| The effect of confinement on the crystallization behavior of polyethylene was studied in spin-cast films as a function of film thickness, annealing temperature, substrate surface energy and microstructure of polymer. The primary morphological tool employed in this study was ex-situ atomic force microscopy (AFM) operated in both contact and tapping modes. For films thicker than ∼200 nm, the crystals were organized in bulk-like banded spherulite morphology irrespective of annealing temperature, and substrate. On the other hand, the thin films (with thicknesses below 200 nm) exhibit changes in the meso-scale organization of the crystals, slowing down of the crystal growth rate and changes in the local organization of the lamellar structure. Most noteworthy is the increase in volumetric number density of crystal nuclei with decreasing film thickness, presumably attributed to the substrate inhomogeneities and the geometric confinement. Detailed imaging of the banded spherulite structure by tapping mode AFM indicates that the band structure composes of periodically branched lamellae that are isochiral. The sheaf-like morphology, prevalent in the ultrathin films, consists of fewer branched edge-on lamellae with successive generations of side lamellae initiating from the edge-on lamellae or nuclei. |
