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- Broadband dielectric spectroscopy (5) (entfernen)
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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.
In times of the energy revolution, the need for energy efficient separation processes promotes the advancement of new high performance materials for use as highly selective separation membranes. Most promising materials in this field, especially for gas separation, are polymers with intrinsic microporosity (PIMs) which were firstly introduced by Budd and McKeown. In this study the permeability of PIM-1 was increased by 439 % by formation of nanocomposites with only 1 wt% of polyhedral oligomeric phenethyl-silsesquioxane (PhE-POSS) within the polymer matrix. As the CO2/CH4 selectivity is fully retained, this leads to a shift in the Robeson plot towards the upper bound. The Robeson plot describes the current state of the art trade-off relation between selectivity and permeability of all known membrane polymers. As molecular mobility is a key factor for gas transport as well as the often observed physical aging of such polymers, our study also includes for the first time, molecular dynamics and conductivity of pure PIM-1 and PIM-1 nanocomposites as investigated by broadband dielectric spectroscopy (BDS).
Especially now, in times of the energy revolution, the need for energy efficient separation processes promote the advancement of new high performance materials for use as highly selective separation membranes. Most promising materials in this field, especially for gas separation, are polymers with intrinsic microporosity (PIMs) which were firstly introduced by Budd and McKeown. Currently widespread application of PIMs for membrane technology is still restrained by their strong tendency to physical aging involving a significant loss of their good gas separation properties. This phenomenom is directly related to the molecular mobility. Here for the first time, molecular dynamics and conductivity in PIM-1, the first synthesized PIM, were investigated by broadband dielectric spectroscopy (BDS). Although an α-relaxation was not observed for PIM-1, as a thermal glass transition temperature has not yet previously been reported below its decomposition temperature at 370°C, surprisingly a conductivity contribution was found which is attributed to intermolecular agglomerates formed by π−π-stacking. Besides the studies on pure PIM-1 a polyhedral oligomeric phenethyl-silsesquioxanes (PhenethylPOSS) was used as nanofiller (0 – 40 wt%) in order to evaluate its ability to improve gas transport properties and probably control physical aging. The molecular mobility of all materials was analyzed by BDS of solution-cast films.
Despite the many controversial discussions about the nanometric confinement effect on the properties of ultra-thin films, much remain not understood and/or experimentally unproven. Here, a combination of 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 a low MW Poly (vinyl methyl ether) (PVME) (thicknesses: 7 nm – 160 nm). 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. For films with thicknesses up to 50 nm, BDS measurements showed two relaxation processes, which can be analyzed for these film thicknesses in details. The process located at higher frequencies coincidence in both, its position and temperature dependence, with the -relaxation of bulk PVME and is therefore assigned to the -relaxation of a bulk-like layer. The temperature dependence of the relaxation rate of this process in independent of film thickness. This is further confirmed by the SHS investigations, which superimpose in its temperature dependence with the BDS results; independent of film thickness. The second process is located at lower frequencies, where it shows a different temperature dependence and ascribed to the relaxation of polymer segments adsorbed at the substrate. The interaction of PVME with SiO2 was further confirmed by contact angle investigations. This adsorbed layer further undergoes a confinement effect that results in a lower Vogel temperature than that of the bulk-like layer. A detailed analysis of the dielectric strengths of both processes reveals that the thickness of the adsorbed layer decreases with increasing temperature, while that of the bulk-like layer increases. As a main conclusion, BDS showed that the glassy dynamics of the bulk-like and the adsorbed layer are thickness independent, which is in agreement with the SHS results. To our knowledge, this is the first probing of the segmental dynamics of an adsorbed layer in ultrathin films.
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.