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SynopsisThe microrheology of polymer blends as influenced by crosslinks induced in the dispersed phase via electron beam irradiation, is systematically investigated for the model system polystyrene/low density polyethylene (PWLDPE).Both break-up of threads and coalescence of particles are delayed to a large extent, but are not inhibited completely and occur faster than would be expected for a nonirradiated material with a comparable viscosity. Small amplitude, dynamic rheological measurements indicated that in the irradiated materials a yield stress could exist. In contrast, direct microrheological measurements showed that this yield stress, which would prevent both break-up and coalescence, could not be realized by EB irradiation. Apparently, the direct study of the microrheology of a blend system is important for the prediction of the development of its morphology and it is not possible to rely only on rheological data obtained via other methods.

A detailed rheological analysis over large shear rate intervals has been performed for electron-beam-irradiated blends of polypropylene (PP) and ethylenepropylene-diene monomer (EPDM) rubber. At high frequencies, a lower viscosity results from irradiation compared with unirradiated blends, which implies that the irradiated blends are easily processable via injection molding. At low shear rates, however, the irradiated blends behave like a network, and the viscosity may even exceed the viscosity of the unirradiated blends. This particular behavior can result in the formation of weak weld lines. Aggregation of the dispersed, cross-linked EPDM particles into a skeletal structure is the most probable explanation. In a first attempt, it was tried to correlate the network behavior to the average (shortest) interparticle distance (ID) between two rubber particles, which takes into account both volume fractions and particle size of the dispersed phase. Provided that the EPDM rubber is sufficiently cross-linked, the network behavior becomes more pronounced: i.e., increase in viscosity with decreasing interparticle distance. Above a critical value of the ID, the viscosity does not change and is determined by the PP matrix. As a vast amount of literature indicates, the rheology of blends proves to be difficult to understand. Because of the more stable morphology, compared with usual blends, induced by irradiation, a more valuable interpretation of the rheological behavior is possible.

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