Characterization

Characterizing nanostructured materials, one faces two challenging tasks: to access nanoparticle characteristics and to get information on nanostructured polymers. Fulfilling these tasks requires using a combination of several modern techniques. The choice of the method is mainly determined by the condition of a polymer matrix (solid state or solution). The following paragraphs give a short description of the major techniques used in characterization of nanostructured polymers with nanoparticles.

Transmission electron microscopy (with magnification 100,000-150,000) is a powerful tool to characterize nanostructured materials with nanoparticles as it allows accessing particle characteristics (size, particle size distribution, particle location) as well as polymer material parameters. Using designated software to process TEM images allows estimation of nanoparticle size and particle size distribution. If one deals with solutions of block copolymer micelles or den-drimers, the TEM sample can be prepared by simply placing a drop of the diluted solution on the electron grid. Micelle or dendrimer size can be obtained from the TEM micrographs along with particle characteristics [42, 43, 74]. Solid materials are normally embedded in the epoxy resin which is then cross-sectioned to allow one to prepare a thin slice. In this case TEM examination allows one to discern the microphase size and shape either in block copolymer bulk or in other solid biphasic materials [7, 38]. The nanopar-ticles or metal salts in these microphases play the role of staining agents, improving the visualization of the material structure. As seen from the sections above, this method is very commonly used.

One of the key advantages of scanning TEM compared to TEM is that the beam damage is limited to the scanned area since only part of the image is exposed to electrons. This feature can be especially valuable for polymeric systems. Other advantages are high resolution of particle imaging and the possibility of using subnanometer probe and high angle annular dark field (Z-contrast) imaging [83]. Despite these advantages this method is not yet widely used for studying the nanostructured polymers with nanoparticles.

High resolution TEM allows one to obtain more detailed information on nanoparticle size and structure [84, 85]. Along with that, distinguishing the shape of nanocrystals permits conclusions on the nucleation and growth mechanism [70].

Powder X-ray diffraction (XRD) is the convenient method to obtain a mean particle size of crystalline particles [45, 86]. Normally, the corresponding Bragg peaks appearing in the XRD profile allow quantitative assessment of a mean particle size, but no reliable information can be obtained on particle size distribution. Besides, this method cannot be applied to amorphous particles.

Small angle X-ray scattering can be used to characterize the nanoparticles within the nanostructured polymers as well as the nanostructured materials [20, 78]. It allows one to obtain particle size and particle size distribution if the ordered polymer structure does not obscure the assessment of the particle characteristics and does not change polymer ordering during incorporation of metal nanoparticles. In the latter case, ASAXS can be used [22, 39]. Here, the polymer matrix structure does not influence the scattering of the particles as the difference scattering (obtained via subtraction of the scattering curves at the different energies near the adsorption edge of the corresponding metal) is used. Both SAXS and ASAXS can be used for solids and solutions.

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spec-troscopies are other methods of choice to characterize nanoparticles and their environments. EXAFS and XANES measurements on the reduced poly(propylene imine) dendrimers with diaminobutane core containing Cu nanoparticles provided additional information on the possible encapsulation of the Cu nanoclusters by the dendrimers and corroborated TEM data [87]. Though these methods are becoming widely used for other types of nanostructured systems (for example, for mesoporous materials with nanoparticles [88]), only a few nanostructured polymers were analyzed in this way. The probable cause of such limited use is the complexity of these methods especially when they are applied to complex nanostructured polymer-nanoparticle systems.

Optical methods (UV absorption and luminescence) are widely used for characterization of the size and size distribution of semiconductor (quantum dots) and some metal (Au, Ag, Cu, etc.) or metal oxide nanoparticles as these parameters determine the optical response [89]. At the same time these methods allow one to characterize the optical properties of nanoparticles [80].

The above methods are designed mainly for particle characterization though TEM and SAXS are also useful for nanostructured polymer description. Other valuable methods for characterization of solutions of nanostructured polymers are DLS [80, 90], static light scattering [91], and sedimentation in an ultracentrifuge [57, 63]. All these methods allow one to obtain sizes of block copolymer micelles, microgels, and dendrimers. Sedimentation in an ultracentrifuge also allows one to distinguish between "empty" polymer nanostructures and those filled with nanoparticles [70].

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