Crystallization and Melting of Oriented Parent-Daughter Lamellae in Sheared Isotactic Poly(propylene)

Alpha isotactic poly(propylene) (alpha-iPP) exhibits a form of lamellar branching that is unique among semicrystalline polymers, where the branches have a distinct orientation relationship with the original crystalline lamellae. This is termed a parent-daughter relationship (PD). By allowing the structure to crystallize in an oriented form, a bimodal orientation of lamellae is developed and the individual contributions of PD lamellae can be observed using wide-angle X-ray scattering (WAXS). The present study investigated oriented PD lamellae during flow-induced crystallization and subsequent melting using time resolved rheo-WAXS. During crystallization the planes of the daughter lamellae were observed to curve towards the flow direction as they grew from their parent lamellae. This was explained by the influence of neighbouring daughter lamellae confining their growth direction. Oriented daughter lamellae were found to melt similar to 5 degrees C lower than oriented parent lamellae, which provides a new explanation for the multiple melting behaviour observed in the melting thermograms of sheared alpha-iPP.