STUDY OF FAST SWITCHING PROCESSES DUE TO ELECTRIC AND MAGNETIC-FIELDS- AN NMR APPROACH

Solid state NMR techniques have been developed to investigate dynamic molecular effects (e.g., molecular reorientations) due to simultaneous ly applied external electric fields on electrically sensitive material s such as liquid crystals (LC), liquid crystalline polymers (LCP) and polymeric electrets. Such effects can be observed only on relatively t hin systems (10-100 mu m). That means that many scans are necessary to achieve a sufficiently high signal-to-noise-ratio in the spectra (500 -1000 scans). If the material is also magnetically sensitive, the elec tric field can be used to orient molecules in a starting orientational state and by switching-off the voltage to access fast reorientation p rocesses in the magnetic field B-0. Until now, the behaviour of orient able molecular systems under the influence of electric fields has been investigated by means of a more or less quasistatic approach (LCP: 10 0 V, electrets: 1 kV) in equilibrium states. The achievable time resol ution depends on the desired signal-to-noise-ratio. For the case of pr oton NMR this means a time resolution of about IO min. However, very o ften switching processes occur on a much shorter time scale. Using con ventional techniques it is impossible to observe fast(ca. 100 mu s) el ectrically or magnetically induced reorientation processes. In this wo rk, we present a concept to overcome the problems outlined above and t o extend the area of our current in situ NMR investigations on thin el ectrically-switched or poled polymeric layers. The basic idea is to in clude synchronized electric pulses during the NMR experiment using the preparation and/or mixing periods of a 1D or 2D pulse sequence for th e application of an orienting field (electric or magnetic) and to use the reversibility of the molecular switching phenomenon to achieve a s ufficient signal-to-noise-ratio. The techniques extend the range of po ssible investigations from about 100 mu s to approximately T-1 for cor related spectra land to longer times of applied fields for uncorrelate d spectral. Results are shown for a nematic LC and a nematic polymer h aving a similar side chain. (C) 1998 Elsevier Science B.V.