7 Molecular glasses as blending materials
7.2 Blends with an azobenzene-containing homopolymer
The second model system investigated, are blends of the molecular glass 7g and homopolymer 17 which contains azobenzene side groups [181]. The chemical structure of the repeating units of the homopolymer 17 is the same as the azobenzene groups in diblock copolymer 18.
Figure 7.4. Position of the maximum of the ππ*-transition in blends of homopolymer 17 and molecular glass 7g as a function of the concentration of the latter.
The blends were prepared by doctor-blading thin films from solution. All samples were amorphous and showed good optical quality without light
7.2 Blends with an azobenzene-containing homopolymer 83 scattering. Different weight fractions of the molecular glass between 0 and 80 w% were employed.
The position of the maximum of the ππ*-transition in the blends shows a red shift with increasing content of the molecular glass, as shown in figure 7.4.
For the pure homopolymer, the maximum is at 327 nm and in blends containing 80 w% of the molecular glass at 346 nm. The latter value is the same as for the pure molecular glass. The pure homopolymer is known to be liquid-crystalline and therefore forms stable holographic gratings. With increasing content of molecular glass, the liquid-crystalline phase is destroyed also the molecular aggregation is suppressed. This leads to the shift of the absorption maximum as discussed above.
Figure 7.5. Maximum refractive-index modulation and time constant of the build-up of the holographic gratings in blends of homopolymer 17 and molecular glass 7g as a function of the concentration of the latter.
The decrease of the refractive-index modulation -as shown in figure 7.5- can be explained by the difference between the maximum refractive-index modulations of the two materials. In contrast to the molecular glass, the azobenzene chromophores in the homopolymer are attached via long spacers.
This enables the formation of a liquid-crystalline phase and an increase of the anisotropy and the order parameter, as discussed in chapter 5. The pure homopolymer 17 has a refractive-index modulation of 0.019 which is higher by a factor 2.5 than the value of 0.0076 for the molecular glass 7g. By blending both materials, a linear decrease of the refractive-index modulation would be expected. The observed nonlinear variation is caused by the
84 7 Molecular glasses as blending materials destruction of the liquid-crystalline phase since the amorphous state of the homopolymer has a lower refractive-index modulation than the liquid-crystalline phase.
The build-up of the refractive-index modulation in the blends can also be fitted with equation 32. The obtained time constants of the build-up are plotted in figure 7.5. With increasing content of the molecular glass, they decrease by more than a factor of 10.
The material sensitivity of the blends -as discussed in section 2.2.3- depends on the refractive-index modulation and the time constant τ1. With the assumption that the temporal increase of the refractive-index modulation is the same for all blends, the sensitivity is directly proportional to the refractive-index modulation and indirectly proportional to τ1. When the content of the molecular glass is varied in this concentration series, the time constant varies more strongly than the refractive-index modulation, leading to an increase of the sensitivity by a factor of 10, as shown in figure 7.6.
Figure 7.6. Material sensitivity of blends of homopolymer 17 and molecular glass 7g as a function of the concentration of the latter.
The observed decrease of the time constant τ1 and the resulting increase of the sensitivity are caused by the photo-sensitivity of 7g. A more detailed discussion of the mechanism is given in the next section. All other possible mechanisms besides photo-induction can be excluded, as shown in the following: Changes of the thermal parameters, in particular a decrease of the glass transition temperature of the blend, would also lead to the observed results. But the glass transition temperature of 7g is 11 °C higher than the Tg
7.2 Blends with an azobenzene-containing homopolymer 85 of the homopolymer (47 °C). Therefore, the glass transition temperature of the blend should increase as compared to that of the pure homopolymer. If only this thermal effect played a role, this would lead -in contrast to the experimental observation- to an increase of the time constant. Additionally, no evidence for changes of the thermal properties was found in DSC experiments. In order to investigate, whether the observed shortening of the time constants in the blends is due to changes in the free volume caused by the molecular-weight compound, the non-photo-addressable low-molecular-weight compound 20 shown in figure 7.7 was used as a blending material. Compound 20 has a molecular weight of 907 g/mol and is, therefore, of similar size as 7g, its Tg is at 83 °C. In holographic experiments, a blend containing 10 w% of compound 20 and 90 w% of homopolymer 17 was investigated. The refractive-index-modulation of the blend decreased slightly as compared to pure 17, since the concentration of the azobenzene is lower, but the time constant of the build-up increased by a factor of 3 in this blend.
20
Figure 7.7. Chemical structure of the non-photo-active low-molecular-weight compound 20.
The temporal behavior of the refractive-index modulation during the first hour after the inscription is shown in figure 7.8. The pure photo-addressable polymer exhibits long-term-stable gratings whose refractive-index modulation shows post-development due to its liquid-crystalline phase. A drawback of the blending approach is that the liquid-crystalline phase is destroyed in blends with high concentrations of molecular glass 7g. In all blends containing more than 25 w% of the molecular glass, the inscribed refractive-index modulation was not long-term stable. With increasing content of the molecular glass, the refractive-index modulation decays faster. But in all blends containing 25 w%
or less of 7g, the slope of the refractive-index modulation was still positive after 2.5 days. This means that the inscribed gratings are long-term stable.
Hence, the blend containing 25 w% of 7g shows the largest improvement of
86 7 Molecular glasses as blending materials the holographic properties without losing its long-term stability. For this composition, τ1 decreases by more than a factor of 5 and the sensitivity increases by more than a factor of 3 as compared to the pure homopolymer.
Figure 7.8. Stability of the refractive-index modulation of volume phase gratings inscribed in blends containing the molecular glass 7g and the photo-addressable homopolymer 17 as a function of time and of the content of 7g. The refractive-index modulation has been normalized with respect to its value at the time when the writing laser was turned off. Note the logarithmic time axis.
With a blend containing 20 w% of the molecular glass, experiments were also performed at elevated constant temperatures. At higher temperatures, the time constant of the build-up decreased further, but at the expense of the stability.
At 40 °C, the gratings are no longer stable.