A possibility was assessed to modify physico-chemical properties of ferronematic mixtures by introducing additional mesogenic and non-mesogenic components. The mixture, consisting of RM734 and DIO at ...the 70:30 weight ratio of the components, was doped with nematic 5CB and its chiral isomer CB15. This allowed a substantial lowering of the ferronematic temperature range, as well as induction of helical twisting in both nematic (N) and ferronematic (N
F
) phases. The effects of 5CB and CB15 on the N
F
to N phase transition were nearly identical, but the isotropic transition temperature decreased more strongly upon addition of CB15. Optical transmission vs. temperature measurements showed lower transmittance in the N
F
phase, probably due to scattering on the domain boundaries. In the vicinity of the N
F
→ N transition, a noticeable range of anomalously low transmission was noted both on heating and cooling, suggesting a complex character of molecular reorientation features. In the chiral N
F
(N*
F
) system, helical twisting was recorded using Grandjean-Cano wedge, with the helical pitch decreasing upon cooling in both N* and N*
F
phases. In the cooling mode, transient undulations emerged in the N*
F
phase. The complex picture of the ferronematic transition is supported by POM images taken at different temperatures.
The structure and microhardness of a multilayer surfaced on an R2M9 high-speed steel in a nitrogen atmosphere and subjected to high tempering and electron-beam treatment are studied. The surfaced ...layer structure consists of a tempered martensite, with the carbides and carbonitrides located along the grain boundaries. It is shown that high tempering increases the steel microhardness and gives rise to a transformation of the retained austenite into martensite. An electron-beam treatment forms a honeycomb structure in the surface layer, which provides a factor of 1.1–1.6 increase in the surface layer microhardness over the initial state and that after tempering.
Optical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At ...small concentrations of CNTs (C ≈ 0.01-0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density D was shown to be essentially non-linear and a minimum at certain concentration of CNTs, C ≈ 0.05-0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced trans-cis-trans isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.
.
Comparative studies of optical transmission of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), dispersed in nematic liquid crystal matrix 5CB, were carried out. ...The data evidence violations of Beer-Lambert-Bouguer (BLB) law both in cell thickness and concentration dependencies. The most striking is the fact that optical transmission dependencies for SWCNTs and MWCNTs were quite different in the nematic phase, but they were practically indistinguishable in the isotropic phase. Monte Carlo simulations of the impact of aggregation on direct transmission and violation of BLB law were also done. The results were discussed accounting for the tortuous shape of CNTs, their physical properties and aggregation, as well as strong impact of perturbations of the nematic 5CB structure inside coils and in the vicinity of CNT aggregates.
Graphical abstract
Liquid crystalline (LC) mixtures of cholesteryl oleyl carbonate (COC) and 4-pentyl-4'-cyanobiphenyl (5CB) as well as dispersions of single-walled carbon nanotubes (NTs) in these mixtures were studied ...by means of selective reflection measurements, differential scanning calorimetry (DSC) and optical microscopy. The relative mass of COC in a mixture X was varied between 0.4 and 1.0, the temperature range of measurements was between 284 and 314 K, and concentration of NTs was fixed at 0.1%. Two important anomalies were noted: (1) the cholesteric to smectic-A transition temperature increased on dilution of COC by non-smectogenic 5CB in the concentration range 0.8 < X < 1 and (2) the reciprocal pitch versus 5CB concentration dependence was essentially linear, in contrast to behaviour commonly observed in nematic-cholesteric mixtures. A model of molecular arrangement in the mixtures, accounting for the possibility of integration of 5CB dimers and monomers between COC molecules and presumably explaining the experimental data, was proposed. The helical pitch of the cholesteric mixtures remained practically unchanged upon doping by NTs, and only slight widening of the selective reflection peaks was noted. The obtained results allow considering the COC + 5CB mixtures as promising matrices for composite materials on the basis of liquid crystals and NTs.
Optical transmittance and electric conductivity were studied for dispersions of multiwall carbon nanotubes (MWCNTs) in nematic liquid crystals of four different chemical classes (cyanobiphenyls, ...Schiff bases, azoxy compounds, and cyclohexanecarboxylic acids). The optical transmittance data showed deep integration of MWCNTs into the orientationally ordered nematic structure. The effect observed was apparently related to interactions between the MWCNTs and mesogen matrix, which appeared to be weaker when the nematic host was formed by non-aromatic molecules. Dependences of the electrical conductivity versus applied DC voltage were noticeably different for different nematic matrices. Voltage amplification of electrical conductivity was observed at relatively higher voltages (above 10
V). The increased threshold voltages for electrohydrodynamical instability in nematics were observed in MWCNT-doped dispersions.
The phase transitions in smectogenic liquid crystal BBBA (4-butoxybenzylidene-4′-butylaniline) doped by multi-walled carbon nanotubes (NTs) were studied by methods of optical transmission, ...differential scanning calorimetry (DSC), measurement of electrical conductivity and analysis of microscopic images. The concentration of NTs was varied within 0-1% wt. Non-monotonous (extremal) changes in temperature, enthalpies and half-width of the DSC peaks of transitions between different phases (smectics, nematic, isotropic) were observed for NT concentrations between 0.05 and 0.1% wt. A noticeable increase of electrical conductivity σ in the same concentration interval evidenced the presence of percolation transition and formation of conductive NT networks. The detailed analysis of σ behavior in the whole concentration interval 0-1% wt revealed the presence of a fuzzy type percolation with multiple thresholds in the studied BBBA + NT suspensions. The percolation behavior was strongly dependent on the temperature, and a noticeable step-like drop of σ in the vicinity of isotropic-nematic transition was observed after the multiple heating-cooling cycles.
This work reports a study on microstructure, electrical conductivity and phase transitions of 5CB(4‐pentyl‐4′‐cyanobiphenyl) + multi‐walled carbon nanotubes (MWCNTs) composite materials. The ...concentration of MWCNTs was varied within 0.025 and 1 wt.%. The unmodified (o) and modified (m) MWCNTs were used. The mild milling was applied for shortening length of MWCNTs. Direct microscopic observation of composites at different concentrations C of MWCNTs evidenced the presence of aggregation and percolation processes. Formation of percolation networks was observed at Cp ≈ 0.025–0.05% wt and C=Cp ≈ 0.1–0.25% wt for o‐MWCNTs and m‐MWCNTs composites, respectively. Increase of the percolation threshold for m‐MWCNTs, possibly, reflected their smaller aspect ratio r. The conductivity exponents were approximately the same (t ≈ 3.3 ± 0.1) for both o‐MWCNTs and m‐MWCNTs. The microstructure of MWCNT aggregates was not stable in time. The initially formed loose aggregates (L‐aggregates) transformed into the more compact C‐aggregates during the incubation time of one week. The incubation resulted in decrease of anisotropy, change in phase behaviour near the nematic‐isotropic transition, and influenced external electric field response of the studied composite material.
In dieser Arbeit werden die Mikrostruktur, die elektrische Leitfähigkeit und die Phasenübergänge in Kompositen aus 5CB (4‐pentyl‐4′‐cyanobiphenyl) und mehrschichtigen Kohlenstoff‐Nanoröhren (MWCNTs) behandelt. Die Konzentration der MWCNTs liegt im Bereich zwischen 0.025 und 1 Gew.‐%. Es wurden unbehandelte (o) und modifizierte (m) MWCNTs verwendet. Um die Länge der MWCNTs zu verkürzen, wurden sie schonend gemahlen. Die direkte mikroskopische Beobachtung der Komposite bei unterschiedlichen Konzentrationen von MWCNTs zeigte das Vorhandensein von Aggregations‐ und Perkolationsprozessen an. Die Bildung eines Perkolationsnetzwerkes wurde im Konzentrationsbereich 0,025–0,05% Gew.‐% für o‐MWCNTS und 0,1–0,25 Gew.‐% für m‐MWCNTs beobachtet. Eine Erhöhung der Perkolationsschwelle für die m‐MWCNTs könnte mit dem kleineren Aspektverhältnis r zusammenhängen. Die Leitfähigkeitsexponenten sind für beide MWCNTs ungefähr gleich (t ≈ 3,3 ± 0,1). Die Mikrostruktur der MWCNT Aggregate war zeitlich nicht stabil. Die urspünglich gebildeten losen Aggregate (L‐Aggregate) wandeln sich in die mehr kompakten Aggregate (C‐Aggregate) während der Inkubation innerhalb von einer Woche um. Die Inkubation führt zu einer Abnahme der Anisotropie, einem Wechsel der Phase in der Nähe der nematischen‐isotropen Umwandlung und beeinflusst die Antwort auf ein externes elektrisches Feld der studierten Kompositmaterialien.
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