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Local structure and thermodynamic properties of model fluids containing rod-like polymers: a comparison of theory and Monte Carlo simulation
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Rigid rod polymers are an interesting and commercially important class of materials. To manipulate these materials on the macroscopic scale, one must understand their behavior on the molecular level. This research investigated model fluids containing rigid rods in two ways. First, Monte Carlo computer simulations were done for mixtures of rigid rods and solvent. Rods were modeled by linear tangent hard spheres, and solvent was modeled by simple monatomic hard spheres. The compressibility factor, Z, excess solvent chemical potential [ ], solvent insertion probability pm, order parameters, and monomer-monomer, rod-monomer, and rod-rod radial distribution functions, g(r), were calculated. Simulations were run over a range of densities (from 11=0. I to 11=0.43) and polymer mole fractions (x=0.5 to x=1.O). Results were compared to theoretical calculations to assess the accuracy of two promising theories which have not been previously applied to rigid rod solutions. The Generalized Flory Dimer Theory (GF-D) was used to predict the compressibility factor, excess solvent chemical potential, and monomer insertion probability. Distribution functions were calculated using the Polymer Reference Interaction Site Model (PRISM) with a Percus-Yevick closure, using an iterative Picard method. GF-D Theory does an excellent job of predicting pn, and m and does almost as well for PVT properties, especially at lower densities. Error increases slightly with both increasing density and increasing polymer mole fraction, since the monomeric solvent is easier to model. PRISM, however, is less accurate at lower densities. For monomer-monomer g(r), PRISM underestimates the contact value relative to simulation results, especially at higher densities, but is qualitatively very similar. For rod-monomer and rod-rod g(r)'s, it overpredicts the contact value, except at high packing fractions, and more importantly, predicts different peaks than simulation results. PRISM calculations show peaks at integer multiples of the hard sphere diameter, while simulations show prominent peaks at @3 (y, and F7(y, and very small peaks at 2.0(y only in some systems. These values are not surprising based on the system geometry. Earlier studies demonstrated that PRISM does quite well for flexible chain systems. However, this research indicates it is less reliable for stiff chains, and loses much of its predictive power.
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Dane, D. Merrill (1997). Local structure and thermodynamic properties of model fluids containing rod-like polymers: a comparison of theory and Monte Carlo simulation. Master's thesis, Texas A&M University. Available electronically from
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