Advanced Mechanical Properties of Amphiphilic Polymer Conetworks through Hierarchical Reinforcement with Peptides and Cellulose Nanocrystals

Amphiphilic polymer conetworks (APCNs) have been explored for various applications, including soft contact lenses, biomaterials, and membranes. They combine important properties of hydrogels and elastomers, including elasticity, transparency, and the capability to swell in water. Moreover, they also swell in organic solvents. However, their mechanical properties could be improved. We developed a two-level, bio-inspired, hierarchical reinforcement of APCNs using cellulose nanocrystals (CNCs) to reinforce peptide-reinforced APCNs formed from hydrophobic poly-β-benzyl-l-aspartate-block-polydimethylsiloxane-block-poly-β-benzyl-l-aspartate (PBLA-b-PDMS-b-PBLA) triblock copolymer crosslinkers and hydrophilic poly(2-hydroxyethyl acrylate) (PHEA) chain segments. Bio-inspired peptide-polymer hybrids combine the structural hierarchy often found in natural materials with synthetic macromolecules, such as block copolymers with soft and hard segments, to enhance their mechanical properties. On the other hand, CNCs provide an additional means to dissipate mechanical energy in polymeric materials, thereby enhancing reinforcement. The key to homogeneously incorporating CNCs into the APCNs is the combination of hydrophobic CNCs (HCNCs) with peptide-blocks in the APCNs, exploiting the hydrogen bonding capability of the peptides to disperse the HCNCs. The effect of HCNCs on the ability of APCNs to swell in water and organic solvents, as well as on their thermal and mechanical properties, was characterized. Additionally, the nanostructure of the materials was analyzed via small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The swellability of the HCNC-containing APCNs was independent of the HCNC concentration, and all samples were highly transparent. The ideal HCNC concentration, in terms of maximal stress, strain, toughness, and reinforcement, was found to be between 6 and 15 wt%. An increase in Young's modulus of up to 500% and toughness of up to 200% was achieved. The hierarchical reinforcement also greatly strengthened the APCNs when swollen in water or n-hexane. Thus, HCNCs and peptide segments can be used to reinforce APCNs and to tailor their properties.