Trilayered Single Crystals with Epitaxial Growth in Poly(ethylene oxide)-block-poly(ε-caprolactone)-block-poly(l‑lactide) Thin Films

Manipulation of crystalline textures of biocompatible block copolymers is critical for the applications in the medical field. Here, we present the control of multiple-crystalline morphologies with flat-on chain orientation in biocompatible poly­(ethylene oxide)-block-poly­(ε-caprolactone)-block-poly­(l-lactide) (PEO–PCL–PLLA) triblock copolymer thin films using melt and solvent-induced crystallizations. Only single-crystalline morphologies of the first-crystallized blocks can be obtained in the melt-crystallized thin films due to the confinement effect. With solvent annealing by PCL-selective toluene, single-crystalline PLLA to double-crystalline PLLA/PCL and to triple-crystalline PLLA/PCL/PEO layered crystals in sequence are observed for the first time. With the control of solvent selectivity, different sequential crystallization involving first-crystallized PCL transferring to double-crystalline PCL/PLLA is obtained using PEO-selective n-hexanol for annealing. Surprisingly, the crystalline growth of the trilayered single crystal exhibits specific layer-by-layer epitaxial relationship. As a result, the multiple-crystalline textures of the PEO–PCL–PLLA thin films can be carried out by controlling solvent and polymer interaction.