High resolution solid state 1H NMR is used to investigate proton mobility of Nafion, Sulfonated Polyether Ether Ketones(S-PEEK) and their composites, which provides better understanding of their proton conductivities. Proton exchange between sulfonic acid groups and water was observed in these materials. The proton mobility is dependent on both the temperature and the water content. Variable temperature experiments were used to determine the activation energy for proton transportation which generally increases with decrease in hydration level. The preparation of Nafion/SiO2 composites can cause large difference in proton diffusion coefficients and proton conductivities in dried states. This indicates that the amount of dopants needs to be optimized to minimize the blocking of proton diffusion pathways by dopant particles. Detailed information on the control of surface hydroxyl groups in Nafion/SiO2 is obtained through the combination of 29Si and 1H NMR. Although hydrated Nafion/ZrP composites show reduced proton activation energy, they present lower proton conductivity at 35°C than unmodified Nafion. For composites dried at 160°C, both the conversion of monohydrogen phosphate into pyrophosphate and the protonation of monohydrogen phosphate have been observed, which could be one of reasons for the decreased proton conductivity after rehydration. Under high humidification, a single or multiple sulfonic acid proton environments was observed in S-PEEKs, which explains the small proton conductivity difference between some of S-PEEKs. However, the observed conductivity difference for S-PEEKs cast from different solvents was attributed to distinct mobilities of polymer chains. In the crosslinked S-PEEK, not all the crosslinkers of ethylene glycol are fully crosslinked. Proton exchange between residual sulfonic acid and hydroxyls of the crosslinker was observed, which is the primary reason that the crosslinked S-PEEK, with very low residual degree of sulfonation (13 %), still shows proton conductivity comparable to those of S-PEEKs. / Thesis / Doctor of Philosophy (PhD)

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The focus of this dissertation was to investigate the influence of sidechain structure and sidechain content on the morphology and physical properties of perfluorosulfonic acid ionomer (PFSA) membranes. One of the primary objectives was to characterize the thermomechanical relaxations for short sidechain PFSAs developed by 3M and Solvay, as well as a new multi-acid sidechain perfluoroimide acid ionomer (PFIA) from 3M. Partial neutralization experiments played a key role in systematically manipulating the strength of the electrostatic interactions between proton exchange groups on each sidechain, leading to the elucidation of the molecular-level motions associated with multiple thermal relaxations observed by dynamic mechanical analysis (DMA). Particularly, 3M PFSA and Solvay Aquivion lack an observable β-relaxation in the sulfonic acid-form that is observed in the long sidechain PFSA, Nafion. By varying the strength of the physically-crosslinked network through exchanging the proton on the sulfonic acid groups for large counterions, we were able to conclude that the shorter sidechain length and increase in ion content in the 3M PFSA and Solvay Aquivion serves to restrict the mobility of the polymer backbone such that the onset of segmental motions of the main chains is not observed at temperatures below the α-relaxation temperature, where destabilization of the physically crosslinked network occurs. As a complementary technique to DMA for probing the relaxations in PFSAs, we introduced a new pretreatment method for differential scanning calorimetry (DSC) measurements that uncover a thermal transition in H+-form 3M PFSA, Aquivion, and Nafion membranes. This thermal transition was determined to be of the same molecular origin as the dynamic mechanical α-relaxation temperature in H+-form PFSAs, and the β-relaxation temperature in tetrabutylammonium (TBA+)-form PFSAs. The thermomechanical relaxations in multi-acid sidechain 3M PFIA were also investigated. Interestingly, the additional acidic site on PFIA led to unexpected differences in thermal and mechanical properties, including the appearance of a distinct glass transition temperature otherwise not seen in PFSA ionomers. We utilized small-angle X-ray scattering (SAXS) studies to probe the differences in aggregate structure between the PFIA and PFSA membranes in order to uncover the morphological origin of the anomalous thermomechanical behavior in PFIA membranes. Larger aggregate structures for PFIA, compared to PFSA, incorporate intervening fluorocarbon chains within the aggregate, resulting in increased spacing between ions that reduce the collective electrostatic interactions between ions such that the onset of chain mobility occurs at lower temperatures than the α-relaxation for PFSA. The SAXS profiles of PFSAs showed two scattering features resulting from scattering between crystalline domains and ionic domains distributed throughout the polymer matrix. In order to fit the "ionomer peak" to models used for the PFIA and PFSA aggregate structure determination, we presented a method of varying the electron density of the ionic domains by using different alkali metal counterions as a tool to make the intercrystalline feature indistinguishable. This allows for isolation of the ionomer peak for better fits to scattering models without any interference from the intercrystalline peak. Lastly, an investigation of annealing PFSAs of different sidechain structures in the tetramethylammonium (TMA+) counterion form above their α-relaxation showed a profound crystalline-like ordering of the TMA+ counterions within the ionic domains. This ordering is maintained after reacidification and leads to improved proton conductivity, which indicates that this method can be used as a simple processing method for obtaining improved morphologies in proton exchange membranes for fuel cell applications. / Doctor of Philosophy / Hydrogen fuel cells offer an environmentally friendly, high efficiency method for powering vehicles, buildings, and portable electronic devices. At the center of a hydrogen fuel cell is a polymer membrane that contains ionic functionalities, which conduct hydrogen ions (protons) from the anode to the cathode while preventing conduction of electrons. The electrons travel through an external circuit to produce electricity, while the protons travel through the polymer membrane and meet with oxygen on the other side to produce water, the only byproduct of a hydrogen fuel cell. The efficiency of this process relies on the ability of the polymer membrane to conduct protons, and the lifetime of a fuel cell depends on the mechanical stability of this membrane. Perfluorosulfonic acid ionomers are good candidates for use as polymer membranes in hydrogen fuel cells due to their Teflon backbone that provides mechanical stability and their sulfonic acid functionalities that form channels for proton conduction. In this work, we probe the structure-property relationships of different perfluorosulfonic acid ionomers for use as fuel cell membranes. We focus on thermal analysis techniques to develop a fundamental understanding of the effect of chemical structure and sulfonic acid content on the temperature-induced mobility of the polymer chains in these ionomers. This mobility at elevated temperatures can be utilized to rearrange the morphological structure of perfluorosulfonic acid ionomer membranes in order to enhance proton conductivity and mechanical integrity.

The focus of this research project was to investigate morphological and mechanical properties of both extruded and dispersion-cast Nafion® membranes. The project can be divided into three primary objectives; obtaining a fundamental understanding of the glass transition temperature of Nafion®, determining the effect of thermal annealing treatments on the morphology and mechanical properties of dispersion-cast Nafion®, and examination of dispersion-cast Nafion® subjected to an ex-situ, Fenton's chemical degradation test. Nafion®, a perfluorosulfonate ionomer, is considered a commercially successful semi-crystalline ionomer with primary applications in chlor-alkali cells and proton exchange membrane fuel cells. With the aid of dynamic mechanical analysis (DMA) and dielectric spectroscopy (DS), we were able to provide definitive evidence for a genuine glass transition in Nafion®. DMA of Nafion® samples that were partially neutralized with tetrabutylammonium counterions showed a strong compositional dependence suggesting that the β-relaxations of H+-form Nafion® and the neutralized ionomers have the same molecular origin with respect to backbone segmental motions. Building upon our previous studies of the molecular and morphological origins of the dynamic mechanical relaxations of Nafion® neutralized with a series of organic ions, the glass transition temperature of H+-form Nafion® is now confirmed to be the weak β-relaxation centered at -20 °C. Dielectric spectra also showed this transition from the perspective of dipole relaxation. The signature of cooperative long range segmental motions in dielectric spectra was seen here, as with other polymers, mainly through the excellent agreement of the β-relaxation time-temperature dependence with the Vogel-Fulcher-Tammann equation. We have also discovered that new dispersion-cast H+ form Nafion® membranes are susceptible to disintegration/dissolution when subjected to boiling methanol. In this work, we have achieved significant decreases in the percent solubility of H+-form Nafion® by either thermally annealing above 175 °C or solution-processing at 180 °C using a high boiling point solvent. Small Angle X ray Scattering (SAXS) displayed a change in the morphology of H+ form membranes with increasing annealing temperature by a shift in the crystalline scattering peak (q â 0.05 Ã 1) to lower q values. Counterion exchange of Nafion® from H+ to Na+ form had no influence on the membrane's susceptibility to disintegration in boiling methanol. In order to achieve mechanical stability in boiling methanol, Na+ form membranes had to be annealed at 275 °C for at least fifteen minutes. The SAXS data of annealed Na+ form membranes showed a dramatic decrease in crystalline order with annealing temperature, ultimately leading to the disappearance of the crystalline scattering peak after fifteen minutes at 275 °C. The onset of methanol stability with the melting of Nafion® crystallites suggests that chain entanglement is an important parameter in obtaining solvent stability. With respect to chemical stability, we performed studies aimed at examining the effects of Fenton's Reagent on the resistance to radical attack of new generation, dispersion-cast Nafion®. Changes in the 19F solid-state NMR spectra of dispersion-cast Nafion® before and after chemical degradation via Fenton's Reagent predicts a rather random attack by ·OH and ·OOH radicals. Several membranes were also thermally annealed between 100-250 °C in an attempt to correlate crystallinity with chemical degradation kinetics of Nafion® via Fenton's Reagent. The results indicate that the effect of counterion exchange into the Na+ form was minimal, but the degree of thermal degradation had a tremendous effect on the fluoride release rate and chemical degradation kinetics. By exchanging the membranes into the Na+ form, thermal degradation was avoided, allowing us to study the role of crystallinity as a function of fluoride release. Ultimately, Nafion® crystallinity was deemed an important factor in deterring peroxide radical attack. As the percent crystallinity decreased with annealing temperature, the fluoride concentration in the resulting Fenton's media increased accordingly, indicating that the amorphous regions of the polymer are more susceptible to chemical degradation via peroxide radical attack. / Ph. D.

Investigation and characterization of polymeric materials are necessary to obtain in-depth understanding of their behavior and properties, which can fuel further development. To illuminate these molecular properties and their coupling to macroscopic behavior, we have performed nuclear magnetic resonance (NMR) studies on a variety of chemical systems. In addition to versatile "traditional" NMR measurements, we took advantage of specialized techniques, such as "rheo-NMR," 2H NMR, and NMR self-diffusion experiments to analyze alignment, orientational order, elaborate rheological behavior, and ion transport in polymer films and complex fluids. We employed self-diffusion and quadrupolar deuterium NMR methods to water-swollen channels in Nafion ionomer films commonly used in fuel cells and actuators. We also correlated water uptake and anisotropic diffusion with differing degrees and types of alignment in Nafion films based on membrane processing methods. Further, we made quantitative measurements of bulk channel alignment in Nafion membranes and determined anisotropic properties such as the biaxiality parameter using these methods. Additionally, our studies made the first direct comparison of directional transport (diffusion) with quantitative orientational order measurements for ionomer membranes. These results lend insight to the importance of water content in ionomer device performance, and showed that increased control over the direction and extent of orientational order of the hydrophilic channels could lead to improved materials design. We used the same techniques, with the addition of "rheo-NMR" and solution rheology measurements, to study the complex rheological behavior of cetyltrimethylammonium bromide wormlike micelle solutions, which behave as nematic liquid crystals at sufficiently high concentration. Amphiphilic solutions of this type are used in myriad applications, from fracturing fluids in oil fields to personal care products. We investigated the phase behavior and dynamics of shear and magnetic field alignment, and made the first observations of a novel bistable shear-activated phase in these solutions. Our first reports of the complex Leslie-Ericksen viscoelastic parameters in wormlike micelles and measurements of diffusion anisotropy show the potential for increased control and understanding of materials used in tissue engineering, oil extraction, personal care products, and advanced lubricants. / Ph. D.

Ionic polymer transducers (IPTs) are soft sensors and actuators which operate through a coupling of micro-scale chemical, electrical, and mechanical mechanisms. The use of ionic liquid as solvent for an IPT has been shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages without electrolysis. This work aims to further the understanding of the dominant mechanisms of IPT actuation and how these are affected when an ionic liquid is used as solvent. A micromechanical model of IPT actuation is developed following a previous approach given by Nemat-Nasser, and the dominant relationships in actuation are demonstrated through an analysis of electrostatic cluster interactions. The elastic modulus of Nafion as a function of ionic liquid uptake is measured using uniaxial tension tests and modeled in a micromechanical framework, showing an excellent fit to the data. Charge transport is modeled by considering both the cation and anion of the ionic liquid as mobile charge carriers, a phenomenon which is unique to ionic liquid IPTs as compared to their water-based counterparts. Numerical simulations are performed using the finite element method, and a modified theory of ion transport is discussed which can be extended to accurately describe electrochemical migration of ionic liquid ions at higher applied voltages. The results presented here demonstrate the dominant mechanisms of IPT actuation and identify those unique to ionic liquid IPTs, giving directions for future research and transducer development. / Master of Science

The purpose of this work was to investigate the effects of structure, humidity and aging on the mechanical behavior of Nafion® and Dais® ionomers. It was determined that the majority of the properties of these membranes were controlled by the formation and growth of the ionic clusters that were the direct result of the ionic nature of these materials. In the process of this study, the properties of Nafion® and sulfonated Dais® polymers were investigated by dynamic mechanical analysis and thermal gravimetric analysis and their water uptake and sorption and desorption isotherms were measured. A mastercurve and a shift factor plot were constructed for 60% sulfonated Dais® membrane. It was determined that an increase in the degree of sulfonation raised the glass transition temperature of these materials by facilitating the formation of the ionic clusters which acted as physical crosslinks, thereby reducing the mobility of polymeric chains. Water was found to effectively plasticize the membranes, especially in the case of Dais® materials, by reducing the storage modulus and decreasing the structural integrity of the ionomers. The effect of pre-treatment of Nafion® was investigated and the glass transition temperature was found to increase as a function of the severity of the treatment procedure. The maximum water uptakes were determined for virgin and aged Nafion® and Dais® membranes and their vapor phase water sorption diffusion coefficients were calculated. The sorption process was found to follow pseudo-Fickian behavior, while the movement of water out of the membranes during the desorption process was determined to be controlled by mechanisms other than diffusion. Lastly, the effect of exposure of Nafion® and 30% sulfonated Dais® membranes to the saturated environment at elevated temperatures was investigated and found to result in the increase in the glass transition temperature of the materials. Results of the exposure effects on the diffusion properties of Nafion® and Dais® were inconclusive. Preliminary findings attributed the changes in the properties of the materials to the counteractive actions of physical aging and the growth of the ionic clusters. / Master of Science

Les membranes fluorées sont utilisées en particulier dans les dénommées piles à combustible à membrane électrolyte polymère. Grâce à sa grande mobilité en protons, le célèbre ionomer Nafion® (Dupont) est un matériau de référence pour les applications liées aux piles à combustible. En présence d’eau ou d’autres solvants hydrophiles la membrane se sépare en une matrice polymérique hydrophobe et une sous-phase aqueuse contenant des clusters d’eau et ions, dont les tailles et la connectivité augmente quand la quantité d’eau augmente [1]. Quelle est la morphologie du Nafion et la structure du solvant, dans de tels systèmes?Il a été récemment montré [2] sur des simulations de large systèmes que plusieurs modèles morphologiques reproduisent les données expérimentales de diffusion, évoquant l’incapacité des mesures de diffusion seules à élucider la véritable structure du Nafion.Néanmoins, un modèle ’aléatoire’ décrit dans [2], c’est à dire l’unique modèle étudié sans présumer d’une structure initiale particulière, n’a pas pu reproduire les données expérimentales.Générer en simulations moléculaires des configurations du système qui soient vraiment décorrélées de la configuration initiale reste un vrai défi statistique. Les échelles de temps réalisables ne permettent simplement pas d’obtenir des mouvements significatifs du polymère (comme des transitions de conformations, repliements de chaînes, etc.). Nous proposons ainsi dans cette étude un nouveau modèle de Nafion à morphologie aléatoire. Un algorithme récemment développé est utilisée pour générer des chaînes de Nafion avec des chemins et des points de départ aléatoires. Une différence majeure avec le modèle aléatoire dans [2] est que nous ne construisons pas nos systèmes à une densité proche de la densité finale. Pour ne pas démarrer avec des chaînes trop enchevêtrées, les systèmes sont initialement préparés à une densité en dessous de la référence expérimentale. La densité après équilibration est de nouveau proche de l’expérience. Bien qu’il soit facilement envisageable d’améliorer les nouveaux algorithmes, nous démontrons ici qu’avec la présente version plusieurs séries de configurations compatibles avec les données expérimentales de diffusion disponibles peuvent être générées et équilibrées. Douze large systèmes de Nafion à morphologie aléatoire sont construits avec des positions initiales des atomes ainsi que des quantités d’eau et des longueurs de chaînes (Nafion/Hyflon) différentes. Ils sont équilibrés puis simulés sur plusieurs dizaines de nanosecondes. Après équilibration, les structures sont, comme indiqué ci-dessus,compatibles avec les données expérimentales de diffusion. En plus nous étudions un modèle ressemblant à celui de Schmidt-Rohr and Chen [3], c’est à-dire le plus récent modèle morphologique. Avec ce modèle, les données expérimentales sont également reproduites de manière satisfaisante, d’où la prolongation du débat sur la structure du Nafion. La cohésion entre les valeurs calculées et celles mesurées expérimentalement incite à des analyses plus en détails de ces configurations obtenues. Nous caractérisons et analysons les structures locales, intermédiaires et à grande échelle avec divers paramètres structuraux et distributions des tailles de domaines. Nous calculons donc, par exemple, des fonctions de distribution radiale (rdf), des facteurs de structure (S(q)) totaux et partiels tout comme des nombres et des tailles de clusters hydrophiles (selon la définition d’un cluster). La dynamique de diverses espèces dans le système est également examinée,par exemple au travers des déplacements carrés moyens (msd) et des coefficients de diffusion. Ces simulations sont probablement à la limite de ce qui est réalisable aujourd’hui avec des simulations ’full-atom’ du type MD. Nous espérons que ce travail fera avancer le débat sur la structure et la dynamique de ces matériaux importants. / Perfluorinated membranes are used in particular in polymer electrolyte fuel cells(PEFC). The well-known ionomer Nafion® (Dupont) is, due to its high proton mobility,a reference material for fuel cell applications. In water or other hydrophilic solvents themembrane segregates into a hydrophobic backbone matrix and a hydrophilic sub-phasecontaining clusters of both water and ions, where the cluster sizes and connectivity increasewith increasing water content [1].What is the Nafion morphology and the structure of the solvent in such systems? It hasbeen shown recently [2] on large simulated systems that several morphological modelsfit the experimental scattering data, suggesting the inability of scattering experimentsalone to elucidate the true structure of Nafion. However, a ’random’ model describedin [2], i.e. the only explored model that did not assume a particular initial structure,could not reproduce the experimental data.It remains a real computational challenge to generate in molecular simulations systemconfigurations which are really decorrelated from the initial one. The time scales thatcan be achieved simply do not allow to obtain significant motions of the polymer (e.g.conformational changes, folding, etc.). We thus propose in this work a new randommodel of Nafion. A newly developped algorithm is used to generate Nafion chains withrandom growth paths and random starting points. A significant difference with therandom model in [2] is that we do not build our systems at a density close to the finalone. In order not to start with too much entangled chains, the systems are initiallybuilt at a density below the experimental one. The density after equilibration is againclose to the experimental one.Even though further improvements of the new algorithms can easily be envisaged,we demonstrate here that with the present version several sets of configurations thatare compatible with the available scattering data can be generated and equilibrated.Twelve large random Nafion systems are built with different initial positions of theatoms as well as different water contents and side chain lengths (Nafion/Hyflon). Theyare equilibrated and then simulated for several ten nanoseconds. After equilibration,the structures are, as mentioned, compatible with the experimental scattering data. Inaddition we study a model similar to the one by Schmidt-Rohr and Chen [3], i.e. thenewest morphological model of Nafion. The experimental scattering data are also satisfactorilyreproduced with this model, hence, the prolonged debate over the structureof Nafion.This agreement gives confidence that a more detailed analysis of the so-obtained configurationsis scientifically warranted. We characterize and analyze the local, intermediateand large-scale structures by various structural parameters and domain size distributions.We therefore compute, for example, radial distribution functions (rdf), total andpartial structure factors (S(q)) as well as numbers and sizes of hydrophilic clusters (dependingon the definition of a cluster). The dynamics of various species in the systemis also investigated, e.g. via the computation of the mean square displacements (msd)and the self-diffusion coefficients. These simulations are probably at the limit of whatcan today be achieved with all-atom molecular simulations of the MD type. We hopethat this work will advance the ongoing debate on the structure and dynamics of theseimportant materials. / Perfluorierte Membranen werden insbesondere in Polymerelectrolyt-Brennstoffzellen(PEFC) eingesetzt. Das wohlbekannte Ionomer Nafion® (Dupont) ist wegen seinerhohen Protonenbeweglichkeit ein Referenzmaterial für solche Anwendungen in Brennstoffzellen.Die Membran separiert in Wasser oder anderen hydrophilen Lösungsmittelin eine hydrophobe Polymermatrix und eine hydrophile Subphase, die Cluster mitWasser und Ionen enthält. Dabei vergroeßern sich die Ausdehnung der Cluster und ihreKonnektivität mit zunehmendem Wassergehalt [1].Welche ist die Morphologie des Nafions und die Struktur des Lösungsmittels in diesenSystemen? Es ist jüngst anhand großer simulierter Systeme gezeigt worden [2], dassmehrere morphologische Modelle die experimentellen Streudaten wiedergeben können,was nahelegt, dass solche Streudaten alleine nicht geeignet sind, die wahre Strukturdes Nafion aufzudecken. Ein in [2] beschriebenes ’Zufallsmodell’, d.h. das einzigeder untersuchten Modelle, das keine besondere Anfangsstruktur annahm, konnte dieexperimentellen Daten allerdings nicht wiedergeben.In molekularen Computersimulationen Konfigurationen zu erzeugen, die wirklich nichtmehr mit der angenommenen Anfangskonfiguration korreliert sind, bleibt eine echteHerausforderung. Die erreichbaren Zeitskalen sind zu kurz, um eine signifikante Bewegungdes Polymers (z.B Konformationsänderungen, Faltungen, usw.) zuzulassen. Indieser Arbeit wird daher ein neues Zufallsmodell für Nafion vorgestellt. Ein neuentwickelterAlgorithmus erzeugt Nafionketten mit zufälligem Wachstumspfad ausgehendvon zufälligen Anfangspunkten. Ein signifikanter Unterschied zu dem Zufallsmodellvon [2] ist, dass hier nicht versucht wird, die Systeme bei einer Dichte vergleichbarder experimentellen Dichte aufzubauen. Anstattdessen werden die Systeme, um alzustarkes Verknäuelung zu vermeiden, anfangs bei einer deutlich kleineren Dichte erzeugt.Nach äquilibrierung ist die Systemdichte wieder in etwa gleich der experimentellen.Wiewohl weitere Verbesserungen des neu Algorithmuses leicht ins Auge gefaßt werdenkönnen, so kann hier doch gezeigt werden, dass mit der gegenwärtigen VersionKonfigurationen erzeugt und äquilibriert werden können, die mit den verfügbarenStreudaten kompatibel sind. Zwölf große Nafion Zufallssysteme, mit verschiedenenAnfangspositionen der Atome, verschiedenem Wassergehalt und Längen der Seitenketten(Nafion/Hyflon) werden aufgebaut. Diese werden äquilibriert und mehrerezehn Nanosekunden lang simuliert. Nach der äquilibrierung sind die Strukturen, wieerwähnt, kompatibel mit den experimentellen Streudaten. Weiterhin wird ein Modellähnlich dem von Schmidt-Rohr und Chen [3], d.h. dem neuesten morphologischen Modellfür Nafion, studiert. Auch hier werden die experimentellen Streudaten zufriedenstellendwiedergegeben, daher die weiterhin bestehende Debatte über die Struktur desNafion.Die gefundenen übereinstimmungen lassen darauf vertrauen, dass eine detaillierte Analyseder simulierten Konfigurationen wissenschaftlich sinnvoll ist. So wird die Strukturder Systeme auf verschiedenen Längenskalen charakterisiert, zum Beispiel durch radialePaarverteilungsfunktionen (rdf), totale und partielle Strukturfaktoren (S(q)) sowieAnzahl- und Größenverteilungen hydrophiler Cluster (abhängig von der Definition einesClusters). Die Dynamik einzelner Spezies im System wird ebenfalls untersucht, zumBeispiel durch die Berechnung der mittleren quadratischen Verschiebungen (msd) undder Selbstdiffusionskoeffizienten. Diese Simulationen sind wahrscheinlich an der Grenzedessen, was heute mit ’all-atom’ molekularen MD-Simulationen möglich ist. Ich vertrauedarauf, dass diese Arbeit dennoch einen Fortschritt in der aktuellen Debatte überdie Struktur und Dynamik dieser wichtigen Materiale darstellt.