Synthesis of Poly(butylene adipate-<i>co</i>-terephthalate) with Branched Monomer for Biodegradable Copolyesters with Enhanced Processability and Rheological Properties

Poly(butylene adipate-<i>co</i>-terephthalate) (PBAT) is one of the most promising biodegradable copolyesters for addressing "white pollution" and has made significant progress in industrial production, particularly in packaging and mulching film applications. However, melt elasticity and the rapid biodegradation rate of linear PBAT limit the broader application. In this study, a series of novel PBAT copolyesters with a low content of branch units (1.3-1.6%) were synthesized from adipic acid (AA), terephthalic acid (PTA), 1,4-butanediol (1,4-BDO), and branching monomers with fixed side chain length through esterification and polycondensation. A comprehensive investigation into the macromolecular structure of branched PBAT was conducted. A "Memoryless Random Model" combined with ¹H-NMR analysis was used to calculate the number-average sequence length, which proved the existence of random block sequences. Both linear and branched PBAT copolymers exhibited relatively high and similar molecular weights of 6.3 × 10⁴ Da and above. The conformational parameters <i>g</i> and <i>B</i> _3<i>w</i> were determined by gel permeation chromatography (GPC) coupled with triple detectors, thereby providing a clear definition of the branching characteristic. Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses revealed two crystalline forms in PBAT, with the β-form predominating in linear PBAT and a decrease in crystallinity in branched PBAT. The enzymatic hydrolysis experiment showed that the branched side chain reduced the hydrolysis rate, and the existence of consecutive sequences (≥3) was proved by LC-MS. The copolymers demonstrated higher complex viscosity and tanδ at the same frequency, indicating enhanced melt viscosity and elastic response. The deviation of the end slope in the Han curve also proved the existence of BA-BT random block sequence and complex relaxation behavior.