Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering

The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EA-TB(CH3) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H2) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m2/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH3) is higher than that for the demethylated counterpart PIM-EA-TB(H2). The additional contribution found for PIM-EA-TB(CH3) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH3) has methyl groups where PIM-EA-TB(H2) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules

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

Räumliche Inhomogenität, Oberflächen und komplexes Verglasungsverhalten von Epoxidharzen und entsprechenden Nanokompositen mittels breitbandiger dielektrischer Spektroskopie und Hyphenated Kalorimetrie

Polymer nanocomposites (PNCs) with inorganic nanofillers dispersed in a polymer matrix have been widely studied from the 1990s, since the pioneering work by Toyota Central Research. The possibility of producing advanced tailor-made, light weight and low-cost materials, inspired academic and commercial research towards numerous potential applications, facilitating PNCs to become a billion-dollar global industry. The introduction of nanoparticles (NPs) to a polymer matrix is expected to result in improved properties. The outstanding performance of PNCs is determined not only by the characteristics of the used components but also by their phase morphology, including the dispersion of NPs and interfacial properties. Understanding of structure-property relationships is particularly important for polymer nanocomposites with high industrial significance, such as epoxy-based materials reinforced with inorganic nanofillers. These PNCs have been successfully adopted by the marine, automotive and aerospace industries, although they are still rarely studied on a fundamental level. Therefore, this thesis aims for a detailed understanding of the structure, molecular mobility and vitrification kinetics first, of two epoxy-based materials with different network structures and second, of the corresponding nanocomposites with different alumina-based nanofillers. The first system considered (EP/T-LDH) was based on bisphenol A diglycidyl ether (DGEBA) cured with diethylenetriamine (DETA) and taurine-modified layered double hydroxide (T-LDH) NPs. The taurine molecule bears additional functionalities that could enhance the interactions between the matrix and the nanofiller, improving the interphase formation. The seconds system (EP/BNP) was based on DGEBA and methyl tetrahydrophtalic acid anhydride (MTHPA) as a hardener, reinforced with boehmite nanoparticles (BNPs). The comparison of the two systems enables for a comparative study on the effect of different hardeners and the morphology and modification of the alumina-based nanofillers on the material behavior. The materials were investigated employing complementary techniques with different sensitivities and frequency windows. The following methodology was used: transmission electron microscopy (TEM), small – and wide – angle X-ray scattering (SAXS/WAXS), broadband dielectric spectroscopy (BDS), calorimetry in a form of conventional DSC and fast scanning calorimetry (FSC), as well as specific heat spectroscopy (SHS) in a form of temperature modulated DSC, temperature modulated FSC and static FSC by calculating the thermal relaxation rates from the cooperativity approach.Polymer-Nanokomposite (PNCs) mit anorganischen Nanofüllstoffen, die in einer Polymermatrix dispergiert sind, werden seit der Pionierarbeit der Toyota-Zentralforschung in den 1990 Jahren, untersucht. Die Möglichkeit der Herstellung fortschrittlicher, maßgeschneiderter, leichtgewichtiger und kostengünstiger Materialien inspirierte die akademische und kommerzielle Forschung. Diese Materialien besitzen zahlreiche potenzielle Anwendungen und machen PNCs zu einem milliardenschweren globalen Markt. Die Einmischung von Nanopartikeln (NPs) in eine Polymermatrix führt zu verbesserten Eigenschaften des Kompositsystems im Vergleich zur ungefüllten Matrix. Die exzellenten Eigenschaften von PNCs werden nicht nur durch die Eigenschaften der verwendeten Komponenten bestimmt, sondern auch durch ihre Phasenmorphologie, einschließlich der Dispersion der NPs in der Matrix und der Grenzflächeneigenschaften der NPs. Ein Verständnis der Struktur-Eigenschafts-Beziehungen ist besonders wichtig für Polymer-Nanokomposite mit hoher industrieller Bedeutung, wie z.B. Materialien auf Epoxidbasis, die mit anorganischen Nanofüllstoffen verstärkt sind. Diese PNCs können erfolgreich von der Schiffs-, Automobil- als auch in der Luft- und Raumfahrtindustrie eingesetzt werden, obwohl sie noch selten grundlegend untersucht werden. Daher zielt diese Arbeit auf ein detailliertes Verständnis der Struktur, der molekularen Beweglichkeit und der Kinetik des Glasübergangs erstens von zwei epoxidbasierten Materialien mit unterschiedlichen Netzwerkstrukturen und zweitens von den entsprechenden Nanokompositen mit unterschiedlichen aluminiumoxidbasierten Nanofüllstoffen ab. Das erste System (EP/T-LDH) basiert auf Bisphenol-A-Diglycidylether (DGEBA), welcher mit Diethylentriamin (DETA) vernetzt wird. Hier wird Taurin-modifiziertes Layered Double Hydroxide (T-LDH) als NP verwendet. Das Taurinmolekül hat zusätzliche Aminfunktionalitäten, die die Wechselwirkungen zwischen der Matrix und dem Nanofüllstoff verstärken und die Interphasenbildung verbessern können. Das zweite System (EP/BNP) basierte auf DGEBA und Methyltetrahydrophtalsäureanhydrid (MTHPA) als Vernetzer. Als Nanopartikel wird Böhmit (BNPs) verwendet. Ein Vergleich der beiden Systeme ermöglicht eine fundierte Studie über die Wirkung der verschiedenen Vernetzer, die Morphologie und Modifikation der Nanofüllstoffe auf das Materialverhalten. Die Materialien wurden mit komplementären Techniken, die unterschiedliche Empfindlichkeiten und Frequenzfenstern besitzen, untersucht. Die folgenden Methoden wurde angewandt: Transmissionselektronenmikroskopie (TEM), Klein- und -weitwinkel-Röntgenstreuung (SAXS/WAXS), Breitbandige dielektrische Spektroskopie (BDS), Kalorimetrie (konventionelle DSC und Fast Scannig Kalorimetrie (FSC)), sowie spezifische Wärmespektroskopie (SHS) (temperaturmodulierte DSC, temperaturmodulierte FSC und statische FSC. Für die letztere Methode wurden die thermischen Relaxationsraten aus den Heizraten berechnet.DFG, 232311024, Wirkprinzipien nanoskaliger Matrixadditive für den Faserverbundleichtba

Confinement and localization effects revealed for thin films of the miscible blend poly(vinyl methyl ether)/polystyrene with a composition of 25/75 wt%

Thin films (200-7nm) of the asymmetric polymer blend poly(vinyl methyl ether) (PVME)/polystyrene (PS) (25/75wt%) were investigated by broadband dielectric spectroscopy (BDS). Thicker samples ($\ge$37 nm) were measured by crossed electrode capacitors (CEC), where the film is capped between Al-electrodes. For thinner films ($\le$37 nm) nanostructured capacitors (NSC) were employed, allowing one free surface in the film. The dielectric spectra of the thick films showed three relaxation processes ( $\alpha_{{2}}^{}$ -, $\alpha_{{1}}^{}$ - and $\alpha^{{\prime}}_{}$ -relaxation), like the bulk, related to PVME fluctuations in local spatial regions with different PS concentrations. The thickness dependence of the $\alpha_{{2}}^{}$ -process for films measured by CECs proved a spatially heterogeneous structure across the film with a PS-adsorption at the Al-electrodes. On the contrary, for the films measured by NSCs a PVME segregation at the free surface was found, resulting in faster dynamics, compared to the CECs. Moreover, for the thinnest films ($\le$26 nm) an additional relaxation process was detected. It was assigned to restricted fluctuations of PVME segments within the loosely bounded part of the adsorbed layer, proving that for NSCs a PVME enrichment takes place also at the polymer/substrate interface

Graphene Oxide/Polyvinyl Alcohol–Formaldehyde Composite Loaded by Pb Ions: Structure and Electrochemical Performance

An immobilization of graphene oxide (GO) into a matrix of polyvinyl formaldehyde (PVF) foam as an eco-friendly, low cost, superior, and easily recovered sorbent of Pb ions from an aqueous solution is described. The relationships between the structure and electrochemical properties of PVF/GO composite with implanted Pb ions are discussed for the first time. The number of alcohol groups decreased by 41% and 63% for PVF/GO and the PVF/GO/Pb composite, respectively, compared to pure PVF. This means that chemical bonds are formed between the Pb ions and the PVF/GO composite based on the OH groups. This bond formation causes an increase in the Tg values attributed to the formation of a strong surface complexation between adjacent layers of PVF/GO composite. The conductivity increases by about 2.8 orders of magnitude compared to the values of the PVF/GO/Pb composite compared to the PVF. This means the presence of Pb ions is the main factor for enhancing the conductivity where the conduction mechanism is changed from ionic for PVF to electronic conduction for PVF/GO and PVF/GO/Pb

The origin of delayed polymorphism in molecular crystals under mechanochemical conditions

Control over ball milling transformations is needed before the transformative potential of mechanochemical processing can be realized. Many parameters are known to affect the outcome of mechanochemical polymorphism, yet the energy of ball milling is itself often overlooked. We here demonstrate how milling energy can exert significant influence over the polymorphic outcome of ball mill grinding and be used to control the overall reaction profile. Milling energy exerts its effect on the reaction profile by changing the rate at which structural defects form in crystalline phases. These defects destabilize a crystal to drive the system step-by-step towards polymorphic transformation. Our results demonstrate decisively that careful design and interpretation of ball milling experiments are necessary to obtain control over mechanochemical polymorphis

The origin of delayed polymorphism in molecular crystals under mechanochemical conditions

We show that mechanochemically driven polymorphic transformations can require extremely long induction periods, which can be tuned from hours to days by changing ball milling energy. The robust design and interpretation of ball milling experiments must account for this unexpected kinetics that arises from energetic phenomena unique to the solid state. Detailed thermal analysis, combined with DFT simulations, indicates that these marked induction periods are associated with processes of mechanical activation. Correspondingly, we show that the pre-activation of reagents can also lead to marked changes in the length of induction periods. Our findings demonstrate a new dimension for exerting control over polymorphic transformations in organic crystals. We expect mechanical activation to have a much broader implication across organic solid-state mechanochemistry

Unraveling the Dynamics of Nanoscopically Confined PVME in Thin Films of a Miscible PVME/PS Blend

Broadband dielectric spectroscopy (BDS) was employed to investigate the glassy dynamics of thin films (7–200 nm) of a poly­(vinyl methyl ether) (PVME)/polystyrene (PS) blend (50:50 wt %). For BDS measurements, nanostructured capacitors (NSCs) were employed, where films are allowed a free surface. This method was applied for film thicknesses up to 36 nm. For thicker films, samples were prepared between crossed electrode capacitors (CECs). The relaxation spectra of the films showed multiple processes. The first process was assigned to the α-relaxation of a bulklike layer. For films measured by NSCs, the rates of α-relaxation were higher compared to those of the bulk blend. This behavior was related to the PVME-rich free surface layer at the polymer/air interface. The second process was observed for all films measured by CECs (process X) and the 36 nm film measured by NSCs (process X2). This process was assigned to fluctuations of constraint PVME segments by PS. Its activation energy was found to be thickness-dependent because of the evidenced thickness dependency of the compositional heterogeneity. Finally, a third process with an activated temperature dependence was observed for all films measured by NSCs (process X1). It resembled the molecular fluctuations in an adsorbed layer found for thin films of pure PVME, and thus, it is assigned accordingly. This process undergoes an extra confinement because of frozen adsorbed PS segments at the polymer/substrate interface. To our knowledge, this is the first example where confinement-induced changes were observed by BDS for blend thin films

Molecular mobility in high‐performance polynorbornenes: A combined broadband dielectric, advanced calorimetry, and neutron scattering investigation*

The molecular dynamics of two addition type polynorbornenes, exo-PNBSi and PTCNSi1, bearing microporosity has been investigated by broadband dielectric spectroscopy, fast scanning calorimetry, and neutron scattering. Both polymers have the same side groups but different backbones. Due to their favorable transport properties, these polymers have potential applications in separation membranes for gases. It is established in literature that molecular fluctuations are important for the diffusion of small molecules through polymers. For exo-PNBSi, two dielectric processes are observed, which are assigned to Maxwell/Wagner/Sillars (MWS) process due to blocking of charge carriers at internal voids or pore walls. For PTCNSi1, one MWS-polarization process is found. This points to a bimodal pore-size distribution for exo-PNBSi. A glass transition for exo-PNBSi and for PTCNSi1 could be evidenced for the first time using fast scanning calorimetry. For Tg and the corresponding apparent activation energy, higher values were found for PTCNSi1 compared to exo-PNBSi. For both polymers, the neutron scattering data reveal one relaxation process. This process is mainly assigned to methyl group rotation probably overlayed by carbon–carbon torsional fluctuations