Polymer Brushes on Cellulose Nanofibers ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

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, FI-00076, Espoo, Finland Nanomicroscopy Center, Department of Applied Physics, School of Science, Aalto University, P.O. Box 15100, FI-00076, Espoo, Finland § Department of Bioproducts and Biosystems, School of Chemical Engineeering, Aalto University, P.O. Box 16300, FI-00076 Espoo, Finland ∥ Physical Chemistry and Centre for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, D-45127 Essen, Germany ‡

S Supporting Information *

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 nanocrystal (CNC) nanorods and bacterial cellulose (BC) networks. However, corresponding SI-ATRP on long and entangled cellulose nanofibers (CNFs), having typically more complex composition and partly disordered 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 CNFs 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 ca. 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 disordered domains of CNF, causing degradation, which can be suppressed using a lower degree of substitution. Therefore, SI-ATRP of CNFs becomes subtler than that of, for example, CNCs, and careful balance has to be achieved between high density of brushes and excessive CNF degradation. KEYWORDS: Cellulose nanofibers, Cellulose degradation, Nanocellulose, SI-ATRP, Surface modification

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INTRODUCTION Cellulose has attracted revived interest due to its wide abundance as a sustainable source for various types of biobased materials ranging from the classic macroscopic wood pulp, paper, and microscopic cellulose particles to advanced nanoobjects 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 highperformance 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 © 2017 American Chemical Society

diameter but are up to 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 to 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. Received: March 30, 2017 Revised: May 26, 2017 Published: July 10, 2017 7642

DOI: 10.1021/acssuschemeng.7b00972 ACS Sustainable Chem. Eng. 2017, 5, 7642−7650

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ACS Sustainable Chemistry & Engineering

Figure 1. Two step modification for high degree of bromination and characterization of CNF-Brhigh 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) FTIR spectra of native CNF (black), CNF-Br after CVD (step 1; red), and CNF-Br 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.

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

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 (60 μm).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 SIATRP. 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 toward 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, nanonetworks, 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 with initiating sites suitable for SIATRP, and the 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 poly-

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RESULTS AND DISCUSSION The surface of CNFs is rich in hydroxyl groups, and CNFs are usually stored in the form of 1−2 wt % hydrogels. For effective surface modification with compounds that are highly reactive toward hydroxyls, all traces of water must be removed from the CNF 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 a small DS, aiming at a 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, for example, 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 7643


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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 CNF-Brhigh (Cr.I. = crystallinity index). Note the lacey carbon grid in b.

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 BriBBr proved essential for high DS during esterification of CNFs in dispersion. The high aspect ratio of the CNFs limits its dispersibility to only low concentrations in DMF because 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 anhydrous DMF. Traces of water immediately react with BriB-Br, and the 40-fold excess guarantees that a 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 CNFBrhigh. The hydroxyl signal at ν = 3000−3600 cm−1 did not change significantly after esterification, which illustrates the still

with bromine on CNFs, two-step modification was used in Method A (see Experimental Section in the Supporting Information for details). In step 1, CNFs are freeze-dried and reacted in the gas phase with α-bromoisobutyryl bromide (BriB-Br) using chemical vapor 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 rodlike 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 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.37,38 Rapid cooling nucleates smaller ice crystals and promotes formation of amorphous ice. This 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 BriB-Br 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, 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 7644


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ACS Sustainable Chemistry & Engineering low content of esterified units on the CNF surfaces compared to the overall number of 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 cryoTEM, which is considerably higher than the actual accessible surface area. The number of hydrophobic initiation sites is sufficient to alter the dispersibility of CNF-Brhigh (Figure 1c). While native CNFs form stable dispersions in water and flocculate in THF, CNF-Brhigh is now dispersible in THF. Figures 2a and b show TEM images of native CNFs prepared from aqueous dispersion and CNF-Brhigh from THF after twostep 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 native CNF in water. More importantly, esterification did not alter the overall morphology. The 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). The 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. 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 α-bromoisobutyrate (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 Figure 4a summarizes a typical SI-ATRP of nBA from CNFBrhigh 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, and 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

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

Figure 4. SI-ATRP of nBA and DMAEMA from CNF-Brhigh (Method A; two-step bromination) in the presence of EBiB. (a) Semilogarithmic plot of the monomer consumption versus time. (b) FTIR spectra of CNF-Brhigh, CNF-ghigh-PnBA, and CNF-ghigh-PDAEMA with pronounced signal at ν = 2800−3000 cm−1 (−CH2−) and ν = 1730 cm−1 (>CO).

of about DPn = 800 repeat units. The semilogarithmic 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 toward the reactive chain end, which can slow down polymerization speed for t > 200 min. Figure 4a shows also polymerization of the pH-responsive and thermoresponsive PDMAEMA brushes from CNF-Brhigh. 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 7645


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Figure 5. Polymer brushes grafted from CNF-Brhigh. (a) 1H NMR spectrum of 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 that at δ = 1.44 ppm is 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.

white piece in Figure 5b). The 1H NMR spectrum of CNFghigh-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 thermoresponsive 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 CNFghigh-PDMAEMA was purified from free polymer, all remaining polymer chains are tethered to the CNF surface and the thermoresponsive behavior now also affects the CNFs. Morphology of CNF Brushes and Degradation Phenomena. Next, we address the CNF−brush 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, for example, 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 CNF-ghigh-PDMAEMA in slightly acidic water with a 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

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 the higher rate of polymerization of DMAEMA (Figure 4a). Its tertiary amine groups 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-ghighPDMAEMA. For FT-IR measurements (Figure 4b), CNFghigh-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 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 7646


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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. 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 us now to faintly resolve the nanocellulose cores (Figure 7a). Single strands of CNFs are

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

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

visible as darker stripes within bundles (see also Supporting Information Figures S2 and S3). The entangled polymer domains appear as brighter spots between the CNF backbones. For comparison, TEM of CNF-gmed-PDMAEMA (medium 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). 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

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 rodlike 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). The morphology of CNF-ghigh-PDMAEMA 7647


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ACS Sustainable Chemistry & Engineering 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 toward 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 nanoplatelets is well established,45,46 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 rodlike 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 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 those for 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 CNFBrmed, but a smaller diameter as compared to CNF-gmedPDMAEMA, which once again confirms successful grafting of PDMAEMA and PnBA, but also a sparser brush of CNF-glowPnBA (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, and 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 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

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

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.

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CONCLUSIONS In this work, we describe the modification of cellulose nanofibers (CNFs) toward 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 freezedrying in liquid propane and esterified with ATRP initiators in a two-step esterification process; Method B: low degree of bromination, based on solvent exchange followed by solution esterification. For both cases, a large excess of initiator proved essential for an efficient degree of substitution. Poly(n-butyl acrylate) and poly(2-(dimethyl amino)ethyl methacrylate) brushes were polymerized from CNFs by controlled SIATRP. First order kinetics indicated controlled polymerization up to chain lengths of DPn = 800. TEM imaging of grafted CNFs showed that hydrophobic CNF-g-PnBA remains 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 toward welldefined 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00972. Experimental, calculations, and additional results (PDF) 7648


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AUTHOR INFORMATION

Corresponding Authors

* André H. Gröschel. E-mail: [email protected]. Ph: +49 (0)201 1832927. *Olli Ikkala. E-mail: olli.ikkala@aalto.fi. Ph: +358 50 4100454. ORCID

André H. Gröschel: 0000-0002-2576-394X Olli Ikkala: 0000-0002-0470-1889 Present Addresses ⊥

Betulium Oy, Tekniikantie 2, FI-02150 Espoo, Finland. Centre de Recherche sur les Macromolécules Végétales (CERMAV) - UPR 5301 CNRS, BP53, 38041 Grenoble Cedex 9, France. ± Nolla Antimicrobial, Viikinkaari 4, FI-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 the experiments. J.M.M. measured cryo-TEM, N.H. and M.M. measured AFM., and 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

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.

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ACKNOWLEDGMENTS We thank Anu Anttila and Ritva Kivelä for extraction of CNFs and Rita Hatakka for measurements of the 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.

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