SUMMARY When crystallisable polymers like isotactic poly(propy1ene) (iPP) or linear low density polyethylene (LLDPE) are finely dispersed in an incompatible matrix like atactic polystyrene (PS) a fractionated crystallisation process will develop if the number of dispersed droplets is greater than the number of active heterogeneities originally present in the bulk polymer. In this work, several PShPP and PSLLDPE blends were prepared in a composition range where PS was always the matrix component and in some cases compatibilizers were used to enhance dispersion. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i.e., those normally active at low supercoolings in the bulk polymer) in every droplet. A detailed characterisation of the particle size distribution was carried out by SEM and the validity of using a Poisson distribution to calculate the concentration of heterogeneities present in one blend system was examined. The calculation of the concentration of heterogeneities can qualitatively explain the presence or absence of particular exotherms in the complex DSC cooling behaviour of some compositions of the PS/LLDPE/SEBS blends. However, the effect of the dispersity of the particle size distribution was found to greatly influence the results. When sufficient amount of a compatibilizer is used to obtain the minimum possible particle size the iPP crystallises exclusively at 45 "C. The origin of such crystallisation and the possibility that it may be interpreted as arising from homogeneous nucleation is discussed along with analogous data for dispersed LLDPE.

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In the present work, the effectiveness of using a styrene/ ethylene-butylene/styrene rubber grafted with maleic anhydride (MA) as toughening material in ternary blends with two polyamide-6 nanocomposites or PP with different molecular weight as matrix phases was evaluated in terms of SEM and TEM morphology, WAXD, capillary rheometry, and tensile properties determination. Two metallocene polyethylenes and two polypropylenes were used as the dispersed phase in these ternary blends, and the effect of the comonomer contents in the former was also evaluated. Results showed that the blends prepared with metallocene polyethylenes are tougher than those prepared with PP as the dispersed phase and no significant changes in the Young's modulus or tensile strength were observed when changing the type of metallocene polyethylenes used. When a blend of SEBS/SEBS-g-MA instead of SEBS-g-MA alone was used as a compatibilizing agent in the ternary PA-6 nanocomposite blends, a major decrease in the Young's modulus and tensile strength and a higher toughness were observed. The formation of PA-OMMT particle clusters seems to improve the toughness/stiffness balance. These results are related to the degree of clay dispersion in the polyamide and to the type of morphology developed in the different blends. POLYM. ENG. SCI., 46:1111-1120, 2006.