Cyano azobenzene-containing block copolymers

The diblock copolymers 13a and 13b contain a cyano azobenzene chromophore that is attached to the polymer backbone via an ester linkage. Both polymer have the same chemical structure and block composition (see chapter 4.5.5) but differ in molecular weight. 13b is used for the following discussion.

DSC traces of second heating and cooling are shown in Figure 4.36. The insets show POM images taken between crossed polarizers at a temperature of 155 °C (right) and 122 °C (left), respectively. On heating as well as cooling only one broad transition can be observed. The heating thermogram might be interpreted as a glass transition temperature of an aged sample as indicated by the “excess peak” at 135 °C, which would result in Tg ≈ 130 °C. However, the POM images have to be taken into account. At temperatures below 130 °C a birefringent phase is evident that is significantly reduced in brightness upon heating above 135 °C, even though the images did not become dark at elevated temperatures. This effect might be attributed to a shear induced birefringence that did not relax fully due to the high viscosity of the sample. The observation, that the possible clearing transition is hardly detectable as well as the glass transition of the functionalized block (T_g(Azo)) cannot be determined might be attributed to low weight fraction of the functionalized block which is only 22 wt% for 13b.

Considering this observation, it is reasonable to interpret the transition observed in the DSC traces as a superimposition of the glass transition of the matrix, expected around 127 °C, and a liquid crystalline to isotropic transition of the azobenzene-containing segment with a maximum about Tcl = 135 °C.

104

0 20 40 60 80 100 120 140 160 180 200

heatflow (endo up)

T/°C

135°C

Figure 4.36: Second heating and cooling DSC traces at a heating rate of 10 K/min under N₂ of cyano azobenzene-containing diblock copolymer 13b. Insets are POM images between crossed polarizers of 13b at 155 °C (right) and at 120 °C (left).

The XRD measurement at 120 °C, i.e. in the temperature range of the liquid crystalline phase, is given in Figure 4.37. The sample was heated to 140 °C for 1h and annealed at 120 °C for 1 h prior to the measurement at the same temperature. No distinct signal is visible in the small angle region thus rendering a smectic mesophase improbable. Two superimposed broad halos can be detected around θ = 6.5° and θ = 9°. These signals are caused by the PMMA matrix (θ = 6.5°) and the polystyrene based backbone of the functionalized segment (θ = 9°). The higher intensity of the former halo compared to the diffractograms obtained for diblock copolymer series 6 and 7 is caused by the higher weight fraction of the PMMA segment in the case of 13b.

Based on the above described results, 13b is assigned to a nematic phase because no indication for a smectic mesophase was evident.

105

Figure 4.37: XRD diffractogram of the cyano azobenzene-containing diblock polymers with PMMA matrix 13b at 120 °C.

The morphology of the diblock copolymers 13 was investigated via TEM analysis, both contain a 22-23 wt% of an azobenzene-containing block. The thin cut samples were prepared and stained with RuO4 to increase the contrast between the two segments as reported for the TEM images of the diblock copolymer series 6 and 7. The only difference was the annealing temperature which was set to 120 °C for 24 h. The resulting images are shown in Figure 4.38. For the block copolymer with the higher molecular weight 13b structures seem to exhibit smaller dimensions compared to 13b. Unfortunately, no images with a higher magnification could be obtained for this sample due to significant blurring caused by radiation damage. However it is reasonable to assume the same morphology is present that was found for 13b.

Figure 4.38: TEM images of 13a

stained with RuO₄; dark areas correspond to the areas to the PMMA.

The triblock copolymer 15 contains a block and a PMMA matrix. From the prec ruPtBS = 16, ruPEHMA = 65 and ru

Analysis of the thermal properties via DSC transition of the PMMA matrix

106

a (top) and 13b (bottom left and right) annealed at 120

; dark areas correspond to the azobenzene-containing .

contains a azobenzene-containing segment, a PEHMA middle From the precursor 14 the repeating units were determined to

= 65 and ruPMMA = 469.

unexpected considering the block copolymer composition (A:B:C = 1:4.1:29.5). The fractions of the functionalized segment as well as the PEHMA middle block are much to minor to be detectable in the thermograms.

0

heatflow (endo up)

Figure 4.39: Second heating and cooling DSC traces at azobenzene-containing

The cyano azobenzene-containing

PMMA matrix 15 was analyzed via TEM. An example of the resulting images is given in Figure 4.40. The present bulk morphology is not easily identified. On the basis of the obtained images a cylindrical morphology of the

assumed. The PMMA and the PEHMA segment cannot be distinguished therefore no conclusion can be drawn if the PEHMA form a shell around the cylinders or if any kind of other possible morphology is present.

Figure 4.40: TEM images of the

azobenzene-107

unexpected considering the block copolymer composition (A:B:C = 1:4.1:29.5). The tions of the functionalized segment as well as the PEHMA middle block are much to minor to be detectable in the thermograms.

50 100 150

containing triblock copolymer with a PEHMA middle block and a was analyzed via TEM. An example of the resulting images is given in . The present bulk morphology is not easily identified. On the basis of the obtained images a cylindrical morphology of the azobenzene-con

assumed. The PMMA and the PEHMA segment cannot be distinguished therefore no conclusion can be drawn if the PEHMA form a shell around the cylinders or if any kind of other possible morphology is present.

TEM images of 15 annealed at 120 °C for 24 h, stained with RuO4; dark areas correspond to -containing segment.

unexpected considering the block copolymer composition (A:B:C = 1:4.1:29.5). The tions of the functionalized segment as well as the PEHMA middle block are much to

150

K/min under N₂ of cyano

triblock copolymer with a PEHMA middle block and a was analyzed via TEM. An example of the resulting images is given in . The present bulk morphology is not easily identified. On the basis of the containing block is assumed. The PMMA and the PEHMA segment cannot be distinguished therefore no conclusion can be drawn if the PEHMA form a shell around the cylinders or if any kind

; dark areas correspond to

108 4.7 Holographic experiments

Holographic measurements were performed by Dr. Hubert Audorff at the Bayreuth Institute of Macromolecular Research (BIMF) in the framework of the Collaborative Research Center 481 (Sonderforschungsbereich (SFB) 481) by the German Research Council (Deutsche Forschungsgemeinschaft, DFG).

Two s-polarized plane waves at 488 nm with an intensity of each 1 W/cm² are brought to interference in the plane of the sample. Due to the s:s-polarization a light-intensity grating is generated in the material. Reading was performed at 685 nm, which is well outside of the absorption of the azobenzene chromophore. From the holographic experiment, the diffraction efficiency is obtained. The refractive index modulation, n1, was calculated according to Kogelniks theory. For details see experimental part, chapter 6.1.1.

In Figure 4.41 a typical temporal evolution of the refractive index modulation is shown.

An important parameter determining the speed of the writing process is the slope near t = 0s. This maximal slope is proportional to the sensitivity of the azobenzene-containing material (see 6.1.1). Other important parameters are a) is the maximum refractive index modulation (n_1max) that is reached at the time t_max , commonly the writinglaser is switched off at this point; b) is the 90% value of n1max and corresponds to the time t90%; c) is the value of n1 after initial relaxation (n1(rel)), this value is used to determine the evolution of n1 after writing laser switch off. For further details see chapter 6.1.1.

0 50 100 150

Figure 4.41: Typical temporal evolution of the refractive index modulation at room temperature with indication of obtained values.

109

4.7.1 Holographic experiments on thin samples of methoxy azobenzene-containing