Polyethylene-Like Blends Amenable to Abiotic Hydrolytic Degradation

Abstract

Long-chain aliphatic polyester-18,18 (PE-18,18) exhibits high density polyethylene-like material properties and, as opposed to high density polyethylene (HDPE), can be recycled in a closed loop via depolymerization to monomers under mild conditions. Despite the in-chain ester groups, its high crystallinity and hydrophobicity render PE-18,18 stable toward hydrolysis even under acidic conditions for one year. Hydrolytic degradability, however, can be a desirable material property as it can serve as a universal backstop to plastic accumulation in the environment. We present an approach to render PE-18,18 hydrolytically degradable by melt blending with long-chain aliphatic poly(H-phosphonate)s (PP). The blends can be processed via common injection molding and 3D printing and exhibit HDPE-like tensile properties, namely, high stiffness (E = 750–940 MPa) and ductility (εtb = 330–460%) over a wide range of blend ratios (0.5–20 wt % PP content). Likewise, the orthorhombic solid-state structure and crystallinity (χ ≈ 70%) of the blends are similar to HDPE. Under aqueous conditions in phosphate-buffered media at 25 °C, the blends’ PP component is hydrolyzed completely to the underlying long-chain diol and phosphorous acid within four months, as evidenced by NMR analyses. Concomitant, the PE-18,18 major blend component is partially hydrolyzed, while neat PE-18,18 is inert under identical conditions. The hydrolysis of the blend components proceeded throughout the bulk of the specimens as confirmed by gel permeation chromatography (GPC) measurements. The significant molar mass reduction upon extended immersion in water (Mn(virgin blends) ≈ 50–70 kg mol–1; Mn(hydrolyzed blends) ≈ 7–11 kg mol–1) resulted in embrittlement and fragmentation of the injection molded specimens. This increases the surface area and is anticipated to promote eventual mineralization by abiotic and biotic pathways of these HDPE-like polyesters in the environment.