Unlocking the thermoplasticity of cellulose via amphiphilic ionic liquid-mediated silk fibroin plasticization

Abstract

Cellulose (CEL) is intrinsically non-thermoplastic, preventing its melt processing into bioplastics. Here, we impart thermoplasticity to CEL by molecular-level integration with silk fibroin (SF) as a macromolecular plasticizer. This is enabled by an amphiphilic ionic liquid solvent with balanced hydrophilicity and lipophilicity that co-dissolves CEL and SF by disrupting hydrogen-bonding networks and hydrophobic interactions. Molecular dynamics simulations and experiments show that dissolution proceeds via chloride anions breaking hydrogen bonds and long alkyl chains disrupting hydrophobic domains. During water-induced regeneration, the ionic liquid impedes the inherent hydrophobically driven crystallization of both CEL and SF, directing the formation of a hydrogen-bond-dominated amorphous co-network. This amorphous structure provides chain mobility, creating a thermal processing window for the two biopolymers. Hot-pressing produces transparent, flexible, and robust free-standing films. The optimal CEL/SF (1 : 1) composite has a tensile strength of up to 55 MPa and biodegrades completely within 42 days. This work provides a sustainable approach to convert natural biopolymers into processable bioplastics, addressing the thermal processability challenge of cellulose.