Secondary-Structure-Mediated Hierarchy and Mechanics in Polyurea-Peptide Hybrids

Peptide-polymer hybrids combine the hierarchy of biological species with synthetic concepts to achieve control over molecular design and material properties. By further incorporating covalent cross-links, the enhancement of molecular complexity is achieved, allowing for both a physical and covalent network. In this work, the structure and function of poly(ethylene glycol) (PEG)-network hybrids are tuned by varying peptide block length and overall peptide content. Here the impact of poly(ε-carbobenzyloxy-l-lysine) (PZLY) units on block interactions and mechanics is explored by probing secondary structure, PEG crystallinity, and hierarchical organization. The incorporation of PZLY reveals a mixture of α-helices and β-sheets at smaller repeat lengths ( n = 5) and selective α-helix formation at a higher peptide molecular weight ( n = 20). Secondary structure variations tailored the solid-state film hierarchy, whereby nanoscale fibers and microscale spherulites varied in size depending on the amount of α-helices and β-sheets. This long-range ordering influenced mechanical properties, resulting in a decrease in elongation-at-break (from 400 to 20%) with increasing spherulite diameter. Furthermore, the reduction in soft segment crystallinity with the addition of PZLY resulted in a decrease in moduli. It was determined that, by controlling PZLY content, a balance of physical associations and self-assembly is obtained, leading to tunable PEG crystallinity, spherulite formation, and mechanics.