Monomaterial Self-Reinforced PHB Copolymer Fibers via Core–Shell Electrospinning and Selective Thermal Coalescence

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Poly(3-hydroxybutyrate) (PHB) is a promising biodegradable polymer; however, its brittleness and narrow processing window limit broader application. Here, a self-reinforcement strategy based on PHB copolymers─poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH)─is developed to address these limitations. Core–shell electrospinning is used to fabricate bicomponent fibers with a PHBV core and a PHBH shell. The distinct melting temperatures of the two copolymers enable selective softening of the PHBH shell, while preserving the structural integrity of the PHBV core. Annealing induces interfiber coalescence and welding, forming interconnected fiber junctions. Consequently, the annealed coaxial electrospun fiber mats achieve an ultimate tensile strength (UTS) of 6.64 ± 0.24 MPa, which is not significantly different from that of neat PHBV fiber mats (6.41 ± 0.40 MPa; Welch’s two-sample t-test, p = 0.30), while exhibiting significantly higher toughness than neat PHBV fiber mats (125.33 ± 18 vs 45.18 ± 11 MJ m–3; Welch’s two-sample t-test, p < 0.001). DMA reveals that annealing increases the storage modulus from about 609 to 1300 MPa while reducing the loss modulus from 332 to 121 MPa, indicating enhanced structural integrity and reduced molecular mobility due to interfiber coalescence. Oxygen permeability also decreased from ∼3.8 ± 0.11×10–20 to ∼2.9 ± 0.10 × 10–20 m·Pa–1·s–1 following annealing, consistent with lower porosity and network densification. These findings demonstrate that a core–shell monomaterial design enables effective self-reinforcement without heterogeneous fillers, providing a design rationale for reduced formulation complexity relative to multicomponent systems containing dissimilar polymer chemistries or inorganic fillers.