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Nanocomposites based on MgAL layered double hydroxides (LDH) and an epoxy resin were prepared and investigated by a combination of complementary methods. As epoxy resin Bisphenol A diglycidyl ether (DGEBA) was used with Diethylenetriamine as curing agent. The LDH was modified with taurine, which acts as an additional crosslinking agent due to its amine groups. The epoxy resin was cured in a presence of the nanofiller, which was added to the system in various concentrations. X-ray scattering, by combination of SAXS and WAXS was used to characterize the morphology of the obtained nanocomposites. These investigations show that the filler is distributed in the matrix as small stacks of ca. 10 layers. The molecular dynamics of the system, as probe for structure, was investigated by broadband dielectric spectroscopy. In addition to the - and -relaxation (dynamic glass transition), characteristic for the unfilled materials, a further process was found which was assigned to localized fluctuations of segments physically adsorbed or chemically bonded to the nanoparticles. The dielectric -relaxation is shifted to higher temperatures for the nanocomposites in comparison to the pure material but depends weakly on the content of nanoparticles. Further, for the first time Flash DSC was employed to a thermosetting system to investigate the glass transition behavior of the nanocomposites. The heating rates were converted in to relaxation rates. For low concentrations of the nanofiller the thermal data overlap more or less with that of the pure epoxy. For higher concentrations the thermal data are shifted significantly to higher temperatures. This is discussed in terms the cooperativity approach to the glass transition.
Inelastic incoherent neutron time-of-flight scattering was employed to investigate the low-frequency vibrational density of states (VDOSs) for a series of glassy Janus-poly(tricyclononenes), which consist of a rigid main chain and flexible alkyl side chains. Here, the length of the flexible side chains was systematically varied from propyl to octyl. Such materials have potential applications as active separation layers in gas separation membranes as a green future technology, especially for the separation of higher hydrocarbons. From the morphological point of view, the Janus polynorbornenes undergo a nanophase separation into alkyl side chain-rich nanodomains surrounded by a rigid polynorbornene matrix. Here, the influence of the nanophase-separated structure on the low-frequency VDOS is investigated from a fundamental point of view. The low-frequency VDOSs of these Janus polynorbornene show excess contributions to the Debye type VDOS known as the Boson peak (BP) for all side chain lengths. Due to the high incoherent scattering cross-section of hydrogen, most of the scattering comes from the alkyl side chain-rich domains.
Compared to conventional glass-forming materials, in the considered Janus polynorbornenes, the BP has a much lower intensity and its frequency position is shifted to higher values. These experimental results are discussed in terms of the nanophase-separated structure where the alkyl chain-rich domains were constrained by the surrounding matrix dominated by the rigid backbone. With increasing alkyl chain length, the size of the alkyl chain-rich domains increases. The frequency position of the BP shifts linearly to lower frequencies with the size of these nanodomains estimated from X-ray measurements. The obtained results support the sound wave interpretation to the BP
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 ([Formula: see text]37 nm) were measured by crossed electrode capacitors (CEC), where the film is capped between Al-electrodes. For thinner films ([Formula: see text]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 ( [Formula: see text] -, [Formula: see text] - and [Formula: see text] -relaxation), like the bulk, related to PVME fluctuations in local spatial regions with different PS concentrations. The thickness dependence of the [Formula: see text] -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 ([Formula: see text]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.
Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. These polymers are characterized by a high permeability and reasonable permselectivity. The latter point is somehow surprising because for microporous systems a more Knudson-like diffusion is expected then a size dependent temperature activated sieving process. It was argued in the framework of a random gate model that molecular fluctuations on a time scale from ps to ns are responsible for the permselectivity.
Here series of polymers of intrinsic microporosity (PIMs) as well as microporous polynorbornenes with bulky Si side groups and a rigid backbone are considered. The polymers have different microporosity characterized by high BET surface area values.
First inelastic time-of-flight neutron scattering measurements were carried out to investigate the low frequency density of state (VDOS). 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. It was found that the frequency of the maximum position of the Boson peak correlates with the BET surface area value.
Secondly elastic scans as well as quasielastic neutron scattering measurements by a combination of neutron time-of-flight and backscattering have been out. A low temperature relaxation process was found for both polymers. This process was assigned to the methyl group rotation. It was analysed in terms of a jump diffusion in a three-fold potential. The analysis of the dependence of the elastic incoherent structure factor on the scattering vector yields the number of methyl groups which might be immobilized.
The neutron scattering experiments were accompanied by fast scanning calorimetry and broadband dielectric investigations as well as atomistic molecular dynamic simulations.
Spatial Inhomogeneity, Interfaces and Complex Vitrification Behavior of Epoxy-based Materials
(2021)
This work deals with an in-depth comparative investigation of the structure, molecular mobility and vitrification kinetics of three bisphenol A diglycidyl ether (DGEBA)-based materials with different nanofillers: taurine-modified layered double hydroxide (T-LDH) and halloysite nanotubes (HNTs). The chosen methodology comprises I) small – and wide – angle X-ray scattering (SAXS/WAXS) II) calorimetry in the forms of a) conventional DSC and b) static fast scanning calorimetry (FSC), III) broadband dielectric spectroscopy (BDS), as well as IV) specific heat spectroscopy in the forms of a) temperature modulated DSC, and b) temperature modulated FSC.
Janus polytricyclononenes (PTCN) with rigid backbones and flexible n-alkyl (n = propyl, butyl, hexyl, octyl, decyl) are novel, innovative materials that show potential in separating hydrocarbons. These superglassy polymers were designed to show an enhanced and controllable gas permeability via flexible alkyl side chains that promote mass transport, as opposed to conventional microporous polymers, where permeability is a function of the free-volume entities.
PTCNs, investigated by small angle X-ray scattering (SAXS), show nanophase separation between the n-alkyl side chains and the backbones. The size of the nanodomains increases with the length of the n-alkyl side groups. In addition, for the alkyl chain-rich nanodomains a distinct α-relaxation was found by means of broadband dielectric spectroscopy (BDS) and temperature modulated DSC (TMDSC). The glass transition of the backbone-rich domains, which is beyond or near to the degradation of the materials, was evidenced by fast scanning calorimetry (FSC) by decoupling it from decomposition, employing high heating rates up to 104 K/s.
Further, Janus PTCNs were studied by quasielastic neutron scattering (QENS) employing the backscattering IN16B (ILL, Grenoble) and time of flight FOCUS (PSI, Villigen) instruments. For an overview of dynamic processes setting in at different temperatures inelastic (IFWS) and elastic fixed window scans (EFWS) were conducted. IFWS showed that the segmental motions of alkyl-rich nanodomains shift to higher temperatures with increasing alkyl chain length, which agrees with SAXS and BDS findings. For the lowest side chain lengths an additional low temperature relaxation process was found, assigned to methyl group rotations.
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 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.
Two epoxy-based nanocomposites with different nanofillers (layered double hydroxide and boehmite) were investigated employing temperature modulated DSC, flash DSC and broadband dielectric spectroscopy. Detailed investigation on the molecular mobility of the two systems showed the effect of the fillers on the structure of the bulk epoxy matrix and the interface formed at the polymer/particle interface