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To address the growing concern of accumulated non-degradable plastic waste in the oceans, a research group at Cornell University, USA, has developed a novel high-strength synthetic polymer called isotactic polypropylene oxide (iPPO).
The new thermoplastic material possesses mechanical properties similar to traditional plastics, making it suitable for commercial applications while being safer for the environment due to its degradation on a realistic time scale when exposed to sunlight.
Despite the large-scale production of synthetic polymers only beginning in the middle of the 20th century, the extraordinary growth in the demand for plastics and their varied applications currently results in the annual global production of several hundred millions of tons.
Industrial Fishing Contributes to the Plastic Waste in the Oceans
A growing environmental concern is non-degradable plastics that accumulate in the oceans, as these materials are often less dense than water and tend to remain in the aquatic systems indefinitely.
Current estimates show that up to 52% of the total plastic waste mass floating in the oceans is composed of lost or discarded plastic fishing nets, ropes, and lines. These waste materials can cause significant damage to aquatic life by trapping and entanglement of marine animals, as well as by being swallowed after the plastic is mistaken for food.
Fishing nets and ropes are commonly made of synthetic polymeric fibers such as isotactic polypropylene (iPP), high-density polyethylene (HDPE), polyethylene terephthalate (PET) or polyamide (PA), materials that are non-biodegradable and typically neutrally buoyant.
Plastics Engineered for Durability Do Not Degrade Easily
Recent efforts to develop degradable engineering plastics showed some promise for ultraviolet (UV) photodegradation of iPP and HDPE. However, the low reactivity of their hydrocarbon backbone chains significantly limits the rate of degradation.
The biodegradable plastics developed and used in food packaging lack the mechanical strength required for more demanding industrial applications.
Scientists from the research group led by Prof. Geoff Coates at the Department of Chemistry and Chemical Biology at Cornell University have taken advantage of a decade-long research experience in the field of synthetic polymer chemistry.
They have developed a very precise and selective polymerization process where the end product is highly-uniform macromolecules of isotactic polypropylene oxide (iPPO), a synthetic polymer structurally similar to iPP.
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Well-Ordered Polymer Chains Increase Mechanical Strength
The ability to control the tacticity of the polymer chains (or the stereochemical arrangement of the side groups attached to the backbone chains) gave the researchers the tools to enhance the mechanical properties of the new polymer.
Highly isotactic polymer chains (with the same orientation of the side groups) can pack together much easier and form crystals and fibers. Higher crystallinity of the material leads to increased mechanical strength.
Direct comparison of the mechanical properties of iPPO and the most widely used synthetic high-strength fibers such as iPP, HDPE and Nylon-6,6 (a commercial PA fiber) revealed that iPPO’s ultimate tensile strength (UTS, the maximum amount of stress a material can sustain before rupturing) was in the range 48 to 75 MPa (depending on the molecular weight of the polymer), which surpassed the UTS of commercial iPP and HDPE fibers. The closest match was Nylon-6,6 where the UTS differed only by a few MPa.
The immense strength of iPPO is enhanced by its strain hardening properties - a process in which the crystalline order in the material under stress is further improved by better alignment of the polymer chains.
UV Photooxidation Can Break Down iPPO Molecules
The presence of ether groups (an oxygen atom connected to two alkyl or aryl groups) that are susceptible to photochemical degradation in the iPPO structure, however, is what makes the new material so interesting from an environmental point of view.
Prof. Coates' team examined the degradation of the polymer when subjected to illumination with UVA light (wavelength of 365 nm) under ambient conditions for 30 days.
The results clearly showed a gradual decrease in the average molecular weight (or the average length of the polymer chains) of the polymer from 93 kDa to 21 kDa due to the fragmentation of the polymer chains, while the same material in the absence of UVA retained its molecular weight unchanged.
Photo- and Biodegradable Plastics with Shorter Persistence in the Aquatic Environment
Although photodegradation can break down the polymer chains, it cannot solve the end-of-life problem for the industrial plastics. This fragmentation can lead to the formation of microplastics (micron-sized plastic particles) that persist in the aquatic environment.
However, the researchers believe that the smaller molecular weight fragments of iPPO can be subjected to additional biodegradation processes, similarly to low molecular weight atactic polypropylene oxide that can be metabolized by microorganisms under aerobic conditions.
The team’s ultimate goal is to create industrial-grade plastic that could eventually degrade without leaving a trace in the oceans.
References and Further Reading
B. M. Lipinski et al., (2020) Isotactic Poly(propylene oxide): A Photodegradable Polymer with Strain Hardening Properties. J. Am. Chem. Soc. 142, 6800-6806. Available at: https://dx.doi.org/10.1021/jacs.0c01768
R. Geyer et al., (2017) Production, use, and fate of all plastics ever made. Science Advances, 3, e1700782. Available at: https://dx.doi.org/10.1126/sciadv.1700782
B. Gewert et al., (2015) Pathways for degradation of plastic polymers floating in the marine environment. Environ. Sci.: Processes Impacts, 17, 1513-1521. Available at: https://dx.doi.org/10.1039/c5em00207a
B. Friedlander (2020) Chemists create faster-degrading plastic for marine uses. [Online] www.phys.org Available at: https://phys.org/news/2020-04-chemists-faster-degrading-plastic-marine.html (Accessed on 22 May 2020).
Polymer that degrades under ultraviolet light could solve ocean plastic pollution. [Online] www.eandt.theiet.org Available at: https://eandt.theiet.org/content/articles/2020/04/polymer-that-degrades-under-ultraviolet-light-could-solve-ocean-plastic-pollution (Accessed on 22 May 2020).
Faster-degrading plastic could promise cleaner seas. [Online] www.sciencedaily.com Available at: https://www.sciencedaily.com/releases/2020/04/200420145031.htm (Accessed on 22 May 2020).