Carbon nanotubes enhanced cellulose nanocrystals films with tailorable electrical conductivity
Introduction
Carbon nanotubes (CNTs) have attracted extreme academic interests due to their unique carbon nanostructures and exceptional intrinsic properties [1], [2]. A Young's modulus of 1.28 ± 0.59 TPa was reported for multi-walled carbon nanotubes (MWCNTs) comparable to the in-plane modulus of 1.06 TPa for graphite, the largest modulus of any known bulk material [3]. Such superlative mechanical properties are extremely attractive for CNT applications in high strength materials. In addition, CNTs also possess desirable electrical and thermal properties suitable for advanced functional material applications such as conductive composite materials and electrochemical devices [4], [5], [6], [7]. Recently, another type of nanoparticles – cellulose nanocrystals (CNCs) have attracted lots of attention due to their intrinsic properties, the abundance and renewable resource for the raw materials, the biodegradability and the potential as next generation reinforcing nanofillers. Since CNCs are typically prepared by removing most of amorphous components (lignin and semicellulose) from cellulosic materials and leaving only the cellulose crystallites, the mechanical properties of CNCs are close to those of cellulose crystallites. The calculated Young's modulus along the chain axis is 167.5 GPa for cellulose crystalline form I (native cellulose) and 162.1 GPa for form II (regenerated cellulose) [8]. The experimentally measured elastic moduli for cellulose crystallites and cellulose nanocrystals cover a wide range from 50 to 220 GPa with variation from cellulose source, preparation and measuring methods [9], [10], [11], [12], [13]. In addition to the attractive mechanical properties, another appeal of CNCs is their surface functionality which could be manipulated by different preparation/surface modification methods, for example, sulfate groups from sulfuric acid hydrolysis [14], carboxylate groups from TEMPO-mediated oxidation [15], acetate ester groups from acid-catalyzed esterification [16], [17] and preserved original hydroxyl groups from mechanical treatment [18]. Decorated with ionic charges on the surface, CNCs prepared via sulfuric acid hydrolysis and TEMPO-mediated oxidation can be easily dispersed in water due to electrostatic repulsion. This enables preparation of CNC films from CNC aqueous suspensions by solvent casting or membrane filtration.
Transparent cellulose nanofiber films with high gas barrier properties were prepared by filtrating TEMPO-oxidized cellulose nanofiber suspensions in water [19]. Ultra-strength cellulose nanopapers were manufactured from cellulose nanofibrils via membrane filtration method [20]. The properties of CNC films could be modified by incorporating other nanoparticles, for example, cellulose nanofiber/clay nanopapers extend the property range of cellulose nanopapers with improved fire retardant and gas barrier properties [21]. CNT enhanced CNC films are worthwhile for investigation because of expected properties from both nanoparticles and potential broad applications in the electronic field, such as electromagnetic interference shielding and electronic circuits [22].
To the best of our knowledge, few reports on CNT enhanced CNC films have been published so far. Salajkova et al. prepared nanopapers by a water based processing route from MWCNTs purified with nitric acid and hydrochloric acid and nanofibrillated cellulose (NFC) derived from a commercial softwood pulp by TEMPO-mediated oxidation [23]. A surfactant – nonylphenol POE-10 phosphate ester was required to stabilize purified MWCNTs in the MWCNT/NFC aqueous suspensions since the MWCNTs settled down without the surfactant. It was pointed out based on SEM and AFM images that the number of contacts between MWCNTs increases with increasing MWCNT content and at 9.1 wt.% the MWCNTS form a continuous network; however, at the maximum MWCNT content used (16.7 wt.%), agglomerates of MWCNTs were observed. Based on the mechanical properties results these authors concluded that MWCNTs could not provide positive contribution to the nanopapers because of the weak or no interaction between MWCNTs and NFC as well as the increased porosity with increasing CNT content. The electrical conductivity improved over more than 4 decades between 6 and 9 wt.% of MWCNTs, indicating a percolation threshold within this range. Most recently, Hamedi et al. prepared semitransparent conductive nanopapers from NFC and single-wall carbon nanotubes (SWCNTs) via a similar water based processing route [24]. They proved that carboxylated NFC could successfully exfoliate SWCNTs into individual nanotubes or small bundles and stabilize them in water. They hypothesized that the driving force for the dispersion of SWCNTs by NFC could be an overall gain in entropy combined with the high nonpolar interactions between the two types of nanoparticles. They also showed that the addition of 10 wt.% SWCNTs led to reduction in modulus, strength and strain-to-failure due to insufficient NFC-SWCNT interactions; nevertheless, the electrical conductivity increased with SWCNT content and reached the highest value of 174 S cm−1 at the dispersion limit of this system (43 wt.% SWCNT).
Hybrid films formed by CNCs and CNTs still need further investigation in terms of CNC-CNT interactions both in aqueous suspensions and solid hybrid films, the hybrid films microstructure and properties. In this study we explore the enhancement effects of CNTs on CNC films and the potential mechanism of CNT reinforcement in the CNC films. The hybrid films made from CNCs and CNTs by a simple vacuum filtration method without the use of surfactants show good mechanical properties and tailorable electrical properties, with potential broad applications in the electronic field.
Section snippets
Materials
Microcrystalline cellulose (MCC) (Sigma–Aldrich) in a form of white fine powder with an average particle size of 20 μm and a bulk density of 0.5 g/cm3 at 25 °C was purchased from Sigma–Aldrich (St. Louis). Multi-walled carbon nanotubes (MWCNTs) (Nanocyl NC7000) with an average diameter of 9.5 nm, an average length of 1.5 μm (rendering an average aspect ratio of 158) and 90% carbon purity as reported by the manufacturer were used. As-produced NC7000 MWCNTs are in the form of entangled micro-size
CNC films
TEMPO-mediated oxidation converts primary hydroxyl groups on the surface of cellulose crystallites into carboxylate groups as well as degrades the links between cellulose crystallites leading to the easy disintegration of oxidized MCC (O-MCC) into CNCs by mechanical forces. Polarized optical microscopy images in Fig. 2(a–b) illustrate the MCC morphology change from intact to irregular after TEMPO-mediated oxidation. The O-MCC was further disintegrated by ultrasonication into CNCs which could
Conclusions
In this study commercial MWCNTs and bio-based CNCs were used to prepare CNT enhanced CNC films via an environmental friendly and simple aqueous suspension vacuum filtration method. The CNT enhanced CNC films were compared with CNC films showing promising tensile and electrical properties. The addition of CNTs allows processing CNC films by a vacuum filtration method which is more convenient than the solvent casting method. The CNT enhanced CNC films, prepared from water suspensions made without
Acknowledgments
We acknowledge the National Science Foundation for the financial support of our research through grant PIRE-1243313. Acknowledgements are also due to Vahab Solouki Bonab and Liang Yue for their help with some experimental work.
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