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In the course of miniaturizing modern technology down to the molecular scale, understanding the materials behavior has to be investigated and deviations from the bulk that might arises from confinement effects has to be understood. Here, a combination of nano-sized relaxation spectroscopies (Broadband dielectric spectroscopy (BDS) and Specific heat spectroscopy (SHS); employing AC nanochip calorimetry) were utilized to investigate the glassy dynamics of ultra-thin films of blends of Poly (vinyl methyl ether) (PVME) / Polystyrene (PS) 50:50 wt-%, which are miscible bulk (thicknesses: 8nm - 200nm, film thickness was controlled by ellipsometry, film topography by AFM). Both methods are sensitive to different probes; where SHS senses entropy fluctuations while BDS measures dipole fluctuations. For BDS measurements, a recently developed nano-structured electrode sample arrangement is employed; where ultra-thin films are spin-coated on an ultra-flat highly conductive silicon wafer, sandwiched between a wafer with nanostructured SiO2 nano-spacers with heights between 35 nm and 70 nm. The method was applied for film thicknesses up to 50 nm. For thicker films, the samples were prepared between crossed electrodes. It was proven that, for 50 nm films, both contact methods will lead to identical results. By measuring the dynamic Tg in dependence of the film thickness, SHS showed that the Tg of the whole film was strongly influenced by a nanometer-thick surface layer at the polymer/air interface due to a self-assembling process [1]. The dynamic Tg obtained from the SHS measurements decrease with decreasing film thickness, evidencing the importance of a mobile surface layer at the polymer/air interface for the glass transition of the whole film. Compared to the SHS results, the BDS measurements show a completely different behavior. At high temperatures, the temperature dependence of the relaxation times of the films follows that of bulk-like PS/PVME; obeying the VFT-law. With decreasing temperature, the temperature dependence deviates from the VFT to an Arrhenius law; where the apparent activation energy decreases with decreasing film thickness. This is the first example where confinement induced changes were observed by BDS for ultra-thin films. All results were analyzed in details in a comprehensive discussion.
Behavior of Ultra-Thin Polymeric Films as Investigated by Dielectric and Thermal Spectroscopy
(2013)
Behavior of Ultra-Thin Polymerich Films as Investigated by a Combination of Complementary Methods
(2014)
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.
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), the chain dynamics of polymers, and the behavior of a high performance polymer were illustrated.
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 illustrated. Further the application of dielectric spectroscopy to advanced functional materials.
Broadband dielectric spectroscopy in the frequency range from 10-1 to 109 Hz is employed to revisit the segmental dynamics of the miscible blend system poly(vinyl methyl ether)/polystyrene (PVME/PS) in dependence on the composition firstly in the bulk state. Here especially the case of high polystyrene concentrations is considered. It is important to note that only the molecular dynamics of PVME segments as affected by PS is observed because the dipole moment of polystyrene is negligible. Three relaxation processes are found which are due to fluctuations of differently constrained or confined PVME segments. The degree of the confinement of PVME segments due to PS is discussed in dependence on the composition.
Further a spatial confinement is considered by investigating thin films of the PVME/PS blend where the film thickness was varied from 100 nm down to 5 nm. Two concentrations of PVME/PS (50/50 wt% and 25/75 wt%) are investigated. To measure nanometer thick films a novel electrode system based on nanostructured electrodes was employed. Nanostructured electrodes can be considered as a cut-edge technology in dielectric spectroscopy. The relaxation spectra of the films showed multiple processes which are discussed in dependence on the film thickness.
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.
In the course of miniaturizing modern technology down to the molecular scale, understanding the materials behavior has to be investigated and deviations from the bulk have to be understood. A combination of nano-sized relaxation spectroscopies (Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS)) employing AC nanochip calorimetry were utilized to investigate the glassy dynamics of ultra-thin films of blends of Poly (vinyl methyl ether) (PVME) / Polystyrene (PS) (50:50 and 25/75 wt-%), which are miscible in the bulk (thicknesses: 8nm - 200nm, film thickness was controlled by ellipsometry, film topography by AFM). Both methods are sensitive to different probes; where SHS senses entropy fluctuations BDS measures dipole fluctuations. For BDS measurements, a recently developed nano-structured electrode sample arrangement is employed. By measuring the dynamic glass transition in dependence of the film thickness, SHS showed that the Tg of the whole film was strongly influenced by a nanometer-thick surface layer at the polymer/air interface due to a self-assembling process. Compared to the SHS results the BDS measurements show a completely different behavior. At high temperatures the temperature dependence of the relaxation times of the films follows that of bulk-like PS/PVME obeying the VFT-law. With decreasing temperature the temperature dependence deviates from the VFT to an Arrhenius law where the apparent activation energy decreases with decreasing film thickness. This is the first example were confinement induced changes were observed by BDS for ultra-thin films. All results were analyzed in detail in a comprehensive discussion.
Semicrystalline polymers have to be described by a three phase model consisting of a mobile amorphous (MAF), a crystalline (CF), and a rigid amorphous fraction (RAF). For nanocomposites based on a semicrystalline polymer the RAF is due to both the crystallites (RAFcrystal) and the filler (RAFfiller). In most cases a separation of both contributions is not possible without further assumptions. Polymer nanocomposite based on poly(L-lactide) and MgAl layered double hydroxide nanofiller were prepared. Due to the low crystallization rate of PLA its crystallization can be suppressed by a high enough cooling rate, and the RAF is due only to the nanofiller. The MAF, CF, and RAF were estimated by Temperature Modulated DSC. For the first time CF, MAF, RAFcrystal, and RAFfiller could be estimated. It was found, that RAFfiller increases linearly with the concentration of the nanofiller. Furthermore, RAFcrystal is only slightly influenced by the presence of the nanofiller.
The molecular dynamics of the discotic liquid crystals like pyrene-1,3,6,8-tetracarboxylic tetra(2-ethylhexyl) ester is studied by dielectric relaxation spectroscopy. Dielectric spectroscopy shows 3 processes: a b-relaxation at low temperatures and an a-relaxation in the temperature range of the mesophases followed by conductivity. The dielectric a-relaxation is assigned to a restricted glassy dynamics in the plastic crystal as well as in the liquid crystalline phase.
Despite the many controversial discussions about the nanometric confinement effect and the predictions of the three-layer model, much remain not understood and/or experimentally unproven. Here, a combination of Broadband Dielectric Spectroscopy (BDS), Specific Heat Spectroscopy (SHS), and ellipsometry was utilized to investigate the glassy dynamics of ultra-thin films of Poly (vinyl methyl ether) (PVME) and PVME/Polystyrene (PS) 50:50 wt-% miscible blend (thicknesses: 8nm - 200nm). For BDS measurements, a recently developed nano-structured sample arrangement; where ultra-thin films are spin-coated on an ultra-flat highly conductive silicon wafer and sandwiched between a wafer with nanostructured silica nano-spacers, was used. For PVME films, two processes were observed and interpreted to be the α-processes of a bulk-like layer and an absorbed layer to the substrate. BDS and SHS showed that glassy dynamics are bulk-like. However, for films lower than 15nm, BDS showed weakly slowed dynamics. For PVME/PS blend, by a self-assembling process, a nanometer-thin surface layer with a higher molecular mobility is formed at the polymer/air interface. By measuring the dynamic Tg in dependence on the film thickness, both BDS and SHS, showed that the Tg of the whole film was strongly influenced by that nanometer thick surface layer, with a lower Tg.
Glassy dynamics of ultra-thin supported polysufone layers as studied by dielectric spectroscopy
(2008)
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.
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.
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.
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 reasonable permselectivity of these materials is somehow surprising because for microporous systems a more Knudsen-like diffusion is expected rather than 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 a series of microporous polynorbornenes with bulky SiMe3 side groups and a rigid backbone is considered which have different microporosities characterized by 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 maximum position of the Boson peak correlates with the BET surface area.
For two selected comparable polynorbornenes elastic scans as well as QENS measurements by a combination of neutron time-of-flight and backscattering were carried 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.
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.
Her a series of microporous polynorbornenes with bulky Si side groups and a rigid backbone are considered which have different microporosity characterized by 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 the maximum position of the Boson peak correlates with the BET surface area value.
For two selected comparable polynorbornenes elastic scans as well as QENS measurements by a combination of neutron time-of-flight and backscattering are carried 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.
Polymers with intrinsic microporosity are promising candidates for the active separation layer in green gas separation membranes and as active materials for electronic applicatons. 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 analysed in detail by quasielastic neutron scattering combining Time-of-Flight and backscattering where the data are discussed with regard to both the q- and the temperature dependence.
Discotic liquid crystals (DLCs) are self-assembled materials where self-assembly is driven by non-covalent intermolecular interactions. The corresponding molecules consist of a flat and rigid aromatic core substituted by flexible aliphatic side chains. While the former is responsible for the π-stacking, the latter gives rise to an increased solubility, processability, and rich thermotropic behavior. The disc-shaped molecules form columns that further assemble into two-dimensional arrays with a hexagonal mesophase. The alkyl chains fill the intracolumnar space giving rise to a nanophase separated state.
Two different homologous systems based on triphenylene derivatives were investigated as model systems for DCLs where the length of the aliphatic side chains is widely varied. Measurements were carried out in the bulk state and confined to the nanometre wide channels of anodic aluminum membranes (Pore diameter 20 nm, 40nm, 80 nm and 180 nm) by means of inelastic and quasielastic neutron scattering. These investigations were accompanied by calorimetry and dielectric relaxation spectroscopy. From the neutron scattering experiments the vibrational density of states (Boson Peak) from neutron time-of-flight spectroscopy, the molecular dynamics on a time scale of ca. 1 ns (elastic scans from neutron backscattering spectrometers) as well as quasielastic neutron scattering by a combination of time-of-flight spectroscopy and backscattering were considered. Structural information is retrieved by X-ray diffraction carried out on a synchrotron.
The dynamics of these unique soft matter materials are discussed in detail considering their structure (length of the aliphatic side chain), the effect of self-confinement (of the aliphatic chains in the intracolumnar space) and of outer confinement (influence of the pore size). Comparison is further made to pyrene based system.
Thin polymeric films are of great importance of high number of high-tech applications for instance in sensors and nanoelectronics. Form the scientific point of view thin films with thickness below 100 nm are ideal model systems to study confinement effects on its properties for instance on the molecular relaxation processes. In this contribution an overview is presented about the behavior of different systems as investigated by nanosized relaxation spectroscopy like broadband dielectric spectroscopy employing nano structured capacitors and AC chip calorimetry complimented by ellipsometry. The systems considered are PVME1, PVME/PS blends2,3 P2VP4, PBAC5 and polysulfone6. Besides the film also the adsorbed layer on the substrate prepared by a leaching approach and investigated by AFM is considered.1,4-7. For these investigationsss it is found that the adsorbed layer itself exhibits a relaxation dynamics which might be assigned either to molecular motions or to adsorptions desorption kinetics.
Discotic liquid crystals (DLCs) are self-assembled materials where self-assembly is driven by non-covalent intermolecular interactions. The corresponding molecules consist of a flat and rigid aromatic core substituted by flexible aliphatic side chains. While the former is responsible for the π-stacking, the latter gives rise to an increased solubility, processability, and rich thermotropic behavior. The disc-shaped molecules form columns that further assemble into two-dimensional arrays with a hexagonal mesophase. The alkyl chains fill the intracolumnar space giving rise to a nanophase separated state. These soft matter materials with highly ordered columnar structures have a great potential in molecular electronic devices such as active semiconductors in organic field-effect transistors and photovoltaic devices with a charge carrier mobility of up to 1.1 cm2/Vs. Two different homologous systems based on triphenylene derivatives were investigated as model systems for DLCs where the length of the aliphatic side chains is widely varied. Structural information is retrieved by X-ray diffraction carried out on a synchrotron. Experiments were carried out in the bulk state and confined to the nanometer wide channels of anodic aluminum membranes (Pore diameter 20 nm, 40nm, 80 nm and 180 nm). The latter case can be considered as a promising way to prepare nanowires. As method broadband dielectric spectroscopy (frequency range 10-2 Hz to 109 Hz) is employed. These experiments were accompanied by thermal investigations (Differential Scanning Calorimetry as well as Thermogravimetric Analysis) as well as inelastic neutron scattering (neutron Time-of-Flight and neutron Backscattering). The dynamics of these unique soft matter materials are discussed in detail considering their structure (length of the aliphatic side chain), the effect of self-confinement (of the aliphatic chains in the intracolumnar space) and of the outer confinement (influence of the pore size). Comparison is further made to a pyrene based system.
Molecules in nanometer confinement investigated by dielectric, thermal and neutron spectroscpoy
(2008)
Molecules in Nanometer Confinement investigated by Dieletric, Thermal and Neutron Spectroscopy
(2013)
Molecules in nanometre confinement investigated by dieletric spectroscopy and related techniques
(2006)
A selected series of dipole functionalized triphenylene-based discotic liquid crystals (DLCs) was synthesized and investigated in a systematic way to reveal the phase behavior and molecular dynamics. The later point is of particular importance to understand the charge transport in such systems which is the key property for their applications such as organic field effect transistors, solar cells or as nanowires in molecular electronics, and also to tune the properties of DLCs. The mesomorphic properties were studied by polarizing optical microscopy, X-ray diffraction, and differential scanning calorimetry, which were compared to the corresponding unfunctionalized DLC. The molecular dynamics were investigated by a combination of state-of-the-art broadband dielectric spectroscopy (BDS) and advanced calorimetry such as fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). Besides localized fluctuations, surprisingly multiple glassy dynamics were detected for all materials for the first time. Glassy dynamics were proven for both processes unambiguously due to the extraordinary broad frequency range covered. The α1-process is attributed to fluctuations of the alky chains in the intercolumnar space because a polyethylene-like glassy dynamics is observed. This corresponds to a glass transition in a confined three-dimensional space. The α2-process found at temperatures lower than α1-process, is assigned to the fluctuation of translational in plane movements of the triphenylene core inside distorted columns. This can be considered as a glass transition of a one-dimensional fluid. Therefore, obtained results are of general importance to understand the glass transition, which is an unsolved problem of condensed matter science.
Hexakis(n-alkyloxy)triphenylene) (HATn) consisting of an aromatic triphenylene core and alkyl side chains are model discotic liquid crystal (DLC) systems forming a columnar mesophase. In the mesophase, the molecules of HATn self-assemble in columns, which has one-dimensional high charge carrier mobility along the columns. Here, a homologous series of HATn with different length of the alkyl chain (n=5,6,8,10,12) is investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques including fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). The investigation of the phase behavior was done utilizing DSC experiments and the influence of the alkyl chain length on the phase behavior was revealed. By the dielectric investigations probing the molecular mobility, a γ-relaxation due to localized fluctuations as well as two glassy dynamics the αcore and αalkyl relaxation were observed in the temperature range of the plastic crystalline phase. Moreover, the observed glassy dynamics were further studied employing advanced calorimetry. All observed relaxation processes are attributed to the possible specific molecular fluctuations and discussed in detail. From the results a transition at around n=8 from a rigid constrained (n=5,6) to a softer system (n=10,12) on a molecular lenght scale was revealed with increasing alkyl chain length. A counterbalance of two competing effects of a polyethylene like behavior of the alkyl chains in the intercolumnar domains and self-organized confinement is discussed in the context of a hindered glass transition.
Hexakis(n-alkyloxy)triphenylene) (HATn) consisting of an aromatic triphenylene core and alkyl side chains are model discotic liquid crystal (DLC) systems forming a columnar mesophase. In the mesophase, the molecules of HATn self-assemble in columns, which has one-dimensional high charge carrier mobility along the columns. Here, a homologous series of HATn with different length of the alkyl chain (n=5,6,8,10,12) is investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques including fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). The investigation of the phase behavior was done utilizing DSC experiments and the influence of the alkyl chain length on the phase behavior was revealed. By the dielectric investigations a γ-relaxation due to localized fluctuations as well as two glassy dynamics the αcore and αalkyl relaxation were observed in the temperature range of the plastic crystalline phase. Moreover, the observed glassy dynamics were further studied employing advanced calorimetry. All observed relaxation processes are attributed to the possible specific molecular fluctuations and discussed in detail. From the results a transition at around n=8 from a rigid constrained (n=5,6) to a softer system (n=10,12) on a molecular lenght scale was revealed with increasing alkyl chain length. A counterbalance of two competing effects of a polyethylene like behavior of the alkyl chains in the intercolumnar domains and self-organized confinement is discussed in the context of a hindered glass transition. The results were confirmed by in- and quasielastic neutron scattering
The effect of a nanometre confinement on the molecular dynamics of liquid crystals was studied by broadband dielectric and neutron spectroscopy. As confining host the molecular sieve Al-MCM-41 (Si/Al=60) with a mean pore diameter of 3 nm was selected. As guest the liquid crystals 8CB and E7 were chosen where the latter does not crystallize but undergoes a glass transition forming a nematic glass.
Dielectric spectroscopy shows that for the confined systems one relaxation process is observed. Ist characteristic relaxation time is much lower compared to that of the bulk. No signature of the Phase transition characteristics of the bulk liquid crystal is detected. The temperature dependence of the relaxation time of this relaxation process has to be described by the Vogel-Fulcher-Tammann equation and thus shows similarities to glassy dynamics.
The vibrational density of states measured by neutron spectroscopy (IN6, ILL; TOFTOF, MLZ) Shows excess contributions with respect to the Debye density of states (boson peak). For the confined systems the low frequency contributions of the boson peak are suppressed, an effect which is also found for conventional glass forming systems.
In addition elastic scans were carried out at the backscattering spectrometer (BS) IN10 at ILL. Such measurements provide an overview about the temperature dependence of the microscopic dynamics.
Assuming a Gaussian form for the elastically scattered intensities the effective mean square displacement was calculated. For the nanoconfined samples the mean square displacement is strongly reduced in comparison to the bulk. Also these measurements show the signature of a glass Transition indicated by a change in the temperature dependence of the mean square displacement. For confined E7 the extracted glass transition temperature is shifted by more than 20 K to higher temperatures.
Broadband inelastic neutron scattering was carried out by combining time-of-flight (IN6, ILL; TOFTOF, MLZ) and backscattering (IN10, ILL; SPHERES, MLZ) experiments in the time domain.
The time dependence of the incoherent intermediate scattering function Sinc(q,t) shows two Relaxation processes which were quantitatively analyzed by fitting the KWW function to the data. The process at shorter times shows a close similarity the methyl group rotation found for polymeric systems. The process at longer times was assigned to glassy dynamics. The obtained data are compared in detail in their temperature dependence to the dielectric results.
Neutron Spectroscopy on nano-confined polymers in comparison with dielectric and thermal results
(2007)
Different types of polymerbased nanocomposites (matrix materials: polyethylene, polypropylene, polylactide, polycarbonate, polystyrene) with a broad variety of nanofillers (fillers: layered silica, layered doubled hydroxides, POSS) are prepared where the concentration of the particles is varied. For layered fillers TEM images show both an intercalated and exfoliated morphology in dependence on the nanofiller and the matrix. All systems are studied in detail by dielectric spectroscopy (BDS) but also complementary methods like WAXS, SAXS, gas transport measurements, calorimetry, and FTIR spectroscopy are employed. Besides the dispersion, the interfacial region between the nanoparticles and the polymer matrix is crucial for the properties of the nanocomposites. Therefore, attention is paid to investigate this interfacial area by BDS because the polar groups of both the surfactant and compatibilizer are located close to the layers. For some systems it is found that the molecular mobility in the interfacial area is essentially higher than in the matrix. In addition a Maxwell/Wagner/Sillars polarization is found due to the blocking of charges at the layers. The time constant of this MWS-process can be correlated with characteristic length scales in the nanocomposites and provides information about the dispersion of the nanofiller.
Polymere in Nano-Confinements: Vergleich von Relaxationsexperimenten und (Neutronen) streuung
(2003)
Polymere in nanoporösen Gläsern: Was können wir von Relaxations- und Streuexperimenten lernen?
(2003)
Polymers in Random Nanoporous Glasses Investigated by Dielectric, Thermal and Neutron Spectroscopy
(2005)