The effects of soft segment structure on the fatigue crack propagation of model polyurethanes.
The present work is a study of the effects of soft segment molecular weight and chemical structure on the fatigue crack propagation of model copoly (ether-urethane-urea)s (PEUU). The PEUU were synthesized using polypropylene glycol (PPG), polytetramethylene glycol (PTMG), and polyethylene glycol (PEG) as the soft segment component. The number average molecular weights of the polyethers were within the range of 1000-2000. Methylene bis (4-phenylisocyanate) (MDI) and ethylene diamine were used as the diisocyanate and the chain extender, respectively. The cyclic loading experiments were carried out using a computerized film stretcher that can conduct sinusoidal operation at a constant strain amplitude, strain rate, and frequency. The Rivlin-Thomas tearing energy, T, and the fatigue crack propagation (FCP) rate were selected to characterize the fatigue behavior of the model polyurethanes. An empirical equation was applied to define the fatigue properties of model polyurethanes and to evaluate the fatigue resistance. To investigate the effect of molecular variables on the FCP, the morphological changes caused by structural differences and cyclic stress were determined using dynamic viscoelastometer (Rheovibron), Small Angle X-ray Scattering (SAXS), and Fourier-Transform Infrared (FT-IR) spectroscopy. Mooney-Rivlin plot was used to determine the crosslink density variation. In addition the orientation behavior at the crack tip was characterized by IR dichroism technique using a polarized FT-IR microscope. The results indicated a reasonable relationship between the FCP rates and the hard segment content, crosslink density, and deformation property at the crack tip. However, the initial stage of phase separation and domain disruption behavior did not show a good correlation with the FCP properties of model polyurethanes. Among the model polyurethanes tested, the PEUU with PTMG (Mn = 1000) exhibited the best fatigue resistance at given test condition.
SELECTION OF MATERIALS FOR VENTRICULAR ASSIST PUMP DEVELOPMENT AND FABRICATION
A study was undertaken to select 2 of the most promising bladder materials from a list of 8 carefully chosen polymers. The polymers chosen for testing included a wide spectrum of urethanes, as well as a polyester resin and a millable polyolefin rubber. The following materials were studied: Adiprene LW-500, Avcothane 51, Biomer, Hexsyn, Hytrel 4055, Mobay 240313, Pellethane 2103-80 AE, and Tecoflex HR. The selection process was based on a test matrix which was divided into segmented steps, each a logical sequel from its predecessor, and each of increasing technical complexity. Hydrolytic stability, flexure endurance, fabricability, tissue compatibility, hemocompatibility and chronic tissue compatibility were evaluated, forming the basis for the acceptance/rejection criteria. As a result of this search, 2 biomaterials were identified as most suitable for use in conventionally flocked bladders; Biomer and Tecoflex HR. Both Biomer and Tecoflex HR bladders have demonstrated impressive results in circulatory analog loop endurance tests, with Tecoflex bladders surpassing 50 million flexes to date, and Biomer bladders (the longest running bladders) exceeding 150 million flexes. Tests are continuing.
Study of the effect of substrate roughness on adhesive joints by SEM image analysis
The effect of adherend surface roughness on epoxy bonded aluminium joints has been studied. Several epoxy adhesives were tested to evaluate the influence of adhesive nature on the roughness effect. The aluminium pre-treatments applied were abrasion and impression processes, generating different texture levels. The abrasion with grinding papers of different grain sizes provides surfaces with high density of low summits. In contrast, the surfaces subjected to impression process present low density of very deep valleys. The roughness measurements were made by image analysis of micrographs obtained by Environmental Scanning Electron Microscopy (ESEM). In addition to the average roughness, other surface descriptors were determined to characterise completely the surfaces, allowing to evaluate the influence on the adhesive strength of different texture variables, such as the height, shape and density of the peaks and valleys which constitute the roughness profile. An optimum value of surface roughness was found for the joint strength measured by the lap shear tensile test. An increase of adherend roughness causes an increase of effective area but, at the same time, decreases the ability for adhesive penetration. It has also been shown that the joint strength depends on the main characteristics of the adherend surface, such as the density and depth of the protuberances. Finally, it has been found that the roughness effect seems be influenced by the adhesive nature. Different epoxy adhesives with similar mechanical properties present different joint strengths due to their different penetration abilities.