Possible mechanism for the influence of magnetic fields on the properties of diamagnetic solids crystallized from melts

Abstract

The purpose of the present work was to turn attention to the possibility of significantly altering the macroscopic properties of diamagnetic molecular solids, particularly polymers, under the influence of a constant magnetic field. It is assumed that thermal equilibrium exists in the system and that paramagnetic particles may be absent. Such an influence is possible in the case in which the substance is placed in a constant magnetic field in the period of crystallization from a melt or a solution or polymerization in a liquid phase. The mechanism underlying the effect involves the orientation of the diamagnetic crystals growing under these conditions under the effect of the magnetic field. Since the mechanical, optical, electrical, and other properties of the substances are greatly dependent on its crystalline ordering, the identical orientation of the crystals during their formation, when they have orientational mobility, may result in significant changes in these properties. It is assumed here that the orientation of small mobile crystals results in a situation in which a considerable portion of the melt is crystallized around these crystals in an oriented manner. It is obvious that the energy of the interaction of a diamagnetic molecule with an external magnetic field U = -(1/2)()~m/N)H 2 is much smaller than the energy for thermal motion kT (here Xm is the molar diamagnetic susceptibility of the substance, N is Avogadro's number, k is the Boltzmann constant, and H is the intensity of the external magnetic field). However, in the case of a crystal containing n molecules, the energy of the interaction with the external magnetic field increases by a factor of n and takes on a value comparable to or greater than kT. The orientation of the crystals when they retain such dimensions and when the difference between the energies for the various orientations of the crystal due to the anisotropy of the diamagnetic susceptibility approximates the value of the thermal energy kT. Let us evaluate the minimum size of the crystals which will be oriented by the external magnetic field. As a typical parameter for organic crystals we shall take the diamagnetic susceptibility for the -CH 2- group )/m = 10-5 cm3" The values of )~m in different crystallographic directions often differ very strongly, especially in compounds containing aromatic rings. For example, in the case of naphthalene, the components of the tensor