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.