Influence of Water on the Structure and Properties of PDMS-Containing Multiblock Polyurethanes

Abstract

Segmented polyurethane multiblock polymers containing polydimethylsiloxane and polyether soft segments form tough and easily processed thermoplastic elastomers. Two commercially available examples, Elast-Eon E2A (denoted as E2A) and PurSil 35 (denoted as P35), were evaluated for molecular and mechanical stability after immersion in buffered water for up to 52 weeks at temperatures ranging from 37 to 85 °C. Dynamic mechanical spectroscopy experiments, performed in tension and shear, were used to characterize the linear viscoelastic properties of compression-molded (dry) specimens. Small-angle X-ray scattering measurements indicated a disorganized microphase-separated morphology for all test conditions. Upon aging in phosphate buffered saline, samples of E2A and P35 were analyzed by size exclusion chromatography (SEC) and tensile testing as a function of time and temperature. The absolute molar mass of each material prior to aging in water was determined by SEC using a multiangle light scattering detector. Aging at 85 °C and 52 weeks lead to a 67% and 50% reduction in molar mass from the original values for E2A and P35, respectively. We attribute the reduction in molar mass to hydrolysis of the polymer backbone and have evaluated the data using a pseudo-zero-order kinetics analysis. The temperature dependence of the extracted rate data is consistent with an activated (i.e., Arrhenius) process, and thus all the molar mass reduction data can be reduced to a single master curve. Concomitant with the reduction in molar mass, E2A and P35 transformed with aging from strain-hardening to strain-softening materials, characterized by substantially reduced tensile strength (stress at failure) and ultimate elongation (strain at failure) relative to the original properties.

Limitations of predictingin vivobiostability of multiphase polyurethane elastomers using temperature-accelerated degradation testing

“…20 In Ref. 20, compression molded E2A and PurSilV R 35 samples were aged in phosphate buffered saline (PBS) at 37, 55, 70, and 85 C for 52 weeks. Molecular weight and tensile properties were evaluated at several time points.…”

“…This led to the prediction of significant loss in molecular weight and tensile properties over time in both materials attributed to hydrolysis of the polymer backbone. 20 A critical assumption in TTS is that all components or phases within a material must respond equally to temperature in the temperature range being tested. 21 In other words, TTS can only be accurately applied when no morphological changes occur in the temperature range being utilized.…”

“…21,22 Because of the complex phase morphology of E2A, we hypothesize that microstructural changes or phase transitions occur in the temperature range between 37 and 85 C, causing the different phases to not respond equivalently to temperature in that range, invalidating the use of TTS. To test this hypothesis, we compared the predicted molecular weight and tensile property changes from the high temperature aging study 20 with those from clinical explants that had been subjected to human biological stresses in real time through 5 years (abstract submitted to Heart Rhythm Society † ). In addition, dynamic mechanical thermal analysis (DMTA), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and SAXS experiments were employed in order to evaluate phase transitions and microstructural changes after aging for 2 weeks in PBS at 37 and 85 C that may help to explain the discrepancy between the in vitro and in vivo results.…”

“…Finally, in the previous in vitro study, an assumption of constant water absorption with temperature was made in the development of the temperature accelerated aging model. 20 In the current work, this assumption was tested using a water absorption study to investigate the effect of temperature on the water absorption level at saturation.…”

Emerging Trends in Heart Valve Engineering: Part I. Solutions for Future

“…16,104 Nevertheless, extended exposure to water promotes degradation of soft segment polyurethanes due to potential hydrolysis of cabamate groups. 24 Polyurethane valves are manufactured primarily by either dip-casting or thermoforming, with very little hemodynamic difference as a result of the two methods, 59 but the dip-casting outperforms thermoforming in durability characteristics. 29 A new development is the polyhedral oligomeric silsesquioxanespolycarbonate soft segment nanocomposite based heart valve with reduced platelet adhesion characteristics.…”