The effect of boehmite nanoparticles (γ‐AlOOH) on nanomechanical and thermomechanical properties correlated to crosslinking density of epoxy

We show that complex physical and chemical interactions between boehmite nanoparticles and epoxy drastically affect matrix properties, which in the future will provide tuning of material properties for further optimization in applications from automotive to aerospace. We utilize intermodulation atomic force microscopy (ImAFM) for probing local stiffness of both particles and polymer matrix. Stiff particles are expected to increase total stiffness of nanocomposites and the stiffness of polymer should remain unchanged. However, ImAFM revealed that stiffness of matrix in epoxy/boehmite nanocomposite is significantly higher than unfilled epoxy. The stiffening effect of the boehmite on epoxy also depends on the particle concentration. To understand the mechanism behind property alteration induced by boehmite nanoparticles, network architecture is investigated using dynamic mechanical thermal analysis (DMTA). It was revealed that although with 15 wt% boehmite nanoparticles the modulus at glassy state increases, crosslinking density of epoxy for this composition is drastically low.DFG, 232311024, FOR 2021: Wirkprinzipien nanoskaliger Matrixadditive für den Faserverbundleichtba

Short- and Long-Range Mechanical and Chemical Interphases Caused by Interaction of Boehmite (γ-AlOOH) with Anhydride-Cured Epoxy Resins

Understanding the interaction between boehmite and epoxy and the formation of their interphases with different mechanical and chemical structures is crucial to predict and optimize the properties of epoxy-boehmite nanocomposites. Probing the interfacial properties with atomic force microscopy (AFM)-based methods, especially particle-matrix long-range interactions, is challenging. This is due to size limitations of various analytical methods in resolving nanoparticles and their interphases, the overlap of interphases, and the effect of buried particles that prevent the accurate interphase property measurement. Here, we develop a layered model system in which the epoxy is cured in contact with a thin layer of hydrothermally synthesized boehmite. Different microscopy methods are employed to evaluate the interfacial properties. With intermodulation atomic force microscopy (ImAFM) and amplitude dependence force spectroscopy (ADFS), which contain information about stiffness, electrostatic, and van der Waals forces, a soft interphase was detected between the epoxy and boehmite. Surface potential maps obtained by scanning Kelvin probe microscopy (SKPM) revealed another interphase about one order of magnitude larger than the mechanical interphase. The AFM-infrared spectroscopy (AFM-IR) technique reveals that the soft interphase consists of unreacted curing agent. The long-range electrical interphase is attributed to the chemical alteration of the bulk epoxy and the formation of new absorption bands.DFG, 232311024, FOR 2021: Wirkprinzipien nanoskaliger Matrixadditive für den Faserverbundleichtba

Rasterkraftmikroskopiebasierte nanoskopische Studien von Epoxid/Böhmit-Nanokomposit

Development of lightweight parts made of fiber-reinforced polymer composites for aerospace and automotive industry has been focus of many studies over the past several decades. Nanofillers added to the epoxy matrices used in such composite parts results in remarkable improvement of matrix-dominated properties. Formation of interphases between inorganic nanofillers and polymer matrices is usually known to have a dominant impact on the properties of the nanocomposite. Sometimes, the effect of nanofillers on the properties of the matrix is not only limited to the immediate vicinities, but a long-range property alteration of the bulk polymer may occur. Underestimating the effect of such short- and long-range interactions between nanofillers and polymer matrices on properties on the nanocomposite, result in misprediction of material behavior. In this thesis the main goal is to understand the interaction between nanofillers and the epoxy matrix, to probe the interphase and its properties and eventually to correlate the nanoscale properties to the macroscopic behavior of the material. Due to size limitations of various analytical methods it is difficult to determine properties of nanofillers and their interphases. This thesis shows that by using multiple atomic force microscopy (AFM) methods, information about physical, chemical and mechanical properties of the nanocomposite at nanometer scale are provided and can be correlated to the macroscopic behavior. Using high resolution intermodulation AFM (ImAFM) results in recording complete force-distance curves from each pixel of the scanned area. By evaluation of both non-contact and contact regions of these curves, maps of electrostatic and van der Waals forces are obtained in addition to stiffness maps, providing complementary information about material composition in addition to the mechanical properties. Scanning Kelvin probe microscopy (SKPM) provides us with maps of potential which correlate to chemical structures of the heterogeneous material. Furthermore, infrared spectroscopy AFM (AFM-IR) is used to investigate the chemical structure of the interphase and heterogenous phases of the bulk matrix. Within the scope of this work, boehmite nanoparticles (BNPs) were selected as nanofillers for the epoxy matrix. The results of this dissertation prove the existence of both short- and long-range interactions of BNPs in epoxy. It was observed that BNPs have a long-range stiffening effect on the bulk epoxy. The short-range influence on the interphase between BNPs and epoxy shows the opposite behavior; the interphase is much softer than the epoxy itself. BNPs affect the network structure of bulk matrix by lowering the crosslinking density. Investigations on interfacial model systems demonstrate the formation of a long-range chemical interphase more than one order of magnitude larger than the short-range mechanical interphase. In the end, it is demonstrated that both the soft interphase and long-range chemical alteration of epoxy result from the preferential absorption of the curing agent (anhydride) towards BNPs. This leads to disturbance of the epoxy-hardener stoichiometric ratio, alteration of curing mechanisms, and modification of bulk properties of the cured matrix.Die Entwicklung von Leichtbauteilen aus faserverstärkten Polymerverbundwerkstoffen für die Luft- und Raumfahrt sowie die Automobilindustrie stand in den letzten Jahrzehnten im Mittelpunkt vieler Studien. Nanofüllstoffe, die den in solchen Verbundteilen verwendeten Epoxidmatrizen zugesetzt werden, führen zu einer bemerkenswerten Verbesserung der matrixdominierten Eigenschaften. Die Bildung von Interphasen zwischen anorganischen Nanofüllstoffen und Polymermatrices hat bekanntlich einen dominanten Einfluss auf die Eigenschaften des Nanokomposits. In manchen Fällen ist die Wirkung von Nanofüllstoffen auf die Eigenschaften der Matrix nicht nur auf die unmittelbare Umgebung beschränkt, sondern es kann auch zu einer weitreichenden Eigenschaftsänderung des Matrixpolymers kommen. Die Unterschätzung der Auswirkungen solcher kurz- und weitreichenden Wechselwirkungen zwischen Nanofüllstoffen und Polymermatrices auf die Eigenschaften des Nanokomposits führt zu einer Fehlinterpretation des Materialverhaltens. In dieser Arbeit ist das Hauptziel, die Interaktion zwischen Nanofüllstoffen und der Epoxidmatrix zu verstehen, die Interphase und ihre Eigenschaften zu untersuchen und schließlich die nanoskaligen Eigenschaften mit dem makroskopischen Verhalten des Materials zu korrelieren. Aufgrund der Größenbeschränkungen verschiedener Analysemethoden ist es eine Herausforderung, die Eigenschaften von Nanofillern und deren Interphasen zu bestimmen. Diese Arbeit zeigt, dass durch den Einsatz von Rasterkraftmikroskopie-Methoden (AFM) Informationen über physikalische, chemische und mechanische Eigenschaften eines Nanokomposits im Nanometerbereich zugänglich werden und mit dem makroskopischen Verhalten korreliert werden können. Die Verwendung von hochauflösendem Intermodulations-AFM (ImAFM) führt zur Aufnahme kompletter Kraft-Weg-Kurven von jedem Pixel des gescannten Bereichs. Durch die Auswertung sowohl der berührungslosen als auch der kontaktierenden Anteile dieser Kurven erhält man neben Steifigkeitskarten auch bildgebende Messungen der elektrostatischen Kräfte und der Van-der-Waals-Kräfte, die neben den mechanischen Eigenschaften auch ergänzende Informationen über die Materialzusammensetzung liefern. Die Kelvin-Sonde Mikroskopie (SKPM) liefert Potenzialkarten, die mit den chemischen Strukturen des heterogenen Materials korrelieren. Darüber hinaus wird die AFM-basierte nano-Infrarotspektroskopie (AFM-IR) eingesetzt, um die chemische Struktur der Interphasen und der heterogenen Phasen der Matrix zu untersuchen. Im Rahmen dieser Arbeit wurden Böhmit-Nanopartikel (BNPs) als Nanofüllstoff für die Epoxidmatrix ausgewählt. Die Ergebnisse dieser Dissertation belegen die Existenz von kurz- und weitreichenden Wechselwirkungen der BNPs im Epoxid. Es wurde beobachtet, dass BNPs einen weitreichenden Einfluss auf die Vernetzung des Epoxids haben und dadurch eine versteifende Wirkung auf das Epoxid haben. Der Kurzreichende Einfluss auf die Interphase zwischen BNPs und Epoxid ist dem entgegengesetzt und die Interphase ist viel weicher als das Epoxid selbst. BNPs beeinflussen die Netzwerkstruktur der Matrix, indem sie die Vernetzungsdichte verringern. Untersuchungen an Grenzflächenmodellsystemen zeigen die Bildung chemischer Interphasen, die mehr als eine Größenordnung größer sind als mechanische Interphasen bei kurzreichenden Wechselwirkungen. Es konnte weiterhin gezeigt werden, dass eine solche weitreichende chemische und strukturelle Veränderung des Epoxids das Ergebnis einer präferentiellen Absorption des Härters am BNP ist. Diese führt zu einer Störung des stöchiometrischen Verhältnisses zwischen Epoxid und Härter, einer Veränderung der Chemie der Aushärtung und damit zur Modifizierung der makroskopischen Eigenschaften der ausgehärteten Matrix

Insights into Nano-Scale Physical and Mechanical Properties of Epoxy/Boehmite Nanocomposite Using Different AFM Modes

Understanding the interaction between nanoparticles and the matrix and the properties of interphase is crucial to predict the macroscopic properties of a nanocomposite system. Here, we investigate the interaction between boehmite nanoparticles (BNPs) and epoxy using different atomic force microscopy (AFM) approaches. We demonstrate benefits of using multifrequency intermodulation AFM (ImAFM) to obtain information about conservative, dissipative and van der Waals tip-surface forces and probing local properties of nanoparticles, matrix and the interphase. We utilize scanning kelvin probe microscopy (SKPM) to probe surface potential as a tool to visualize material contrast with a physical parameter, which is independent from the mechanics of the surface. Combining the information from ImAFM stiffness and SKPM surface potential results in a precise characterization of interfacial region, demonstrating that the interphase is softer than epoxy and boehmite nanoparticles. Further, we investigated the effect of boehmite nanoparticles on the bulk properties of epoxy matrix. ImAFM stiffness maps revealed the significant stiffening effect of boehmite nanoparticles on anhydride-cured epoxy matrix. The energy dissipation of epoxy matrix locally measured by ImAFM shows a considerable increase compared to that of neat epoxy. These measurements suggest a substantial alteration of epoxy structure induced by the presence of boehmite

Nanomechanical study of polycarbonate/boehmite nanoparticles/epoxy ternary composite and their interphases

Thermoplastic modified thermosets are of great interest especially due to their improved fracture toughness. Comparable enhancements have been achieved by adding different nanofillers including inorganic particles such as nanosized boehmite. Here, we present a nanomechanical study of two composite systems, the first comprising a polycarbonate (PC) layer in contact with epoxy resin (EP) and the second consisting of a PC layer containing boehmite nanoparticles (BNP) which is also in contact with an EP layer. The interaction between PC and EP monomer is tested by in situ Fourier transformed infrared (FT‐IR) analysis, from which a reaction induced phase separation of the PC phase is inferred. Both systems are explored by atomic force microscopy (AFM) force spectroscopy. AFM force‐distance curves (FDC) show no alteration of the mechanical properties of EP at the interface to PC. However, when a PC phase loaded with BNP is put in contact with an epoxy system during curing, a considerable mechanical improvement exceeding the rule of mixture was detected. The trend of BNP to agglomerate preferentially around EP dominated regions and the stiffening effect of BNP on EP shown by spatial resolved measurements of Young's modulus, suggest the effective presence of BNP within the EP phase.DFG, 232311024, FOR 2021: Wirkprinzipien nanoskaliger Matrixadditive für den Faserverbundleichtba

Carrier Fibers for the Safe Dosage of Nanoparticles in Nanocomposites: Nanomechanical and Thermomechanical Study on Polycarbonate/Boehmite Electrospun Fibers Embedded in Epoxy Resin

The reinforcing effect of boehmite nanoparticles (BNP) in epoxy resins for fiber composite lightweight construction is related to the formation of a soft but bound interphase between filler and polymer. The interphase is able to dissipate crack propagation energy and consequently increases the fracture toughness of the epoxy resin. Usually, the nanoparticles are dispersed in the resin and then mixed with the hardener to form an applicable mixture to impregnate the fibers. If one wishes to locally increase the fracture toughness at particularly stressed positions of the fiber-reinforced polymer composites (FRPC), this could be done by spraying nanoparticles from a suspension. However, this would entail high costs for removing the nanoparticles from the ambient air. We propose that a fiber fleece containing bound nanoparticles be inserted at exposed locations. For the present proof-of-concept study, an electrospun polycarbonate nonwoven and taurine modified BNP are proposed. After fabrication of suitable PC/EP/BNP composites, the thermomechanical properties were tested by dynamic mechanical analysis (DMA). Comparatively, the local nanomechanical properties such as stiffness and elastic modulus were determined by atomic force microscopy (AFM). An additional investigation of the distribution of the nanoparticles in the epoxy matrix, which is a prerequisite for an effective nanocomposite, is carried out by scanning electron microscopy in transmission mode (TSEM). From the results it can be concluded that the concept of carrier fibers for nanoparticles is viable.DFG, 232311024, FOR 2021: Wirkprinzipien nanoskaliger Matrixadditive für den Faserverbundleichtba

Carrier Fibers for the Safe Dosage of Nanoparticles in Nanocomposites: Nanomechanical and Thermomechanical Study on Polycarbonate/Boehmite Electrospun Fibers Embedded in Epoxy Resin

The reinforcing effect of boehmite nanoparticles (BNP) in epoxy resins for fiber composite lightweight construction is related to the formation of a soft but bound interphase between filler and polymer. The interphase is able to dissipate crack propagation energy and consequently increases the fracture toughness of the epoxy resin. Usually, the nanoparticles are dispersed in the resin and then mixed with the hardener to form an applicable mixture to impregnate the fibers. If one wishes to locally increase the fracture toughness at particularly stressed positions of the fiber-reinforced polymer composites (FRPC), this could be done by spraying nanoparticles from a suspension. However, this would entail high costs for removing the nanoparticles from the ambient air. We propose that a fiber fleece containing bound nanoparticles be inserted at exposed locations. For the present proof-of-concept study, an electrospun polycarbonate nonwoven and taurine modified BNP are proposed. After fabrication of suitable PC/EP/BNP composites, the thermomechanical properties were tested by dynamic mechanical analysis (DMA). Comparatively, the local nanomechanical properties such as stiffness and elastic modulus were determined by atomic force microscopy (AFM). An additional investigation of the distribution of the nanoparticles in the epoxy matrix, which is a prerequisite for an effective nanocomposite, is carried out by scanning electron microscopy in transmission mode (TSEM). From the results it can be concluded that the concept of carrier fibers for nanoparticles is viable