**4.1. Characterization of MNPs**

When synthesizing PSMNPs the first parameter to determine is the metal content of the polymer-metal nanocomposite.

The composition of MNPs (even that of a single nanoparticle) can be determined by High Resolution TEM or SEM analysis coupled with EDS techniques. The microscopic techniques allow for the selection of the nanoobject(s) whereas the EDS provides the composition analysis. These methods usually gives qualitative or semi-quantitative results which can be useful to have an estimation of the composition differences in a sole sample. An example of this analysis is showed in in Figure 14, where a crossed-section resin bead sample is shown, prepared as described afterwards.

**Figure 14.** SEM image of an Ion Exchange resin with PdNPs and the corresponding EDS analysis.

If a quantitative analysis is required, the total metal content in the matrix can be determined by using and ICP coupled to an Atomic Emission Spectrometer (for metal concentrations between 0.1 to 500 ppb, depending on the metal) or coupled to a Mass Spectrometer (for metal concentrations between 0.1 to 10 ppb, depending on the metal)[58] to analyse the solution obtained after the treatment of a known amount of the nanocomposite with aqua regia to completely dissolve the MNPs and degrade the polymeric matrix.

The size and the shape of the MNPs obtained are important parameters allowing determining the nanocomposite characteristics. Further development of Nanotechnology needs a better understanding of nanomaterial properties and implies a better characterization of the above parameters, which are evaluated as a rule by using TEM technique.

In this sense, depending on the solubility properties of the polymeric matrices different sample preparation methodologies have to be considered. If the polymeric matrix is soluble in a volatile organic solvent (e.g. DMF, CHCl3, THF) it is possible to prepare MNPs suspensions or "inks" (5% mass solutions) in adequate organic solvents which can be deposited onto a TEM grid to perform the microscopic characterization. On the contrary, if the MNPs are stabilized in a non-soluble matrix (e.g. Nafion), the preparation of the sample is more difficult (although allows us to determine the distribution of the MNPs in the matrix), since it requires to cut thin or ultrathin slices (thickness about 1µm or less) of the nanocomposite material and deposit them onto a TEM grid. An example of this procedure and the final image obtained is showed in Figure 15.

Unfortunately, in practice, in many instances the quality of TEM images appear to be quite low due to the high noise and low contrast, making their processing a challenging task to accomplish. Also, the quantitative treatment of TEM images is often carried out by manual measurements of high number of nanoparticles, a task that is highly subjective and time consuming. During the last years, several computer imaging particle analysis software tools have been conceived to achieve a more accurate assessment of the size and frequency (size distribution) of nanoparticles.

technique.

**Figure 14.** SEM image of an Ion Exchange resin with PdNPs and the corresponding EDS analysis.

regia to completely dissolve the MNPs and degrade the polymeric matrix.

and the final image obtained is showed in Figure 15.

distribution) of nanoparticles.

If a quantitative analysis is required, the total metal content in the matrix can be determined by using and ICP coupled to an Atomic Emission Spectrometer (for metal concentrations between 0.1 to 500 ppb, depending on the metal) or coupled to a Mass Spectrometer (for metal concentrations between 0.1 to 10 ppb, depending on the metal)[58] to analyse the solution obtained after the treatment of a known amount of the nanocomposite with aqua

The size and the shape of the MNPs obtained are important parameters allowing determining the nanocomposite characteristics. Further development of Nanotechnology needs a better understanding of nanomaterial properties and implies a better characterization of the above parameters, which are evaluated as a rule by using TEM

In this sense, depending on the solubility properties of the polymeric matrices different sample preparation methodologies have to be considered. If the polymeric matrix is soluble in a volatile organic solvent (e.g. DMF, CHCl3, THF) it is possible to prepare MNPs suspensions or "inks" (5% mass solutions) in adequate organic solvents which can be deposited onto a TEM grid to perform the microscopic characterization. On the contrary, if the MNPs are stabilized in a non-soluble matrix (e.g. Nafion), the preparation of the sample is more difficult (although allows us to determine the distribution of the MNPs in the matrix), since it requires to cut thin or ultrathin slices (thickness about 1µm or less) of the nanocomposite material and deposit them onto a TEM grid. An example of this procedure

Unfortunately, in practice, in many instances the quality of TEM images appear to be quite low due to the high noise and low contrast, making their processing a challenging task to accomplish. Also, the quantitative treatment of TEM images is often carried out by manual measurements of high number of nanoparticles, a task that is highly subjective and time consuming. During the last years, several computer imaging particle analysis software tools have been conceived to achieve a more accurate assessment of the size and frequency (size

**Figure 15.** Sample preparation of cross-sectioned with microtome of a sulfonated polyetehersulphone (SPES-C) membrane with AgNPs and the corresponding TEM image obtained.

Through the image analysis of TEM micrographs (either manually or automatically) it is possible to make size distribution histograms from the sample data as the one shown in Figure 16. The obtained data can be used fitted to a 3-parameter Gaussian curve (14) where *a* is the height of Gaussian peak, dm is the position of the center of the peak (corresponding to the most frequent diameter), and is the standard deviation.

$$\text{by} \mathbf{a} \cdot \exp\left[ \text{-0.5} \left( \frac{\mathbf{d} \cdot \mathbf{d}\_m}{\sigma} \right)^2 \right] \tag{14}$$

**Figure 16.** TEM image of an ink of PdNPs obtained in a SPES-C membrane with the corresponding histogram adjusted by a 3 parameter Gaussian curve.

#### **4.2. Metal–polymer nanocomposite morphology**

The loading of MNPs in the polymeric matrix may cause important changes in the polymer morphology. For this reason, a characterization of the matrix previous and after the stabilization of the MNPs on it is required.

In this sense and among other techniques, SEM has been established as a referent in the field of the surface characterization. Because polymeric matrices usually are not conductive, in some cases (in matrices without MNPs or with a low content of MNPs) it is imperative to prepare the sample for the study, by a sputter coating with gold, carbon or palladium layers of about 50Å of thickness.

For membranes and films, cross-section images can be obtained by cutting the samples under liquid N2. For resin nanocomposites, it is necessary to embed the material in an epoxy resin to cut it transversally with a microtome, as shown in Figure 17.

**Figure 17.** Sample preparation of a resin bead for SEM characterization.

**Figure 18.** SEM images of (a) SPES-C membrane surface, (b) bare SPES-C membrane, (c) SPES-C membrane with PdNPs, (d) Blend membrane surface, (e) bare Blend membrane and (f) Blend membrane with PdNPs.[32]

Figure 18 shows SEM images of bare and coated SPES-C and Blend membranes (prepared with sulfonated and non sulfonated Polyethersulphone). Whereas in the case of SPES-C membranes there is an absence of porosity and surface defects in the case of Blend membranes it can be observed a finger structure porosity, and the existence of defects on the surface. In both cases, the load with PdNPs do not affect the final structure of the membrane.

The difference in porosity between the two types of membranes can be explained by an increase in the hidrofobicity of the final polymer: adding PES-C polymer increases the capacity of repulsion of H2O molecules thus, when preparing the membranes by wet phase inversion method by immersion in a non-solvent such as water, pores are generated during the precipitation of the membrane.

In this way, changing the ratio of PES-C / SPES-C in the final polymer blend membranes with different morphology and, therefore, different final application can be obtained.
