Polymer processing nuclear magnetic resonance

Perhaps the most revolutionary development has been the application of on-line mass spectroscopic detection for compositional analysis. Polymer composition can be inferred from column retention time or from viscometric and other indirect detection methods, but mass spectroscopy has reduced much of the ambiguity associated with that process. Quantitation of end groups and of co-polymer composition can now be accomplished directly through mass spectroscopy. Mass spectroscopy is particularly well suited as an on-line GPC technique, since common GPC solvents interfere with other on-line detectors, including UV-VIS absorbance, nuclear magnetic resonance and infrared spectroscopic detectors. By contrast, common GPC solvents are readily adaptable to mass spectroscopic interfaces. No detection technique offers a combination of universality of analyte detection, specificity of information, and ease of use comparable to that of mass spectroscopy. 

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state [1]. Undoubtedly the widest application of NMR spectroscopy is in the field of structure determination. The identification of certain atoms or groups in a molecule as well as their position relative to each other can be obtained by one-, two-, and three-dimensional NMR. Of importance to polymerization of vinyl monomers is the orientation of each vinyl monomer unit to the growing chain tacticity. The time scale involved in NMR measurements makes it possible to study certain rate processes, including chemical reaction rates. Other applications are isomerism, internal relaxation, conformational analysis, and tautomerism. 

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

For the investigation of the molecular dynamics in polymers, deuteron solid-state nuclear magnetic resonance (2D-NMR) spectroscopy has been shown to be a powerful method [1]. In the field of viscoelastic polymers, segmental dynamics of poly(urethanes) has been studied intensively by 2D-NMR [78, 79]. In addition to ID NMR spectroscopy, 2D NMR exchange spectroscopy was used to extend the time scale of molecular dynamics up to the order of milliseconds or even seconds. In combination with line-shape simulation, this technique allows one to obtain correlation times and correlation-time distributions of the molecular mobility as well as detailed information about the geometry of the motional process [1]. 

In the past decades, nuclear magnetic resonance (NMR) spectroscopy has been used extensively to study various aspects of polymer chemistry and engineering. Fig. 1 shows the relationship among polymerization conditions, polymer structure, and the material s physical structure and end uses. Solution, solid state, and imaging NMR techniques contribute to imderstanding the physical and chemical aspects of the route from raw materials to final product. Solution NMR provides information about all aspects of the polymerization reactions and the final structure of the synthesized polymer. This information can be correlated with the material s final properties and provide feedback to control the initial polymerization process so that the fraction of structures responsible for desirable properties can be controlled in a systematic way. 

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. 

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