Phase transitions, intermolecular interactions and electrical conductivity behavior in carbon multiwalled nanotubes/nematic liquid crystal composites

The phase transitions, intermolecular interactions and electrical percolation behavior in the composites of multiwalled carbon nanotubes (MWNTs) and nematic LC (p-ethoxybenzylidene-p′-butylaniline, EBBA) are experimentally investigated in this work. The concentration of nanotubes was between 0 and 1% (wt), and the temperature interval was 273–363 K. The experimental data of differential scanning calorimetry (DSC) and FTIR adsorption spectra evidence the presence of strong interactions inside MWNTs/EBBA nanocomposites. An increase of both isotropic-to-nematic (by 0.5-1 K) and nematic-to-crystalline (by 1.5-3 K) phase transition temperatures was observed for EBBA filled with MWNTs. The FTIR spectroscopic data evidence an enhancement of the strong binding interaction between MWNTs and EBBA with increase of the concentration of MWNTs. It was shown that intermolecular interactions affect noticeably the electrical conductivity of MWNTs/EBBA composites. The percolation transition from non-conductive to high conductive state, and the effect of positive temperature coefficient (PTC effect) were observed at MWNT concentrations exceeding 0.05-0.1 wt.% Within the nematic phase temperature range, the electrical conductivity of the composites reveals a noticeable heating–cooling hysteresis and follows Arrhenius behavior. The activation energy decreases when MWNTs concentration increases, which supports the dominating role of the junction mechanism related to the thermal motion of nanotubes. The electric field dependence of the electrical conductivity was explained by the existence of the field-enhanced charge transport through hopping junctions in the LC gaps.