Confined crystallization and polymorphism in iPP thin films

The morphological and structural evolution of isotactic polypropylene (iPP) in thin and ultrathin films casted by hot-solution spin coating is investigated by means of atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). When reducing the film thickness from 300 nm to 15 nm, changes from a 3D to a quasi-2D morphology are observed. The quasi-2D confined morphology is found in the 220-40 nm thickness range and is characterized by a predominant edge-on orientation (EO) of the lamellae. A transition to the flat-on (FO) orientation paired to a drastic decrease of the sample roughness is observed for thicknesses below 30 nm. Surprisingly, a mixed morphology where orthorhombic γ-phase branches develop from FO α-phase lamellae is observed at the lowest prepared thickness of 15 nm. The unexpected appearance of the γ-phase for the iPP grade used here is due to the favourable flat-on lamellar orientation at the lowest film thickness. Thanks to a quantitative analysis of the GIWAXS patterns, the relative amount between edge-on and flat-on oriented lamellae (χ_EO/χ_FO) is calculated, providing useful insights on the real morphological composition of the iPP thin films, a task that is very challenging to achieve with microscopy techniques.