Crystallization of linear low density polyethylene on an in situ oriented isotactic polypropylene substrate manipulated by an extensional flow field†
In this work, we demonstrate that utilization of extensional flow with different intensities can regulate the flow-induced crystallization and epitaxially surface-induced crystallization simultaneously in crystalline–crystalline immiscible blends, leading to improved interfacial adhesion and thus enhanced mechanical properties, which provides a versatile methodology to industrially achieve polymer blends with advanced performance. An accessible methodology, i.e., “extrusion–hot stretching–quenching”, was applied to fulfill the scalable achievement of an epitaxial interface for a linear low density polyethylene (LLDPE)/isotactic polypropylene (iPP) blend, where LLDPE could epitaxially grow on an oriented iPP substrate but greatly influenced by the flow field, with its chains and lamellae aligned abnormally off the flow direction revealed by wide angle X-ray diffraction and small angle X-ray scattering, respectively. Depending on the intensity of flow, the above effect of flow can be divided into two types: under a strong flow field, the LLDPE chains prefer to align along the flow direction, inducing the formation of a shish-kebab structure. For another type, i.e., under a weak flow field, the pre-oriented LLDPE chains can relax quickly and epitaxially nucleate on the surface of the oriented iPP substrate. During further growth, the epitaxial LLDPE lamellae deform and reorient along the flow direction under the mechanism of flow-induced block slips, fragmentation and reorientation. Moreover, it is believed that incomplete lamellar twist also occurs under flow. Mechanical property tests demonstrate that an epitaxial structure significantly improves the interfacial adhesion between LLDPE and iPP, showing remarkable enhancements in both strength and toughness.