Equation of state for polymer liquid crystals: Theory and experiment

H. H. Strey, V. A. Parsegian, and R. Podgornik
Phys. Rev. E 59, 999 – Published 1 January 1999
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Abstract

The first part of this paper develops a theory for the free energy of lyotropic polymer nematic liquid crystals. We use a continuum model with macroscopic elastic moduli for a polymer nematic phase. By evaluating the partition function, considering only harmonic fluctuations, we derive an expression for the free energy of the system. We find that the configurational entropic part of the free energy enhances the effective repulsive interactions between the chains. This configurational contribution goes as the fourth root of the direct interactions. Enhancement originates from the coupling between bending fluctuations and the compressibility of the nematic array normal to the average director. In the second part of the paper we use osmotic stress to measure the equation of state for DNA liquid crystals in 0.1M to 1M NaCl solutions. These measurements cover five orders of magnitude in DNA osmotic pressure. At high osmotic pressures the equation of state, dominated by exponentially decaying hydration repulsion, is independent of the ionic strength. At lower pressures the equation of state is dominated by fluctuation enhanced electrostatic double layer repulsion. The measured equation of state for DNA fits well with our theory for all salt concentrations. We are able to extract the strength of the direct electrostatic double layer repulsion. This is an alternative way of measuring effective charge densities along semiflexible polyelectrolytes.

  • Received 2 March 1998

DOI:https://doi.org/10.1103/PhysRevE.59.999

©1999 American Physical Society

Authors & Affiliations

H. H. Strey*, V. A. Parsegian, and R. Podgornik

  • National Institutes of Health, National Institute of Child Health and Human Development, Laboratory of Physical and Structural Biology, Building 12A/2041, Bethesda, Maryland 20892-5626

  • *Permanent address: Department of Polymer Science and Engineering, University of Massachusetts—Amherst, Amherst, MA 01003.
  • On leave from Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana and Department of Theoretical Physics, “J. Stefan” Institute, Ljubljana, Slovenia.

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Vol. 59, Iss. 1 — January 1999

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