Estimation of molecular orientations in disordered samples by a proton-NMR-based method

We introduce a procedure on the basis of proton nuclear magnetic resonance (NMR) for investigation of the orientation state of disordered samples like amorphous or nematic polymers. Advantageous features of this method are the following: (i) disorder of the sample is not a problem (other than in the case of X-ray); (ii) the method works faster than multidimensional NMR techniques; (iii) this procedure can be implemented also at more simple and inexpensive NMR spectrometers; and (iv) for the data evaluation it will be not necessary to know the molecular geometry. The latter is possible by introducing the expressions “relative orientation distribution” and “relative orientation degree” which characterize the difference of the orientation of the current sample in comparison to a reference sample. Contrary to the absolute orientation degrees the relative ones are easily available from wide-line proton NMR spectra. The method is demonstrated by applying it to monitor the qualitatively different behavior of the director fields of two liquid-crystalline polymer samples with different molecular weights which are exposed to a suddenly switched magnetic field. A temporary asymmetry of the orientation distribution could be detected and quantified.