Noncovalent Surface Modification of Cellulose Nanopapers by

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Non-Covalent Surface Modification of Cellulose Nanopapers by Adsorption of Polymers from Aprotic Solvents Katri S. Kontturi, Karolina W. Biegaj, Andreas Mautner, Robert T. Woodward, Benjamin Paul Wilson, Leena-Sisko Johansson, Koon-Yang Lee, Jerry Y.Y. Heng, Alexander Bismarck, and Eero Kontturi Langmuir, Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

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Non-Covalent Surface Modification of Cellulose Nanopapers by Adsorption of Polymers from Aprotic Solvents Katri S. Kontturi,1,2 Karolina Biegaj,3 Andreas Mautner,4 Robert T. Woodward,1 Benjamin P. Wilson,5 Leena-Sisko Johansson,5 Koon-Yang Lee,6 Jerry Y. Y. Heng,3 Alexander Bismarck,*,1,4 Eero Kontturi*,1,4,5 1

Polymer and Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

2

Biocomposites and Processing, VTT Technical Research Centre of Finland Ltd, 02150 Espoo, Finland

3

Surfaces and Particle Engineering Laboratory (SPEL), Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK 4

Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and

Research, Faculty of Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria 5

Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, P.O. Box 16300, Aalto, Finland

ACS Paragon Plus Environment

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The Composites Centre, Department of Aeronautics, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

ABSTRACT. Basic adsorption of hydrophobic polymers from aprotic solvents was introduced as a platform technology to modify exclusively the surfaces of cellulose nanopapers. Dynamic vapor sorption demonstrated that the water vapor uptake ability of the nanopapers remained unperturbed, despite strong repellency to liquid water caused by the adsorbed hydrophobic polymer on the surface. This was enabled by the fact that the aprotic solvents used for adsorption did not swell the nanopaper unlike water that is generally applied as the adsorption medium in such systems. As case examples, the adsorptions of polystyrene (PS) and poly(trifluoroethylene) (PF3E) were followed by x-ray photoelectron spectroscopy and water contact angle measurements, backed up with morphological analysis by atomic force microscopy. The resulting nanopapers are useful in applications like moisture buffers where repellence to liquid water and ability for moisture sorption are desired qualities.

INTRODUCTION Nanopapers are networks of nanosized filaments or sheets such as carbon nanotubes or graphene.1 Recently, nanopapers from polymeric materials have gained more ground and renewable polymers, such as chitin2 or cellulose,3-6 are at the forefront of this trend. Strong and lightweight nature of cellulose nanofibers in particular has resulted in multiple proposed applications as membranes, electronic supports, or composites.3-6 To expand the scope of applications for renewable nanopapers, however, facile modification of their surface chemistry is required. Here, we show that simple exposure of nanopapers to aprotic solutions of hydrophobic


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polymers results in adsorption and subsequently, a controlled surface modification of the nanopapers. Cellulose, the structural component of green plants, is an attractive building block for nanopapers. Nanosized and mechanically strong microfibrils are formed in the plant cell wall and modern technology enables their isolation as cellulose nanofibers (CNFs)7 which are very hygroscopic8 because cellulose is a polysaccharide made up exclusively of anhydroglucose units. Many applications of CNF nanopapers see the abundant water uptake of cellulose as a disadvantage because absorbed water often deteriorates their mechanical properties.9 Some applications, however, make use of the specific water or moisture uptake of CNFs.10,11 This article introduces polymer adsorption from non-aqueous, aprotic solvents as a means to modify the surface of CNF nanopapers. With some exceptions, dissolved polymers usually adsorb on most surfaces. Moreover, the high interfacial energy of cellulose results in remarkably high surface excess concentrations12,13 but adsorption in hydrophobic and/or aprotic systems is virtually never reported. In this proof-of-concept, we demonstrate the case of hydrophobizing the nanopaper surface with polystyrene (PS) and poly(trifluoroethylene) (PF3E). PS was chosen because of its popularity and availability and PF3E because it has been successfully applied to hydrophobize flat silica surfaces.14 As the nanopaper substrate, we have employed CNFs from bacterial cellulose because, unlike plant-based CNF grades, they consist of pure cellulose,15 which is particularly suitable for a fundamental study like this one. Making paper more hydrophobic with, for instance, sizing chemicals (for cardboard cups) or plastic films (for milk cartons) is of course an age-old affair.16 In a more sophisticated framework, chemical modification of individual CNFs for, e.g., better compatibility in composite matrices has been recently reported.17 Among such approaches,


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polymer grafting is a particular favorite.18-20 However, few of these methods generally have the capability to modify exclusively the surface of the fibrous network, i.e. the nanopaper.20 Contrary to aqueous systems which swell CNF nanopapers, the use of non-swelling solvents enabled the essentially non-porous CNF nanopaper to retain its integrity while immersed in the polymer solution during the adsorption. This resulted in CNF nanopapers with tunable hydrophobicity on the surface yet the water vapor absorption capacity was demonstrated to be similar to pristine, untreated nanopapers. Furthermore, adsorption is a simple, robust, and scalable method and the macroscopic nanopapers can be purified by mere rinsing with pure solvent, skipping the laborious purification steps usually involved when chemically modifying nanomaterials. Adsorption is also a mild treatment and does not compromise the integrity of nanocellulose, which occasionally can be a problem with covalent reactions.

EXPERIMENTAL Materials. Toluene (>99.3%, Sigma-Aldrich), anhydrous THF (99%, Riedel-de Haen/SigmaAldrich) and anhydrous heptane (99%, Aldrich) were of analytical grade, and used without further purification. Bacterial cellulose (BC) was supplied by fzmb GmbH (Bad Langensalza, Germany) in wet pellicle form containing 94 wt.-% water. Poly(chlorotrifluoroethylene) (PCTFE), tributyltin hydride (Bu3SnH) and 2,2’-azobis(2-methylpropionitrile) (AIBN) (98%) were purchased from Aldrich. Polystyrene (Mw ~350000, average Mn ~170000) (Aldrich) was used without further purification. Synthesis and characterization of poly(trifluoroethylene) (PF3E) is explained in Supporting information. Nanopaper preparation. Nanopapers of 50 g/m2 BC were prepared by filtration from diluted aqueous suspension of BC. The homogeneous BC suspension of 0.1 wt.-% concentration was


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prepared by cutting the BC pellicles into small pieces (with a length of approximately 5 mm) and blended (Breville VBL065/01, Oldham, UK) with deionised water for 2 min. An amount of the suspension was vacuum-filtered onto a cellulose filter paper (VWR 413, 5-13 μm pore size, Lutterworth, UK) using a funnel with a glass frit (Schott, porosity No. 1, Mainz, Germany). After filtration, the wet filter cake was removed from the filter paper and sandwiched between blotting papers (Whatman 3MM Chr, VWR, Lutterworth, UK) and wet-pressed under a weight of 10 kg for 10 min to absorb the excess water. After this, the wet filter cakes were dried and consolidated in a hot-press (25-12-2H, Carver Inc., Wabash, USA) under a compression weight of 1 t for 4 h at 120 °C, sandwiched between fresh blotting papers and metal plates. Polymer adsorption. Solvents used for polystyrene (PS) were toluene and 50:50 vol-% toluene/heptane mixture. Solvents used for PF3E were THF and a 30:70 vol-% THF/toluene mixture. The polymer adsorption treatments were conducted in concentrations of 0, 0.1, 0.5, 1.0, and 2.5 g/l. During adsorption treatment the nanopaper sample was immersed into polymer solution for 15 minutes, after which the sample was immersed in two consequent baths of pure solvent for 10 minutes in order to remove any unadsorbed polymer molecules from the surface. After this, the sample was allowed to dry in air in room temperature. X-ray photoelectron spectroscopy (XPS). Elemental composition of the samples was evaluated using AXIS Ultra instrument (Kratos Analytical, UK). Samples were mounted on a linear sample holder with UHH compatible carbon tape and pre-evacuated overnight. A fresh piece of pure cellulosic filter paper (Whatman) was mounted and analyzed with each sample batch as an in-situ reference. Measurements were performed using monochromated Al Kα irradiation at 100 W and under neutralization. Wide scans as well as high resolution regions of C 1s, O 1s and N 1s were recorded on 3-4 locations for each sample, with nominal analysis area of 400×800 µm2.


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Analysis depth of the method is less than 5 nm. Data analysis was performed using CasaXPS: first, high resolution carbon C 1s regions were fitted with symmetric Gaussian peaks according to ref. 21 then all binding energies were charge corrected using the fitted C-O component at 286.7 eV as a reference value.22 Charge corrected wide scans were used for elemental analysis. Conditions in UHV remained satisfactory throughout the analysis. The low and stable contamination levels observed in the in-situ reference sample which was measured before and after each experiment, justified the analytical use of the C-C component in high resolution C 1s spectra. Coverage of cellulose with adsorbed polymer was calculated utilizing the detected elemental composition of the surface by XPS and the theoretical elemental ratios of the components involved. Characteristic markers used for the components were C-C bonds for polystyrene, fluorine (F 1s) for PF3E, and C-O bonds for cellulose. The theoretical correlations between the markers and the calculated surface coverage are presented in Supporting Information (Figure S2). Contact angle measurement. The contact angles were measured on a DSA30 (Krüss), with the software package Drop Shape Analysis 4. A drop of 5 µL was placed on the sample at 100 µl/min. Thereafter the drop volume was increased to 20 µL at 15 µL/min and subsequently decreased again to 5 µL again at 15 µl/min. Data points were collected every two seconds with the "tangent 2" method on the right and the left side of the drop. The advancing contact angle was determined as average from up to 4 measurements from the data points after 60 to 62 seconds and the receding contact angle after 100 to 102 seconds. Figure S3 in Supporting Information presents an example of the contact angle data obtained with this procedure. Atomic force microscopy (AFM). The imaging was performed with AFM MultiMode 8 scanning probe microscope from Bruker AXS Inc. (Madison, WI, USA) with an E scanner in


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tapping mode. The cantilevers used were NSC15/AIBS silicon cantilevers from Ultrasharp µmasch (Tallinn, Estonia). Dynamic vapor sorption (DVS). Dynamic water vapor sorption by the nanopaper samples was characterised using DVS-1000 (Surface Measurement Systems, London, UK). The measurements were performed for the nanopaper samples treated with 2.5 g/l solution of PS in toluene/heptane and PF3E in THF/toluene (being the samples most influenced by the polymer adsorption as characterized by other techniques). The water vapor uptake was followed at 25 °C by monitoring the change in mass as the sample was exposed to 0-95-0% RH cycle in steps (0, 20, 40, 60, 80 and 95% RH) for 3 h at each step. Sample size of 4-6 mg and the flow rate of 100 sccm were applied.

RESULTS AND DISCUSSION The nanopaper substrates were exposed to the polymer solutions for 15 minutes, after which they were carefully washed with pure solvent to remove any excess, non-adsorbed polymer from the surface. Figure 1a shows the surface sensitive x-ray photoelectron spectroscopy (XPS) data of nanopapers modified by immersion in PF3E solutions of ascending concentrations in two different solvents: tetrahydrofuran (THF, a good solvent) and 30/70 vol-% THF/toluene (a poor solvent). The increased F 1s peak indicates that the use of THF/toluene solvent mixture resulted in superior retention of PF3E on the nanopaper surface. However, even the good solvent system with THF led to detectable adsorption of PF3E. Figure 1b, on the other hand, shows the XPS data of PS modified nanopapers where the increased contribution of saturated (C-C) C 1s peak indicates an increasing amount of PS retained on the nanopaper surface. It should be noted that all carbons in a cellulose molecule are bound to at least one oxygen, thereby contributing to the resolved C 1s emissions at higher binding energies. Again, it is easy to notice that the poor solvent for PS, i.e. 50/50 vol-%


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toluene/heptane mixture, amounted to a higher extent of adsorption of PS than the good solvent (toluene).

Figure 1. XPS data on the nanopaper after adsorption of polymer from different solvents and concentrations; (a) spectra of nanopaper with PF3E adsorbed from toluene/heptane (top 5 spectra)


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and toluene (bottom 5 spectra); (b) high resolution C 1s spectra with component fits of nanopaper with PS adsorbed from THF/toluene (top 5 spectra), THF (bottom 5 spectra), and a control reference (utmost bottom spectrum); (c) coverage of the nanopaper surface with polymer calculated based on the XPS data: F 1s/O 1s and C-C/C-O are used as a marker for PF3E and PS on cellulose, respectively. For calculations, see Supporting Information. Adsorption isotherms calculated from the XPS data are plotted in Figure 1c (see Supporting Information for calculations). It appears that the behavior followed the Langmuir adsorption isotherm where the plateau is reached already at fairly low concentrations.23 The only exception was the adsorption of PF3E from THF/toluene where a notable increase was observed at a polymer concentration of 1.0 g/l, after seemingly reaching the plateau at 0.1 g/l concentration. Such behavior is characteristic of multilayer adsorption.23 Interestingly, on silica substrates, multilayer adsorption of PF3E was reported to occur only at 2.5 g/l concentration.14 Here, we must emphasize the distinction in what are labeled poor solvents in this study. Toluene/heptane (50/50 v/v) mixture is actually a theta solvent for PS24 whereas THF/toluene (70/30 v/v) mixture appears a much poorer than a theta solvent for PF3E. The latter was chosen because of the superior adsorbed amounts in an aforementioned study for silica substrates.14 The distinction underlines the importance of the solvent system: Neither polystyrene nor PF3E are likely to possess notable affinity to cellulose but aside the solution concentration, the choice of solvent is a potent way to control the adsorbed amount. Atomic force microscopy (AFM) images of the treated nanopapers back up the XPS data (Figure 2). Surfaces at maximum adsorption load were identical to the untreated control sample (Figure 2a-2d), indicative of molecular adsorption which does not cause detectable alterations to the morphology of rougher samples. The only exception was the sample that had been exposed to 2.5


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g/l PF3E in THF/toluene. Clearly discernible particles of PF3E were visible on the surface (Figure 2e). We want to emphasize that the observed phenomenon is still adsorption, not precipitation, as the excess polymer has been rinsed off from the surface after the treatment. Although THF/toluene is a poor solvent for PF3E, it is still a solvent, capable of dissolving up to 2.5 g/l of the polymer. In case of precipitation, the minute amounts of precipitated PF3E would be washed away in the rinsing step. The apparent reminiscence of the adsorbed granular structures (Figure 2e) to precipitated ones may occur from the multilayer adsorption which is not a case of one layer neatly piling up after another in case of rough polymeric substrates (nanopaper) and polymeric adsorbents in solution.

Figure 2. 5×5 µm AFM height images of nanopaper (a) without treatment; after adsorption from 2.5 g/l solution of (b) PS from toluene; (c) PS from toluene/heptane; (d) PF3E from THF; (e) PF3E from THF/toluene. A large amount of literature on the adsorption of polymers on various types of cellulose substrates exists but nearly all of them are restricted to aqueous systems. Two main categories of such studies can be distinguished: (i) electrostatically driven adsorption of cationic polyelectrolytes to a cellulosic surface that is often slightly anionic25 and (ii) adsorption of watersoluble polysaccharides on a cellulose surface.12,13 It is difficult to compare any result with our dense CNF nanopapers in non-swelling aprotic solvents because the adsorbed amount depends heavily on the porosity of the cellulose substrate26 and the ability of the solvent to swell it.


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Nevertheless, a 2-22% coverage observed here is not entirely dissimilar with many accounts on polymer adsorption on cellulose although they are generally reported as mass percentage values.8,12,13,25,26 However, the >50% coverage for PF3E in THF/toluene is certainly unusually high, again highlighting the impact of the solvent system on adsorption. The hydrophobizing effect of the adsorbed polymer is clearly visible as increased water-in-air contact angles of the nanopapers after exposures particularly to higher polymer concentrations (Table 1). Although the water contact angle of single crystal native cellulose is theoretically over 40,27 in many cases the contact angle could not effectively be measured because the droplet was actually imbibed by the hygroscopic porous nanopaper structure. Only when the adsorbed amounts were significant, the water absorption was blocked and the hydrophobizing effect started to show. Genuine hydrophobic nanopapers were observed only at very high PF3E coverages, namely after exposure to solutions of 1.0 and 2.5 g/l solutions. However, a ~90 static contact angle is what can be reached with a pure homogeneous PF3E surface in the first place,28 which indicates that much of the hydrophobic nature of the treated nanopapers originates from the surface roughness of the modified nanopapers since PF3E does not exhibit full coverage by any means (Figure 1 and 2). Anac et al.14 also reported around 90 water contact angles after PF3E adsorption on flat silica but the coverage was not established. Because of the water imbibition by the unmodified hygroscopic nanopaper, the Cassie-Baxter relationship could not be established for these samples and the interplay of chemical composition and roughness remained unquantified.

Table 1. Advancing a /receding r contact angle of water on nanopapers with adsorbed PS and PF3E from different concentrations and solvents. Standard deviation of the contact angle measurements is ±10%.


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PS adsorbed Polymer concentration

PF3E adsorbed

from

from

from

from

toluene/

THF a/r

THF/

(g/l)

toluene a/r

0

n.a.

n.a.

n.a.

n.a.

0.1

n.a.

n.a.

n.a.

51°/25°

0.5

n.a.

28°/13°

n.a.

62°/30°

1

n.a.

45°/21°

n.a.

96°/62°

2.5

36°/24°

32°/14°

31°/22°

88°/59°

heptane a/r

toluene a/r

Essentially, the nanopapers where the contact angles could be measured (Table 1) were water repelling, that is, water droplets on the surface no longer penetrated into the treated nanopaper structure. This is an important quality when one wants to retain the integrity of nanopapers under moist conditions: like paper, nanopaper is also weakened and even disintegrated when exposed to excess liquid water. In contrast to liquid water on the surface, bulk water vapor uptake of the modified nanopapers as determined by DVS stayed virtually unaffected, even with the highest amounts of PF3E adsorbed, (Figure 3). This simple experiment strongly indicated that the polymer is adsorbed exclusively on the nanopaper’s surface. If the hydrophobic PS or PF3E were sorbed also inside the CNF network, the water vapor uptake of the nanopaper would be severely decreased – just like the imbibition of water is hindered on the surface after surface modification (Table 1). Adsorption is of course a surface phenomenon but in most cases cellulose substrates are porous when exposed to water. Therefore, the hydrophilic polymer adsorption in water results in enrichment of the polymer also inside the cellulose matrix unless very high molecular weight polymers are used.25 The same applies also to generic modification methods based on adsorption of modified polysaccharides in water.29,30 (It can still be deemed as adsorption because the


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polymers enrich on the surface of cellulose microfibrils although they are inside a microfibril network.) Unless highly polar, most aprotic solvents induce minimal swelling of cellulosic materials31 and therefore, they are optimal if one wants to treat only the external nanopaper surface while retaining an unaltered interior. Sorption of the polymers inside the nanopaper beyond the mere surface would require a swelling solvent like water. Curiously, what was claimed an exclusively topochemical acetylation of cellulose nanopaper did result in reduced moisture sorption,32 suggesting the reaction was not entirely confined on the surface.

Figure 3. Dynamic vapor sorption of nanopaper without a treatment and after treatment with adsorption of 2.5 g/l PS in 50/50 vol-% toluene/heptane and 2.5 g/l PF3E in 30/70 vol-% THF/toluene. Polymer adsorption as a means for surface modification has certainly been utilized before, for example, in layer-by-layer deposition which usually relies on electrostatic attraction in water.33 Polymer surfaces modified by polymer adsorption in aprotic solvents are not as common.34 Moreover, covalent methods for surface modification of CNFs have been intensively investigated in recent years.17-19 All these efforts, however, result in CNFs that are modified at their surfaces and in turn, when subjected to nanopaper preparation, they form a CNF network that consists of modified CNFs throughout. Here, we have presented a simple method to directly modify the CNF


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nanopaper exclusively on its surface while the interior remains untreated by the polymer. Therefore, the strong mechanical properties, based partially on the hydrogen bonding between the CNFs,35 have been retained. In addition, the vapor transmission of the modified nanopapers was comparable to the unmodified nanopaper, which can be favorable in applications such as textiles or building insulations.11

CONCLUSIONS This study has established that the previously uninvestigated system of hydrophobic polymer adsorption on cellulose in non-aqueous systems is feasible and possesses major potential. Beyond this proof-of-concept study, the method in principle enables the attachment of any dissolving polymer on the CNF nanopaper surface.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org The SI includes additional details for PF3E synthesis and characterization, XPS data analysis, and contact angle measurements. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected]


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ACKNOWLEDGMENT EK acknowledges Academy of Finland (project 259500) and AB acknowledges the UK Engineering and Physical Sciences Research Council EPSRC (EP/K014676/1 and EP/J014974/1) for funding KSK and RTW. In addition, KSK acknowledges Finnish Cultural Foundation for funding. Funding by the University of Vienna is acknowledged by AB, AM and EK for the latter’s visiting professorship. Dr. J.M. Campbell is acknowledged for performing the XPS measurements. REFERENCES (1) Huang, W.; Ouyang, X.; Lee, L. J. High-performance nanopapers based on benzenesulfonic functionalized graphenes. ACS Nano 2012, 6, 10178-10185. (2) Jin, J.; Lee, D.; Im, H.-G.; Han, Y. C.; Jeong, E. G.; Rolandi, M.; Choi, K. C.; Bae, B.-S. Chitin nanofiber transparent paper for flexible green electronics. Adv. Mater. 2016, 28, 51695175. (3) Zhou, L.; Yang, Z.; Luo, W.; Han, X.; Jang, S.-H.; Dai, J.; Yang, B.; Hu, L. Thermally conductive, electrical insulating, optically transparent bi-layer nanopaper. ACS Appl. Mater. Interfaces 2016, 8, 28838-28843. (4) Inui, T.; Koga, H.; Nogi, M.; Komoda, N.; Suganuma, K. A miniaturized flexible antenna printed on a high dielectric constant nanopaper composite. Adv. Mater. 2015, 27, 1112-1116. (5) Mautner, A.; Lee, K.-Y.; Lahtinen, P.; Hakalahti, M.; Tammelin, T.; Li, K.; Bismarck, A. Nanopapers for organic solvent nanofiltration. Chem. Commun. 2014, 50, 5778-5781.


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(6) Lee, K.-Y.; Tammelin, T.; Schulfter, K.; Kiiskinen, H.; Samela, J.; Bismarck, A. High performance cellulose nanocomposites: comparing the reinforcing ability of bacterial cellulose and nanofibrillated cellulose. ACS Appl. Mater. Interfaces 2012, 4, 4078-4086. (7) Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: a new family of nature-based materials. Angew. Chem. Int. Ed. 2011, 50, 5438-5466. (8) Kontturi, K. S.; Kontturi, E.; Laine, J. Specific water uptake of thin films from nanofibrillar cellulose. J. Mater. Chem. A 2013, 1, 13655-13663. (9) Siró, I.; Plackett, D. Microfibrillated cellulose and new nanocomposite materias: a review. Cellulose 2010, 17, 459-494. (10) Olsson, R. T.; Azizi Samir, M. A. S.; Salazar-Alvarez, G.; Belova, L.; Ström, B; Berglund, L. A.; Ikkala, O.; Nogués, J.; Gedde, U. W. Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates. Nature Nanotechnol. 2010, 5, 584-588. (11) Lozhechnikova, A.; Vahtikari, K.; Hughes, M.; Österberg, M. Toward energy efficiency through an optimized use of wood: The development of natural hydrophobic coatings that retain moisture-buffering ability. Energy Buildings 2015, 105, 37-42. (12) Kargl, R.; Mohan, T.; Bracic, M.; Kulterer, M.; Doliska, A.; Stana-Kleinschek, K.; Ribitsch, V. Adsorption of carboxymethyl cellulose on polymer surfaces: evidence of a specific interaction with cellulose. Langmuir 2012, 28, 11440-11447.


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Noncovalent Surface Modification of Cellulose Nanopapers by

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