Synergistic Enhancement of Melt Strength and Elastic Recovery in Poly(butylene adipate- <i>co</i> -terephthalate) Foams via Transesterified Vitrimer-Derived Branching

High Resolution Image Download MS PowerPoint Slide Poly(butylene adipate- co -terephthalate) (PBAT) was synthesized via a one-pot method using dimethyl terephthalate (DMT), adipic acid (AA), and 1,4-butanediol (BDO), followed by the addition of a transesterified vitrimer-derived segment solution during the polycondensation stage to obtain PBAT copolymers containing transesterified vitrimer-derived segments. Nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and intrinsic viscosity measurements confirmed the successful incorporation of vitrimer-derived segments. Thermogravimetric analysis (TGA) showed that all samples exhibited a temperature at 5% weight loss ( T d–5% ) of around 360 °C, indicating good thermal stability. Differential scanning calorimetry (DSC) revealed that increasing vitrimer-derived segment content progressively decreased the crystallization temperature ( T c ), crystallization enthalpy (Δ H c ), and melting enthalpy (Δ H m ), indicating suppressed crystallization due to increased branching and chain entanglement. Dynamic mechanical analysis (DMA) showed a decreased glass transition temperature ( T g ) due to disrupted chain packing and increased free volume, while β-relaxation analysis confirmed restricted side-chain mobility at higher vitrimer-derived segment contents. Rheological tests showed enhanced storage modulus ( G ′), loss modulus ( G ″), and complex viscosity (η*), with no evidence of a fully cross-linked network, indicating that vitrimer-derived segments increased melt viscoelasticity through branched and entangled chain structures. Supercritical carbon dioxide (CO 2 ) foaming experiments demonstrated improved foamability after incorporation of transesterified vitrimer-derived segments. Among all compositions, the PBAT+0.5 wt % sample exhibited the highest expansion ratio, increasing from 8.075 for neat PBAT to approximately 13.8, whereas the foam density progressively decreased and reached a minimum of 0.076 g/cm 3 for the PBAT+1.0 wt % sample. Scanning electron microscopy (SEM) images confirmed enhanced melt strength and stable foam-cell structures due to branched entanglements. Notably, the PBAT sample containing 0.5 wt % vitrimer-derived segments showed the most favorable compression–recovery behavior, achieving an optimal balance between melt strength and crystallinity.