**5. Conclusions**

**5. Conclusions** 

**fabricated at pH 10, before and after heating to 190C.** 

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 In this chapter we have discussed how nonlinear optical methods, and in particular secondharmonic 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 infor‐ mation 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 anisotropic and inhomogeneous with nitrogen-flow drying.

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) 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 that all layers keep the same relative

thickness and fabrication conditions, especially the pH of the assembling/rinsing solutions.

In contrast to previous reports in the literature, we did not find that all layers keep the same

relative orientation, leading to a linear increase of the optical nonlinearity with thickness.

Instead, we find that for films fabricated at low or high pH, the nonlinearity tends to

decrease for thick films (~10 bilayers). Films fabricated at neutral pH generate an SHG

We have also investigated how the molecular ordering depends on the film

drying, while it becomes anisotropic and inhomogeneous with nitrogen-flow drying.

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orientation, leading to a linear increase of the optical nonlinearity with thickness. Instead, we find that for films fabricated at low or high pH, the nonlinearity tends to decrease for thick films (~10 bilayers). Films fabricated at neutral pH generate an SHG signal that does not vary significantly with thickness, except for a slight alternation in intensity for films with odd or even number of layers. These results are due to the influence of adjacent polyelectrolyte layers on the order of an adsorbed layer, that is, the order of the last adsorbed layer is different than that for layers within the film. Phase measurements of the SHG signal confirm the reorientation of polymer groups in the last layer after adsorption of an additional polyelectrolyte layer. Finally, we have also studied the thermal stability of the molecular arrangement by SHG measurements as a function of sample temperature. We found that the nonlinear response presents a gradual and significant reduction upon heating, so that a clear glass transition temperature cannot be defined for these ultrathin layer-by-layer films. Again, the thermal stability of the samples depends on their fabrication conditions (pH and thickness), with higher charge density in the polyelectrolytes and substrate promoting better complexation and improving their thermal stability. We also noted that the disordering effect of heating is reversible, and the SHG signal is recovered upon cooling. However, a few samples had their molecular arrangement becoming anisotropic after a heating/cooling cycle, as a result of aggregation and formation of molecular domains at the scale of tens of micrometers. We hope that these examples of SHG applied to the study of thin nonlinear optical polymer films have shown how powerful the technique can be to obtain information about the film structure at the molecular level, with also important consequences for their applications in optical devices.
