SynopsisA simple superposition model was used to define the relationship between molecular weight distribution and shear viscosity for linear polymeric systems. Gel permeation chromatography data for molecular weight distributions were fitted using statistical distribution functions. A simple superposition model was then employed to calculate the shear viscosity for the systems investigated. The effect of polydispersity on the shape of the flow curves was calculated. The simplicity of the model makes feasible its use in numerical simulations of complex geometries as encountered in polymer processing equipment. The present study also sheds some light on the relationship between entanglement and disentanglement phenomena in polymeric systems. © 1998 The Society of Rheology. ͓S0148-6055͑98͒00404-0͔Colby et al. ͑1987͔͒. At values of molecular weight greater than a critical molecular weight, M c , 0 becomes proportional to M 3.4 as proposed in 1951 by Fox and Flory ͓Fox and Flory ͑1951͔͒ on the basis of measurements on narrow distribution fractions of polystyrene and polyisobutylene. In noncrosslinked systems, physical knots between chains, commonly referred to as entanglements, were invoked to explain, beyond M c , both the sharp increase in the molecular weight dependence of zero shear viscosity and the concomitant appearance of the shear thinning behavior.

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The rich behavior of thermoplastic polyurethanes (TPUs) undergoing phase transition is investigated by means of combined rheological and DSC techniques. In particular, the kinetics of micro-phase transition of a commercial TPU are followed under both isothermal conditions (crystallization from the melt), and constant temperature rate heating ramps (melting). In rheological tests, the time evolution of the storage modulus is monitored by using a strain rateϪcontrolled rheometer. The rheological results are compared with those obtained by more traditional calorimetric techniques (DSC) under the same thermal histories. It is shown that rheological measurements can give additional (and, in some cases, more accurate) information than that obtained from the more classical thermal analysis. Polym. Eng. Sci. 44:1514 -1521, 2004

A viscosity model showing a direct correlation between molecular weight distributions and shear thinning behavior of polymeric melts was used in 3D numerical simulations of an extrusion process. The equipment analyzed was a single screw extruder with an annular die attached. The material of choice in the simulations was polystyrene in a range of molecular weights and degrees of polydispersity. The influence of material parameters (molecular weight and polydispersity) on the system operating point, power consumption and residence time distribution was analyzed. The results were generalized and a window of processing conditions for resins with different properties was analyzed. The effect of blending homologous polymers with different molecular parameters on material processability was also presented. The model can be extended to other materials and various processing equipment.

The free volume fraction of a macromolecular structure can be assessed theoretically by using a suitable model; however, it can also be evaluated from experimental data obtained from dilatometry and positron annihilation lifetime spectra. In this second case, a regular geometry of the sub-nanometric cavities forming the free volume has to be assumed, although in fact they are irregularly shaped. The most popular approach is to guess spherical holes, which implies an isotropic growth of these last with temperature. In this work, we compared the free volume fraction, as obtained from experiments in a set of polybutadiene and polyisoprene cured rubbers and their blends, with the analogous quantity expected by using the lattice-hole model. The results allowed us to obtain insights on the approximate shape of the holes. Indeed, a cylindrical flattened geometry of the cavities produced a better agreement with the theory than the spherical shape. Furthermore, the best fit was obtained for holes that expanded preferentially in the radial direction, with a consequent decrease of the aspect ratio with temperature.