Article Cite This: Chem. Mater. 2018, 30, 2322−2330
pubs.acs.org/cm
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,∥ and 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 S Supporting Information *
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 were applied to obtain i-CNF dispersions in water. Nematic ordering caused by repulsive forces between iCNF surface carboxylate groups was set by acid-induced hydrogen bonding and gelation, respectively. Solvent exchange to acetone, impregnation with the PMMA, scCO2-mediated antisolvent 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 patterns 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 virtually does 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 toward moisture as apparent from the high water contact angle (119.4° ± 7.5). As PMMA imparts the aerogel stiffness and hydrophobicity, aggregation of nanofibrils in moist environment or under vacuum conditions can be avoided even at ultralow densities as low as 9.6 mg cm−3.
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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 surface,15,16 low thermal conductivity,15−17 high transparency,15,16,18 superabsorption,19,20 and excellent mechanical properties in tension18 and compression16 while lacking the brittleness of their inorganic counterparts. Specific physical and chemical features supporting self-assembly to sophisticated hierarchical
INTRODUCTION
Ultralightweight 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., organosilanes,2 chalcogens,3 boron nitride4) and organic polymers (e.g., crosslinked polyimides,5 Kevlar-type polyamides6). However, the recent progress in second-generation biorefinery and renewable-based material research has shown that many biopolymers, such as polysaccharides (cellulose,7 hemicellulose,8 pectin,9 chitosan10) and proteins (whey,11 soy,12 silk fibroin13) or © 2018 American Chemical Society
Received: December 17, 2017 Revised: March 21, 2018 Published: March 22, 2018 2322
DOI: 10.1021/acs.chemmater.7b05226 Chem. Mater. 2018, 30, 2322−2330
Article
Chemistry of Materials structures, biocompatibility, biodegradability, and flexibility inherent to many of these materials even let it be expected that bioaerogels will conquer new applications in the near future. 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 solution 22−24 and dispersion15,25,26 state. The latter is particularly interesting for anisometric cellulose nanoparticles like cellulose nanocrystals (CNCs)26 or individualized cellulose nanofibers (i-CNFs)15 which can form different types of liquidcrystalline phases in dispersion state because of 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 purpose,27−29 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 6carboxyl cellulose. Recently the preparation of transparent nematic cellulosic aerogels from nanofibrillated 6-carboxylcellulose15 and 2,3-dicarboxyl-cellulose,16 respectively, has been reported, including a facile uniaxial densification approach capable of affording mechanically stable, superinsulating i-CNF aerogels.16 Further applications of these novel classes of CNCand 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 especially the case for light soft materials like i-CNF aerogels as their fragility, extensive network of nanocapillaries, along with their large internal surface invite 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 iCNF 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 wet-chemical derivatization techniques. The core concept combines scCO2 antisolvent 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 solute.31 Appropriate PMMA loading and scCO2-antisolvent coating conditions being 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 thatdifferent from poly(lactic acid) or poly(caprolactone) PMMA does not foam or agglomerate in scCO2,32 has good affinity to cellulosic surfaces, and renders cellulosic aerogels particularly biocompatible.33 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.
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EXPERIMENTAL SECTION
Never-dried bisulfite hardwood dissolving pulp (50% w/w H2O, CCOA34 24.3 μmol g−1 CO, FDAM35 13.9 μmol g−1 COOH, Mw 303.7 kg mol−1) was used as cellulose starting material. Poly(methyl methacrylate) (PMMA, Mw 350.0 kg mol−1), 2,2,6,6tetramethylpiperidine-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). Deionized 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 previously,36 however, with the difference that NaClO was continuously added instead of at once to avoid partial overoxidation. In brief, 8 g (dry weight) of dissolving pulp (50% w/w H2O) was suspended in 800 mL of DI H2O and disintegrated using a household blender for 1 min. For selective oxidation of the primary (C6) hydroxyl groups, 128 mg of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 800 mg of NaBr were added. Thereafter, a total of 12 mL of 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 0.1 M 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). For conversion of potentially formed carbonyl moieties into carboxyl groups, the never-dried oxidized pulp was postoxidized 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 laboratory homogenizer (APV 1000, APV Manufacturing Sp. z o.o.). 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) instrument equipped with a swing-out-rotor (17541778 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 elsewhere.37 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 addition of 2.75 mL 0.1 M hydrochloric acid solution, the titration was carried out under constant stirring by adding a total of 5 mL of 0.1 M NaOH with an increment of 25 μL every 30 s. Precise dosing was accomplished using an automated titration apparatus 800 Dosino device connected to an 856 conductivity module equipped with an 801 stirrer (all Metrohm). 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; formerly Veeco) equipped with an 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 coils. A PMMA solution in acetone corresponding 2323
DOI: 10.1021/acs.chemmater.7b05226 Chem. Mater. 2018, 30, 2322−2330
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Chemistry of Materials 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. Then, 100 data points were acquired using an acquisition time of 5 s. The parameters dn/dc = 0.134038 and A2 = 9.51 10 × 5 mol mL g−239 were used to determine the hydrodynamic radius and corresponding diameter of the polymer random coils. Preparation of Nematic i-CNF Aerogels. After rotoevaporation of the i-CNF dispersions to the 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 1 h. 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 h of 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 h each) soaked in acetone before being transferred into a loading bath (50 times gel volume, residence time 24 h) 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 zeroa 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 toward higher temperatures and pressures with increasing amount of acetone (e.g., to 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 3 h to dry the samples. After completion of acetone extraction, the system was isothermally depressurized at 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. 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
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 3.3−3.7 mg cm−3 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 cm−2 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 nanomorphological (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 28−43% of PMMA (range reflects variance of i-CNF cross-sectional dimensions) related to the mass of cellulose only, which fits well with the experimental cellulose-to-PMMA ratios (Table 1). Accessibility of virtually the entirely interconnected pore system (99.3% porosity) of iCNF lyogels toward 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 from 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 θ 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 2326
DOI: 10.1021/acs.chemmater.7b05226 Chem. Mater. 2018, 30, 2322−2330
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Chemistry of Materials generated ultrathin PMMA layers. While water droplets deposited on the surface of unmodified i-CNF aerogels 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 films.62 The occurrence of super-hydrophobicity 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 drasticallytriggered by surpassing the critical pressure of the binary phaseand 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 nonpolar 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
