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

The effect of maleic-anhydride-grafted polypropylene compatibilizer on the crystallization behavior of two isotactic polypropylene homopolymers is experimentally investigated under both quiescent and shear flow conditions. A traditional combination of optical microscopy and calorimetric techniques is used to quantify crystal nucleation and growth rates and suggests a minute increase in nucleation density when the compatibilizer is added. The flow properties of these systems are assessed by means of oscillatory shear rheometry. The altered flow characteristics can be explained based on the molecular weight distribution of the individual blend components, and no influence of maleic anhydride incorporation on the rheological properties is found. While the addition of a small amount of this compatibilizer thus leads to only a slight acceleration of the crystallization kinetics in quiescent conditions, it markedly enhances the crystallization rate when a mild (and strong) shear flow is applied. In the latter case, the resulting morphology and crystal modification are considerably different as compared to crystallization conditions without the presence of flow; in addition to having significantly faster flow-induced crystallization kinetics (I), when the system contains maleic anhydride compatibilization, the formation of oriented structures is hindered (II), and the appearance of the β-modification is suppressed (III) with respect to the homopolymers, which in turn affects the mechanical properties of the material. This result highlights the importance of understanding the crystallization kinetics under processing relevant conditions in order to get a step closer toward full control over the crystallizing microstructure and the subsequent mechanical performance of polyolefin-based composites.