NMR OF LIQUID POLYMERS

Phillips Petroleum Company, Research and Development, Bartlesville, OK 74004 

Carbon 13 nuclear magnetic resonance can be used quantitatively in analyses of polymers to measure conveniently comonomer concentrations, average sequence lengths, run numbers and comonomer triad distributions. 

The identification of both short chain and long chain branches in polyethylene at concentrations of 1 per 10,000 carbon atoms has become feasible with the availability of improved probes and improved computer hardware/ software capabilities. Reviewed in this chapter are the methods and computations as well as the basic requirements for sound quantitative analyses namely, correct choice of solvent, a consideration of concentration effect on line widths and satisfying nuclear Overhauser effects and spin lattice relaxation time requirements. Finally, the NMR generated structural information is put to use in correlations with polyethylene physical properties and measurements of number average molecular weight. 

Improvements in instrumentation have led to increased spectral sensitivity and to more reliable methods for data integration. These improvements coupled with larger, more efficient and faster computers have given quantitative l c NMR applications the boost required for broad useage. A sensitivity of at least one structural unit per 10,000 carbon atoms is required for useful molecular weight and branching measurements and to allow species produced by oxidation and irradiation (12)(13) to be detected early in the process. 

There are a number of considerations that must be addressed when formulating quantitative 13c NMR procedures - these include solvent effects, spectral overlap, line widths, dynamic and nuclear Overhauser effects and detailed assignments. The steps required to develop sound quantitative methods will be the subject of this chapter. It is imperative that excellent quantitative methods be established so that NMR can be utilized in studies of polymer structure-property relationships. Polymer molecular structure needs to be related to the incipient solid state structure and ultimately to observed solid state physical properties such as density, flexural moduli, environmental stress cracking behavior, to name a few. 

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