Network structure chemical analysis

A tested example of a practical traceability structure has been described which has already proved useful in several fields of chemical analysis. It makes use of a national calibration service as a successful and efficient dissemination mechanism. An essential part of the structure is a network of high-level chemistry institutes at the national standards level providing the end points of traceability and coordinated by the national metrology institute. The need for a network of competence seems to be typical for metrology in chemistry for which in most countries the resources are largely to be found outside of the national metrology institute. 

Analysis of chemical conversion and cure chemistry is another way of studying network structures. Several techniques are used for this purpose, e.g., optical spectroscopy [12], high-resolution NMR spectroscopy and titration of non-reacted functional groups. The spectroscopic methods can be used for quantitative analysis of crosslinks [13-15]. Chemical conversion is usually closely related to the network density. However, no exact quantitative information on the network structure can be obtained because reacted groups can form... 

Solid-state NMR magnetisation relaxation experiments provide a good method for the analysis of network structures. In the past two decades considerable progress has been made in the field of elastomer characterisation using transverse or spin-spin (T2) relaxation data [36-42]. The principle of the use of such relaxation experiments is based on the high sensitivity of the relaxation process to chain dynamics involving large spatial-scale chain motion in elastomers at temperatures well above the Tg and in swollen networks. Since chain motion is closely coupled to elastomer structure, chemical information can also be obtained in this way. 

Network structure analysis is discussed in Chapters 7, 8,10 and 13. These chapters deal with the characterisation of the structure of chemical and physical networks, rubber-filler physical network, network defects and its heterogeneity using NMR relaxation techniques and NMR imaging. 

The glass transition temperature Tg is one of the most important structural and technical characteristics of amorphous solids. The correlations of Tg of linear polymers with their chemical composition, molecular weight, rigidity and symmetry of chains, as well as some other characteristics of macromolecules are well documented 57,58) Thg information on networks is much poorer. At present, for networks there exists mainly one parameter in structure-Tg correlations. It is the concentration of crosslinks — a parameter which is very insufficient, since in networks there are chemical crosslinks of different functionality (connectivity) which are distinguished by their molecular mobility. This means that the topological aspect of the network structure should be taken into account in the Tg analysis. Another difficulty connected with Tg determination of polymers lies in vitrification occurring during polymer formation (Sect. 6). 

The elastic properties of rubbers are primarily governed by the density of netw ork junctions and their ability to fluctuate [35]. Therefore, knowledge of the network structure composed of chemical, adsorption and topological junctions in filled elastomers as well as their relative weight is of a great interest. The H T2 NMR relaxation experiment is a well established method for the quantitative determination of the network structure in the elastomer matrix outside the adsorption layer [14, 36]. The method is especially attractive for the analysis of the network structure in filled elastomers since filler particles are "invisible" in this experiment due to the low fraction of protons at the Aerosil surface as compared with those in the host matrix. 

Even if the methods of structural and morphological analysis suggest that the gross features of the ACE preparations are those expected of a plain mixed system (at least up to the transition to the a-Al203 phase, catalytically inactive), surface physico-chemical analysis methods indicate that, on a microscopic scale, the situation is quite different. In fact, the two oxidic systems do affect one another appreciably, (i) The presence of the transition-phase alumina induces in the Ce02 network smaller crystallites size and enhanced acidity of the surface... 

The chemical analysis of the structure of crossllnked polymer networks Is complex In nature and various approaches have been given in an effort to understand these systems (1- ). The usual chemical or physical methods are limited because of the insolubility and infusibility of the system. However, recently CP/MASS (cross-polarlzatlon magic-angle satple spinning) has made It possible to obtain a high resolution NMR spectrum of these Insoluble solid polymers (9,10). 

With regard to the mechanical reactirai of a polymer network to a stress applied, it is important that loose ends of macromolecules in a network structure are as shmrt as possible and/or their concentration is low. As these ends mostly extend out of the lamellas of crystallites then, while crossUnking is taking place in an amorphous phase and with the simultaneous presence of crystallites, a network with small loose ends should be formed. The crosslink junctions stabilize the natural molecular network (entanglements and crystallites), and every chain in the system is potentially elastically operative and can contribute to the stress in a tensile experiment [33]. The stabilization effect of chemical crosslinks on entanglements and crystallites may be the direct cause of observed differences in the determination of the amount of chemical crosslinks from mechanical property measurements and sol-gel analysis of the cross-linked polymer. 

Figure 11.3 ATR-FTIR analysis of commercial silicone elastomeric foam (Dow Corning RTV 5370). Although the overall chemical functionalities of the silicone are observable using vibrational spectroscopy, they provide only broad average of the materials structure and provide little information on the network structure and morphology.

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