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1
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Cu_3(tmen)_3(tma)_2(H_2O)_2 ·6.5H_2O - New S = 1/2 "Sawtooth" Chain?

100%
Vrábel P., Orendáčová A., Černák J., Orendáč M.
Acta Physica Polonica A
|
2010
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vol. 118
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issue 5
993-994
EN
The results of the investigation of magnetic susceptibility, magnetization and specific heat of Cu_3(tmen)_3(tma)_2(H_2O)_2·6.5H_2O (tmen = N,N,N',N'-tetramethylethane-1,2 diamine; H_3tma = 1,3,5-benzenetricarboxylic acid) are reported. The spatial arrangement of magnetic Cu(II) ions and network of covalent bonds suggest that the studied material might be a representative of S = 1/2 sawtooth chain with moderate exchange coupling J/k_{B}. The investigation of the temperature dependence of susceptibility and magnetic field dependence of magnetization yielding J/k_{B} ≈ - 0.63 K is consistent with the structural features. In addition, specific heat data reveal short-range correlations in milikelvin temperature range and indicate long-range ordering below 150 mK.
2
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Solid State ¹³C Nuclear Magnetic Resonance Study of Morphology and Molecular Mobility in Quenched Poly(3-hydroxybutyrate)

100%
Baran A., Vrábel P., Olčák D.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1144-1146
EN
Single pulse magic angle spinning ¹³C NMR spectra and the carbon spin-lattice relaxation times T₁(¹³C) were used for the study of morphology and molecular mobility in poly(3-hydroxybutyrate) quenched in cold water after melting in a twin-screw extruder. The crystallinity of the quenched sample was found to be comparable with that of virgin material, and the decrease in dimensions of the crystallites grown after quenching was deduced from the single pulse magic angle spinning ¹³C NMR spectra. The spin-lattice relaxation curves for the carbons of the CO, CH, CH₂ and CH₃ groups were found to be bi-exponential, which enables the molecular mobility within crystalline and amorphous domains to be studied. The spin-lattice relaxation times T₁(¹³C) estimated for the carbons of the CO, CH, CH₂ groups show the chain mobility enhancement within crystalline domains of the quenched sample, but the influence of the quenching on the CH₃ group rotation and on the main chain motion within amorphous regions was not observed.
3
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Estimation of the Degree of Crystallinity of Partially Crystalline Polypropylenes Using ^{13}C NMR

88%
Hronský V., Koval'aková M., Vrábel P., Uhrínová M., Olčák D.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
409-410
EN
The paper deals with a method for the estimation of the degree of crystallinity for partially-crystalline isotactic-polypropylenes (i-PP) using high-resolution solid-state ^{13}C NMR. For this purpose direct polarization ^{13}C MAS NMR spectra were measured for i-PP samples with different degrees of crystallinity at 98°C. The areas beneath the resonance lines in these spectra correspond to the number of carbons in particular functional groups, while the widths and shapes of the lines reflect the degree of crystallinity, crystalline modifications, distribution of chain conformations and the chain mobility. The ^{13}C MAS NMR spectra, measured using appropriate combination of delay time and high proton decoupling field, made it possible to detect only amorphous domains in the sample. This enabled identification of the lines associated with the amorphous domains in the complete ^{13}C MAS NMR spectra and provided sufficient information for reliable estimation of the degree of crystallinity. The heteronuclear Overhauser enhancement of the ^{13}C NMR signals due to short delay time was taken into account in our calculations.
4
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Spin Glass State in S=3/2 Kagomé Antiferromagnet Co(N_{3})_{2}(bpg)[(CH_{3})_{2}(NCOH)]_{4/3}

88%
Vrábel P., Čižmár E., Orendáčová A., Gao S., Orendáč M.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
266-267
EN
Specific heat, magnetic susceptibility and magnetization of Co(N_{3})_{2}(bpg)[(CH_{3})_{2}(NCOH)]_{4/3}, representing S=3/2 kagomé system are reported. Long-range ordering at 16 K was revealed, however, at lower temperatures slow spin dynamics is still found. The analysis of alternating susceptibility suggests the onset of glassy state. The study of the time dependence of magnetization revealed the existence of more relaxation channels with pronounced different relaxation times. The observed behaviour is consistent with the formation of topological spin glass in which relaxation is governed by both spin and chiral degrees of freedom.
5
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Solid State ^{13}C NMR Study of Modified Polyhydroxybutyrate

76%
Vrábel P., Hronský V., Fričová O., Koval'aková M., Chodák I., Alexy P.
Acta Physica Polonica A
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2014
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vol. 126
|
issue 1
419-420
EN
Structural changes in polyhydroxybutyrate (PHB) brought about by addition of chain extender and/or plasticizer were studied using ^{13}C MAS NMR spectra, which were measured using the direct polarization technique at 30 and 98°C. Four resonances associated with the CO, CH, CH_{2} and CH_{3} groups of PHB were observed in the spectra. The CH_{3} resonance appearing at 98°C shows splitting into two lines related to the crystalline and amorphous regions. Analysis of the CH_{3} resonances made it possible to estimate the influence of applied modifications on crystallinity and distribution of conformations in amorphous and crystalline domains of PHB samples.
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