Shear-Induced Structure Formation in MAH-g-PP Compatibilized Polypropylenes

The effect of maleic-anhydride-grafted polypropylenecompatibilizeron the crystallization behavior of two isotactic polypropylene homopolymersis experimentally investigated under both quiescent and shear flowconditions. A traditional combination of optical microscopy and calorimetrictechniques is used to quantify crystal nucleation and growth ratesand suggests a minute increase in nucleation density when the compatibilizeris added. The flow properties of these systems are assessed by meansof oscillatory shear rheometry. The altered flow characteristics canbe explained based on the molecular weight distribution of the individualblend components, and no influence of maleic anhydride incorporationon the rheological properties is found. While the addition of a smallamount of this compatibilizer thus leads to only a slight accelerationof the crystallization kinetics in quiescent conditions, it markedlyenhances the crystallization rate when a mild (and strong) shear flowis applied. In the latter case, the resulting morphology and crystalmodification are considerably different as compared to crystallizationconditions without the presence of flow; in addition to having significantlyfaster flow-induced crystallization kinetics (I), when the systemcontains maleic anhydride compatibilization, the formation of orientedstructures is hindered (II), and the appearance of the & beta;-modificationis suppressed (III) with respect to the homopolymers, which in turnaffects the mechanical properties of the material. This result highlightsthe importance of understanding the crystallization kinetics underprocessing relevant conditions in order to get a step closer towardfull control over the crystallizing microstructure and the subsequentmechanical performance of polyolefin-based composites.