Interface-Induced Crystallization of Polycaprolactone on Graphite via First-Order Prewetting of the Crystalline Phase

Abstract

Interface-induced crystallization of a liquid on a solid substrate can occur either via heterogeneous nucleation or via prefreezing, i.e., the formation of a crystalline layer, which is stable already above the bulk melting temperature. Upon cooling this layer acts as an "ideal crystal nucleus". Using in situ AFM measurements, we here show that polycaprolactone exhibits prefreezing on graphite. Because of the special wetting behavior of the liquid phase, which is different from the previously investigated system polyethylene on graphite, it is possible to directly measure the thickness of the prefrozen layer as a function of temperature. A thin crystalline layer forms with a finite thickness which increases and theoretically diverges upon approaching the phase transition liquid-solid. The results of our experiments show that prefreezing is a first-order transition.