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 d- 10%  = 275.3 °C vs. 179.2 °C), migration resistance (<1 % mass loss in petroleum ether), and mechanical performance (elongation at break, ε b  = 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.