Direct Surface Force Measurements of Polyelectrolyte Multilayer Films

Publication Date (Web): October 6, 2010 ... multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make ...
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Direct Surface Force Measurements of Polyelectrolyte Multilayer Films Containing Nanocrystalline Cellulose Emily D. Cranston,*,†,‡ Derek G. Gray,† and Mark W. Rutland‡ †

Department of Chemistry, McGill University, Montr eal, QC, H3A 2A7 Canada, and Department of Chemistry, Surface and Corrosion Science, Royal Institute of Technology, SE-100 44, Stockholm, Sweden

‡

Received August 2, 2010. Revised Manuscript Received September 21, 2010 Polyelectrolyte multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make up a new class of nanostructured composite with applications ranging from coatings to biomedical devices. Moreover, these materials are amenable to surface force studies using colloid-probe atomic force microscopy (CPAFM). For electrostatically assembled films with either NCC or PAH as the outermost layer, surface morphology was investigated by AFM and wettability was examined by contact angle measurements. By varying the surrounding ionic strength and pH, the relative contributions from electrostatic, van der Waals, steric, and polymer bridging interactions were evaluated. The ionic cross-linking in these films rendered them stable under all solution conditions studied although swelling at low pH and high ionic strength was inferred. The underlying polymer layer in the multilayered film was found to dictate the dominant surface forces when polymer migration and chain extension were facilitated. The precontact normal forces between a silica probe and an NCC-capped multilayer film were monotonically repulsive at pH values where the material surfaces were similarly and fully charged. In contrast, at pH 3.5, the anionic surfaces were weakly charged but the underlying layer of cationic PAH was fully charged and attractive forces dominated due to polymer bridging from extended PAH chains. The interaction with an anionic carboxylic acid probe showed similar behavior to the silica probe; however, for a cationic amine probe with an anionic NCC-capped film, electrostatic doublelayer attraction at low pH, and electrostatic double-layer repulsion at high pH, were observed. Finally, the effect of the capping layer was studied with an anionic probe, which indicated that NCC-capped films exhibited purely repulsive forces which were larger in magnitude than the combination of electrostatic double-layer attraction and steric repulsion, measured for PAH-capped films. Wherever possible, DLVO theory was used to fit the measured surface forces and apparent surface potentials and surface charge densities were calculated.

Introduction Surface interactions in cellulose systems have great industrial and fundamental importance. Recent advances in the colloidprobe atomic force microscopy technique (CP-AFM) of Ducker et al.1 have allowed for detailed interactions between materials to be easily measured. In this method, direct surface forces, both attractive and repulsive, can be detected by precisely moving a colloidal particle attached to an AFM cantilever toward a surface and recording the deflection of the cantilever. AFM provides the subnanometer spatial resolution and piconewton force resolution necessary to measure these types of interactions. Polymeric cellulose, β-1,4-linked D-glucose, is biosynthesized in plants, algae, bacteria, and some marine animals and is a ubiquitous natural resource. This makes it attractive as a filler in nanocomposites, especially since cellulose has favorable mechanical and optical properties and is environmentally acceptable. Conventionally, cellulose is used as a construction material (wood), as a natural textile (cotton and flax), and for paper and board. Moreover, high performance cellulose-based materials are used throughout industry and everyday life. In all of these applications, cellulose-cellulose and cellulose-polymer interactions are vital, yet poorly understood. *To whom correspondence should be addressed. E-mail: [email protected]. (1) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (2) Edgar, C. D.; Gray, D. G. Cellulose 2003, 10, 299. (3) Kontturi, E.; Th€une, P. C.; Niemantsverdriet, J. W. H. Langmuir 2003, 19, 5735. (4) Notley, S. M.; Wa˚gberg, L. Biomacromolecules 2005, 6, 1586.

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Various cellulose systems have been developed to study surface interactions.2-10 Cellulose is not amenable to solvent or melt casting since it does not dissolve in common solvents and decomposes before melting.11 One option is to work with nanocrystalline cellulose (NCC) which is prepared through acid hydrolysis of cotton or pulp. During the hydrolysis reaction, the amorphous cellulose regions are degraded leaving small crystallites and preserving the natural cellulose I crystal form.12 Smooth cellulose I surfaces can be made from these aqueous suspensions of charged rod-like particles and are suitable for most modern characterization techniques.2 Previously, we have described the preparation of cellulose I and polyelectrolyte films with internal structure and variable surface orientation through layer-by-layer (LbL) electrostatic assembly of NCC and cationic polyelectrolytes.13-15 This low-cost technique is (5) Gunnars, S.; Wa˚gberg, L.; Cohen Stuart, M. A. Cellulose 2002, 9, 239. (6) Carambassis, A.; Rutland, M. W. Langmuir 1999, 15, 5584. (7) Notley, S. M.; Pettersson, B.; Wa˚gberg, L. J. Am. Chem. Soc. 2004, 126, 13930. (8) Neuman, R. D.; Berg, J. M.; Claesson, P. M. Nord. Pulp Paper Res. J. 1993, 8, 96. € (9) Holmberg, M.; Berg, J.; Stemme, S.; Odberg, L.; Rasmusson, J.; Claesson, P. M. J. Colloid Interface Sci. 1997, 186, 369. € (10) Kontturi, E.; Tammelin, T.; Osterberg, M. Chem. Soc. Rev. 2006, 35, 1287. (11) Klemm, D.; Philipp, B.; Heinze, T.; Heinze, U.; Wagenknecht, W. Comprehensive Cellulose Chemistry Vol. 1: Fundamental and Analytical Methods; WileyVCH: Weinheim, Germany, 1998; Vol. 1, p 260. (12) Revol, J. F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (13) Cranston, E. D.; Gray, D. G. Biomacromolecules 2006, 7, 2522. (14) Cranston, E. D.; Gray, D. G. Sci. Technol. Adv. Mater. 2006, 7, 319. (15) Cranston, E. D.; Gray, D. G. Colloids Surf., A 2008, 325, 44.

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ideally suited to creating defect-free smooth films of controlled chemistry. The LbL method was first introduced by Decher16 and consists of the adsorption of alternately charged polyelectrolytes with successive rinsing steps. Applications of LbL films include sensors,16-18 nonlinear optical devices,19 antireflective coatings,20,21 separation technologies,16 dielectric mirrors,22 drug-delivery systems,22 biofouling agents, and functional coatings.23 The LbL technique has been extended to include multiplecharged nanoparticles such as nanotubes, quantum dots, clays, and proteins.23 Cellulose nanocrystals are amenable to LbL assembly because they possess anionic surface sulfate ester groups as a result of the hydrolysis reaction used for their preparation. Their dimensions, ca. 5-10 nm wide by 100-350 nm long, allow for high loading of cellulose and fast film build-up. Films incorporating linear polycations and NCC have been prepared by conventional LbL solution-dipping,13,18,21,24-26 as well as by a spin-coating variant.13 Both techniques give rise to uniform and stable thin films where the large aspect-ratio cellulose nanocrystals serve to strengthen the elastic polymer matrix or can be tailored to create highly porous, low refractive index coatings.21 Here we present direct surface force measurements of polyelectrolyte multilayers containing NCC and poly(allylamine hydrochloride) (PAH). We examined the interactions between films with different capping layers and silica colloid-probes as well as amine- and carboxylic acid-functionalized probes. Measurements were made in aqueous solutions and the effects of pH and ionic strength on the multilayer structure and behavior were observed. The intersurface forces were recorded as a function of separation and are compared to the classical Derjaguin-Landau-VerweyOverbeek (DLVO) theory. This theory accounts for van der Waals dispersion forces and electrostatic double-layer forces and takes them to be additive.27 The double-layer force was calculated from the nonlinear Poisson-Boltzmann equation for asymmetric systems and originates from Coulombic interactions and repulsive osmotic pressure that arises when counterions distributed around a charged surface (diffuse electric double-layer) are forced together. The ubiquitous, though weaker, attractive van der Waals interaction was calculated for the nonretarded case, for sphere-plane geometry, according to the Hamaker summation method.27 In many applications, such as papermaking and composite manufacturing, it is likely that a variety of surface forces are operating between cellulose, polymers, and charged colloids, which are not restricted to DLVO-type forces. Interactions to consider between macroscopic surfaces are both long-range (van der Waals, electrostatic, polymer bridging, and steric forces) and short-range (which are harder to discern, such as solvation and other steric forces). (16) Decher, G. Science 1997, 277, 1232. (17) Jiang, C.; Markutsa, S.; Pikus, Y.; Tsukruk, V., V. Nat. Mater. 2004, 3, 721. (18) Jean, B.; Dubreuil, F.; Heux, L.; Cousin, F. Langmuir 2008, 24, 3452. (19) Ding, B.; Fujimoto, K.; Shiratori, S. Thin Solid Films 2005, 491, 23. (20) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (21) Podsiadlo, P.; Sui, L.; Elkasabi, Y.; Burgardt, P.; Lee, J.; Miryala, A.; Kusumaatmaja, W.; Carman, M. R.; Shtein, M.; Kieffer, J.; Lahann, J.; Kotov, N. A. Langmuir 2007, 23, 7901. (22) Zhai, L.; Nolte, A. J.; Cohen, R. E.; Rubner, M. F. Macromolecules 2004, 37, 6113. (23) Decher, G.; Schlenoff, J. B. Multilayer Thin Films, 1 ed.; Wiley-VCH: Weinheim, Germany, 2002; Vol. 1, p 524. (24) Podsiadlo, P.; Choi, S.-Y.; Shim, B.; Lee, J.; Cuddihy, M.; Kotov, N. A. Biomacromolecules 2005, 6, 2914. (25) Shim, B. S.; Podsiadlo, P.; Lilly, D. G.; Agarwal, A.; Lee, J.; Tang, Z.; Ho, S.; Ingle, P.; Paterson, D.; Lu, W.; Kotov, N. A. Nano Lett. 2007, 7, 3266. (26) Jean, B.; Heux, L.; Dubreuil, F.; Chambat, G.; Cousin, F. Langmuir 2009, 25, 3920. (27) Israelachvili, J. Intermolecular & Surface Forces, 2 ed.; Academic Press Inc.: London, 1992; Vol. 1, p 450.

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The build-up of LbL films28,29 and the adsorbed structure of polyelectrolytes have previously been investigated through direct surface force measurements, providing a good starting point for the current experiments.30-32 Additionally, cellulose systems have been extensively characterized by AFM,33,34 CP-AFM,4,6,7,35-49 and the surface force apparatus (SFA).8,9,48 To obtain reproducible force data, surfaces need to be smooth, stable and minimally prone to swelling and molecular fibrillation, which points to highly crystalline films as a possible solution. The latest CP-AFM studies systematically measured the forces (attractive, repulsive, adhesive and frictional) between an amorphous cellulose sphere and cellulose surfaces of varying crystallinity.35,36,49 Cellulosecellulose interactions were found to be more dependent on how the surfaces were prepared than on the crystal structure. Variations in roughness, charge and the tendency to swell led to the variation in surface forces: NCC films showed monotonically repulsive forces (from surface charges that led to electrostatic double-layer interactions),35,36 cellulose II films showed both attractive and repulsive DLVO forces under opposing solution conditions,7,35,36,47 and amorphous films were dominated by steric forces from extended cellulose chains.35,36 Short-range steric repulsion that could be screened by added salt and long-range double-layer interactions were generally operative for nonswelling cellulose films under charged solution conditions. Building on past work, here we measure the surface forces between a cellulose-polymer LbL nanocomposite film and charged colloidal-probes. In contrast to other studies, the polyelectrolyte component is not just adsorbed on the surface but is part of the multilayer structure and, hence, is less likely to desorb or lead to purely repulsive steric forces. In addition to the useful material properties, these LbL films present a novel challenge for surface force analysis due to the anisotropy of the film structure. Since one component is a rigid anionic rod and the other a cationic flexible molecule, it is reasonable to assume that the capping layer will significantly dictate the manner in which the composite interacts with its surroundings. The focus of this work (28) Blomberg, E.; Poptoshev, E.; Claesson, P. M.; Caruso, F. Langmuir 2004, 20, 5432. (29) Blomberg, E.; Poptoshev, E.; Caruso, F. Langmuir 2006, 22, 4153. (30) Notley, S. M.; Biggs, S.; Craig, V. S. J.; Wa˚gberg, L. Phys. Chem. Chem. Phys. 2004, 6, 2379. (31) Rojas, O. J.; Claesson, P. M.; Muller, D.; Neuman, R. D. J. Colloid Interface Sci. 1998, 205, 77. (32) Plunkett, M.; Feiler, A.; Rutland, M. Langmuir 2003, 19, 4180. (33) Pang, L.; Gray, D. G. J. Pulp Paper Sci. 1998, 24, 369. (34) Lefebvre, J.; Gray, D. G. Cellulose 2005, 12, 127. (35) Notley, S. M.; Eriksson, M.; Wa˚gberg, L.; Beck-Candanedo, S.; Gray, D. G. Langmuir 2006, 22, 3154. (36) Stiernstedt, J.; Nordgren, N.; Wa˚gberg, L.; Brummer, H. I.; Gray, D. G.; Rutland, M. W. J. Colloid Interface Sci. 2006, 303, 117. (37) Leporatti, S.; Sczech, R.; Riegler, H.; Bruzzano, S.; Storsberg, J.; Loth, F.; Jaeger, W.; Laschewsky, A.; Eichhorn, S. J.; Donath, E. J. Colloid Interface Sci. 2005, 281, 101. (38) Radtchenko, I. L.; Papastavrou, G.; Borkovec, M. Biomacromolecules 2005, 6, 3057. (39) Notley, S. M.; Norgren, M. Langmuir 2006, 22, 11199. (40) Stiernstedt, J.; Brummer, H. I.; Zhou, Q.; Teeri, T. T.; Rutland, M. W. Biomacromolecules 2006, 7, 2147. (41) Rutland, M. W.; Carambassis, A.; Willing, G. A.; Neuman, R. D. Colloids Surf., A 1997, 123-124, 369. (42) Theander, K.; Pugh, R. J.; Rutland, M. W. J. Colloid Interface Sci. 2005, 291, 361. (43) Zauscher, S.; Klingenberg, D. J. J. Colloid Interface Sci. 2000, 229, 497. (44) Zauscher, S.; Klingenberg, D. J. Colloids Surf., A 2001, 178, 213. (45) Bogdanovic, G.; Tiberg, F.; Rutland, M. W. Langmuir 2001, 17, 5911. (46) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. Adv. Mater. 1993, 5, 919. (47) Notley, S.; Wa˚gberg, L. 13th Fundamental Research Symposium, Cambridge, September 2005, p 1337. (48) Holmberg, M.; Wigren, R.; Erlandsson, R.; Claesson, P. M. Colloids Surf., A 1997, 129-130, 175. € (49) Ahola, S.; Salmi, J.; Johansson, L.-S.; Laine, J.; Osterberg, M. Biomacromolecules 2008, 9, 1273.

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is therefore to address both how the choice of capping layer, as well as the underlying layers, affect the interactions of such LbL composite films.

Experimental Details Materials. Cationic poly(allylamine hydrochloride) (PAH), Mw = 60 000 g/mol, was obtained from Polysciences (Warrington, PA) and used as received. All aqueous solutions and rinsing procedures used deionized water (18.2 MΩ cm, Millipore MilliQ Purification System). Anionic nanocrystalline cellulose (NCC) was prepared, as described previously,13 by sulfuric acid hydrolysis of cotton from Whatman cellulose filter aid (ashless powder, catalog no. 1700025). The nanocrystals were cleaned of residual acid by centrifugation and dialysis and sonicated to make stable aqueous suspensions. NCC was converted from the protonated sulfate ester form to the sodium salt by conductometric titration (Orion conductivity cell 018010, with cell constant K = 0.987 cm-1 attached to a Fisher Scientific accumet pH meter 50) with NaOH. The surface charge density of cellulose nanocrystals was calculated to be 0.41 ( 0.05 e/nm2 as measured by conductometric titration. Standard solutions of HCl, NaOH, and NaCl were acquired from Sigma-Aldrich and used to make the controlled pH and ionic strength solutions needed for the CP-AFM measurements. Surface Preparation. Polyelectrolyte multilayer films were spin-assembled (Spin Processor, Laurell Technologies) on polished silicon Æ100æ wafers obtained from WaferNet, Inc. The silicon wafers were cut to the desired dimensions (ca. 1.5  4.5 cm) and cleaned in a piranha bath (3:1 concentrated sulfuric acid to hydrogen peroxide mixture) for 0.5 h, followed by continuous rinsing with deionized water. LbL films were prepared as follows: 0.5 mL of 10-2 M PAH was poured onto a stationary wafer which was then accelerated at 1260 rpm/s and spun at 3000 rpm for 40 s. The film was rinsed with 1.0 mL of deionized water and spun at 3000 rpm for 40 s. This rinsing procedure was repeated three times. A 1% (w/w) NCC suspension was filtered through a 0.8 μm filter unit (Millipore) directly onto the PAH layer, spun, and rinsed following the same protocol. One bilayer is defined as a single deposition step of PAH followed by rinsing and an adsorption step with cellulose with rinsing; thus, integer bilayers have NCC as the outermost layer and half-integer bilayers end with PAH. Multilayered films containing 15 bilayers, (PAH/ NCC)15, and 15.5 bilayers, (PAH/NCC)15PAH, were assembled for normal force measurements. Additionally, for comparison purposes, films containing only NCC were prepared by spincoating. Without rinsing, 30 spin-deposition steps of 1% (w/w) NCC suspension gave thin films of approximately the same thickness as (PAH/NCC)15. All films were dried in an oven at 80 C for 25 min to stabilize and ensure that they did not redisperse in the aqueous solutions; this post-treatment is also assumed to reduce the number of sulfate ester groups on the NCC surface.34,50 AFM images of films, before and after drying and before and after force measurements, were acquired and indicated that there was no morphological difference or significant change in surface roughness throughout the experiments. Atomic Force Microscopy Imaging (AFM). An MFP-3D atomic force microscope (Asylum Research) was used to image surface morphology. Images were collected in tapping mode using Ultrasharp Si tips (spring constant 5.7 N/m, resonant frequency ca. 160 kHz, NSC14 series, MikroMasch). Root-mean-squared (rms) roughness was calculated by averaging 10 values from 1 μm2 areas on AFM images of 2, 5, and 10 μm2. Reported values are therefore an average of 30 replicate measurements with confidence intervals of 95%. Ellipsometry. Film thickness was measured by ellipsometry (Gaertner ellipsometer model L116C, Gaertner Scientific Corporation) with average “isotropic” refractive index values (50) Eriksson, M.; Notley, S. M.; Wa˚gberg, L. Biomacromolecules 2007, 8, 912.

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set as previously determined.15 Reported values are an average of 10 replicate measurements with confidence intervals of 95%. Contact Angle Measurements. Contact angle measurements, according to the static sessile drop method, were conducted using a goniometer (Contact Angle System OCA20, Dataphysics) at room temperature, in air at a relative humidity of 10-25%. A drop of deionized water (8 μL) from a microsyringe (Hamilton-Bonaduz) was placed on the surface of the dry cellulose films. The sample stage and CCD camera position were adjusted so that the image of the drop was clearly obtained on the computer monitor, and the image was captured 5 s after depositing the drop. The shape of the droplet was determined by the Young-Laplace equation and the angle formed between the liquid drop and film was recorded as the contact angle. Reported contact angle values are an average of 10 replicate measurements with confidence intervals of 95%. Zeta Potential Measurements. The electrophoretic mobility of cellulose nanocrystals in water at neutral pH was measured on a Microelectrophoresis Apparatus Mk II (Rank Brothers). Mobility values were converted to ζ-potentials using the Smoluchowski equation and the reported value is an average of 20 measurements with confidence intervals of 95%. Surface Force Measurements. The interactions between silica, or functionalized silica, colloid-probes and cellulose thin films were measured using CP-AFM according to the method of Ducker et al.1 An MFP-3D atomic force microscope (Asylum Research) on a vibration-dampening table in an isolation hood was used for all force experiments. Silica colloid-probes (1 μm diameter) were purchased from Novascan Technologies and were individually sealed under Ar gas. The probes were used immediately after removal from packaging. The functionalized silica probes were coated with gold and an alkanethiol self-assembled monolayer (SAM) terminating in either NH3 (11-mer) or COOH (16-mer). The deflection sensitivity was determined from the region of constant compliance (where the linear deflection of the cantilever corresponds to the linear motion of the piezoelectric scanner) on silicon wafers or glass microscope slides, in air and in deionized water. The zero point of separation between the probe and the film is defined as the extrapolated intersection of the region of constant compliance and the region where there is no deflection of the cantilever (at far distances from the surface). Care should be taken in analyzing AFM force data for deformable materials,51 such as the highly swollen films measured here. This has also been discussed for other cellulose surfaces;36,52 however, in our experiments, no change in the slope of the region of constant compliance was observed with increasing ionic strength indicating that constant compliance (or consistent pseudoconstant compliance) was reached and the determination of the zero of separation was precise. The cantilever spring constants (ca. 0.12 N/m or 0.32 N/ m) were calibrated in air and in water from the spectral power density of the cantilever fluctuations due to thermal noise according to the resonance frequency method.53,54 Nine solutions were prepared at pH values of 3.5, 5.5, and 8 by adjusting deionized water (with a starting pH of 5.5) with HCl or NaOH and the total ionic strengths were set to 0.5, 1, and 10 mM by adding NaCl. The value for the double-layer thickness was calculated from the known solution conditions and was not a fitting parameter in the theoretical analysis. An “open fluid cell” was used for the AFM force measurements; a few drops of prepared solution were placed onto both the film and the colloid-probe, and the cantilever was lowered near to the surface. The system was left to equilibrate for 15-30 min (51) Rutland, M. W.; Tyrrell, J. W. G.; Attard, P. J. Adhes. Sci. Technol. 2004, 18, 1199. € (52) Nordgren, N.; Eronen, P.; Osterberg, M.; Laine, J.; Rutland, M. W. Biomacromolecules 2009, 10, 645. (53) Hutter, J. L.; Bechhoefer, J. Rev. Sci. Instrum. 1993, 64, 1868. (54) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403.

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Figure 1. AFM height images (AC-mode and atmospheric conditions) of films containing NCC: (a) (PAH/NCC)15, (b) (PAH/NCC)15PAH, and (c) spin-coated NCC. before measurements were made. Force-distance curves were obtained with a z-scan size of 300-800 nm at a rate of 0.8-1.0 Hz and 20 curves were collected in six different locations on the film. A minimum of 70 force-distance curves were averaged for all plots shown. (For more information about force curve reproducibility, see the Supporting Information.) For comparison purposes, measured forces between the spherical probe and planar film, FSP, were then normalized by 2πR, where R is the probe radius. According to the Derjaguin approximation, this is equivalent to the interaction energy per unit area between two planes, VPP VPP ¼

FSP 2πR

ð1Þ

The Derjaguin approximation is valid in this case because R/κ-1 > 35, where κ-1 is the double-layer thickness.55 Selected normalized force curves were fitted to DLVO theory in the limiting cases of constant surface charge and constant surface potential for the solution to the nonlinear Poisson-Boltzmann equation.56-58 The algorithm was provided by Dr. Johan Fr€ oberg (formerly KTH/YKI, Sweden) and allowed for fitting as a function of ionic strength, surface potential, and Hamaker constant in symmetric electrolytes. Data was simulated for the interactions between a probe and a film with different surface potentials (asymmetric interaction) and van der Waals attraction, FvdW, was calculated with a Hamaker constant, A, of 5  10-21 J,35 with sphere-plane surface geometry, i.e. FvdW ¼ -

AR 6D2

ð2Þ

where D is the probe-film separation distance. The surface potentials obtained from the DLVO fitting should be regarded as apparent values and moreover, non-DLVO forces (bridging, solvation, steric and hydrophobic forces), charge regulation, ion-ion correlation, and ion size effects are not accounted for in the model.27,59

Results and Discussion Surface Properties of Nanocrystalline Cellulose Thin Films. Surface force studies require cellulose films that are smooth and resistant to swelling with minimal molecular fibrillation. NCC films have proven to be one such surface2,35,36 and polyelectrolyte multilayers incorporating NCC and PAH have similar properties. The rms roughness and surface morphology of spin-coated LbL films with different capping layers, either NCC ((PAH/NCC)15) or PAH ((PAH/NCC)15PAH), were found to be indistinguishable. AC-mode AFM images of the LbL films are compared to a spin-coated film composed of pure NCC in Figure 1. (55) Ducker, W. A.; Senden, T. J. Langmuir 1992, 8, 1831. (56) Bell, G. M.; Peterson, J. J. Colloid Interface Sci. 1972, 77, 542. (57) Devereux, O. F.; de Bruyn, P. L. Interaction of Plane-Parallel Double Layers: MIT Press: Cambridge, MA, 1963. (58) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283. (59) Israelachvili, J.; Gourdon, D. Science 2001, 292, 867.

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Table 1. Film Thickness, RMS Surface Roughness, and Water Contact Angle of Films Containing NCC film

film thickness (nm)

rms surface roughness (nm)

contact angle (degrees)

(PAH/NCC)15 (PAH/NCC)15PAH spin-coated NCC

121 ( 4 148 ( 3 169 ( 1

6.2 ( 0.1 6.2 ( 0.1 4.6 ( 0.1

45 ( 3 58 ( 3 30 ( 2

The prepared films were smooth and stable and AFM analysis before and after heat-treatment and exposure to aqueous solution did not show any morphological changes. Features in AFM images of NCC films are often less resolved than in LbL films, presumably due to less surface charge and consequently closer packing of the nanocrystals. We prepared relatively thick films, measured in the dry state by ellipsometry to be ca. 150 nm, to avoid substrate effects contributing to the measured surface forces. The rms surface roughness, film thickness and contact angles for LbL films with different capping layers are compared to a NCC film in Table 1. Contact angle measurements were used to quantify wettability and show that the cellulose films have different surface energies despite having similar chemistry and morphology. Cellulose is hydrophilic, however, a continuum of contact angles has been reported, depending on crystallinity and how the surfaces are prepared.60 Amorphous cellulose is the least hydrophilic (with the highest water contact angles ranging from 35 to 50)9,60-62 and crystalline cellulose is the most hydrophilic. Previously reported contact angles were 19 for dry-cast NCC films50,63 and 23.7 for spin-coated NCC films.60 The contact angle values measured here (Table 1) are within the correct range and importantly, indicate that the surfaces become more hydrophobic with increasing PAH content. Furthermore, it is evident that even when NCC is the outermost layer in the LbL film, some PAH chains are present at the surface contributing to the material’s surface energy. Normal Surface Forces. Interaction Between a Silica Probe and a (PAH/NCC)15 Multilayer Film. Measuring precontact normal surface forces gives useful physicochemical information, such as the surface potential, and provides insight into the dominant intersurface interactions. To demonstrate that LbL films containing NCC and PAH were suitable for direct surface force measurements by CP-AFM, we started with silica probes which have been extensively characterized in the past.55,64,65 Figure 2 shows the interaction between (PAH/NCC)15 and a silica probe as a function of ionic strength, at pH 5.5. DLVO theory fits the € (60) Aulin, C.; Ahola, S.; Josefsson, P.; Nishino, T.; Hirose, Y.; Osterberg, M.; Wa˚gberg, L. Langmuir 2009, 25, 7675–7685. (61) Luner, P.; Sandell, M. J. Polym. Sci. Pol. Lett. 1969, 115. (62) Dankovich, T.; Hsieh, Y.-L. Cellulose 2007, 14, 469. (63) Yamane, C.; Aoyagi, T.; Ago, M.; Sato, K.; Okajima, K.; Takahashi, T. Polym. J. 2006, 38, 819. (64) Giesbers, M.; Kleijn, J. M.; Cohen Stuart, M. A. J. Colloid Interface Sci. 2002, 248, 88. (65) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207.

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Cranston et al. Table 2. Fitted Apparent Surface Potentials and Calculated Surface Charge Densities for the Interaction between (PAH/NCC)15 and a Silica Probe at pH 5.5 apparent surface apparent surface apparent surface potential for potential for charge density ionic strength silica probe LbL film for LbL film (mM) (mV) (mV) (e/nm2) 0.5 1.0 10

Figure 2. Averaged normalized force-distance curves for the interaction of (PAH/NCC)15 and a silica probe at pH 5.5 and ionic strengths of 0.5 (circles), 1 (squares) and 10 mM (triangles). Solid lines are theoretical fits for the limit of constant surface charge and dotted lines are for the limit of constant surface potential. The corresponding fitted apparent surface potentials are reported in Table 2. Inset: Force-distance curves on a log-linear scale, with fits.

averaged experimental data well (in between the limits of constant surface potential and constant surface charge). Two points are noteworthy: the observed forces were monotonically repulsive, as expected, and the forces were larger when solution conditions allowed for a thicker double-layer. Cellulose nanocrystals have an anionic surface from sulfate ester groups and may contain some residual carboxylic acid groups as well. The apparent pKa’s of these moieties are approximately 2 and 5, respectively.35,39 Therefore at pH 5.5, the LbL film surface is highly charged and the silica probe is also anionic (above a pH of 3 from surface SiO- groups).65,66 Purely repulsive electrostatic double-layer forces were observed due to the overlap of diffuse electric double-layers. Increasing the concentration of background NaCl in the aqueous environment (i.e., increasing the screening) decreases the double-layer thickness and results in less repulsion at all separations. This trend was clearly seen (Figure 2) as the ionic strength was increased from 0.5 to 10 mM. The fitted apparent surface potentials for Figure 2 and the calculated surface charge densities are shown in Table 2. The same forces, trends and apparent surface potential values were also found at pH 8 and are shown in the Supporting Information. (The term “apparent” is used in recognition of the limitations of the model, and it should be noted that the sign of the surface potential cannot be deduced from the model but is known e.g., from ζ-potential measurements.) The absolute value of the LbL film surface potential decreased with increasing ionic strength (for low ionic strengths, i.e., 0.5 and 1 mM); this is qualitatively in accordance with the Grahame equation for low potentials under constant surface charge conditions.27 However, at higher salt concentrations (10 mM) the film surface potential is significantly larger. The surface potential of -30 mV for the silica probe at 10 mM agrees with previous studies of symmetric silica systems,64 nonetheless the exact choice of surface potential is arguably within the range of -30 to -40 mV which would correspond to a LbL film surface potential that ranges from -70 to -55 mV; in all cases a substantial increase in magnitude of the film surface potential is (66) Wang, J.; Feldberg, S. W.; Bard, A. J. J. Phys. Chem. B 2002, 106, 10440.

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-60 -50 -30

-40 -30 -70

0.014 0.014 0.130

required to fit the 10 mM data. This is attributed to swelling, which is known to be extensive in LbL films,23,29,67 resulting in pronounced Donnan equilibrium effects along with other charge regulating effects.35 The trend in apparent surface potentials for the LbL film is similar to that measured for a pure NCC film (with an amorphous cellulose probe) by Notley et al.35 The calculated surface potential should be related to the measured ζ-potential and depends on the exact location of the plane of charge, which cannot be determined precisely in CPAFM measurements.64,65 By assuming that the charge lies at the hard-wall contact point, our fitted potentials should be larger than the determined ζ-potential.28 The ζ-potential value for NCC, measured by electrophoresis at neutral pH with no added salt was -39 ( 3 mV, therefore the potentials in Table 2 appear highly reasonable. At high ionic strength, substantial swelling of the LbL film leads to the largest discrepancy between fitted values and ζ-potential, as seen previously.28 Additionally, we can compare calculated values of the surface charge to those determined by conductometric titration. For this NCC in suspension, the surface charge density at pH 7 with ionic strength >10 mM was 0.41 ( 0.05 e/nm2 which is larger than the DLVO calculated value, 0.13 ( 0.05 e/nm2, but the fitted charge density is still within a physically reasonable range. Charge densities vary depending on the NCC preparation conditions,68 and moreover, the heat process used to stabilize the prepared films is expected to substantially reduce the surface charge.34,50 Additionally, there are many charge regulating effects, either from counterions or the cationic polymer within the film, or from species binding to the surface which may cause the surface charge density to be lower than that determined for NCC in solution.27,64 In contrast, at an acidic pH of 3.5, attractive forces dominated between the LbL film and the silica probe (Figure 3). Under these solution conditions both NCC and the silica probe are weakly anionic, however, PAH, the underlying layer in the LbL film, is cationic and fully charged. (The pKa of PAH is approximately 8.8 and according to previous work it is fully ionized at pH values lower than 6.69,70) The observed attractive forces are attributed to polymer bridging between PAH and the silica probe. To a lesser degree there is likely a contribution from both electrostatic double-layer attraction (between PAH and the probe) and steric repulsion. Extensive swelling of the multilayer film at low pH is expected due to a reduction in the cross-link density from the minimally charged NCC. Again, as seen at pH 5.5 and 8, Donnan equilibrium effects and screening of surface charges are pronounced at high ionic strength which may also lead to film swelling. Under such conditions, PAH is less strongly bound within the film. A swollen film, with a minimally charged NCC capping layer could (67) Tanchak, O. M.; Barrett, C. J. Chem. Mater. 2004, 16, 2734–2739. (68) Beck-Candanedo, S.; Roman, M.; Gray, D. G. Biomacromolecules 2005, 6, 1048. (69) Goicoechea, J.; Arregui, F. J.; Corres, J. M.; Matias, I. R. J. Sens. 2008, 142854, 1. (70) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116.

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Figure 3. Average normalized force-distance curves for the interaction of (PAH/NCC)15 and a silica probe at pH 3.5 and ionic strengths of 0.5 (circles), 1 (squares), and 10 mM (triangles). Inset shows a schematic representation of the PAH chains extending and bridging more at high ionic strength at an approximate separation distance of 40 nm. (NCC: blue rods, PAH: green line, not drawn to scale.)

facilitate PAH polymer chain migration from within the film to the surface allowing PAH chains to extend even further into solution. It should be kept in mind that even at pH 5.5 some surface PAH chains are present for a NCC-capped film, as indicated by contact angle measurements. Although every LbL system reported in the literature has its uniqueness, it is generally believed that substantial interpenetration between layers occurs;23 however, the actual vertical migration of species within the film has been complicated to probe. Successful attempts have included monitoring fluorescently labeled polyelectrolytes by confocal laser scanning microscopy or fluorescence resonance energy transfer.71-73 These studies concluded that in LbL films, some types of polyelectrolytes may migrate throughout the entire film (nanometers to micrometers in thickness)71 while others remain stationary, such as hyaluronic acid in hyaluronic acid/poly(L-lysine) films.72 In all cases, movement is dependent on pH, ionic strength, and the charge of the capping layer but the interplay between these factors is unclear. Overall, it appears that the polymer mobility is increased when the amount of uncompensated charge within the film increases.72 In our case, the geometrical constraints of the large NCC crystals must limit diffusion within the film to a certain extent; however, polyelectrolyte migration to the surface is expected when the LbL film is less dense, the charge on NCC is low (and thus the amount of uncompensated cationic charge is high), and when the surrounding media is a “good” solvent for the polyelectrolyte. All of these criteria are met at low pH and high ionic strength in NaCl solution implying that migration of PAH to the surface of the LbL films is highly probable. Once the polyelectrolyte has migrated to the surface, polymer bridging can then be expected when there is an attractive interaction between the polymer chains and the opposing surface (in this case, the probe). The extent of bridging is highly dependent on the availability of binding sites on the probe, and the surface coverage of the polymer coated surface. When the polymer coated (71) Lundin, M.; Blomberg, E.; Tilton, R. D. Langmuir 2010, 26, 3242–3251. (72) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J.-C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159–1162. (73) Richert, L.; Lavalle, P.; Payan, E.; Shu, X. Z.; Prestwich, G. D.; Stoltz, J.-F.; Schaaf, P.; Voegel, J.-C.; Picart, C. Langmuir 2004, 20, 448–458.

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surface is brought close to the probe, an initial attractive force is measured due to the favorable PAH-probe interaction which then switches to an overall repulsive force due to chain confinement and steric hindrance. This type of bridging interaction can be strong and long ranged and exceeds van der Waals attraction in most cases.27 The observed interaction in Figure 3 is not due to van der Waals forces or simple electrostatic double-layer attraction, as explained in the Supporting Information. The attractive forces measured at pH 3.5 were strongly ionic strength dependent. The shift from low to high ionic strength corresponded to an increase in attractive forces which indicates stronger bridging from polymer chains which were more loosely held in the film at high ionic strength. A PAH chain which reaches further from the film surface due to its swollen nature, would be able to bridge more effectively at long distances and give stronger attractive forces, as seen in Figure 3. To our knowledge, this is the first study showing that underlying layers in a multilayer film can dictate the dominant surface interactions. The approximate contour length of a fully extended 60 kD PAH chain is 160 nm (640 repeat units  0.25 nm/repeat unit). In the case of high ionic strength, where the film is swollen and PAH is loosely bound, Figure 3 shows that the bridging attraction starts at 45 nm from the surface, which seems physically reasonable. Higher ionic strengths lead to larger measured attractive forces and the onset of the bridging force occurs at larger separation distances. Experimentally and theoretically, bridging forces have been shown to decrease exponentially with distance with a decay length that is close to the value of the unperturbed radius of gyration (Rg) of the polymer on the surface.27 Exponential fitting of the curves in Figure 3 gives Rg values of 13, 22, and 46 nm for 0.5, 1, and 10 mM NaCl, respectively. In most other CP-AFM studies of pure NCC films, attractive forces were not observed or were quickly screened by steric repulsion,35,36 implying that this is an effect of the composite material and not just the capping layer. Those studies showed that long-range steric interactions and possible redispersion of cellulose nanocrystals in solution were problematic despite the high crystallinity of the samples.36 For this reason, our films were heat treated and imaged before and after measurements to ensure film stability. We also collected force curves on a clean Si wafer after each set of experiments to check for material transfer from the film to the probe. This leads us to believe that PAH was able to extend relatively far from the LbL film, and that the extension increased with the degree of swelling, however, very little desorption of the polymer from the film occurred. Moreover, we can conclude that the ionic-cross-linking and extensive interpenetration of layers, responsible in general for LbL film stability, has led us to cellulose-polymer surfaces which are able to swell and rearrange, without delaminating or disassembling, which is ideal for CP-AFM measurements. As such, these LbL films have given us new insight into the role of the underlying layers on surface interactions. Interaction Between an Amine-Functionalized Probe and a (PAH/NCC)15 Multilayer Film. Quantitative force measurements and chemical force microscopy require functionalized probes with well-defined surface chemistry74 which is often achieved by functionalizing gold-coated probes with self-assembled monolayers (SAMs). The SAMs are made-up of organic thiols terminated with selected chemical groups, in this case an 11 carbon chain alkanethiol ending with an amine group was used. The interaction profile between an amine-functionalized probe and a (PAH/NCC)15 multilayer film at pH 3.5 and 8 is shown in (74) Noy, A. Surf. Interface Anal. 2006, 38, 1429.

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Figure 4. Averaged normalized force-distance curves for the

Figure 5. Averaged normalized force-distance curves for the

interaction of (PAH/NCC)15 and an amine-functionalized probe at an ionic strength of 0.5 mM and pH 3.5 (triangles) and pH 8 (squares). The theoretical fits shown are for -60 mV probe/-40 mV LbL film (pH 8) and þ20 mV probe/-30 mV LbL film (pH 3.5) in the limit of constant surface charge (solid lines) and the limit of constant surface potential (dotted lines).

interaction of (PAH/NCC)15 and a carboxylic acid-functionalized probe at an ionic strength of 0.5 mM and pH 3.5 (triangles) and pH 8 (squares). The theoretical fit for a probe apparent surface potential of -70 mV and LbL film apparent surface potential of -40 mV in the limit of constant surface charge (solid line) and constant surface potential (dotted line) is also shown.

Figure 4. The sign of the forces at both pH values is the same as for the silica-LbL film interaction, i.e., repulsive at pH 8 and attractive at pH 3.5, although the explanation of these dominant forces is different. At pH 3.5 the attractive forces are due to electrostatic doublelayer attraction between the weakly charged cationic probe and anionic LbL film. DLVO fitting gives an apparent surface potential for the probe of þ30 mV, which agrees with ζ-potential measurements of amine SAMs at low pH,75 and -20 mV for the LbL film. The jump-to-contact instability in the curve, due to the oppositely charged surfaces, makes accurate fitting difficult.66 A slight repulsion in the pH 3.5 curve (Figure 4) around 15-30 nm is likely a combination of steric repulsion and electrostatic doublelayer repulsion between cationic PAH chains and the cationic probe. (The onset of this repulsion corresponds to the same distance at which bridging attraction begins between the LbL film and the silica probe at pH 3.5 and 0.5 mM, Figure 3.) At pH 8, monotonically repulsive forces of a double-layer repulsion-type are observed which correspond to apparent surface potentials of -60 and -40 mV for the probe and film, respectively. While somewhat surprising that the amine probe is negatively charged, these findings concur with previous work showing that amine SAMs75,76 and adsorbed amine-functionalized surfactants77,78 are negatively charged when subjected to pH conditions well above their pKa. Moreover, the exact pKa for this SAM system is unknown since the acid-base equilibrium shifts when chemical species are adsorbed on a surface and the characterization of such functionalized colloidal particles is limited.79 While amines in solution generally have a pKa of 9-10, the apparent pKa of amine SAMs has been found to range from 3-5.75,80 Thus the cationic charge density of both PAH in the film and the amine-functionalized probes decreases with increasing pH, although the exact isolelectric points are unknown. More generally, we note that the dramatically reduced pKa of amine (75) Shyue, J.-J.; Guire, M. R. D. Langmuir 2004, 20, 8693. (76) Wallwork, M. L.; Smith, D. A. Langmuir 2001, 17, 1126. (77) Claesson, P. M.; Herder, P. C.; Rutland, M. W.; Waltermo, A˚.; Anhede, B. Prog. Colloid Polym. Sci. 1992, 88, 64. (78) Castro, S. H.; Vurdela, R. M.; Laskowski, J. S. Colloids Surf. 1986, 21, 87. (79) Burke, S.; Barrett, C. J. Langmuir 2003, 19, 3297. (80) Sheikh, K. H.; Evans, S. D.; Christenson, H. K. Langmuir 2007, 23, 6893.

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SAMs significantly limits the range of pH over which they can be applied as model cationic surfaces. Interaction between a Carboxylic Acid-Functionalized Probe and a (PAH/NCC)15 Multilayer Film. The interaction profile between a carboxylic acid-functionalized probe and a (PAH/NCC)15 multilayer film at pH 3.5 and 8 is shown in Figure 5 and indicates that the charge density on a COOH SAM-coated probe is larger than for a silica probe. Again, the dominant forces at pH 8 are repulsive and at pH 3.5 are attractive. The doublelayer repulsive forces are larger in magnitude than in the case of the silica probe giving fitted apparent surface potential values of -70 (probe) and -40 mV (film). The attractive forces at pH 3.5 are a result of polymer bridging from the underlying layer but are longer ranged and stronger than with the silica probe. The larger surface potential of the COOH probe, compared to the silica probe, combined with the possibility of hydrogen bonding between undissociated COOH and PAH evidently leads to a more stable bridging force, manifested as a deeper minimum in the attractive curve (-0.1 mN/m for the COOH probe, Figure 5, and -0.06 mN/m for the silica probe, Figure 3). Comparing LbL Capping Layer Interactions With a Carboxylic Acid-Functionalized Probe. The effect of the LbL capping layer on the dominant surface interactions was investigated with a carboxylic acid probe at pH 8 (Figure 6). The NCC and probe are fully anionic and PAH is weakly cationic, under these solution conditions. The forces are mainly repulsive, due to electrostatic double-layer interactions, despite the Coulombic attraction between cationic PAH and anionic COO- groups. The purely repulsive nature of the curve for the NCC cappedfilm in Figure 6 is due to electrostatic double-layer repulsion as described for Figure 5. The force curve for the PAH-capped film shows a shallow but long-range attraction and a strong shortrange repulsion. These interactions can be ascribed to a combination of electrostatic double-layer attraction (between weekly cationic PAH and anionic COOH) and steric repulsion (between the dense, but collapsed at high ionic strength, polymer layer). The diffuse nature of PAH-coated surfaces leads to force curves that are not described by DLVO theory. From these results we can clearly see that the dominant interaction in polymer covered surfaces is dependent on both Langmuir 2010, 26(22), 17190–17197

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Figure 6. Averaged normalized force-distance curves for the

interaction of (PAH/NCC)15 (.) and (PAH/NCC)15PAH (solid :) with a carboxylic acid probe at pH 8 and an ionic strength of 10 mM.

the polymer chain density and the charge density; strong attraction due to bridging is operative for low density and high charge (i.e., when only a few polymer chains extend past the capping NCC layer, Figure 5, pH 3.5) but weak attraction followed by repulsion due to steric hindrance and compression of the polymer layer is seen at high polymer chain density and low charge (i.e., when PAH is the outermost layer, Figure 6, lower curve). In general we observe that for both capping layers, stronger repulsive forces were measured at low ionic strength, where the double-layers are larger and PAH chains are more extended. The effect of ionic strength is more pronounced for the PAH-capped films as a result of the greater collapsibility of the PAH chains (data not shown). Importantly, Figure 6 implies that the dominant surface interactions can be tailored in nanocrystalline cellulose systems by the addition of polyelectrolytes, particularly when the capping layer contains the polymer.

Conclusions Polyelectrolyte multilayer films containing NCC and PAH were found to be ideally suited for surface force measurements using the CP-AFM technique. The characteristics of the forcedistance curves were strongly dependent on how the solution conditions affected the composite film structure. Monotonically repulsive forces were observed between highly charged surfaces of the same sign and were easily fitted by DLVO theory in between the limits of constant surface charge and constant surface potential. Repulsive forces were due to electrostatic double-layer interactions which were screened at high ionic strength. Swelling and Donnan equilibrium effects were non-negligible under acidic conditions, and at high salt concentrations, and

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should not be overlooked for LbL films of all types. The underlying PAH layer played an increasingly important role when the films were swollen and chain extension/migration was facilitated: For negatively charged probes and NCC-capped films, attractive forces dominated at pH 3.5 where the film was swollen and highly charged PAH chains were able to extend into solution. This attractive interaction was attributed to polymer bridging enhanced by electrostatic double-layer attraction between the oppositely charged PAH chains and the probe. When the density of surface PAH chains was low and the surface charge high, (i.e., NCC-capped films at low pH) bridging forces dominated; however, for PAH-capped films where the surface chain density was high and surface charge was low, steric repulsion dominated. The fitted apparent surface potential values for the silica probe agreed with previous studies and the apparent surface potentials for the LbL films were on the order of -50 mV for an NCCcapped film. Varying the solution conditions greatly affected the surface charge density for both silica and SAM-functionalized probes. Moreover, a negatively charged probe tended to be better for measuring negatively charged surfaces and vice versa, avoiding jump-to-contact type instabilities in the force curves. Overall, the interactions between such LbL films are more complicated than those that would be displayed by the capping layers alone. The underlying layers also contribute; the interface behaves as a composite material and interacts selectively with the opposing surface. Depending on solution conditions, steric forces and electrostatic double-layer forces have different relative contributions to the interaction. The ability to tailor surface interactions between cellulose, polyelectrolytes and charged particles is promising. The responsive behavior of these LbL films, leading to consistent and strong attractive forces only at low pH, may prove useful in further applications and fundamental studies of cellulose nanocomposites. Acknowledgment. We thank NSERC Canada, FPInnovations Paprican, FQRNT, and Biomime, the Swedish Centre for Biomimetic Fibre Engineering (www.biomime.org) for financial support and Professors R. B. Lennox and T. van de Ven for use of laboratory equipment. The DLVO simulation program was generously provided by Dr. J. Fr€oberg (KTH/YKI, Sweden). Useful discussions with Dr. N. Nordgren, Dr. K. Yager, and Prof. T. van de Ven are gratefully acknowledged. M.R. is a fellow of the Swedish Research Council. Supporting Information Available: Description of force curve processing and data reproducibility, interaction between a silica probe and a (PAH/NCC)15 multilayer film at pH 8, and further discussion of silica probe and (PAH/ NCC)15 multilayer film interactions at pH 3.5. This material is available free of charge via the Internet at http://pubs.acs.org.

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