BLENDS OF AMORPHOUS-CRYSTALLINE BLOCK-COPOLYMERS WITH AMORPHOUS HOMOPOLYMERS .2. SYNTHESIS AND CHARACTERIZATION OF POLY(ETHYLENE-PROPYLENE) DIBLOCK COPOLYMER AND CRYSTALLIZATION KINETICS FOR THE BLEND WITH ATACTIC POLYPROPYLENE

A symmetric diblock ethylene-propylene copolymer (DEP) was synthesized through hydrogenation of an anionically polymerized polybutadiene-poly(2-methyl-1,3-pentadiene) diblock copolymer (PBD-b-PMPD). IR and NMR measurements showed the hydrogenation to be more than 98 % complete. The resultant diblock copolymer consists of an atactic polypropylene block and a polyethylene block (b-PE) containing 3 mol % of ethyl branches. Differential scanning calorimetry (DSC) was carried out for three series of binary blends made from DEP (M(w) = 113 x 10(3)) and three atactic polypropylene (APP) samples with molecular weights of M(w) = 15, 39, and 190 x 10(3). Blending did not affect the melting behavior of b-PE but drastically altered the crystallization behavior depending on the molecular weight and composition of the blend. Adding APP to DEP caused the primary crystallization peak to shift to lower temperature. In the composition range of more than 50 wt % of APP, another new crystallization peak appeared around 70 degrees C for all blends. This feature was especially pronounced for the blends with the lowest molecular weight APP. Isothermal crystallization studies were carried out for the blends and the data were analyzed by the Avrami theory. The analysis indicates that the dimensionality of the crystallization growth geometry was reduced by blending and each crystallization peak could be correlated with a different dimensionality in the growth geometry. These results are consistent with the framework of the currently accepted microphase separation model for homopolymer/diblock copolymer blends.