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- BDS broadband dielectric spectroscopy (1)
- Broadband dielectric spectroscopy (1)
- COOH rich surfaces (1)
- Confinement (1)
- Cyanophenyl molecules (1)
- Density of states (1)
- Dynamic glass transition (1)
- Glass transition (1)
- Matrimid (1)
- Membrane (1)
The effect of a nanometer scale confinement (pore sizes 7.5 nm down to 2.5 nm) on the vibrational density of states (VDOS) and on the molecular dynamics of Poly(dimethyl siloxane) (PDMS) and Poly(-methyl phenyl siloxane) (PMPS) is studied by inelastic Neutron scattering. The high penetration depth of neutrons makes neutron scattering a suitable tool for the study of confined systems. Moreover, Neutrons are sensitive to light nuclei therefore the confined polymers can be investigated directly, more or less independently of the confining host. Resulting findings are firstly, a reduction of the low frequency contributions to the VDOS below the Boson Peak frequency for both polymers. Including literature data, this reduction can be regarded as a more general feature for glass-forming systems confined by hard walls. Secondly, clear deviations in the temperature dependence of the mean squared displacement of
the confined molecules compared to the bulk were found close to the thermal glass transition temperature, whereas localized methyl group rotations were only weakly influenced. Furthermore, the molecular dynamics is accessed. The combination of neutron Time-of-Flight with neutron backscattering, thus covering a broad dynamical range from sub ps to ns, reveal clear influence from confinement on the intermediate incoherent scattering function S(q,t). The latter was obtained by combining the inverse Fourier transform of the individual dynamical structure factors measured by the both methods. The time and q dependence of S(q,t) are discussed in detail for the local methyl group rotations and the segmental dynamics, considering both the interaction of the segments with the pore walls and possible geometrical confinement effects.
In contrast to most of the existing literature on plasma polymerization of acrylic acid (AA), not only the chemical structure and film thickness of the deposits were studied, but also a new way to obtain COOH-rich surfaces. This is an important aspect when applying the obtained coatings for biomedical and electronics applications. Therefore, acrylic acid/CO2 polymer films with a thickness of ca. 150 nm were deposited in the pulsed plasma regime onto polyethylene and aluminium as organic and inorganic substrates. Their structure–property relationships were studied in dependence on the mixture ratio of acrylic acid monomer and CO2 gas. The influence of the CO2 gas on the regularity and functionality of plasma deposited poly(acrylic acid) (PAA) was studied in detail using bulk-sensitive (dielectric spectroscopy and Fourier transforms infrared spectroscopy) as well as surface-sensitive methods such as X-ray photoelectron spectroscopy. For a clear identification of the functionality of the deposited films, the COOH groups were estimated after derivatization with trifluoroethanol by XPS measurement as well as by the quantitative analysis of the FTIR data. Results show, in presence of a small amount of CO2 gas within the acrylic acid plasma, a structure of poly(acrylic acid) with high concentration of COOH groups was obtained. However, the excess of CO2 gas in the plasma chamber leads to fragmentation of the monomer or/and degradation of the layer which is already deposited. A polymer network is obtained with an increasing abundance of branched groups for AA/CO2 with increasing CO2 gas in the mixture.
Matrimid and Matrimid/PhenethylPOSS nanocomposites films were prepared by solution casting. They can be considered as potential membrane materials for gas separation. Theire dielectric properties were investigated using Broadband Dielectric Spectroscopy (BDS) in combination with standard techniques. Matrimid shows one relaxation process assigned as beta*-relaxation and a conductivity contribution. The relaxation process has a high activation energy of 99 kJ/mol. Thus this process is supposed to be of cooperative nature due to a π-π stacking of the phenyl rings of Matrimid. The influence of the thermal history on Matrimid was analyzed with BDS as well where an annealing effect is found. The Matrimid/PhenethylPOSS nanocomposites show up to a concentration of about 4 wt% PhenethylPOSS a miscibility on a molecular level. For higher concentrations a phase separated structure was indicated. The conductivity of both systems is explained by π-π stacking of the phenyl rings which enhances charge transport.
The increasing demand for energy efficient separation processes fosters the development of new high performance polymers as selective separation layers for membranes. PIM-1 is the archetypal representative of the class of polymers of intrinsic microporosity (PIM) which are considered most promising in this sector, especially for gas separations. Since their introduction, PIMs stimulated a vast amount of research in this field and meanwhile evolved to the state of the art in membrane technology for gas separation. The major obstacle for extending the practical membrane application is their strong tendency to physical aging. For the first time, investigations by broadband dielectric spectroscopy (BDS) addressing molecular dynamics and conductivity in PIM 1 are presented. As chain packing during film formation from the casting solution and physical aging are key factors determining the separation performance of PIMs as membrane materials, characterization of the molecular mobility in such materials as revealed by BDS will provide valuable information for further development and optimization.
Surface layers have already been observed by broadband dielectric spectroscopy for Composite systems formed by adsorption of rod-like cyanophenyl derivates as probe molecules on the surface of oxide particles. In this work, features of the surface layer are reported; samples with different amounts of the probe molecules adsorbed onto oxide (nano) particles were prepared in order to study their interactions with the surface. Thermogravimetric analysis (TGA) was applied to analyze the amount of loaded probe molecules.
The density of the surface species ns was introduced and its values were estimated from quantitative Fourier transform infrared spectroscopy (FTIR) coupled with TGA. This parameter allows discriminating the composites into several groups assuming a similar interaction of the probe molecules with the hosts of a given group. An influence factor H is further proposed as the ratio of the number of molecules in the surface layer showing a glassy dynamics and the number of molecules adsorbed tightly on the surface of the support: It was found for aerosil composites and used for calculating the maximum filling degree of partially filled silica MCM-41 composites showing only one dielectric process characteristic for glass-forming liquids and a bulk behavior for higher filling degrees.
In the recent years, highly crosslinked plasma polymers have started to unveil their potential in numerous biomedical applications in thin-films form. However, conventional diagnostic methods often fail due to their diverse molecular dynamics conformations. Here, glassy dynamics and the melting transition of thin PEO-like plasma assisted deposited (ppPEO) films (thickness 100 nm) were In situ studied by a combination of specific heat spectroscopy, utilizing pJ/K sensitive AC-calorimeter chip, and composition analytical techniques. Different crosslinking densities were obtained by different plasma powers during the deposition of the films. Glassy dynamics was observed for all values of the plasma power. It was found that the glassy dynamics slows down with increasing the plasma power. Moreover, the underlying relaxation time spectra broaden indicating that the molecular motions become more heterogeneous with increasing plasma power. In a second set of experiment, the melting behavior of the ppPEO films was studied. The melting temperature of ppPEO was found to be decreasing with increasing plasma power. This was explained by a decrease of the order in the crystals due to formation of chemical defects during the plasma process.