Franklin Lonberg

Measurements of the anisotropic viscous and elastic properties of lyotropic polymer nematics

We summarize the current status of our studies of the macroscopic linear mechanical properties of nematic liquid crystals formed from solutions of rigid or semirigid rod-like polymers. The polymer system we have studied is a racemic mixture of poly(benzyl glutamate) dissolved in a mixture of dioxane and methylene chloride. We have also studied nematics formed from colloidal suspensions of tobacco mosaic virus which may be viewed as an ideal model system representing a rigid-rod polymer solution. We review briefly the current theoretical understanding of the elastic and viscous parameters characterizing a nematic. Then we discuss our experiments, both Frederiks-transition studies and quasielastic Rayleigh scattering. The Frederiks transition in its well known simple forms is not easily utilized in these systems, mainly because of the intervention of a number of phenomena not normally encountered in low-molecular-weight liquid crystals. However, these phenomena have been analysed theoretically and can be used to extract information on elastic and viscous properties. Quasielastic Rayleigh scattering on well oriented single crystals has proved to be an excellent technique for measuring ratios of elastic and viscous parameters. We describe the scattering geometries we have used and our results.

Cholesteric Liquid Crystal Smart Reflectors

Abstract We present results concerning the development of liquid crystal smart reflectors, consisting of a cholesteric liquid crystal with temperature sensitive helix pitch, in combination with a light absorbing dye. Light entering the liquid crystal is absorbed by the dye, generating heat which raises the temperature of the liquid crystal. The resulting change in the helix pitch of the cholesteric causes an increase in its reflectivity, reducing the intensity of light that can be absorbed by the dye. This negative feedback stabilizes the reflector for a given light intensity. The smart reflector thus achieves a reflectivity which increases with increasing intensity of incident light. We report on two configurations of the device, with both experimental measurements and mathematical models of the system.

Light Scattering Studies of a Nematic to Smectic — A Phase Transition in Rigid Rod Polymer Solutions

An important approach to understanding phase transitions in liquid crystals is to study systems in which the dominant interparticle interactions are hard repulsion, rather than the complicated interparticle potentials found in most liquid crystals. Computer simulations have shown [1] that a nematic to smectic A phase transition may exist in a system as simple as monodisperse hard rods. Experimentally, Xin Wen, Robert Meyer and Donald Caspar [2] have reported the observation of the smectic A ordering in tobacco mosaic virus (TMV) suspensions, in which the dominant interparticle interactions are screened electrostatic repulsion. In this paper we present the first quantitative measurements of the nature of the nematic to smectic A phase transition in TMV. In solutions in which the concentration is just below the transition concentration to the smectic phase, large pretransitional smectic fluctuations are observed. We will describe the methods of measuring the correlation lengths of these presmectic fluctuations by light scattering. In addition, the concentration difference between the smectic and the nematic phase close to the transition point is determined to be very small, implying that the transition is very close to second order. This result is consistent with the conclusion of numerous theoretical approaches [3], such as density functional theory and scale particle theory, which have concluded that a nematic to smectic A transition in a system of rigid rods should be second order or very close to second order.

Liquid crystal smart reflectors

We report on the development of liquid crystal smart reflectors, consisting of a cholesteric liquid crystal with temperature sensitive helix pitch, in combination with a light absorbing dye. Light entering the liquid crystal is absorbed by the dye, generating heat which raises the temperature of the liquid crystal. The resulting change in the helix pitch of the cholesteric causes an increase in its reflectivity, reducing the intensity of light that can be absorbed by the dye. This negative feedback stabilizes the reflector for a given light intensity. The smart reflector thus achieves a reflectivity which increases with increasing intensity of incident light. We report on two configurations of the device, with both experimental measurements and mathematical models of the system.

Periodic Director Patterns in Twist and Splay Frederiks Transitions in Tobacco Mosaic Virus Liouid Crystals

Abstract When a magnetic field is applied in the plane and perpendicular to the unperturbed director of a uniform planar-aligned liquid crystal, a twist Frederiks transition occurs. For fields greater than a critical value, periodic bend-modulated twist distortions are dynamically favored over the uniform twist distortion. This is because the periodic flow pattern associated with the bend twist distortion lowers the effective viscosity more than it raises the elastic restoring torque. Under crossed polarizers the sample has a striped texture with the stripes perpendicular to the initial director. The wavelength of the stripe texture is field dependent. When the magnetic field is applied perpendicular to the uniform planar sample, a similar striped distortion occurs above a critical field value, dominated by splay rather than twist. The most striking difference from the twist transition is that there is a thickness-dependent angle between the stripes and the original director whereas in thinner samples, th...