History of Rheo-NMR
The first suggestion of using NMR to measure rheological properties was by Martins et al. in 19861. In this paper it was pointed out that the NMR spectrum of a nematic liquid crystal would slowly evolve if the sample was physically reoriented inside the NMR magnet due to director realignment in the magnetic field. The rate of reorientation could give information about the rotational visco-elastic properties of the fluid.
This Rheo-NMR method was later applied by Goncalves et al.2 who used proton NMR spectra to monitor the magnetic field reorientation of an initially aligned sample of a nematic polymer liquid crystal, following the sudden physical rotation. By carefully fitting these spectra, rotational (Leslie) viscosities were calculated along with elastic constants associated with defect formation.
In 1990, Nakatani, Polliks and Samulski3 described a quite different Rheo-NMR experiment in which the proton NMR spectrum of a polymer melt was investigated under shear. They acquired their signal from a 100 kD polydimethylsiloxane sample which was contained in the gap of a cone-and-plate cell located in the NMR electromagnet, and in which the cone was driven to produce a shear rate of 4 s-1 .In this experiment the proton NMR spectrum broadened under shear and slowly relaxed once shearing ceased.
In 1991, another approach to Rheo-NMR was reported by Xia and Callaghan (1991)4 who showed that NMR microscopy could be used to measure shear-thinning effects in the semi-dilute solution of a high molecular weight polymer, in particular by observing the deviation from Poiseuille flow apparent when the fluid was pumped through a narrow (700 Ám diameter) pipe.
Then, in 1994, Grabowski and Schmidt5 demonstrated the use of the deuterium quadrupole interaction to investigate molecular order in studying the alignment of nematic liquid crystalline directors under shear, finding the equilibrium angle in the competition between the magnetic torque and the shear. This type of experiment proved able to yield the ratio of specific Leslie viscosities to the magnetic anisotropy.
1. A. F. Martins, P. Esnault, and F. Volino, Phys. Rev. Lett., 1986, 57, 1745.
2. L. N. Goncalvs, J. P. Casquilho, J. Figueirinhas, C. Cruz, and A. F. Martins, Liquid Crystals, 1993, 14, 1485.
3. A. I. Nakatani, M. D. Poliks, and E. T. Samulski, Macromolecules, 1990, 23, 2686.
4. Y. Xia and P. T. Callaghan, Macromolecules, 1991, 24, 4777.
5. D. A. Grabowski and C. Schimdt, Macromolecules, 1994, 27, 2632.