Structural Characterization Techniques

As has been described above, dendrimers present synthetic and structural peculiarities, which require a variety of techniques for their adequate characterization. These techniques not only have to confirm the presence of the different moieties of the macromolecules, but also of possible defects or of different isomers formed in their functionalizations. Furthermore, information about the 3D structure, size of dendrimer and cavities inside them, charge distribution, etc., is important to depict completely the macromolecule. 

This section will overview several characterization techniques. Their principles will not be discussed here, as they can be found in more specialized documents. The attraction of dendrimers has generated several reviews related to their synthesis and characterization, and thus in this chapter a special focus on techniques, or the data obtained from them, that emphasize the presence of the cationic charge on dendrimers will be presented. 

Purification of dendrimers is, of course, of maximum importance for characterization. In this sense, chromatographic and electrophoretic methods are very helpful procedures to achieve pure dendrimers. Furthermore, the use of these procedures, together with characterization techniques such as mass spectrometry, UV, etc., gives complete important information about the purity of dendrimers, taking into account the presence of not fully functionalized branches in the macromolecule that would not be identified properly without these treatments. 

1 Chromatography. Chromatographic techniques for dendrimers include liquid chromatography (LC), thin-layer chromatography 

HPLC has been helpful for identifying major structural defects of PAMAM dendrimers of different generations with different terminal groups and a folic acid-dendrimer conjugate. Findings from this study indicate that HPLC is an effective technique not only for characterization and separation of functionalized PAMAM dendrimers and conjugates, but also for investigation of the interaction between dendrimers and biomolecules.  

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Elemental and Structural Characterization Many oxidation reactions occur on mixed oxides of complex composition, such as SbSn(Fe)0, VPO, FePO, heteropolycompounds, etc. Very often the active surfaces are not simple terminations of the three dimensional structure of the bulk phases. There is need to extensively apply structural characterization techniques to the study of catalysts, if possible in their working state. 

We have briefly covered some of the important developments in structural characterization techniques. There are many other techniques such as Mossbauer spectroscopy, positron annihilation and Rutherford backscattering which have wide applications. Mossbauer spectroscopy is specially useful to investigate different oxidation states, spin-states and coordinations of metal ions, phase transitions, magnetic ordering. 

Since most zeolites are only available as polycrystalline phases, powder diffraction is an essential structural characterization technique. A powder diffraction pattern can be used to identify a material, to determine unit cell parameters, to estimate the quality of a sample, to monitor phase transitions, to evaluate whether or not a post-synthesis treatment has induced structural changes, to establish whether or not impurities (amorphous or crystalline) are present, or to recognize the presence of a significant level of faulting. 

Full equilibration of ions at a known temperature in IMS allows measuring temperature-dependent rate constants for structural transitions, from which accurate activation energies and preexponential factors could be determined in an assumption-free manner using Arrhenius plots. In contrast, structural characterization techniques implemented in vacuum, such as various laser spectroscopies (threshold photoionization,photodissociation,or photoelectron spectroscopy ), MS/MS by collisional or other dissociation, or chemical reactivity studies lack a direct ion thermometer. In those methods, ion temperature is estimated as the source temperature (possibly with semiempirical adjustments) or gauged using various indirect thermometers, and vibrationally or electronically hot ions are the ever-... 

Therefore, this chapter covers the investigation of the crystallization kinetics of amorphous materials by studying the crystallization mechanism in terms of isothermal and non-isothermal methods and describing different thermal analysis techniques used in crystallization kinetic studies and explaining the structural characterization techniques used to determine the crystallization mechanism... 

Solid-liquid extraction is the most commonly used technique to isolate flavonoids from Cornus spp. fruits, the extraetion solvents, the purification methods and the structural characterization techniques being the same as in the case of anthoeyanins. 

For the detailed information, pore structure porosimetry techniques are used. These methods enable measurement of pore diameter, pore shape, pore volume, and pore distribution in the electrode catalyst and gas diffusion layers. However, for PEMFC, these layers have hydrophobic and hydrophilic pores and there is no suitable technique available for characterization of such complex pore structures. Combination of multiple porosimetry techniques are employed to characterize layers with both hydrophobic and hydrophilic pores. The pore structure characterization techniques include capillary flow porosimetry, water intrusion porosimetry, and mercury intrusion porosimetry (Jena and Gupta, 2002). In water... 

Hoffman [64] studied monodisperse suspensions of polyvinyl chloride and styrene-acrylonitrile copolymer particles of diameter 0.4 to 1.25 pm using both rheological and structural characterization techniques as shown in Figure 2.1. The shear viscosities showed striking viscosity increases at critical shear rates. Structurally, the suspensions at low shear rates were found to exhibit a hexagonal crystalline lattice. At a critical shear rate, this lattice structure broke up into less-oriented arrays with a jump increase in shear viscosity. 

It is no exaggCTation to say that the development of structure characterization techniques for molecular solids has revolutionized the study of organic solid state chemistry. It has allowed for the first time a rationalization of observed properties and transformations (including reactivity) which were previously unexplained by simple chemical means. The principal method used has been diffraction, particularly of X-rays, but also, more recently, of electrons and neutrons. This chapter will first give a basic introduction to crystal symmetry and then describe the use of X-ray, neutron, and electron diffraction, as well as of EXAFS and of vibrational spectroscopy in the study of molecular crystals. 

The capabilities of SECCM are demonstrated most powerfully when it is used in the imaging mode, where a surface of interest is scanned with the SECCM probe meniscus to produce x-y maps of surface reactivity, simultaneously with topography and ion conductance (between the two QRCEs). SECCM images can be complemented with a variety of structural characterization techniques, applied to the same region of the surface, such as FE-SEM, micro-Raman, AFM, and electron backscatter diffraction (EBSD). This type of multimicroscopy approach provides a rich and powerful platform for elucidating how local reactivity is influenced by the properties of the surface under investigation. 

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