Cooperative and selective self-assembly behaviors of diblock copolypeptides in nanoscale thin films

Abstract

A series of diblock copolypeptides with various compositions (PBLG_m-b-PBCL_n) was synthesized through the living ring-opening polymerizations of γ-benzyl-L-glutamate and ε-(benzyloxycarbonyl)-L-lysine N-carboxyanhydrides with the aid of a nickel catalyst system. They were found to be stable up to around 150 °C and easily processable. Their chain conformations and morphologies in nanoscale thin films were characterized in detail by using infrared spectroscopy, atomic force microscopy, and in situ synchrotron grazing incidence X-ray scattering. In particular, quantitative X-ray scattering analysis was used to provide for the first time the morphological structures and orientation details of the diblock copolypeptides in thin films. Fibrils are present in the thin films of the copolypeptides; interestingly, the films are composed of two different rotationally isomeric hexagonally (HEX) packed cylinder structures that are preferentially oriented in the film plane. Further, the HEX structures consist of two substructural block units: one consisting of PBLG block chain cylinders and the other consisting of PBCL block chain cylinders. The block chains in the substructural units were found to interdigitate partially via the side groups. Thus the cylinders' interdigitation takes place selectively between block chains of the same kind rather than between different kinds of block chains. It was also confirmed that this high interdigitation selectivity occurs in the blend films of the homopolypeptides. These results show that in diblock copolypeptide films such selective interdigitation can override any thermodynamic penalties associated with the high chain rigidity due to the α-helical conformation and the effects of confinement in the connected diblock architecture, which leads to phase separation and the formation of well-defined, integrated HEX cylinder structures. These cooperatively and selectively formed HEX cylinder structures were found to be stable up to the degradation temperature. Molecular structure models are presented for the copolypeptide thin films as well as for the homopolypeptide blend films.