Microstructure and physical properties of polymer composites with inclusions aligned by electric fields.

The topics of this thesis are the microstructure, its formation, and the physical properties of particulate polymer composites formed under an electric field. A possible application of the composites is for dental restoration. Particles in a photopolymerizable fluid were aligned by an electric field, and the microstructure was examined at different angles with respect to the alignment axis after polymerization. A variety of rigid particles were examined. Factors affecting the aligned structure were particle size, size distribution, shape, concentration, and dielectric constant. The formation of the aligned structure was studied by observing the microstructure after various field application times. Particle strings and columns formed in distinct stages by longitudinal and lateral particle motion, respectively. Cross sections of the columns showed them to form by aggregation of strings into ribbons. Measurement of the mechanical properties of these composites showed that the compressive elastic modulus along the alignment axis was as much as 35% greater than that of the unaligned composite. Hardness and wear resistance were also increased. The contribution of the aligned particulate structure to the shear viscosity of the fluid under an electric field was also examined. The presence of strings correlated with increased viscosity: strings in a quiescent fluid caused a high initial shear stress when shearing began, and the magnitude of the shear stress increased with increasing string length and thickness. For most shear rates, except those below a critical value, the shear stress subsequently fell. The critical shear rate depends on volume fraction and may be determined by where the shear rate becomes comparable with the rate of string formation. Aligned gels formed by the crystallization of poly(ethylene oxide) were also studied, by applying an electric field across solutions of poly(ethylene oxide) in a photopolymerizable solvent and allowing the solutions to cool until gels formed. After hardening the solvent, the structure examined in thin sections and at fracture surfaces. Both aligned spherical and branched multi-lamellae crystalline phases of poly(ethylene oxide) were observed. Lamellae planes were aligned parallel to the electric field. This method may enable the tailoring of special anisotropic crystalline morphologies.PhDApplied SciencesDentistryHealth and Environmental SciencesMaterials sciencePolymer chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130325/2/9722058.pd