Materials

As materials, an azobenzene-functionalized molecular glass, three photo-orientable polymers (see Figure 77) and polystyrene were selected. To demonstrate how the orientation rate of azobenzene moieties in functionalized polymers is affected different photoactive are blended with photo-active MG1.

Figure 77: Chemical structure of the investigated methoxyazobenzene-functionalized molecular glass 1 of the investigated methoxyazobenzene-functionalized homopolymer 2 and block copolymers 3. Block copolymer 4 comprises additional non-photoactive mesogenic side groups.

Compound MG1 was synthesized by R. Walker and is based on a 1,3,5-substituted cyclohexane core with three azobenzene arms attached by an esterification reaction similar as reported previously.^[142] The required hydroxyl-functionalized azobenzene moieties are accessible by a common azo-coupling reaction as described in basic literature.^[85] Figure 78 depicts DSC traces of second heating and first cooling from 30°C to 250°C for MG1.

Figure 78: DSC traces of compound MG1 during heating and cooling at a scan rate of 10 K/min under a N2 -flow of 50 mL/min.

MG1 H1 BC1 BC2

50 75 100 125 150 175 200 225 250

T_m = 180°C

T_recryst. = 150°C T_recryst. = 108°C

T_g = 56°C

1^st cooling 2^nd heating

Heat flow (endo up)

Temperature / °C

Molecular Glasses

MG1 features a glass transition temperature of 56°C. MG1 tends to recrystallize between 90-120°C and 135-165°C and eventually melts at 180°C. During cooling, however, no crystallization occurs, so that the material can easily be quenched into a stable amorphous phase. It has been selected for its high orientation rate and its ability to form a stable amorphous phase at room temperature.^[92]

In the neat material, holographically inscribed gratings are not long-term stable.

The polymers were synthesized by R. Walker and were chosen to feature good processability to yield transparent thin films, and to possess a good miscibility with molecular glass MG1. The synthesis of these polymers has been described elsewhere.^[38,143] All three selected photo-orientable azobenzene polymers feature methoxy-azobenzene chromophores, which were attached to the backbone as side groups. They exhibit good processability and yield transparent films. Azobenzene-containing homopolymer H1 has a molecular weight Mn of 200000 g/mol, a PDI of 1.4 and a glass transition temperature of 47 °C. Azobenzene block copolymer BC1 comprises a polystyrene majority block and a methoxy-azobenzene side group block. The molecular weight Mn

of the block copolymer is 59000 g/mol, the PDI is 1.04. The weight fraction of the majority block and the azobenzene-containing minority block is 82.5 wt.-% and 17.5 wt.-%, respectively. Block copolymer BC2 has a molecular weight Mn of 127000 g/mol and a PDI of 1.05. It features a polystyrene majority block and a minority block, which is composed of methoxy-azobenzene and non-photoactive mesogenic side groups. The weight fraction of the majority block and the azobenzene-containing minority block is 72 wt.-% and 28 wt.-%, respectively. Within the minority block, the ratio of azobenzene units to non-photoactive units is 46:54 mol-%. Owing to the liquid-crystalline properties of the non-photoactive mesogenic side groups, the refractive-index modulation in block copolymers of this type is increased and, more importantly, also stabilized.^[54]

As mentioned above, in polystyrene and homopolymer H1, the molecular glass is distributed randomly. Figure 78 depicts the differential scanning calorimetry scans of photoactive homopolymer H1. Upon heating, H1 shows two melting points at 83°C and 115°C. After the first melting, the polymer recrystallizes at 95°C. Upon cooling, the polymer crystallizes at 94°C and 78°C.

The DSC experiments in combination with polarization microscopy investigations did not show a liquid crystalline phase, which, owing to the form anisometric shape of the chromophore, could have been expected. Homogeneous transparent films with a stable amorphous phase at room temperature can be obtained via spin-coating.

Figure 79: DSC traces of compound H1 during heating and cooling at a scan rate of 10 K/min under a N2-flow of 50 mL/min.

Block copolymer BC1 features a non-photoactive polystyrene block and a methoxy-azobenzene-functionalized minority segment. The weight ratio of the polystyrene block is approximately 80 %.

Figure 80 depicts DSC traces of BC1 in the three heating and cooling cycles carried out from 30°C–

150°C at a scan rate of 10 K/min. The first heating of the block copolymer reveals a broad melting peak, which is assigned to the minority phase. Moreover, the polymer features a glass transition at a temperature of 101°C caused by chain segment mobility of the majority block.

Figure 80: DSC traces of compound BC1 during heating and cooling at a scan rate of 10 K/min under a N2-flow of 50 mL/min.

Upon cooling, the polymer remains amorphous. The minor peak at 78°C during second heating can be allocated to the azobenzene-functionalized minority block and could might be assigned to the glass transition temperature of the same.

Besides photoactive azobenzene side groups, block copolymer 4 also contains non-photoactive mesogenic moieties in the minority block. Owing to these liquid crystalline moieties, the

refractive-50 75 100 125 150

T_cryst. = 78 °C

T_cryst. = 94 °C T_m = 115 °C

T_recryst. = 95 °C T_m = 83 °C

1^st cooling

Heat flow (endo up)

Temperature / °C

1^st heating

50 75 100 125 150

T_g = 101°C

3^rd cooling 2^nd

Heat flow (endo up)

Temperature / °C 1^st

3^rd heating 2^nd

1^st

Molecular Glasses

index modulation in this block copolymer is increased and more importantly stabilized.^[143] Figure 81 depicts the second heating and first heating DSC traces of BC2 carried out from 30°C–250°C at a scan rate of 10 K/min.

Figure 81: DSC traces of compound BC2 during heating and cooling at a scan rate of 10 K/min under a N2-flow of 50 mL/min.

The amorphous block copolymer has a glass transition temperature of 107°C, which is attributed to the PS block and can easily be transferred to homogenous, transparent films. Even though the weight of the minority block is about 30 %, the glass transition of the minority block is not visible in the DSC traces.

The thermal data of the compounds used in this chapter are summarized in Table 9.

Table 9: Characterization data of investigated azobenzene molecular glass MG1, photo-orientable azobenzene homopolymer H1 and the azobenzene block copolymers without (BC1) and with (BC2) additional non-photoactive mesogenic side groups.