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S. I. Moseenkov¹, A. V. Zavorin^1,2, and V. L. Kuznetsov^1,2

¹ Boreskov Institute of Catalysis SB RAS, Lavrentiev ave. 5, 630090 Novosibirsk, Russia

² Novosibirsk State University, Pirogova str. 2, 630090 Novosibirsk, Russia

kuznet@catalysis.ru

In this paper we suggested a method for evaluating the uniformity of the nanotube distribution in the MWCNT-polymer composites based on sequential measurements of their current-voltage-conductivity (СVС) characteristics in a wide range of applied voltages (E, up to 10³ V/mm). The MWCNTs in the composites form ohmic contacts (direct contacts between the nanotubes) and non-ohmic contacts (nanotubes in the contact are separated by several polymer chains). In our study we investigated composites with polyethylene and poly(methyl methacrylate) matrixes produced using MWCNTs with different aspect ratio (AR, 36 to 3000). In composites with uniform distribution of nanotubes (near the percolation threshold), large number of non-ohmic contacts results in high specific resistivity to 10¹³-10¹⁴ Ω・cm. This makes it difficult to measure the resistance at low E and impairs reproducibility of the results because partial transformation of contacts due to the heat release under electrical current takes place during the measurements already at E = 0.3 V/mm and current density 4・10^-8 A/cm². Furthermore, in the case of a high applied voltage, the decrease in resistance can reach 10⁵ due to the formation of new ohmic contacts between nanotubes. The number of ohmic contacts in the composites also increases when the conductivity and I–V characteristics are measured due to irreversible transformation of non-ohmic contacts into ohmic contacts under the action of electrical thermal breakdown. This effect increases together with the number of non-ohmic contacts in the composite, which was demonstrated for composites modified by MWCNTs with AR values ranging from 36 to 3000. Therefore, the MWCNT percolation threshold largely depends on measurement conditions and on the sample's "history". We demonstrated that cyclic I–V measurements can be used to characterize the nature of the contacts between the nanotubes in polymer composites, in particular, to determine the presence of non-ohmic and ohmic contacts, transformations of the former into the latter, and to control the conductivity of MWCNT based composites using electric fields with a strength higher than 1 V/mm.

The suggested method for activation of isolated contacts between nanotubes can be used for controlled modification of MWCNT based composites for production of functional materials or devices on their basis (e.g. pressure sensors or deformation sensors, etc.) [1,2].

Acknowledgments. The work was carried out within the framework of the RFBR project No. 20-33-70120.

References:

[1] Moseenkov S.I., et al., Journal of Structural Chemistry. 2020. V.61. N4. P. 628–639.

[2] Moseenkov S.I., et al., eXPRESS Polymer Letters. 2019. V.13. N12. P. 1057–1070.