**4.3. Effect of temperature on the ordering of azopolymer films**

orientation.

Now, we shall discuss the effect of heating on the ordering of azopolymer films. Figure 15 shows results for PAH/PS-119 films fabricated from solutions at pH 3.5, which are illustrative of the general behavior of molecular ordering as a function of heating. We begin at room temperature (~ 20°C) and ramp the temperature up to 190°C. As can be seen, there is no abrupt variation of SHG signal, but a gradual and significant decrease of intensity, even for thicker films with 5 or 10 bilayers. Similar behavior was observed for PDDA/PAZO films [15]. **4.3. Effect of temperature on the ordering of azopolymer films** Now, we shall discuss the effect of heating on the ordering of azopolymer films. Figure 15 shows results for PAH/PS-119 films fabricated from solutions at pH 3.5, which are illustrative of the general behavior of molecular ordering as a function of heating. We begin at room temperature (~ 20°C) and ramp the temperature up to 190°C. As can be seen, there is no abrupt variation of SHG signal, but a gradual and significant decrease of intensity, even for thicker films with 5 or 10 bilayers. Similar behavior was observed for PDDA/PAZO films [15].

Figure 15. SHG intensity as a function of temperature for PAH/PS-119 films (pH 3.5). **Figure 15.** SHG intensity as a function of temperature for PAH/PS-119 films (pH 3.5).

It is interesting to note that the literature reports that these films are quite thermally stable. They state that the SHG signal decreases about 20% from the initial value at room temperature, for temperatures above Tg (~ 140°C). Clearly, Figure 15 shows that thermal stability was not confirmed. The SHG intensity goes to almost zero in some cases, such as for the (PAH/PS-119)/PAH film (result not shown). The SHG signal for the 1 bilayer film at 150°C is only ~ 25% of initial signal at 20°C: a reduction of 75%, instead of only 20% as previously reported [16, 38, 40]. However, these authors do not mention how this Tg was measured. It is not clear if it is for the LbL film, including the substrate effect, or for complexed molecules in the bulk. The fact that we do not observe an abrupt decrease of the SHG signal (indicating a glass transition temperature) can be It is interesting to note that the literature reports that these films are quite thermally stable. They state that the SHG signal decreases about 20% from the initial value at room temperature, for temperatures above Tg (~ 140°C). Clearly, Figure 15 shows that thermal stability was not confirmed. The SHG intensity goes to almost zero in some cases, such as for the (PAH/PS-119)/ PAH film (result not shown). The SHG signal for the 1 bilayer film at 150°C is only ~ 25% of initial signal at 20°C: a reduction of 75%, instead of only 20% as previously reported [16, 38, 40]. However, these authors do not mention how this Tg was measured. It is not clear if it is for the LbL film, including the substrate effect, or for complexed molecules in the bulk.

> due interactions in the film that are different from those in the bulk materials, like lateral interactions (intralayer) or interlayer interactions. Furthermore, our films are formed by two different molecules and we should also consider the substrate effect, because the films studied here have only a few layers and the substrate/polymer electrostatic interaction is considerable. Our results on films fabricated at pH 7.0 show this influence on the film adsorption (data not shown [45]). For these films, it was observed that the SHG signal vanishes at high temperatures for a 2-bilayer film, but not for

thicker films. This suggests that these thicker films, with a more efficient complexation between layers, have an

complexation effect between layers and the thermal stability for these thin films. For thicker films, the

Figure 16 shows the ratio of χ(2) at 180°C and 30°C as a function of the number of bilayers, for films fabricated at both pH 7 and 3.5. Thicker films are more stable at pH 7, but for films at pH 3.5, that ratio was quite independent of thickness. However, there is an important difference in the temperature at which the SHG signal reaches the lowest value for pH 3.5 films. For a 1-bilayer film, this temperature is around 180°C, but for films with 5 and 10 bilayers, this temperature is near 160°C. This behavior suggests a significant influence of substrate charge density on the first layers, increasing the

complexation/interpenetration of layers is not as disturbed as for thin films and the thermal stability increases. However, at pH 3.5 the silica substrate is less charged and has less influence on the complexation of layers, resulting in a thermal

improved thermal stability because it is more difficult to thermally induce disorder.

opposite to the substrate, causing a reversal of the phase of the SHG signal. Therefore, films with an even number of layers have the same phase, with azo-groups pointing toward the substrate, while the 2.5 layer film had the opposite net The fact that we do not observe an abrupt decrease of the SHG signal (indicating a glass transition temperature) can be due interactions in the film that are different from those in the bulk materials, like lateral (intralayer) or interlayer interactions. Furthermore, our films are formed by two different molecules and we should also consider the substrate effect because the films studied here have only a few layers and the substrate/polymer electrostatic interac‐ tion is considerable. Our results on films fabricated at pH 7.0 show this influence on the film adsorption (data not shown [45]). For these films, it was observed that the SHG signal vanishes at high temperatures for a 2-bilayer film, but not for thicker films. This suggests that these thicker films, with a more efficient complexation between layers, have an improved thermal stability because it is more difficult to thermally induce disorder.

For films with 2 or 2.5 bilayers, whose signals are less intense, the phase measurement, while difficult to be performed, confirms that the reordering of the azo-groups also occurs, as shown in Figure 14(b). When the film is finished with a layer of PAH, PS-119 chromophores undergo reorientation and then acquire a small average ordering in the direction opposite to the substrate, causing a reversal of the phase of the SHG signal. Therefore, films with an even number of layers have the same phase, with azo-groups pointing toward the substrate, while

Now, we shall discuss the effect of heating on the ordering of azopolymer films. Figure 15 shows results for PAH/PS-119 films fabricated from solutions at pH 3.5, which are illustrative of the general behavior of molecular ordering as a function of heating. We begin at room temperature (~ 20°C) and ramp the temperature up to 190°C. As can be seen, there is no abrupt variation of SHG signal, but a gradual and significant decrease of intensity, even for thicker films with 5 or 10 bilayers. Similar behavior was observed for PDDA/PAZO films [15].

10 bilayers. Similar behavior was observed for PDDA/PAZO films [15].

(PAH/PS-119)5

Temperature / <sup>O</sup>

Figure 15. SHG intensity as a function of temperature for PAH/PS-119 films (pH 3.5).

It is interesting to note that the literature reports that these films are quite thermally stable. They state that the SHG signal decreases about 20% from the initial value at room temperature, for temperatures above Tg (~ 140°C). Clearly, Figure 15 shows that thermal stability was not confirmed. The SHG intensity goes to almost zero in some cases, such as for the (PAH/PS-119)/ PAH film (result not shown). The SHG signal for the 1 bilayer film at 150°C is only ~ 25% of initial signal at 20°C: a reduction of 75%, instead of only 20% as previously reported [16, 38, 40]. However, these authors do not mention how this Tg was measured. It is not clear if it is for the LbL film, including the substrate effect, or for complexed molecules in the bulk.

30 60 90 120 150 180 30 60 90 120 150 180

(PAH/PS-119)10

It is interesting to note that the literature reports that these films are quite thermally stable. They state that the SHG signal decreases about 20% from the initial value at room temperature, for temperatures above Tg (~ 140°C). Clearly,

due interactions in the film that are different from those in the bulk materials, like lateral interactions (intralayer) or interlayer interactions. Furthermore, our films are formed by two different molecules and we should also consider the substrate effect, because the films studied here have only a few layers and the substrate/polymer electrostatic interaction is considerable. Our results on films fabricated at pH 7.0 show this influence on the film adsorption (data not shown [45]). For these films, it was observed that the SHG signal vanishes at high temperatures for a 2-bilayer film, but not for

thicker films. This suggests that these thicker films, with a more efficient complexation between layers, have an

complexation effect between layers and the thermal stability for these thin films. For thicker films, the

Figure 16 shows the ratio of χ(2) at 180°C and 30°C as a function of the number of bilayers, for films fabricated at both pH 7 and 3.5. Thicker films are more stable at pH 7, but for films at pH 3.5, that ratio was quite independent of thickness. However, there is an important difference in the temperature at which the SHG signal reaches the lowest value for pH 3.5 films. For a 1-bilayer film, this temperature is around 180°C, but for films with 5 and 10 bilayers, this temperature is near 160°C. This behavior suggests a significant influence of substrate charge density on the first layers, increasing the

complexation/interpenetration of layers is not as disturbed as for thin films and the thermal stability increases. However, at pH 3.5 the silica substrate is less charged and has less influence on the complexation of layers, resulting in a thermal

improved thermal stability because it is more difficult to thermally induce disorder.

C

**4.3. Effect of temperature on the ordering of azopolymer films**

the 2.5 layer film had the opposite net orientation.

orientation.

52 Advanced Electromagnetic Waves

**4.3. Effect of temperature on the ordering of azopolymer films**

(PAH/PS-119)1

30 60 90 120 150 180

**Figure 15.** SHG intensity as a function of temperature for PAH/PS-119 films (pH 3.5).

0

molecules in the bulk.

2

4

6

8

SHG signal / arb. unities

10

12

14

16

Now, we shall discuss the effect of heating on the ordering of azopolymer films. Figure 15 shows results for PAH/PS-119 films fabricated from solutions at pH 3.5, which are illustrative of the general behavior of molecular ordering as a function of heating. We begin at room temperature (~ 20°C) and ramp the temperature up to 190°C. As can be seen, there is no abrupt variation of SHG signal, but a gradual and significant decrease of intensity, even for thicker films with 5 or Figure 16 shows the ratio of χ(2) at 180°C and 30°C as a function of the number of bilayers, for films fabricated at both pH 7 and 3.5. Thicker films are more stable at pH 7, but for films at pH 3.5, that ratio was quite independent of thickness. However, there is an important difference in the temperature at which the SHG signal reaches the lowest value for pH 3.5 films. For a 1 bilayer film, this temperature is around 180°C, but for films with 5 and 10 bilayers, this temperature is near 160°C. This behavior suggests a significant influence of substrate charge density on the first layers, increasing the complexation effect between layers and the thermal stability for these thin films. For thicker films, the complexation/interpenetration of layers is not as disturbed as for thin films and the thermal stability increases. However, at pH 3.5 the silica substrate is less charged and has less influence on the complexation of layers, resulting in a thermal stability which is independent of film thickness, with only a slight reduction in the stabilization temperature for thicker films. For films fabricated at pH 7, the substrate charge is higher, which promotes more efficient complexation between the polyelectrolytes and yields more thermally stable films (except for a 2-bilayer film that presents an anomalous behavior). stability which is independent of film thickness, with only a slight reduction in the stabilization temperature for thicker films. For films fabricated at pH 7, the substrate charge is higher, which promotes more efficient complexation between the polyelectrolytes and yields more thermally stable films (except for a 2-bilayer film that presents an anomalous behavior).

Figure 15 shows that thermal stability was not confirmed. The SHG intensity goes to almost zero in some cases, such as for the (PAH/PS-119)/PAH film (result not shown). The SHG signal for the 1 bilayer film at 150°C is only ~ 25% of initial signal at 20°C: a reduction of 75%, instead of only 20% as previously reported [16, 38, 40]. However, these authors do not Figure 16. Reduction of χ(2) due to heating as a function of number of layers, for films fabricated at pH 7.0 and 3.5. The points are the ratio of χ(2) measured at 180°C to that at 30°C, and the lines are guides to the eye. **Figure 16.** Reduction of χ(2) due to heating as a function of number of layers, for films fabricated at pH 7.0 and 3.5. The points are the ratio of χ(2) measured at 180°C to that at 30°C, and the lines are guides to the eye.

In order to verify the effect of heating on the structure of films, we compare the SHG signal for films at pH 3.5 before

profile before and after heating. On the other hand, the same was not verified for films fabricated at pH 10, where we can observe that after heating the ordering is no longer isotropic, as shown in Figure 17 for a 1-bilayer PAH/PS-119 film. This suggests that the films fabricated at this pH value have larger mobility than those at pH 3.5 or 7, which allows the rearrangement of chains to form macroscopic domains (~ hundreds of micrometers) with preferential orientation along

Figure 17. SHG signal in SS and SP polarization combination for a one-bilayer PAH/PS-119 film fabricated at pH 10, before and after

In this chapter we have discussed how nonlinear optical methods, and in particular second-harmonic generation (SHG), can be used to investigate the molecular order in polyelectrolyte layer-by-layer films containing azopolymers. After a brief outline of the basic theory of SHG for interface studies, we have shown how its polarization dependence can be used to obtain quantitative information about the orientational distribution function of azo-groups in these thin films. However, even a qualitative analysis of the SHG signal can give important information about the film structure. For example, the SHG dependence on the azimuthal rotation of the sample has shown that the way the films are dried has a marked influence of their molecular arrangement, which is isotropic for slow (spontaneous) drying, while it becomes

We have also investigated how the molecular ordering depends on the film thickness and fabrication conditions, especially the pH of the assembling/rinsing solutions. In contrast to previous reports in the literature, we did not find

mention how this Tg was measured. It is not clear if it is for the LbL film, including the substrate effect, or for complexed The fact that we do not observe an abrupt decrease of the SHG signal (indicating a glass transition temperature) can be heating, and after slow cooling to room temperature. Results indicate that thermally induced disordering is not permanent because the SHG signal is restored after slow cooling. This behavior is similar to what happens in the spontaneous drying assembly, since as the film cools the chains are losing mobility, but slowly enough for them to recover the best configuration induced by electrostatic interaction, thus recovering the order and restoring the SHG signal. For the films fabricated at pH 3.5, the SHG signal as a function of the azimuthal angle has the same isotropic In order to verify the effect of heating on the structure of films, we compare the SHG signal for films at pH 3.5 before heating, and after slow cooling to room temperature. Results indicate that thermally induced disordering is not permanent because the SHG signal is restored after

anisotropic and inhomogeneous with nitrogen-flow drying.

the substrate plane.

heating to 190°C. **5. Conclusions**

slow cooling. This behavior is similar to what happens in the spontaneous drying assembly, since as the film cools the chains are losing mobility, but slowly enough for them to recover the best configuration induced by electrostatic interaction, thus recovering the order and restoring the SHG signal. For the films fabricated at pH 3.5, the SHG signal as a function of the azimuthal angle has the same isotropic profile before and after heating. On the other hand, the same was not verified for films fabricated at pH 10, where we can observe that after heating the ordering is no longer isotropic, as shown in Figure 17 for a 1-bilayer PAH/PS-119 film. This suggests that the films fabricated at this pH value have larger mobility than those at pH 3.5 or 7, which allows the rearrangement of chains to form macroscopic domains (~ hundreds of micrometers) with preferential orientation along the substrate plane. profile before and after heating. On the other hand, the same was not verified for films fabricated at pH 10, where we can observe that after heating the ordering is no longer isotropic, as shown in Figure 17 for a 1-bilayer PAH/PS-119 film. This suggests that the films fabricated at this pH value have larger mobility than those at pH 3.5 or 7, which allows the rearrangement of chains to form macroscopic domains (~ hundreds of

micrometers) with preferential orientation along the substrate plane.

**Figure 17: SHG signal in SS and SP polarization combination for a one-bilayer PAH/PS-119 film Figure 17.** SHG signal in SS and SP polarization combination for a one-bilayer PAH/PS-119 film fabricated at pH 10, before and after heating to 190°C.
