Polyelectrolyte Brushes Grafted from Cellulose Nanocrystals Using Cu

ARTICLE pubs.acs.org/Biomac

Polyelectrolyte Brushes Grafted from Cellulose Nanocrystals Using Cu-Mediated Surface-Initiated Controlled Radical Polymerization Johanna Majoinen,† Andreas Walther,*,†,|| Jason R. McKee,† Eero Kontturi,‡ Vladimir Aseyev,§ Jani Markus Malho,†,^ Janne Ruokolainen,† and Olli Ikkala*,† †

Molecular Materials, Department of Applied Physics, Aalto University (formerly Helsinki University of Technology), P.O. Box 15100, FIN-00076 Aalto, Espoo, Finland ‡ Aalto University, Department of Forest Products Technology, P.O. Box 16300, FIN-00076 Aalto, Espoo, Finland § Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 HY, Helsinki, Finland

bS Supporting Information ABSTRACT: Herein we report the synthesis of cellulose nanocrystals (CNCs) grafted with poly(acrylic acid) (PAA) chains of different lengths using Cu-mediated surface initiatedcontrolled radical polymerization (SI-CRP). First, poly(tertbutylacrylate) (PtBA) brushes were synthesized; then, subsequent acid hydrolysis was used to furnish PAA brushes tethered onto the CNC surfaces. The CNCs were chemically modified to create initiator moieties on the CNC surfaces using chemical vapor deposition (CVD) and continued in solvent phase in DMF. A density of initiator groups of 4.6 bromine ester groups/ nm2 on the CNC surface was reached, suggesting a dense functionalization and a promising starting point for the controlled/living radical polymerization. The SI-CRP of tert-butylacrylate proceeded in a well-controlled manner with the aid of added sacrificial initiator, yielding polymer brushes with polydispersity values typically well below 1.12. We calculated the polymer brush grafting density to almost 0.3 chains/nm2, corresponding to high grafting densities and dense polymer brush formation on the nanocrystals. Successful rapid acid hydrolysis to remove the tert-butyl groups yielded pH-responsive PAA-polyelectrolyte brushes bound to the CNC surface. Individually dispersed rod-like nanoparticles with brushes of PtBA or PAA were clearly visualized by AFM and TEM imaging.

’ INTRODUCTION Cellulose is the most abundant organic polymer in nature, and it has inspired us to study not only its remarkable properties but also how to utilize it in novel sustainable advanced applications in material science. Native cellulose fibers extracted from wood consist of nanofibers with randomly alternating amorphous and crystalline domains according to the so-called fringe-fibrillar model.1 Cellulose can be used in biocomposites in many different forms, where the rod-shaped cellulose nanocrystals (CNCs) are of growing importance because of their extraordinary mechanical properties, high aspect ratio, and high surface area. Because of the tight packing and hydrogen bonding of the individual molecular cellulose chains, CNCs can exhibit a remarkably high modulus of up to 134 GPa, as shown for tunicate whiskers,2,3 and the tensile strength is expected to be in the gigapascal range. Although plantbased CNCs used in this study display smaller aspect ratios compared with long tunicate whiskers, they have the important advantage of being drastically more accessible.4 CNCs are prepared by inorganic acid hydrolysis to destroy the amorphous parts in the wood microfibrils.5
7 One of the most popular, accessible, and reproducible sources for CNCs is cotton filter paper, having an R-cellulose content of >98%. Typical dimensions of CNCs prepared from filter paper are in the range of r 2011 American Chemical Society

5
8 nm in diameter and 50 to 500 nm in length. This leads to aspect ratios (length/diameter, L/d) of 10
100. Upon reaching a critical concentration, colloidal CNC dispersions separate spontaneously into an isotropic and an anisotropic phase, where the anisotropic phase forms a chiral (cholesteric) nematic liquid crystalline structure.6 Because of the nanoscale dimensions, the CNCs possess a very large specific surface area nominally reaching 300 m2/g. CNCs are mainly prepared by sulphuric acid hydrolysis, which introduces sulfate groups on the crystallite surface. Charged sulfate groups enable stable dispersions in water due to electrostatic repulsion, suppressing the inherent tendency of aggregation. This property restricts colloidal CNC system to aqueous solutions and hinders efficient dispersion in nonpolar polymers or nonaqueous solvents. At this point, the proper chemical surface modification plays a crucial role. Therefore, it has been of great interest to find ways to modify CNCs toward compatibility with organic media and to enable new applications.8
12 Received: May 3, 2011 Revised: July 6, 2011 Published: July 09, 2011 2997

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Biomacromolecules Surface modifications of CNCs can be achieved by attaching small molecules or polymers via covalent bonds or physical interactions.13 Additionally, chemical surface modification in the gas phase offers an elegant way to overcome the challenging redispersion procedures of CNCs in organic solvents.14 A particularly versatile approach is to provide easily accessible surface functionalities with a high density of functional groups via grafting of polymer brushes. Polymer brushes are polymer coatings consisting of polymer chains that are tethered with one chain end to an interface.15 They are the key components to impart controlled and interactive properties to colloidal materials, for instance to enable tight binding of CNCs into another matrix, to induce stimuliresponsive character to the colloidal stability, or to entrap and immobilize inorganic materials.16
19 Two main approaches, “grafting-to” or “grafting-from” the CNC surface, can be used to fabricate polymer brushes on surfaces. In the first approach, one cannot expect high grafting densities because of steric hindrance and blocking of reactive sites by the already grafted polymer chains. As examples of the “grafting-to” process, Kloser et al. grafted polyethylene oxide (PEO) to the CNCs to change the aqueous colloidal dispersion of pristine CNCs from an electrostatically stabilized to a sterically stabilized system.20 Similarly, Azzam et al. report on grafting ethylene oxide and propylene oxide copolymers to CNCs using a peptide coupling reaction.21 Grafting-from has proven to be a very effective way to create high grafting densities and well-controlled polymer structures on different kinds of surfaces.22
24 We chose Cu-mediated controlled radical polymerization for its versatility with respect to monomer choice and ease of synthesis. Depending in the reaction conditions, two mechanisms, atom transfer radical polymerization (ATRP) and single electron transfer-living radical polymerization (SETLRP), are distinguished.25
28,38 In both cases, Cu(I) salts can be added to control the polymerization. SET-LRP is favored in polar reaction media and for strongly coordinating ligands, whereupon Cu(I) spontaneously and near-quantitatively disproportionates into Cu(0) and Cu(II).25
28 Subsequently, an outer-sphere electron transfer is thought to occur, and Cu(0) activates the dormant chains. On the contrary, in ATRP, the Cu(I) salt directly reacts with the alkyl halide initiator fragment, favorable under conditions where Cu(I) is stable, for example, in less polar solvents. First reports on using SI-ATRP to form polymer brushes from CNC surfaces were published by Yi et al. concerning polystyrene, poly(N,N-dimethylaminoethylmethacrylate), and poly[6-(4-(4methoxyphenylazo)phenoxy)hexyl methacrylate] brushes.29
31 Unfortunately, analysis of successful ATRP was not clearly demonstrated in any of their reports. Morandi et al. reported results from optimizing ATRP initiator modification on CNCs and controlled SI-ATRP of polystyrene brushes tethered from the CNC surface.32 Although a large excess of the harmful reactant, 2-bromoisobutyryl bromide (Br-iBBr), was used, a full coverage with ATRP initiator molecules was still not achieved. These examples demonstrate the challenge of immobilizing the nonpolar initiator units onto the highly polar CNC materials. Thermoresponsive poly(N-isopropylacrylamide) brushes were grafted from CNCs utilizing SI-SET-LRP, where an analysis of the molecular weight distribution of the polymers revealed a broad size distribution.33 In summary, despite progress, we identify that simple pathways toward well-defined polymer-grafted CNCs with high-density of CRP initiator groups and polymer chains, as well as convincing imaging data on the resulting hybrid materials,

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have remained a challenge, and thus a more refined synthetic effort is called for. Herein, we demonstrate a facile way of creating well-defined polymer brush architectures from CNC surfaces to yield polyelectrolyte brushes of poly(acrylic acid) (PAA) by applying the Cumediated SI-CRP method in combination with sacrificial initiator. Furthermore, we demonstrate an effective method to create very high initiator densities for CNC surfaces to promote high grafting densities. Chemical vapor deposition (CVD) was used as a pretreatment method before continued esterification in solution and allowed us to fully functionalize the CNC surface hydroxyl groups without affecting the integrity of the CNC crystal. Well-defined poly(tert-butyl acrylate) (PtBA) brushes with high grafting densities were first synthesized from the CNC surfaces, followed by the acid hydrolysis of their tertiary alkyl functionalities to provide PAA brushes. The materials are thoroughly characterized for their chemical and morphological features by applying different spectroscopic and microscopic techniques. Such polyelectrolyte brushes on modified CNCs can have many benefits in designing novel hybrid materials as based on sustainable and renewable natural resources.

’ EXPERIMENTAL SECTION Materials. Tert-butylacrylate (tBA, 99%, Aldrich) was purified by filtration through basic alumina. N,N,N0 ,N0 ,N0 -Pentamethyldiethylenetriamine (PMDETA, 99+%) and copper(I)bromide (99.99%) were purchased from Aldrich and used without further purification. All other reagents and solvents were obtained from Aldrich and used without further purification. Instrumentation. Molecular weights and molecular weight distributions of polymers were obtained by gel permeation chromatography (GPC). GPC was performed with a Waters chromatograph equipped with three Styragel columns (HR2, HR4, HR6) and a Waters 410 differential refractometer (Waters Instruments, Rochester, MN). Tetrahydrofuran (THF) was used as an eluent with a flow rate of 0.8 mL/min. Polystyrene standards (PSS Polymer Standards Service GmbH) were used for the calibration. Matrix-assisted laser desorption ionization
time of flight (MALDI-TOF) mass spectrometric analysis was performed on a Bruker microflex for isolated polymer samples to determine the absolute molecular weights. Indol acrylic acid (IAA) was used as matrix. Sodium trifluoroacetate (NaTFA) was used as a salt for ion formation. Samples were prepared from THF solution by mixing matrix (20 mg/mL), ionizing salt (5 mg/mL), and the sample (5 mg/mL) in a ratio of 40:10:1. 1 H Nuclear magnetic resonance (NMR) spectra were recorded with Bruker 300 MHz instrument. The samples were dissolved in CDCl3. Fourier transform-infrared (FT-IR) spectra were recorded on a Nicolet 380 instrument using an ATR cell. Thermal gravimetric analysis (TGA) was determined using Mettler M3 TG50 thermobalance gravimetric analysis (Mettler Instrumente AG., Switzerland). Samples were heated from 40 to 550 C at a heating rate of 20 C/min in air. Elemental analyses were performed in the microanalysis service at Mikroanalytisches Laboratory Pascher, Germany. Transmission electron microscopy (TEM) was performed on a JEOL JEM-3200FSC Cryo-TEM, operating at liquid nitrogen temperature. Zero-loss filtered images were obtained at an acceleration voltage of 300 kV. All images were registered digitally by a bottom-mounted CCD camera system (Ultrascan 4000, Gatan) combined and processed with a digital imaging processing system (Gatan Digital Micrograph 3.9 for GMS 1.4). For unmodified CNCs, 0.5 wt % water suspension was vitrified using a FEI Vitrobot in 100% humidity, and the sample was imaged in the cryostate. CNC-g-PAA material was imaged in the dry state using CsCl 2998

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Figure 1. Characterization of the pristine CNCs. (a) AFM height image with a height-range of 14 nm. (b) Statistical analysis of the height of the CNCs illustrating the average diameter of the CNCs (upper diagram). Statistical analysis of the average length of the CNCs (lower diagram). (c
d) SEM images of the top-surface of a CNC film. White arrow indicates the local alignment. (100 mM) and CsOH (pH 8) solution to enhance the contrast of the polymer after the sample was dried on the grid. Scanning electron microscope (SEM) images were acquired with a JEOL JSM-7500F scanning electron microscope operating at 2 kV. Prior to imaging, the samples were coated with a thin Pt/Au layer. Atomic force microscopy (AFM) characterizations were performed on a Veeco Dimension 5000 scanning probe microscope with a Nanoscope V controller (Digital Instruments). All samples were spin coated from dilute solutions of water or organic solvents onto freshly cleaved mica. Tips of μmash NSC 15 type were used with a typical resonance frequency of 325 kHz, spring constants of 40 N/m, and a tip radius 10 nm) compared with Br-ester-modified CNCs also displays the created polymer brush layer on top of the CNC. We can also observe indications of a nonuniform polymer layer on the top surface of the CNC-g-PtBA. This phenomenon was also observed by Wu et al. for single-walled carbon nanotubes with poly(n-butyl acrylate) brushes synthesized using SI-ATRP and might be due to a nonperfect distribution of the initiator grafting density or unequal polymer growth from the surface due to possible steric hindrance of neighboring brushes affecting the catalytic system.53 It is worth noting that to the best of our knowledge these are the first examples of AFM images of individually grafted CNC
polymer hybrids. Water-Soluble Polyelectrolyte-Grafted Cellulose Nanocrystals. The hydrophobic PtBA brushes were converted into water-soluble PAA brushes via fast acid-catalyzed hydrolysis using methylsulfonic acid. Again, FT-IR and the altered dispersibility from hydrophobic to hydrophilic solvents clearly 3003

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Figure 7. (a) Cryo-TEM image of pristine CNCs. (b) Gray scale analysis showing the width of a single pristine CNC marked by a black line in part a. The white boundaries originate from a slight defocus, necessary for proper contrast conditions. (c) TEM image of CNC-g-PAA (PAA molecular weight 17.7 kg 3 mol
1 from MALDI TOF of PtBA) obtained for a solution containing 100 mM CsCl and adjusted to pH 8 with CsOH. (d) Gray scale analysis of CNC-g-PAA marked by a black line in part c.

demonstrate the new functionality of the material (Figure 6a,b). Whereas CNC-g-PtBA does not disperse in water, the hydrolyzed product readily suspends into stable colloidal solutions. FT-IR displays a shift of the carbonyl peak to a lower wavenumber, 1700 cm
1, and the peak corresponding to the tert-butyl group disappears. Additionally, a broad band related to carboxylic acids appears in the region of 3750
2400 cm
1. DLS experiments were carried out to assess the size distribution of the CNC-g-PAA particles in aqueous solution. Figure 6c depicts the CONTIN plots of the apparent hydrodynamic radii for a sample with grafted brushes of degree of polymerization of ca. 230 for brush length. The intensity-weighted distribution shows a broad curve with a z-average z of 334 nm. A broad distribution is expected because the starting material CNC is a polydisperse biocolloid (Figure 1b,c). However, larger particles are drastically overrepresented in broad CONTIN distributions as intensity in DLS scales with R6. Therefore, we also show the mass-weighted distribution54 from which it is possible to infer a more narrow size distribution with an average of w = 85 nm. This is in a very reasonable range with the experimental results of the imaging. The morphology of the CNC-g-PAA material was further investigated using TEM and AFM (Figures 7 and 8). The cryoTEM image of the pristine CNCs clearly shows the rod-like character of the crystals with an average diameter of ∼7 nm (Figure 7a,b). Despite numerous attempts, it unfortunately remained impossible to visualize clearly the CNC-g-PAA brushes using cryo-TEM. Therefore, we turned to standard TEM for nanoparticles deposited from aqueous solution. The corresponding

Figure 8. (a) AFM height image of a CNC-g-PAA sample with Mn = 16.7 kg 3 mol
1 (MALDI-TOF of PtBA). (b) Height profile of the section analysis of the pink line indicated in part a.

images in Figure 7c,d illustrate very clearly a ca. 5 nm wide collapsed PAA (17.7 kg 3 mol
1 from MALDI-TOF of PtBA) shell around the crystalline CNC particle. Note that we enhanced the contrast by staining the PAA brushes with cesium ions that bind to the polyacid chains. To confirm further the presence of a grafted PAA-shell surrounding the CNCs, we also recorded AFM images. Figure 8a exhibits a flat polymer corona extending from the rod-like particles and thus confirming the presence of densely grafted PAA chains. The height profile also illustrates the roughness of the PAA brush layer covering the CNCs (Figure 8b). Collapsed brushes can often manifest in blobs on the surfaces,53 thus partially explaining the surface heterogeneities on top of the imaged CNC-g-PAA samples. 3004

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’ CONCLUSIONS Hybrid nanoparticles consisting of crystalline CNCs tethered with PAA polyelectrolyte brushes were prepared by SI-CRP of tert-butyl acrylate and subsequent hydrolysis. Both brushes, hydrophobic PtBA and hydrophilic PAA, were characterized for their correct chemical functionalities, solution properties, and morphological features. The surface-grafted polymer chains are well-defined with very narrow PDIs and can be controlled in length in a broad range. On the basis of an improved route for the initiator modification, we could create high polymer grafting densities close to 0.3 chains/nm2. This new modification route enables further advanced targeted use for CNCs by exploiting supramolecular ionic complexations of the surface-grafted polyelectrolyte chains toward functionalities and applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Grafting density calculations for the Br-iBBr-ester functionalities and for the polymer brushes grafted from the CNC surface. Comment on the polymerization mechanism. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; Olli.Ikkala@tkk.fi. )

Present Addresses

DWI at the RWTH Aachen, Forckenbeckstr. 50, D-52056 Aachen, Germany. ^ Nanobiomaterials, VTT, TT2 Tietotie 2, P.O. Box 1000, FIN02044 VTT, Finland.

’ ACKNOWLEDGMENT We would like to thank Felix Plamper, Lauri Valtola, Erno Karjalainen, Heikki Tenhu, Eija Ahonen, and Mari Granstr€om for their help in the measurements and discussions about the results. This work was supported by the Finnish Funding Agency for Technology and Innovation (NASEVA) and Academy of Finland and made use of the facilities of the Finnish Nanomicroscopy Center. We further appreciate Nikolay Houbenov for the graphical design of Scheme 1. ’ REFERENCES (1) Kr€assig, H. A. Cellulose, Structure, Accessibility and Reactivity; Gordon and Breach Publishers: Philadelphia, 1993. (2) Sturcova, A.; Davies, G. R.; Eichhorn, S. J. Biomacromolecules 2005, 6, 1055–1061. (3) Iwamoto, S.; Kai, W. H.; Isogai, A.; Iwata, T. Biomacromolecules 2009, 10, 2571–2576. (4) Eichhorn, S. J. Soft Matter 2011, 7, 303–315.  (5) Ranby, B. G. Discuss. Faraday Soc. 1951, 11, 158–164. (6) Revol, J.-F.; B., H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170–172. (7) Edgar, C. D.; Gray, D. G. Cellulose 2003, 10, 299–306. (8) Heux, L.; Chauve, G.; Bonini, C. Langmuir 2000, 16, 8210–8212. (9) Bonini, C.; Heux, L.; Cavaille, J. Y.; Lindner, P.; Dewhurst, C.; Terech, P. Langmuir 2002, 18, 3311–3314. (10) Yuan, H. H.; Nishiyama, Y.; Wada, M.; Kuga, S. Biomacromolecules 2006, 7, 696–700. (11) van den Berg, O.; Capadona, J. R.; Weder, C. Biomacromolecules 2007, 8, 1353–1357.

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