Rheology and reptation of linear polymers. Ultrahigh molecular weight chain dynamics in the melt

The melt rheology of ultrahigh molecular weight polymeric materials characterized by a narrow molecular weight distribution has been analyzed. Ultrahigh molecular weight polyethylene obtained from a metallocene catalyst shows a well-developed “plateau” modulus in a range of angular frequency of more than 3 decades. The characteristic value of the plateau modulus (∼2 MPa) is in close agreement with those reported for a model high molecular weight monodisperse polyethylene. From this value one can determine a characteristic molecular weight between entanglements of 1200 g mol⁻¹. The molecular weight dependency of different, experimentally based relaxation times obtained from the linear viscoelastic response exhibits an exponent power law close to 3.0 for these materials. This seems to contradict the 3.4 dependence observed in the usual molecular weight range, which is based on the chain contour length fluctuation approach, but is in agreement with the latest reptation-based models. These models predict a crossover from the 3.4 to a 3.0 exponent for very long chains as used here at a constant critical value of the molecular weight $Mr$ close to $100Mc$ $(200Mc$ when using the well accepted relationship $Mc=2Me).$ This predicted crossover is independent of the polymer’s chemical composition. However, combining results from our experiments with results from literature shows that the experimental values of $Mr$ extend from $15Mc$ for polystyrene, $25Mc$ for polyisobutilene, $100Mc$ for polybutadyene to $220Mc$ for polyethylene. These results are not predicted by molecular models and demand for new theoretical considerations of chain dynamics, in which the chemical structure is, most probably, a key factor that should be taken into account. It should be noticed that the influence of the molecular weight distribution on the differences observed is not understood. Unfortunately, it is very difficult to obtain monodisperse samples of ultrahigh molecular weight polyethylene and, therefore, the use of the samples studied here the best choice possible, up to now to test and revisit basic and novel aspects of the rheology of polyolefin’s.