Study on Concentrated Solutions of Stiff-Chain Polymers : Thermodynamic and Viscoelastic Properties

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The isotropic-liquid crystal phase boundary concentrations, osmotic pressure, and zero-shear viscosity were measured for concentrated solutions of two stiff-chain polymers, poly (hexyl isocyanate) (PHIC) and xanthan, a double-helical polysaccharide. The former is a neutral polymer, while the latter is a polyelectrolyte. It turned out that all the experimental data obtained cannot be described by current theories for rod-like and semi-flexible polymer solutions. We modified the current statistical-mechanical theories and also proposed a new dynamic theory of stiff-chain polymers in isotropic solution as described below.1. The scaled particle (SP) theory of Cotter for neutral straight spherocylinders was extended to semiflexible polymer solutions to calculate the isotropic-liquid crystal phase boundaries and osmotic pressure of the solution. Both quantities calculated were compared favorably with the experimental data PHIC and schizophyllan (a triple helical polysaccharide) solutions.2. The Helmholtz free energy of rod-like and worm-like polyelectrolyte solutions was formulated from a perturbation theory, where the hard core and electrostatic potentials between two polymers were taken as the pair potentials of the reference system and the perturbation, respectively. The free energy of the reference system was obtained from the SP theory, while the perturbation terms were evaluated by the second virial approximation. The agreement between theory and experiment of aqueous xanthan was almost quantitative in the phase boundaries, but not good in osmotic pressure.3. A viscosity equation for stiff-chain polymer solutions dilution through concentration was formulated by replacing the stiff chain by the "fuzzy cylinder" model. This equation describe well the dependence of the zero-shear viscosity on polymer concentration C and molecular weight M for isotropic soltions of xanthan and schizophyllan over wide ranges of C and M.