Ann-Kristin Flieger, M. Schulz, and T. Thurn-Albrecht
Experimental Polymer Physics, Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany
Interface-induced crystallization of a liquid on a solid substrate can either occur via heterogeneous nucleation or via prefreezing. Whereas heterogeneous nucleation takes place at finite supercooling below the melting temperature Tm, in prefreezing a crystalline layer is formed at the surface of a solid substrate already above Tm. Wetting theory predicts a jump in thickness at the formation and a divergence upon approaching coexistence. However, the thickness of the prefreezing layer has not been experimentally measured so far.
We studied ultrathin films of polycaprolactone (PCL) during the crystallization on graphite. With in-situ AFM-measurements we observe prefreezing instead of heterogeneous nucleation. The corresponding crystalline layer is formed at a temperature above the bulk melting temperature. Similar observations were already made for polyethylene on graphite . In that case however, a direct measurement of the thickness of the prefreezing layer was not possible. Here, we show directly the finite thickness of the prefreezing layer for PCL. It forms with a thickness of a few nanometers which further increases during cooling. This observation demonstrates the transition is of first order, as expected for a prewetting transition.
The results prove that prefreezing can be described by common wetting theory. The studied system PCL-graphite is of importance for applications since graphitic materials are widely used as fillers for PCL.
 A.-K. Löhmann, T. Henze, and T. Thurn-Albrecht, PNAS 49, 17368-17372 (2014). (link)