Magnetic field induced instabilities in nematic solutions of polyhexylisocyanates

摘要

Magnetic field induced Freedericksz transition in the twist geometry of an initialy well-aligned nematic solution of a rod-like polymer, polyhexylisocyanate (PHIC), is studied. For the angle #alpha# between the magnetic the magnetic field and the unperturbed director N_0, one finds a homogeneous distortion if this angle is less than 45 deg C. For angles greater than 45 deg C, the reorientation is inhomogeneous, and is coupled to secondary flow, giving rise to spatially periodic structures. The director deformation involves an out-of-plane tilt at short times. which manifests itself as a phase grating for transmitted polarized light. It is found that the wabelength of the instability is independednt of the magnetic field strength in the experimental range, contrary to prior results ofr nematic solutions of polybenzylglutamate. We postulate that the insensitivity of the wavelength of the periodic structure is due to nonflow-aligning character of PHIC solutions.