Polycarbonate-Urethane Molar Mass Distributions

The Application Notebook

The Application Notebook, The Application Notebook-06-01-2013, Volume 0, Issue 0

Wyatt Technology Corporation

Polycarbonate urethanes (PCUs) are a class of biocompatible polyurethanes that show great promise for use in orthopaedic implants, such as artificial hips.

Traditional implant wear surfaces made from ultrahigh molecular weight polyethylene (UHMWPE) generate wear debris that induce osteolysis, causing the implant to loosen and ultimately require surgical replacement. PCU has been found to have lower friction properties than UHMWPE and has a modulus of elasticity similar to cartilage, resulting in PCU having superior wear behavior compared to UHMWPE (1). The molar mass distribution of PCU is of critical importance for its performance in medical device applications.

The molar mass distribution of a PCU resin was characterized using size exclusion chromatography (SEC) in N,N-dimethylacetamide (DMAc) containing 0.1 M lithium bromide. The SEC separation employed a pair of Tosoh Bioscience TSK-Gel Alpha-M columns, followed by Wyatt miniDAWN TREOS and Optilab rEX detectors connected in series. The molar mass distributions of PCU samples were measured using SEC-MALS, and also using conventional column calibration with a series of narrow polystyrene standards.

Figure 1: Comparison of polycarbonate-urethane absolute molar mass measured using SEC-MALS, versus apparent molar mass relative to polystyrene standards. The 90° light scattering (solid line) and refractive index (dashed line) chromatograms are overlaid.

The PCU molar mass calculated using MALS is plotted versus retention volume in Figure 1. The column calibration plot using polystyrene standards is shown in Figure 1 for comparison. The differential molar mass distribution plots from light scattering and column calibration are compared in Figure 2. The molar mass was greatly overestimated using column calibration with polystyrene standards, due to the significant difference in the size in solution of a polystyrene molecule compared to a PCU molecule of the same molar mass. The Mw value of the PCU resin as measured by column calibration was 134,600 g/mol, compared to the actual value of 60,800 g/mol measured by SEC-MALS.

Figure 2: Comparison of polycarbonate-urethane absolute molar mass measured using SEC-MALS, versus apparent molar mass relative to polystyrene standards. The 90° light scattering (solid line) and refractive index (dashed line) chromatograms are overlaid.

References

(1) R.W. Treharne and A.H. Greene, "The Case for the Use of Polycarbonate-Urethane in Orthopedic Implants," Med-Tech Precision (2008).

This note was graciously submitted by Jason Todd, Polymer Solutions Incorporated, Blacksburg, VA.

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