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One of the biggest problems in high-voltage silicone rubber insulation cable accessories is the damage on electrical treeing, initiated by partial discharges. The electrical treeing starts at unavoidable imperfections inside the material or at interfaces. The damage is usually irreversible and leads around the starting points to a partial destruction of the material. To prolong the lifetime and thereby to increase the assurance of the structural component commonly, for mechanical improvement constituted filler is modified to obtain a self-healing silicone rubber. Damage mechanism is analysed to address the filler to the damage mode.
Der Vortrag gibt den aktuellen Stand der Normung im Bereich der Oberflächen-technik insbesondere der Oberflächenanalytik und der Oberflächenmess- und -prüftechnik wider.
Vorgestellt wird der Status technischer Berichte, Normenentwürfe und Normen u.a. zur Charakterisierung von funktionellen Glasoberflächen für bioanalytische Anwendungen, zur Messung der Schichtdicke von Nanomaterialien mittels Ellipsometrie, zur Kalibrierung von Interferometern und Interferenzmikroskopen für die Formmessung, zur Rockwelleindringprüfung zur Bewertung der Schicht-haftung, zu den Grundlagen der Ellipsometrie, zur Messung der Schichtdicke mittels Wirbelstromverfahren, zur Schichtpotentialmessung von Mehrfach-Nickelschichten, zu Filtern und Augenschutzgeräten gegen Laserstrahlung sowie zur Bestimmung der Schichthaftung mittels Zugversuch.
Branched Polyurethanes Based on Synthetic Polyhydroxybutyrate with Tunable Structure and Properties
(2018)
Branched, aliphatic polyurethanes (PURs) were synthesized and compared to linear analogues. The influence of polycaprolactonetriol and synthetic poly([R,S]-3-hydroxybutyrate) (R,S-PHB) in soft segments on structure, thermal and sorptive properties of PURs was determined.
Using FTIR and Raman spectroscopies it was found that increasing the R,S-PHB amount in the structure of branched PURs reduced a tendency of urethane groups to hydrogen bonding. Melting enthalpies (on DSC thermograms) of both soft and hard segments of linear PURs were higher than branched PURs, suggesting that linear PURs were more crystalline. Oil sorption by samples of linear and branched PURs, containing only polycaprolactone chains in soft segments, was higher than in the case of samples with R,S-PHB in their structure. Branched PUR without R,S-PHB absorbed the highest amount of oil. Introducing R,S-PHB into the PUR structure increased water sorption.
Thus, by operating the number of branching and the amount of poly([R,S]-3-hydroxybutyrate) in soft segments thermal and sorptive properties of aliphatic PURs could be controlled.
The presence of polycarboxylate ether (PCE) based superplasticizers (SPs) has an enormous influence on the early hydration of cement. Therefore, the talk presents first experimental results about the influence of a delayed addition time of PCE SPs on the hydration of cement and tricalcium aluminate (C3A) pastes, investigated by isothermal heat flow calorimetry and in-situ XRD.
The state of fresh concrete is predominantly determined by the rheological properties of the cement paste. In order to control performance spectra and requirements of novel concretes and to better understand macroscopic phenomena, comprehensive knowledge of the material behavior of fresh cement suspensions as well as of the complex relationships of mechanisms at the nano and micro scale are necessary. This work focuses on micro and nano rheology of suspensions of cementitious model systems and the influence of polycarboxylate-based admixtures on the rheology. The phenomena are driven by multiple parameters such as adsorption and particle interactions. Hence, the first part examines the interaction between polycarboxylate ether (PCE) and synthesized clinker phases and hydration products as model systems with regard to early hydration products.
The presence of polycarboxylate ether (PCE) based superplasticizers (SPs) has an enormous influence on the early hydration of cement. The hydration is retarded and the timing of formation and the morphology of hydrates is affected. This short paper presents experimental results about the influence of delayed addition time of PCE SPs on hydration of cement and alite pastes, investigated by isothermal heat flow calorimetry. For cement as well as for alite pastes the hydration is retarded with SP, whereby the high charge PCE has a stronger retarding effect than the low charge PCE. The retardation caused by PCE is much more pronounced for alite than for cement mixes. If PCE is added later to the mix, the induction period is shortened and the hydration is accelerated compared to simultaneous addition. This applies for cement and alite pastes. With delayed PCE addition the alite shows a clearly less retarded setting and main hydration than after simultaneous addition. It is obvious that for alite pastes there is less retardation the later the addition of SP.
The presence of polycarboxylate ether (PCE) based superplasticizers (SPs) has an enormous influence on the early hydration of cement. The hydration is retarded and the timing of formation and the morphology of hydrates is affected. This short paper presents experimental results about the influence of delayed Addition time of PCE SPs on hydration of cement and tricalcium aluminate (C3A) pastes, investigated by isothermal heat flow calorimetry. For cement pastes the hydration is retarded with SP, whereby the high charge PCE has a stronger retarding effect than the low charge PCE. With delayed PCE addition the cement shows a less retarded setting than with simultaneous addition. The alteration caused by PCE is much more pronounced for C3A and gypsum mixes than for cement. If the SP is added simultaneous, the exothermic peak of C3A is retarded. However, with delayed addition of SP the hydration is shortened, the gypsum depletion is fastened and the exothermic peak occurs less retarded or even accelerated compared to simultaneous addition. It is obvious that for C3A pastes there is less retardation the later the Addition of SP. Furthermore, the PCE alter the hydration of C3A when added delayed and exhibit changes in kinetics and hydration rates. The rate of reaction in the second stage is lower, discernible in decreased slopes and broader peaks. Besides this, a distinct ramp in the C3A heat flow curves within the first stage of C3A hydration occurs for all pastes with delayed addition of SP, which suggests an accelerated ettringite formation.
In well-annealed thin polymer films, with non-repulsive polymer/substrate interactions, an irreversibly adsorbed layer is expected to form. These adsorbed layers have shown great potential for technological applications. However, their growth kinetics and molecular dynamics are still not fully understood. This is partially due to the hard accessibility of these layers in thin films. Here, the irreversibly adsorbed layers of homopolymer thin films are revealed by solvent-leaching experiments. First, the growth kinetics of these layers is investigated as a function of annealing times and original film thickness. The thickness, topography and quality of the adsorbed layer is controlled with Atomic Force Microscopy (AFM). Secondly, the molecular mobility of the adsorbed layer is investigated with Broadband Dielectric Spectroscopy (BDS). A recently developed nanostructured capacitor is employed to measure the adsorbed layers with a free surface layer. The results are quantitatively compared and discussed with respect to recently published work.
In well-annealed thin polymer films, with non-repulsive polymer/substrate interactions, an irreversibly adsorbed layer is expected to form. These adsorbed layers have shown great potential for technological applications. However, their growth kinetics and molecular dynamics are still not fully understood. This is partially due to the hard accessibility of these layers in thin films. Here, the irreversibly adsorbed layers of homopolymer thin films are revealed by solvent-leaching experiments. First, the growth kinetics of these layers is investigated as a function of annealing times and original film thickness. The thickness, topography and quality of the adsorbed layer is controlled with Atomic Force Microscopy (AFM). Secondly, the molecular mobility of the adsorbed layer is investigated with Broadband Dielectric Spectroscopy (BDS). A recently developed nanostructured capacitor is employed to measure the adsorbed layers with a free surface layer. The results are quantitatively compared and discussed with respect to recently published work.
In this presentation, we report on the Determination of the glass transition temperature of thin layers of Polyvinylmethylether (PVME) depending on the thickness of the polymer layer. The glass Transition of thin polymer layers is currently under much investigation due to the nano-confinement effects proposed to appear in dependence on the layer thicknesses in the nm range. The properties of the polymer, the temperature range, as well as the thicknesses range of the polymer layers pose a serious challenge to the investigation by means of
spectroscopic ellipsometry. By careful choice of experimental parameters, we were able to investigate the thickness change by temperature of PVME layers in the range between 2 and 300 nm and in the temperature range between 200 K and 340 K. By optimizing the analysis process, we were able to determine Tg values within this parameter range with sufficient accuracy to investigate the Tg change due to confinement effects.
Alongside the change of position of the glass transition with thickness, we discuss the details of the ellipsometric analysis and its implications for the resulting thermal properties of the thin polymer layers as well as the accuracy of the Tg value with respect to the method used in the analysis process.
Commercial grade-1 titanium samples (Ti, 99.6%) were treated using three alternative methods, (i) femtosecond laser processing, (ii) thermal heat treatment, and (iii) electrochemical anodization, respectively, resulting in the formation of differently conditioned superficial titanium oxide layers. The laser processing (i) was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning setup for the processing of several square-millimeters large surface areas covered homogeneously by these nanostructures. The differently oxidized titanium surfaces were characterized by optical microscopy, micro Raman spectroscopy, variable angle spectroscopic ellipsometry, and instrumented indentation testing. The tribological performance was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in fully formulated engine oil as lubricant. The specific tribological performance of the differently treated surfaces is discussed with respect to possible physical and chemical mechanisms.
Die Rheologie von fließfähigen zementären Systemen mit Fließmitteln wird durch eine Vielzahl parallel stattfindender Effekte beeinflusst. Zu diesen Effekten zählen Wechselwirkungen zwischen den Polymeren und Ionen in der Porenlösung, frühe Phasenbildung, zeitabhängige und kompetitive Adsorption zwischen anionischen Polymeren und Sulfationen, Bildung von Phasen in der Porenlösung sowie Morphologieänderungen an Partikeloberflächen. Die frühe Hydratation von Zement, die durch Lösungs- und Fällungsprozesse angetrieben wird, beeinflusst diese Effekte erheblich. Das permanente Ungleichgewicht der Porenlösung führt zu Veränderungen der Partikeloberflächen, welches widerum zur Folge hat, dass rheometrische Messungen dieser Zementleime anfällig für Streuungen sind. Um die Einflüsse aus der Zementhydratation zu minimieren, wurden die rheometrischen Untersuchungen mit Zement in Wasser, wässriger Phase von Zementleim bzw. mit vorhydratisiertem Zement durchgeführt. Zusätzlich wurden die gleichen Systeme mit Zugabe von Polycarboxylatethern untersucht.
Die Ergebnisse zeigen, dass die Leime mit Wasser niedrigere Werte für Fließgrenze und plastische Viskosität aufweisen, als die Systeme mit der wässrigen Phase von Zementleim. Während die Polymere eine Verminderung der Fließgrenze zur Folge hatte, war die Wirkung von Polymeren auf die plastische Viskosität vernachlässigbar. Zusätzlich wurde die frühe Hydratation unter Verwendung von Wärmeflusskalorimetrie, Rasterelektronenmikroskopie und Nadeleindringtiefe beobachtet.
Die Rheologie von fließfähigen zementären Systemen mit Fließmitteln wird durch eine Vielzahl parallel stattfindender Effekte beeinflusst. Zu diesen Effekten zählen Wechselwirkungen zwischen den Polymeren und Ionen in der Porenlösung, frühe Phasenbildung, zeitabhängige und kompetitive Adsorption zwischen anionischen Polymeren und Sulfationen, Bildung von Phasen in der Porenlösung sowie Morphologieänderungen an Partikeloberflächen. Die frühe Hydratation von Zement, die durch Lösungs- und Fällungsprozesse angetrieben wird, beeinflusst diese Effekte erheblich. Das permanente Ungleichgewicht der Porenlösung führt zu Veränderungen der Partikeloberflächen, welches widerum zur Folge hat, dass rheometrische Messungen dieser Zementleime anfällig für Streuungen sind. Um die Einflüsse aus der Zementhydratation zu minimieren, wurden die rheometrischen Untersuchungen mit Zement in Porenlösung durchgeführt. Die Experimente wurden mit verschiedenen Feststoffvolumenfraktionen durchgeführt und mit den Ergebnissen identischer Systeme mit Wasser anstelle von Porenlösung verglichen. Zusätzlich wurden die gleichen Systeme mit Zugabe von Polycarboxylatethern untersucht.
Die Ergebnisse zeigen, dass die Leime mit Wasser niedrigere Werte für Fließgrenze und plastische Viskosität aufweisen, als die Systeme mit Porenlösung. Während die Polymere eine Verminderung der Fließgrenze zur Folge hatte, war die Wirkung von Polymeren auf die plastische Viskosität vernachlässigbar. Zusätzlich wurde die frühe Hydratation unter Verwendung von Wärmeflusskalorimetrie, Rasterelektronenmikroskopie und Nadeleindringtiefe beobachtet.
Hyperbranched PAMAM/ Kaolinite Nanocomposites: Decoupling phenomenon and conductivity mechanism
(2018)
Increasing demands of the daily life requires a continuous discovering of new and tailored properties of materials that can be utilized in covering the requirements in several fields. Hyperbranched polymers (HBPs) are macromolecules that are characterized by a highly branched structure and multiplicity of reactive end groups, which could be promising for numerous applications. Here, hyperbranched poly(amidoamine) (HPAMAM)/ Ka nanocomposites was prepared via an in-situ polymerization and an ex-situ method. The latter approach leads to a partly intercalated structure of the nanocomposites, while the former method results in an exfoliated morphology. Α combination of different techniques such as broadband dielectric spectroscopy (DBS), SXAS, FTIR, TEM, and DSC are employed to study the prepared samples. For the HPAMAM/ Ka-DCA nanocomposites (an ex-situ samples), the results indicated that the dc conductivity is increased by 4 orders of magnitude, with increasing concentration of the Ka-DCA. As an interesting result is that a significant decoupling between the characteristic time for conductivity relaxation and the segmental dynamics was observed, which depends on the concentration of the nanofiller. For the HPAMAM/ EDA nanocomposites (an in-situ polymerization), the dc conductivity is also increased with increasing the concentration of the Ka-EDA. The decoupling phenomenon and conductivity mechanism are discussed in detail.
Hyperbranched polyamine ester (HPAE)/ kaolinite nanocomposites were papered via an ex situ (solution-based) method. The kaolinite has been modified by dodecylamine (DCA). SAXS measurements revealed that the Ka interlayer space increased from 0.71 to 3.6 nm-1. A partly exfoliated structure of the HPA/Ka-DCA nanocomposites was proved by SAXS and TEM. By a combination of BDS and SHS, the relaxation properties of the nanocomposites were investigated in dependence on frequency and temperature. The activation energies of γ-relaxation for the nanocomposites were lower than the values found for the pure HPAE. The segmental dynamics (α- relaxation) was found to be screened out by the conductivity contribution. While it is retrieved by SHS employing AC-chip calorimetry. A systematic change of the dynamic glass transition estimated by AC-chip calorimetry was observed, which is in agreement with a behavior expected for a confined sample. The confinement effect of the Ka-DCA nanofillers reduces the glass transition temperature Tg and enhances, meanwhile, the electrical conductivity of the polymer. By comparing the temperature dependence of the dynamic glass transition measured with SHS and that of the dc conductivity measured by dielectric spectroscopy, a decoupling in their temperature dependencies was found.
With increasing concentration of the nanofiller, which results in a stronger glass-formation behavior, this decoupling becomes weaker.
Dielectric spectroscopy (BDS) was employed to investigate the dynamics of thin films (7 – 200 nm) of a Poly (vinyl methyl ether) (PVME) / Polystyrene (PS) blend (50:50 wt%). For the BDS measurements Nano-Structured Capacitors (NSC) were employed, where films have a free surface. This method was applied for film thicknesses up to 36 nm. Thicker films were prepared between Crossed Electrodes Capacitors (CEC). The spectra of the films showed multiple processes. The first process was assigned to the -relaxation of a bulk-like layer. For films measured by NSC, its rates were higher compared to that of the bulk blend. This behavior was related to a PVME-rich free-surface layer. A second process was observed for films measured by CEC (process X) and the 36 nm film measured by NSC (process X2). This process was assigned to fluctuations of PVME constraint by PS. Its activation energy was found to be thickness dependent, due to the evidenced thickness dependency of the compositional heterogeneity. Finally, a third process with an activated temperature-dependence was observed for all films measured by NSC (process X1). It resembled the molecular fluctuations in an adsorbed layer found for films of pure PVME.
Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. Here, by means of inelastic neutron scattering, the vibrational density of states (VDOS) and the molecular mobility were investigated for PIM-1, the prototypical polymer with intrinsic microporosity. The results are compared to data measured for a more conventional high-performance polyimide used in gas separation membranes (Matrimid).
The measured data show the characteristic low frequency excess contribution to the VDOS above the Debye sound wave level, generally known as the Boson peak in glass-forming materials. In comparison to the Boson peak of Matrimid, that of PIM-1 is shifted to lower frequencies. This shift is discussed considering the microporous, sponge-like structure of PIM-1 as providing a higher compressibility at the molecular scale than for conventional polymers.
Elastic fixed window scans were measured on a neutron backscattering spectrometer to have an overview about the molecular dynamics at a time scale of ca. 1 ns. The temperature dependence of the estimated mean squared displacement shows a step-like increase in the temperature range from 100 K to 250 K indicating the onset of some molecular mobility. The nature of this motional process was analyzed in detail by quasielastic neutron scattering where the data are discussed with regard to both the q- and the temperature dependence.
In well-annealed thin polymer films, with non-repulsive polymer/substrate interactions, an irreversibly adsorbed layer is expected to from. These adsorbed layers have shown greate potential for technological applications. However, their growth kinetics and molecular dynamics are still not fully understood. This is partly due to the hard accessibility of these layers in thin films. Here, the irreversibly adsorbed layers of homopolymer thin films are revealed by solvent-leaching experiments. First, the growth kinetics of these layers is investigated as a function of annealing time, annealing temperature, leaching time and the original filme thickness. 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 recently developed nanostructured capacitor arrangement is employed to measure the layer with a free surface. The results are quantitatively compared and discussed with respect to recently published work.
The basics of broadband dielectric spectroscopy were introduced in detail. The analysis of the data was discussed. As application of broadband dielectric spectroscopy the alpha-relaxation (dynamic glass transition) and the chain dynamics of polymers were ilustrated. Further the application of dielectric spectroscopy to polymerbased nanocomposites and high preformance polymers was discussed in detail.
Polymers at interfaces play a major role in a broad variety of applications ranging from engineering purposes (for instance polymer based nanocomposites) to high tech implications (for instance light emitting diodes).
Here, thin films with thicknesses down to few nanometers are prepared on different substrates as model systems for polymer composites. The thin films are investigated by a combination of surface analytical and volume sensitive methods. As surface analytical methods atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements (CAM) are employed. As volume sensitive broadband dielectric spectroscopy (BDS), specific heat spectroscopy (SHS), and ellipsometry are used. Especially attention is paid to understand the glass transition behavior of thin films because the glass transition is the key phenomenon determines the application of polymers also in thin films.
To understand the glass transition behavior of thin films, which is controversially discussed in literature, a three layer model is discussed. Firstly, a mobile surface layer is assumed at polymer air interface of the film due to missing of segment/segment interactions. Secondly, in the middle of the film a bulk-like layer should be expected. Thirdly, for polymers having non-repulsive interactions with the substrate an irreversibly adsorbed layer is expected to be formed. Due the adsorption the molecular mobility of the segments in this layer is slowed down. What is measured for the glass transition of thin films is a complicated average of all of these effects. The different layers are hardly to address separately. Therefore, in the presentation especially model systems are selected and investigated to verify the layer model.
A combination of different complementary methods is employed to investigate scaling of the molecular dynamics of two different liquid crystals. Each method is sensitive to different kind of fluctuations and provides therefore a different window to look at the molecular dynamics. In detail, broadband dielectric spectroscopy is combined with specific heat spectroscopy and neutron scattering. As systems the nematic liquid crystal E7 and a discotic liquid crystalline pyrene are considered. First of all it was proven that both systems show all peculiarities which are characteristic for glassy dynamics and the glassy state. Especially for the nematic liquid crystal E7 it could be unambiguously shown by a combination of dielectric and specific heat spectroscopy that the tumbling mode is the underlying motional process responsible for glassy dynamics. Dielectric investigations on the discotic liquid crystalline pyrene reveal that at the phase transition from the plastic crystalline to the hexagonal columnar liquid crystalline phase the molecular dynamics changes from a more strong to fragile temperature dependence of the relaxation rates. Moreover a combination of results obtained by specific heat spectroscopy with structural methods allows an estimation of the length scale relevant for the glass transition.
Disklike molecules with aromatic cores spontaneously stack up in linear columns with high, onedimensional charge carrier mobilities along the columnar axes, making them prominent model systems for functional, self-organized matter.We show by high-resolution optical birefringence and synchrotron-based x-ray diffraction that confining a thermotropic discotic liquid crystal in cylindrical nanopores induces a quantized formation of annular layers consisting of concentric circular bent columns, unknown in the bulk state. Starting from the walls this ring self-assembly propagates layer by layer towards the pore center in the supercooled domain of the bulk isotropic-columnar transition and thus allows one to switch on and off reversibly single, nanosized rings through small temperature variations. By establishing a Gibbs free energy phase diagram we trace the phase transition quantization to the discreteness of the layers’ excess bend deformation energies in comparison to the thermal energy, even for this near room-temperature system.
Monte Carlo simulations yielding spatially resolved nematic order parameters, density maps, and bondorientational order parameters corroborate the universality and robustness of the confinement-induced columnar ring formation as well as its quantized nature.
In recent years, substantial efforts have been devoted to investigating nanoscopic confinement of polymers, and its effect on glassy dynamics. Broadband Dielectric Spectroscopy (BDS) was used to study the dynamics of ultra-thin films of PVME/PS 50/50 wt% blend, employing a novel nano-structured capacitor sample arrangement. The investigated system shows a complex dynamic behavior. First, an α-relaxation, related to a bulk-like layer was found. Second, an α’-relaxation was observed, characteristic for dynamically asymmetric blends, where the out of equilibrium dynamics is attributed to weakly-cooperative PVME segments relaxing within a frozen environment of PS segments. Thirdly, for thinnest films, an Arrhenius-like process was dominant in the dielectric spectra, indicating localized fluctuations of the segments. Relaxation rates of this process resembled that of the degenerated α-relaxation of the adsorbed layer, found for pure PVME, thus it was assigned accordingly. For thinnest films, this process undergoes a further confinement, due to the topological constraints, introduced by PS. Such multiple confinement effect has not been reported for ultra-thin films of polymer blends, before this study.
The polymer matrix region near a filler surface, termed as the interface, witnessed increasing interest, due to its possible influence on the macroscopic properties of the nanocomposite. The interphase is expecting to have different segmental dynamic, as compared to the pure matrix, which can percolate into the entire system. Here, the segmental dynamics of epoxy/Boehmite nanocomposite was studied by Broadband Dielectric Spectroscopy. It was found that an artificial relaxation process is present in the nanocomposite, on the contrary to the pure epoxy system. It was assigned to constrained fluctuations of polymer chains in the interfacial region, due to the nanofiller. However, the overall dynamic Tg of the system decreased with increasing filler concentration, indicating higher segmental mobility. This was in accordance with Temperature Modulated DSC investigations of specific heat capacity of the system, which was found to increase with increasing filler concentration, up to 10 wt%, indicating increasing mobility of the polymer matrix segments. Surprisingly, for the highest filler content, the heat capacity decreases, implying a formation of an immobilized rigid amorphous phase in the interfacial region.
The structure and molecular dynamics of thin polymer films are of topical interest of soft matter-physics. Commonly, spatial structural heterogeneities of 1D confined thin films (surface, bulk-like and adsorbed layer), are expected to alter the glassy dynamics, compared to the bulk. Here, Specific Heat Spectroscopy (SHS) was used, to investigate the glassy dynamics of thin films of an asymmetric miscible PVME/PS 25/75 wt% blend. SHS measurements showed a non-monotonous thickness dependence of the dynamic Tg, on the contrary to the previously investigated PVME/PS 50/50 wt%. For PVME/PS 25/75 wt% thin films (> 30 nm), due to the presence of PVME-rich adsorbed and surface layers, the bulk-like layer experienced a thickness dependent increase of PS concentration. This led to a systematic increase of dynamic Tg. Further decrease of the film thickness (< 30 nm), resulted in a decrease of dynamic Tg, ascribed to the influence of the surface layer, which has a high molecular mobility. This is the first study, which shows deviations of dynamic Tg of thin films, compared to the bulk, resulting from the counterbalance of the free surface and adsorbed layer.
The interphase between the inorganic filler and the polymer matrix is considered as one of the most important characteristics of inorganic/polymer nanocomposites. The segmental dynamics of this interphase is expected to be altered as compared to the pure matrix, which might percolate into the entire system. For instance, it was found that a so-called Rigid Amorphous Phase (RAF) is formed by adsorption of segments onto the nanoparticles yielding to its immobilization. The RAF is available from the decrease of the specific heat capacity Δcp in the glass transition region of the nanocomposites. Here, precise Temperature Modulated DSC (TMDSC) was employed to study Δcp of epoxy/Boehmite nanocomposites with different nanofiller concentrations. Surprisingly, the investigated system showed an increase of Δcp with increasing filler concentration up to 10 wt%. This implies an increased fraction of mobile segments, and is in accordance with the found decreased value of the glass transition temperature Tg. Although for higher filler contents Tg further slightly decreases, Δcp decreases in contrary, indicating a formation of RAF. This behavior was discussed as a competition of mobilization effects, due to an incomplete crosslinking reaction, and the formation of RAF.
Over the past decades research on the molecular dynamics of miscible polymer blends are of topical interest in the literature, in an attempt to understand the segmental mobilty of individual components, as it is affected by blending. In general, miscible polymer blends exhibit a complex behavior of the molecular mobility. For an A/B blend the relaxation times of component A and component B are affected by the spatial local compositional heterogeneity, present in binary systems on a microscopic level, regardless of the macroscopic homogeneity. Here, a combination of broadband dielectric and specific heat spectroscopy was employed to study the dynamically asymmetric PVME/PS blend with seven different compositions, focusing on samples with high PS contents. Considering that PS is dielectrically invisible, BDS is a powerful technique to study the response of PVME, as it is affected by PS segments. In this work the well-known binary relaxation times distribution of PVME in a blend, originating from the spatial local heterogeneity, was studied over ten decades in frequency, for the first time in literature. Secondly, one of the detected processes, α’-relaxation, shows a crossover from high-temperature behavior (system in equilibrium) towards a low temperature regime, where PS undergoes the thermal glass transition, resulting in confined segmental dynamics of PVME within a frozen network of PS. Here, we introduce a precise mathematical tool to distinguish between the temperature dependency regimes of the process, and examine the composition dependence of the crossover temperature, detected by dielectric spectroscopy. Moreover, the dielectric data was compared in detail with results obtained by specific heat spectroscopy. This comparison provides new insights in the dynamics and dynamic heterogeneity of the PVME/PS blend system.
Dynamics of nanoscopically confined PVME in thin films of an asymmetric miscible PVME/PS blend
(2018)
In recent years, substantial efforts have been devoted to investigating nanoscopic confinement of polymers, and its effect on glassy dynamics. Broadband Dielectric Spectroscopy (BDS) was used to study the dynamics of ultra-thin films of PVME/PS 50/50 wt% blend, employing a novel nano-structured capacitor sample arrangement. The investigated system shows a complex dynamic behavior. First, an α-relaxation, related to a bulk-like layer was found. Second, an α’-relaxation was observed, characteristic for dynamically asymmetric blends, where the out of equilibrium dynamics is attributed to weakly-cooperative PVME segments relaxing within a frozen environment of PS segments. Third, for thinnest films, an Arrhenius-like process was dominant in the dielectric spectra, indicating localized fluctuations of the segments. Relaxation rates of this process resembled that of the degenerated α-relaxation of the adsorbed layer, found for pure PVME [1], thus it was assigned accordingly. For thinnest films, this process undergoes a further confinement, due to the topological constraints, introduced by PS. Such multiple confinement effect has not been reported for ultra-thin films of polymer blends, before this study [2].
[1] Madkour, S. et al. ACS Appl. Mater. Interfaces 2017, 9, 7535.
[2] Madkour, S. et al. ACS Appl. Mater. Interfaces 2017, 9, 37289.
In inorganic/polymer nanocomposites the polymer matrix region near a filler surface, termed as the interphase, is of topical interest due to its possible influence on the macroscopic properties of the material. The segmental dynamics of this interphase is expected to be altered, as compared to the pure matrix, which might percolate into the entire system. It was found that a so-called Rigid Amorphous Phase (RAF) is formed in the interfacial region by adsorption of the polymer segments onto the nanoparticles, yielding in their immobilization. Here, we employed a combination of two relaxation spectroscopy techniques (Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS) in a form of Temperature Modulated DSC (TMDSC)) to investigate the structure and molecular mobility of Epoxy/Inorganic nanofiller composites with different nanoparticles geometries and contents. The two techniques show different perspectives on the glassy dynamics; BDS is sensitive to dipole fluctuations, whereas SHS senses entropy fluctuations.
First, our dielectric relaxation investigations proved an existence of an additional process in nanocomposites, which is not present in the pure material. Due to the increasing intensity of the process with increasing filler content it was assigned as the α-process related to the segmental dynamics of polymer chains adsorbed onto the nanoparticles. Considering the expected high conductivity effects of the material, the dielectric data were analyzed by fitting a derivative of the HN function to a “conduction-free” loss spectra: ε''deriv=-(∂ε'/∂logω).
Second, TMDSC measurements were used to study the specific heat capacity of nanocomposites in its nanofiller content dependence. Assuming that RAF is proportional to the decrease of the specific heat capacity step (Δcp) in the glass transition region of the nanocomposites, comparing to the pure material, the inorganic/polymer interphase was quantitatively analyzed and the amount of RAF estimated.
The effects of weathering exposure on unfilled and filled thermoplastic polyurethanes (TPU) materials are described as performed under different humidity conditions. For this purpose, a weathering device was used with UV-A 340 nm lamps at a constant temperature of 40 °C. The effects of environmental (UV and humidity condition) degradation on the frictional properties of TPU materials are presented along with surface analyses to characterize the chemistry of the degradative process. Photooxidative degradation of unfilled polymer leads to deterioration of physical and mechanical properties, which affects its tribological behavior significantly. Due to crosslinking, the stiffness of the material increases, reducing drastically the friction coefficient of unfilled TPUs. The frictional behavior of glass fiber reinforced TPU is less affected by radiation.
Hybrid materials have attracted growing interest during the last decade, particularly due to their extraordinary properties. Cycloalyphatic-epoxy oligosiloxane (CEO) resin was shown to be a good candidate as a barrier material for the encapsulation purposes. Incorporation of inorganic nanoparticles such as Boehmite (BA) into polymers was observed to modify their specific characteristics, in particular, thermal, thermo-oxidative and barrier ones. In this work, novel BA-embedded organic inorganic hybrid nanocomposite material was engineered by combining the advantageous properties of hybrid polymers and nanoparticle enhancement effect. Impacts of particles on the photocuring kinetics, degree of crosslinking and the resultant changes in the thermal properties of the cured films were investigated. CEO synthesis via condensation reaction was confirmed by 1H and 29Si NMR. The particle distribution within the films was verified by SEM including transmission mode coupled with EDX elemental analysis. Photocuring kinetics and thermal properties of the films were studied by in situ FTIR spectroscopy and DSC with TGA, respectively.
Organic-inorganic nanostructured materials have drawn much attention over the past decade, particularly due to their versatile and outstanding properties. Possessing the properties between those of polymers and those of glasses, siloxane-based resins are non-toxic, easy to synthesize and process hybrid materials, that hold a promising potential in the field of advanced coatings.
Photocurable resins are nowadays widely used as coatings due to their unique advantages. In particular, cationic ring-opening curing is not inhibited by oxygen, leads to low degree of shrinkage and superior adhesion. The most important, it enables production of high-quality coatings within short exposure times without applying temperature, thus, minimizing the presence of the thermal stress in the substrate. Photocured Cycloaliphatic-Epoxy Oligosiloxane (CEO) resin was reported to be used as encapsulation material for organic electronics. However, further reinforcement of CEO properties is desired to achieve requested device lifetime. One of the common approaches to improve material characteristics is by embedding inorganic nanoparticles into polymer matrix. It has been shown that the resulted nanocomposites exhibit enhanced functional properties included but not limited by optical, mechanical, thermal and barrier ones.
In this work we focused on the incorporation of Boehmite nanoparticles (BA) into CEO matrix as a tool to strengthen the film properties and to study the main changes occurred in the material behavior with regard to its photocuring kinetics, thermal stability and glass transition. Particular interest was focused on the role of particle surface in nanocomposite properties. Hence, BA particles without (HP14) and with organic surface modifier (OS1) at different loadings
(up to 10 wt%) were applied in this study. Morphology investigation with SEM operated in transmission mode showed good BA dispersion forming
network-like structure. At the same time, distribution of particles differed for HP14 and OS1 as a result of different interaction in CEO-solvent-particles system. CEO structure obtained via non-hydrolytic sol-gel reaction was verified by 13C and 29Si NMR. In situ monitoring of film curing was performed using
RT-IR spectroscopy. A slight increase of final convention degree with particle incorporation was observed in contrast to the considerable decrease of curing efficiency reported previously for similar system. Further, the cured hybrid nanocomposite films were analyzed by TGA and DSC, which revealed impact of surface modifier on thermal stability and glass transition temperature.
UV-curing coatings are nowadays widely used due to their unique advantages. High-quality coatings can be obtained at short curing times and low temperatures so that thermal stress to the substrate is minimised. Photocured Cycloaliphatic-Epoxy Oligosiloxane (CEO) resin was reported to be used as encapsulation material for organic electronics. However, further reinforcement of CEO film properties is desired to achieve requested device lifetime.
In this work we introduced Boehmite nanoparticles (BA) into CEO matrix in order to modify the film properties and study the main changes of the material behaviour with regard to its photocuring kinetics, thermal stability and glass transition. Particular interest was focused on the role of particle surface in nanocomposite properties. Hence, BA particles without (HP14) and with organic surface modifier (OS1) at different loadings (up to 10 wt%) were applied in this study. Morphology investigation with SEM operated in transmission mode showed good BA dispersion forming network-like structure. At the same time, distribution of particles differed for HP14 and OS1 as a result of different interaction in CEO-solvent-particles system. CEO structure obtained via non-hydrolytic sol-gel reaction was verified by 13C and 29Si NMR. In situ monitoring of film curing was performed using RT-IR spectroscopy. No significant modification of final convention degree with particle incorporation was observed in contrast to considerable decrease of curing efficiency reported previously for similar system by Esposito et al.,2008. Further, cured hybrid nanocomposite films were analysed by TGA and DSC, which revealed impact of surface modifier on film thermal properties.
For the first time, the molecular mobility of two linear-shaped tetramethylated guanidinium triflates ionic liquid crystals (ILCs) having different length of alkyl chains was investigated by a combination of broadband dielectric spectroscopy (BDS) and specific heat spectroscopy (SHS). SHS was carried out by differential AC-chip calorimetry at higher frequencies and temperature modulated DSC at lower frequencies. These ILCs can form a hexagonal ordered columnar mesophase. Two relaxation processes were found by BDS for both samples. At low temperatures, a γ-processes is observed which is assigned to specific localized fluctuations. At higher temperatures, α1-processes take place. α2 processes were also detected by SHS but with a completely different temperature dependence of the relaxation times. Different molecular assignments of α1- and α2-processes are suggested. At even higher temperatures, conductivity was detected by BDS.
Collective Orientational Order and Phase Behavior of a Discotic Liquid Crystal under Confinement
(2018)
Discotic liquid crystals (DLCs) are a promising class of soft matter for electronic applications. This is due to their ability to self-organize into columns in a hexagonal columnar mesophase, driven by the overlapping of the π orbitals of their aromatic cores. This leads to a high charge-carrier mobility along the column axis. Previous studies on DLCs showed that their properties, such as phase transition temperatures and enthalpies, are susceptible to nanoconfinement. In this study, 2,3,6,7,10,11 hexakis[hexyloxy] triphenylene (HAT6) was confined into parallel aligned cylindrical nanopores of anodic aluminum oxide (AAO) membranes by melt infiltration. Furthermore, the pore surfaces of a series of membranes were chemically modified, resulting in a more hydrophobic pore surface than the unmodified ones. Collective orientational order and phase behavior of HAT6 confined into modified and unmodified nanopores of AAO were investigated by broadband dielectric spectroscopy and differential scanning calorimetry respectively.
The molecular dynamics and ionic conductivity of ionic liquid crystals (ILCs) forming a columnar mesophase, two linear-shaped tetramethylated guanidinium triflates ILCs having different lengths of alkyl chains, were investigated by a combination of Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS). Three processes were detected by BDS; at low temperature γ-process, at higher temperatures α-process and at even higher temperatures conductivity. The γ-process indicates localized fluctuations, and the α-process designates cooperative fluctuations. Slightly different restrictions were found for conductivity processes of LC536 and LC537 due to the slightly different lengths of alky chains. The conductivity mechanisms in the plastic crystalline and the columnar mesophase have been revealed by BDS and different charge carriers were assigned for the different phases. Furthermore, the cooperative dynamics were also probed by SHS. The cooperative dynamics probed by the different techniques (BDS and SHS) compared, and assigned to the different restrictions on the cooperativity due to the difference in the sensitivity of the techniques.
Collective Orientational Order and Phase Behavior of a Discotic Liquid Crystal under Confinement
(2018)
Discotic liquid crystals (DLCs) are a promising class of soft matter for electronic applications. This is due to their ability to self-organize into columns in a hexagonal columnar mesophase, driven by the overlapping of the π orbitals of their aromatic cores. This leads to a high charge-carrier mobility along the column axis. Embedding liquid crystals into nanopores of anodic aluminum oxide (AAO) results in a 2D nanoconfinement of these materials. This confinement affects their properties, compared to the bulk, such as phase transition temperatures and enthalpies, molecular mobility, and crystallization.
In this study, 2,3,6,7,10,11 hexakis[hexyloxy] triphenylene (HAT6) was confined into parallel aligned cylindrical nanopores of AAO membranes by melt infiltration. The membrane as confining hosts used have varying pore diameters, from 10 nm to 160 nm, covering a broad pore size range, thus, a better understanding of the confinement effect on phase behavior and molecular configuration in the pores. Furthermore, it is aimed to obtain axial ordering or to increase degree of axial ordering by chemically modifying the surfaces of the pores. Therefore, the pore surfaces the membranes were chemically modified, resulting in a more hydrophobic pore surface than the unmodified ones. The phase behavior was explored by a power-compensated DSC allowing the detecting of small changes in the phase behavior. In the literature, dielectric spectroscopy was demonstrated as a method to monitor molecular order inside the pores. Here, we also investigate the collective orientational order, corresponding to dominating molecular ordering, by dielectric spectroscopy.
Since discovery of discotic liquid crystals (DLCs), consisting of a disklike rigid aromatic core and flexible alkyl chains attached to the core, dating back to Chandrasekhar’s work in 1977, they have been extensively investigated to reveal their fundamental properties and potential for applications. The researches on DLCs in last decades showed that DLCs can be considered as promising materials for organic electronic applications since they exhibit one dimensional high charge mobility along the column axis in a columnar mesophase. The mobilies of the rigid aromatic core and the flexible alkyl chains can influence their application properites, e.g. the charge carrier mobility, therefore, it needs to be explored in detailed.
In this study, a series of dipole functionalized triphenylene-based discotics, forming a columnar mesophase, were investigated to reveal the influence of the functionalization on phase behavior, molecular dynamics and as well as conductivity. The molecular mobility of the discotics was probed by broadband dielectric spectroscopy (BDS). In addition to conductivity and localized dynamics, glassy dynamics were also observed. The phase behavior of the material was explored by a power-compansated differential scanning calorimetry (DSC). Beside the phase transition temperatures and enthalpies, thermal glass transitions were found for all the materials. Moreover, the glassy dynamics were further investigated by Flash DSC, which is a chip-based calorimetry technique allows fast heating and cooling rates as high as 10000K/s.
For the first time, the molecular mobility of two linear-shaped tetramethylated guanidinium triflates ionic liquid crystals (ILCs) having different length of alkyl chains was investigated by a combination of broadband dielectric spectroscopy (BDS) and specific heat spectroscopy (SHS). By self-assembly, these ILCs can form a hexagonal ordered mesophase besides plastic crystalline phases and the isotropic state. SHS was carried out by differential AC-chip calorimetry at higher frequencies and temperature modulated DSC at lower frequencies.
Two relaxation processes were found by BDS for both samples. At low temperatures, a γ-processes is observed which is assigned to specific localized fluctuations. At higher temperatures, α1-processes take place. α2 processes were also detected by SHS but with a completely different temperature dependence of the relaxation times. Different molecular assignments of α1- and α2-processes are suggested. At even higher temperatures, conductivity was detected by BDS. An increase in the DC conductivity by four orders of magnitude at the phase transition from the plastic crystalline to the hexagonal columnar mesophase is found. This result is traced to a change in the charge transport mechanism from a delocalized electron hopping in the stacked aromatic systems (in the plastic phase) to one dominated by an ionic conduction in the quasi-1D ion channels formed along the supermolecular columns in the ILC hexagonal mesophases.
Dynamics and ionic conductivity of ionic liquid crystals forming a hexagonal columnar mesophase
(2018)
For the first time, the molecular mobility of two linear-shaped tetramethylated guanidinium triflate ionic liquid crystals (ILCs) having different length of alkyl chains were investigated by a combination of broadband dielectric spectroscopy (BDS) and specific heat spectroscopy (SHS). By self-assembly, these ILCs can form a hexagonal ordered mesophase besides plastic crystalline phases and the isotropic state. Three dielectric active processes were found by BDS for both samples. At low temperatures, a γ-process in the plastic crystalline state is observed which is assigned to localized fluctuations of methyl groups including nitrogen atoms in the guanidinium head. At higher temperatures but still in the plastic crystalline state, an α1-process takes place. An α2 process was detected by SHS but with a completely different temperature dependence of the relaxation times than that of the α1-relaxation. This result is discussed in detail, and different molecular assignments of the processes are suggested. At even higher temperatures, electrical conductivity is detected and an increase in the DC conductivity by four orders of magnitude at the phase transition from the plastic crystalline to the hexagonal columnar mesophase is found. This result is traced to a change in the charge transport mechanism from a delocalized electron hopping in the stacked aromatic systems (in the plastic phase) to one dominated by an ionic conduction in the quasi-1D ion channels formed along the supermolecular columns in the ILCs hexagonal mesophases.
Polymers with intrinsic microporosity (PIMs) represent a novel, innovative class of materials with great potential in various applications from high-performance gas separation membranes to electronic devices. Here for the first time, for PIM-1, as the archetypal PIM, fast scanning calorimetry provides definitive evidence for a glass transition (Tg=715 K, heating rate 3·10^4 K/s) by decoupling the time-scales responsible for glass transition and decomposition. As the rigid molecular structure of PIM-1 prevents any conformational changes, small-scale bend and flex fluctuations must be considered the origin of its glass transition. This result has strong implications for the fundamental understanding of the glass transition and for the physical aging of PIMs and other complex polymers, both topical problems of materials science.
Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. Here, the vibrational density of states (VDOS) for PIM-1, the prototypical polymer with intrinsic microporosity, is investigated by means of inelastic neutron scattering. The results are compared to data measured for a more conventional high-performance polyimide used in gas separation membranes (Matrimid). The measured data show the characteristic low frequency excess contribution to VDOS above the Debye sound wave level, generally known as the Boson peak in glass-forming materials. In comparison to the Boson peak of Matrimid, that of PIM-1 is shifted to lower frequencies. This shift is discussed considering the microporous, sponge-like structure of PIM-1 as providing a higher compressibility at the molecular scale than for conventional polymers. For an annealed PIM-1 sample, the Boson peak shifts to higher frequencies in comparison to the un-annealed sample. These changes in the VDOS of the annealed PIM-1 sample are related to changes in the microporous structure as confirmed by X-ray scattering.