Conformal ultrathin coating by scCO2-mediated PMMA deposition: A

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Conformal ultrathin coating by scCO-mediated PMMA deposition: A facile approach to add moisture resistance to lightweight ordered nanocellulose aerogels Sven F. Plappert, Sakeena Quraishi, Jean-Marie Nedelec, Johannes Konnerth, Harald Rennhofer, Helga C. Lichtenegger, and Falk W. Liebner Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05226 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Chemistry of Materials

Conformal ultrathin coating by scCO2-mediated PMMA deposition: A facile approach to add moisture resistance to lightweight ordered nanocellulose aerogels Sven F. Plappert†, Sakeena Quraishi†, Jean-Marie Nedelec‡, Johannes Konnerth§, 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, A-3430 Tulln, Austria

‡

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

§

Institute of Wood Technology and Renewable Materials University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, A-3430 Tulln, Austria

∥

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

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Table of Content (ToC) Graphic

Abstract A facile approach for adding moisture resistance to transparent nematic aerogels composed of individualized cellulose nanofibers (i-CNF) at full preservation of the anisotropic aerogel structure is presented. Sequential nitroxide-mediated oxidation and mechanical cellulose fiber delamination was applied to obtain i-CNF dispersions in water. Nematic ordering caused by repulsive forces between i-CNF surface carboxylate groups was set by acid-induced hydrogen bonding and gelation, respectively. Solvent exchange to acetone, impregnation with the PMMA, scCO2mediated anti-solvent precipitation of the secondary polymer and scCO2 extraction of interstitial acetone afforded highly hydrophobic nanocomposite aerogels. Birefringence studies utilizing polarized light and the Michel-Levy Chart to evaluate interference pattern revealed that nematic i-CNF ordering is virtually not affected by the respective surface modification and scCO2 drying steps. Morphological studies provide evidence that the large internal surface (> 500 m2 g-1) of the hybrid aerogels consists of a homogeneous ultrathin PMMA (mono)layer that does virtually not affect the high porosity (≥ 99 %) and transparency (> 77 % transmission, 600 nm, 2.13 mm thickness) inherent to i-CNF aerogels. The obtained materials exhibited excellent resistance towards moisture as apparent from the high water contact angle (119.4° ± 7.5). As PMMA imparts 2


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the aerogels stiffness and hydrophobicity, aggregation of nanofibrils in moist environment or under vacuum conditions can be avoided even at ultra-low densities as low as 9.6 mg cm-3.

Introduction Ultra-lightweight materials of tailored interconnected nanoscale porosity, high specific surface area and target-specific surface chemistry are of immense interest for many fields of applications including thermal and acoustic insulation, gas separation, adsorption, catalysis, tissue engineering, theranostics or optoelectronics.1 Depending on the target application and desired properties, respective aerogels can be prepared from a vast variety of engineered inorganic (e.g., organosilanes2, chalcogens3, boron nitride4) and organic polymers (e.g. cross-linked polyimides5, Kevlar-type polyamides6). However, the recent progress in second-generation biorefinery and renewables-based material research has shown that many biopolymers, such as polysaccharides (cellulose7, hemicellulose8, pectin9, chitosan10) and proteins (whey11, soy12, silk fibroin13) or lignin14 can be used as valuable source materials for aerogels as well. From the perspective of sustainability and cost-efficiency, biopolymer aerogels are already considered viable competitors of traditional aerogels for some applications. In particular nanocellulose aerogels7 have shown remarkable potential in this respect, as they can feature high specific surface15, 16, low thermal conductivity15-17, high transparence15, 16, 18, superabsorption19, 20 and excellent mechanical properties in tension18 and compression16 while lacking brittleness of their inorganic counterparts. Specific physical and chemical features supporting self-assembly to sophisticated hierarchical structures, biocompatibility, biodegradability and flexibility inherent to many of these materials even let expect bio-aerogels to conquer new applications in near future.

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Cellulose as the most abundant biopolymer on earth21 is of particular interest as it is readily accessible even at high purity, offers a multitude of opportunities for selective modification and features an intriguing self-assembling behavior in both solution22-24 and dispersion15, 25, 26 state. The latter is particularly interesting for anisometric cellulose nanoparticles like cellulose nanocrystals (CNC)26 or individualized cellulose nanofibres (i-CNF)15 which can form different types of liquid-crystalline phases in dispersion state due to repulsive surface charges. i-CNF of negative net charge can be obtained by oxidation of bulk cellulose and subsequent mechanical exfoliation and nanofibrillation of the macrofibrils, respectively. Nitroxide-mediated oxidation using the stable 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical is the most frequently applied approach for this purpose27-29, capable of selectively oxidizing outward facing C6 hydroxyls of the cellulose fibrils. Sequential periodate and chlorite oxidation is an alternative approach affording 2,3-dicarboxyl cellulose instead of 6-carboxyl cellulose. Recently the preparation of transparent nematic cellulosic aerogels from nanofibrillated 6-carboxyl-cellulose15 and 2,3-dicarboxylcellulose16, respectively, has been reported, including a facile uniaxial densification approach capable of affording mechanically stable, superinsulating i-CNF aerogels16. Further applications of these novel classes of CNC and i-CNF based aerogels are currently intensively explored.7 However, the hydrophilicity of cellulose, similar to other natural or synthetic organic polymers is an obstacle that needs to be overcome to make respective aerogels ready for real-world applications. This is in particular the case for light soft materials like i-CNF aerogels as their fragility, extensive network of nanocapillaries along with their large internal surface literally invites for moisture sorption and shrinkage triggered by capillary forces. Effective techniques are therefore required that impart moisture resistance but fully preserve the specific nanomorphology of i-CNF

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aerogels in terms of interconnected porosity, internal surface and nematic arrangement of the amphiphilic cellulose colloids as well as their transparency. This study proposes a facile hydrophobization method that can be applied to virtually any type of moisture-sensitive aerogels and circumvents elaborate chemical vapor deposition30 or wetchemical derivatization techniques. The core concept combines scCO2 anti-solvent precipitation of the secondary polymer poly(methyl methacrylate) (PMMA) from solution state with scCO2 drying of the i-CNF/PMMA hybrid aerogels in one single batch, just by isothermal variation of the CO2 pressure. This is possible as the solvent power of a mixture of solvent and CO2 sharply decreases during transition of the binary mixture to supercritical state and turns CO2 to act as an antisolvent for the solute31. Appropriate PMMA loading and scCO2-antisolvent coating conditions provided, homogeneous deposition of a very thin PMMA (mono)layer on the large internal surface of i-CNF aerogels was anticipated that would not interfere with the nanomorphological characteristics of the parent material. The choice of PMMA as secondary polymer was the result of previous studies demonstrating that – different from poly(lactic acid) or poly(caprolactone) – PMMA does not foam or agglomerate in scCO232, has good affinity to cellulosic surfaces and renders cellulosic aerogels particularly biocompatible33. Hence, the implementation of this facile and inexpensive technique could give a great impetus to aerogel research in general and help polysaccharide aerogels to enter new applications, including true volumetric displays, thermal superinsulation, wound dressings, tissue engineering or optoelectronics.

Experimental Section Never-dried bisulfite hardwood dissolving pulp (50 % w/w H2O, CCOA34 24.3 µmol g-1 C=O, FDAM35 13.9 µmol g-1 COOH, Mw 303.7 kg mol-1) was used as cellulose starting material. 5



Poly(methyl methacrylate) (PMMA, Mw 350.0 kg mol-1), 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), NaBr, NaClO solution (available chlorine 10 - 15 %), NaClO2, ethanol and acetone were purchased at the highest available grade and used without further purification. Aqueous solutions of NaOH and HCl were prepared from respective volumetric concentrates (Fixanal). Deinonized water (DI H2O) millipore grade was used for all experiments.

Preparation of TEMPO-oxidized individualized cellulose nanofibrils (i-CNF) TEMPO-mediated oxidation of cellulose was accomplished as described previously36, however, with the difference that NaClO was continuously added instead at once to avoid partial overoxidation. In brief 8 g (dry weight) dissolving pulp (50 % w/w H2O) was suspended in 800 mL of DI H2O and disintegrated using a household blender for 1 min. To achieve selective oxidation of the primary (C6) hydroxyl groups 128 mg 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 800 mg NaBr were added. Thereafter, a total of 12 mL NaClO solution was added at a rate of 200 µL min-1 at room temperature under constant stirring (1000 rpm). pH was constantly adjusted to pH 10 by addition of 0.1M NaOH. After completion of NaClO addition, a reaction time of 20 min was granted before the oxidized cellulose was filtrated and washed with DI H2O multiple times (4 washing steps with 1 L each). To convert potentially formed carbonyl moieties into carboxyl groups, the never dried oxidized pulp was post-oxidized in three successive batches by suspending the material in 800 mL of 0.1 M sodium acetate buffer (pH 4.8), adding 2 g of NaClO2 and constant stirring (1000 rpm) at room temperature for 4, 6 and 15 h, respectively. Between the batches, the oxidized material was thoroughly washed by vacuum filtration. The obtained suspension of 6-carboxyl cellulose was diluted to 0.5 % w/w and pH was adjusted to 8 by addition of 0.1 M NaOH. The suspension was then homogenized using a high pressure 6


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laboratory homogenizer (APV 1000, APV Manufacturing Sp. z o.o., Poland). After 5 passes at 80 MPa, the dispersion was further diluted to 0.125 % w/w and 3 more passes at 80 MPa were conducted. After homogenization the i-CNF dispersions were centrifuged using a Rotina 380 (Hettich Lab Technology, Germany) equipped with a swing-out-rotor (1754-1778 R13) for 30 min at 5000 rpm to remove any residual agglomerates and unfibrillated material.

Characterization of i-CNF: The amount of introduced carboxyl groups was determined by conductometric titration as detailed elsewhere37. In brief 0.065 g of i-CNF that was beforehand subjected to centrifugation (see above) was dispersed in 52.25 mL of DI H2O by continued magnetic stirring (1400 rpm, 15 min). After adding 2.75 mL 0.1 M hydrochloric acid solution, the titration was carried out under constant stirring by adding a total of 5 mL 0.1 M NaOH with an increment of 25 µL every 30 seconds. Precise dosing was accomplished using an automated titration apparatus 800 Dosino device connected to an 856 Conductivity Module equipped with a 801 Stirrer (all Metrohm, Switzerland). AFM-topography imaging of a diluted i-CNF dispersion dried on a mica plate was performed with a Dimension Icon Scanning Probe Microscope (Bruker AXS, France; formerly Veeco) equipped with OTESPA cantilever used in tapping mode and a NanoScope V control station. Gwyddion 2.40 software was employed for image processing.

Characterization of PMMA Dynamic laser light scattering (658 nm laser, scattering angle 90°) using Wyatt DLS DynaPro NanoStar equipment was performed to determine the hydrodynamic radius of the PMMA random 7



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coils. A PMMA solution in acetone corresponding to the PMMA loading bath concentration of 4 mg mL-1 was transferred into a disposable cuvette, placed inside the DLS compartment and equilibrated at 40 °C. 100 data points were acquired using an acquisition time of 5 seconds. The parameters dn/dc = 0.134038 and A2 = 9.51 10 × 5 mol mL g-2

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were used to determine the hy-

drodynamic radius and corresponding diameter of the polymer random coils.

Preparation of nematic i-CNF aerogels After rotoevaporation of the i-CNF dispersions to desired solid content (4.8, 8.1 or 9.5 mg mL-1 i-CNF), an appropriate amount was poured into cylindrical teflon molds (Ø 10 mm, height: 10 mm) and immersed in 1 M HCl for one hour. After acid-induced gelation, water was replaced by aqueous ethanol of incrementally increasing ethanol content until the water content equaled zero (four solvent exchanges in total with 24 hours residence time for each step) and the samples were ready for scCO2 drying using SF-1 supercritical fluid extraction equipment (Separex, Champigneulles, France). For PMMA coating, alcogels were repeatedly (three times, 3 hours each) soaked in acetone before transferred into a loading bath (50-times gel volume, residence time 24 hours) containing 4 mg mL-1 PMMA dissolved in acetone. Isothermal (40°C) scCO2 antisolvent precipitation of PMMA from solution state was accomplished by slowly crossing the supercritical boundary of the binary mixture acetone/CO2 by increasing the pressure beyond 8 MPa. As the interfacial tension between liquid and supercritical phase is here already close to zero – a prerequisite for supercritical extraction – scCO2 antisolvent precipitation of PMMA and supercritical extraction of acetone can be performed simultaneously. The supercritical point of the CO2 phase is slightly shifted towards higher temperatures and pressures with increasing amount of acetone (e.g. to 8


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8.04±0.07 MPa and 38.7±0.5 °C for 2.3 %mol acetone).40 To ensure quantitative precipitation of the secondary polymer and extraction of the solvent, the following conditions were used: The autoclave was pressurized until a supercritical phase started to become visible after about 240 s. Then the system was slowly further pressurized up to the drying pressure of 9.5 MPa within 300 s and a flow of 40 g min-1 CO2 (at 40 °C with Pmax 9.5 MPa) was applied for three hours to dry the samples. After completion of acetone extraction, the system was isothermally depressurized at < 0.1 MPa min-1.

Characterization of i-CNF aerogels: Light transmittance of both the i-CNF/PMMA hybrid aerogels and their non-hydrophobized counterparts in the range of 200-1000 nm was measured using a PerkinElmer Lambda 35 UV/VIS spectrometer and a scanning speed of 480 nm min-1. Birefringence of the colloidal i-CNF dispersions, the lyogels and aerogels was investigated by illuminating the samples placed between crossed polarizers (at 90°) with white light and evaluating the observed color pattern using the Michel-Lévi chart41. While both the i-CNF dispersions and respective hydrogels were measured at a thickness of about 10 mm, sample thickness was reduced to 2-4 mm for the aerogels due to their stronger birefringence. Density and shrinkage of the aerogels was determined after exposure to 50 % relative humidity for 10 minutes using a precision balance and electronic caliper, respectively. Mechanical response towards uniaxial compression was investigated using a Zwick-Roell Materials Testing Machine Z020 (Zwick GmbH & Co. KG, Ulm, Germany). Elastic (Young’s) modulus was calculated from the range of 2-4% strain. The strain required to achieve 80% compression was measured with a 500 N load cell. 9



Field emission scanning electron microscopy was used to visualize the morphology of one representative i-CNF/PMMA aerogel. The sample was imaged on a Zeiss Supra 55VP using a 0.75 kV beam after gold sputtering. Nitrogen sorption experiments at 77 K were performed using TriStar II PLUS equipment (Micromeritics, Aachen, Germany). The specific surface area of the aerogels was calculated according to the Brunauer–Emmett–Teller42 (BET) equation from the adsorption branch (relative pressure P/P0 = 0.05-0.343). All samples were degassed at 50 °C for 20 hours prior to the sorption experiments. Small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments were performed to investigate the nanomorphology of the cellulose scaffold forming units including cellulose crystallinity. One representative sample was selected for each of the aerogel types (iCNF aerogel: ρB = 16.4 mg cm-3; i-CNF/PMMA aerogel: ρB = 12.6 mg cm-3). For purposes of comparison never-dried dissolving pulp was included in the studies. A respective sample was immersed in absolute ethanol and subjected to scCO2 extraction using the same protocol as applied for preparation of the i-CNF aerogels. X-ray scattering was performed on a Rigaku S-Max 3000 (Rigaku Co., Tokyo, Japan) equipped with MM002+ Cu-Kα source (wavelength k = 0.1542 nm). A two-dimensional gas filled TRITON200 multi-wire detector was used for SAXS experiments while an FUJI image plate detector was employed in WAXS studies. The obtained two-dimensional scattering images were integrated and background corrected prior to further evaluation to obtain the scattering curves, where the scattered intensity I(q) is a function of the scattering vector q. The latter is related to the scattering angle 2Θ as described by Braggs law: λ = 4 π / q ⋅ (sin Θ).

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Small-angle X-ray scattering curves have been evaluated using a modified Guinier approach44 that considers the fibrillar structure of i-CNF and yields, hence, the cylinder radius of gyration Rc of the cellulose nanorods as network building structures. In the low q regime the SAXS scattering curve follows a power law: I(q) ~ A ⋅ q-n with A being a constant and n the slope of the curve in a double logarithmic plot, which is an indication for the fractal dimension of the network45. A value of n = 1 would be indicative for a structure composed of loosely connected nanoscale rod-like building blocks while values of 2 and 3 would be caused by interlinked chains or clustered networks, respectively. The results of WAXS were used to calculate cellulose crystallinity (equation 1) based on a comparison of the intensities of the main visible peak at about 20° (Icryst) and of a peak at a position, which is assumed to show scattering from amorphous phases only, such as that at 13.2 nm-1, i.e. 18° (Iamorph)46.

CrystallinityWAXS =

Icryst - Iamorph Icryst

· 100%

(equation 1)

The contact angle (θ) of water on both i-CNF and i-CNF/PMMA aerogels was measured using a drop shape analyzer (DSA30, KRÜSS Optronic GmbH, Hamburg, Germany) complemented by a CCD camera device (Sony 93D, Model XC-77CE, 2/3 Zoll CCD, 11 x 11 µm pixel size) and an adjustable background lighting. The measurements were performed in controlled environment of temperature (23°C) and relative humidity (50 %RH) using DI H2O as test medium. The angle between water drop and surface was determined with KRÜSS DSA1 drop shape analysis software.

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Results and discussion TEMPO mediated oxidation of never-dried hardwood bisulfite dissolving pulp in alkaline medium followed by mechanical disintegration using high pressure homogenization equipment afforded aqueous dispersions of individualized cellulose nanofibers (i-CNF) largely uniform in size. The amount of carboxyl groups introduced was 1.29 mmol g-1 as determined by conductometric titration which equals a degree of oxidation (DOCOOH) of 20.9 % related to the total count of anhydroglucose units (AGU). Aiming to narrow the particle size distribution in favor of small fibril diameters and to remove insufficiently fibrillated cellulose, centrifugation was applied (5000 rpm, 30 min) which removed a sedimentable fraction of 12.5 % related to the total mass of dispersed i-CNF. Atomic force micrographs revealed that the edge length of the nearly square cross-sectional area of the obtained i-CNF nanofibers was about 2.4 ± 0.5 nm (n = 45) which corresponds well with literature values for cellulose elementary microfibrils47 and is in good agreement with other AFM studies48. Transmission electron spectroscopy data sometimes suggest somewhat higher values for comparable materials15, 27. However, these deviations on the upper picometer scale are assumed to be rather due to methodological differences as the cross-sectional dimension of the elemental fibril is rather independent of the wood species used as pulp source27. This is quite different for the longitudinal dimension of the nanofibers which depend on various factors. This concerns primarily the severity of cellulose degradation during the isolation (pulping) and purification (bleaching) processes owing to their strong impact on molecular weight characteristics21, 49, 50

potentially limiting the length of nanofibers. High-pressure fibrillation can also provoke a quite

severe reduction of the longitudinal nanofiber dimension, mainly caused by strong shear forces27,

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51

. The i-CNF obtained in this study had an average length of 940 ± 250 nm (AFM, n = 15; Fig-

ure 1).

Figure 1: AFM image of i-CNF (a) and FEG-SEM images of the fracture surface of an i-CNF aerogel (density: 18.2 mg cm-3, b) and i-CNF/PMMA hybrid aerogel (density: 14.8 mg cm-3, c)

As detailed in the introduction, i-CNF can easily self-assemble in aqueous dispersion owing to the rod-like dimension, high aspect ratio and negative electrical surface charge. Using crossed polarizers, the typical color pattern of nematic liquid-crystalline phases can be observed already at Ccr,min values as low as 0.1 wt%15, 52. However, free self-assembly of i-CNF is limited to a comparatively narrow concentration window as beyond a solid content of about 2 wt% the strongly increasing viscosity of the dispersion would supposedly require shear forces for further alignment. Within this small concentration range of Ccr, min < x < Ccr,max, dispersions of i-CNF can freely flow and, therefore, easily cast. Immersion of the cast i-CNF in acidic medium sets the anisotropic phase by protonation of glucuronic acid moieties located at the surface of the nanoparticles. This, in turn, triggers reduction of electrostatic repulsion53, inter-particulate hydrogen bonding and eventually results in rapid gelation and formation of free-standing highly transparent aquogels. It is worthwhile mentioning that 13



according to the same order of birefringence as obtained for equally thick samples and using the revised Michel-Levy color chart41 for evaluation, nematic ordering of the nanoparticles can be fully preserved upon immersion of cast i-CNF dispersions in aqueous HCl. Intriguingly, optical anisotropy is also maintained throughout the entire process of converting hydrogels into nanocomposite aerogels. This includes replacement of interstitial water occupying the voids of i-CNF hydrogels by ethanol of incrementally decreasing water content and eventual extraction of anhydrous ethanol using scCO2. Compared to hydrogel state, birefringence was even about 2.5 times higher for the obtained aerogels. This is most obviously due to the different refractive indices of water and air, occupying the voids in hydrogel and aerogel state, respectively. The refractive index of bulk i-CNF films is about 1.54527, whereas liquid solvents like acetone, ethanol and water have respective refractive indices of 1.354054, 1.356854 and 1.327754 at 20 °C. Compared to liquids, gases have lower refractive indices as exemplarily given for nitrogen (1.000355) and carbon dioxide (1.000456). Therefore based on Snell’s law57 the refraction by the nanofiber-skeleton in aerogels is higher compared to the respective refraction in lyogels. However the bulk reflection is higher for the lyogels. This phenomenon has also been observed in porous materials derived from chiral nematic CNC structures58, 59. In these materials a strong decrease of their respective birefringence and circular dichroism - differential absorption of leftand right-handed light - was observed when the respective gas filling their pores was replaced by liquids which had a similar refractive index like the corresponding solid matrix. Since hydrophilicity and fragility, and hence moisture adsorption, shrinkage and biodegradation have hitherto prevented lightweight cellulosic aerogels from real-world applications, surface coating is considered a viable approach to overcome these obstacles. The i-CNF aerogels of this study have been therefore subjected to 1) impregnation with a solution of PMMA in acetone, 2) 14


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scCO2-antisolvent precipitation of PMMA and 3) extraction of acetone using scCO2. As steps 2 and 3 require similar conditions they can be performed in one single batch just by increasing the pressure when step 2 has been finished. Interestingly, deposition of PMMA did virtually not affect the extent of birefringence of i-CNF aerogels. This suggests that anisotropic ordering of the nanofibrils as established by self-alignment in dispersion state is largely preserved throughout incorporation of the internal PMMA surface layer.

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Figure 2: Picture series showing the single-batch joint scCO2 antisolvent PMMA precipitation and acetone extraction processes for a cylindrical i-CNF/PMMA lyogel (Ø = 10 mm) through a sapphire window of the autoclave

Combined scCO2 antisolvent precipitation of PMMA and extraction of acetone was visually observed by placing respective samples in an autoclave equipped with sapphire windows and camera. The picture series shown in Figure 2 confirms that no sample buckling occurs during conversion of the lyogels into cellulose/PMMA hybrid aerogels even though during pressurization already at sub-supercritical conditions larger quantities of CO2 solubilize60 into the outer acetone layer and PMMA precipitation can be seen (Figure 2d). Blurry clouds of acetone visible when approaching and crossing the supercritical point indicate the release of the (CO2-expanded) acetone phase. The appearance of Rayleigh scattering which starts with the extraction of acetone by scCO2 is caused by the drop in refractive index as discussed above. PMMA impregnation of the i-CNF lyogels (water is replaced by acetone beforehand) was accomplished using a PMMA loading bath concentration of 4 mg mL-1 for all experiments as the variation in specific surface area among i-CNF aerogels was virtually negligible within the studied range of apparent densities. According to the weight gain (compared to PMMA-free samples) after loading, solvent exchange and joint scCO2 antisolvent precipitation / scCO2 drying, and considering volumetric shrinkage, an effective PMMA loading of 3.3 to 3.7 mg per cubic centimeter of i-CNF/PMMA hybrid aerogel was calculated. This corresponds to a PMMA loading efficiency of 82.5-92.5%. Taking the volume-normalized specific surface of i-CNF aerogels into account (7.6 and 8.5 m2 cm-3 for aerogel bulk densities of 12.0 and 16.4 mg cm-3, respectively), a loading of about 43 ng per square centimeter of internal surface can be derived. This would be equal to a 3.6 Å thick PMMA monolayer assuming a PMMA bulk density of 1.2 g cm-3 and quantitative coating of the surface. Prior discussion of the results obtained from nanomorphologi16


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cal (WAXS, SAXS, FEG-SEM), optical and mechanical studies which all support the implementation of a nearly monolayer PMMA coating, it should be noted that the latter requires about 2843% of PMMA (range reflects variance of i-CNF cross-sectional dimensions) related to the mass of cellulose only which fits well the experimental cellulose-to-PMMA ratios (Table 1). Accessibility of the virtually entirely interconnected pore system (99.3 % porosity) of i-CNF lyogels towards PMMA should not be a limiting factor, too, as suggested by nitrogen sorption experiments (cf. Figure S3) and dynamic light scattering experiments. While the former confirmed that i-CNF aerogels are largely macroporous comprising mainly the pore diameter range of 50 to several hundred nanometers, the latter revealed that the PMMA used had an average hydrodynamic diameter of 9.1 nm at a polydispersity of 1.6 at loading bath conditions. This is in good agreement with the theoretical hydrodynamic diameter (10 nm) of PMMA random coils in theta solvents under consideration of the Kuhn length of PMMA61 and suggests that the great majority of the aerogel’s surface is accessible to the PMMA used. The assumption that the applied scCO2 antisolvent process affords an evenly spread PMMA monolayer across the internal surface of i-CNF aerogels is also strongly supported by FEG-SEM images (cf. Figure 1). This is concordant with a recent study32 which demonstrated that bacterial cellulose aerogels can be reinforced by interpenetrating networks of biocompatible polymers, such as cellulose acetate or PMMA using a similar CO2 antisolvent approach but significantly higher biopolymer-to-cellulose ratios compared to this study. The same study also revealed that unlike poly(lactic acid) or poly(caprolactone), PMMA does not agglomerate or foam as reconfirmed here. In contrast to the above-cited study which was aiming to add mechanical stability to ultra-lightweigth cellulosic aerogels, imparting moisture resistance and hydrophobicity to similarly lightweight but transparent, nematic i-CNF aerogels requires ultrathin spreading of the second17



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ary polymer only and is mandatory to preserve interconnectivity, porosity, accessible internal surface, transparence and nematic ordering. Intriguingly, the thin PMMA (mono)layers deposited onto the large internal surface of i-CNF aerogels turned out to protect the latter very efficiently from moisture-induced shrinkage. While non-hydrophobized samples exposed to moderate relative humidity (50 % RH) suffered from rapid water uptake and volumetric shrinkage (2.2 % weight gain and 30-45 % volume reduction within 10 minutes, Table 1), their PMMA-coated counterparts were largely stable under these conditions. Even though this can be seen already as substantial progress with regard to the use of cellulosic aerogels in real-world applications, full preservation of other features inherent to i-CNF aerogels like nematic orientation of individualized cellulose nanofibrils, interconnected porosity or transparence needs to be demonstrated in the further course.

Table 1. Bulk properties of i-CNF and i-CNF/PMMA aerogels i-CNF content lyogel [mg cm-3]

CNF:PMMA Ratio

Total shrinkage*

Bulk density

Youngs modulus

Specific modulus

Porosity

i-CNF aerogels

4.8 8.1 9.5

i-CNF/PMMA aerogels

4.8 8.1 9.5

[%]

Specific pore volume [cm3 g-1]

Specific surface area [m2 g-1]

[v%]

[mg cm-3]

[kPa]

[MPa cm3 g-1]

-

56 46 42

12.0 16.4 18.2

108±25 315±16 389±62

9.0 19.2 21.4

99.3 99.0 98.9

60 54

465 466

1.35 2.44 2.60

12 14 11

9.6 12.6 14.8

87±15 208±47 360±47

9.0 16.5 24.4

99.3 99.1 99.0

80 67

523 508

Suppression of shrinkage as imparted to i-CNF aerogels by PMMA coating translated into distinctly lower apparent densities of the final aerogels (10-15 mg cm-3) compared to their nonhydrophobized counterparts (Table 1), even though this was not necessarily to be expected at a first glance as loading of the secondary polymer increases the bulk density in lyogel state as a matter of course. However, it is plausible if one considers that the density gain accomplished by 18


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introduction of the PMMA monolayer is small compared to the density gain caused by excessive shrinkage of the non-hydrophobized samples. Suppression of shrinkage has a positive impact on specific surface area and specific pore volume, too, which both increased distinctly for the i-CNF/PMMA hybrid aerogels. The high porosity of all aerogels (> 99 %) is not affected by incorporation of PMMA, since the strongly reduced shrinkage of the hybrid aerogels overcompensates for the loss of volume associated with the introduction of PMMA.

Figure 3: Visualized appearance and properties of i-CNF alcogels (a, b; 9.5 mg mL-1 i-CNF) and corresponding i-CNF/PMMA composite aerogels (c, d, e, f; ρB = 14.8 mg cm-3): Nematic orientation as revealed by the birefringence pattern obtained under crossed polarizers (b, d), transparence (e; disc thickness 2 mm) and hydrophobicity as seen from the shape of four water droplets on an aerogel disc surface (f)

Water contact angle measurements as performed on the surface of i-CNF/PMMA aerogel discs (ρB = 14.8 mg cm-3) confirmed the excellent hydrophobization capabilities of the generated ultrathin PMMA layers. While water droplets deposited on the surface of unmodified i-CNF aero19



gels immediately start to penetrate the highly fragile materials and lead to full destruction by capillary forces, the hybrid materials entirely resisted the penetration of water. Particularly noteworthy is that the observed contact angles were significantly higher (119.4° ± 7.5, n=5) compared to that of bulk PMMA (ca. 68°). Even higher values of up to 154° had been previously reported for nanostructured porous PMMA films62. The occurrence of superhydrophobicity had been proposed to be the result of both the presence of a nanostructured surface (lotus effect) and a stereoselective arrangement of the two organic substituents (methyl, methoxycarbonyl) attached to every second carbon atom of the PMMA polymer backbone. This seems to be similar for i-CNF/PMMA aerogels. We assume that cellulose exerts a directing effect on PMMA orientation in statu secernendi, i.e. the moment when the solvent power of the binary mixture acetone/CO2 starts to drop drastically – triggered by surpassing the critical pressure of the binary phase – and forces PMMA to precipitate. Governed by the abundant hydroxyl (and fewer carboxyl) groups on the surface of i-CNF nanoparticles, hydrogen bonding as well as dipole-dipole and possibly some ion-dipole interactions with methoxycarbonyl moieties of the secondary polymer are established. As a result, most of the non-polar methyl groups are assumed to turn away from the i-CNF surface to form a highly hydrophobic shell. The pronounced nanoscale surface roughness of i-CNF/PMMA hybrid aerogels is likely to contribute as well to the high water contact angles, even though their “papillae” and contact points with water, respectively, have much smaller dimensions (strut thickness < 10 nm) and form a much denser (ca. 100 nm x 100 nm) grid compared to that on the surface of Nelumbo (lotus flower) leaves. Even though in-situ polymerization of PMMA using bacterial cellulose for example as templating scaffold can afford hydrophobic nanocellulose membranes of similarly high water contact angles63, the proposed PMMA coating approach employing the principles of scCO2 antisolvent 20


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precipitation holds a series of advantages. This is in particular the case for transparent i-CNF aerogels of partially nematic nano-architecture that needs to be preserved and requires therefore generation of ultrathin PMMA layers. Advantages include coating with PMMA of defined degree of polymerization, material efficiency, simplicity of combining scCO2 precipitation with scCO2 drying and purity as the obtained hybrid materials are free of by-products and do not contain any catalyst for example. The mechanical behavior of the nanocomposite aerogels is very similar to that of the nonPMMA-modified reference materials recently prepared from nanofibrillated 2,3-dicarboxyl cellulose16. Upon uniaxial compression they respond first in an elastic way (< 5-7% strain, depending on bulk density) before irreversible plastic deformation sets in (cf. Figure 4a). Beyond 50 % strain, accordion-wise densification of nanofibrils leads to pronounced strain hardening which considerably increases strength, stiffness and toughness of the otherwise rather fragile materials. As compaction and folding of fibrils under reduction of the inter-fibril angles happens at the expense of macropores and in favor of a narrow mesopore size distribution, materials of low thermal conductivity can be obtained.16

Figure 4: a) Response of i-CNF/PMMA composite aerogels towards compressive stress (shadowed areas represent the standard deviation; n= 5); b) Density-dependence of the Young’s modulus for different i-CNF and i-CNF/PMMA aerogels; c) Light transmittance of i-CNF aerogels (density: 16.4 mg cm-3; thickness: 1.9 mm) and i-CNF/PMMA hybrid aerogels (density: 9.6 mg cm-3; thickness: 2.13 mm).

21



Owing to virtually full compensation of the mass gain caused by PMMA modification by lower apparent density due to largely suppressed shrinkage, the specific mechanical properties of unmodified and modified aerogels are comparable (Figure 4b). The linear correlation between bulk density and modulus of elasticity as obtained for the reference i-CNF aerogels was in good agreement with a previous report15, however due to the small density range studied (targeting superinsulating materials) an allometric power law correlation for higher densities as reported for respective materials64-66 cannot be precluded. i-CNF/PMMA hybrid aerogels featured a similar correlation, however at slightly higher offset (cf. Figure 4b) caused by the minor reinforcing effect of the secondary polymer. PMMA surface coating had nearly no negative impact on the good light transmittance inherent to ultra-lightweight i-CNF aerogels; respective sample pairs were visually indistinguishable and even the bluish appearance caused by Rayleigh scattering (cf. Figure 3c) was identical. At a disc thickness of 2.1 mm (ρB 9.6 mg cm-3), transmission in the yellow-orange range of the visible light (600 nm) was approximately 77%. Nitrogen sorption experiments at 77 K revealed high specific surface areas for the hybrid aerogels which reached values as high as 510-520 m2 g-1 while those of their PMMA-free counterparts were at around 460-470 m² g-1 (cf. Table 1). Since the precipitation of the secondary polymer prevents shrinkage and, hence, aggregation of the nanofibers, it also increases the specific surface area of the aerogels. According to the pattern observed by wide-angle X-ray scattering experiments, all samples clearly featured cellulose I crystallinity (Figure 5a). Taking the cellulose Iβ structure into account the following peaks have been identified: 1-10, 110, 200, 004. Applying the peak height method developed by Segal67 and coworkers a crystallinity index of 76 % for the pulp starting material and 22


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of about 65 % for the oxidized, nanofibrillated, gelled, solvent-exchanged and scCO2-dried material was extracted from the WAXS curves, revealing a minor decrease in crystallinity throughout the entire pulp modification process only. Table 2. Structural parameters determined from SAXS Bulk density

Rc

n

[mg cm-3]

[nm]

[-]

scCO2 dried pulp

765

2.01 ± 0.04

2.14 ± 0.07

i-CNF aerogels

16.4

2.77 ± 0.04

1.56 ± 0.07

i-CNF/PMMA aerogels

12.6

2.63 ± 0.06

1.50 ± 0.07

Evaluation of small-angle X-ray scattering (SAXS) data revealed significant nanomorphological differences between the parent pulp fibrils and the i-CNF units forming the network architecture of the respective aerogels. While the cylinder radius of gyration was higher for the i-CNF particles constituting the aerogel struts, the power law exponent decreased from n = 2.14 (pulp) to n = 1.56 (i-CNF) suggesting transition from a rather two-dimensional interlinked structure to a more one-dimensional structure (Table 2,). In line with previous studies16, 68 the cylinder radius of gyration was interpreted as diameter of the rod-like particles (fibrils) constituting both the parent pulp and the i-CNF aerogels. The obtained values for i-CNF samples turned out to be in good agreement with the average nanofiber diameter determined by atomic force microscopy which unambiguously confirms that the nanofibers do not aggregate in the course of gelation, solvent exchange or scCO2 drying. It is worth to note that hydrophobic coating of the i-CNF scaffolds did also not afford bigger diameters of the rod-like nanoscale building units. This can be seen as a further proof for both successful homogenous

23



monolayer coating and suppression of fibril aggregation, the latter accomplished by inhibition of water sorption, collapsing of pores and eventual shrinkage of the fragile non-coated samples.

Figure 5: a) WAXS (a) and SAXS (b) profiles of scCO2 dried pulp and i-CNF aerogels optionally hydrophobic coated with PMMA (SAXS only).

Conclusion Supercritical carbon dioxide antisolvent precipitation of PMMA onto the large internal surface of transparent, ultra-lightweight, nematic i-CNF lyogels and subsequent scCO2 drying of the hybrid materials combined in one single batch is a facile and highly effective method to afford hydrophobic aerogels at full preservation of transparency, mechanical properties, and nano-structured nematic architecture. This is particularly useful with regard to many real-world applications of cellulosic aerogels which were hitherto not possible due to the poor dimensional stability of the lightweight materials, in particular in humid environment. This approach however is not limited to cellulose based materials but rather showcases a facile tool to impart various hydrophilic porous materials with moisture resistance. Regarding the iCNF/PMMA hybrid aerogels, the preservation of their transparency and high internal surface for example render them attractive as matrix material for true volumetric displays, as transducer ma-

24


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terial for fluorescence-based bio-sensing applications, as carrier material for catalysts or as superinsulating materials.

Supporting Information Conductometric titration curve used to calculate the amount of (surface) carboxyl moieties anchored on i-CNF; Full nitrogen sorption isotherms of i-CNF aerogels (ρB 16.4 mg cm-3 and 18.2 mg cm-3) and i-CNF/PMMA nanocomposite aerogels (ρB 12.6 mg cm-3 and 14.8 mg cm-3); pore size distribution of i-CNF aerogel and i-CNF/PMMA nanocomposite aerogel as calculated from nitrogen desorption isotherm; pore size distribution of i-CNF/PMMA nanocomposite aerogel after uniaxial densification to demonstrate pore size reduction by post-drying uniaxialdensification.

Acknowledgements The financial support by the Austrian Science Fund (FWF: I848-N17), the French Agence Nationale de la Recherché (ANR-11-IS08-0002; Austrian-French Project CAP-Bone) and the Austrian BMLFUW Ministry (WoodWisdom project AeroWood) is gratefully acknowledged. Furthermore, we thank cordially Tiina Nypelö for performing AFM analyses (Institute of Wood Technology and Renewable Materials, Department of Material Sciences and Process Engineering, University of Natural Resources and Life Sciences Vienna, Austria) and Irina Sulaeva (Division of Chemistry of Renewable Resources, Department of Chemistry, University of Natural Resources and Life Sciences Vienna) for helping with dynamic light scattering measurements.

25



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Conformal ultrathin coating by scCO2-mediated PMMA deposition: A

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