396 research outputs found
Spontaneous thermal expansion of nematic elastomers
We study the monodomain (single-crystal) nematic elastomer materials, all
side-chain siloxane polymers with the same mesogenic groups and crosslinking
density, but differing in the type of crosslinking. Increasing the proportion
of long di-functional segments of main-chain nematic polymer, acting as network
crosslinking, results in dramatic changes in the uniaxial equilibrium thermal
expansion on cooling from isotropic phase. At higher concentration of main
chains their behaviour dominates the elastomer properties. At low concentration
of main-chain material, we detect two distinct transitions at different
temperatures, one attributed to the main-chain, the other to the side-chain
component. The effective uniaxial anisotropy of nematic rubber, r(T)
proportional to the effective nematic order parameter Q(T), is given by the
average of the two components and thus reflects the two-transition nature of
thermal expansion. The experimental data is compared with the theoretical model
of ideal nematic elastomers; applications in high-amplitude thermal actuators
are discussed in the end
UV-isomerisation in nematic elastomers as a route to photo-mechanical transducer
The macroscopic shape of liquid crystalline elastomers strongly depends on
the order parameter of the mesogenic groups. This order can be manipulated if
photoisomerisable groups, e.g. containing N=N bonds, are introduced into the
material. We have explored the large photo-mechanical response of such an
azobenzene-containing nematic elastomer at different temperatures, using force
and optical birefringence measurements, and focusing on fundamental aspects of
population dynamics and the related speed and repeatability of the response.
The characteristic time of ``on'' and ``off'' regimes strongly depends on
temperature, but is generally found to be very long. We were able to verify
that the macroscopic relaxation of the elastomer is determined by the nematic
order dynamics and not, for instance, by the polymer network relaxation.Comment: Latex (EPJE class) 12 figure
Fast microwave-assisted oxidation of 1,4-dihydropyridines with FeCl 3.SiO 2
Pyridine derivatives are easily obtained in high yields by microwave-promoted rapid oxidation of the corresponding 1,4-dihydropyridines with ferric chloride hexahydrate and silica gel under solvent-free conditions
Nematic elastomers with aligned carbon nanotubes: new electromechanical actuators
We demonstrate, for the first time, the large electromechanical response in
nematic liquid crystalline elastomers filled with a very low (~0.01%)
concentration of carbon nanotubes, aligned along the nematic director at
preparation. The nanotubes create a very large effective dielectric anisotropy
of the composite. Their local field-induced torque is transmitted to the
rubber-elastic network and is registered as the exerted uniaxial stress of
order ~1kPa in response to a constant field of order ~1MV/m. We investigate the
dependence of the effect on field strength, nanotube concentration and
reproducibility under multiple field-on and -off cycles. The results indicate
the potential of the nanotube-nematic elastomer composites as electrically
driven actuators
Detection of some virulence factors in Staphylococcus aureus isolated from clinical and subclinical bovine mastitis in Iran
Mastitis is one of the common diseases of dairy cattle and an inflammatory response of the mammary glands tissue. Mastitis causes considerable loss to the dairy industry. Among several bacterialpathogens that can cause mastitis, Staphylococcus aureus is probably the most lethal agent because it causes chronic and deep infection in the mammary glands that is extremely difficult to be cured. The present study was to detect some of the virulence factors in the S. aureus isolated from 360 mastitis milk samples in Chaharmahel va Bakhtiari and Isfahan provinces of Iran via PCR by using specific primers. Among a 360 raw milk samples, 86 samples contained 1250 bp fragment of the 23srRNA gene,42 samples contained coa gene, 63 samples contained clfA gene, 69 samples contained IgG binding region gene, 22 samples contained X region coding gene protein A, 3 sample contained Toxic shock syndrome toxin gene (tst), 16 samples contained the exfoliative toxin A and B genes, 10 samples contained agrI gene, 42 samples contained agrII gene, 19 samples contained agrIII gene and 15 samples contained agrIV gene
Titanium dioxide nanoparticles catalyzed synthesis of Hantzsch esters and polyhydroquinoline derivatives
1,4-Dihydropyridine and polyhydroquinoline derivatives have been prepared efficiently in a one-pot synthesis via Hantzsch condensation using nanosized titanium dioxide as a heterogeneous catalyst. The present methodology offers several advantages such as excellent yields, short reaction times (30-120 min), environmentally benign, and mild reaction conditions. The catalyst can be readily separated from the reaction products and recovered in excellent purity for direct reuse
Photonic gaps in cholesteric elastomers under deformation
Cholesteric liquid crystal elastomers have interesting and potentially very
useful photonic properties. In an ideal monodomain configuration of these
materials, one finds a Bragg-reflection of light in a narrow wavelength range
and a particular circular polarization. This is due to the periodic structure
of the material along one dimension. In many practical cases, the cholesteric
rubber possesses a sufficient degree of quenched disorder, which makes the
selective reflection broadband. We investigate experimentally the problem of
how the transmittance of light is affected by mechanical deformation of the
elastomer, and the relation to changes in liquid crystalline structure. We
explore a series of samples which have been synthesized with photonic stop-gaps
across the visible range. This allows us to compare results with detailed
theoretical predictions regarding the evolution of stop-gaps in cholesteric
elastomers
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The occipital lateral plate mesoderm is a novel source for vertebrate neck musculature
In vertebrates, body musculature originates from somites, whereas head muscles originate from the cranial mesoderm. Neck muscles are located in the transition between these regions. We show that the chick occipital lateral plate mesoderm has myogenic capacity and gives rise to large muscles located in the neck and thorax. We present molecular and genetic evidence to show that these muscles not only have a unique origin, but additionally display a distinct temporal development, forming later than any other muscle group described to date. We further report that these muscles, found in the body of the animal, develop
like head musculature rather than deploying the programme used by the trunk muscles. Using mouse genetics we reveal that these muscles are formed in trunk muscle mutants but are absent in head muscle mutants. In concordance with this conclusion, their connective tissue is neural crest in origin. Finally, we provide evidence that the mechanism by which these neck muscles develop is conserved in vertebrates
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