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Simultaneously Tailoring Surface Energies and Thermal Stabilities of Cellulose Nanocrystals Using Ion Exchange: Effects on Polymer Composites Properties for Transportation, Infrastructure, and Renewable Energy Applications Douglas M. Fox, Rebeca S Rodriguez, Mackenzie N Devilbiss, Jeremiah W. Woodcock, Chelsea Simone Davis, Robert Sinko, Sinan Keten, and Jeffrey W. Gilman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06083 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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ACS Applied Materials & Interfaces

Simultaneously Tailoring Surface Energies and Thermal Stabilities of Cellulose Nanocrystals Using Ion Exchange: Effects on Polymer Composites Properties for Transportation, Infrastructure, and Renewable Energy Applications Douglas M. Fox,*,† Rebeca S. Rodriguez,† Mackenzie N. Devilbiss,† Jeremiah Woodcock,‡ Chelsea S. Davis,‡ Robert Sinko,§ Sinan Keten,§ and Jeffrey W. Gilman.‡

† - Department of Chemistry, American University, Washington, DC 20016-8014, [email protected]. ‡ - Materials Science and Engineering Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, 20899-8664. § - Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208-3109. Keywords: Cellulose nanocrystals, ionic liquids, polymer composites, epoxy, extrusion, dispersion.

ACS Paragon Plus Environment

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ABSTRACT Cellulose nanocrystals (CNCs) have great potential as sustainable reinforcing materials for polymers, but there are a number of obstacles to commercialization that must first be overcome. High levels of water absorption, low thermal stabilities, poor miscibility with non-polar polymers, and irreversible aggregation of the dried CNCs are among the greatest challenges to producing cellulose nanocrystal – polymer nanocomposites. A simple, scalable technique to modify sulfated cellulose nanocrystals (Na-CNCs) has been developed to address all of these issues. By using an ion exchange process to replace Na+ with imidazolium or phosphonium cations, the surface energy is altered, the thermal stability is increased, and the miscibility of dried CNCs with a non-polar polymer (epoxy and polystyrene) is enhanced. Characterization of the resulting ion exchanged CNCs (IE-CNCs) using potentiometry, inverse gas chromatography, dynamic vapor sorption, and laser scanning confocal microscopy reveals that the IE-CNCs have lower surface energies, adsorb less water, and have thermal stabilities of up to 100 °C higher than prepared protonated cellulose nanocrystals (H-CNC) and 40 °C higher than neutralized Na-CNC. Methyl(triphenyl)phosphonium exchanged cellulose nanocrystals (MePh3P-CNC) adsorbed 30% less water than Na-CNC, retained less water during desorption, and were used to prepare well dispersed epoxy composites without the aid of a solvent and well dispersed polystyrene nanocomposites using a melt blending technique at 195 °C. Predictions of dispersion quality and glass transition temperatures from molecular modeling experiments match experimental observations. These fiber-reinforced polymers can be used to lightweight composites in transportation, infrastructure, and renewable energy applications. INTRODUCTION


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There is an increasing demand for sustainable materials in manufactured products. At the same time, the performance, weight, environmental health and safety, and production costs of the sustainable materials must match or exceed conventional, petrochemically-derived materials. In light of these demands, cellulose nanocrystals have been identified as a potential next generation of performance fillers for reinforcing polymers.1, 2 Cellulose, the most abundant polymer found in nature, is a high molecular weight homopolymer comprised of β-1,4-linked anhydro-Dglucose units in a hierarchical crystalline and amorphous structure. Cellulose can be isolated from wood in the form of fibrillated microfibers through a pulping and bleaching process. When cellulosic materials are treated with a strong acid at elevated temperatures, they undergo hydrolysis, predominantly at the disordered regions. The resulting cellulose nanocrystals are short, stiff, predominantly crystalline structures with high aspect ratios. CNCs prepared from mineral acids, such as HCl or HBr, have the same chemical structure as the native cellulose. When dried, the crystals form an extensive hydrogen bond network with each other, leading to large aggregated particles regardless of the drying method. Attempts to re-disperse these dried crystals in water have generally failed, because the entropic penalty cannot be overcome by the fairly equal enthalpic interactions between cellulose and water.3-5 Adding NaCl or leaving the crystals partially wet can aid in re-dispersion at the expense of having an impurity or higher weight. When preparing CNCs from oxyanionic acids, such as H2SO4 or H3PO4, in addition to the acid hydrolysis at the disordered regions of the fiber, a side esterification reaction occurs on the surface of the nanocrystals.6, 7 This leads to incorporation of anionic sites periodically along the CNCs. There have been numerous attempts to blend CNCs with polymers. The four critical obstacles preventing practical realization of the theoretical performance of nanocellulose/polymer


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composites are high levels of water absorption, low thermal stabilities, poor miscibility with nonpolar polymers, and irreversible aggregation of the dried CNCs. The inability to separate the cellulose nanocrystals once they have been dried is due to the strong hydrogen bonding between adjacent cellulose chains. The low thermal stability of the cellulose nanocrystals is attributed to the mobile protons present as a result of the esterification side reaction with bisulfate ions. To disrupt these interactions, most researchers have either used surfactants or covalently attached less polar molecules to the hydroxyl groups of the cellulose.8-15 Never-dried Na-CNCs have been successfully dispersed in water-soluble polymers, such as polyvinyl alcohol16-18 or polyethylene oxide.19 These composites have then been used as surfactant – cellulose matrices in more hydrophobic polymers.20, 21 In addition, t-butanol (t-BuOH) and tetraalkylammonium salts have been physisorbed onto the cellulose surface as surfactants for more hydrophobic media.22-24 These approaches have had limited success due the high content of surfactant needed, which leads to phase separation issues and changes in composite properties.9, 15 The most common surface modification methods are esterification,14, 23, 25, 26 carboxymethylation,14, 25 silylation,14, 25 and urethanization.14, 25 These methods almost always require liquid – liquid extractions, heating, and purification steps, significantly adding operating costs to any scale up process. In addition, the mechanical property enhancement of the composites is often significantly lowered after the modification due to an inability to form a 3-dimensional (3D) reinforcing network.8, 9, 15 For all of these methods, the thermal stabilities of the modified celluloses remain low, limiting composite preparations to in-situ polymerization and solvent casting techniques. Foster and coworkers used phosphorylated CNCs (CNC-PO4) to overcome the thermal stability limit while maintaining the ability to disperse dried nanocrystals.27 CNC-PO4 could be dried and dispersed


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in Nylon 12 using a melt-extrusion process. The crystals still have high polarity, so they maintain high water absorption and their use is limited to polymers with fairly high polarities. Another possible solution is to change the counterion to the sulfate groups on cellulose nanocrystals. Ionic liquids have been successfully used as surfactants to improve the dispersion of a wide range of materials in polymers. Ionic liquids have been used to assist in the dispersion of clay,28, 29 silica,30 carbon nanotubes,31, 32 and graphene.33 When melt-blending with polymers, the thermal stability of the surfactant is critical to prepare well-dispersed materials. Imidazolium and phosphonium based ionic liquids are typically more thermally stable than ammonium based ones.34, 35 The use of ionic liquid type surfactants have been used with cellulose nanocrystals as well. Recently, Cranston added surfactant and dialyzed the solutions, relying on equilibrium and CNC selectivity towards the surfactant cation to prepare mostly exchanged CNCs. 36, 37 This method is effective for only a few cations, which are typically the less thermally stable alkylammonium ones, and requires the tedious process of dialysis. We developed a method using ion exchange resins, which significantly increases the number of potential cations that can be exchanged and has the potential to be scaled up industrially. In this study, strong acid ion exchange resins were loaded with ionic liquid cations and used to exchange sodium ions on sulfated cellulose nanocrystals. The ionic liquid cations investigated were methyl(triphenyl)phosphonium (MePh3P+), 1,2,3-trimethylimidazolium (Me3Im+), 1-hexyl2,3-dimethylimidazolium (HxMe2Im+), and 1-hexadecyl-2,3-dimethylimidazolium (HdMe2Im+). The surface energies and water adsorption of freeze-dried crystals were characterized using inverse gas chromatography and dynamic vapor sorption. The thermal stabilities were examined using microcombustion calorimetry. The freeze-dried crystals were then mixed with a two component epoxy or melt-blended with polystyrene, and the composites were characterized by


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confocal fluorescent microscopy, UV-VIS spectroscopy, water absorption, and tensile testing. The ion exchange process is shown to significantly reduce aggregation and improve dispersion in hydrophobic polymeric systems. EXPERIMENTAL SECTION* Materials.

Methyl(triphenyl)phosphonium

bromide

(MePh3PBr,

98+%)

and

1,2,3-

trimethylimidazolium methylsulfate (Me3ImMeSO4, 95%) were obtained from Alfa Aesar, 1hexadecyl-2,3-dimethylimidazolium chloride (HdMe2ImCl) was obtained from Merck, 1-hexyl2,3-dimethylimidazolium chloride (HxMe2ImCl) and sodium hydroxide (NaOH, 98.5%) were obtained from Acros Organics, and ammmonium hydroxide (NH4OH, ACS Plus grade) and hydrochloric acid (HCl, ACS Plus grade) were obtained from Fisher Scientific. Diglycidyl ether of bisphenol A (DGEBA, %) and poly(propylether)diamine (JA230, 230 g/mol) were obtained from Sigma Aldrich. De-ionized water (> 18.2 MΩ) was collected from a ELGA LabWater PURELAB flex 2-stage water purification system. Cellulose nanocrystals (Na-CNC) in solution form and as a freeze-dried product were obtained from the University of Maine. These were prepared using sulfuric acid, neutralized to the sodium form, and contained 0.95% mass fraction sulfur on a dry basis. The nanocrystals had an average diameter of 6 nm and average length of 130 nm.38 The received Na-CNC solutions were diluted from 6.2 wt% mass fraction to 2.1% mass fraction using de-ionized water and the freeze-dried powder, which was dried from a solution containing a 9% mass fraction t-butanol, was used as received. Dowex 50W-X2, 50 –

*

The policy of NIST is to use metric units of measurement in all its publications, and to provide statements of uncertainty for all original measurements. In this document however, data from organizations outside NIST are shown, which may include measurements in non-metric units or measurements without uncertainty statements.


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100 mesh, H+ form cation exchange resin was obtained from Sigma Aldrich. The resin was washed sequentially in 1 M HCl, deionized water, 1 M NaOH, and deionized water to remove metal impurities and convert the resin to Na+ form. Polystyrene (PS, 210,000 g/mol, melt flow index 7 g/10min at 200 °C) was obtained from Scientific Polymer Products and was dried 1 h at 90 °C prior to use. Ion exchange of cellulose nanocrystals. H+ or Na+ form cation exchange resin was loaded into a glass chromatography column. The column was charged at a rate of 40 mL/h with a 0.3 M surfactant (ionic liquid) solution until the eluent contained no more H+ or Na+. It was then rinsed with deionized water until the eluent contained no more surfactant molecules. Na-CNC solutions were passed through the column gravimetrically at an average rate of 20 mL/h.

The level of

exchange in the sodium cellulose was monitored in 4 mL aliquots. To maintain processing consistency, the Na-CNC samples used in this paper were passed through a Na+ loaded ion exchange column. Portions of the collected samples were flash frozen in liquid nitrogen and freeze-dried for 2 – 3 days using a Labconco FreeZone lyophilizer. The cellulose content was determined gravimetrically from the freeze dried samples. The H+ content was measured using a pH electrode connected to a VersaStar multimeter. The Na+ content of the solutions were determined using an Orion Ross Na+ ion selective electrode connected to a VersaStar multimeter. Ionic strength adjuster was added at a ratio of 1:100 relative to the cellulose solution to prevent signal interference from protons.

The surfactant cation content was determined using a

Shimadzu UV-2550 UV-VIS Spectrophotometer. Quartz cuvettes with a 1 cm pathlength were used. Spectra were collected between 200 nm and 800 nm, with peaks of interest at 211 nm for imidazolium cations and 267 nm for the methyl(triphenyl)phosphonium cations. Spectra were


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baseline corrected from Na-CNC solutions of equivalent concentration.

All cation

concentrations were measured with an uncertainty of σ = ±0.3 mM. Nanocellulose characterization. The surface energies were measured using inverse gas chromatography (iGC). All surface energy analyses were carried out using the iGC Surface Energy Analyzer (SEA, SMS, Alperton, UK) and the data were analyzed using both standard and advanced SEA Analysis Software. For all experiments, approximately 45 mg of each sample were packed into individual silanized glass columns (300 mm long by 4 mm inner diameter) using the SMS Column Packing Accessory. Each column was conditioned for a period of 2 hours at 30 °C and 0 % relative humidity (RH) with helium gas prior to any measurements. All experiments were conducted at 30 °C with 25 cm3/min total flow rate of helium gas, using methane for dead volume corrections. Samples were run at a series of surface coverages with nalkanes (decane, nonane, octane, and heptane; Aldrich, HPLC grade) and polar probe molecules (acetone, ethanol, acetonitrile, ethyl acetate, and dichloromethane; Aldrich, HPLC grade) to determine the dispersive surface energy as well as the specific free energies of adsorption, respectively. The complete iGC experiment over all surface coverages measured takes approximately 24 hours for one sample. Repeat experiments were completed in succession on the same column to investigate if the elapsed time or exposure to vapors caused any measureable surface changes. The dispersive surface energy analysis was performed by measuring the net retention volume, VN (measured retention volume – dead volume) for a series of alkane elutants (heptane, octane, nonane, and decane). The dead-volume was determined by methane, which does not interact significantly with the samples under the chosen conditions. For the analysis, the method of Schultz et al. was applied.39 ܽ(ߛ௅ ஽ )ଵ/ଶ = ܴ݈ܶ݊(ܸே ) ∗ 2ܰ஺ (ߛௌ ஽ )ଵ/ଶ

(Eqn 1)


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In this method, a plot of RTln(VN) versus a(γLD)1/2 should produce a straight line with a slope equal to 2NA(γSD)1/2, where γSD is the dispersive component of the solid surface energy, a is the molecular area of the probe molecule, γLD is the dispersive component of the surface energy of the liquid elutant (surface tension), R is the ideal gas law, T is the temperature, and NA is Avogadro’s number. The specific contribution to the total surface energy is obtained via iGC by first measuring the specific free energies of desorption for different polar probe molecules. These values were determined by measuring the retention volume of polar probe molecules (acetone, acetonitrile, ethyl acetate, ethanol, and, dichloromethane) on the samples. Points representing a polar probe are located above the alkane straight line in the RTln(VN) versus a(γLD)1/2 plot. The distance to the straight line is equal to the specific component of the free energy. The probes were chosen to span a range of interactions (acidity, basicity, hydrophilicity, hydrophobicity, etc.). Surface energy values were repeatable within 2.1 %. Dynamic Vapor Sorption (DVS) experiments were conducted using a DVS Intrinsic-1 (SMS, Alperton, UK) at 25 °C. The instrument measures the uptake and loss of vapor gravimetrically using the SMS UltraBalance with a mass resolution of ±0.1 µg. The vapor partial pressure around the sample is generated by mixing saturated and dry carrier gas streams using electronic mass flow controllers. A total flow rate of 200 mL/minute was used. Sample mass was 4 mg to 8 mg. Samples were initially dried at 0 % RH. Then, the samples were exposed to the following relative humidity profile: 0 %RH to 95 %RH to 0 %RH in 10 %RH increments. The change in mass per unit time used was 0.005 %/minute. This is the mass slope setting used to determine when equilibrium has been reached, and the experiment proceeds to the next relative humidity step. The thermal stabilities of the cellulose were analyzed using microcombustion calorimetry (MCC). In an MCC, samples are pyrolyzed in nitrogen at a heating rate of 1 °C/min. The evolved gases enter a combustion


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chamber at 900 °C, where they are oxidized in air. The heat released is calculated from the reduction in oxygen using the assumption that gaseous fuels will generate 13.1 kJ of heat per 1 g of oxygen consumed.40, 41 The total heat released (THR) is the integrated heat loss rate over the entire experiment and the heat release capacity (HRC) is the peak heat release rate divided by the actual heating rate of the experiment.42 An MCC is best suited for analyzing materials that reduce flammability using a condensed phase mechanism, since materials inhibiting flames in the gas phase will usually react with oxygen and generate an artificially high heat release rate. In these samples, no components are expected to act in the gas phase, so the peaks are assumed to reveal the actual changes to heat released during combustion. Epoxy Composites. The epoxy composites were blended using a FlakTek high speed, centrifugal mixer for 10 min at 2500 rpm. Na-CNC crystals obtained from the University of Maine were first blended with the diamine curing agent. The epoxy resin was added and the components were mixed a second time. For the ion exchanged crystals, the order of addition was reversed, adding epoxy resin to the crystals first. After combining all three components, the mixture was degassed for 5 minutes under vacuum and immediately transferred to silicone molds for tensile properties (type V, ASTM D 638-02a), UV-VIS transmission (22mm diameter, 1 mm thick) and water absorption analysis (51 mm diameter x 3.2 mm thick). All epoxy samples were cured at room temperature for 24 h and 80°C for 2 h. Tensile tests were performed on an MST Criterion Model 45 hydraulically driven test frame with a 5kN load cell in accordance to the ASTM D 638-02a norm at a speed rate of 0.05 mm/sec with a gauge length of 14 mm. All tests were averaged over a minimum of five measurements for each sample with a standard deviation of σ = ±10%. Water absorption tests were conducted at 23 °C according to ASTM D570 standards. Duplicate samples were calculated to have water absorptions of ±0.05% mass fraction.


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Curing properties and glass transition temperatures were measured using a TA Instruments Q2000 Differential Scanning Calorimeter. Samples (4.8 mg to 6.4 mg) were loaded into open aluminum pans and tests were conducted under a stream of N2 at 50 mL/min. For curing properties, mixed samples were heated from 20 °C to 250 °C at 10 °C/min. For glass transition temperatures, cured epoxies were heated and cooled at a rate of 10 °C/min between -20 °C and 250 °C. The temperature was held at each endpoint for 2 min before reversing the heat flow. Each sample was cycled a total of 3 times, with the first scan revealing the percent of incomplete curing and the last 2 showing no further changes in thermal behavior. The error in the reported temperature was σ = ±1.5 °C.

Polystyrene Composites. For polystyrene composites, all CNC samples were prepared by flowing Na-CNC solution as described in the ion exchange procedure described earlier. Freeze dried CNC samples were melt-blended with polystyrene at 195 °C and 200 rpm for 3 min using an Xplore twin screw extruder (DSM, Geleen, Netherlands). Composites were prepared by adding a mass fraction of 1 % or 3% of freeze dried Na-CNC, HdMe2Im-CNC, or MePh3P-CNC to polystyrene pellets and transferring the coated pellets to the extruder. A Na-CNC dispersion (a mass fraction of 6.2 %) was mixed with PVOH aqueous solution (a mass fraction of 6.2 %) to prepare a 1:4 Na-CNC:PVOH (by mass) solution and was then freeze dried for 2 days. This mixture was also melt-blended with polystyrene to produce a 1% mass fraction of Na-CNC. Glass transition temperatures were determined in the same manner as the epoxy composites.. Composite Imaging. A confocal laser scanning microscope (LSM 510 META Carl Zeiss, Germany) operating in reflectance mode was used to examine the aggregation and dispersion of cellulose in polystyrene. The excitation source was a 405 nm diode laser (30 mW) and an


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emission band pass filter (420 nm to 480 nm) was used. Cellulose nanocrystals were detected as dark regions within the polystyrene matrix, which has intrinsic fluorescence. Images were collected at 5x and 10x magnification. A Leica Confocal TC5 SP5 microscope operating in transmission mode was used to image the epoxy composites. Images were collected at 5x and 20x magnification using a 514 nm laser with no filter. Transmission through all composites was examined using the Shimadzu UV-2550 UV-VIS Spectrophotometer. PS samples were hot pressed at 160 °C for 5 min and cooled in-situ using cold water into 1 mm thick discs using a Carver hydraulic press (Model #3912). Epoxy composites were cured in 1 mm thick silicone molds. Polymer samples were mounted in front of the incident beam. Spectra were collected between 200 nm and 800 nm. RESULTS AND DISCUSSION Cellulose surface energies. The cation exchange resulted in changes in the surface energies of the CNCs. Solutions of exchanged CNCs were as stable in water as the unmodified Na-CNC solutions. Dried CNCs could be re-dispersed in water, and had the same visual appearance as the Na-CNC solutions. Dried, modified CNCs could be temporarily dispersed in organic solvents, such as ethanol or toluene, whereas the dried Na-CNCs remained aggregated, indicating changes in surface energy properties. The surface energies were measured using inverse gas chromatography (iGC), as shown in Figure 1. The total surface energy of a material (γST ) is often divided into two components: dispersive (γSD) (London dispersion, van der Waals, and Liftschitz interactions) and acid-base (γSAB) (specific, polar interactions). The addition of these two components equals the total surface energy (γST = γSAB + γSD ). For the acid-base component of the surface energy (Figure


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1b), the Good-van Oss theory was used.43 The γS+ (electron acceptor) and γS- (electron donor) numbers based on the Good-van Oss concept can then be calculated from the iGC free energy values (∆G). The derivation of the acid base component of the surface energy is provided in the supplemental section. One of the advantages of using iGC for surface energy measurements is that the heterogeneity of the surface energy can be examined. By measuring the adsorption of carrier gas over time, the surface energy as a function of percent of the sample surface covered by the gas can be probed. The highest energy sites are covered first, and steeper curves indicate a more heterogeneous surface.44 For the unmodified Na-CNCs, both the dispersive and acid-base surface energies show higher energy sites covering about 5% of the crystal surface and fairly equal energy sites covering the next 15% of the surface. The relative change in surface energy from the highest energy sites to the plateau was greater for the acid base component of the surface energy. This has been attributed most often to trace surface contaminants, such as lignin, but could also be due to crystallinity differences at the surface, surface morphology, or diffusion of probes into the bulk volume.45 The values obtained (40 mJ/m2 to 55 mJ/m2 for the dispersive component and 3 mJ/m2 to 8 mJ/m2 for the geometric mean of the donor and acceptor components) are consistent with what has been reported by others.38, 45-47 Sacui and coworkers used the same source nanocrystals in their work and obtained nearly identical iGC curves for Na-CNC (labeled “wood sulfate” in their paper).38 Since our Na-CNC were passed through an ion exchange column loaded with Na+ ions, this shows that the ion exchange process itself does not change the surface energies of the crystals. Therefore, any changes in surface energy observed here can be attributed to the exchange of cation. The electron acceptor (acid) component and electron donor (base) component in the unmodified Na-CNCs are about equal, with the crystals exhibiting a


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slightly more basic behavior than acidic behavior. It may seem counterintuitive that the dispersion component is greater than the acid-base component; however, it has been argued that the crystals expose large hydrophobic regions along the planar ringed surface.48 Replacing Na+ results in a decrease in the dispersive component of the surface energy for the other cations (Figure 1a). Polyolefins have dispersive components near 30 mJ/m2 and polystyrene has a dispersive component near 37 mJ/m2,47 so if the only factor was cation intermolecular forces, it would be expected that the hydrophobic cations would reduce the dispersive component of the cellulose. The dispersive surface energy for the crystals follows the order of cation hydrophobicity, accordingly. The changes in surface energy may also be affected by steric influences. The crystal structure of cellulose is planar, with the hydrophobic rings lying parallel along the plane surface. This creates a large, flat, hydrophobic surface for adsorbing the alkane probes. The MePh3P+ cation extends out of the plane, reducing the surface area available along the crystal plane for alkane adsorption. This creates a steeper change in surface energy as the surface coverage of probe molecules increases. The acid base character of ion exchanged crystals did not change appreciably relative to the Na-CNCs (Figure 1b). It may seem counterintuitive that MePh3P+ has an acid-base component that is on par with Na+. More insight can be gained by examining the individual donor and acceptor properties that comprise the acid-base component of the surface energy (Figure 1c). The MePh3P+ cation significantly increases the electron donor interactions (γ-) while reducing the electron acceptor interactions (γ+). The cation contains three phenyl rings, and the delocalized π electrons increase the Lewis basicity and effectively screen the cationic charge on the P atom. iGC can also be used to examine the surface energies under different relative humidity conditions (Figures 1d). At 50% RH, the dispersive component of Na-CNC decreases by about


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5 mJ/m2 at all surface coverage while the acid-base component increases by about 3 mJ/m2. This is consistent with water molecules forming a monolayer along the planar crystal surface. The surface energies for MePh3P-CNC do not change much after equilibration at 50% RH, indicating that less water is present along the crystal surface than on the Na-CNC crystal surface. This also suggests that any water that does adsorb may not change the interfacial bond strength between the cellulose and polymer, since the surface energy does not change much in the presence of moisture. To further examine the ability of the modified CNCs to lower water absorption, dynamic water vapor adsorption experiments were conducted.

(a)

(b)

(c) (d) Figure 1. (a) Dispersive and (b) acid-base surface energies at 0% RH for Na-CNC, Me3Im-CNC, HxMe2Im-CNC, and MePh3P-CNC; (c) acid-base surface energies at 0% RH of Na-CNC and


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MePh3P-CNC; and (d) dispersive surface energies at 0% relative humidity and 50% relative humidity of Na-CNC MePh3P-CNC, measured using iGC. The coverage is expressed as a fraction, where n is the number of sites occupied by probe molecules and nm is the total number of sites to form a monolayer over the entire sample surface. γ+ is the electron acceptor and γ- is the electron donor component of the surface energies.

Water absorption and thermal stability of exchanged cellulose. Dynamic Vapor Sorption (DVS) experiments were conducted on unmodified CNCs and CNCs modified with Me3Im+, HxIm2Im+, and MePh3P+. As shown in Figure 2a, the amount of water absorbed by the crystals decreases as the hydrophobicity of the exchanged cation increases. The cation exchange resulted in a 30% reduction in equilibrium water uptake at 70% RH for MePh3P-CNC relative to NaCNC, which is remarkable considering how few exchange sites are available on a cellulose crystal surface. In desorption tests, the relative humidity of the chamber is lowered and the sample is allowed to reach equilibrium by releasing bound water. All samples showed a slight hysteresis during the desorption tests. The sorption-desorption profiles as the relative humidity was adjusted between 0 %RH and 95 %RH for the unmodified CNCs and the MePh3P-CNCs are shown in Figure 2b. The additional amount of water retained during desorption was also correlated to the hydrophobicity of the cation, with the most hydrophobic cations retaining the least water.


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(a) (b) Figure 2. Dynamic vapor adsorption of Na-CNCs, Me3Im-CNCs, HxMe2Im-CNCs, and MePh3P-CNCs, illustrating the reduced water adsorption and lower desorption hysteresis of the modified CNCs.

The thermal stabilities of the cellulose were assessed using microcombustion calorimetry (MCC). Microcombustion calorimetry measures total heat released (THR) and heat release capacity (HRC) of materials during combustion. Since samples are pyrolyzed under nitrogen and only the pyrolysis gases are exposed to oxygen, the residue after the experiment represents the pyrolysis char yield of the sample. The MCC results for several exchanged CNCs are shown in Figure 3. The exchange efficiency, average total surface energy, THR, HRC, and char yield for all the CNCs examined in this study are provided in Table I. The most striking feature here is the marked change in decomposition onset temperatures as the cation is changed. The acid form of CNCs has the lowest thermal stability, showing decomposition at temperatures well below 200 °C. Neutralized CNCs (Na-CNC) have thermal stabilities that are 50 °C to 60 °C higher than the protonated CNCs. This is consistent with what has been observed by others.49-51 Replacing the Na+ with Me3Im+ did not change the onset of degradation temperature, but it did result in a


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secondary decomposition temperature about 25 °C higher than the peak decomposition temperature. Exchange of the Na+ with either HdMe2Im+ or MePh3P+ ions increased both the onset degradation temperature and the peak degradation temperature by 20 °C to 40 °C. It has been hypothesized by several authors that an “active cellulose” precursor forms prior to cellulose decomposition, which exists in a liquid-like state.52-54 We attribute the increased thermal stability to an inhibition of this precursor liquid-like active cellulose intermediate. Since the liquid-like state of the intermediate is water soluble, the hydrophilic interactions likely dominate. It is not surprising, then, that the highest thermal stabilities occur for the most hydrophobic crystals. Sulfate moieties and sodium ions typically enhance char, reducing the consumption of material. As a result, the heat release data are lower for the sulfated nanocrystals than pure cellulose, which typically has a peak heat release rate at 380 °C.55 The exchanged cations are organic, which adds fuel to the crystals. As a result, both the THR and HRC of the crystals increase as the exchanged cations get larger.

Figure 3. Heat release rates of modified CNCs using microcombustion calorimetry.


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Table I. Exchange efficiencies, average overall surface energies, heat release rates, peak decomposition temperatures, and char yields of modified CNCs.

CNC

Exchange %

THR (kJ/g)

HRC (J/gK)

Tonset

% Char

60 121

(°C) 181 217

4.5 4.6

24 21

H-CNC NH -CNC

100 100

γs (mJ/m2) n.t. n.t.

HdMe2Im/H-CNC Na-CNC Me3Im-CNC

70 --98

n.t. 47 52

5.8 6.5 10.0

100 131 171

205 245 249

21 22 12

HxMe Im-CNC

99

50

12.0

203

260

9

HdMe2Im/Na-CNC

10

n.t.

7.8

277

274

15

MePh3P-CNC

83

44

12.6

195

273

12

4

2

Epoxy Composites. Ion exchanged cellulose nanocrystals were used to prepare well dispersed cellulose – epoxy composites without the aid of a solvent or surfactant molecule. Na-CNC that were acquired as freeze dried materials (from t-BuOH) were used for the epoxy composites, because the Na-CNC freeze dried in our laboratory produced composites with very large aggregates on the mm length scale. Initial tests were conducted on the curing behavior with and without the addition of cellulose. There were no appreciable differences in cure behavior between the different samples. The peak cure temperature in the DSC scan (not pictured) was between 132 °C and 135 °C and the heat of curing was calculated to be between 387 J/g and 409 J/g for all composites. Post cured temperature profiles revealed that only about 0.5% to 1% of the reactive groups were not crosslinked during the initial curing, with the one exception of 5% Na-CNC, which had about 2% unreacted material. This indicates that cellulose did not appreciably interfere with the epoxy cross-linking, despite the large number of available hydroxyls. Previous studies have shown that cellulose nanocrystals react with DGEBA at much higher temperatures.56


19


The tensile properties of the cured epoxies are shown in Figure 4. The addition of cellulose nanocrystals modestly increased the modulus and is consistent with other studies.57, 58 The tensile strength was reduced, which was in part due to a different failure mechanism. The neat epoxy yielded and then deformed plastically, while the filled samples all failed in a brittle manner. As such, the peak stress was a yield stress value for the neat epoxy, but was a fracture stress for the composites. Since the curing chemistry of the filled composites appears to be similar to the neat epoxy, it is likely that the nanocrystals create defects in the crosslinking structure, leading to embrittlement. MePh3P-CNC filled epoxies had statistically greater moduli and tensile strength than the Na-CNC filled epoxies. Indeed, half of the tensile strength lost by adding Na-CNC was recovered when using MePh3P-CNC. Reduced self-aggregation of these modified crystals and reduce aspect ratio of the aggregates can reduce the size and number of defects in the crosslinked matrix.

Peak Stress

Modulus 2.5

60 2.0 50 1.5

40 30

1.0

20

Modulus (GPa)

Peak Stress (MPa)

70

0.5 10 0

0.0

3

5%MePh PCNC

5%NaCNC

3

1%MePh PCNC

1%NaCNC

Neat Epoxy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 4. Tensile properties of cellulose nanocrystal – epoxy composites.


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Water absorption has been found to have a negative impact on the mechanical properties of composites. As stated earlier, one of the major drawbacks to using cellulose as a reinforcing material in composites is its high moisture absorption properties. As shown in Figure 5, the addition of a small amount of unmodified Na-CNC substantially increased the water absorption of the epoxy composite. After 5 days, the 1% Na-CNC composites absorb 50% more water than the neat epoxy. As a result of the greater hydrophobicity of the MePh3P-CNC, 1% MePh3PCNC composites only absorb 10% more water than the neat epoxy.

Figure 5. Water absorption of epoxy composites using the ASTM D570 standard method. The dispersion quality and degree of self-aggregation is shown in the confocal microscope images provided in Figure 6. Dispersion in all composites is quite good, but the average aggregate size is dependent on the counterion of the cellulose. Very large aggregates, several hundred microns across, are dispersed in the Na-CNC composites. The average aggregate size is at least an order of magnitude smaller in the MePh3P-CNC composites. The MePh3P+ modification also helps preserve the high aspect ratio of crystalline cellulose, with almost all


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aggregates having a rod like shape. This is in sharp contrast to the Na-CNC aggregates, which clump together forming many edge – edge interactions.

(a)

(b)

(c)

(d)

Figure 6. Confocal microscope images of (a) 1% Na-CNC at 5x, (b) 1% Na-CNC at 20x, (c) 1% MePh3P-CNC at 5x, and (d) 1% MePh3P-CNC at 20x. Polystyrene composites. Several freeze dried CNC samples were melt-blended with polystyrene using a twin screw extruder. Previous studies have shown that CNC can be thermally protected and dispersed in hydrophobic polymers by melt blending when they are first mixed and dried with either polyvinyl alcohol (PVOH)20 or polyethylene oxide (PEO).59-61 Dufresne and co-workers60 have already shown that CNC-PEO have thermal stabilities around 150 °C and resulted in poor dispersions in PS, so we used PVOH as a test compatibilizer. Figure 5a shows images of pure PS, PS + 1% Na-CNC, PS + 1% HdMe2Im-CNC, and PS + 1% MePh3P-CNC. The dispersion quality and thermal stabilities of the cellulose was visibly evident. Na-CNC produced large, partially degraded aggregates when melt blended with polystyrene. PVOH compatibilized Na-CNC appeared to have better dispersion, but the cellulose still degraded (brown color) and the composite lost all transparency. In contrast, Me2HdIm-CNC and MePh3P-CNC were well dispersed in polystyrene and retained optical transparency, as shown in Figures 7a and 8. Me2HdIm-CNC did show some dark specks, indicating aggregation. These crystals were only 10 % exchanged, so achieving a higher exchange rate might further improve


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their dispersion. MePh3P-CNC did not show any visible signs of aggregation. Even at a mass fraction of 3% MePh3P-CNC, aggregates were not visible without magnification and the optical transparency remained quite high relative to the PS + unmodified Na-CNC. Composites containing exchanged cellulose even had better dispersion and optical clarity than composites containing Na-CNC that were freeze dried from aqueous t-butanol solutions, which represent the best possible dispersion of unmodified sulfated cellulose nanocrystals. The comparison in total transmission between PS (Figure 8a) and epoxy composites (Figure 8b) show that the larger crystal aggregation and not the cellulose degradation is the major factor leading to lower transparency for the Na-CNC containing composites.

(a)

(b)

500 µm

500 µm

500 µm

Figure 7. (a) Photographs and (b) laser scanning confocal microscope images of PS + 1% NaCNC, PS + 1% HdMe2Im-CNC, and PS + 1% MePh3P-CNC at 5x magnification.


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(a)

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(b)

Figure 8. UV-VIS transmission through 1 mm thick discs of (a) PS composites and (b) epoxy composites.

To obtain a better picture of the dispersion quality in PS, images were obtained using a laser scanning confocal microscope, shown in Figure 7b. The images confirm the macroscopic visual observation: Na-CNC produces micro-sized aggregates. Exchanging Na+ with MePh3P+ eliminates the largest aggregates. Exchanging Na+ with HdMe2Im+ also reduces the size of aggregates by at least an order of magnitude. There still appears to be some aggregation of CNCs after surface modification. This, however, may be due to the drying process used. Jin and coworkers,62 Oksman and co-workers,22 and Wu and co-workers63 have shown that freeze dried CNCs will produce mesoscale platelets of crystals, roughly 0.5 mm to 3 mm in diameter. If they are first solvent exchanged in t-BuOH), then these platelets are more loosely assembled22. It has been postulated that the water crystals that form during freeze drying force cellulose nanocrystals together,63 and that t-BuOH can reduce the size of flash frozen crystals, resulting in CNC aggregates which are smaller in size.23 Despite the improvement using t-BuOH, the problem of aggregation during the drying process has yet to be resolved for any dried CNC sample.


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Changes in glass transition. The glass transition of all the composites was measured using DSC. (cf Figure 9) The glass transition temperature of epoxy increased with the addition of NaCNC.

This has been observed previously for epoxy composites filled with cellulose

nanomaterials.57,

64

The reduction in mobility in epoxy upon the addition of fillers is often

attributed to interfacial adhesion between the filler and the epoxy matrix. The glass transition temperature of the epoxy upon addition of MePh3P-CNC did not change significantly, suggesting a lower adhesion energy between the modified cellulose and the epoxy. For PS samples, glass transition temperatures did not change outside of the measurement uncertainty. It is likely that there was too little filler for a noticeable difference.

Due to the structure of the

poly(propylether)diamine, it would be expected that the interfacial interactions would be greater in epoxy and cellulose additions would have a larger effect on Tg. 100 95 Tg (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 85 80 75 0

2 4 Mass Fraction CNC (%)

6

Figure 9. Glass transition temperatures measured using DSC of epoxy + NaCNC (▫), epoxy + MePh3PCNC (▪), PS + NaCNC (○), and PS + MePh3PCNC (●).


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Dispersion factor. The observed dispersion of modified CNCs in polystyrene can be explained by quantifying the so-called dispersion factor of different surface modifications. This dispersion factor D, explained previously by Khoshkava and Kamal26, is a ratio of the fillermatrix surface energy (ܹிெ ) to the filler-filler surface energy (ܹிி ) defined by Eqn 2.

D=

W FM W FF

(Eqn 2)

A large value of the dispersion factor indicates a well-dispersed nanocomposite, while a low value of the dispersion factor indicates poor dispersion and the formation of larger microaggregates. In order to measure these interfacial surface energies, molecular dynamics simulations of both CNC-polystyrene and CNC-CNC interfaces were conducted. Molecular dynamics simulations were carried out using the NAMD molecular dynamics package65 and a combination of the generalized CHARMM force field66, 67 and the CHARMM force field for carbohydrates68. Details are provided in the Supporting Information. To obtain the interfacial energy between CNCs and polystyrene, a (110) surface of a CNC having a cross-sectional area of 74.984 Å x 103.8 Å was brought into equilibrium with a PMMA thin film with a thickness of approximately 10 nm. The configuration of this system is shown in Figure 10a. To obtain CNCCNC interfacial energies, two CNCs having the same dimensions as the CNC placed on top of the polymer film were arranged as shown in Figure 10b. Here, two cases of interfacial alignment are considered. The first case is referred to as an aligned interface (aCNC-CNC) where the exchanged ions are directly aligned across the interface and interact directly with one another. The second case is referred to as an interdigitated interface (iCNC-CNC) where the exchanged ions are offset from one another and the ions interact with the opposing CNC surface. It should be noted that in all cases 10% of the total exposed hydroxyl groups on the CNC surfaces are


26

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replaced by sulfate groups and sodium ions. Further, when replacing sodium ions with the larger ions, 100% exchange is assumed for MePh3P, HxMe2Im, and Me3Im while only 14% exchange is assumed for HdMe2Im to accurately replicate experiments. Upon setting up the systems, simulations were run for 5 nanoseconds in order to bring the interfaces into equilibrium. Once the systems reached their equilibrium state, the interaction energy across the interface was measured using the NAMD Energy plugin provided in VMD69. Using this analysis tool, one is able to further break down the interfacial energy into both electrostatic and van der Waals (vdW) components. The results of these calculations are shown in Figure 10c. For the filler-matrix interface, the addition of the ion-exchanged surface modifications lowers the interfacial energy by only 5-10% compared to the Na-CNC case. However, CNC-CNC interfacial interactions show a much more dramatic change as the ionexchanged modifications reduced interfacial energies from 65-80% with the MePh3P-CNCs exhibiting the largest reduction. This reduction can be attributed to two factors, with the degree of importance for each depending on the specific surface modification. First, van der Waals interactions are reduced as the ions physically increase the distance between CNC surfaces at equilibrium, thus reducing non-bonded interactions that scale with the square of inverse distance70. Secondly, electrostatic interactions are greatly reduced due to the increased distance between surfaces that prevents the formation of hydrogen bonds across the interface. Additionally, at interfacial locations where ions are present, hydrogen bonds cannot be formed as the ions act as a physical barrier or shield and prevent the CNC surfaces from interacting electrostatically (i.e. hydrogen bonding). Using the values of the CNC-polystyrene and CNC-CNC interfacial energies, the dispersion factor for each of these different modifications is calculated according to Eqn 2. Here, the CNC-


27


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CNC interfacial energy is taken as the arithmetic average of the aligned and interdigitated cases. The dispersion factor is plotted in Figure 10c. According to this calculation, one would expect the Na-CNCs to exhibit poor dispersion in polystyrene in comparison to each of the other surface modifications. Additionally, one can also conclude that MePh3P ions should exhibit the best dispersion out of all of the cases examined here. These predictions agree well with the experimentally observed dispersion pictured in Figure 7, thereby suggesting that tailoring surface energies, specifically the CNC-CNC adhesion, is a key strategy for controlling dispersion of CNCs in polymers.

Figure 10. (a) CNC-polystyrene systems showing the initial configuration of the system (left) and the equilibrated interface (right). (b) CNC-CNC systems showing the different configurations for aligned and interdigitated configurations. (c) Interfacial energies from MD simulations for each of the different surface modifications for CNC-polystyrene interfaces (red


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outline), aligned CNC-CNC interfaces (green outline) and interdigitated CNC-CNC interfaces (gold outline). Black portions of the bars are contributions from electrostatic interactions (Elec.) and gray portions are contributions from van der Waals interactions (vdW). The blue line and circles show the dispersion factor calculated from Eqn 2. Surface energies, in addition to controlling the dispersion of fillers, also play a role in dictating thermomechanical properties of nanocomposites. Our recent computational study71 has shown that the nanocomposite glass transition temperature (Tg) is largely controlled by the interfacial adhesion energy between the filler and matrix as well as the interparticle distance. Effective interparticle distance is related to weight percent of nanoparticles and their dispersion state. It was shown that as the filler-matrix adhesion is increased, there is a greater appreciation of the Tg. However, as most modifications that improve filler-matrix adhesion energy also increase the CNC-CNC interaction energy, dispersion may become more poor, limiting the Tg appreciation effect. This was observed in the Tg of 5%Na-CNC in epoxy, which has a lower Tg than the 2%Na-CNC in epoxy (Figure 9).

Confocal images (not shown) confirm larger

aggregates in the composite with higher loading. As the dispersion of nanoparticles is decreased and microaggregates are formed, the effective interparticle distance is increased and a lower appreciation of Tg is expected. In the context of current experiments, composites with Na-CNCs have been shown to exhibit a higher Tg appreciation than those embedded with MePh3P. Based on simulations, this is to be expected due to the higher CNC-polystyrene surface energy for NaCNCs. However, the poor dispersion of Na-CNCs is most likely limiting these Tg appreciations, so it is desirable to improve dispersion while also maintaining the strong interaction between CNCs and the polymer matrix. One potential strategy for doing so, beyond examining other potential ions, would be to create CNC surfaces with lower ion exchange percentages. For


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example, instead of exchanging 100% of sodium ions for the larger ions (i.e. MePh3P, HxMe2Im, etc.), only 50% could be exchanged. By doing so, one would still decrease the CNC-CNC interfacial energy to improve dispersion over the Na-CNC case. However, by retaining some sodium ions, this would provide for improved CNC-polymer adhesion that would lead to greater appreciations in Tg. Overall, this demonstrates that tuning surface energies, and specifically balancing filler-filler and filler-matrix energies, is important for a number of different aspects of nanocomposite development. CONCLUSIONS A novel method for modifying the surface of cellulose nanocrystals has been developed. The simple ion exchange method improves the thermal stability while lowering the surface energies of CNCs to allow for melt blending with hydrophobic polymers. This process also can be used to prepare partially exchanged CNCs, so the surface energy of the crystals can be tailored to decrease self-aggregation while retaining strong adhesion with polymers. The crystals prepared potentially retain the ability to form a percolated network, since the modifications are sequestered to the periodic sulfate groups that decorate the polymer chains. Modeling predicts that partial exchange can reduce self-aggregation while maintaining strong interfacial adhesion with the polymer. Imidazolium cations and MePh3P+ were successfully used to exchange the Na+ on CNCs. The exchanged CNCs had thermal stabilities up to 40 °C higher than Na-CNC. The MePh3P-CNC were found to absorb 30% less water than Na-CNC and were used to prepare well dispersed epoxy composites without the aid of a solvent and well dispersed melt blended polystyrene composites without cellulose degradation. The composites retained high optical transparency and modulus. Cellulose self-aggregation and composite tensile strength reduction was minimized when using ion exchanged cellulose. Modeling experiments show that CNC –


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CNC interactions are dramatically reduced while CNC – polymer interactions are only slightly reduced when exchanging Na+ ions for more hydrophobic cations and can be used as a predictive tool for cellulose dispersions in polymer composites. Water absorption in epoxy composites was reduced for ion exchanged cellulose nanocrystals relative to the Na-CNCs.

Some self-

aggregation remains, but may be eliminated through either a tailored exchange of cations or improved drying methods. The results show tremendous potential for using ion exchanged cellulose nanocrystals to overcome the four major obstacles to producing cellulose nanocrystal composites on a commercial scale. This would be useful to the transportation, infrastructure and renewable energy industries, which use fiber-reinforced composites for weight reduction. ACKNOWLEDGEMENTS Research was carried out at the National Institute of Standards and Technology (NIST), an agency of the U. S. government and by statute is not subject to copyright in the United States. Certain commercial equipment, instruments, materials or companies are identified in this paper in order to adequately specify the experimental procedure. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for this purpose. Supporting Information Available: A detailed derivation of the surface energy equations and detailed background on the molecular modeling used in this study are provided in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. References


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