Effect of Interactions, Molecular and Phase Structure on the Properties of Polyurethane Elastomers

Linear polyurethanes with various stoichiometry were prepared from 4,4'-methylenebis(phenyl isocyanate), 1,4-butanediol and poly(tetrahydrofuran) in melt reaction. The molecular weight of the polymers was estimated from the equilibrium torque measured during the reaction, mechanical properties were characterized by tensile testing and DMA measurements, while phase separation by the determination of transparency. Changes in the NCO/OH ratio led to the expected changes in molecular weight. Both strength and elongation-at-break increased with increasing isocyanate content up to the equimolar ratio of the two functional groups, but decreased only slightly at larger NCO/OH ratios. On the other hand, modulus assumed minimum value in the same composition range. The extent of phase separation is the smallest and the number of mobile soft segments the largest at around the equimolar ratio of the two functional groups.While viscosity depends only on molecular weight, strength and deformability are strongly influenced also by interactions, which on the other hand drive phase separation and completely determine stiffness

Preparation and Characterization of Barium Sulfate Particles as Contrast Materials for Surgery

Barium sulfate particles were prepared from BaCl2 and Na2SO4 under various conditions to explore their potential application as contrast material in endovascular surgery. Ethanol was added for the stabilization of the suspension obtained. Particle characteristics of the samples were determined by SEM and light scattering, while the stability of the suspensions was characterized by sedimentation experiments. Particle characteristics were shown to depend very much on reaction conditions. Particles were obtained in spherical and plate like shape and in a variety of sizes. Primary particles of nanometer size formed aggregates in most cases. Some of the samples had bimodal particle size distribution. Particle characteristics determined from SEM micrographs, by laser light scattering measurements and calculated from sedimentation experiments often differed from each other. The discrepancy could be explained by the dissimilar behavior of various fractions and the different sensitivity of the measurements to them. Some of the samples showed aging phenomena in DMSO leading either to the formation of larger particles or to the break down of aggregates