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  • 1
    ISSN: 1435-1536
    Keywords: NMR ; polymers ; molecular motions ; entanglements
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Zusammenfassung Die Solid-Echo-Technik wurde auf die Bestimmung der restdipolaren und restquadrupolaren Kopplung in Schmelzen von Polyäthylen (PE) und deuteriertem PE angewandt. Die Restkopplung, definiert durch die Wurzel aus dem zweiten Moment, beträgt etwa 1% der entsprechenden Kopplung im festen Zustand. Sie nimmt mit steigendem Molekular-gewicht zu, nimmt aber ab mit steigender Temperatur. Eine theoretische Behandlung von zeitabhängigen Restkopplungen ergibt eine Zerfallzeit, die in PE-Schmelzen etwa 0.1 ms beträgt und die innerhalb der Fehlergrenzen unabhängig von Molekulargewicht und Temperatur ist. Die Restkopplung wird im Zusammenhang mit der Kettenverhakung in PE diskutiert. Experimentell bestimmte Spin-Gitter- und Spin-Spin-Relaxationszeiten werden mit der schnelleren segmentalen Bewegung in PE in Beziehung gebracht.
    Notes: Summary The solid echo technique is applied to determining residual dipolar and quadrupolar couplings in molten polyethylene (PE) and deuterated PE, respectively. The residual coupling defined by the square root of the second moment is about 1% of the corresponding quantity in the solid. It increases with molecular weight, and decreases with rising temperature. A theoretical treatment of time dependent residual couplings yields a decay time that is found experimentally to be about 0.1 ms in molten PE, and independent of molecular weight and temperature within our limits of accuracy. The residual coupling is discussed in relation with chain entanglement in PE. Measurements of the spin-lattice and spin-spin relaxation time are related with the more rapid segmental motion of PE chains.
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  • 2
    ISSN: 1435-1536
    Keywords: Solid polymers ; molecular motion ; solid state NMR ; polycarbonate ; additives
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Pulsed deuteron NMR line shapes have been analysed to characterize type and time scale of the phenylene group motion in glassy bisphenol-A polycarbonate. The motional mechanism involvesπ-flips about theC 1 C 4 axis augmented by small angle fulctuations about the same axis, reaching a rms amplitude of ±35
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  • 3
    ISSN: 1435-1536
    Keywords: Bisphenol-A-polycarbonate ; diphenylene propaneorientation ; birefringence ; order parameter
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Two-dimensional magic-angle-spinning (2D-MAS) NMR has been used to measure the orientation parameter, P2, of the diphenylene propane unit in bisphenol-A polycarbonate oriented by stretching to various extension ratios,λ, atT=295K andT= 403 K.P 2 is proportional to the birefringenceΔn, with a maximum birefringenceΔn 0= 0.189. There is some evidence that the order parameterP 2 ″ of the DPP units with respect to the chain axis deviates from unity.Δn 0 is therefore expected to be different from the birefringence,Δn 0 ′, of perfectly aligned chains of polycarbonate. The experimental results obtained forP 2(λ) are compared to those predicted by the aggregate model.
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  • 4
    ISSN: 1435-1536
    Keywords: Deuteron NMR ; polymeric liquid crystals ; molecular dynamics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Pulsed deuteron NMR spectroscopy has been used to examine the axial motion and the side-chain conformation in both oriented and unoriented mesophases of discotic liquid crystal main-chain polymers based on pentyloxy or heptyloxy substituted triphenylenes. Lineshape simulations show that the rotational motion of the triphenylene rings about the column axes can be described by an inhomogeneous distribution of reorientation angles around 45°. However, only about 60% of the discs are involved in such large amplitude motions; the remaining 40% have reorientation angles below 10°. This illustrates the severe restrictions imposed on the rotation of the discs by the interlinkage of the columns via the alkylene spacers. Furthermore, the simulations demonstrate that, at the α-carbon, the side chains show very little fast internal motion, but have a relatively complex conformation involving a disorder which does not change on the microsecond timescale. Since such a disorder is not present in the corresponding monomeric samples it is ascribed to the presence of the spacers. These results are also consistent with the presence of large sterical hindrances between the first side chains segments of adjacent discs, and they indicate a correlated reorientation of the discotic units within a column.
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  • 5
    ISSN: 1435-1536
    Keywords: Key words Polyacrylate ; Super absorber ; Diffusion NMR imaging
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract  The use of NMR imaging techniques for the characterization of superabsorbing polymers is explored. Spatial differences in the cross-link densities of polyacrylate-based superabsorbing particles are investigated by parameter-selective 1H NMR imaging of samples swollen with water. Images of the transverse relaxation time and of the self-diffusion coefficient of the absorbed water provide valuable information on the local water mobility and, thus, indirectly also on the polymer. The time evolution of the swelling process was also studied by NMR images acquired after different swelling times. Moreover, using magic-angle-spinning imaging, the influence of mechanical load on the swollen particle was investigated by taking advantage of the centrifugal forces on the spinning sample. These experiments indicate the presence of two water components, one strongly adsorbed, while the other is relatively mobile.
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  • 6
    ISSN: 1435-1536
    Keywords: Dielectricspectroscopy ; polymericliquidcrystal ; moleculardynamics indiscoticliquidcrystals
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Broadband dielectric spectroscopy is employed for the first time to study the molecular dynamics in discotic liquid crystalline polymers. One dielectric relaxation process is found which is strongly broadened and asymmetric. It is assigned a local hindered rotation of the ester groups attaching the spacer to the discotic mesogen. This assignment is supported by NMR measurements on the identical substance.
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  • 7
    ISSN: 1435-1536
    Keywords: Orientation ; deformation ; thermoplastic elastomer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The orientation of polybutadiene chains in thermoplastic elastomers based on hydrogen bonding complexes is investigated under uniaxial deformation by two-dimensional small-angle neutron scattering (SANS), deuteron magnetic resonance spectroscopy (2H-NMR), optical birefringence and infrared dichroism spectroscopy (FTIR-D). While SANS probes orientation on the length scale of the radius of gyration,2H-NMR, birefringence and FTIR-D monitor orientation on a segmental scale. The deformation of the elastomer chains appears to be affine on the different length scales.
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  • 8
    ISSN: 1435-1536
    Keywords: Key words Latex dispersions ; Film formation ; 1H and 2H solid-state nuclear magnetic resonance ; Water in polymer films
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract  The properties of polymer films prepared from latex dispersions are influenced by the drying or film formation process. In order to investigate this process, various systems of aqueous latex dispersions were dried until a specific solid content was reached. The samples investigated were based on vinyl acetate, vinyl acetate/ethylene and pure acrylics employing different surfactants and polyelectrolytes as stabilisers of the dispersions. The role of water in these partially dried films was investigated using 1H and 2H solid-state NMR spectroscopy. Different types of water could be distinguished in the spectra. The drying latex films were found to contain interfacial external water, water at ionic and nonionic groups at surfactants in the polymer/water interface and also water inside the swollen polymer. These different types of water were examined separately using various NMR techniques.
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  • 9
    ISSN: 1435-1536
    Keywords: Poly(diethyl siloxane) ; molecularmotions ; NMR ; temperatureinfluence ; differentphases ; chainmobility
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Molecular motions in poly(diethyl siloxane) were studied by solid-state29Si-NMR in the temperature range 180–350 K. In this temperature range two solid phasesβ 1 andβ 2, a mesophaseα m, and an amorphous isotropic phase exist. The nature of the chain mobility in the different phases was deduced from the resulting changes in the NMR line-shape governed by anisotropic chemical shift. In the intermediate solid phaseβ 2 its anisotropy is reduced by 25% compared with the low temperature phaseβ 1 due to the onset of oscillations around the chain axis and conformational transitions. In the mesophaseα m the polymer chain rotates about its long axis yielding an axially symmetric chemical shift tensor opposite in sign to that in theβ 1,β 2 phases. The broad transition of the mesophase into the isotropic phase is accompanied by an increase in a narrow Lorentzian line arising from the amorphous phase. The results are compared with previous1H NMR, Raman-spectroscopy and x-ray measurements.
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  • 10
    ISSN: 1435-1536
    Keywords: Polycarbonates ; molecular motions ; 2H NMR ; tensile stress
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Deuteron NMR was utilized to study phenylene group motions in glassy polycarbonate (PC) as a function of tensile stress and chemical modification. A special stretching device was constructed allowing the application of forces up to 1500 N during the measurement. While cold drawing of PC effects motional restrictions equivalent to a temperature shift of 10 K, a reversible strain near the yield point enhances the ring dynamics slightly. Methyl group substitution at the ortho positions gives rise to even more constraints, shifting the onset of fast ring flips by about 180 K.
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