Prefreezing of Different Folding States of Linear Polyethylene on Graphite

Prefreezing is the interface induced crystallization of a melt on a solid substrate. In contrast to heterogeneous nucleation, prefreezing is an equilibrium phenomenon that refers to the reversible and abrupt formation of a crystalline layer at a temperature T_max above the bulk melting point T_m. Recent experimental results evidenced that thin films of oligomeric linear polyethylene prefrozen on graphite have a complex structure consisting of a thin layer of extended chain (EC) crystals directly at the graphite and folded chain (FC) crystals on top of this layer. Temperature dependent AFM experiments showed that EC crystals prefreeze at a higher T_max than that of FC crystals: T_max,EC>T_max,FC. To explain this behavior, we extend the recently developed phenomenological theory of prefreezing to the case when a melt can crystallize in phases of different thermal stability with melting temperatures T_m1>T_m2. Our analytical results indicate that while the more stable crystal phase prefreezes at a higher temperature T_max1, prefreezing of the less stable phase is thermodynamically preferred at lower temperatures, T_max2<T_max1. This behavior is in contrast to bulk crystallization, where crystallization of the less stable FC crystals is often kinetically preferred.