Biobased poly(ester amide)s surpass polyolefins in tensile strength

Researchers in Japan have developed biobased poly(ester amide)s derived from non-edible renewable resources that exhibit superior mechanical properties compared to commodity plastics. The new materials, which can be easily chemically recycled, surpass conventional petroleum-based polymers such as polyethylene and polypropylene in both tensile strength and elongation at break in film form.

The development was led by Professor Kotohiro Nomura of Tokyo Metropolitan University. He worked in cooperation with the research group of Director Hiroshi Hirano and Senior Researcher Seiji Higashi at the Osaka Research Institute of Industrial Science and Technology, as well as Associate Professor Hiroki Takeshita from The University of Shiga Prefecture.

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Overcoming the molecular weight barrier

Creating high-performance sustainable plastics is a core requirement for a circular economy, but biobased alternatives have historically struggled to match the mechanical requirements of petroleum-derived plastics. According to Eurekalert, conventional polycondensation methods had long faced a pending issue: the difficulty of synthesizing high molecular weight, long-chain polymers.

To bypass this, the research team utilized an olefin metathesis polymerization method involving a high-performance molybdenum catalyst. This specific process is a polycondensation that synthesizes polymers while producing ethylene as a by-product, a method known as acyclic diene metathesis polymerization. In this context, olefin refers to hydrocarbons with one carbon-carbon double bond, and metathesis refers to the substitution or recombination of substituents on that double bond.

The resulting polymers are synthesized from inedible vegetable oils, amino acids, and sugars, including glucose. Miragenews reports that the poly(ester amide) containing phenylalanine also displays unique fast self-healing properties at ambient temperature.

Defying polymer physics

The new material challenges standard behavioral patterns in polymer science. Typically, there is an antinomic or inverse relationship between tensile strength and elongation at break in polymer films; as one increases, the other tends to decrease. Furthermore, increasing the molecular weight usually leads to a decrease in elongation at break.

However, the polymer films developed by Nomura's team demonstrate that both tensile strength and elongation at break increase alongside the molecular weight. This allows the material to exceed the performance of common plastics including polystyrene (PS), poly(lactic acid) (PLA), high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP), according to Eurekalert.

Chemical recyclability and biodegradability

Unlike many high-performance polymers that are difficult to break down, these biobased polyesters are designed for chemical recyclability. They can be converted back into their starting organic monomers through transesterification, a catalytic reaction with alcohol that involves quantitative bond formation and dissociation, according to Miragenews.

The broader context of bio-based materials highlights a recurring tension between performance and decay. A review published by PMC notes that while there is an intrinsic expectation that polymers made from biorenewable feedstock are biodegradable, there is no guarantee. Changes in crosslink density or copolymerization can result in materials that do not exhibit significant biodegradability.

The PMC report further distinguishes between thermoplastic polymers, which can melt and flow when heated, and thermosetting polymers, which are interconnected through covalent bonds. Thermosetting polymers often have superior mechanical properties but are physically harder to degrade and require harsher conditions for recycling. This is because they do not melt when heated and cannot be dissolved in a solvent.

Material Comparison and Future Outlook

Property	Conventional Polyolefins (PE/PP)	New Biobased Poly(ester amide)s
Source Material	Petroleum	Inedible plant resources (oils, sugars, amino acids)
Tensile Strength	Standard baseline	Superior/Beyond commodity plastics
Elongation at Break	Inversely related to strength	Increases with molecular weight
Recyclability	Variable/Mechanical	Chemical recyclability via transesterification

The research was conducted as part of the JST CREST program under the research area Precise Material Science for Degradation and Stability and the research theme Development of Bio-Based Advanced Polymers and their Depolymerization, Chemical Recycle.

The researchers believe this development is a breakthrough for the circular economy. They indicate that the properties of these films can be further improved through the combination of the polymer with naturally derived fibers, such as cellulose nanofibers.