of the side group increases. This might cause an increase in the degree of order of the smectic phase becomes more stable as can be seen chapter 4.5.3. This leads to longer writing times and an increase of the refractive index modulations of the annealed diblock copolymers as shown in Figure 4.45.

The maximum refractive index modulation is not significantly influenced by the length of the spacers and, thus, the degree of order of the smectic phase. All annealed samples of the block copolymers exhibit a n1(max) in the range of 6.6x10^-3 to 10.3x10^-3 with no apparent dependency on the spacer length.

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Figure 4.45: Time to reach 90 % of the maximum refractive index modulation (left) and respective 90 % of the maximum refractive index modulation (right) for methoxy azobenzene-containing diblock polymers 6a–7c for holographic experiment at room temperature. (a): sample annealed (1h at 150 C, 2h at 120 °C).

Amorphous quenched samples were only investigated for the block copolymers 6a-6c and 7c. In Figure 4.46 the writing times and refractive index modulation for these measurements are given. Comparing the results in the series 6 the writing time until 90 % of the refractive index modulation is reached (t90%) exhibits the same trends as observed for the smectic samples although the absolute values are significantly reduced. The reduction in writing times is higher for the poloymers with shorter spacers. Thus, the reduction by the factor 12 is observed for 6c whereas the writing time is reduced by the factor 100 for the azobenzene-containing block copolymer with the four-membered spacer 6a. The block copolymer containing a mixture of two spacer lengths 7c also exhibits a reduced writing time although it is only lower by the factor 6 compared to the smectic sample.

The 90 % values of the maximum refractive index modulation are lower in the amorphous quenched samples compared to the smectic samples as also observed for the homopolymer III. No dependency on the spacer lengths can be observed. Nevertheless the difference to the smectic samples is lower for the ten membered spacer 6c and block copolymer 7c with the mixture of six and eight membered spacers.

All samples of initially amorphous quenched azobenzene-containing block copolymers exhibit good long-term stability. For 7c a slight postdevelopment effect can be observed.

This might indicate the formation of a liquid crystalline phase induced in the initially amorphous samples. As shown in Figure 4.47, POM images between crossed polarizers of the irradiated sample show birefringence only in the irradiated region, a possible indicating for the formation of a liquid crystalline mesophase due to the photo-orientation.

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Figure 4.46. Time to reach 90 % of the maximum refractive index modulation (left) and respective 90 % of the maximum refractive index modulation (right) for methoxy azobenzene-containing diblock polymers 6a–6c and 7c for holographic experiment at room temperature. (q) quenched sample, (heated to 170 °C, rapidly cooled below T_g on copper bock in liquid N₂).

Figure 4.47: POM images between crossed polarizers of an amorphous quenched sample (heated to 170 °C, rapidly cooled below Tg on copper bock in liquid N2) of the methoxy azobenzene-containing block copolymer 7c after irradiation at room temperature. The irradiated area appears bright.

In conclusion, a liquid-crystalline phase can be induced by the holographic light grating and stable holographic gratings can be inscribed. In the initially amorphous sample, the reorientation occurs faster leading to writing times one order of magnitude shorter as compared to the annealed sample, whereas the refractive index modulation does not change so strongly. Therefore, the sensitivity to light increases in the quenched samples.

irradiated area

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4.7.1.4 Influence of the writing temperature on thin films of methoxy