Characterization of microphase separated structure, interaction with blood components such as lipids, and fatigue behavior after immersion in blood components were carried out for segmented poly(urethaneureas). The materials studied were Biomer and segmented poly(urethaneurea) (TU‐Mn) composed of hard segment with 4,4′‐diphenylmethane diisocyanate (MDI)‐ethylenediamine (EDA) or 4,4′‐diaminodiphenylmethane (DAM) and soft segment with MDI‐polytetramethylene glycol (PTMG) [Mn of 856, 1350, and 2000]. Small‐angle x‐ray scattering, wide‐angle x‐ray diffraction, and dynamic viscoelastic measurements revealed that these materials showed the state of microphase separation. TU‐Mn with PTMG of Mn = 856 shows the partial phase mixing between hard and soft segments, and phase separation was improved with an increase of Mn of PTMG. Biomer has the characteristics of stronger aggregation of hard segment than that of TU‐Mn. All the specimen showed lipid absorption, but the amount of absorption decreased remarkably after precoating on the specimen surface with serum albumin. Lipid absorption of the specimen was confirmed by dynamic viscoelastic and IR measurements. In the case of segmented poly(urethaneurea) which did not immersed in lipids solution, their fatigue strength are sufficient for application to artificial heart systems. However, fatigue strength of the TU‐Mn series was decreased drastically after absorption of lipids. On the other hand, Biomer did not show a decrease of fatigue strength after lipid absorption. The reduction of fatigue strength in the TU‐Mn series after lipid absorption will be attributed to the weak aggregation of hard segment domain. This reduction of fatigue strength in the TU‐Mn series is characterized by formation of microcrack and mirror zone in fatigue fractured specimen. As the precoating of the specimen surface with serum albumin inhibits the absorption of lipids, the reduction of fatigue strength was not observed for the specimen precoated with serum albumin even after immersing the TU‐Mn series in lipids solution for 96 days.
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