A.I. Sibilev

NSE-study of fullerene-containing polymers

Abstract Stars of Poly(styrene) chains grafted to C60 have been studied by SANS and NSE below glass transition (TG∼94∘C). In stars the arms are not condensed at fullerene surface, spreading away from C60-core. The interaction between fullerene core and polymer chains slows down the β-relaxation in arms at T=−20 to +70 ∘ C

Magnetic phase ordering in ferrogels under applied field

Abstract A new cross linked polymer compound (hydrogel) with fine Fe3O4-grains embedded in it has been investigated by neutron scattering. We studied the nanoscale structures of these ferrogels synthesized in a magnetic field and without a magnetic field. By means of polarized neutron scattering we studied the self-assembly of the particles under the influence of the magnetic field. Even in a weak field (H∼65 Oe ) the ferrogel showed strong spatial correlations of the particles; a strong field (H∼4 kOe ) induced slow growth of the interference effect. Over the observation time (several tens of hours) a cluster formation of particles was observed. This magnetic ordering is reversible: it disappears when switching off the external field.

Segmental dynamics in stars of poly(ethylene oxide) chains grafted to fullerene

Abstract We have studied the structure and dynamics of star-shaped poly(ethylene-oxide) (PEO), grafted to fullerene, in D2O-solutions as compared to linear polymer. The neutron scattering gives the radius of stars having a value close to the end-to-end distance of their PEO arms. The spin-echo measurements revealed a strong damping of segmental diffusion in stars showing stretched relaxation behavior.

NSE-study of poly(N-vinylcaprolactam) by coil–globule transition

Abstract Poly(N-vinylcaprolactam) was studied by SANS and NSE around the coil–globule transition (TC=32°C) at the threshold of overlapping of molecules in D2O-solution ( C=0.5 wt%, mass M=1×106). For T⩾TC chain units’ interaction grown by temperature increase makes coils collapsed. In the dynamics, an anomalous behaviour was observed: together with a slowing down of the diffusion as in the Rouse model, in the globular state the chains demonstrate fast stretched oscillations.

Magnetic-field-induced slowing-down of molecular rotation in C60 crystals

The molecular dynamics of C60 crystals was studied by inelastic neutron scattering at T=290 K, i.e., above the first-order phase transition temperature (TC≈260 K), in the region of free C60-spheroid rotation in the lattice. The energy broadening of the original neutron spectrum 2Γ0≈0.1 meV for a momentum transfer q=2 Å−1 is in agreement with NMR data on the rotational relaxation time of the molecule τ∼10−11 s∼ ℏΓ0. This effect was observed to decrease in magnetic fields H=2.5–4.5 kOe applied along the scattering vector: ΓH=0.7Γ0. The slowing-down of the molecular rotation is discussed in connection with the interaction of a magnetic field with the molecular currents, which fluctuate when the C60 cage rotates.

Dynamics of concentrated ferrofluid with labelled particles

In the study of ferrofluid dynamics the separation of self and pair correlation is the essential problem at both intermediate and high concentrations of the magnetic phase. The neutron spin-echo (NSE) total scattering function S t (q, t) = βS p (q, t) + (1 - β)S s (q, t) contains both contributions, where the parameter β = σ p /(σ p + σ s ) depends on the corresponding cross sections σ s.p . To distinguish these different kinds of dynamics we eliminated one of them by using a mixture of magnetic particles with different scattering amplitudes and concentrations. The NSE experiments (LLB, CE-Saclay) at momentum transfer q 1 0.4 nm ∼ 1/D p (D p is the particle diameter) and q 2 0.7 nm > 1/D p have been carried out for these opposite situations. The functions S p (q, t), S s (q, t), from these data behave quite differenly. The autocorrelations reveal a stretched relaxation at low q: S s (q, t) = exp[- q 2 Γ(t)/2], where Γ(t)= 2Dt α is the squared particle displacement, the exponent α = 0.5 ± 0.1 at q 1 , and α = 0.8 ± 0, at q 2 . The magnitude α < 1 indicates the strong influence of dipole forces on the motion of a particle. It resembles the segmental relaxation in a polymer chain. On the other hand, the pair correlations show the oscillations (period ∼ 40 ns, amplitude ∼ 1 nm) mixed with diffusion.