Influence of the writing temperature on thin films of methoxy azobenzene- azobenzene-containing block copolymer 7c

The influence of the writing temperature depending on the sample preparation on the holographic behavior was investigated on azobenzene-containing block copolymer 7c.

The diblock copolymer 7c featuring a mixture of a six-membered and an eight-membered spacers. This block copolymer exhibits a smectic mesophase between 105 °C and 141 °C.

As described above smectic annealed samples and amorphous quenched samples were prepared. The temporal evolution of the refractive index modulation for a smectic annealed thin film of 7c at different temperatures (20 °C to 100 °C) is shown in Figure 4.48. On the left side the temporal evolution up to 4000 s is shown to accommodate the exceptionally long writing time until the maximum refractive index modulation is reached at room temperature. Both values are very high compared to the measurements at elevated temperatures but well within range of the variance discussed above. At temperatures above 23 °C the writing time as well as the maximum refractive index modulation are significantly reduced thus on the right side the relevant magnification is shown. For easier comparison the relevant values are extracted and plotted in separate graphs (Figure 4.50, Figure 4.51, and Figure 4.52). From Figure 4.48 it can be seen, that the sensitivity at an early stage increases with increasing writing temperature and, thus, the system becomes faster.

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0 50 100 150 200 250 300

0,000 0,002 0,004 0,006 0,008

annealed

n1

time/s

40 60 80 100

Figure 4.48: Temporal evolution of the refractive index modulation of an annealed (150°C for 1h and at 130°C for 2h) thin film (d = 0.67 µm) of 7c at different writing temperatures. Arrows indicate writing laser switch off.

To compare the influence of sample preparation the temporal evolution of the refractive index modulation for a for initially amorphously quenched thin film (quenched) of 7c at different temperatures (40 °C to 100 °C) is shown in Figure 4.49. The times were the writing laser was switched off are indicated by arrows. For this sample the differences between measurements at room temperature and at elevated temperatures are not as drastic as described for the annealed sample. In the following the extracted significant values are compared to the results obtained for the annealed sample (Figure 4.50 (t90%), Figure 4.51 (n1(90%)), Figure 4.52(n1(4000s)/n1(rel))). For the amorphous quenched sample an increase in sensitivity is observed as described for the smecitc annealed sample.

122 at different writing temperatures. Arrows indicate writing laser switch off.

As presented before, writing times at which 90 % of the maximum refractive index modulation were reached (t90%) were compared and discussed. Figure 4.50 reveals the pronounced temperature-dependence of the writing time t90 on a logarithmic scale. With increasing temperature from (40 °C to 100 °C), the t90% decreased by two orders of magnitude for the annealed as well as the quenched sample. At 100 °C the quenched sample reaches a t_90% of 1 s whereas the writing time of annealed sample amounts for 6 s.

40 60 80 100

Figure 4.50: Temperature dependence of time to reach 90 % of n_1(max) of inscribed holographic gratings for amorphous quenched (heated to 170 °C, rapidly cooled below T_g on copper bock in liquid N₂) and annealed (1h at 150 C, 2h at 130 °C) samples of the methoxy azobenzene-containing diblock copolymer 7c.

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In contrast to the writing time the results for the temperature dependence of the 90 % value of the refractive index modulation (n1(90%)) do not show a clear trend. For 40 °C and 100 °C the n1(90%) of the quenchedsample is higher than for the annealedsample. It can be noted that for the smectic annealed the n1(90%) decreases with increasing temperature. For the quenched sample a local minimum at 60 °C is apparent although theses minor differences in n1(90%) might be attributed to the variance in the experimental results. For this sample the 90 % value of the refractive index modulation is reduced by a factor 0.6 when the writing temperature is increased from 40°C to 100°C.

40 60 80 100

0,000 0,002 0,004 0,006 0,008

n1(90%)

T /°C

annealed quenched

Figure 4.51: Temperature dependence of the refractive index modulation (n1(90%)) of inscribed holographic gratings for amorphous quenched (heated to 170 °C, rapidly cooled below T_g on copper bock in liquid N₂) and annealed (1h at 150 C, 2h at 120 °C) samples of 7c.

The temperature dependence of the temporal evolution of the refractive-index modulation after writing laser is switched off (n1(4000s)/n1(rel)) is presented in Figure 4.52. For the annealed sample the inscribed gratings are stable at room temperature. A slight postdevelopment is observed with increasing temperature, reaching a maximum value of 122 % at a temperature of 100 °C. The amorphous quenched samples shows approximately the same trend, an increase of postdevelopment with increasing temperature, although at 60 °C a negative deviation is apparent that might be attributed to variance in the measurement. In this sample the postdevelopment reaches 136 % at a temperature of 100 °C. From Figure 4.48 and Figure 4.49 it can be seen that the refractive index modulation increases directly after the initial relaxation for temperatures 80 °C and 100 °C. The rate of the postdevelopment is higher at 100 °C for both samples.

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40 60 80 100

80 100 120

140 annealed quenched

n1(rel)/n1(4000s) /%

T/°C

Figure 4.52: Temperature dependence of refractive index modulation of inscribed holographic gratings 4000 s after writing laser switch off for amorphous quenched (heated to 170 °C, rapidly cooled below T_g on copper bock in liquid N₂) and annealed (1h at 150 C, 2h at 120 °C) samples of 7c.

The above described results for the azobenzene-containing diblock copolymer indicate that the orientation process of the chromophores induced by the holographic experiment is facilitated by elevated temperatures. Thus the writing times are drastically reduced as well as the postdevelopment is amplified with increasing temperatures. The increased thermal relaxation at elevated temperatures and thus a decrease of the refractive index modulation after writing laser switch off that was reported by other groups[188,204–206]

could not be observed in this work.

Different results were obtained for the azobenzene-containing homopolymer III compared to the block copolymer 7c. While both showed improvement in writing times and postdevelopment at elevated temperatures with significant performance increases around the glass transition temperatures, the observed trend for the refractive index modulation are. For the homopolymer III an increase of the refractive index modulation is observed that is in agreement with the result in literature for the block copolymer 7c a decreasing trend was observed.

The results of holographic experiments on thin films for methoxy azobenzene-containing polymer can be summarized as follows.

1. Influence of the sample preparation (smectic vs. amorphous quenched):

In quenched samples the writing times as well as the refractive index modulation is reduced while the postdevelopment is slightly enhanced. The sensitivity in the early stage of the smectic and the amorphous quenched samples does not differ significantly.

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2. Influence of the writing temperature on thin films:

The sensitivity sensitivity in the early stage increases at elevated temperatures.

Writing times decrease and postdevelopment is amplified with increasing temperatures. Refractive index modulation increases with temperature for homopolymer III and decreases for block copolymer 7c. Significant faster orientation is achieved at writing temperatures in the range of the glass transition of the photoaddressable segment.

3. Influence of spacer length:

Writing times increase with increasing spacer length due to higher order of the smectic mesophase. Mixing of two different spacer length results in lower order of the mesophase, and also lower writing times. No influence on the refractive index modulation is apparent.

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