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Rheological properties of liquid crystalline star-block copolymers
Abstract
This dissertation details the study of the properties of some unique radial, or 'star', liquid-crystalline polymers: block copolymers with poly(dimethyl siloxane) (at the arm centers) (PDMS) and poly(p-benzamide) (at the ends) (PBA). They were studied in the solid state and in solutions with dimethylacetamide/lithium chloride. The critical variable was the molecular weight of the siloxane, which was varied from 650/arm to 4250/arm. The molecular weight of the PBA was held constant, at an Mn = 7000/arm, for all the star-block copolymers. In total, there were four such radial copolymers for study, as well as one linear block copolymer, used as a standard for comparison. Characterization of the solid copolymers was performed using Thermogravimetric Analysis (TGA) and X-Ray Scattering. The copolymers are soluble with difficulty. The solutions obtained were characterized in the quiescent state by polarized-light microscopy (PLM), and by Wide- and Small-Angle X-Ray Scattering (WAXS, SAXS). It was found by PLM that the PDMS spacer length greatly influenced the ability of the polymers to form a liquid-crystalline solution. The block copolymers of highest PDMS molecular weight (largest flexible spacer), above 1500/arm, formed anisotropic solutions. Below this molecular weight, the solutions were isotropic at concentrations up to saturation. Rheological characterization was performed by steady-shear solution rheology in both the cone-plate and parallel-disk modes. The viscosity and normal-stress behavior of the solutions were evaluated with respect to concentration and liquid-crystallinity. It was found that both the isotropic and mesophasic solutions were sharply shear-thinning, with no low-shear Newtonian plateau. The liquid-crystalline polymers were found to exhibit a "rheological phase diagram", a peak in viscosity vs. concentration, confirming the phase behavior observed by polarized-light microscopy. The solutions showed a positive primary normal-stress coefficient which decreased with increasing shear rate, and which also was dependent on the presence or absence of liquid-crystallinity in the solutions. To understand further the deformation behavior of the solutions, fibers were spun from the solutions and the results characterized by X-Ray Scattering. It was found that the fibers spun from anisotropic solutions showed a greater degree of crystalline orientation.
Subject Area
Polymers
Recommended Citation
Muir, Matthew Campbell, "Rheological properties of liquid crystalline star-block copolymers" (1989). Doctoral Dissertations Available from Proquest. AAI9001542.
https://scholarworks.umass.edu/dissertations/AAI9001542