Article pubs.acs.org/IECR
Effect of Softwood Kraft Lignin Fractionation on the Dispersion of Multiwalled Carbon Nanotubes Nai-Yu Teng, Ian Dallmeyer, and John F. Kadla* Advanced Biomaterials Chemistry, Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada S Supporting Information *
ABSTRACT: Dispersion of carbon nanotubes has been a major obstacle for the application and utilization in composites. In this study, it was observed that a small amount of softwood Kraft lignin (SKL) could facilitate the dispersion of multiwalled carbon nanotubes (MWCNTs) in dimethylformamide (DMF) solutions. Classification of the technical SKL by solvent fractionation revealed distinct differences in MWCNT dispersibility. Using Raman spectroscopy the efficacy of the various SKL fractions to disperse MWCNT’s in DMF was studied. Of the fractions investigated it was found that the methanol/methylene chloride (70/ 30, v/v) soluble fraction (F4SKL) performed the best. Characterization of the various fractions indicates that lignin structure and propensity to form intermolecular (π− π and hydrogen bonding) interactions is critical for MWCNT dispersion.
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CNTs,5 the effect of lignin structure on the dispersion of CNTs is not well understood. We hypothesized that specific structures with specific properties such as molecular weight and hydroxyl group content within Kraft lignin are responsible for its ability to act as a dispersant for CNT’s. Kraft lignin is a complex heterogeneous mixture of degraded native lignin fragments, which vary greatly in molecular weight, types of interunit linkages, and functional groups. As well lignin macromolecules are known to undergo associative interactions which result in large supramacromolecular complexes.20 Thus the structure of lignin will naturally affect the predominant types of interactions contributing to its propensity to associate, along with the dispersion of CNTs. To study the effect of lignin structure on CNT dispersion a technical softwood Kraft lignin (SKL) was fractionated by sequential extraction with organic solvents.21 The chemical structure and hydroxyl group content (aliphatic and aromatic) as well as molecular weight were analyzed and shown to vary by fraction. As such the extent and type of intermolecular interactions such as π−π interactions and hydrogen bonding with the oxidized CNTs varied.
INTRODUCTION Lignin is a complex aromatic heteropolymer occurring in the xylem of most land plants, making up about 20 − 30% of the terrestrial woody biomass.1 Delignification of wood is a key step in pulp and paper and biorefinery processes, which aim to isolate the carbohydrate components for conversion to traditional pulp and paper products or biofuels. In the predominant Kraft pulping process, lignin is typically burnt for energy and chemical recovery.1,2 However, there also exists an opportunity to precipitate some lignin from black liquor for conversion to value-added products.1 Furthermore, opportunities exist for traditional applications of lignin to be expanded into new areas. While dispersants are a well-known application of lignin,2,3 a relatively new application of lignin-based dispersants is for preparing suspensions of carbon nanotubes (CNTs).4,5 Since their discovery in 1991 by Iijima6 CNTs have been studied extensively for the preparation of nanocomposite materials with enhanced properties.7,8 However, due to their natural tendency to form bundles through van der Waals forces, dispersion of CNTs has been a major obstacle for further application and utilization.9 There are two general methods to disperse CNTs. One is chemical modification of CNTs to form functionalized surfaces.10 A disadvantage of this strategy is the disruption of π-networks within CNTs, leading to a decrease in mechanical and electrical properties.11 The second approach is the addition of polymers or surfactants capable of “wrapping” CNTs through noncovalent interactions.4,12,13 It was found that π-interactions and specifically the coupling of π-electrons between CNTs and aromatic molecules could modify the electronic and transport properties of CNTs14,15 and contribute to CNT dispersion.16,17 Similarly, intermolecular hydrogen bonding through the addition of polar polymers to functionalized CNTs has also been shown to enable CNT dispersion.18,19 Using noncovalent interactions to facilitate dispersion has the potential advantage of reducing the deterioration of the intrinsic properties of CNTs. While it has been shown that Kraft lignin can act as a dispersing agent for © 2013 American Chemical Society
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EXPERIMENTAL SECTION Materials. Softwood kraft lignin (Indulin-AT, SKL) was obtained from MeadWestvaco (Glen Allen, VA, USA), repeatedly washed (five times) with aqueous hydrochloric acid (HCl) at pH 2 to exchange sodium counterions, and airdried at 105 °C. Poly(ethylene oxide) (PEO) with an average molecular weight, Mv of 1 × 106 g/mol, 4-nitrobenzaldehyde, pyridine, and chromium acetylacetonate were obtained from Sigma-Aldrich. N,N-Dimethylformamide (DMF), methylene chloride, n-propanol, methanol, acetic anhydride, and potassium bromide (KBr) were obtained from Fisher Scientific Received: Revised: Accepted: Published: 6311
November 26, 2012 April 8, 2013 April 20, 2013 April 20, 2013 dx.doi.org/10.1021/ie303261z | Ind. Eng. Chem. Res. 2013, 52, 6311−6317
Industrial & Engineering Chemistry Research
Article 1
H and 13C NMR spectra were obtained using a Bruker Avance 300 MHz spectrometer equipped with a BBO probe. In quantitative 1H NMR, acetylated lignin was accurately weighed (∼5 mg) and mixed with an internal standard 4-nitrobenzaldehyde (∼1 mg) in 500 μL of deuterated chloroform (CDCl3) to enable quantitative determination of hydroxyl group content (Supporting Information Table S1). The NMR spectra were recorded at 25 °C with a 90° pulse width and a 1.3 s acquisition time. A 7 s relaxation delay (d1) was used to ensure complete relaxation of the aldehyde protons. A total of 128 scans were collected. Quantitative 13C NMR spectroscopy was performed using a lignin concentration of 15 wt % in DMSOd6. Relaxation was facilitated by the addition of chromium acetylacetonate.22 Conditions for analysis were a 90° pulse width with a 0.9 s acquisition time and a 1.7 s of relaxation delay (d1). A total of 20 000 scans were collected. Suspension Preparation and Characterization. MWCNT suspensions were prepared by sonication of glass vials containing MWCNT, DMF, and SKL or SKL fractions: 1 mg lignin and 3.1 mg MWCNTs were added to 1 mL DMF unless otherwise stated. The vial was placed into a cup with an immersed sonicating cone (Fisher Scientific, Sonicator Dismembrator, model 500) for 3 h (sonicating program: 1 min pulse on and 30 s pulse off) at a power level of 30 W. Dispersion of the MWCNTs was characterized by optical microscopy (Olympus BX41) and Raman spectroscopy (RM1000, Renishaw, Gloucestershire U.K) equipped with a 785 nm laser using a 20× objective and a laser power density of approximately 1.0 × 104 W/cm2. Samples were mounted on a cavity glass slide, and measurements were taken at five different spots for each sample.
(Ottawa, ON). Deuterated dimethyl sulfoxide (DMSOd6) was obtained from Cambridge Isotope Laboratories, Inc. (MA, USA). Multiwalled carbon nanotubes (MWCNTs) were obtained from CheapTubes.com (Brattleboro, VT, USA) and reported to contain
