Nuclear magnetic resonance spectroscopy polymer characterization using

The instrumental analytical techniques discussed in this chapter are those used most frequently to identify plastics in collections. There are many techniques that are used in the plastics industry or university research laboratory which provide extensive information about synthetic materials, but which have not yet found a place in the conservation workshop. This may be attributed to the high cost of the instruments and their maintenance. For this reason they have not been included here. High resolution solid state nuclear magnetic resonance spectroscopy is one such technique which may well be found in many museum laboratories by 2015, but is not available to such institutions today (Lambert et al., 2000). Descriptions of instrumental analytical techniques have been divided into those used to identify polymers, those to examine fillers and those to characterize plasticizers, stabilizers and flame retardants. 

The characterization methods used for polymers of any kind are naturally also usable for conducting polymers. For identification purposes different spectroscopic methods such as infrared spectroscopy and nuclear magnetic resonance (NMR) may be used. If the polymer is soluble, gel filtration methods may be used for the determination of molecular weight. Simple elemental analysis gives information on the stoichiometry etc. However, the extraordinary properties of conducting polymers allow the use of several methods that are novel, at least in the field of polymer analysis. 

The properties of some polymers are dependent on their microstructure for example isotactic polypropylene is crystalline whereas atactic polypropylene is amorphous. Microstructure effects are also exemplified by polybutadienes, where the mode of addition of the diene to the growing chain leads to 1,2-addition, 1,3-addition and 1,4-addition, which may be as or trans. The fraction of different addition species changes the mechanical properties of the polymer. Another example is provided by the chemical composition of a copolymer and its sequence distribution, which together determine its ultimate properties. It is thus of great importance to be able to characterize polymer micro structure. This is generally done using spectroscopic methods, specifically infrared spectroscopy and nuclear magnetic resonance spectroscopy. 

The use of spectroscopic methods to analyse polymers was a natural extension of their initial use for studying the structures of low molar mass compounds. There now are available an enormous number of fully-interpreted spectra of low molar mass compounds and of polymers, and these provide a firm foundation for structural characterization. In the following sections, spectroscopic characterization of polymers is reviewed with particular emphasis upon infrared spectroscopy and nuclear magnetic resonance spectroscopy, since they are most widely used. Fundamental aspects, and descriptions of the instrumentation and experimental procedures used, will be treated only briefly because a full account would require a disproportionate amount of space and there already exist many excellent texts on spectroscopy which deal with these topics. Also due to the limitations of space, only a few spectra and a brief survey of the vast potential of these and other spectroscopic methods for characterization of polymers are given. The reader is referred to specialist texts on spectroscopy of polymers for a more complete appreciation of their uses. 

IR spectroscopy is the most widely-used method for characterizing the molecular structures of polymers, principally because it provides a lot of information and is relatively inexpensive and easy to perform. However, it is not simple to interpret absolutely the more subtle features of IR spectra, such as those due to differences in tacticity. Such interpretations are usually made on the basis of information obtained from other techniques, in particular nuclear magnetic resonance spectroscopy which is by far the most powerful method for determining the detailed molecular microstructures of polymers. 

Nuclear magnetic resonance spectroscopy and Fourier transform infrared (FT-IR) spectroscopy are two common techniques for structural characterizations of organic compounds and polymers, and they have been widely used to characterize the... 

Since many aliphatic PEMs were prepared from some new synthesized polymers in researches, nuclear magnetic resonance (NMR) is a useful tool to characterize the molecular structure of these polymers. In most cases, it was used to verify the coherence of the designed and actual structure of the new polymer [41,47]. Sometimes, it has also been used to identify a series of new polymers, when the researchers need to study on the effect of polymer structure on the properties of PEMs [65]. Confirmation of quaternization was also done by IH NMR spectroscopy (Figure 10.4), such as in cross-linked quaternized PVA (QPVA) [66] and cross-linked quaternized-CS [67] AEMs by observing the chemical shifts (ppm) of related functional groups (OH, CH, CH3) or increase in these peak intensities. 

For both copolymers and stereoregular polymers, experimental methods for characterizing the products often involve spectroscopy. We shall see that nuclear magnetic resonance (NMR) spectra are particularly well suited for the study of tacticity. This method is also used for the analysis of copolymers. 

Fourier transform infrared (FTIR) spectroscopy was performed oj a Nicolet 10DX spectrometer. Nuclear magnetic resonance ( H) characterization was accomplished using an IBM 270 SL. Both techniques can successfully be utilized to analyze both the diblock precursors as well as the derived acid containing polymers. 

Various methods may be used to examine configurations of polysilanes, but 29Si NMR spectroscopy has been the most useful. Silicon-29, like carbon-13, has spin 1/2 and a relatively low abundance, 4.7%. Nuclear magnetic resonance (NMR) spectroscopy using 29Si has been important for the characterization of siloxane polymers, and is proving to be equally useful for polysilanes. 

In terms of characterizing the microstrac-ture of polymer chains, the two most useful techniques are infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectroscopy. Commercial infrared spectrometers were introduced after the end of the second world war and quickly became the workhorse of all polymer synthesis laboratories, providing a routine tool for identification and, to a certain degree, the characterization of microstructure (e.g., the detection of short chain branches in polyethylene). In this regard it can no longer compete with the level of detail provided by modem NMR methods. Nevertheless, IR remains useful or more convenient for certain analytical tasks (and a powerful tool for studying other types of problems). So here we will first describe both techniques and then move on to consider how they can be applied to specific problems in the determination of microstructure. 

The widespread use of synthetic polymers has led to the development of a considerable number of analytical tools for polymer characterization and analysis. Analytical pyrolysis, consisting of pyrolysis coupled with an analytical technique, is one of these tools. The technique can be invaluable in solving many practical problems in polymer analysis. It can be used alone or can provide complementary information to other techniques such as thermal analysis, infrared spectroscopy, or even nuclear magnetic resonance. 

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

Characterization of the chemical structure of highly cross-linked polymers, and of the chemical changes that accompany degradation processes, relies on spectroscopic methods. Solid-state nuclear magnetic resonance techniques have the potential to allow a more detailed characterization than before possible of the chemical environment and structure of chemical crosslinks in elastomers and thermoset epoxies. Degradation processes in cross-linked systems have been studied by using infrared spectroscopy, solid-state NMR, and electron spin resonance. 

Nuclear magnetic resonance (NMR) spectroscopy is now well established as one of the most useful instrumental techniques for characterization of adhesives and for the study of polymeric adhesive structure-property relationships [34]. The reasons are that (1) individual chemical groups in adhesive often give signals that can be resolved, (2) the NMR signals are sensitive to environment, and (3) the theory is well understood and the relationship between speetral parameters and the informa tion of interest (such as concentration or structure) is relatively straightforward. Polymer scientists and technologists have been... 

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