Nuclear Magnetic Resonance Studies of Elastomers

The remarkable property that we associate with rubberlike elasticity, the high degree of elastic deformability, has long been known to arise from molecular motion. In fact, Joule recognized a century ago that the retractive force in stretched rubber stems from thermal motions of molecules rather than from attractive forces between molecules, a conclusion which was all the more remarkable because Joule had no idea of the polymeric nature of rubber. This review tells of the newest technique for studying molecular motion, nuclear magnetic resonance spectroscopy (NMR), and of its application to studies of rubberlike substances. Appropriately, the most important measurements of rubberlike elasticity have been mechanical—creep, stress relaxation, dynamic response. The visco-elastic properties have been studied theoretically and have been measured profusely. They have told us much about the spectra of relaxation processes, which range over many decades of frequency. However, the mechanical experiments occur at the macroscopic level. Conclusions as to behavior at the molecular level depend upon the soundness of models. Plainly it is also valuable to examine motion directly at the molecular level. There are several techniques that accomplish this end. Infrared spectroscopy and dielectric relaxation studies are two kinds of measurement that directly indicate the motion of atoms and molecules. To these techniques is added nuclear magnetic resonance spectroscopy. This method responds to molecular behavior quite differently from other kinds of measurement, and avoids some of the restrictions encountered in these other techniques. For example, the requirement of a permanent electric dipole moment effectively excludes dielectric measurements for the study of pure natural rubber and other hydrocarbons, yet motion in such substances is readily seen by NMR. On the other hand, there are distinct limitations to the use of nuclear resonance, as we shall note. In this paper, we shall review the phenomenon of nuclear magnetic resonance, with emphasis on its use in studies of molecular motion in elastomers. It would be wrong to say that NMR has achieved the importance of the principal physical techniques used to study elastomers. Indeed, the information on elastomers yielded by NMR consists largely of isolated examples. Still, we shall seek to show that the method is powerful and has great potentialities. For a more detailed review of the fundamental physics than is given here, the reader is referred to the excellent paper by Pake. A comprehensive survey of NMR studies of polymers is given by Powles.