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Solution viscosity and phase behavior for blends of lyotropic liquid crystalline and flexible polymers
Abstract
Lyotropic liquid crystalline polymer solutions containing rod-like polymer can be oriented changing the concentration and/or applying a shear field. These solutions can be processed into fibers and films with well-developed, nearly uniaxial orientation providing high stiffness and rigidity. The solutions studied in this dissertation were comprised of rod-like poly-$p$-phenylene benzobisthiazole (PBZT) and coil-like Zytel${\sp{\circler}}$330 (Z33) in methane sulfonic acid and a methane sulfonic acid/poly-phosphoric acid blend. The isotropic-nematic phase boundary for the binary, ternary and quaternary systems was determined from the concentrations corresponding to the maxima in zero-shear viscosity. These "critical concentrations" were found to be in close proximity to the concentrations corresponding to the appearance of a nematic phase as determined by optical microscopy. For the binary and ternary solutions (in addition to many case studies), the theory of Flory for the prediction of the isotropic-nematic phase boundary based on thermodynamics provided an adequate fit of the concentrations corresponding to the viscosity maxima in addition to the critical concentrations observed by microscopy. For quaternary solutions, the aspect ratio of the PBZT component corresponding to the theoretical fit was significantly lower than that expected based on molecular structure. The unique structure of the PPA solvent component is believed to be attributed to this observation (non-negligible energies of interaction). For all of the solutions, a partitioning of the phases was found to occur in which the addition of the coil-like Z33 decreased the amount of rod-like PBZT necessary for the appearance of a biphasic region. The zero-shear viscosity for the isotropic solutions was determined almost entirely by the concentration of the PBZT component and not significantly influenced by the concentration of the Z33 component. However, the critical concentration was significantly reduced by the addition of the Z33. The Doi theory did not provide an adequate fit of the concentration dependence of the zero-shear viscosity for binary and ternary solutions while it was quite successful for the quaternary solutions. The viscosity for solutions of higher total polymer concentration were found to be predominantly dependent on deformational history. This suggests that molecular ordering attained by the concentrated solutions through shearing does not relax significantly to result in a system that is independent of deformational history. The zero-shear viscosity for a concentrated solution is therefore an "apparent" one since it cannot be entirely retained after being subjected to higher shear rates.
Subject Area
Chemical engineering|Plastics|Materials science
Recommended Citation
Magliochetti, Michael James, "Solution viscosity and phase behavior for blends of lyotropic liquid crystalline and flexible polymers" (1990). Doctoral Dissertations Available from Proquest. AAI9022712.
https://scholarworks.umass.edu/dissertations/AAI9022712