Electrical conductivity, optical transmittance and microstructure of multiwalled carbon nanotubes (MWCNTs) dispersed in nematic liquid crystal 4-ethoxybenzylidene-4′-n-butylaniline (EBBA) were studied in the temperature range between 287 and 363 K. The concentration C of MWCNTs was varied within 0.01–1% wt. The percolation threshold with a noticeable increase in electrical conductivity (by many orders of magnitude) was observed in the vicinity of C ≈ 0.1% wt. The heating–cooling hysteretic behaviour of electrical conductivity and optical transmittance thermal pre-history effects were studied. These effects reflected strong agglomeration and rearrangement of nanotubes during the thermal incubation. The estimates show that transient behaviour during the thermal incubation can be caused by Brownian motion of MWCNTs. The solidification of MWCNT + EBBA composite in the nematic range extended by conditions of supercooling was also studied as a function of temperature using electrical conductivity measurements. The solidification lag-time dependence on supercooling temperature followed the classical heterogeneous nucleation law, with MWCNTs serving as centres of EBBA solidification.
This work reports a study on microstructure, electrical conductivity and phase transitions of 5CB(4-pentyl-49-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 C p L 0.025 -0.05% wt and C=C p L 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 L 3.3 l 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. 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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