POLYMER AGGREGATES WITH CRYSTALLINE CORES - THE SYSTEM POLYETHYLENE-POLY(ETHYLENEPROPYLENE)

We have studied the aggregation behavior of polyethylene-poly(ethylene propylene) (PE-PEP) diblock copolymers dissolved in decane. For this p urpose PE-PEP diblock copolymers of various molecular weights, composi tions, and degrees of deuteration were synthesized via an anionic rout e. The structure and morphology of the aggregates was studied by small angle neutron scattering varying both the contrast as well as the pol ymer labeling. Sire found a hierarchy of structures: The PE component crystallizes in lamellar sheets (thickness 40-80 Angstrom) surrounded on both sides by a PEP brush which exhibits a close to parabolic densi ty profile. Different aggregates form macroaggregates of needlelike sh ape with the PE lamellar planes in the long direction. This macroaggre gation is well described by a paracrystalline structure factor. The st ructural parameters depending on composition and molecular weights can be well understood in terms of a free energy of formation based on a scaling model. A quantitative evaluation of the different contribution s to the free energy reveals an important role of defect structures re sulting from the ethylene side branches in the polyethylene component. Finally, we show in a semiquantitative approach that the van der Waal s energy between the brushes is large enough to facilitate macroaggreg ation.