Covalent-adaptable networks enabled by epoxy-functionalized citrate plasticizers for enhanced 3D printing of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) bioplastics.

The advancement of high-performance poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) bioplastics is limited by their inherent brittleness and narrow processing window. In this study, a novel class of reactive citrate-based plasticizers was developed by combining short-chain butyl esters with long-chain polyethylene glycol diglycidyl ether (PEGDE). Three derivatives-TE1B2, TE2B1, and TE3-were synthesized, with TE3 containing three epoxy groups for covalent network formation. Comprehensive characterization showed that TE3 significantly outperforms commercial tributyl citrate (TBC) in thermal stability (T = 275.3 °C vs. 179.2 °C), migration resistance (<1 % mass loss in petroleum ether), and mechanical performance (elongation at break, ε = 51.2 %; tensile strength, σ = 15.8 MPa). The PEGDE segments of TE3 enable (1) covalent crosslinking with P34HB terminals via epoxy-carboxyl and epoxy-hydroxyl reactions and (2) topological entanglement maintaining the storage modulus of neat P34HB. This molecular architecture suppressed spherulite formation and established a stress-dissipating network, as evidenced by SEM. Rheological analysis further revealed TE3's shear-thinning behavior with high-frequency elasticity retention, enabling reliable fused deposition modeling (FDM). By overcoming the traditional strength-ductility trade-off, this work introduces a synergistic strategy combining chemical crosslinking and physical entanglement, offering new insights into covalently anchored plasticization and network dynamics in biopolymer design.