Structure and properties of metal-free conductive tracks on polyethylene/multiwalled carbon nanotube composites as obtained by laser stimulated percolation
Introduction
Composites materials based on conducting fillers such as metal particles, carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene, dispersed in insulating polymeric matrices are attracting a considerable interest for their use in polymeric and flexible electronics, anti-static and electromagnetic interference shielding, sensoring and structural applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].
Due to the excellent electric properties of CNTs [12], CNT/polymer composites have been extensively investigated and low density polyethylene (LDPE), high and ultra high density polyethylene (HDPE and UHMWPE) have been used as matrices [13]. The resulting electric properties are mainly determined by the filler concentration. Composites characterized by filler concentration lower than ≈0.5–5 wt.% are characterized by very low conductivity, while composites with concentrations higher than the percolation threshold (i.e., the concentration associated with the formation of a continuous net of CNTs), are characterized by conductivity several order of magnitude larger.
In particular, considerable efforts have been made on the fabrication of conducting tracks based on metals (e.g., Ag) and/or carbon [7], [14], [15], [16], doped polymers (e.g., polyacetylene, polythiophene, pentacene, etc.) [17], on non conducting matrices.
As for conductive tracks obtained by thermal treatment of low concentrated CNT/polymer matrices, primarily constituted by immiscible polymer blends and containing CNTs in low amount and additives, information was first reported in an exhibition fair [18] and in a patent [19]. According to these reports, upon thermal heating by means of a wide range of methods including laser treatment, the nonconductive surface of CNT/polymer blends loaded with additives is becoming conductive. In these reports no detailed information about the structure and properties characterization at micro-nano level of the produced systems is reported. For sake of completeness, it is useful to recall that the laser writing technique has also proved its utility for in situ localized reduction of graphite oxide [16].
In this paper, we describe the formation of conductive paths obtained by CO2-pulsed laser irradiation (laser writing) of the surface of multiwalled carbon nanotubes (MWCNTs)/polymer composites constituted by a single polymer phase (high density polyethylene, HDPE) and MWCNTs with concentration ranging in the 0.5–5 wt.% interval. No additives and additional phases are present in our system, which so appears ideal for a basic investigation at the micro/nano level of the laser induced percolation process and on the structure of the conductive tracks. It will be shown by means of several techniques that in the regions stimulated by the laser beam, the polymer melting is induced together with the formation of an accumulation layer, where the percolation of nanotubes is occurring. This leads to the formation of tracks characterized by an enhancement of conductivity of several orders of magnitude. The obtained tracks are stable because the accumulation layer is firmly adhering to the unperturbed polymer phase. This result is highly relevant because a selective control of the conductivity along tracks of any selected pattern, surrounded by an insulating bulk, is important in many industrial applications, where the dispersion of the electric signal through the composite must be avoided.
Section snippets
Material preparation
The carbon nanotubes used in this work were previously and extensively characterized in other papers [20], [21]. The CNTs are highly-crystalline multiwalled carbon nanotubes, 10–15 μm in length and with external diameters of 30–90 nm. The high-cristallinity and purity of the material are confirmed by Raman spectroscopy and AE-ICP spectrometry (Fe 0.61 ± 0.01, Al 0.04 ± 0.05, Ni 0.003 ± 0.001). MWCNTs/polymer composites were prepared by the melt compounding of High Density Polyethylene (HDPE) in a mixer
The laser printing methodology and the morphological modifications
The tracks have been produced under controlled atmosphere by means of a focalized laser beam impinging on the surface of composites plates containing a variable concentration of carbon nanotubes. The scheme of the laser manifold is illustrated in Fig. 1a, while details concerning the preparation and the structure of the composites with concentration ranging in the 1–5 wt.% interval are reported in Supplementary data. The effect of the laser beam is the localized melting of the polymer phase with
Conclusions
The fabrication of metal-free conductive tracks, firmly adhering on the surface of HDPE/MWCNT composites, by means of a laser stimulated localized percolation of carbon nanotubes is illustrated. The IR-based laser beam, being strongly absorbed by the carbon nanotubes, causes the local melting of the composite with formation of v-shaped channels. The concentration of MWCNTs in the external layer of the walls of the v-shaped tracks increases overcoming the percolation concentration even on low
Acknowledgements
This work was supported by NANOCONTACT project (Progetto CIPE 07- Converging Technologies (2009-2012)), Regione Piemonte (Italy). The authors thank: Dr. Alessandro Damin (University of Torino, Dept. of Chemistry) for the Raman investigation and for the technical support; Dr. Gerald Kada and Dr. Matthias A. Fenner (Agilent Technologies, Frankfurt – Germany) for the Kelvin Probe Force and Current Sensing Atomic Force Microscopies, respectively.
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These authors contributed equally to this work.