Polyethylene-Like Blends Amenable to Abiotic Hydrolytic Degradation

Abstract

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 (ε tb = 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 (M n (virgin blends) ≈ 50−70 kg mol −1 ; M n (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.

Closed-Loop Recyclable and Nonpersistent Polyethylene-like Polyesters

“…Even the addition of a hydrolyzing enzyme did not lead to significant cleavage of the inchain ester bonds. While this hydrolytic stability, which we also found for PE-18,18 in acidic and basic media, 46 can be beneficial for certain applications, it at the same time impedes biodegradability, desirable as a backstop for plastic accumulation in the environment. Abiotic hydrolytic degradation in the environment is expected to be slower than biodegradation under optimum conditions in general, but it can be less dependent on the specific microbial environment present.…”

“…We found that waxes based on the repeat unit motif of PE-2,18 (or 12,12) with an average M n of 10 3 to 10 4 g mol −1 can have a profile of properties relevant for applications that compares well to their petrochemical PE-wax counterparts. 60 For the aforementioned mixed-monomer chain-length polyesters, (PE-2,X ± Y) 1 degradation was also studied in an agricultural soil. Compared to the harsh conditions in industrial compost in terms of a particularly broad microbial consortium being present and the elevated temperature (58 °C), this represents milder conditions of biodegradation.…”

Synthesis and Deconstruction of Polyethylene-type Materials

“…Notwithstanding, their long-chain structure renders poly-H-phosphonates compatible with PE-18,18 in blends. Upon immersion of these HDPE-like blends in water, the minor hydrolytically labile component degrades completely over the course of months and the liberated phosphoric acid also initiates hydrolysis of the PE-18,18 matrix, resulting in embrittlement and significant molecular weight decrease …”

“…Upon immersion of these HDPE-like blends in water, the minor hydrolytically labile component degrades completely over the course of months and the liberated phosphoric acid also initiates hydrolysis of the PE-18,18 matrix, resulting in embrittlement and significant molecular weight decrease. 218 Photolytically cleavable breaking points are another approach toward nonpersistent polymers. Exposure of PElike keto-polyethylenes from catalytic and free-radical ethylene-CO copolymerization to (simulated) sunlight resulted in a clear onset of degradation, as evidenced by embrittlement, mass loss, and substantial decrease of molecular weight.…”

Controlling the Marine Biodegradation Profile and Mechanical Properties of Poly(ε-caprolactone) with Hydrophobic Water-Responsive Linkages

“…These trade-offs can be overcome by introducing stimuli-responsive linkages into the backbones of linear polymers or at the cross-linking points in polymer networks. − Once the material is exposed to certain stimuli after being discarded in nature, the linkages would quickly dissociate, and the polymer chains or networks would disintegrate into low-molecular-weight (MW) species that can be readily assimilated by microorganisms. To this end, the stimuli that trigger the dissociation of the linkages must be mild and ubiquitous in nature.…”