Material Properties of Complex Synthetic Macromolecules Containing Secondary Structures

Proteins are key functional biomacromolecules whose unique properties are derived from the organization of secondary structures into three-dimensional tertiary structures. In this dissertation, helical linear polypeptides, polypeptide-grafted brush polymers and block copolypeptides designed to undergo an α → β transformation are studied to explore the influence of secondary structures on material properties. In solvent-cast poly(γ-benzyl-L-glutamate) (PBLG) films, depending on the casting solvent, α-helices assume distinct molecular packing configurations, which are independent of chain length, and chain-length dependent stacking of phenyl rings associated with distinct viscoelastic properties. Compression-molded PBLG films have less crystallinity and orientation compared to solution-derived films; however, they have a higher storage modulus, % elongation at break and strength at break. In solvent-cast and thermally processed films, storage modulus, % elongation, strength at break and Young’s modulus increase with increase in chain length. These observations could result from the entanglement of random coil residues present in interrupted helices. In molecular brushes with poly(γ-benzyl-L-glutamate) (PBLG) side chains and a poly(norbornene) (PN) backbone, the grafted PBLG is entirely in the α-helical conformation; however, the molecular brushes do not show appreciable molecular packing or orientation. The backbone lengths studied probably result in difference in conformation, with short backbones maintaining a spherical conformation and longer backbones maintaining a cylindrical conformation. The increase in DP of the backbone lowers the storage modulus profile probably due to a difference in conformation. Increase in DP of PBLG shifts the plateau modulus upwards, probably due to the interactions associated with stacking of phenyl rings, and increases the % elongation at break and strength at break, probably due to the entanglement of random coil residues present in interrupted helices. Block copolypeptides of poly(γ-benzyl-L-glutamate)-b-poly(γ-benzyl-L-tyrosine) block copolymers (PBLG-b-PBLT) undergo an α → β transformation when subjected to heat and mechanical stress. The α → β transformation is aided by attaching a poly(ethylene glycol) (PEG) segment to the PBLG block. Interestingly, the formation of β-sheets inhibits the folding of PEG. However, the structural change is precluded by a triblock architecture of the form PBLG-b-PBLT-b-PBLG. The structural change generates materials with higher modulus and the capacity to withstand high temperatures.