Direct observation of prefreezing at the interface melt–solid in polymer crystallization

Abstract

Crystallization is almost always initiated at an interface to a solid. This observation is classically explained by the assumption of a reduced barrier for crystal nucleation at the interface. However, an interface can also induce crystallization by prefreezing (i.e., the formation of a crystalline layer that is already stable above the bulk melting temperature). We present an atomic force microscopy (AFM)-based in situ observation of a prefreezing process at the interface of a polymeric model system and a crystalline solid. Explicitly, we show an interfacial ordered layer that forms well above the bulk melting temperature with thickness that increases on approaching melt-solid coexistence. Below the melting temperature, the ordered layer initiates crystal growth into the bulk, leading to an oriented, homogeneous semicrystalline structure.semicrystalline polymers | AFM | thin films | epitaxy T he fundamental process of crystallization from the liquid or gaseous state is of importance in many areas of condensed matter physics and materials science. In practice, crystallization is, in most cases, initiated at an interface to a solid. Crystal growth on solid substrates from the gaseous state has been studied in depth, and detailed understanding of different growth modes as well as interfacial thermodynamics has been achieved (1-3). Much less experimental data are available for crystallization occurring at the interface from the solid to the melt. Generally, crystallization can be initiated at the solid-melt interface by two processes: heterogeneous nucleation or formation of a crystalline wetting layer (so-called prefreezing) (4-6). In terms of thermodynamics, these processes are very different. Whereas nucleation takes place under nonequilibrium conditions at finite supercooling below the melting temperature T m of the bulk material, the formation of a wetting layer is an equilibrium phenomenon taking place above T m (4). It is often assumed that heterogeneous nucleation is the more relevant process (7), but in simulations, nucleation as well as prefreezing have been shown to occur (4,8). Prefreezing is expected for strongly attractive surfaces or epitaxial systems for which the lattices of the substrate and the crystallizing materials match well (9-12). In the case of polymers, prefreezing can also manifest itself in the conformational degrees of freedom, leading to an interfacial layer with nematic order, which was recently shown in simulations (13). Because of the difficult accessibility of the buried interface between a melt and a solid, direct observation of crystallization of molecular systems at the interface is lacking, and there is only limited, indirect evidence that, in some cases, prefreezing at the solid interface exists (e.g., for the growth of aluminum crystals on TiB 2 particles) (14, 15). Recently, it has been suggested that prefreezing also plays a role during epitaxial crystallization in some polymeric systems (16). It is well-known, however, that one or sometimes several ordered layers of organ...

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