Kasyap Seethamraju

Long‐term in vitro hydrolytic stability of thermoplastic polyurethanes

Long-term in vitro stability of thermoplastic polyurethanes (TPUs) was studied for up to 52 weeks in phosphate buffer solution at 37, 55, and 80°C. Water uptake, molecular weights, and tensile properties were measured at regular intervals of 4, 8, 16, 32, and 52 weeks. The rate of molecular weight reduction increased with increasing temperature, and after 52 weeks at 80°C, all commercial polycarbonate (Bionate-55D, Quadrathane-80A, and Chronoflex-80A), poly(dimethylsiloxane) (ElastEon-2A) and polyether (Elasthane-55D) TPUs showed significant (43-51%) molecular weight (Mn ) reduction. The polyisobutylene (PIB)-based TPU exhibited a significantly lower decrease in Mn (26%) after 52 weeks at 80°C. For Bionate-55D and ElastEon-2A, at 80°C in dry nitrogen atmosphere substantial thermal degradation was observed, while for the other TPUs the effect of thermal degradation is small. The temperature dependent reduction of molecular weight was interpreted by simple second order kinetics. From the approximately linear Arrhenius plots the activation energies were calculated, which were highest for PIB-PU-020 and lowest for ElastEon-2A. For Elasthane-55D the in vitro molecular weight reduction was compared with that of explanted leads. The molecular weight reduction in vivo was much smaller than that predicted from in vitro data, which may suggest that the in vitro model does not adequately describe the hydrolysis in vivo. In the absence of validation for the other TPUs that in vitro methods closely reproduce in vivo degradation, it is unknown how these results correlate with in vivo performance.

Structural characterization of telechelic polyisobutylene diol

The chemical homogeneity of telechelic polyisobutylene diol (PIB-diol), prepared by hydroboration-oxidation of allyl telechelic PIB obtained by reacting living PIB with allyltrimethylsilane, was investigated by liquid chromatography at critical conditions (LCCC) and HPLC coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). A normal phase gradient HPLC method was developed that was able to separate the as-synthesized PIB-diol into three components; PIB-diol, PIB-monool and PIB without any OH functionality. These were analyzed by MALDI-TOF MS, which suggested that the reaction of living PIB with allyltrimethylsilane was incomplete. LCCC using refractive index (RI) detector as a concentration detector allowed separation and quantification of PIB species according to their chemical heterogeneity (PIB-diol=95.3%, PIB-monool=3.3%, non-functional PIB=1.4%). The calculated number average functionality (Fn) of PIB-diol=1.94 suggests high quality of PIB-diol suitable for high molecular weight polyurethane synthesis.

Effects of Catalyst Concentration on the Morphology and Mechanical Properties of Polyisobutylene-based Thermoplastic Polyurethanes

We studied the morphology and mechanical properties of a series of polyisobutylene-based thermoplastic polyurethanes (TPUs) with the same composition, but synthesized with tin (II) 2-ethylhexanoate catalyst concentrations ranging from 0.04 to 1 mol% relative to 4,4′-methylenebis(phenyl isocyanate) (MDI). All of the TPUs were found to have incomplete microphase separation between the soft segment (SS) and hard segment (HS), and the degree of microphase separation was not significantly affected by catalyst concentration. However, changing the catalyst concentration resulted in a substantial change in the microphase separated structure. Such differences may arise from the difference in the average chain length distribution of HS, which was confirmed by oxidation of the TPUs. Interestingly, the catalyst concentration also had a dramatic effect on the mechanical properties of these TPUs. When the catalyst concentration was at or below 0.1 mol%, the TPUs exhibited ultimate tensile strength of 20–21 MPa, compared to 10 MPa when the catalyst concentration was at or above 0.4 mol%. These results may help better understand the structure-property relationships of TPUs, and more importantly, develop biostable TPUs with high mechanical strength.

Morphology and Mechanical Properties of Thermoplastic Polyurethanes Containing Polyisobutylene/Poly(tetramethylene oxide) Mixed Segments

We investigated the thermal properties, microphase separated structure and mechanical properties of a series of thermoplastic polyurethanes (TPUs) containing both polyisobutylene (PIB) and poly(tetramethylene oxide) (PTMO) diols in the soft segment (SS). A series of TPUs were prepared with the same weight fraction of the SS but different ratio between PIB and PTMO diols. Molecular weight of the PTMO diol and chemical structure of the hard segment (HS) also varied. Dynamic mechanical analysis (DMA) measurements did not reveal strong microphase separation between PIB and PTMO in the SS. While it has been assumed that incorporating PTMO diol into the SS can enhance the phase mixing between the hard segment (HS) and SS, our results indicated that, in most cases, the degree of microphase separation of TPUs based on mixed diols is slightly higher than that of TPUs based on only PIB diol.