Strain Hardening and Pore Size Harmonization by Uniaxial

Article

Strain hardening and pore size harmonization by uniaxial densification: A facile approach towards superinsulating aerogels from nematic nanofibrillated 2,3-dicarboxyl cellulose Sven F. Plappert, Jean-Marie Nedelec, Harald Rennhofer, Helga C. Lichtenegger, and Falk W. Liebner Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00787 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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Chemistry of Materials

Strain hardening and pore size harmonization by uniaxial densification: A facile approach towards superinsulating aerogels from nematic nanofibrillated 2,3dicarboxyl cellulose Sven F. Plappert†, Jean-Marie Nedelec‡, Harald Rennhofer§, Helga C. Lichtenegger§, Falk W. Liebner†* †

‡

Division of Chemistry of Renewable Resources, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria

Université Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 Clermont-Ferrand, France §

Institute of Physics and Material Sciences, University of Natural Resources and Life Sciences Vienna, Peter-Jordan-Straße 82, 1190 Vienna, Austria *corresponding author: [email protected]

ABSTRACT Dissolving pulp has been subjected to consecutive periodate / chlorite treatments to afford 2,3-dicarboxyl cellulose (DCC, 1.02 mmol g-1 COOH). Subsequent nanofibrillation afforded stable nematic nf-DCC dispersions (average particle size 2.1 nm x 525 nm) at significantly lower energy input compared to TEMPO-oxidation. Acidinduced gelation triggered by extensive hydrogen bonding sets the ordered state and affords free-standing hydrogels that can be converted to highly transparent birefringent aerogels by scCO2 drying. Uniaxial compression of the obtained ultra-lightweight ductile nf-DCC aerogels down to 5 % of their original volume intriguingly preserves nematic orientation and transparence. Simultaneously, strain hardening translates into exceptionally good mechanical properties, such as toughness at nearly zero Poisson’s ratio. Uniaxial compression has been furthermore demonstrated to be a facile and efficient means for converting nf-DCC aerogels of broad, multi-scale pore size distribution into entirely micro/mesoporous scaffolds of narrow size distribution at farreaching preservation of porosity. Following this approach, thermally super-insulating nf-DCC aerogels (λ = 0.018 W m-1 K-1) have been prepared whose intriguing mechanical properties, transparence and nematic ordering bear great potential for other applications as well.

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INTRODUCTION Self-assembly and liquid-crystalline ordering of nano-scale cellulose fibers promoted by charged surfaces moieties is an intriguing approach towards nano-structured bio-based materials. Surface charges are capable of increasing the effective volume of such anisometric nanoparticles promoting their self-assembly into liquid crystal phases as first described by Onsager 1. This phenomenon has so far been used in the preparation of liquid crystal templates 2, 3, photonic materials 4, 5, nanocomposites 6 and transparent aerogels 7. Cellulose aerogels of narrow pore size distribution in the mesopores range (< 50 nm diameter) are potential super-insulating materials as heat conduction via the gas phase as the main transport mechanisms of heat in organic aerogels is largely hindered due to the Knudsen effect 8. Predominantly silica-based aerogels have been hitherto explored as thermal super-insulating materials (λ ≥ 14 mW m-1 K-1) 9 since their porosity in terms of size and narrow distribution can be easily controlled in the required range by sol-gel chemistry. Recently, both synthetic and natural polymer based aerogels have increasingly moved into the focus of thermal insulation, such as polyurethanes (e.g., polyurethane aerogels from BASF; λ ≥ 19.8 mW m-1 K-1) 10 or pectin (e.g., Aeropectin; λ ≥ 20 mW m-1 K-1) 11. Different from silica, cellulose-based aerogels whose synthesis does not follow the principles of the classic Teichner sol-gel chemistry exhibit rather wide pore-size distributions and consist of voids whose diameters frequently distinctly exceed 100 nm rendering them less suited for insulation applications. Therefore a variety of recent studies have investigated templating approaches that combine morphological features of inorganic and entirely mesoporous nanostructured materials with the particular properties of cellulose 8, 12-15. Nanocellulose is particularly useful in this respect as it imparts the composite materials strength and flexibility, and can simultaneously compensate the brittleness inherent to most inorganic aerogels 13, 14. Kobayashi et al. 7 recently presented an approach towards thermal superinsulating cellulosic aerogels that circumvents the use of templating inorganic constituents. It is based on acidinduced gelation of low-concentrated aqueous dispersions of cellulose nanofibrils obtained by TEMPO-oxidation (2,2,6,6-tetramethylpiperidinyloxyl) of pulp and subsequent mechanical nanofibrillation (TOCN). Conversion of the obtained hydrogels into aerogels employing supercritical carbon dioxide drying (scCO2) afforded a novel type of lightweight aerogels which was reported to feature thermal super-insulation properties for a certain density window owing to the presence of highly ordered, narrow-mashed TOCN networks and the low heat conduction of the solid phase 7. Although respective aerogels contain a certain volume fraction of mesopores, the size of the voids accounting for most of the pore volume is not in the desired range (diameter 50-100 nm). Beyond that, the low bulk densities of these TOCN aerogels owing to the poor castability of dispersions having a TOCN content significantly greater than 20 mg cm-3 results in somewhat tough but rather poor mechanical properties. Therefore, this study investigates and proposes uniaxial compression as a means of poresize control and strain-hardening to obtain entirely mesoporous, fully biopolymer-based and mechanical strong nanocellulose aerogels aiming at super-insulating properties. Furthermore it investigated in how far key features, such as nematic ordering, specific surface area, birefringence and transparency – rather elusive for other cellulose-based aerogels or porous materials – can be preserved throughout uniaxial compression, as this would be of great benefit to a broad variety of applications no matter whether 3D materials (aerogels: thermal ACS Paragon Plus Environment

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superinsulation, sensing, matrices for optical devices, e.g.) or 2D materials (membranes: filtration, fractionation of nanoparticles, sensing, e.g.) are considered 16. While the above cited work on TOCN aerogels is based on heterogeneous TEMPO-mediated oxidation affording 6-carboxyl cellulose, we followed a different approach relying on sequential periodate and chlorite oxidation as chemical pulp pretreatments. Unlike TEMPOoxidation, periodate regio-selectively converts the vicinal secondary hydroxyl groups in C2 and C3 position of the anhydroglucose units (AGU) into aldehyde groups which are subsequently further oxidized by chlorite to carboxyl moieties, hence affording 2,3-dicarboxyl cellulose (DCC). As periodate oxidation occurs under cleavage of the pyran ring (between C2 and C3), conformational freedom is supposedly added to that introduced by creation of negative charges alongside the cellulose chains which is not the case for TEMPO-mediated oxidation. Thereby, it was expected that nanofibrillation of DCC could afford smaller particles compared to TOCN and would require less energy. The good recyclability of sodium periodate by simple ozone treatment 17 along with the fact that neither the preparation of 3D materials like aerogels nor self-alignment of nanofibrillated 2,3-dicarboxyl cellulose (nf-DCC) in aqueous dispersion have been hitherto reported were further motifs of this study. To the best of our knowledge, DCC nanofibrils have been hitherto only investigated for the preparation of 2D materials for packaging, oxygen barrier 18 and filtration 19 applications.

EXPERIMENTAL SECTION Materials. Never-dried, sulfite dissolving pulp (fagus spp., TCF bleached, CCOA 24.3 µmol g-1 C=O, FDAM 13.9 µmol g-1 COOH, Mw 303.7 kg mol-1, 50 %w) was used as cellulosic starting material. Sodium periodate, acetic acid, sodium chlorite was purchased at the highest grade available and used without further purification. Preparation of 2,3-dialdehyde cellulose (DAC). 24 g dissolving pulp (50 %w) was disintegrated in 1 L of deionized water using a conventional kitchen blender for one minute. Then 200 mL of 0.23 M NaIO4 was added and the reaction mixture was stirred at 600 rpm and 50°C for three hours. Oxidation was stopped by filtering-off the formed 2,3-dialdehyde cellulose from the aqueous oxidant solution followed by immediate thorough washing of DAC with deionized water. Storage of the never-dried DAC until further processing (within 3 days) was accomplished at 4 °C and a solid content of 5.5 %w. Determination of aldehyde content. The degree of oxidation of DAC was determined according to the method by Sirviö at al. 20 relying on quantitative oximation of the carbonyl moieties and subsequent determination of the nitrogen content by elemental analysis (Carlo Erba EA 1108 CHNS-O instrument). Preparation of nanofibrillated 2,3-dicarboxyl cellulose (nf-DCC). 2 g of DAC was dispersed in 200 mL 1 M acetic acid. After addition of 2.013 g NaClO2 the reaction mixture was stirred (600 rpm) at 20°C for 16.5 hours. Oxidation was stopped by filtering-off the formed 2,3-dicarboxyl cellulose (DCC) from the aqueous oxidant solution followed by immediate and exhaustive washing of DCC with deionized water. Nanofibrillation of DCC was accomplished by repeated homogenization (2 times at 50 MPa then 2 times at 80 MPa) of aqueous 0.25 %w dispersions of DCC which had been adjusted to pH 7.5 by addition of 0.1 M NaOH. After fibrillation the dispersion was centrifuged at 5000 rpm (∼ 4830 g) for one hour to remove possible non-fibrillated residues. The obtained nf-DCC dispersion was subsequently concentrated to ≥1 %w using a vacuum rotavapor. ACS Paragon Plus Environment


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Determination of carboxyl content. The amount of carboxyl groups on nf-DCC was determined by conductometric titration. In brief 100 mg of freeze dried DCC nanofibrils were disintegrated in 50 mL of distilled water. After adding 2.75 mL of 0.1 M hydrochloric acid and continued stirring (30 min), the equilibrium H+ concentration was measured (pH 2.75). Conductometric titration of the dispersion was then started and maintained under continued stirring until a total of 5 mL aqueous 0.1 M NaOH at an increment of 25 µL every 30 seconds had been added. An automated titration apparatus consisting of a 800 Dosino dosing device connected to an 856 conductivity module (Metrohm, Switzerland) was used. Atomic force microscopy (AFM) of nf-DCC. AFM scans were made from diluted nf-DCC dispersions dried on mica discs using a Dimension Icon Scanning Probe Microscope (Bruker AXS, France; formerly Veeco) equipped with OTESPA cantilever operated in tapping mode and a NanoScope V control station. Gwyddion 2.40 software was employed for image processing. Preparation of nf-DCC aerogels. Aqueous dispersions of nf-DCC (≥1 %w/v) were poured into molds of various size and geometry. Acid-induced gelation was accomplished by immersing the molds in aqueous 1 M HCl for 1 hour per 1 cm mold depth. The gels were then transferred into aqueous 50% ethanol and finally to absolute ethanol with four solvent exchanges every 24 hours. The resulting anhydrous alcogels were subjected to an ethanol extraction using supercritical CO2 (“scCO2 drying”; 9.5 MPa, 40 °C, flow rate of 40 g min-1, 23 hours). Drying was stopped by isothermal depressurization at a rate of 50 nm) and preservation of a minimum density (low λsolid) is hence an effective approach to reduce heat conductivity. This has been confirmed for the densified nf-DCC aerogels as exemplarily demonstrated for an nf-DCC ACS Paragon Plus Environment

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aerogel of ultra-low density (23 mg cm-3). Uniaxial compression to 20% of its original height (ρB = 87 mg cm-3) affords a mechanically robust material that has a lower thermal conductivity than air (17.7 vs. 26 mW m-1 K-1 51 at 25 °C and ambient pressure). Nondensified nematically ordered aerogels prepared from nanofibrillated TEMPO-oxidized cellulose have been reported to feature similarly low thermal conductivities 7. However the latter exhibit these properties at very low density (17 mg cm-3) only, coinciding with rather weak mechanical properties. Also, compared with other biopolymer-based superinsulating aerogels, such as aeropectin 11 or even mesoporous silica aerogels 52, 53, uniaxially compressed nf-DCC aerogels are regarded superior due to their exceptionally good mechanical properties, ductility, transparence and nematically ordered particulate network morphology which invite to be exploited in traditional and novel applications.

Figure 8: Thermal conductivity (λ, 25°C) of nf-DCC aerogels of different initial densities after uniaxial compression to 20 % of their initial heights

CONCLUSION The presented approach comprising A) periodate oxidation of dissolving pulp to obtain DAC, B) conversion of DAC to DCC by chlorite oxidation, C) nanofibrillation of DCC in aqueous dispersion to afford self-aligning nf-DCC and D) acid-induced gelation of highly diluted nf-DCC dispersions gives access to transparent, nematic hydrogels. Replacement of water by ethanol followed by scCO2 drying of the anhydrous alcogels affords ultra-lightweigth, transparent and birefringent nf-DCC aerogels at otherwise preserved nematic orientation and solid network features. Compared to TEMPO oxidation and subsequent nanofibrillation which is hitherto the only alternative to afford carboxyl cellulose aerogels of comparable transparence, the two-step nf-DCC approach is superior for various reasons, including the lower size of the nanofibrils accessible already at lower energy input or the good recyclability of the oxidizing agent. Uniaxial densification has been tested to be a facile technique to impart nf-DCC aerogels mechanical properties inviting for real-world cellulose aerogel applications as in addition to strain-hardening the porosity and specific surface of the aerogels can be largely preserved even at 90% compression. Since uniaxial densification simultaneously harmonizes the multi-scale pore-size distributions typical of cellulosic aerogels in favor of mesopores and at the expense of macropores, the presented approach affords materials of ultralow thermal conductivity due to the largely suppressed heat conductivity through the gas phase (Knudsen effect). The preservation of nematic ordering



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perpendicular to the compression axis, the alignment of mesopores as evident from the distinct capillary condensation of nitrogen at a specific relative pressure, and the compression-dependent transparency amplify the application potential of DCC aerogels and encourage for further studies.

ACKNOWLEDGEMENTS The financial support by the Austrian Science Fund (FWF: I848-N17) and the French Agence Nationale de la Recherche (ANR-11-IS08-0002; Austrian-French Project CAP-Bone) as well as the Austrian Federal Ministry for Agriculture, Forestry, Environment and Water Management (BMLFUW) through the WoodWisdom Net+ project AeroWood is gratefully acknowledged. Johannes Konnerth is gratefully acknowledged for his support on mechanical testing of the materials, as is Tiina Nypelö for performing AFM analysis of nf-DCC (Institute of Wood Technology and Renewable Materials, Department of Material Sciences and Process Engineering, University of Natural Resources and Life Sciences Vienna, Austria).

SUPPORTING INFORMATION The supporting information contains the detailed procedure applied to prepare TEMPOoxidized cellulose nanofibrils (TOCN) and a comparison between TOCN and nf-DCC with regard to fibril dimensions (AFM) and degree of oxidation (conductometric titration). Furthermore a visualization of the mechanical data concerning strain hardening of nf-DCC aerogels upon uniaxial densification is given.

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Strain Hardening and Pore Size Harmonization by Uniaxial

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