Filtern
Dokumenttyp
Sprache
- Englisch (5)
Schlagworte
- Molecular mobility (5) (entfernen)
Organisationseinheit der BAM
Dielectric Study of Molecular Mobility in Poly(propylene-graft-maleic anhydride)/Clay Nanocomposites
(2005)
Polymer/clay nanocomposite materials based on poly(propylene-graft-maleic anhydride) (PPgMAH) and two different organophilic modified clays were investigated by dielectric relaxation spectroscopy (DRS). In contrast to ungrafted polypropylene (PP), PPgMAH shows a dielectrically active relaxation process which can be assigned to localized fluctuations of the polar maleic anhydride groups. Its relaxation rate exhibits an unusual temperature dependence, which could be attributed to a redistribution of water molecules in the polymeric matrix. This is confirmed by a combination of Raman spectroscopy and thermogravimetric experiments (TGA) with real-time dielectric measurements under controlled atmospheres. In the nanocomposites this relaxation process is shifted to higher frequencies up to 3 orders of magnitude compared to the unfilled polymer. This indicates a significantly enhanced molecular mobility in the interfacial regions. In the nanocomposite materials a separate high-temperature process due to Maxwell-Wagner-Sillars (MWS) polarization was observed. The time constant of this MWS process can be correlated with characteristic length scales in nanocomposites and therefore provides additional information on dispersion and delamination/exfoliation of clay platelets in these materials. These properties also influence the diffusivity of the water molecules as revealed by real-time dielectric investigations.
According to basic phenomenological models describing the solution-diffusion based mechanism of penetrant diffusion in dense polymers, a connection between the diffusive transport of gas molecules in a polymeric matrix and the molecular mobility of that matrix on a certain length scale is, in principle, established for a long time. However, experimental data directly showing this correlation are rare. The investigation of a series of nanocomposites based on a polyhedral oligomeric silsesquioxane (POSS) and a polycarbonate matrix allows a systematic change of the molecular mobility on a local length scale (β-relaxation) and of the corresponding activation energy EA, both determined by broadband dielectric spectroscopy. Independently, activation energies of penetrant diffusion (ED) of these nanocomposites were determined for N2, O2, CO2, and CH4 and a clear linear correlation between the two activation energies was established for the first time.
We report dielectric and calorimetric studies on metathesis and addition-type polytricyclononenes, both based on the same monomer bearing three pendant OSiMe3 groups. For the addition-type polymer, dielectric spectroscopy reveals a β*-process related to the microporosity, whereas for its metathesis counterpart, the segmental dynamics manifests as an α-process related to a glass transition. Besides active dielectric processes, a significant conductivity contribution is detected for both samples which for the microporous additiontype polymer is three orders of magnitude greater than for the metathesis polymer. The broadband dielectric spectroscopy is complemented by detailed calorimetric investigations, comprising DSC, FSC, and TMDSC. The calorimetric methods detected the glass transition for the metathesis polymer in agreement with the observed dielectric α-process. Furthermore, the already reported gas transport properties for both polymers are compared, setting them in correlation with the observed molecular mobility and conductivity behavior. The discussed results reflect significant differences in molecular mobility of the two polymers affecting the appearance of microporosity which strongly determines the gas transport properties.
Polymeric membranes represent a cost- and energy efficient solution for gas separation. Recently superglassy polymers with high free volume outperform many conventional dense polymers in terms of gas permeability and selectivity. However, such polymers are prone to pronounced physical aging, resulting in a dramatic reduction in the gas permeability. Molecular mobility of polymer segments plays an important role in the physical aging and the gas transport performance of polymeric membranes. Molecular mobility and physical aging of a representative superglassy polynorbornene with very high gas permeability, PTCNSi2g, was monitored by using dielectric spectroscopy with state-of-the-art high-resolution analyzers. This work helps to shed some light on the structure−property relationship of superglassy polymers on a molecular level and to provide practical “design rules” for the development of high performance polymers for gas separation.
Superglassy polymers with a large fractional free volume have emerged as novel materials with a broad range of applications, especially in the field of membrane separations. Highly permeable addition-type substituted polynorbornenes with high thermal resistance and chemical stability are among the most promising materials. The major obstacle for extending the practical membrane application is their strong tendency to physical aging, leading to a partial decline in their superior transport performance over time. In the present study, broadband dielectric spectroscopy with complementary X-ray scattering techniques were employed to reveal changes in microporous structure, molecular mobility, and conductivity by systematic
comparison of two polynorbornenes with different numbers of trimethylsilyl side groups. Their response upon heating (aging) was compared in terms of structure, dynamics, and charge transport behavior. Furthermore, a detailed analysis of the observed
Maxwell−Wagner−Sillars polarization at internal interfaces provides unique information about the microporous structure in the
solid films. The knowledge obtained from the experiments will guide and unlock potential in synthesizing addition-type polynorbornenes with versatile properties.