Characterization and Rheological Properties of Ultra-High Molecular Weight Polyethylenes.

The molecular characteristics and rheological properties of three UHMWPE samples were investigated. The high-temperature GPC method was used for characterizing UHMWPE samples used. The interpretation of the measurement results was based on calibration using the PS standard and the approximation of the PS data by linear and cubic polynomials, as well as on the data for linear PE. The assessment of the average MW and MWD depends on the choice of calibration method, so that different methods give different results. Only the results obtained using PS with cubic approximation are close to the characteristics offered by the manufacturer. It was also shown that the obtained MW characteristics depend on the dissolution time. The reason for this may be the presence of any processing-aid compounds or destruction of macromolecules. Measurements of the rheological properties were performed in creep modes for a wide range of shear stresses and harmonic oscillations. It was shown that even at 210 °C, UHMWPE does not flow, and the observed irreversible deformations are due to the plasticity of the polymer, i.e., UHMWPE is in an elastic-plastic state. The ultimate plastic deformations drop sharply with increasing MW of the polymer. The plasticity modulus for the highest molecular weight UHMWPE samples does not depend on stress. Measurements of viscoelastic characteristics confirmed that the terminal region of viscous flow cannot be reached under any conditions. Increasing the duration of holding the polymer at high temperature leads not to flow, but to the destruction of macromolecules.