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

Amphiphilic polymer conetworks (APCNs) have been explored for applications, including soft contact lenses, biomaterials, and membranes. They combine several important properties of hydrogels and elastomers, including elasticity, transparency, and 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 peptide-containing triblock copolymers and cellulose nanocrystals (CNCs). Bio-inspired peptide-polymer hybrids combine the structural hierarchy often found in natural materials with synthetic macromolecules, e.g. having soft and hard segments like in multiblock copolymers, to strengthen the mechanical properties. On the other hand, CNCs provide an additional way to dissipate mechanical energy in polymeric materials and, therefore, for reinforcement. The key to achieving homogeneous incorporation of CNCs into the APCNs is the combination of hydrophobic CNCs (HCNCs) with peptide-modified APCNs, exploiting the hydrogen bonding present in the peptides to disperse the HCNCs. The effect of HCNCs on the ability of APCNs to swell in water and organic solvents and on the 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 CNC-containing APCNs exhibited similar swellability independent of the CNC concentration, and all the samples were highly transparent. The ideal CNC concentration, in terms of maximal stress, strain, toughness, and reinforcement was found to be between 6 and 15 wt%, and an increase of Young´s modulus by up to 500% and of toughness of up to 200% was achieved. The hierarchical reinforcement also greatly strengthened the APCNs when swollen in water and n-hexane. Thus, CNCs and peptide segments can be used to reinforce APCNs and to tailor their properties.