Interfacial effects in blends with a liquid crystalline dispersed phase.

摘要

The wide range of applications of liquid crystalline materials has created new areas of academic and industrial research, including multiphase liquid crystalline systems. One of the most important new developments in display technology is the emergence of polymer-dispersed liquid crystals for applications in flat panel television technology and switchable windows. Dispersed liquid crystalline polymers act as "flow modifiers" for conventional thermoplastics, effecting substantial reduction in extrusion pressure at very low concentrations (5%). A similar effect is also observed in fiber spinning, in which spin speed is increased.; We have studied dispersions of two biphenylcarbonitriles, 5CB and 8CB, in polydimethylsiloxane as model systems to explore the properties of liquid crystal dispersions. 5CB exhibits nematic and isotropic phases, while 8CB exhibits smectic, nematic, and isotropic phases.; The interfacial tensions between 5CB and 8CB and polydimethylsiloxane were measured as functions of temperature using pendant drop tensiometry enhanced by video image digitization. The interfacial tensions are increasing functions of temperature, an apparent consequence of homeotropic orientation in the nematic phase and decreasing nematic order with increasing temperature. Nematic order near the interface persists above the bulk nematic-isotropic transition temperature. The interfacial tension in the smectic phase is too low to obtain a stable droplet.; We describe dielectric spectroscopy measurements on dispersions of 5CB and 8CB in a polydimethylsiloxane matrix. The spectra of the dispersions exhibit a temperature-dependent dielectric relaxation in the range from 100 to 1,000 Hz, with relaxation times that depend strongly on whether the dispersed phase is isotropic, nematic, or smectic. The dielectric relaxation time also depends on the viscosity of the matrix fluid and the droplet size. These results suggest a coupling between the electric field and the mechanics of the interface that affects the spectrum of the dispersed phase and shifts the Maxwell-Wagner interfacial polarization peak.; To simulate the dielectric measurements, we examine the director fluctuations in a radial nematic droplet subjected to an oscillating electric field using the Leslie-Ericksen-Parodi continuum model. The velocity and director fields are obtained analytically for planar and slightly curved nematic systems through a perturbation analysis. The deformation of the planar interface seems to be too small to affect the dielectric measurements.