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.
Morphology of block copolyurethanes. III. Effect of polyether molecular weight on the mechanical properties of solvent cast films
A homologous series of copolyether–urethane–ureas were synthesized with polypropylene glycol segments having molecular weights of approximately 200, 400, 700, 1000, 2000, and 3000. These copolymers were characterized using tensile measurements and dynamic mechanical spectroscopy. The dynamic mechanical measurements at low frequencies (≤ 1 Hz) showed evidence of the existence of an α-relaxation process. This was attributed to constrained polyether segments located at the interfacial region between the hard urea segments and the soft polypropylene glycol segments.
Analyses of a silicone elastomer heart ball valve.
This work represents a preliminary study of one case history of a silicone heart valve. The laboratory techniques employed have proven useful in characterizing the polymer before and after implantation. Before any model explaining ball variance can be proposed it must be determined whether the polymer loses its integrity because specific constituents in the body fluids interact with primary valence bonds in the polymer chains, or if the fluids simply cause swelling and plasticization of the crosslinked system and the damage is the result of mechanical work on the weaker polymer system. Detailed kinetic and thermodynamic studies are underway to answer these questions. 
 
 
 
The work summarized in this paper refers to the evaluation of only one specimen and should not be taken to mean that every silicone ball from a heart valve prosthesis will show this variance. The incidence of functionally variant silicone balls as reported to one manufacturer represents approximately 1.5% of the aortic and less than 0.05% of the mitral prostheses which have been supplied by this manufacturer. A small percent of valves of this type have the potential of becoming functionally variant and this is one failure mode which should be considered in the implantation of silicone ball valves.