Electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding
Graphical abstract
Introduction
Graphene, monolayer of carbon atoms arranged in a honeycomb network, has recently gained revolutionary aspirations [1], [2], [3], [4], [5] because of its remarkable electronic [4], [6], [7], thermal [8], and mechanical properties [9]. These unique properties make it a choice as inorganic fillers to improve electrical, thermal and mechanical properties of composite materials [5], [10]. Several effective techniques have been developed for preparing graphene nanosheets, including chemical [5] and mechanical exfoliation [11], alkali metals intercalation and expansion [12], microwave chemical vapor deposition [13], substrate-based thermal decomposition [7], and thermal exfoliation of graphite oxide (GO) [14]. Among them, the thermal exfoliation and in situ reduction method can conveniently produce graphene nanosheets for mass production. More importantly, although insulating graphite oxide was converted to conducting graphene, the graphene nanosheets resulted through thermal exfoliation still contained some oxygen-containing groups [14], which will facilitate the dispersion of the nanosheets in polar polymers [10].
Graphene has exhibited its potential in improving electrical conductivity of polymers [15], [16], [17]. The polystyrene/graphene nanocomposites prepared by chemical modification and reduction in solution exhibited a percolation threshold as low as 0.1 vol.% [5], comparable to those observed in single-walled carbon nanotubes-based nanocomposites [18], [19]. However, un-fully exfoliated graphite led to a much higher threshold (>0.6 vol.%) and a much lower electrical conductivity (<10−2 S/m) even at high loading of 6.0 vol.% [20], [21], [22].
Compared to in situ exfoliation and solution mixing, melt compounding using commercial resins and conventional compounding devices such as extruder and mixer is more attractive because this approach provides manufacturers many degrees of freedom with regard to the selection of polymer grades and choice of graphene content. It is believed that melt compounding would be more economical and suitable for mass production than solution mixing. Actually, melt compounding has been successfully used to prepare conductive polymeric composites by using conducting fillers such as carbon nanotube (CNT) [23], carbon black and expanded graphite [24], [25]. It has been confirmed that CNTs can substantially increase the electrical conductivity of PET nanocomposites with low filler loading, but it is expensive. As for cheap carbon black, higher loading is usually required to make a polymer electrically conductive. The low price and availability of pristine graphite in large quantities, coupled with the relative simple fabrication process make graphene a potential choice as conductive fillers in the preparation of conductive PET nanocomposites. To the best of our knowledge, few papers have been published on PET/graphene nanocomposites prepared by conventional melt compounding. In this study, graphene nanosheets were prepared by complete oxidation of pristine graphite followed by thermal exfoliation and reduction. Subsequently, polyethylene terephthalate/graphene nanocomposites were fabricated by melt compounding. The microstructure of graphene, its dispersion in PET matrix, and the electrically conductive behavior of the resulting nanocomposites were studied.
Section snippets
Materials
PET pellets were purchased and dried in vacuum oven at 150 °C for 5 h before use. Flaky pristine graphite with a mean size of 45 μm was bought from Qingdao Huatai Lubricant Sealing S&T (China). Graphene density is assumed as the theoretical graphite density of 2.28 g/cm3 [18], [26], and PET density is 1.34 g/cm3 [27]. Fuming nitric acid (63%), sulfuric acid (98%), potassium chlorate (98%) and hydrochloric acid (37%) were obtained from Sinopharm Chemical Reagent (China).
Preparation of graphene
A technique, similar to that
Characterization of graphene
Fig. 1 shows the XRD spectra of pristine graphite, graphite oxide and graphene. The strong and sharp diffraction peak of pristine graphite at 26.6° completely disappeared after oxidization and instead a new peak at 13.9° appeared, indicating a complete oxidization of graphite [28], [30], which is a prerequisite to obtain exfoliated graphene nanosheets by ultrasonication [5] or thermal expansion [14], [28]. After thermal exfoliation of the completely oxidized graphite (GO), there was no apparent
Conclusions
Graphene sheets were prepared by chemical oxidation of pristine graphite flakes followed by thermal exfoliation and reduction of graphite oxide. PET/graphene nanocomposites prepared by melt compounding exhibit superior electrical conductivity with a low percolation threshold of 0.47 vol.%. A high electrical conductivity of 2.11 S/m of PET nanocomposite was achieved with only 3.0 vol.% of graphene, which is even adequate for EMI shielding. The low percolation threshold and superior electrical
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