News

1. Biodegradable plastics: from product to end-of-life

   Korean researchers investigate degradation of commercial materials in soil and seawater

   Global plastic production is at an all-time high. In 2019, we produced 460 million tonnes of it – that’s close to the weight of a thousand Burj Khalifas (the world’s tallest building). And as plastic production has increased, so too has waste. The UN predicts that by 2040, around 30 million tonnes of plastic waste will have entered Earth’s marine ecosystems. In the past year alone, microplastics have been reported in the air above cities, in remote regions of Antarctica , in nearly all American food proteins, and on the deep ocean floor. The situation is dire, and so the hunt for alternative materials is on. Biodegradable plastics have been proposed as one such solution to the rapidly worsening problem of plastic pollution. Currently a small percentage of the total plastics market (1.14 million tons were produced in 2022), these materials look set to grow in popularity. One of the bottlenecks to their widespread adoption is an as-yet-incomplete understanding of how these plastics degrade in all environments.

   In a new paper published in the latest issue of Polymer Testing [DOI: 10.1016/j.polymertesting.2024.108338], Korean scientists report on the performance of three classes of biodegrade plastics in soil and seawater conditions. Their materials of choice were polycaprolactone (PCL), poly(butylene succinate) (PBS), and poly(butylene adipate-co-terephthalate) (PBAT) which were all synthesised using lab-scale systems. The synthesized polymers were then used to fabricate films that were cut into a range of sizes and shapes suited to each test.

   For soil degradation experiments, films of each polymer were placed in either horticultural topsoil mixed with water, or fertilized soil (a mixture of topsoil, vermicompost, and water). Every three weeks for six months, the samples were temporarily removed from the soil, cleaned and dried and their weight measured. To evaluate any changes in molecular weight of the polymers over time, powered samples of each polymer were added to both soils. At three-week intervals, 3g of the soil containing the powder was collected and dissolved in chloroform to extract the biodegradable plastic.

   The PCL and PBS films showed little-to-no visible changes in horticultural soil. In contrast, in fertilized soil, PCL exhibited substantial degradation and weight loss, with “large, highly curved holes” forming on its surface within the first three months; PBS exhibited extensive surface cracking, and later, some weight loss, while the sample itself deformed and bent. PBAT did not show any major changes in either soil environment. In addition, the authors found that of the molecular weight of biodegradable plastics showed no significant decrease after 6 months, regardless of soil type. In terms of microbial growth, the higher nutrient content of the fertilised soil was seen to provide “a more favorable environment.”

   To investigate degradability in seawater conditions, samples of PCL – which exhibited the fastest rate of decomposition in soil – were placed in either a coarse or fine fishing net before being submerged in an aquarium that was filled with warm seawater and inhabited by marine fish. Every three weeks for one year, the researchers temporarily removed each sample and dried it, weighing it to monitor any losses. They found that PCL samples submerged within coarse nets exhibited a considerably faster rate of decomposition than those submerged in fine nets. In addition, there was a greater abundance of microbes on the surface of the PCL in the coarse nets. The authors attribute this to an increased degree of aeration and circulation of seawater, facilitated by the coarse nets. Vibro species represented close to 95% of all microbes present on the samples, regardless of whether coarse or fine nets were used.

   All three biodegradable plastics were subjected to a real-world marine test. Dog-bone-shaped samples of each were prepared and placed inside a coarse net and then enclosed in a larger fishing net and submerged in coastal seas off South Korea. Every two months for a period of one year, samples were temporarily retrieved from the fishing net, washed, dried and weighed. Tensile tests were also carried out after each retrieval. The researchers found that the PCL films decomposed 2 – 2.5 times faster in the marine environment than in the aquarium, and that marine samples also exhibited a wider variety of microorganisms. The PBAT sample exhibited the slowest rate of biodegradation among the tested specimens. The authors attribute this to the chemical structures of the plastics.

   Finally, the authors 3D printed fishing jars – typical of those used in Cephalopod-farming – from PCL. The goal was to investigate whether biodegradable plastics might be suitable for use in the fishery industry. They write, “The mechanical properties of the samples were maintained for several months before significant weakening occurred, which can prevent ghost fishing [where marine species are killed by waste nets and plastic] supporting the potential application of biodegradable plastics as replacements for nondegradable materials in fishing gear.”

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   Junhyeok Lee, Semin Kim, Sung Bae Park, Mira Shin, Soyoun Kim, Min-Sun Kim, Giyoung Shin, Taewook Kang, Hyo Jeong Kim, Dongyeop X. Oh, Jeyoung Park. “Mimicking real-field degradation of biodegradable plastics in soil and marine environments: From product utility to end-of-life analysis,” Polymer Testing 131 (2024) 108338. DOI: 10.1016/j.polymertesting.2024.108338

2. MAXimising the lubricity of shape-memory polymers

   Addition of titanium carbide-based materials significantly reduces friction and wear

   Anywhere there are surfaces in relative motion, there will be friction and wear to contend with. And they come at an astonishing cost. Almost a quarter (23%) of all energy consumed worldwide (119 EJ) originates from tribological contacts. The vast majority (20%) goes towards overcoming friction, with the remaining 3% used to remanufacture components lost due to wear and wear-related failures. Finding ways to control and reduce friction is therefore a global priority. Complex lubricant formulations, coupled with direct surface modification techniques (e.g. coatings and plasma treatment) have been hugely successful in many applications. There is also a growing interest in using composite materials that have been specifically designed to have optimised tribological properties.

   A paper from scientists at India’s CSIR-AMPRI has added some new knowledge to this effort. Writing in the latest issue of Carbon [DOI: 10.1016/j.carbon.2024.118790], they report on a polymer composite that displays low friction, reduced wear, and damage-healing capability. Their composite uses a shape-memory polyurethane (SMPU) as the model polymer matrix and titanium carbide-based MAX and MXene materials as fillers.

   MAX materials are functional ceramics – typically based on ternary carbide or nitride – with a layered structure. They exhibit high mechanical strength and favourable electrical and thermal conductivities. MXenes, which are 2D materials exfoliated from the MAX phase, retain many of those same properties. In addition, they are hydrophilic, which allows them to form strong bonds with a range of matrix materials, and they have low shear strength. These characteristics suggest that, as fillers, these materials could improve the wear resistance and frictional performance of composites.

   For this study, the team chose two MAX phases – Ti₂AlC (MAX₁), and Ti₃AlC₂ (MAX₂) – and one MXene, Ti₃C₂, and employed filler concentrations of 0.25, 0.5, 1.0, and 2.0 wt%. A sample of pristine SMPU was also produced, and acted as a reference.

   To evaluate the sliding friction and wear properties of the composites, the researchers employed a ball-on-disk tribometer. In these tests, the friction coefficient of the SMPU was seen to be initially similar to that of the SMPU-MAX₁, SMPU-MAX₂, and SMPU-MXene composites. However, beyond ~700 cycles, the friction coefficient of the SMPU increased considerably, reaching an average of 0.35. The authors credit this to the onset of frictional heating in the polymer, which led to deformation and increased adhesion forces. In contrast, the average friction coefficient of all composites stayed below 0.13, at all filler loadings. In fact, the percentage loading had no significant impact – similar friction reduction was seen for 0.25 wt% and 2.0 wt% composites.

   Images of each sample and the ball probes were collected following the tribometer tests. They conformed the presence of a large wear track on the pristine SMPU samples, and significant transfer of material onto the ball. The wear tracks seen on the composite samples were both narrower and shallower, with substantially less debris transfer to the balls. The authors then used 3D optical surface profilometry to estimate the wear rate of each sample. The addition of fillers significantly enhanced the wear resistance (compared to pristine SMPU). For SMPU-MAX₁ (0.25 wt%), the enhancement was ~50 times. For SMPU-MAX₂ (0.25 wt%), it was ~100 times, and for SMPU-MXene (0.25 wt%), it was 500 times. From this, they concluded that “…as a filler material for PU, the Ti₃C₂ MXene phase material is superior to MAX phase material for tribological applications.” Elemental analysis of the wear tracks also revealed traces of constituent elements of MAX/MXene and SMPU (e.g. Ti, Al, C, and O) suggesting that these elements likely contributed to the low sliding friction they’d measured.

    

   Finally, the authors tested the self-healing capabilities of the samples through the use of a universal tensile machine to intentionally dent the sample surface. Pristine SMPU contains an abundance of reversible physical crosslinks and thermo-reversible bonds. When an external stimulus (e.g. heat energy) is applied to a damaged piece of SMPU, these polymer chains rapidly diffuse, allowing it to re-form or ‘heal’. Their analysis showed that all three of the composites retained this ability. They conclude, “The results suggest that by using these composites, not only the friction and wear but also the frequent replacement of sliding components can be minimized, which is crucial for cost-saving and environmentally sustainable technologies.”

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   Shubham Jaiswal, Jeet Vishwakarma, Shubham Bhatt, Reuben J. Yeo, Rahul Mishra, Chetna Dhand, Neeraj Dwivedi. “Enhancing the lubricity and wear resistance of shape-memory-polymer via titanium carbide-based MAX and MXene,” Carbon 219 (2024) 118790. DOI: 10.1016/j.carbon.2024.118790

    

3. Robert Cahn Award Nomination Deadline 1 March 2024

   Robert Cahn Award

   The purpose of the award is to recognise an outstanding scientist who has:

   • a high scientific profile in the field of nuclear materials
   • the ability to communicate science to a broad audience
   • demonstrated interest in breaking down barriers between different scientific disciplines

    

   The winner of the 2024 award will be invited to give a Plenary lecture at the next NuMat Conference in Singapore, 14–17 October 2024.

    

   The nomination should contain the name of your nominee, his/her CV, and a summary of his/her scientific merit and research impact. The award is open to anyone in the field, although unfortunately self-nominations cannot be accepted.

    

   Please send nominations to the Publisher of the journal, Rachel Garland ().

    

   The deadline for nominations is 1 March 2024 at midnight BST, and notification of the award winner will appear on the websites of both the journal, Journal of Nuclear Materials and the conference, NuMat 2024. The award will be presented at the NuMat 2024 Conference, Singapore, 14–17 October 2024.

    

   Deadline for nominations: 1 March 2024 at midnight BST

    

   

4. How does graphene self-fold?

   Experimental study proposes mechanism for crumpled graphene formation and growth

   Plasma synthesis of free-standing, few-layer graphene (FLG) was first reported in 2007. Since then, the process has undergone further development and refinement, which has led to overall improvements in yield. However, there has been a lack of experimental studies into the specifics of FLG synthesis – namely, the underlying details of FLG formation and structural growth. Numerous theoretical and computational investigations have proposed a ‘stacking’ mechanism for graphene sheets, but this has yet to be confirmed experimentally. In addition, no experimental studies have explained the ever-present crumpling observed in free-standing FLG structures.

   A group of researchers from the University of Duisburg-Essen in Germany set out to fill that information gap. Writing in the latest issue of Carbon [DOI: 10.1016/j.carbon.2023.118732], they report on a conceptual model they have developed, via experiment, to accurately describe what happens during the initial steps of FLG formation.

   Their experimental setup – first described in a paper published in December 2023 – consisted of a microwave-plasma reactor with a gas inlet, a thermophoretic sampling device, and a fibre filter for product harvesting. The reactant vapour was ethanol, which is known to produce high-quality crumpled FLG. A mix of argon and hydrogen was used as the sheath gas. Ethanol vapor was injected into the centre of the microwave plasma zone at a fixed flow rate. Downstream of that, hot gases carrying the atomic and molecular decomposition products cooled, forming FLG via nucleation and growth. In this system, FLG formation began at <12 cm from the plasma nozzle.

   The synthesized particles were harvested from the post-plasma flow via thermophoretic sampling. This involved rapidly inserting a transmission electron microscopy (TEM) grid to specific positions in the reactor; in this case, at 12.4, 14.4, 19.4, and 24.4 cm from the plasma nozzle. The exposure period at each position was ~10 ms. Each grid was inserted at the same position 20 times to increase the amount of collected material. To ensure that the TEM grid material had no influence on the morphology of the FLG, samples were collected on both lacey carbon TEM grids and closed carbon-film-supported copper grids. This approach allowed the researchers to get ‘snapshots’ of the graphene formation process.

   TEM analysis of these samples revealed a progressive pattern of morphology evolution for single- and few-layer graphene. Firstly, carbonaceous species nucleate and grow to form rounded, single-layer graphene sheets. These sheets continue to grow in stage two, while retaining their shape. In the absence of any interactions with a substrate, and beyond a sheet diameter of ≤370 nm, each laterally-growing graphene sheet becomes unstable, and begins to change shape. The team found that this change follows one of two pathways – the graphene sheet starts to crumple randomly, or the graphene self-folds as an envelope and/or it curls onto itself. Their results suggest that at different nozzle distances, curled, self-folded, and crumpled flake structures co-exist.

   An additional finding was that when an FLG flake folds ‘neatly’, it forms sharp edges, at which no further graphene forms. Evidence of two consecutive self-folding events was also seen, and it led to the creation of ~90° flake edges. However, reactive positions (i.e., dangling bonds or hydrogen bonds) form away from those terminated edges, which is what allows growth to continue. It’s this growth that leads to the final form of arbitrarily crumpled FLG.

   The authors write that, “All the observations from this work align with the most recent literature published using MD [molecular dynamics] simulation approaches, yet showing for the first time the FLG evolution using an experimental approach.

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   Claudia-F. López-Cámara, Paolo Fortugno, Markus Heidelmann, Hartmut Wiggers, Christof Schulz. “Graphene self-folding: Evolution of free-standing few-layer graphene in plasma synthesis,” Carbon 218 (2024) 118732. DOI: 10.1016/j.carbon.2023.118732

5. Novel nanolubricant reduces friction and wear at elevated temperatures

   UK researchers say this opens the door for more environmentally-friendly lubricants

   Tribology (the study of friction and wear) might not be the first thing that comes to mind when considering the energy crisis but close to a quarter (23%, or 119 EJ) of the world’s total energy consumption originates from tribological contacts. 20% (103 EJ) goes towards overcoming friction, and the remaining 3% (16 EJ) accounts for the replacement of components that have failed due to wear. The main route to reducing this energy loss is through the design of effective, efficient lubricants that can form a protective barrier between rubbing surfaces.

   Writing in the latest issue of Carbon [DOI: 10.1016/j.carbon.2023.118742] researchers from the University of Leeds report on a novel lubricant formulation that incorporates nanodiamonds which, they say, significantly reduces the coefficient of friction and improves the wear performance of steel plates at industrially-relevant temperatures.

   They started with fairly typical constituents – a polyalphaolefin (PAO) base oil, which is the most common synthetic base oil employed in industrial and automotive lubricants, and glycerol monooleate (GMO); an extensively-used organic friction modifier. The primary additive was zinc dialkyl dithiophosphate (ZDDP). While ZDDP has long been utilised as a lubricant additive in car engines due to its impressive anti-friction and anti-wear performance, in high concentrations it can have a negative impact on the fuel combustion catalysts that reduce emissions. The researchers chose to incorporate a maximum of 0.2 wt% ZDDP in their test formulations. As a secondary additive, they used nanodiamonds (NDs). These ultrafine carbon-based particles are known to have outstanding thermal and chemical stability, and are extremely hard, imparting excellent wear resistance to surfaces. NDs were added at 0.05 wt% to four of the nine test formulations.

   The tem used a range of analytical techniques, including high-resolution TEM, Fourier transform infrared spectroscopy, dynamic light scattering, and rheometry to characterise each of the lubricants. This analysis showed that in the formulations that included NDs, the NDs were carboxylated, which helped them disperse well in the oil medium. Evidence of particle agglomeration was found in some samples after 30 days. As a result, all tribological tests were carried out immediately following preparation of the nanolubricants.

   A reciprocating (sliding) tribometer with steel surfaces was used to measure the tribological performance of each lubricant. In order to simulate real engine conditions in internal combustion vehicles and hybrid vehicles, two elevated test temperatures were chosen: 50 °C and 80 °C. PAO oil alone exhibited the highest overall coefficient of friction at both temperatures; close to 0.11. In contrast, nanolubricant PGZN (98.75 wt% PAO, 1 % GMO, 0.2% ZDDP, 0.05% ND) had a measured friction coefficient of 0.06. This was the lowest value measured at 50 °C. The nanolubricant’s performance was even more impressive at 80 °C, where an average friction coefficient of 0.04 was measured. In all cases, the formulations that contained nanodiamonds outperformed those without NDs.

   Following the sliding tests, the resulting wear surfaces were characterised. Here, ZDDP was the star. The additive was shown to help form thick, solid, pad-like tribofilm on the rubbing surfaces, preventing metal-to-metal contact and minimising the volume of material lost through wear. The inclusion of NDs had a polishing effect on the contacting

   surfaces, further protecting them from wear. Structural analysis of the tribofilm formed between rubbing surfaces at 80 °C showed that the inclusion of NDs resulted in a significantly thicker film – on average, PGZ’s tribofilm was 3-5 nm thick after two hours of sliding, while PGZN’s was 20-22 nm thick. The authors attribute the nanolubricant’s low friction coefficient to the formation of this thick layer.

   Their proposed mechanism for tribofilm formation involves adsorption of GMO onto the steel surface, followed by sliding-induced decomposition of ZDDP to form a film. This film then acted as a matrix for the NDs to embed and mechanically interlock themselves in place, increasing the film hardness and robustness, and reducing the measured friction coefficient. They write, “The experimental findings presented demonstrate the potential to develop environmentally friendly low sulphated ash, phosphorus, sulphur (SAPS) lubricants by utilising ND/GMO/low concentration ZDDP synergy.”

   Future work will include developing a molecular dynamic simulation of the mechanism in order to verify their experimental results.

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   A.K. Piya, L. Yang, A. Al Sheikh Omar, N. Emami, A. Morina. “Synergistic lubrication mechanism of nanodiamonds with organic friction modifier,” Carbon 218 (2024) 118742. DOI: 10.1016/j.carbon.2023.118742