62 research outputs found
End-chain segment ordering in lamellar sublayers of a diblock copolymer
The segmental ordering within a lamellar polystyrene-polydimethylsiloxane
diblock (PS-PDMS) was examined by NMR. A non-uniform stretching of the
PDMS chains was evidenced. Although chain segments displaying isotropic reorientational
motions are present along the chains, relaxation measurements on the free PDMS
extremities demonstrate that these latter are oriented and submitted to a non-zero
constraint. This specific result allows to precise some features of the order distribution
within the lamellae
Self-confined polymer dynamics in miscible binary blends
The segmental dynamics of PVME within the single-phase state of poly(styrene)/poly(vinyl methyl ether) blends (PS/PVME) was examined by dielectric spectroscopy. A particular attention has been given to the high PS concentration regime. In this latter, rather localized, weakly cooperative motions of the PVME segments are detected at low temperatures, in addition of the secondary relaxation processes. This feature is attributed to confinement effects induced by the PS chains on the PVME
Chain segment ordering in lamellar sublayers of block copolymers: A NMR study
The chain segment dynamics in the bulk lamellar phase of
polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The
experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with
twice the molecular weight. In the diblock, at room temperature, the PDMS block segments
undergo uniaxial reorientations around the normal to the lamellae. In the triblock,
the reorientational motions exhibit a lower degree of symmetry: deviations from a
uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both
PDMS chain ends into the PS glassy layers
Heterogeneity of the chain segment dynamics and ordering in lamellar layers of a symmetric triblock copolymer
The segmental ordering within the sublayers of a lamellar
polystyrene-poly dimethylsiloxane-polystyrene triblock
(PS-PDMS-PS) is examined by NMR. The carbon and deuterium NMR
line shapes of the PDMS sequence (soft block) reveal clearly a
wide distribution of segmental orientational order S in the
lamellae and thus a distribution of constraints probed by the
chains. It is observed that this distribution is mainly due to
the coexistence of positive and negative S, resulting from a
competitive ordering effect induced by the presence of glassy
interfaces ( in their vicinity) and the chain-end anchoring
junctions ()
Segmental order and dynamics of polymer chains confined in block copolymer lamellar mesophases: NMR and dielectric relaxation studies
The PDMS lamellar sublayers of a poly(styrene)-poly(dimethylsiloxane) diblock (PS-PDMS) and PS-PDMS-PS triblocks are investigated by NMR and dielectric spectroscopy. Some segments of the confined PDMS chains display anisotropic orientational fluctuations along the interfaces with the PS glassy blocks, whereas the others display fluctuations rather parallel to the lamellae normal. This coexistence results from a competitive ordering effect induced by the glassy interfaces and the chain-end anchoring junctions. The distribution of PDMS relaxation times within the sublayers is also examined: in particular, a slowing down of the segmental motions, together with a broadening of this distribution, are detected
Structural reinvestigation of Li3FeN2: Evidence of cationic disorder through XRD, solid-state NMR and Mössbauer spectroscopy
International audienceA significant cationic disorder is evidenced on Li3FeN2 prepared through solid-state reaction under controlled atmosphere. This derivative anti fluorite type structure (orthorhombic, space group Ibam, a=4.870(1) Å, b=9.652(1) Å and c=4.789(1) Å), solved first through single crystal X-ray diffraction [7], is usually described by Li+ and Fe+3 ordered distribution in tetrahedral sites formed by the nitrogen network, leading to [FeN4/2]3− edge-sharing tetrahedral chains. From 7Li/6Li Nuclear Magnetic Resonance spectroscopy, 57Fe Mössbauer spectroscopy and powder X-ray and neutron diffraction, we demonstrate that about 4% of lithium sites are filled by iron and about 11% of iron sites are occupied by Li, which can explain the discrepancy within the Gudat's model observed on larger scale solid-state synthesis samples
- …