The results of a dilatometric study of the crystallization kinetics of molecular weight fractions of linear polyethylene are reported. These data encompass t?ie molecular weight range 4 X 103-8 X 106 7. Although the usual deviations from the Avrami or Goler-Sachs free-growth formulations are observed, these deviations are systematic with molecular weight and become more pronounced as the molecular weight is increased. There is a one to one correlation between the level of crystallinity at which these deviations occur and the final level of crystallinity that can be attained for each molecular weight. These kinetic results in turn explain the very wide range in values that is observed for all properties. To explain the isotherm shape, we note that over the region of experimental adherance the Avrami exponent is an integral number and varies from four to two as the molecular weight increases. The exponent is independent of temperature except for the very highest crystallization temperatures. At the very highest crystallization temperatures, closer adherance to theory is found, with the implication that if the experiments could be conducted at still higher temperatures the Avrami-type theory would be obeyed. At a fixed temperature the crystallization rate goes through a maximum as a function of molecular weight and the location of the maximum is dependent on the undercooling. Although the very marked negative temperature coefficient gives strong support to nucleation control, it is demonstrated that the rudimentary nucleation theories that have been proposed are not obeyed. irtually all thermodynamic, spectral, mechanical and physical properties of linear polyethylene are dependent on molecular weight and on the crystallization conditions.2-7 A wide range in the values of any specific property is observed for the same chemically constituted polymer. For

Blends of isotactic polypropylene (iPP) with the polyamide nylon-6 (N6), prepared by extrusion, were studied with a composition of up to 30% by weight polyamide. In the case of a 70/30 iPP/N6 blend, the influence of a compatibilizing agent based on polypropylene functionalized with maleic anhydride (PP-g-MA), with compositions of 1, 3, 5, and 10% by weight in polypropylene, was followed. The influence of the concentration of N6 and the compatibilizing agent on the rheological and thermal properties, and the morphology of the blends, was analyzed by monitoring the melt viscosity at different shear rates, differential scanning calorimetry, and polarized light microscopy. Vibrational spectroscopy was used to characterize the blends and to study the effect of the compatibilizing agent. The viscosity-composition curves for the iPP/ N6 blends, in the composition and shear rate ranges analyzed, show a negative deviation from the additive rule, while the opposite trend is observed for the blends compatibilized with PP-g-MA. Important variations in the spectroscopic behavior was observed between compatibilized and noncompatibilized blends, which varied as a function of the compatibilizing agent concentration. The crystallization rates of iPP in the iPP/N6 blends, under both dynamic and isothermal conditions, are much greater than are those observed for pure iPP and are directly related to the nucleating activity of the polyamide. This effect is much smaller in the presence of the compatibilizing agent. The isothermal crystallization of the polyamide N6 in compatibilized blends is affected by the presence of iPP, reducing the crystallization rate due to the diluent effect of the polypropylene. de la Cierva 3, 28006-Madrid, Spain. Contract grant sponsor: Spanish Ministerio de Educación polymers is an inmiscible system composed of two y Ciencia. phases, where the component with higher concen-

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