Other Characterization Techniques

A series of additional characterization techniques for macromolecules is available. All should be familiar to the polymer scientist. Ultracentrifugation is a special technique, although in principle well understood, it is more difficult to make absolute measurements on synthetic polymers than for the more compact globular proteins for which the method was originally developed. 

Neutrons and electrons are two of the elemental particles of matter. A neutron in its free state is unstable (P-decay to an electron and a proton within a haif-iife of about 12 minutes it has a rest mass of 1.675 10-27 kg and no eieotric oharge). The eiectron has oniy 1/1839 of the neutron mass (9.109x10-31 kg), and its charge is negative (- 1). 

Finally, one must consider the differences in scattering mechanism. Electromagnetic waves excite oscillations in the electrons in the structure that emit the scattered light. Neutrons interact with nuclei. Electrons interact with the charges of electrons and nuclei of atoms. 

The end-group determination can be done with many analytical tools, such as infrared analysis, nuclear magnetic resonance, or the detection of radioactive end groups which were included during the synthesis. The end-group determination for the purpose of measurement of the molar mass of macromolecules, great precision is necessary. 

Calorimetry and dilatometry were used to estimate super-high molar masses for some crystallizable polymers. For such high molar masses, crystallization is increasingly impeded and the fraction of the molecules able to crystallize has been calibrated with respect to molar mass for polytetrafluoroethylene [25,26]. 

---

After a temptative structure-based classification of different kinds of polymorphism, a description of possible crystallization and interconversion conditions is presented. The influence on the polymorphic behavior of comonomeric units and of a second polymeric component in miscible blends is described for some polymer systems. It is also shown that other characterization techniques, besides diffraction techniques, can be useful in the study of polymorphism in polymers. Finally, some effects of polymorphism on the properties of polymeric materials are discussed. 

Although the diffraction techniques are unique in providing detailed information on the structural organization at the molecular level in the different crystalline forms, there are other characterization techniques which are sensitive to the chain conformation and in some cases to the chain packing, which can be used advantageously (and in some case more efficiently than diffraction techniques) in the recognition and quantification of the different polymorphs in polymeric materials. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

Other characterization techniques are devoted to the determination of the CMC. They have been reviewed elsewhere [2,14] and will not be discussed here. Various NMR, fluorescence, and stop-flow techniques have also been used to characterize different aspects related to the dynamics of block copolymer micelles, as will be discussed in the next section. 

The set of results from TPR is consistent with the interpretations made so far, based on other characterization techniques used. In aU cases, the reduction in tin oxides occurs at temperatures much lower than those in the literature, where values above 900 K are reported for Sn02 [59]. The selectivity of the preparation reaction leads to a close relationship between M and Sn atoms transition metal atoms generate atomic hydrogen by dissociative adsorption, these hydrogen atoms being able to reduce tin oxides closely related to transition metals. Thus, the reduction temperature of the transition metal is the one that controls the global reduction process. As Pt is the metal that can be reduced most easily, the PtSn-BM catalyst exhibits the highest reducibiUty (lower reduction temperature) of aU the bimetalUc systems studied. 

With the ability to obtain information about the concentrations of various types of metal surface sites in complex metal nanocluster catalysts, HRTEM provides new opportunities to include nanoparticle structure and dynamics into fundamental descriptions of the catalyst properties. This chapter is a survey of recent HRTEM investigations that illustrate the possibilities for characterization of catalysts in the functioning state. This chapter is not intended to be a comprehensive review of the applications of TEM to characterize catalysts in reactive atmospheres such reviews are available elsewhere (e.g., 1,8,9 )). Rather, the aim here is to demonstrate the future potential of the technique used in combination with surface science techniques, density functional theory (DFT), other characterization techniques, and catalyst testing. 

In addition to identifying and characterizing known crystalline phases, single crystal, and more recently, powder diffraction methods can be used to determine the atomic positions within the crystal structures of new and uncharacterized materials. An excellent demonstration of how XRD structure determination methods can be applied in concert with other characterization techniques for... 

A number of crystalline VOPO4 phases were detected in the VPO catalysts by different research groups [11,12] that displayed vastly different catalytic behavior during on-stream conditioning (Figure 2). Poorly crystalline and amorphous VOPO4 phases present in the catalysts are often undetected by XRD. The presence of such disordered phases may be confirmed by other characterization techniques, such as Raman spectroscopy and P spin echo NMR. The incomplete phase analysis of VPO catalysts is one of the major reasons responsible for several alternative definitions of the equilibrated state and the nature of the active VPO phase [10-12]. In many cases, only short-term VPO conditioning... 

Structural information obtained from ET should be complemented with other characterization techniques in view of the intrinsically poor statistics of electron microscopy. 

Only a limited amount of work on high materials has been performed in the UHV environment. However, a few references to UHV design are included [6.44-6.51]. A few commercial instruments are available either as a complete system (JEOL, for example) or as an add-on to a custom UHV chamber containing other characterization techniques and/or in situ sample preparation (cleaning, annealing, cleaving, pealing) or film deposition capabilities. 

Catalyst activities for CO oxidation were measured in a continuous flow system. Other characterization techniques employed were X-ray diffraction, Temperature Programed Reduction, Auger spectroscopy, and Scanning Electron Microscopy. 

In addition, there are some other characterization techniques that can be used to examine the carbon-Ti02 composites, including Raman spectroscopy [40,123], atomic force microscopy (AFM) [106,123], thermogravimetric and differential thermal analysis (TGA-DTA) [26,125], determination of pHpzc [19,29], and electron paramagnetic resonance (EPR) [126,127],... 

---