Supramolecular structures from self-assembled poly(γ-benzyl-l-glutamate)-polydimethylsiloxane-poly(γ-benzyl-l-glutamate) triblock copolypeptides in thin films

We describe the self-assembly of A-B-A triblock copolymers in thin films composed of a soft polydimethylsiloxane (PDMS) central block (B) and two polypeptidic (A) blocks, poly(γ-benzyl)-l-glutamate (PBLG). The PBLG segment exhibits depending on the chain length two distinct secondary conformations either a β-sheet or a α-helical conformation. The direct relationship between the surface morphology and the secondary conformation of the polypeptide segment has been evidenced by atomic force microscopy. For chain lengths below 20 U the polypeptide segments adopt preferentially a β-sheet secondary structure and the triblock copolymer self-assembled in fibers. Moreover, the fiber diameters increased with the chain length of the triblock copolymer. For chain lengths above 20, the α-helical structure is stabilized and a lamellar morphology is formed driven by rod-rod interactions in spite of the very asymmetric composition of the triblock copolymer. However, decreasing the film thickness from 25 to 8 nm, i.e., below the L/2 and due to the preferential attraction of the polypeptide block for the hydrophilic substrate employed, instead of a lamellar morphology a rod-like morphology could be found. Thus, the use of hybrid block copolymer containing polypeptides with particular secondary structures offers novel alternatives to control the self-assembly in thin films compared to traditional amorphous block copolymers.