SAXS/WAXS Studies of Flow-Induced Crystallization of LDPE under Uniaxial Extensional Flow

We report flow-induced crystallization studies of LDPE under uniaxial extensional flow. Flow was applied using an SER housed in a custom-built oven designed to facilitate in situ synchrotron x-ray experiments. Samples loaded onto the fixture were first heated well into the melt, and then cooled to selected crystallization temperatures under which negligible quiescent crystallization occurred on reasonable time scales. Two types of flow protocols were employed: short-term and continuous. Under short-term flow, a short burst of flow was applied at temperatures where samples crystallized isothermally after the application of flow under varied extension rates and Hencky strains. Under continuous flow, samples crystallized isothermally at selected temperatures during the application of flow under varied extension rates. The extent of crystallization and orientation of crystallites were monitored using SAXS and WAXS measurements. WAXS measurements are generally consistent with each other and indicate alignment of unit cells along the flow direction in both short-term and continuous flows. SAXS patterns under short-term flow show diverse and complex evolutions in the lamellar scale morphologies; those under continuous flow show distinct features of shish-kebab crystalline structure.