Polymer Brushes on Cellulose Nanofibers: Modification, SI-ATRP, and

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Polymer Brushes on Cellulose Nanofibers: Modification, SI-ATRP, and Unexpected Degradation Processes Maria Morits, Jason McKee, Johanna Majoinen, Jani-Markus Malho, Nikolay Houbenov, jani Seitsonen, Janne Laine, Andre H. Gröschel, and Olli Ikkala ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b00972 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Sustainable Chemistry & Engineering

Polymer Brushes on Cellulose Nanofibers: Modification, SI-ATRP, and Unexpected Degradation Processes Maria Morits,† Jason R. McKee,†£ Johanna Majoinen,†§ Jani-Markus Malho,†± Nikolay Houbenov,† Jani Seitsonen,⊥ Janne Laine,∥ André H. Gröschel,†§* and Olli Ikkala,†∥* † Molecular Materials, Department of Applied Physics, School of Science, Aalto University, P.O. Box 15100, FIN-00076, Espoo, Finland ⊥ Nanomicroscopy Center, Department of Applied Physics, School of Science, Aalto University, P.O. Box 15100, FIN-00076, Espoo, Finland ∥ Department of Forest Products Technology, School of Chemical Technology, Aalto University, P.O. Box 16300, FIN-00076 Espoo, Finland § Physical Chemistry and Centre for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, D-45127 Essen, Germany * André H. Gröschel. E-mail: [email protected] * Olli Ikkala. E-mail: [email protected]

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KEYWORDS: cellulose nanofibers, cellulose degradation, nanocellulose, SI-ATRP, surface modification.

ABSTRACT Controlled surface-initiated atom transfer radical polymerization (SI-ATRP) has previously been described as a versatile method that allows grafting polymer brushes on purely cellulosic forms of nanocelluloses, i.e., cellulose nanocrystals (CNC) nanorods and bacterial cellulose (BC) networks. However, corresponding SI-ATRP on long and entangled cellulose nanofibers (CNF), having typically more complex composition and partly amorphous structure, has been only little reported due to practical and synthetic challenges, in spite of technical need. In this work, the feasibility of SI-ATRP on CNF is exemplified on the polymerization of poly(n-butyl acrylate) and poly(2-(dimethyl amino)ethyl methacrylate) brushes, both of which showed first order polymerization kinetics up to a chain length of 800 repeat units. By constructing high and low initiator densities on CNF surfaces, we also show that, surprisingly, a higher grafting density of polymer brushes around CNF causes noticeable degradation of the CNF nanofibrillar backbone, whereas lower grafting densities retained the structural integrity of the CNF. We tentatively suggest that the side-chain brushes strain the amorphous domains of CNF causing degradation, which can be suppressed using a low degree of substitution. Therefore SI-ATRP of CNF becomes subtler than that of e.g. CNC, and careful balance has to be achieved between high density of brushes and excessive CNF degradation.

INTRODUCTION Cellulose has attracted revived interest due to its wide abundance as a sustainable source for various types of bio-based materials ranging from the classic macroscopic wood pulp, paper, and


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microscopic cellulose particles to advanced nano-objects down to well-known cellulosic polymer chains. In particular, the emerging field of nanocelluloses (diameter below 100 nm) suggests novel applications as viscosity modifiers, mechanically strong and modifiable components for high-performance lightweight biocomposites, as well as building blocks for templating of functional materials and bioscaffolding.1-5 Related to nanocelluloses, the major types involving native crystalline structures are colloidal cellulose nanocrystals (CNCs),6-8 high aspect ratio cellulose nanofibers (CNFs),2,3 and networks of bacterial cellulose (BC)9. While CNCs are rodlike nanoparticles mostly with a length of 50-300 nm (depending on the source),6 native CNFs have comparable diameter but are several micrometers in length leading to physical entanglements, typically involving also hemicelluloses (depending on the preparation conditions). Note that CNFs are often also denoted as nanofibrillated cellulose (NFC) or microfibrillated cellulose (MFC). Additionally, a specific class of CNFs is nanofibers involving carboxylates on the surfaces based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation.10-12 Finally, BC refers pure and continuous fibrillar cellulose networks produced by certain bacteria. Therefore, each of them has distinct chemical and physical properties (morphology, viscosity, gelation, etc.) requiring fundamentally different protocols for handling and processing. Cellulose modification with well-defined polymer brushes allows detailed tuning of the material and interfacial properties. Therein, SI-ATRP13 has previously been applied to prepare polymer brushes on dialysis membranes,14 filter paper,15-23 cellulose nanopaper,24 and cellulose microfibers (60µm)25. For nanocelluloses, SI-ATRP has been utilized to graft polymer brushes from BC and CNCs.26-31 They are both purely cellulosic materials without major other constituent components, which obviously promotes well-defined brushes by SI-ATRP.


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Surprisingly, polymer brush modification of native CNFs by SI-ATRP in solvent dispersion has so far not received much attention. The most likely reason is the complicated processing and complex composition of the heterogeneous system, which will be part of the discussions in this study. In more detail, CNFs consist of several micrometers long, flexible and entangled cellulosic nanofibers with few nanometer cross-section.2,3,32 Homogenization through mechanical refinement33-35 towards separated – yet still entangled – nanofibers leads to hydrogel formation already at concentrations below 1.0 wt.-%. The rheological behavior is not only caused by the nanofibers, but rather a complex interplay between CNF bundles, nano-networks and strong interfibrillar interactions; the solid content further contains roughly 30 wt.-% of hemicellulose. Since hemicellulose covers and stabilizes CNFs, purification from it leads to more pronounced aggregation of the nanofibers.36 Nevertheless, the remarkable properties of CNFs are attractive for a variety of applications and methods for versatile surface modification are in demand. Here, we describe protocols for the modification of solvent dispersed native CNFs, decoration with initiating sites suitable for SI-ATRP, and subsequent grafting of polymer brushes. Each processing step was characterized with spectroscopic and microscopic techniques. We find important differences for surface modification as compared to purely cellulosic CNC nanorods, but also changes to the CNF morphology after polymerization. Molecular weight distributions of the polysaccharides and X-ray diffraction (XRD) of CNFs both show that degradation already occurs during surface anchoring of initiator molecules, yet these damages do not lead to noticeable infringement of structural integrity. For generality, well-defined model brushes of soft, hydrophobic poly(n-butyl acrylate) (PnBA) and multifunctional, hydrophilic poly(2(dimethyl amino)ethyl methacrylate) were polymerized from the surface of dispersed CNFs.


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Importantly, we find that a high degree of substitution (DS) and thus grafting density led to noticeable fragmentation of the CNFs after polymerization, whereas low DS much better preserved the structural CNF nanofibrillar integrity.

RESULTS AND DISCUSSION The surface of CNFs is hydrophilic and CNFs are usually stored in form of 1-2 wt.-% hydrogels. For effective surface modification with compounds that are highly reactive towards hydroxyls, all traces of water must be removed from the CNFs hydrogel. In the following, we first discuss one procedure (Method A) that yields a high DS, aiming at dense CNF-brush polymerization, whereas next we introduce a second procedure (Method B) that leads to small DS, aiming at sparse set of polymer brushes, which turns out to preserve the CNF nanofibrillar structure. Note a comment for the use of DMF solvent during modification and purification of the material. While DMF is not well compatible with sustainability, the main focus of this manuscript is to explore functionalized nanocelluloses as a sustainable alternative, potentially replacing synthetic polymers of the petroleum-based industry. The current study is an attempt to explore the conceptual effects occurring during CNF modification. Improvements to the protocol will be investigated in the future, e.g., using more sustainable solvents and modification processes. High Degree of Bromination and DS by Two-Step Surface Modification of CNFs (Method A). For high DS with bromine on CNF, two-step modification was used in Method A (see Experimental Section in Supporting Information for details). In step 1, CNF is freeze-dried and reacted in the gas phase with α-bromoisobutyryl bromide (BriB-Br) using chemical vapor


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deposition (CVD); In step 2, the bromination is continued in the organic solvent phase using BriB-Br. While straightforward related protocols have been successfully demonstrated for the shorter rod-like CNCs,26-28 in CNF the freeze-drying poses several obstacles. For instance, using liquid nitrogen therein results in unwanted aggregation of CNFs. Gas formation at the nitrogen/gel interphase (Leidenfrost effect) slows cooling and the growing large ice crystals compress the CNFs into dense sheets instead of preserving the fibrillar structures (see Supporting Information Figure S1a, b). The effect is particularly pronounced for the sample sizes exceeding a few millimeters due to slow and inhomogeneous cooling of the interior parts. To prevent irreversible bundling of the nanofibers, we freeze-dry hydrogels from liquid propane.37 Rapid cooling nucleates smaller ice crystals and promotes formation of amorphous ice. That facilitates formation of a fibrillar aerogel (Figure S1c, d) with high porosity and surface area, both crucial for efficient CVD. The highly porous CNF aerogel was first esterified with an acid bromide in the gas phase reaction to covalently attach ATRP initiation sites to the surface (Figure 1). CVD alters a small number of hydroxyl groups hydrophobic due to bromination, thereby increasing the dispersibility in organic solvents after step 1, relevant for the subsequent more effective solution esterification in step 2.27 CNF aerogels adsorb impurities also from surrounding air and should thus always be produced freshly and used immediately. After CVD, the CNF-Br aerogel was dispersed in DMF and esterified using 40-fold excess of BriB-Br (see Supporting Information for details). The comparably large excess of BriB-Br proved essential for the successful esterification of CNFs in dispersion, and is one of the central findings of this work. The high aspect ratio of the CNFs limits its dispersibility to only low concentrations in DMF as otherwise the dispersion becomes highly viscous or forms a gel. We used a concentration of 1 mg/mL and in order to esterify 500 mg of CNFs this requires 500 ml of


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anhydrous DMF. Traces of water immediately react with BriB-Br and the 40-fold excess guarantees that sufficient amount of reactants remain active for CNF surface modification when working with large volumes. We evaluated the DS of the hydroxyl groups using FT-IR (Figure 1b). For the native CNFs, the characteristic carbonyl-stretching band at ν = 1730 cm-1 is absent, but after CVD, the appearance of a small peak indicates ester bond formation through covalent anchoring of ATRP initiator to the CNF surface. The emerging carbonyl signal was relatively weak after CVD, as expected from previous literature about CVD on CNCs.27,31 After the second step in DMF, a clear increase in the carbonyl signal corroborates a substantial increase of the DS. Elemental analysis indicates a weight fraction of bromine f(Br) = 5.45 wt.-%, which equals about 1.8 initiator units per nm2 and corresponds to a DS = 0.43. This is a relatively large density and therefore we denoted the resulting material CNF-Brhigh. The hydroxyl signal at ν = 30003600 cm-1 did not change significantly after esterification, which illustrates the still low content of esterified units on the CNF surfaces compared to overall hydroxyl groups (including also those inside the CNF cores). These values mark the lower limit of initiator density and DS, because for calculations (see Supporting Information) we use the maximum possible surface area of well-dispersed native CNFs as measured from cryo-TEM that is considerably higher than the actual accessible surface area. The number of hydrophobic initiation sites is sufficient to alter dispersibility of CNF-Brhigh (Figure 2c). While native CNFs form stable dispersions in water and flocculate in THF, CNF-Brhigh is now dispersible in THF.


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Figure 1. Two step modification for high degree of bromination and characterization of CNFBrhigh for SI-ATRP (Method A). (a) Scheme for chemical vapor deposition (CVD) of BriB-Br initiators onto the CNF aerogel (step 1) followed by solution esterification in DMF (step 2). (b) FT-IR spectra of native CNF (black), CNF-Br after CVD (step 1; red), and after solution esterification CNF-Brhigh (step 2; blue). (c) Photographs showing well-dispersed native CNF in water, collapsed native CNF in THF, and dispersed CNF-Brhigh in THF.

Figures 2a and b show TEM images of native CNFs prepared from aqueous dispersion and CNF-Brhigh from THF after two-step bromination. Dispersions were drop-cast onto copper grids with lacey carbon allowing the solvent to pass through and preventing dense film formation of the fibrils. For both native CNFs and CNF-Brhigh, we found a loosely connected nanofibrillar network typical for dried CNFs. The absence of microscopic fiber bundles in Figure 2b suggests that the esterification procedure did not induce noticeable aggregation in THF as compared to the


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native CNF in water. More importantly, esterification did not lead to seeming fragmentation or alteration of the overall morphology. Molecular weight distributions of polysaccharide contained in native CNF and CNF-Brhigh were analyzed with size exclusion chromatography (SEC). For this purpose, CNFs were dissolved into individual cellulose polymer chains in LiCl/DMAc before SEC analysis (see Supporting Information). Molecular weight distribution of chains originating from native CNF shows two peaks (Figure 2c), where the higher molecular weight peak corresponds to cellulose polymer chains and the lower molecular weight peak to hemicellulose. In contrast to native CNF, the molecular weight distribution of CNF-Brhigh has only one peak. We assume that the hemicellulose fraction was hydrophobized by attaching much of the ATRP initiator, which was then removed during the purification of CNF-Brhigh. An alternative explanation could be that hemicellulose was chemically disintegrated. Moreover, the weight average molecular weight of CNF-Br polysaccharide chains decreased from Mw = 290,000 Da to Mw = 85,900 Da, which we attribute to damage to the polysaccharide chains caused by the esterification reaction during anchoring of the SI-ATRP initiator to CNF (see further discussion later). Investigation of the CNF-Brhigh by XRD showed decrease of the crystallinity index from Cr.I.=68% to Cr.I.=59% as compared to native CNFs. However, the damage of CNF-Brhigh does not clearly resolve in TEM imaging (Figure 2a, b), where no drastic differences between CNF-Brhigh in THF and aqueous CNF are resolved.


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Figure 2. Comparison of native CNF and CNF-Brhigh prepared by the two-step bromination (Method A). (a) TEM images of native CNF from water and (b) CNF-Brhigh from THF. (c) Elugrams with Molecular Weight Distribution and (d) XRD patterns of native CNF and CNFBrhigh (Cr.I.=crystallinity index). Note the holey carbon grid in b).

Polymer Brushes from CNF-Brhigh by SI-ATRP. Figure 3 exemplifies conditions for the polymerization of n-butyl acrylate (nBA) from CNF-Br in the presence of ethyl αbromoisobutyrate (EBiB) as a sacrificial initiator. The reason for the use of sacrificial initiator is to gather information about chain length distributions without the need to cleave polymer chains from the CNF surface.28


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Figure 3. Reaction scheme for SI-ATRP from CNF-Br in the presence of sacrificial initiator, EBiB, exemplified for of n-butyl acrylate (nBA). The polymerization takes places similarly for DMAEMA monomers and CNFs initially brominated using Method A and Method B.

Figure 4a summarizes a typical SI-ATRP of nBA from CNF-Brhigh to prepare polymer brushes. The catalytic system consists of the Cu(I)Br/PMDETA complex as well as Cu(II)Br2 to set the equilibrium conditions. The initial molar ratios of monomer ([M]0), macroinitiator ([MI]0), EBIB, Cu(I)Br, Cu(II)Br2, PMDETA ([L]) were 8000 : 1 : 1 : 1.9 : 0.1 : 3. Polymerization was conducted at 75 °C and allowed to proceed for 6 h to reach a conversion of xp = 10.3%, which corresponds to chain lengths of about Pn = 800 repeat units. The semi-logarithmic plot of nBA consumption versus time progresses linearly up to a conversion of xp = 7.7 % (t = 200 min) corroborating first order kinetics and controlled polymerization conditions. The growing polymer inside small CNF bundles may complicate diffusion of monomer towards the reactive chain end, which can slow down polymerization speed for t > 200 min. Figure 4a shows also polymerization of the pH- and thermo-responsive PDMAEMA brushes from CNF-Brhigh.


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Figure 4. SI-ATRP of nBA and DMAEMA from CNF-Brhigh (Method A; two-step bromination) in the presence of EBiB. (a) Semi-logarithmic plot of the monomer consumption versus time with first order kinetics up to conversion of xp = 11 % (linear region). (b) FT-IR spectra of CNFBrhigh, CNF-ghigh-PnBA and CNF-ghigh-PDAEMA with pronounced signal at ν = 2800-3000 cm-1 (-CH2-) and ν = 1730 cm-1 (>C=O).

Also, the polymerization of PDMAEMA was conducted at 75 °C and allowed to proceed for 6 h to reach a conversion xp = 11.8% for DMAEMA, i.e. ca. 800 repeat units. SI-ATRP of DMAEMA demonstrates first order kinetics throughout the reaction. It is known that DMAEMA supports polymerization equilibrium due to its ability to reduce copper. This fact might be one of the reasons of higher rate of polymerization of DMAEMA (Figure 4a). Its tertiary amine groups


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are reactive binding sites and can interact, for example, with cations of metals to produce various types of metal nanoparticles.39 Moreover, hybrid particles grafted with PDMAEMA have found applications in binding and separation of biomolecules, microelectronics and ultrafiltration. Subsequently, the colloidal brushes are denoted as CNF-ghigh-PnBA and CNF-ghigh-PDMAEMA. For FT-IR measurements (Figure 4b), CNF-ghigh-PnBA and CNF-ghigh-PDMAEMA were purified from the free polymer by consecutive centrifugation cycles using THF and methanol (both good solvents for PnBA and PDMAEMA). After washing, any recorded signals in FT-IR should originate only from CNF-ghigh-PnBA or CNF-ghigh-PDMAEMA. We thus attribute the increased carbonyl signal at ν = 1730 cm-1 to the presence of ester-groups of polymer repeating units attached to CNFs. The -CH2- signal at ν = 2800-3000 cm-1 becomes more pronounced further indicating the presence of the polymer backbone. In 1H-NMR spectra (Figure 5a), we observe characteristic PnBA signals at chemical shifts,

δ = 2.2 ppm and δ = 4.0 ppm, for the methylene groups (-CH2-) adjacent to the ester unit. Also, the dispersibility of CNFs changes considerably with the attached hydrophobic PnBA brush. While CNF-ghigh-PnBA is dispersible in THF, the tethered polymer chains immediately collapse from water and cause precipitation of CNF-ghigh-PnBA (floating white piece in Figure 5b). The 1

H-NMR spectrum of CNF-ghigh-PDMAEMA (Figure 5c) presents signals at δ = 1.02 ppm for

the methyl (-CH3) and δ = 1.44 ppm for the methylene groups (-CH2-) of the polymer backbone,

δ = 2.25 ppm for the methyl groups adjacent to amine, and δ = 2.03 ppm and δ = 4.02 ppm for the methylene groups (-CH2-) adjacent to the ester unit. Figure 5d qualitatively shows the thermo-responsive behavior of the CNF-ghigh-PDMAEMA. It is dispersible in cold water well below the cloud point of PDMAEMA (e.g. T ≈ 5 °C), but precipitates at elevated temperatures (e.g. T > 60 °C) according to the well-known LCST behavior.40 Since the CNF-ghigh-PDMAEMA


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was purified from free polymer, all remaining polymer chains are tethered to the CNF surface and the thermo-responsive behavior now also affects the CNFs.

Figure 5. Polymer brushes grafted from CNF-Brhigh. (a) 1H-NMR spectrum CNF-ghigh-PnBA with characteristic PnBA peaks at δ = 4.0 ppm and δ = 2.2 ppm for methylene groups adjacent to the ester unit. (b) CNF-ghigh-PnBA is dispersible in THF, but completely phase separates in water. (c) 1H-NMR spectrum of CNF-ghigh-PDMAEMA. The signal at δ = 1.02 ppm originates from the methyl (-CH3) and δ = 1.44 ppm from the methylene groups (-CH2-) of the polymer backbone. (d) The CNF-ghigh-PDMAEMA brushes are dispersible in cold water well below the cloud point of PDMAEMA (e.g. T ≈ 5 °C), but precipitate at elevated temperatures.


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Morphology of CNF Brushes and Degradation Phenomena. Next, we address the CNFbrush morphology as characterized by TEM. The unmodified native CNFs form loosely entangled aqueous nanofiber networks as visualized in cryo-TEM in Figure 6a. The nanofibers show "kinks" (Figure 6a inset), as also observed previously e.g. in a negatively charged CNF.41 This could agree with the so-called fringed-micellar structure where the highly crystalline domains are separated by more disordered or amorphous flexible domains. Cryo-TEM of CNFghigh-PDMAEMA in slightly acidic water with high degree of substitution DS = 0.43 showed drastic morphological changes, see Figure 6b. We expected to observe a nanocellulose core and surrounding "cloud" of the polymer brushes with soft borders based on the studies on related polymer-grafted CNCs.27,42 Accordingly, the present CNF-ghigh-PDMAEMA (DS = 0.43) indeed showed elongated overall shapes with soft boundaries due to the surrounding polymer brushes, but surprisingly the CNF core could not be resolved. The objects remain extended, but they were considerably shorter than the original CNFs. In order to achieve a better distinction between the polymer brushes and CNF, we prepared TEM samples also in the dry state by blotting (Figure 6c). In this case, the nanofibrillar cores and the surrounding polymer brushes can be better resolved. However, we do not observe CNF network formation any more. Instead, we see very short nanocellulose fragments that may be attributed to crystalline domains about the length scale of the rod-like CNCs (≈ 50 - 300 nm), whereas some pieces are even shorter than the usually observed CNC lengths (note that CNCs can be produced from CNF through acid-driven fragmentation). In conclusion, the morphology of CNF-ghigh-PDMAEMA clearly has changed as compared to the native CNFs, but also compared to CNF-Brhigh, indicating that either the polymerization process itself or the growing polymer chains played a role in further CNF degradation.


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Figure 6. TEM comparison of unmodified CNF and polymer-grafted CNF-ghigh-PDMAEMA. a) Cryo-TEM of native CNFs in water at c = 0.1g/L. b) Cryo-TEM of CNF-ghigh-PDMAEMA at c = 0.1g/L. c) TEM of CNF-ghigh-PDMAEMA blotted from THF. Note the holey carbon grid in b).


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As the above findings suggest that polymer grafting using the high degree of substitution (DS = 0.43) caused degradation of the CNF, we next reduced the DS slightly still using Method A. Cryo-TEM of CNF-gmed-PDMAEMA with a medium DS = 0.28 in slightly acidic water allows now to faintly resolve the nanocellulose cores (Figure 7a). Single strands of CNFs are visible as darker stripes within bundles (see also Supporting Information Figures S2 and S3). The entangled polymer domains appear as brighter spots in between the CNF backbones. For comparison, TEM of CNF-gmed-PDMAEMA (DS = 0.28) by blotting on grids showed more clearly the CNF cores and the surrounding polymer brushes (Figure 7b). In this case, the CNFs still do not form networks, but the nanofibrillar cores are distinctly visible and noticeably longer than for DS = 0.43. It is reasonable to assume that the medium DS caused less fragmentation and suggests exploring considerably lower DS for more pronounced effects (see Method B later).


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Figure 7. TEM of CNF-gmed-PDMAEMA with medium DS = 0.28. a) Cryo-TEM in water at c = 0.1g/L. b) TEM of CNF-gmed-PDMAEMA blotted from THF.

A possible explanation may be found in the combination of crystalline and more disordered or amorphous domains within the CNFs. 6,43,44 We observed noticeable disintegration of the CNFs after polymerization from CNF-Brhigh with high DS and high polymer brush density. The surface modification in CNF may follow different reaction kinetics in the crystalline and disordered/amorphous domains, i.e., heterogeneous modification of the CNFs with preference for the amorphous parts. This would also be in line with the recent observations related to hairy cellulose nanocrystalloids by van de Ven et al.43 The densely packed cellulosic polymer chains in the crystalline domains are sterically more blocked for internal (core) modification or at the very least exhibit slowed reaction kinetics towards the acid bromide. On the contrary, the loosely bundled cellulosic polymer chains of the more disordered or amorphous domains facilitate access to a higher concentration of reactive sites across the entire cross-section of the CNF. Polymerization-induced exfoliation of nano-platelets is well established45,46 and similar mechanisms may support interpretations for the present degradation of CNFs. The localized increase of DS in the amorphous domains causes considerable expansion through the volume demand of growing polymer chains with progressing conversion. The expanding chains exert strain on the cellulosic polymer chains either affecting crystalline domains directly or inducing fragmentation of CNFs into rod-like segments. The volume requirement of the growing polymer brush could induce gradual separation of individual chains loosening up the crystalline regions like a zipper. Morphological changes through modification have been observed before for hydrophobization with excess of silylation agents,47 which led to the partial dissolution of CNFs in THF. Heux et al. further found vastly different modification behavior of bacterial cellulose


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and tunicate whiskers, and demonstrated that amorphous domains facilitate access for the modification of the crystalline domains.48 Low Degree of Bromination of CNFs for CNF-Brushes with Promoted Stability (Method B). Finally, reference studies were made to explore whether the high DS of bromination and subsequent dense and strained brush architecture could indeed be a reason for the CNF partial disintegration. To that end, we also conducted a polymerization using never-dried CNFs with overall lower DS as obtained from modification “Method B” (see Experimental Section for details). Here, we directly esterify BriB-Br to never-dried CNFs, i.e., esterification takes place in solution after gradual solvent exchange from water to DMSO. Although this mild process reduces the probability for nanofiber aggregation (as compared to e.g. drying), the DS after esterification is comparably low according to elemental analysis. A weight fraction of bromine f(Br) = 0.5 wt.-% equals about 0.15 initiator units per nm2 and corresponds to a DS = 0.035, which is an order of magnitude smaller than CNF-Brhigh and CNF-Brmed. The subsequent brominated nanofiber is denoted as CNF-Brlow. The nanofiber network of never-dried CNF most likely prevents sufficient water removal or solvent exchange still causes some aggregation despite the polar DMSO. Therefore, CNF-Brlow will be grafted with a sparse brush of PnBA (see Experimental Section). AFM measurements show an increase in diameter of CNF-glow-PnBA in comparison with the CNF-Brmed, but a smaller diameter as compared to CNF-gmed-PDMAEMA, which once again confirms successful grafting of PDMAEMA and PnBA, but also a sparser brush of CNF-glow-PnBA (see Supporting Information Figure S4). As for the degradation, indeed, the CNF-glow-PnBA backbone remained intact, as seen from the long dark fibrils in Figure 8 that exhibits similar morphology as compared to the CNF-Br macroinitiator of Figure 2b. The CNF-glow-PnBA forms a strongly entangled nanofiber network owing to the long PnBA


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chains of the polymer brush. Longer brushes allow stronger interaction among polymer chains of neighboring nanofibers due to interdigitation of chains (i.e., in the confined space between entangled CNFs).13 During polymerization, the growing polymer chains wrap around the nanofibers and tighten the CNF network by formation of a second, finely woven polymer web. The low DS not only keeps the structural integrity of CNFs intact, but the resulting lower grafting density of polymer chains after SI-ATRP might even be beneficial to enhance interactions with polymer matrices (so-called wet brush regime), which is currently studied in detail in ongoing projects.

Figure 8. CNF-brush with very low grafting density (Method B). TEM of CNF-glow-PnBA using DS = 0.035.

CONCLUSIONS In this work, we describe the modification of cellulose nanofibers (CNFs) towards the aim of surface-initiated grafting of polymer brushes with SI-ATRP. CNFs were modified using two routes: Method A: High degree of bromination, based on hydrogel conversion into nanofiber aerogels through freeze-drying in liquid propane and esterified with ATRP initiators in a twostep esterification process; Method B: low degree of bromination, based on solvent exchange


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followed by solution esterification. For both cases, a large excess of initiator proved essential for efficient degree of substitution. Poly(n-butyl acrylate) and poly(2-(dimethyl amino)ethyl methacrylate) brushes were polymerized from CNFs by controlled SI-ATRP. First order kinetics indicated controlled polymerization up to chain lengths of Pn = 800. TEM imaging of grafted CNFs showed that hydrophobic CNF-g-PnBA retain entangled with the polymer brushes. We found that modification has its subtle challenges during the process itself, but also because there is evidence of CNF degradation induced by the too high DS and polymer brush density. In perspective, the quality of the material could be improved using centrifugation49 at early stages of preparation to separate entangled CNF bundles from single nanofibers towards well-defined single nanofiber polymer brushes. In future works, we further aim to study the observed degradation phenomena in more detail employing a larger spectrum of initiators for modification and record the effect of grafting density and brush length on amorphous and crystalline domains.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental, calculations and additional results (PDF) Experimental Calculation of Initiator Density on CNF surface Calculations of Degree of Substitution Figure S1 Comparison of CNF networks after freeze-drying. Figure S2 Supporting cryo-TEM images of CNF-gmed-PDMAEMA. Figure S3 Supporting normal TEM images of CNF-gmed-PDMAEMA. Figure S4 Tapping mode AFM characterization of grafted CNFs. Figure S5 Tapping mode AFM characterization of grafted CNF-ghigh-PDMAEMA Figure S6 TEM-EDX and carbon and bromine mapping of CNF-Brhigh Supporting References

S2 S5 S5 S6 S7 S8 S9 S10 S11 S12


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AUTHOR INFORMATION Corresponding Authors André H. Gröschel. E-mail: [email protected]. Ph: +49 (0)172 4194096. Olli Ikkala. E-mail: [email protected]. Ph: +358 50 4100454.

Present Addresses £

Betulium Oy, Tekniikantie 2, FIN-02150 Espoo, Finland.

§ Centre de Recherche sur les Macromolécules Végétales (CERMAV) - UPR 5301 CNRS

BP53, 38041 Grenoble Cedex 9, France. ±Nolla

Antimicrobial, Viikinkaari 4, 00790 Helsinki, Finland.

Author Contributions A.H.G., O.I., J.R.M. and M.M. designed all experiments. A.H.G., J.R.M. and J.M. advised experimental work and analysis of results. M.M. prepared the samples and performed most of experiments. J-M.M. measured cryo-TEM, N.H. and M.M. measured AFM. J.S. and M.M. measured EDX spectroscopy and elemental mapping. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. A.H.G. and O.I. supervised the project Funding Sources


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This work was carried out under the Academy of Finland's Centre of Excellence Programme (2014-2019) and 256530 CellAssembly, WoodWisdom-Net. The work was further supported by ERC-2011-AdG (291364-MIMEFUN). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Anu Anttila and Ritva Kivelä for extraction of CNFs and Rita Hatakka for measurements of molecular weight distribution of polysaccharides in CNFs. We thank Taneli Tiittanen for the XRD measurements. For characterization, the authors made use of the Aalto University Nanomicroscopy Center (Aalto-NMC) premises and of the Aalto University Bioeconomy Facilities.

REFERENCES (1)

Spence, K.; Habibi, Y.; Dufresne, A. Cellulose Fibers: Bio- and Nano-Polymer Composites; Kalia, S., Kaith, B. S., Kaur, I., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2011.

(2)

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, DOI: 10.1002/anie.201001273.


23


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

(3)

Page 24 of 32

Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40, DOI: 10.1039/C0CS00108B.

(4)

Capadona, J. R.; Shanmuganathan, K.; Tyler, D. J.; Rowan, S. J.; Weder, C. Stimuliresponsive polymer nanocomposites inspired by the sea cucumber dermis. Science 2008, 319, DOI: 10.1126/science.1153307.

(5)

Laaksonen, P.; Walther, A.; Malho, J.-M.; Kainlauri, M.; Ikkala, O.; Linder, M. B. Genetic engineering of biomimetic nanocomposites: diblock proteins, graphene, and nanofibrillated cellulose. Angew. Chem. Int. Ed. 2011, 50, DOI: 10.1002/anie.201102973.

(6)

Habibi, Y.; Lucia, L. A; Rojas, O. J. Cellulose nanocrystals: chemistry, self-assembly, and applications. Chem. Rev. 2010, 110. DOI: 10.1021/cr900339w.

(7)

Eyley, S.; Thielemans, W. Surface modification of cellulose nanocrystals. Nanoscale 2014, 6, DOI: 10.1039/C4NR01756K.

(8)

Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gind, W.;Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund; L.A.; Peijs, T. Review: Current international research into cellulose nanofibres and nanocomposites. J. Mater. Sci. 2010, 45, DOI: 10.1007/s10853-009-3874-0.

(9)

Schramm, M.; Hestrin, S. Factors affecting production of cellulose at the Air/ liquid interface of a culture of Acetobacter xylinum. J. Gen. Microbiol. 1954, 11, DOI: 10.1099/00221287-11-1-123.


24

Page 25 of 32

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


(10)

Isogai, A.; Saito, T.; Fukuzumi, H. TEMPO-oxidized cellulose nanofibers. Nanoscale 2011, 3, DOI: 10.1039/C0NR00583E.

(11)

Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007, 8, DOI: 10.1021/bm0703970.

(12)

Saito, T.; Nishiyama, Y.; Putaux, J. L.; Vignon, M.; Isogai, A. Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules 2006, 7, DOI: 10.1021/bm060154s.

(13)

Hui, C. M.; Pietrasik, J.; Schmitt, M.; Mahoney, C.; Choi, J.; Bockstaller, M. R.; Matyjaszewski,

K.

Surface-initiated

polymerization

as

an

enabling

tool

for

multifunctional (nano-)engineered hybrid materials. Chem. Mater. 2014, 26, DOI: 10.1021/cm4023634. (14)

Lindqvist, J.; Malmström, E. Surface modification of natural substrates by atom transfer radical polymerization. J. Appl. Polym. Sci. 2006, 100, DOI: 10.1002/app.23457.

(15)

Hansson, S.; Östmark, E.; Carlmark, A.; Malmström, E. ARGET ATRP for versatile grafting of cellulose using various monomers. ACS Appl. Mater. Interfaces 2009, 1, DOI: 10.1021/am900547g.

(16)

Carlmark, A.; Malmström, E. E. ATRP grafting from cellulose fibers to create blockcopolymer grafts. Biomacromolecules 2003, 4, DOI: 10.1021/bm030046v.

(17)

Lee, S. B.; Koepsel, R. R.; Morley, S. W.; Matyjaszewski, K.; Sun, Y.; Russell, A. J. Permanent, nonleaching antibacterial surfaces, 1. Synthesis by atom transfer radical polymerization. Biomacromolecules 2004, 5, DOI: 10.1021/bm034352k.


25


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

(18)

Page 26 of 32

Carlmark, A.; Malmström, E. Atom transfer radical polymerization from cellulose fibers at ambient temperature. J. Am. Chem. Soc. 2002, 124, DOI: 10.1021/ja016582h.

(19)

Lindqvist, J.; Nyström, D.; Östmark, E.; Antoni, P.; Carlmark, A.; Johansson, M.; Hult, A.; Malmström, E. Intelligent Dual-responsive cellulose surfaces via surface-initiated ATRP. Biomacromolecules 2008, 9, DOI: 10.1021/bm800193n.

(20)

Nyström, D.; Lindqvist, J.; Ostmark, E.; Hult, A.; Malmström, E. Superhydrophobic biofibre surfaces via tailored grafting architecture. Chem. Commun. 2006, DOI: 10.1039/B607411A.

(21)

Lönnberg, H.; Zhou, Q.; Brumer, H.; Teeri, T. T.; Malmström, E.; Hult, A. Grafting of cellulose fibers with poly(epsilon-caprolactone) and poly(L-lactic acid) via ring-opening polymerization. Biomacromolecules 2006, 7, DOI: 10.1021/bm060178z.

(22)

Westlund, R.; Carlmark, A.; Hult, A.; Malmström, E.; Saez, I. M. Grafting liquid crystalline polymers from cellulose substrates using atom transfer radical polymerization. Soft Matter 2007, 3, DOI: 10.1039/B700630F.

(23)

Hansson, S.; Trouillet, V.; Tischer, T.; Goldmann, A. S.; Carlmark, A.; Barner-Kowollik, C.; Malmström, E. Grafting efficiency of synthetic polymers onto biomaterials: a comparative study of grafting-from versus grafting-to. Biomacromolecules 2013, 14, DOI: 10.1021/bm3013132.

(24)

Boujemaoui, A.; Carlsson, L.; Malmström, E.; Lahcini, M.; Berglund, L.; Sehaqui, H.; Carlmark, A. Facile preparation route for nanostructured composites: Surface-initiated ring-opening polymerization of ε-caprolactone from high-surface-area nanopaper. ACS Appl. Mater. Interfaces 2012, 4, DOI: 10.1021/am300537h.


26

Page 27 of 32

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


(25)

Xiao, M.; Li, S.; Chanklin, W.; Zheng, A.; Xiao, H. Surface-initiated atom transfer radical polymerization of butyl acrylate on cellulose microfibrils. Carbohydr. Polym. 2011, 83, DOI: 10.1016/j.carbpol.2010.08.011.

(26)

Morandi, G.; Heath, L.; Thielemans, W. Cellulose nanocrystals grafted with polystyrene chains through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP). Langmuir 2009, 25, DOI: 10.1021/la900452a.

(27)

Majoinen, J.; Walther, A.; McKee, J. R.; Kontturi, E.; Aseyev, V.; Malho, J. M.; Ruokolainen, J.; Ikkala, O. Polyelectrolyte brushes grafted from cellulose nanocrystals using

Cu-mediated

surface-initiated

controlled

radical

polymerization.

Biomacromolecules 2011, 12, DOI: 10.1021/bm200613y. (28)

Morandi, G.; Thielemans, W. Synthesis of cellulose nanocrystals bearing photocleavable grafts by ATRP. Polym. Chem. 2012, 3, DOI: 10.1039/C2PY20069D.

(29)

Lacerda, P. S. S.; Barros-Timmons, A. M. M. V; Freire, C. S. R.; Silvestre, A. J. D.; Neto, C. P. Nanostructured composites obtained by ATRP sleeving of bacterial cellulose nanofibers

with

acrylate

polymers.

Biomacromolecules

2013,

14,

DOI:

10.1021/bm400432b. (30)

McKee, J. R.; Huokuna, J.; Martikainen, L.; Karesoja, M.; Nykänen, A.; Kontturi, E.; Tenhu, H.; Ruokolainen, J.; Ikkala, O. Molecular engineering of fracture energy dissipating sacrificial bonds into cellulose nanocrystal nanocomposites. Angew. Chemie Int. Ed. 2014, 53, DOI: 10.1002/ange.201401072.

(31)

Rosilo, H.; McKee, J. R.; Kontturi, E.; Koho, T.; Hytönen, V. P.; Ikkala, O.; Kostiainen, M. A. Cationic polymer brush-modified cellulose nanocrystals for high-affinity virus binding. Nanoscale 2014, 6, DOI: 10.1039/C4NR03584D.


27


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

(32)

Page 28 of 32

Sacui, I. A.; Nieuwendaal, R. C.; Burnett, D. J.; Stranick, S. J.; Jor, M.; Weder, C.; Foster, E. J.; Olsson, R. T.; Gilman, W. Comparison of the properties of cellulose nanocrystals and cellulose nanofibrils isolated from bacteria, tunicate, and wood processed using acid, enzymatic, mechanical, and oxidative methods. ACS Appl. Mater. Interfaces 2014, 6, DOI: 10.1021/am500359f.

(33)

Pääkko, M.; Ankerfors, M.; Kosonen, H.; Nykänen, A.; Ahola, S.; Österberg, M.; Ruokolainen, J.; Laine, J.; Larsson, P. T.; Ikkala, O.; Lindström, T. Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels. Biomacromolecules 2007, 8, DOI: 10.1021/bm061215p.

(34)

Ferrer, A.; Filpponen, I.; Rodríguez, A.; Laine, J.; Rojas, O. J. Valorization of residual Empty Palm Fruit Bunch Fibers (EPFBF) by microfluidization: Production of nanofibrillated cellulose and EPFBF nanopaper. Bioresour. Technol. 2012, 125, DOI: 10.1016/j.biortech.2012.08.108.

(35)

Abdul Khalil, H. P. S.; Davoudpour, Y.; Islam, M. N.; Mustapha, A.; Sudesh, K.; Dungani, R.; Jawaid, M. Production and modification of nanofibrillated cellulose using various mechanical processes: A review. Carbohyd. Polym., 2014, 99, DOI: 10.1016/j.carbpol.2013.08.069.

(36)

Arola, S.; Malho, J.; Laaksonen, P.; Lille, M.; Linder, M. B. The role of hemicellulose in nanofibrillated cellulose networks. Soft Matter 2013, 9, DOI: 10.1039/C2SM26932E.

(37)

Korhonen, J. T.; Hiekkataipale, P.; Malm, J.; Karppinen, M.; Ikkala, O.; Ras, R. H. A. Inorganic hollow nanotube aerogels by atomic layer deposition onto native nanocellulose templates. ACS Nano 2011, 5, DOI: 10.1021/nn200108s.


28

Page 29 of 32

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


(38)

Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Controlled graft polymerization of methyl methacrylate on silicon substrate by the combined use of the Langmuir−Blodgett

and

Atom

Transfer

Radical

Polymerization

techniques.

Macromolecules 1998, 31, 5934–5936. (39)

Yuan, J.; Schacher, F.; Drechsler, M.; Hanisch, A.; Lu, Y.; Ballauff, M.; Möller, A. H. E. Stimuli-responsive organosilica hybrid nanowires decorated with metal nanoparticles. Chem. Mater. 2010, 22, DOI: 10.1021/cm9038076.

(40)

Plamper, F. A.; Ruppel, M.; Schmalz, A.; Borisov, O.; Ballauff, M.; Müller, A. H. E. Tuning the thermoresponsive properties of weak polyelectrolytes: Aqueous solutions of star-shaped and linear poly(N,N-dimethylaminoethyl methacrylate). Macromolecules, 2007, 40, DOI: 10.1021/ma071203b.

(41)

Wågberg, L.; Decher, G.; Norgren, M.; Lindström, T.; Ankerfors, M.; Axnäs, K. The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes. Langmuir 2008, 24, DOI: 10.1021/la702481v.

(42)

Malho, J.-M.; Morits, M.; Löbling, T. I.; Nonappa, Majoinen, J.; Schacher, F. H.; O. Ikkala, O.; Gröschel, A. G. Rod-Like nanoparticles with striped and helical topography. ACS Macro Letters, 2016, 5, DOI: 10.1021/acsmacrolett.6b00645.

(43)

van de Ven, T. G. M.; Sheikhi, A. Hairy cellulose nanocrystalloids: A novel class of nanocellulose. Nanoscale 2016, 8, DOI: 10.1039/C6NR01570K.

(44)

Fink, H.-P.; Walenta, E. X-ray-diffraction investigations of cellulose supramolecular structure at processing. Papier 1994, 48, 739–748.


29


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

(45)

Page 30 of 32

Tudor, J.; Willington, L.; O’Hare, D.; Royan, B. Intercalation of catalytically active metal complexes in phyllosilicates and their application as propene polymerisation catalysts. Chem. Commun. 1996, DOI: 10.1039/CC9960002031.

(46)

Choi, Y. S.; Wang, K. H.; Xu, M.; Chung, I. J. Synthesis of exfoliated polyacrylonitrile/ Na-MMT nanocomposites via emulsion polymerization. Chem. Mater. 2002, 14, DOI: 10.1021/cm0116020.

(47)

Andresen, M.; Johansson, L.-S.; Tanem, B. S.; Stenius, P. Properties and characterization of hydrophobized microfibrillated cellulose. Cellulose 2006, 13, DOI: 10.1007/s10570006-9072-1.

(48)

Berlioz, S.; Molina-Boisseau, S.; Nishiyama, Y.; Heux, L. Gas-phase surface esterification of cellulose microfibrils and whiskers. Biomacromolecules 2009, 10, DOI: 10.1021/bm900319k.

(49)

Toivonen, M. S.; Kurki-Suonio, S.; Schacher, F. H.; Hietala, S.; Rojas, O. J.; Ikkala, O. Water-resistant, transparent hybrid nanopaper by physical cross-linking with chitosan. Biomacromolecules 2015, 16, DOI: 10.1021/acs.biomac.5b00145.


30

Page 31 of 32

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For Table of Contents Use Only

SYNOPSIS Cellulose nanofibers are sustainable materials with a wide spectrum of applications. This paper discusses problems and solution of cellulose nanofiber modification.


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TOC Graphic 84x47mm (300 x 300 DPI)


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Polymer Brushes on Cellulose Nanofibers: Modification, SI-ATRP, and

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