2,383 research outputs found
The Effect of Age, Sex, Strain, Species and Dose Level Differences Upon the Metabolism of 2-Naphthylamine in Rodents
THE production of bladder tumours by aromatic amines has been correlated with the excretion of ortho-hydroxylated metabolites (Clayson, 1962). In the case of 2-naphthylamine, Bonser, Clayson and Jull (1951) showed that the susceptibility of a species to bladder tumour induction on feeding this agent appeared to depend on the proportion of a dose metabolised to give 2-amino-1-naphthol and its conjugates. It was further shown that 2-amino-i-naphthol was highly carcinogenic upon implantation into the bladders of mice whilst the amine itself was only feebly carcinogenic under these circumstances (Bonser
Diffusive processes in polyacrylic acid hydrogels
The aim of this work was to characterise the diffusive properties of superabsorbent polymer systems prepared by free radical crosslinking polymerisation of acrylic acid. The polyelectrolyte nature of these hydrogels gives rise to high swelling capacities, and their ability to absorb and retain water is highly dependent on the precise network structure. Modifying the synthesis formulation results in considerable changes to the dynamic and structural properties of these gels, providing the motive for the work presented here. The influence of two factors, namely the extent of monomer neutralisation and the level of crosslinker in the pre-gel solution, were investigated. The dynamic properties of gels were examined using Quasi-Elastic Light Scattering (QELS), from which the cooperative diffusion coefficient and degree of heterogeneity could be determined. The former was found to increase linearly with neutralisation, due to the introduction of electrostatic interactions. The diffusion coefficient initially remained constant with the addition of crosslinker, due to the dominating influence of physical entanglements, but increased above a threshold crosslinking degree, corresponding to a reduction of the network mesh size. The extent of large-scale inhomogeneity increased for higher ionisations, as both the crosslinker solubility and the efficiency of monomer-crosslinker reaction decreased. However, there was a tendency for concentration fluctuations to be minimised for higher neutralisations, making the gel more microscopically homogeneous. Kinetics of swelling experiments were used to investigate gels of varying composition. The macroscopic diffusion coefficient was found to increase rapidly with increasing neutralisation until the monomer was approximately 35% neutralised, beyond which point counterion condensation caused insignificant variation. This trend was also reflected in the equilibrium swelling ratios, and mode of diffusion. For the majority of gels, the diffusion process was characterised as case II transport. Variation of crosslinking degree caused an overall increase in the diffusion coefficient, reflecting the trend observed in the QELS studies. Nuclear Reaction Analysis (NRA) was used to probe the penetration of heavy water into dry network slabs. The concentration-depth profiles revealed a discontinuity in the diffusion coefficient, corresponding to the transition between glassy and rubbery states, for which the diffusivities differed by several orders of magnitude. The kinetics of plasticisation was assumed to be the rate determining factor in the swelling process, on the timescale of the NRA experiments. The diffusion coefficient for the swollen rubbery region, representative of the macroscopic diffusion process, was found to increase linearly with neutralisation, and decrease with crosslinking degree. The latter observation was explained as due to a reduction in the free volume available for solvent diffusion with higher levels of crosslinker
Static and Dynamical Susceptibility of LaO1-xFxFeAs
The mechanism of superconductivity and magnetism and their possible interplay
have recently been under debate in pnictides. A likely pairing mechanism
includes an important role of spin fluctuations and can be expressed in terms
of the magnetic susceptibility chi. The latter is therefore a key quantity in
the determination of both the magnetic properties of the system in the normal
state, and of the contribution of spin fluctuations to the pairing potential. A
basic ingredient to obtain chi is the independent-electron susceptibility chi0.
Using LaO1-xFxFeAs as a prototype material, in this report we present a
detailed ab-initio study of chi0(q,omega), as a function of doping and of the
internal atomic positions. The resulting static chi0(q,0) is consistent with
both the observed M-point related magnetic stripe phase in the parent compound,
and with the existence of incommensurate magnetic structures predicted by
ab-initio calculations upon doping.Comment: 15 pages, 8 figure
Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of FeCoSi
We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron
Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of
the helimagnetic transition in FeCoSi with = 0.30. In contrast
to the sharp transition observed in the archetype chiral magnet MnSi, the
transition in FeCoSi is gradual and long-range helimagnetic
ordering coexists with short-range correlations over a wide temperature range.
The dynamics are more complex than in MnSi and involve long relaxation times
with a stretched exponential relaxation which persists even under magnetic
field. These results in conjunction with an analysis of the hierarchy of the
relevant length scales show that the helimagnetic transition in
FeCoSi differs substantially from the transition in MnSi and
question the validity of a universal approach to the helimagnetic transition in
chiral magnets
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