**1. Introduction**

The molecular organization and thermal stability of self-assembled thin films containing azopolymers are important to their applications, such as organic diodes [1, 2], optical storage

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[3], and biosensors [4, 5], to name a few. Here we are interested in probing the molecular organization of azopolymer thin films fabricated by the layer-by-layer technique resulting from their intermolecular and interlayer interactions. The thermal stability of these films will also be studied by measuring the nonlinear optical signal from second-harmonic generation (SHG) while the sample is heated.

Polymeric thin films may be very different from thick films. Moreover, the surface properties of thick films can be very different from those of their bulk [6]. For a full characterization of polymeric thin films, it is necessary to probe their thermal behavior, including the glass transition temperature, Tg, that is, the temperature where molecules acquire a more mobile state, which leads to lower film viscosity. Particularly, the Tg of polymeric materials gives us information about the intermolecular interactions. Furthermore, glass transition temperatures have practical importance for optical storage devices because near Tg the information recorded in the molecular arrangement is lost due to increased motion, which leads to molecular disorder.

In this sense, several techniques have been used to characterize these polymeric materials, and the most common are differential scanning calorimetric (DSC), thermal gravimetric analysis (TGA), and dynamic mechanical analysis (DMA). However, these thermal techniques charac‐ terize bulk samples but are ineffective for studying films with thickness on the order of a few nanometers. Some authors have applied these techniques to probe free-standing films of polystyrene (PS) [7], but for films adsorbed on solid substrates, such as layer-by-layer (LbL) and Langmuir–Blodgett (LB) films, or on liquid interfaces, such as Langmuir films, those techniques cannot be applied.

Recently, Lutkenhaus *et al.* have reported an alternative methodology for using traditional techniques (DSC and TGA) to study LbL films fabricated by secondary interactions [8], like Hbonding for PEO/PAA films, and PAH/PAA films fabricated by electrostatic interaction [9]. The method is based on using films with many (around 100 or 200) layers, and average thickness per bilayer about 80 nm. These films were removed from the inert substrate (Teflon) to be investigated by conventional techniques. Curiously, for PEO/PAA films (assembled by secondary interactions) it was possible to find a Tg, but not for PAH/PAA (strongly bound by electrostatic interaction). However, those films are not exactly what we could call ultrathin LbL films, because both films are very thick (2–8 μm), and the methodology removes the influence of the substrate. This influence is retained only in the conformation of the initial layers, but their contribution is negligible in the thermal analysis. It is therefore quite chal‐ lenging to investigate the Tg of ultrathin polymeric films, and in particular of LbL films with only a few bilayers.

Optical techniques such as ellipsometry and Raman spectroscopy [10] have been used to probe the Tg of thin films, but they do not allow to investigate how the molecular arrangement is changed during the glass transition, although their results show excellent agreement with the values of Tg obtained by other techniques. Because Tg is associated with a transition to a state of molecular disorder, it is natural to apply second-order nonlinear optical techniques, such as second-harmonic generation (SHG), which are quite sensitive to molecular orientation [11– 13]. With increasing temperature, the molecules become increasingly mobile (more random and dynamic conformation), resulting in increased disorder and reduced SHG signal, which is dependent on orientational ordering. Therefore, for a system consisting of only one material, we would expect a sudden variation of the signal when the Tg of the material is reached [14].

Another issue that is interesting is the thermal stability of the films. Here, we do not consider thermal degradation of the film, but the film's ability to maintain its molecular arrangement while temperature varies. Lvov *et al*. showed that the SHG signal from of PDDA/PAZO LbL films was substantially reduced when heated to about 120°C [15], showing that the ordering of the films was destroyed and they were not thermally stable. It is worth mentioning that, unlike the work of Han *et al*. [14], ref. [15] does not directly aim at measuring the Tg of LbL films, but intends only to check their thermal stability. However, some authors have described thermally stable layer-by-layer films, showing stability around 20% [16].

In this chapter, we will describe the basic theory for SHG in thin films, showing how its polarization dependence can be used to determine the orientation of chromophores in azopolymer LbL films, and how phase measurements of the SHG signal can probe the molecular reorientation after azopolymer adsorption. SHG measurements as a function of number of layers, of sample azimuthal angle, and of pH of the self-assembly solution reveal how the molecular organization depends on sample fabrication conditions. We also probe the thermal stability and effect of heat treatment on molecular ordering.
