Intrinsic Interfacial Adhesion Enables Compatibilizer-Free Reinforcement of an Immiscible HDPE/Biobased Copolyester Blend

Blends of high-density polyethylene (HDPE) with an isosorbide-containing biobased copolyester (PEICT) were prepared by melt compounding and injection molding, and the role of the intrinsic HDPE/PEICT interface in determining the blend properties was investigated. Although HDPE and PEICT are immiscible and form a phase-separated droplet morphology, interfacial thermodynamic analysis suggested favorable interfacial affinity. The aliphatic cyclohexylene and isosorbide units of PEICT may contribute to the high dispersive surface energy, favoring dispersive interaction with nonpolar HDPE and a solid-state work of adhesion of 82.63 mN/m. The results showed that the Young’s modulus of the noncompatibilized blends exceeded the Halpin–Tsai prediction for spherical inclusions by up to 10.6%, and the estimated heat-deflection temperature increased by up to 13.2%, while X-ray diffraction confirmed that the HDPE crystalline structure was preserved. A Pukánszky analysis of the tensile strength yielded an interaction parameter of B ≈ 3.7 for the noncompatibilized blends, with similar values after compatibilizer addition. The maleic-anhydride-grafted polyethylene compatibilizer reduced the tensile modulus, while its effects on flexural properties and estimated heat-deflection temperature depended on composition. These results show that HDPE/PEICT blends can achieve improved stiffness and estimated heat resistance without compatibilizer addition under the conditions studied.