Hard segment-tuned polyurethane prepolymers boost interfacial compatibility of high-content starch/poly (butylene adipate-co-terephthalate) biodegradable composite films

To overcome the interfacial incompatibility in high-starch-content starch/ poly (butylene adipate-co-terephthalate) (PBAT) biodegradable composites, we designed four structure-defined polyurethane prepolymers (PUPs) as in-situ reactive compatibilizers. Specifically, four PUPs with uniform poly (butylene succinate) diol soft segments and structurally distinct diisocyanate (MDI, TDI, HDI, IPDI) hard segments were precisely synthesized, with well-matched molecular weights and isocyanate (NCO) contents to eliminate interference from non-structural variables. A systematic relationship between PUP hard-segment structure, interfacial organization, and composite performance was established.The PUPs were used to modify starch via internal mixing, followed by blending with PBAT at a fixed 55:45 (modified starch: PBAT) mass ratio, with composite sheets and films fabricated via flat-plate molding and extrusion blow-molding, respectively. Our results indicated that the TDI-based polyurethane prepolymer (TDIPUP)-modified starch/PBAT composite film exhibited the best interfacial compatibility, achieving an exceptional tensile strength of 10.05 MPa and elongation at break of 475.97% at 55% modified starch content. Moreover, the TDIPUP-compatibilized composite reached 88.8% biodegradation after 94 days of controlled composting. These results demonstrate that tailoring PUP hard-segment structure provides an effective strategy for producing high-starch-content PBAT films with balanced mechanical, barrier, processing, and biodegradation performance.