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Application note helps to characterise polyurethanes used in medical applications

Wyatt Technology : 10 April, 2008  (Company News)
A new, free-of-charge application note has been published by Wyatt Technology to demonstrate the accurate and reliable characterisation of polyurethanes used in medical applications.
Wyatt's powerful Multi-Angle Light Scattering (MALS) detectors were found to provide vital information about the molecular structure of polyurethanes when coupled to the company's innovative on-line ViscoStar viscometer.

Polyurethanes are increasingly being used in medical applications, outperforming many other materials with regards to robustness, biocompatibility, haemocompatibility, flexibility, abrasion resistance and processing versatility. Silicone, for example, is difficult to extrude and does not bond easily to non-silicone components.

PVC is associated with increased concerns regarding leachable plasticizers while becoming brittle at low temperatures. Applications include short-term implants, artificial hearts, Catheter and general-purpose tubing, surgical drains, intra-aortic balloon pumps, dialysis devices and medical garments. It is of utmost importance to test the structure-property relationships of polyurethanes prior to production in order to ensure that factors such as chemical composition, molar mass and molecular structure are within specifications to avoid possible fatal consequences to the device users.

Valuable information about the polymer structure and polymer-solvent interactions can be obtained from the log-log relation between the intrinsic viscosity and molar mass, called a Mark-Houwink plot. The typical way to determine a Mark-Houwink plot involves preparing narrow fractions and their characterisation with batch light scattering and a capillary viscometer.

However, this procedure is laborious and unsuitable for routine work. An efficient and rapid way to generate Mark-Houwink plots is to couple a MALS detector to an on-line viscometer and Size Exclusion Chromatograph. Such a configuration can achieve excellent repeatability and accuracy for sensitive comparison of slightly different samples.

For this particular application, Wyatt's SEC-MALS instrument and ViscoStar viscometer were combined to measure the Mark Houwink parameters of two polyurethanes. The curvature of the two Mark Houwink plots indicated branching in both polymers. In addition, the plots overlapped in the region of lower molar masses, indicating an identical degree of branching.

Deviation of the plots towards higher molar masses revealed a higher branching degree of the red polymer, the plot of which became flat at the region of higher molar masses. The fact was explained by the presence of compact sphere-like fractions, which may be important for the final properties of the material.
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