Most polymers solidify below a glass transition temperature (T g), which is important for the fabrication of polymeric materials. The glass transition dynamics (GTD) of polymers alters their physical properties and therefore the range of applications suitable for the particular materials. In this regard, most GTD studies were oriented to the thermodynamics of amorphous polymer systems, while little studies were known for semicrystalline polymers. Here, we focus on the glassy and crystalline properties of semicrystalline polymers such as poly(l-lactic acid) (PLLA) and envisage to control the nanostructure of free-standing PLLA ultrathin films (referred as “PLLA nanosheets”), via thermodynamic rearrangement of polymer chains entangled in a quasi-two-dimensional interface during the GTD process. The annealing process on the PLLA nanosheets (<100 nm thick) resulted in the formation of semicrystalline domains and microscopic apertures with polymer chains (∼100 nm in size). Such nanostructure surprisingly induced selective molecular permeability, which was controlled as a function of film thickness and inherent crystallinity. The present methodology demonstrates the direct conversion of thermodynamic properties of semicrystalline polymers into the functional nanostructured polymeric materials.

Polymer crystallization affects structural and mechanical properties of polymer thin films. In this study, we focused on the thermal annealing-induced crystallization in semi-crystalline poly(l-lactic acid) (PLLA) ultrathin films (referred as nanosheets) was investigated in terms of interfacial interaction of PLLA with air and substrate. The surface structure of the PLLA nanosheets observed by atomic force microscopy showed that roughness of the air-side surface increased due to crystallization of PLLA under thermal annealing, whereas that of the substrate-side surface changed little. The elastic moduli and the physical adhesiveness of the nanosheets also changed only on the surface of the air side from crystallization, in contrast to the substrate side. The X-ray diffraction studies of the PLLA nanosheets with different thickness showed that the crystalline contents steeply increased below ca. 200 nm. These results indicated that the crystallization was enhanced near the surface of the air side and restricted near that of the substrate side due to the different interfacial association of the polymer chains in the nanosheet.