Nanoscale surface lamellar orientation and lamellar doubling in ultrathin UHMW-PE films

Nanostructured, highly oriented, ultrathin ultrahigh-molecular weight polyethylene (UHMW-PE) films have been generated by melt drawing at temperatures in proximity to the mean polymer melting point (142.3 +/- 0.2 degrees C, as determined by differential scanning calorimetry). In this temperature regime, the crystal nuclei size, the rheology, and the mobility of melted polymer chains strongly depend on the polymer temperature and therefore affect the development of the crystal morphology of the film. By stepwise increasing of the polymer processing temperature from 138 to 146 degrees C in steps of 2 degrees C, the mean lamellar thickness increases from 25 +/- 2 to 32 +/- 5 nm, as observed with tapping mode atomic force microscopy (AFM). Parallel with increasing thickness, the lamellae arrangement in the drawn films change from staggered to stacked and uncoordinated. On the basis of this observed surface crystal morphology, a model is proposed describing the temperature-dependent evolution of the lamellar arrangement in the melt drawn films. In sufficiently thin films, i.e., thickness of the order of a lamellar width (approximate to 100 nm), discrete lamellae thickness values of 16 +/- 2, 29 +/- 2, and 62 +/- 2 nm are observed in the UHMW-PE film drawn at a temperature of 140 degrees C, indicating the onset of lamellar doubling. For the first time, indications for a mechanical strain-induced lamellar doubling during film drawing are reported.