6.6 Physik und chemische Analytik der Polymere
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Every day, there are new headlines in the media about microplastics (1-1000 µm, ISO/TR 21960:2020) and nanoplastics (< 1 µm, ISO/TR 21960:2020) findings all over the planet with high variations in particle number and mass. The challenges in analytics are very complex, e.g. representative sampling, non-destructive sample preparation with concentrated particles and homogeneous distribution and true detection. All together lead to lacks in harmonization and results, which are hardly comparable. On the other hand, monitoring of microplastics is mandatory in the future strictly regulated by the EU commission in the Drinking water and Wastewater Framework Directive. One step to accurate and precise results will be the development of suitable reference materials mimicking particles in the environment.
BAM developed test materials, which are produced by mixing a small portion of microplastic particles with a water-soluble matrix. After solid phase dilution and homogenisation small portions are pressed into tablets and bottled in glass vials (Figure 1). These tablets are well characterized with particle size distribution and SEM images. Additionally, they are tested as reference material candidate according to homogeneity and stability for particle number with µ-IR and µ-Raman as well as on particle mass with Py-GC/MS and TED-GC/MS after ISO Guide 35. Results are promising. The material passed the homogeneity control. No changes are observed within 6 months of storage.
The same tested reference material is finally used in sample preparation experiments, where environmental suspended particular matter from surface water or baby milk powders are spiked with the tablets.
Over the last 20 years, many researchers, politicians and citizens have become increasingly aware of the growing plastic problem of our time. A lack of recycling concepts and plastic collection points as well as careless dumping lead to accumulation of plastic products in the environment. Natural weathering can cause these plastics to degrade and fractionate, meaning that microplastics (1 1,000 µm, ISO/TR 21960:2020) and nanoplastics (< 1 µm, ISO/TR 21960:2020) of various synthetic polymer materials can now be detected in all parts of the world. Whether microplastics or nanoplastics pose a toxicological hazard is being investigated in a variety of ways. Valid results are still pending. However, the EU precautionary principle applies to micro- and nanoplastics. Monitoring of microplastics is already required in the revision of the Drinking Water and Wastewater Framework Directive.
Reliable monitoring of rivers can be carried out by sampling with sedimentation boxes and microplastic detection by using thermal extraction desorption gas chromatography/mass spectrometry (TED-GC/MS) in routine operation (Figure 1). The river Rhine was sampled for microplastic masses at three different sampling locations over a period of one year and in addition the Danube at randomized sampling locations The TED-GC/MS results showed that various synthetic polymers frequently produced in industry, such as polyethylene, polypropylene or polystyrene as well as the tire compound styrene-butadiene rubber were found. The work not only shows a possible workflow for monitoring concepts, but also provides information on environmentally relevant concentrations of microplastics and tire components in surface waters. This in turn is necessary for ecotoxicological studies.
Microplastic determination in food and surface waters will be increasingly carried out in the course of future directives and regulations, such as EU drinking water directive and wastewater directive. In addition to unique identification, this also includes reliable quantification. Two different methodological approaches are used for the quantification. With vibrational spectroscopic techniques such as µ-FTIR and µ-Raman, results are obtained in the form of particle number, size and shape. Instead, with thermal analytical technics as TED-GC/MS and Py-GC-MS the results are expressed as mass concentration.. Both concepts offer different information variables. In terms of routine monitoring, it is necessary to obtain a rapid sequence from sampling to the detection result. For this reason, this study focuses on TED-GC/MS and Py-GC-MS. Even though thermal analytical methods generally require less sample preparation, some matrices require it. This is usually done if the analyte concentration is too low or if strong matrix effects such as signal suppression and false positive signals occur.
This talk presents advanced sample preparation for baby milk powder as food example and density separation for microplastic analysis in surface waters.
A preparation protocol based on citric acid was selected for the milk to remove as much as possible of the matrix. The low pH value leads to a fast and effective protein precipitation and minimizes filter cake formation, making filtration possible, reduces the number and the amount of compounds in detection and hence, simplifies the evaluation.
Microplastics in surface waters and sediments must be concentrated not only because of their low microplastics mass content but also in terms of homogeneity. For this purpose, a density separation was carried out using a concentrated sodium iodide solution, which led to a high reduction of the inorganic mineral matrix and made possible to obtain a representative subsample of initial masses of up to 80 g.
Das Ziel dieses Grundlagenforschungsprojektes war, die Abbaubarkeit syntheti-scher Polymere unter realitätsnahen Umweltbedingungen durch eine Mischung bzw. Blend eines Hydrolyse- mit einem Oxidations-empfindlichen Polymer zu verbessern, während er unter Nutzungsbedingungen stabil sein sollte. Die Arbeits-hypothese war, dass sich die Polymere gegenseitig in ihrem Abbau beschleunigen, wenn sie der alternierenden Wechselbeanspruchung aus UV-Strahlung und hydrolytischer Beanspruchung ausgesetzt sind. Hierfür wurden vier Blends aus Polylactid (PLA) und Polystyrol (PS) oder Polymethylmethacrylat (PMMA) in unterschiedlichen Mischverhältnissen und Phasenstrukturen mittels additiver Fertigung hergestellt. Die Blends wurden über acht Wochen bei 45 °C mittels alternierender Wechselbeanspruchung aus je fünf Tagen UV-Bestrahlung und zwei Tagen hydrolytischer Beanspruchung beschleunigt gealtert. Für eine Basis-charakterisierung und als Vergleichsgrundlage wurden zusätzlich die Ausgangs-polymere PLA, PS und PMMA jeweils einzeln der hydrolytischen Beanspruchung, UV-Beanspruchung und alternierenden Wechselbeanspruchung unterzogen. Neben der Bestimmung der physiko-chemischen Effekte auf Prüfkörperoberflächen und Bulk wurden erstmalig auch die Wassermedien auf Mikroplastikgehalte und ökotoxikologische Wirkungen analysiert.
Die Ergebnisse ergaben, dass reines PLA kaum degradierte, aber der PLA-Abbau sowohl durch Mischung mit PS als auch PMMA verstärkt wurde. Entgegen der Hypothese wurde dadurch allerdings sowohl die Degradation von PS als auch von PMMA gehemmt. Es konnte gezeigt werden, dass sich PS und PMMA im Blend mit PLA durch radikalische Depolymerisation abbauten und die dabei entstehenden Radikale die PLA-Hydrolyse beschleunigten, was wiederum den Abbau der PS- und PMMA-Phasen verlangsamte. Die Ergebnisse zeigten, dass bei dem PLA/PS-Schichtblend (PLA/PS-50) der höchste PS-Gehalt von 50 % den stärksten Effekt auf den PLA-Abbau hatte. Insgesamt war allerdings selbst in diesem Fall die PLA-Degradation nach achtwöchiger Beanspruchung als gering einzustufen. Der totale Masseverlust von PLA/PS-50 entsprach nach achtwöchiger Wechselbeanspruchung -2,6 ± 0,2 mg absolut bzw. ca. -1,5 ± 0,1 % der Prüfkörpermasse, wovon 0,001 % in partikuläres Mikroplastik fragmentierte (PLA: 0,2 – 2,7 μg/g; PS: 3,0 – 3,4 μg/g). Folglich war der größte Teil des Masseverlustes der Prüfkörper auf die Bildung gelöster und/oder gasförmiger Degradationsprodukte zurückzuführen.
Ferner zeigten ökotoxikologische Untersuchungen der wechselbeanspruchten Wassermedien von PLA/PS-50 eine zunehmende Hemmung des Algenwachstums der Grünalge Desmodesmus subspicatus, die nach sechs Wochen Wechsel-beanspruchung zu 100 % Hemmung und nach acht Wochen zu einem Absterben der eingesetzten Algenzellen führte. Ergänzende Algenwachstumshemmungstests mit
2
Prüfkörpern der Ausgangspolymere PLA und PS zeigten, dass dies nicht auf der Bildung von partikulärem Mikroplastik, sondern gelösten PS-Degradations-produkten basierte, wie z. B. Benzaldehyd oder Benzoesäure.
Zusammenfassend lässt sich sagen, dass nur eine geringe Beschleunigung des PLA-Abbaus bei gleichzeitiger Hemmung des PS- bzw. PMMA-Abbaus stattfand - und das auch erst bei hohen PS- bzw. PMMA-Gehalten. Weiterführende Experimente zeigten, dass dies speziell für PS höchst bedenklich und nicht empfehlenswert ist, da es abhängig von der Konzentration der gelösten PS-Degradationsprodukte zu starken ökotoxikologischen Effekten kommen kann, die von der Hemmung des Algen-wachstums bis zum Algenabsterben reichen können. Folglich konnte die Arbeits-hypothese nur sehr eingeschränkt verifiziert werden und ist mit den hier untersuchten Polymeren als nicht zielführend einzuschätzen.
In well annealed thin polymer films with non-repulsive polymer/substrate interaction with a substrate an irreversibly adsorbed layer is expected to form. These adsorbed layers have shown a great potential for technological applications [1]. However, the growth kinetics and the molecular mobility of the adsorbed layer is still not fully understood. This concerns also the influence of the adsorbed layer on the thickness dependence of the glass transition temperature of thin films. This is partly due to the difficult accessibility of these layers in thin films. Here, the irreversibly adsorbed layers of semi-rigid mail chain polymers like polycarbonate or polysulfone are revealed by solvent-leaching experiments. First, the growth kinetics of these layers is investigated as a function of annealing time. The film thickness, topography and the quality of the adsorbed layer is controlled by Atomic Force Microscopy (AFM). Secondly, the molecular mobility of the adsorbed layer is investigated by Broadband Dielectric Spectroscopy (BDS). A developed nanostructured capacitor arrangement is employed to measure the layer with a free surface. In addition to the dielectric experiments, spectroscopic Ellipsometry measurements are carried out to estimate the glass transition of the thin films. The thickness dependence of the glass transition of the thin films is correlated with the adsorbed layer [2,3].
Acknowledgments
D. Hülagü and G. Hidde thanked for the help with the ellipsometry measurements.
This talk presents the EU funded project PlasticTrace. It shows the challenges in micro- and nanoplastic reference materials and gives some solutions regarding spectroscipc methods such as Raman or IR microscopy to determine the particle numbers. Materials are prepared by cryo milling to get some powder that is later pressed into tablets. The polymer type used is PET, because that is highly important for drinking water directive of EU commission.
A combination of neutron time-of-flight and neutron backscattering spectroscopy was used to investigate the molecular dynamics of Janus polynorbornenes (Janus poly(tricyclononenes)) on a microscopic level.
These Janus polynorbornenes, denoted as PTCNSiOR, have a semirigid backbone with −Si(OR)3 side groups attached to it. R represents the length of the alkyl side chain. Here side chain lengths of R = 3 (propyl) and R = 8 (octyl) were considered. It is worth mentioning that these polymers have some potential as active layers in gas separation membranes, especially for the separation of higher hydrocarbons. The combination of time-of-flight and backscattering will ensure a reasonably broad time window for analysis where the incoherent intermediate scattering function SInc(q,t) is considered. Previously, it was shown by X-ray investigations that the system undergoes a nanophase separation into alkyl side chain-rich domains surrounded by a backbone-rich matrix. For PTCNSiOPr (R = 3), the alkyl side-chain-rich domains are truly isolated in the backbone-rich matrix, whereas for PTCNSiOOc (R = 8) these domains percolate through the matrix. Further, it was also previously shown that the alkyl side-chain-rich domains undergo a glass transition. The advantage of neutron scattering experiments discussed here is that besides temporal also spatial information is obtained which will allow conclusions to be drawn about the type of molecular fluctuations. At the lowest measured temperature, the decay in Sinc(q,t) is due to the methyl group rotation. The methyl group dynamics is analyzed in terms of a modified jump-diffusion in a 3-fold potential and yields to a reasonable fraction of hydrogens which contribute to the methyl group rotation. At higher temperatures, the decay in SInc(q,t) is due to both the methyl group rotation and the segmental dynamics in the alkyl side-chain-rich domains. The segmental diffusion is modeled by a sublinear diffusion. For the analysis of the scattering function SInc(q,t) of PTCNSiOPr an elastic scattering due to the immobilized backbone-rich matrix must be taken into account. The analysis reveals that the segmental dynamics is confined by the finite size of alkyl chain-rich domains and that it is intrinsically heterogeneous in nature. Both effects are more pronounced for PTCNSiOPr in comparison to those of PTCNSiOOc.
We simulate a homologous series of guanidinium-based columnar ionic liquid crystals (ILCs) using coarse-grained molecular dynamics (MD) simulations with the Martini force field. We systematically vary the length of alkyl side chains, ILC-n (n = 8, 12, 16), and compare our results with previous experimental findings. Experimentally, ILC-8 exhibits a narrow mesophase window and weak columnar order, while ILC-12 and ILC-16 display a broad mesophase window and high columnar order. The MD simulations show that ILC-8 forms a percolated structure, whereas the longer chain analogues self-assemble into columns, with columnar assembly becoming more prominent as the side chain length increases, in qualitative agreement with the experiments. Furthermore, the intercolumnar distance increases monotonically with increasing side chain length and decreases with increasing temperature. Finally, we find that the diffusion coefficient and ionic conductivity decrease substantially with increasing chain length, consistent with experimental observations. We attribute this decrease in mobility to the formation of hexagonally ordered columns, which restrict transport more than percolated networks.
This work studies the influence of the adsorbed layer on the glass transition of thin films of polysulfone. Therefore, the growth kinetics of the irreversibly adsorbed layer of polysulfone on silicon substrates was first investigated using the solvent leaching approach, and the thickness of the remaining layer was measured with atomic force microscopy. Annealing conditions before leaching were varied in temperature and time (0–336 h). The growth kinetics showed three distinct regions: a pre-growth step where it was assumed that phenyl rings align parallel to the substrate at the shortest annealing times, a linear growth region, and a crossover from linear to logarithmic growth observed at higher temperatures for the longest annealing times. No signs of desorption were observed, pointing to the formation of a strongly adsorbed layer.
Second, the glass transition of thin polysulfone films was studied in dependence on the film thickness using spectroscopic ellipsometry. Three annealing conditions were compared: two with only a tightly bound layer formed in the linear growth regime and one with both tightly bound and loosely adsorbed layers formed in the logarithmic growth regime. The onset thickness and increase in the glass transition temperature increases with annealing time and temperature. These differences were attributed to the distinct conformations of the formed adsorbed layers.
AbstractRecent studies have hypothesized that the reinforcing effects of boehmite nanoparticles (BNPs) in polymer nanocomposites (PNCs) are partly related to the particles themselves and partly to the water released from the BNP during curing. In this work, PNCs made from dried BNP (dBNP) with concentrations up to 15 wt% are investigated to differentiate particle and water related effects. The observed trend of the storage modulus in dynamic mechanical thermal analysis measurements was found to be independent of the drying procedure. Stiffness maps from intermodulation atomic force microscopy showed that dBNP leads to a stiffening of the interphase surrounding the particles compared with the unaffected epoxy matrix, while a softer interphase was reported for PNCs with as received BNP. A slight decrease in the glass transition temperature was observed by broadband dielectric spectroscopy related to a lowered crosslink density due to the particles. A significantly higher decrease was reported for PNCs with BNP, attributed to water influencing the curing process. In conclusion, the stiffening of PNC with BNP is related to the particles themselves, while the release of water causes the formation of a soft interphase in the vicinity of the particles and a significant decrease in crosslink density.