Shivam Saretia, Rainhard Gabriel Machatschek, Thanga Bhuvanesh Vijaya Bhaskar, Andreas Lendlein

• The thermal behavior of ultrathin, semi‐crystalline films of oligo(ε‐caprolactone)s (OCLs) with hydroxy or methacrylate end groups, is studied by the Langmuir technique in dependence on mean molecular areas and crystallization temperatures. The films on solid substrate as obtained by Langmuir–Schaefer transfer exhibit different lamellar thicknesses, crystal number densities, and lateral sizes. The melting temperature of OCL single crystals at the water and solid surface is proportional to the inverse crystal thickness and generally lower than in bulk PCL. An influence of OCL end groups on the melting behavior is observed mainly at the air–solid interface, where methacrylate end capped OCL melts at lower temperatures than hydroxy end capped OCL. Comparing the underlying substrate, melting/recrystallization of OCL ultrathin films is achievable at lower temperatures at the air–water interface than at the air–solid interface, where recrystallization is not identifiable. Recrystallization at the air–water interface generally occurs atThe thermal behavior of ultrathin, semi‐crystalline films of oligo(ε‐caprolactone)s (OCLs) with hydroxy or methacrylate end groups, is studied by the Langmuir technique in dependence on mean molecular areas and crystallization temperatures. The films on solid substrate as obtained by Langmuir–Schaefer transfer exhibit different lamellar thicknesses, crystal number densities, and lateral sizes. The melting temperature of OCL single crystals at the water and solid surface is proportional to the inverse crystal thickness and generally lower than in bulk PCL. An influence of OCL end groups on the melting behavior is observed mainly at the air–solid interface, where methacrylate end capped OCL melts at lower temperatures than hydroxy end capped OCL. Comparing the underlying substrate, melting/recrystallization of OCL ultrathin films is achievable at lower temperatures at the air–water interface than at the air–solid interface, where recrystallization is not identifiable. Recrystallization at the air–water interface generally occurs at higher temperature than the initial crystallization temperature. The surface pressure, as an additional thermodynamic variable, seems to further affect the crystallization behavior, with crystal thickness and lateral growth rate increasing with surface pressure. The results presented here are important when designing temperature‐sensitive or active nanostructured materials or interfaces based on OCL.…