Counterion Size and Nature Control Structural and Mechanical

Article pubs.acs.org/Biomac

Counterion Size and Nature Control Structural and Mechanical Response in Cellulose Nanofibril Nanopapers Alejandro J. Benítez and Andreas Walther* Institute for Macromolecular Chemistry, Albert-Ludwigs-University Freiburg, Stefan-Meier-Straße 31, 79104 Freiburg, Germany Freiburg Materials Research Center, Albert-Ludwigs-University Freiburg, Stefan-Meier-Straße 21, 79104 Freiburg, Germany Freiburg Center for Interactive Materials and Bioinspired Technologies, Albert-Ludwigs-University Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany S Supporting Information *

ABSTRACT: Nanopapers formed from aqueous dispersions of cellulose nanofibrils (CNFs) combine stiffness, strength, and toughness. Yet, delicate interactions operate between the CNFs during nanopaper formation and mechanical deformation. We unravel in detail how counterions, being either of the organic alkyl ammonium kind (NR4+) or of the earth metal series (Li+, Na+, Cs+), need to be chosen to achieve outstanding combinations of stiffness, strength, and toughness, extending to previously unreached territories. We relate structure formation processes in terms of colloidal stabilization to nanostructural details such as porosity and ability for swelling, as well as to interfibrillar interactions in bulk and macroscale mechanical properties. We demonstrate that our understanding also leads to new levels of ductility in bioinspired CNF/polymer nanocomposites at high levels of reinforcements. These results contribute to future rational design of CNF-based highperformance materials.

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INTRODUCTION Cellulose nanofibrils (CNFs) emerge as an attractive building block for sustainable nanoscience applications due their renewable origin, simple preparation, easy functionalization, and facile processing into films, fibers, nanocomposites, and 3Dprinted materials.1−11 CNFs can form nanopapers with excellent mechanical properties by drying from aqueous suspension, whereupon the nanofibrils form an entangled network (colloidal glass) with strong interfibrillar hydrogen bonding.11−16 Various factors influence the final structure and interactions. For instance, the fibrillation method controls surface chemistry, aspect ratio, size distribution, and crystallinity of the nanofibers.17 The film preparation method, i.e., film casting or filtration, addition of stabilizing polymers, and the pH strongly influence the nanopaper structure beyond simple entangled networks.11,16,18−20 Preventing excess flocculation during the processing aids in realizing better mechanical properties (in particular strain-to-failure, toughness).12,16,18 While there are increasing articles addressing mechanical properties, it is still a challenge to comprehensively understand the design criteria for a rational mechanical performance increase of nanopapers. For example, rationally designed enhancements in toughness by combining CNFs with polymers can be brought forward by changing the glass transition temperatures or the volume fraction of the polymers, as long as coagulation or illdefined structures are prevented.11,21,22 The general intention is to introduce soft components capable of improving frictional interfibrillar sliding. It was also clearly shown that ill-defined © 2017 American Chemical Society

nanopaper structures formed from coagulated dispersions lead to inferior properties, while tailored porosities in defined networks can enhance ductility, yet at the expense of stiffness and strength.11−13,16,19,23 Increase in strength and stiffness by combining CNFs with multivalent metal ions, clay, or carbon nanotubes has been a viable approach.24−29 Furthermore, orientation by cold drawing or wet stretching has been a key approach to enhance mechanical properties in a unidirectional way by alignment of the CNFs.7,28−30 Two key aspects emerge when aiming at improved mechanical properties, that is, well-defined structure formation and providing components to manage the interfibrillar interactions.4,12,18,31 For both factors, it is important to control the colloidal solution properties, that primarily depend upon the chemical pretreatment during pulp processing.32 TEMPO oxidation is one of the most common and straightforward pretreatments, leading to very thin CNFs having carboxylate groups on the surface that introduce pH-dependent electric double layer repulsion in dispersions and improve compatibility in bulk materials.5,33,34 Nanopapers derived from TEMPO− CNF exhibit remarkable mechanical properties with a Young’s modulus between 13 and 15 GPa and strength of 230−300 MPa.5,16 Recently, Shimizu et al. showed that ammonium counterions with different alkyl chain length (methyl to butyl) Received: February 21, 2017 Revised: March 27, 2017 Published: March 28, 2017 1642

DOI: 10.1021/acs.biomac.7b00263 Biomacromolecules 2017, 18, 1642−1653

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The Na+ ions in the original CNF-Na+ suspension were exchanged as follows: (i) 0.25 wt % of CNF-Na+ dispersions were prepared by addition of Milli-Q water, (ii) adjustment to pH = 3 with HCl to form CNF-COOH, (iii) centrifugation for 60 min at 8000 rpm to collect a pellet of protonated CNF-COOH and remove excess NaCl (this step was done twice), (iv) redispersion of CNF-COOH to pH 9 with hydroxide solutions of the different counterions forming CNF-Me+ or CNF-QA+ for metals and quaternized ammonium counterions, respectively. Then, (v) the dispersions were placed in dialysis at pH 9 (for 72 h) with the corresponding hydroxide solution to ensure maximum ion exchange and removal of impurities. Finally, (vi) the CNFs dispersions were filtered using a 50 μm pore size mesh sieve to remove dust and other large particles, followed by passing the dispersions through a 20 μm pore size mesh. Atomic Force Microscopy (AFM). A NanoScope V AFM (Digital Instruments Veeco Instruments, Santa Barbara, CA) operating in tapping mode was used to record the micrographs. The samples were obtained by dip-coating from diluted suspensions in water (ca. 0.005 wt %) onto freshly cleaved mica. Preparation of the Nanopapers. Vacuum Filtration (Nanopapers). Typically, 250 mL of a 0.25 wt % dispersions were filtered through 1.0 μm Omnipore filters during ca. 24 h. Afterward, the hydrogel films were dried in an oven at 60 °C for 5 days under pressure (approximately 2.5 kPa) to yield films with thicknesses of 30−40 μm. The densities were measured by weighing a piece of nanopapers of known dimensions at 50% RH. Adsorbed water is included in the density value. Solvent Casting (Polymer/CNF Nanocomposites). 60 mL of the 0.25 wt % suspensions were added to Petri dishes at room temperature to give films of ca. 22 μm thickness. Elemental Analysis. Alkali earth metal content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a PerkinElmer Plasma 400 spectrometer. Nitrogen content was calculated using a Vario EL Element analyzer. Tensile Testing. Mechanical tests were carried out on a DEBEN minitester equipped with a 200 N load cell at room temperature. The specimen sizes used were typically in the range of 10 mm × 1.25 mm × 40−50 μm. At least five specimens were tested for each condition. A nominal strain rate of 1.5 mm/min was used. The slope of the linear region of the stress−strain curves was used to determine the Young’s modulus (E). The yield points were determined by the intersection of the two lines of the elastic region with the strain hardening region.11 The samples were conditioned at different relative humidities (RH) in the range of 20 to 99% for at least 2 days and measured at this RH. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX). The cross sections of fractured samples were imaged using a Hitachi S-4800 field emission microscope (1−1.5 kV) after sputter-coating a thin Au/Pd layer. EDX spectra were recorded in a Hitachi S-3000N SEM at 30 kV acceleration voltage. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded using a Thermo Nicolet Nexus 470 spectrometer with a smart split ATR single reflection Si crystal from 1800 to 1500 cm−1 with a resolution of 4 cm−1. Water Sorption. Water sorption isotherms were recorded with a HIDEN ISOCHEMA IGAsorp device at 25 °C in a range of 5−95% of RH. Samples (ca. 20 mg) were exhaustively dried at 60 °C for 24 h before starting the test. Thermoporosity. Nanopapers were cut into ca. 4 × 4 mm and placed in aluminum pans with Milli-Q water with a moisture content between 1−1.5 g/g for 24 h until reaching equilibrium swelling. Samples (ca. 10 mg) were encapsulated in aluminum sealed pans, and ultradry high purity dry nitrogen was used as an inert atmosphere for all tests in a PerkinElmer DSC8500 calibrated with cyclohexane, indium, and zinc. The range span from −35 to 20 °C with scanning rates of 1 and 10 °C/ min for the isothermal step and dynamic (heating and cooling) tests, respectively.38 The following procedure was carried out twice for each sample: Isothermal Steps for Porosity Size Distribution (PSD). 1. The sample was cooled from 20 °C, at 10 °C/min, to −35 °C to crystallize all water in the sample.

can lead to elongation enhancements by providing interfibrillar slippage. However, the results indicated a nonlinear relationship between the alkyl chain length and the mechanical properties. Maximum elongation was found for ethyl (and not butyl), which is counterintuitive and remains difficult to understand.35 Hence, while the toughness enhancements are promising, more understanding on the origins of such nonlinear behavior is very desirable. At the same time, we hypothesized that the modification of the ionic binding strength between the carboxylic groups on the CNF surface and their counterions may be a crucial aspect to modify the system behavior beyond interfibrillar interactions in bulk. It also determines CNF suspension stability and hence structure formation during filtration. In a previous study, we demonstrated the importance of the colloidal state by showing that CNF suspensions at low pH form aggregates and flocs that are translated to grains inside the nanopapers reducing density, strength, and toughness.16 Alkali metal counterions (Li+, Na+, K+, Cs+) are known to have a complex adsorption behavior that often depends on the detailed surface under study, whether hydrophilic or hydrophobic and regardless of what other chemical groups are present. Hofmeister series on hydrophilic nanoparticles in suspensions show higher affinity for the smaller counterions.36,37 Since colloidal stability is decisive in defining structures and porosity during nanopaper formation or filtration, it is fundamentally important to comprehensively understand mechanical behavior beyond reduction to interfibrillar interactions. Herein, we present a detailed study on the effect of different counterionsa full series of alkali metal ions and alkyl ammonium ionson the colloidal solution properties, the structural and mechanical response, and deformation mechanisms of bulk nanopapers prepared by vacuum filtration. From thermoporosity, swollen nanopapers present a pore formation capability depending on the counterion size, showing that pores act as potential energy dissipators during deformation to improve toughness. We present a systematic study of the colloid stability by checking light transmittance, total interaction potential energy from DLVO theory, aggregation by salinity, and sedimentation speed to differentiate dispersion stabilities and explain the observed nanostructures and properties from different angles. Finally, a comparison with CNF/polymer bionanocomposites illustrates advantages of the counterion exchange, and links advances in nanopapers to advantages in nanocomposite science.

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EXPERIMENTAL SECTION

Materials and Counterion Exchange. Never dried TEMPOoxidized Kraft pulp oxidized under neutral conditions was diluted to 1.4 wt % and set to pH 9 with NaOH. The content of carboxyl groups is 0.44 mmol/g, the degree of polymerization measured by viscosimetry (DPv) is 725 and crystallinity is 77%.11 The suspension is called CNF-Na+, and was homogenized in a microfluidizer MRT CR5 by applying four shear cycles (1400, 1000, 1000, and 1000 bar). Standard sodium hydroxide (NaOH, 1 M), standard hydrochloric acid (HCl 1 M), lithium hydroxide monohydrate (LiOH·H2O, solid), cesium hydroxide monohydrate (CsOH·H2O, solid), ammonium hydroxide (AOH, 25 wt % in H2O), tetraethylammonium hydroxide (TEAOH, 40 wt % in H2O), tetrabutyl ammonium hydroxide (TBAOH, 40 wt % in H2O), tetrahexyl ammonium hydroxide (THAOH, 40 wt % in H2O), sodium chloride (NaCl), lithium chloride (LiCl), cesium chloride (CsCl), ammonium chloride (ACl), tetraethylammonium chloride (TEACl), tetrabutyl ammonium chloride (TBACl), and tetrahexyl ammonium chloride (THACl) were purchased from SigmaAldrich in the highest purity available. 1643


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Biomacromolecules Scheme 1. Preparation of CNF Nanopapers with Different Counterionsa

a

Washed and dialyzed CNF-COOH suspensions are neutralized to pH = 9 using alkali metal (Li+, Na+, Cs+) or quaternized ammonium (A+, TEA+, TBA+, THA+) hydroxide solutions. The counterions locate as a sheath around the CNF in bulk after filtration.

2. Isothermal melting step: the sample was heated at 1 °C/min from −35 °C to the selected isothermal temperature (Tm). Then the sample was held for enough time to complete the melting transition. This step was repeated for the isothermal temperatures listed in Table S1. The full program is presented in Figure S1a with CNF-Na+ as example. The melting enthalpy of each step (Hm) was calculated for the area under the curve and used to determine the amount of water via the specific enthalpy of water (334 J/g). Dynamic Step for Bound Water Content Determination.

velocities of CNF dispersions with salt solutions. The dispersions were placed in PC 110-132XX cells and centrifuged at 2000 rpm and 25 °C.

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RESULTS AND DISCUSSION Ion Exchange Efficiency. The first step in our study is to understand the counterion exchange efficiency of the CNFs in dispersion and their final content after nanopaper preparation (filtration), which we analyzed using Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX), and elemental analysis. The CNFs are prepared by TEMPO-mediated oxidation at neutral conditions with a charge density of 0.44 mmol/g, and subsequent microfluidization, yielding micrometer-long CNFs with diameters of 2.0−2.5 nm (Figure S3a).11 We applied a stringent and highly standardized protocol to ensure the counterion exchange. First, we purposely induced aggregation by setting 0.25 wt % dispersions of CNFNa+ to pH = 3 with HCl. This protonates the COO− groups on the CNF surface, and the CNF-COOH dispersion tends to coagulate and can be pelletized by centrifugation.40 This pellet can be redispersed easily using the appropriate metal and alkyl ammonium hydroxide solutions set to pH = 9 (Scheme 1). Repeating the precipitation and redispersion step and additional dialysis against the hydroxide solutions (pH = 9) ensure complete counterion exchange. The corresponding samples are called CNF-Me+ (Me+ = Li+, Na+, Cs+) for the alkali metal ions, and CNF-QA+ (QA+ = ammonium (A+), tetraethylammonium (TEA+), tetrabutyl ammonium (TBA+) and tetrahexyl ammonium (THA+)) for nanopapers with quaternized ammonium counterions. Finally, we placed the different dispersions into vacuum filtration units to obtain transparent nanopapers (Figure S3). From previous studies it is known that well-stabilized CNFs with homogeneous and clear dispersions produce better structured nanopapers.16 This necessitates performing the vacuum filtration from high pH (pH = 9). We chose vacuum filtration over film casting because there is no concentration increase (only at the filter cake), preventing agglomeration of the CNFs in the bulk solution, and because excess hydroxide solution can be washed out. After vacuum filtration, we trace the sodium content with EDX spectroscopy (Figure 1a,b). Sodium (Kα = 1.041 keV) is detectable only for CNF-Na+ and eliminated for the other

1. After the last isothermal step at −0.2 °C, the sample was frozen to −35 °C, held for 5 min for stabilization, and then ramped to 25 °C to completely melt the water contained in the pan. The program is presented in the Figure S1b using CNF-Na+ as the example. Two overlapping peaks were obtained (related to freeze bound and free water) and separated by deconvolution using the Gaussian method (Figure S2), and giving two parameters: (i) the area of the complete overlapped peak provides total amount of freezing water (TFW), and (ii) the integration of the peak molten at lower temperature represents the frozen water within the pores. Specific Surface Area. An ASAP 2020 Plus-Physisorption device was employed for Kr gas adsorption experiments. CNF nanopapers were cut into ca. 4 × 4 mm, dried at 60 °C in a convection oven for 24 h, and degassed at 95 °C for 24 h to ensure no moisture effects.39 Adsorption isotherms were recorded at 77 K with relative pressures (P/P0) in the range of 0.05−0.3. Stability of CNF/Water Dispersions. The stability of the CNFs in suspension was evaluated by adding suitable amounts of the corresponding salt solutions to 0.25 wt % of CNF dispersions. The added salts were in correspondence with the counterion previously exchanged, that is, solutions of MeCl or QACl were added to dispersions of CNF-Me+ or CNF-QA+, respectively. Optical Transmittance. Light transmittance of the dispersions (0.25 wt %) was measured at 600 nm using an Ocean Optics UV−vis spectrophotometer with a HL-2000-FHSA light source at 25 °C. Zeta Potential. Zeta potentials of CNF dispersions were measured using a Zetasizer Nano ZS from Malvern at 25 °C. Measurements were performed three times in disposable capillary cells. pH Titration. Titration of CNF-COOH was recorded on a Methrom Titrando 90 at 25 °C. The pH was adjusted in the range of 3−10 with NaOH (0.1 M). Sedimentation Speed. An analytical photocentrifuge LUMiSizer with a wavelength of 865 nm was used to evaluate sedimentation 1644


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anionic carboxylates and the cationic QA+ as the alkyl chain length is increased.41 Finally, the ion substitution can be followed by elemental analysis: for instance, the used TEMPO−CNF possesses 0.44 mmol(COOH)/g. A titration showed that 2.5 mmol/L (∼1 mmol/gCNF) of NaOH is necessary for reaching pH = 9 when titrating from pH = 3 (Figure S4a). After vacuum filtration, its nanopaper shows 0.30 mmol/g of Na+ content by elemental analysis with inductively coupled plasma optical emission spectroscopy (ICP-OES), which demonstrates that excess counterions and base are indeed removed during vacuum filtration. The dried films of CNF-Li+, and all CNF-QA+ have a similar exchange efficiency as judged from the Li content (0.31 mmol/g) and the nitrogen content for QA+ counterions (0.29− 0.31 mmol/g; see Table S2; Cs cannot be measured due to lack of sensitivity). Therefore, these protocols demonstrate similarly efficient and near quantitative counterion exchange. Mechanical Properties and Deformation Mechanisms. Deformation mechanisms in CNF nanopapers are complex, as we deal with a material structured on several length scales, and the presence of counterions on the surface modifies the interfibrillar interactions. We start our analysis by performing tensile tests at a relative humidity (RH) of 50% to break down the effect of each counterion. Second, we observe cross sections of the fractured samples to illustrate further the deformation mechanisms and interfibrillar interactions. After ion substitution, the variation in mechanical response of the nanopapers should be discussed in terms of chemical interactions and the nanostructure. In general, increasing the size of the counterion reduces the bond strength and the ability of forming interfibrillar hydrogen bonds (Me+ and QA+). Interfibrillar hydrogen bonds are further hindered by the introduction of bulky hydrophobic components like the alkyl chains (QA+). The stiffness, Young’s modulus (E), shows low values for CNF-COOH (∼10 GPa; prepared from the aggregated state at pH = 3), that increase drastically when the CNF dispersions are ion-exchanged and processed from high pH to reduce floc formation (Figure 2c and Table S3). The highest values correspond to the smallest ions (Li+ and A+) producing stronger interfibrillar bonds. A subtle decrease of E is obtained as the counterion size increases for the alkali metals. This drop is stronger for CNF-QA+ nanopapers due to the larger size (in comparison with Me+) and further hindrance of interfibrillar hydrogen bonding as the alkyl chains grow and shield surfaces. This agrees with data on large cationic surfactants such as

Figure 1. Analysis of the ion exchange of CNF-Me+ and CNF-QA+ nanopapers. (a,b) EDX spectra and (c,d) FTIR spectra, respectively. CNF-COOH is used as a reference.

nanopapers after ion exchange. The cesium signal (Lα = 4.2865) for CNF-Cs+ indicates proper substitution. Since the signals for lithium and nitrogen (Kα of 0.0544 and 0.392, respectively) overlap with carbon and oxygen, it is only possible to confirm the sodium removal for the other samples. Subsequently, FTIR spectroscopy reveals a high ion exchange efficiency, as most of the carboxyl groups are present as carboxylate, COO− (Figure 1c,d). Differences in ionization can be followed via the absorption bands (CO stretching) for COOH groups at 1720−1740 cm−1 (gray curve for CNFCOOH) and COO− at approximately 1600 cm−1 (shifting from pH 3 to pH 9, respectively). Once the pH is increased (using Me+OH and QA+OH), the relative intensity of the COOH band inverts, and the COO− functionality is dominant (Figure 1c). Nanopapers in the form of CNF-Me+ present no shift for the COO− band as a function of the metal counterion, hence they seem unaffected by the counterion size in bulk (Figure 1c). By contrast, CNF-QA+ nanopapers show a subtle shift from 1600 to 1615 cm−1 in the COO− band for longer alkyl chains (Figure 1d). This shift is evidence of weakened ionic bonds between the

Figure 2. Mechanical tensile properties of nanopapers at RH 50%. Tensile curves for nanopapers with (a) metal CNF-Me+ and (b) quaternized amines CNF-QA+ counterions. (c) Young’s modulus (E, empty symbols) and work of fracture (Ut, filled symbols) as a function of the different counterions. Black arrows in panels a and b show yield points. 1645


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Figure 3. Comparison of stiffness (Young’s modulus E), (a) strength (σb) and (b) toughness (Ut) of CNF nanopapers in Ashby plots: CNF with small ions (green), CNF/clay (blue), CNF/polymer nanocomposites (CNF/EGxDMAmy (purple), and CNF/HEC (magenta)). Our samples are circled in black lines and Shimizu et al.35 samples are in white lines.

modified nanopapers (methyl to butyl).35 We discuss it further below. The work of fracture or toughness (Ut), calculated from the area under the σ−ε curve, shows a continuous increase with ion size (Figure 2c). CNF-COOH shows drastically lower Ut, corroborating that a poor dispersion leads to structures with low mechanical performance.16 The introduction of alkyl chains in the counterions improves the inelastic deformation, yielding tougher nanopapers with outstanding ductility up to a strain-tofailure of above 20% and close to 300 MPa in tensile strength. To comprehensively compare to previous data, we plot E versus σb and E versus Ut in two Ashby plots to provide an instructive comparison between stiffness, strength, and ductility (Figure 3 and Table S5). In general, the best materials are located on the top right of the graphs. The literature contains a wide range of CNF nanopapers (green data) substituted with small ions (mainly with sodium; see Table S5), where it is hardly useful to go into further details because of the large differences in preparation, carboxylate content, fibril size, and aspect ratio. The strongest nanopapers are typically obtained in combination with nanoclays (blue data, 340−510 MPa) and span toughness between 4 and 30 MJ/m3. Finally, CNF/polymer nanocomposites show that promoting deformation with a softer response (lower E and σb) depends on the polymer content (CNF/HEC) or thermal glass transitions (CNF/EGxDMAmy). Our new materials maintain the highest combination of strength and toughness in comparison to any other nanopapers with small ions. CNF-Me+ and CNF-A+ have a competitive stiffness (E = 17−19 GPa) with the best CNF/clay nanopapers, and the CNF-THA+ nanopaper has the highest toughness reported (38 MJ/m3). Specifically, A+ (i.e., NH4+) cations can interact with the carboxylic groups via directional bonding and can form multiple hydrogen bonds.45 The most related work by Shimizu et al. has found a nonlinear evolution of properties in CNF-QA+ nanopapers prepared by solvent casting.35 Specifically, an increase in strain to failure up to 14−18% with a strength of 240−150 MPa for methyl and ethyl substituents (CNF-TMA and CNF-TEA marked with white borders in Figure 3), respectively, while propyl- (CNF-TPA) and butyl-ammonium (CNF-TBA) nanopapers drop to 8% and 12% elongation, respectively, with a strength of 130−140 MPa (see Table S5). This is not the case for our samples, which follow a trend in all properties. We believe that the differences to our

cetyltrimethylammonium bromide, which lead to reduced hydrogen bonding of CNF in bulk and add hydrophobicity as the alkyl length is increased.42−44 CNF-Me+ nanopapers show a correlation of the yield strength, the post yield behavior, and strength at break (σy, σb, respectively) with the size of the ion in the order of Li+ > Na+ > Cs+ (Figure 2a and Table S3). The smaller ion (Li+) allows higher interfibrillar hydrogen bonding and ultimate strength of ∼301 MPa (see Table S3). This is visible in particular in the higher yield strengths and the larger slope of the post-yield behavior. The larger ions (Na+ and Cs+) allow for higher deformability and increase the strain (11−13%, respectively), while leading to lower tensile strength. Increasing the length of the alkyl chains in CNF-QA+ nanopapers shows a similar softening but with further enhancement in deformation (Figure 2b and Table S3): The yield strengths, σy, are reduced from 125 to 105 MPa by increasing the alkyl chain length (A+ to THA+), while σb is maintained (∼300 MPa). The alkyl chains ease interfibrillar sliding allowing to reach an ultimate deformation up to 22% for CNF-THA+ (hexyl chain substituent) without losing mechanical strength. It is worth mentioning, that deformation at yielding (εy) is around ∼1% for all nanopapers, confirming it to be a structural parameter, while yield stress depends on the interfibrillar bonding (type of counterion). The alkyl chains reduce the hydrogen bonding and act as plasticizers. Strikingly, we can observe a second yield point (σy2, εy2) for the largest metal counterion, CNF-Cs+, and for nanopapers substituted with QA+ with alkyl chains (TEA+, TBA+ and THA+). A second yield point represents the activation of motion of a second order structure with lower dynamics (grains and mesoscopic layers).16 This is prevented for the smallest ions (Li+, Na+ and A+) due to the strong combination of hydrogen bonds and ionic bonds. This result clearly shows that increasing the counterion size improves interfibrillar sliding, allowing further deformation without losing strength. Moreover, εy2 is similar for CNF-Cs+, CNF-TEA+, and CNF-TBA+, indicating again its relation to a similar nanostructure (Table S4). Interestingly, CNF-THA+ is the only nanopaper able to translate the second yield point, εy2, to higher values. This phenomenon is an indication for a slightly different nanostructure. It is important to realize that our results for the QA+ nanopapers do not show nonlinearities, as found in an earlier study for alkyl ammonium1646


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Figure 4. Cross-section micrographs of nanopapers fractured at RH 50%. CNF-Me+: (a) Li+, (b) Na+, and (c) Cs+. CNF-QA+: (d) A+, (e) TEA+, (f) TBA+, and (g) THA+ (scale bar = 5 μm).

CNF-QA+ samples arise mainly from the preparation method. Longer preparation times by solvent casting for hydrophobically modified CNFs (e.g., CNF-TBA) may lead to flocculation in the dispersion due to constant concentration increase. Additionally, excess counterions are not washed out. Next, we analyze the fracture surfaces to understand differences in deformation mechanisms (Figure 4). In general, a layered structure is observed with different levels of pull-out phenomena. More pronounced nanofibril and mesoscale layer pullout phenomena occur as the counterion size increases. The CNF-Me+ nanopapers with the smallest counterions present a similar structure (CNF-Li+ and CNF-Na+ in Figure 4a and b, respectively), but CNF-Cs+ shows pull-out of mesoscopic layers (50−100 nm thick layers) and some bundles of individual fibrils being pull out (Figure 4c). The effect is more evident for alkyl substituted nanopapers (CNF-QA+ in Figure 4d-g). CNF-A+ shows a layered structure similar to small metal ions (Figure 4d), while CNF-TEA+, CNF-TBA+, and CNF-THA+ present pull-out of the mesoscale layers and individual fibrils (Figure 4e−g). These results strongly correlate with the presence of a second yield point, εy2, observed only in tough nanopapers with Cs+ and QA+ alkyl-substituted counterions. The cross-section images of a

strongly toughened CNF-Na+ nanopaper plasticized by water sorption at RH 99% present pull-out of fibrils and mesoscale layers (Figure S5), confirming the relation of nano/mesostructure deformation and toughness with weakened interfibrillar interactions. These observations are an interesting complement to earlier studies, where it could be shown that increased pullout on several length scales originated from weakened interfibrillar interactions by (a) increased humidity or (b) for changing the thermomechanical properties of the polymers from glassy to ductile in CNF/polymer nanocomposites.11,16,46 The small ions advantageously preserve the inner strength of the CNF and promote deformability at the same time, while nanocomposites with polymers or clay typically lose strength or strain (except for nanocomposites with optimized conditions of small amounts of clay content (5−10%)).25,47,48 Overall, this allows for reaching new areas in the structure/property chart above (Figure 3). Nanostructure Analysis. It is known that mechanical response is not only related to chemical makeup in such CNFbased hierarchically structured entangled glasses, but is also intimately connected to the nanostructure formed during the film preparation and to how the structure responds during 1647


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Finally, we employ thermoporosimetry to gain more information about the interfibrillar interactions in hydrated state by the ability to swell and form nanopores. In thermoporosimetry, the nanopapers swell until equilibrium, and a series of isothermal melting steps allows identification of depressed melting temperatures of water within nanopores, enabling evaluation of the pore size distribution (Table S1). The water content is divided into three categories: free water, nonfreezing water (NFW), and total freeze bound water (TBW). Free water is the unbound water with thermodynamic properties similar to bulk water. NFW is adsorbed on the cellulose surfaces and bound to the cellulose chains by strong hydrogen bonds restricting mobility and ability to melt (2−3 layers of water molecules).38 TBW is inside the nanopores, and has depressed melting temperatures as a function of pore size. The content of the NFW has an average of ∼0.45 g/g and is independent of the counterion presence, size, or nature (Table S6). Regarding the most interesting parameter, TBW, the CNFCOOH nanopaper has the highest TBW (porosity) arising from the aggregate structure with lower density (Figure 6a). For both CNF-Me+ and CNF-QA+, a proportional increase in TBW is observed for larger ion sizes (Li+ < Na+ < Cs+ and A+ < TEA+ < TBA+ < THA+, respectively). This arises from a decreased bond strength and easier dissociation between the carboxylic groups and the counterions in the presence of hydration, yielding higher swelling. This can be further related to the colloidal properties and DLVO theory (below). Based on the isothermal melting isotherms it is possible to calculate the pore size distribution (PSD) in wet state (Figure 6bc). Not surprisingly, CNF-COOH shows large pores with a continuous increase in the range of 10−216 nm pore size. Pores larger than 216 nm cannot be separated from free water with thermoporosimetry. After ion substitution, the PSD shows a transition to lower pore sizes in the range of 10−100 nm. The PSDs increase in pore size and content consistently with the counterion size. The pore size increase up to 100 nm for Cs+, which is the largest metal counterion evaluated (Figure 6b). CNF-QA+ samples show a consistent increase in larger pore fractions, yet the pores only range up to ca. 30 nm, indicating the hydrophobicity introduced to the nanopaper.49 From the PSD, it can thus be inferred that larger counterions ease interfibrillar debonding by providing a larger tendency for forming larger pores under stress (water ingress or mechanical stress at ambient RH). This can, on one hand, be related to a higher tendency for hydration (hydrophobic shielding of interfibrillar interactions; see zeta potential data below) and on the other hand may be related to slight changes in the

exposure to humidity and mechanical deformation. This all connects to interfibrillar interactions modified by the counterions (Scheme 1). Consequently, to provide a better understanding, we access the nanostructural details with a range of complementary methods: specific surface area (SSA), apparent density (ρ), water uptake, and porosity during swelling (see Table S6 for a summary of the obtained parameters). First, measurements on the specific surface area (SSA), measured under fully dry conditions, shows a very low SSA of ∼0.04 m2/g for all nanopapers prepared with counterions, while CNF-COOH has 0.17 m2/g (Figure 6a). In agreement, the apparent density (ρ) at RH 50% is fairly constant around ∼1.42 g/cm3 for CNF-Me+ and CNF-QA+, while it is 1.35 g/cm3 for CNF-COOH. This corresponds to a porosity of around 10% and 15%, respectively, and a significantly more porous structure for CNF-COOH (1.58 g/cm3 was used for crystalline CNF and the porosity is estimated as 1 − ρ/ρcrystalline). These results indicate the formation of a tightly packed fibril network independent of the counterion for dispersions set to pH = 9. On the contrary, the uncontrolled aggregation at low pH in CNF-COOH leads to entrapping of porous structures and the typical mechanical response of nanopapers composed of poorly connected flocs, that is, lower mechanical strength. Other porous CNF papers with more controlled pore formation processes contain better connected porous fibrillar networks and partly show larger deformations.12,13,16 Going away from fully dry conditions, we first turn to water sorption experiments. Interestingly, the amount of water uptake is similar for all counterion-containing nanopapers (Figure 5). Hence, differences in the water content can be excluded as a strong factor (H2O-induced weakening of interfibrillar interactions) influencing the mechanical response.

Figure 5. Water uptake of the nanopapers as a function of relative humidity.

Figure 6. Nanostructure characterization of the nanopapers. (a) Specific surface area by gas adsorption of Kr (SSA, empty symbols) and total bound water within the pores (TBW, filled symbols). (b,c) Pore size distribution (PSD) of nanopapers (b) NFC-Me+ and (c) NFC-QA+. 1648


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Figure 7. Electric double layer repulsion in CNF dispersions. (a) Zeta potential (ZP, filled symbols) and light transmittance at 600 nm (LT, empty symbols) for CNF dispersions. (b,c) DLVO theory for crossed cylinders.

nanostructure (see sedimentation data, Figure 9). Importantly, CNF-A+ shows the lowest distribution (PSD) and porosity (TBW), indicating the most packed nanostructure correlated with the highest stiffness and strength (Figure 3) owing to the ammonium counterions capable of additional hydrogen bonds and providing strong bond with the carboxylic groups on the surface of the CNF. This effect is further demonstrated by DLVO in the next section. Colloid Stability. In this part, we want to shed light on the colloidal stability of the various CNF species to understand how different dispersion stabilities can aid in explaining the observed properties. First, we evaluate the effect of the pH and type of counterion by measuring Zeta Potential (ZP) and light transmittance (LT) to determine the electric double layer repulsion and aggregation state of the nanofibrils in dispersion, respectively. Then, we calculate the total interaction potential energy using DLVO theory to determine the effect of the repulsion forces and bond strength. Finally, we induce aggregation by adding salt to the dispersions, and measure the sedimentation speed, which depends on the size and nature of the counterions. CNF-COOH at pH = 3 exhibits a ZP of −27 mV and a LT of 71.5% (0.25 wt %), because the carboxylic groups are largely protonated inducing interfibrillar attraction in the dispersion. When adding sodium hydroxide up to pH = 9 (CNF-Na+), the ZP decreases to −77 mV and the LT of the dispersion reaches much higher transparency up to 85% (Figure 7, full titration in Figure S4b). Here, the effective repulsion between the colloidal nanofibrils strongly diminish aggregates and produce transparent dispersions. For all CNF counterion samples at pH = 9, the ZP shows strongly negative values in the range of −71 to −87 mV and a concomitantly high LT above 84% (Figure 7a; Table S7). Increasingly negative ZP values are observed for larger counterions, indicating augmented counterion separation, higher dissociation, and double layer repulsion breaking the aggregates. Correspondingly, light transmittance increases. Hence, counterions with higher diameter are more separated from the surface of the fibril.45,50 For alkali cations, the overall charge of the nanofibril increases following the Hofmeister series (Cs+ > Na+ > Li+),36,51 indicating less screening effect and leading to higher electric double layer repulsion.36,52,53 The analysis is analogous for QA+ counterions with a concomitant increase in dissociation produced by the bulkiness of the alkyl chains. Note that the weaker binding for larger QA+ counterions correlates with the shift of the COO− peak in FTIR measurements showing weakened ionic bonds for bulk materials in Figure 1d.41

To further illustrate the ionic strength variation, we express the total interaction potential energy between the nanofibers (V, eq 1 and Figure 7b,c) using DLVO theory, assuming CNFs to be two crossed cylindrical rods with a fixed diameter (average D = 2.25 nm from AFM).11,54 V can be predicted quantitatively for D/2 ≫ H, with H as the distance between the particles. Our CNF dispersions are in the limit, as the maximum potential is around H ∼ 0.3 nm. Nevertheless, the literature shows that we gain a qualitative understanding of the counterion size effect.31,40,55−57 V = VR + VA

(1)

where VR (eq 2) corresponds to the repulsive energy from (electric) double layer interactions, and VA (eq 4) corresponds to the attraction van der Waals energy. VR = 64π

D nkBTγ 2e−κH κ2

⎛ ψ0 ⎞ ⎟ γ = tanh⎜ ⎝ 102.75 ⎠ VA =

−AD 12H

(2)

[J· m−1] at 25°C

(3)

(4)

where κ corresponds to the Debye length (0.1627 nm−1, eq S6 in the SI) for a cation/anion molar ratio of 1:1 with a concentration of 2.5 mmol/L (∼1 mmol/gCNF) corresponding to dispersions set to pH = 9 (Figure S4a), n is the number density of ions (∼0.0015 nm−3), kB is the Boltzmann constant, T is the absolute temperature, γ is an interaction constant that depends on the temperature (25 °C) and the surface potential of the nanofibers (ψ0, eq 3), which is approximated to the measured ZP, and A is the Hamaker constant for cellulose surfaces (3.5 × 10−21 J)58 (see Figure S6 for more details of the DLVO theory for a crossed cylinder geometry). By expressing the potential as V/kBT, the two rod nanofibers must overcome a maximum barrier sufficiently higher than kBT.56 The CNF-COOH dispersion shows the lowest potential barrier correlated with the observed aggregation state. Then, CNF-Me+ dispersions show a maximum potential in the range of 8.8−9.5 kBT, following Li+< Na+< Cs+ (Figure 7a and Table S7). Similarly, CNF-QA+ shows a stronger increase of the maximum of the potential with the alkyl chains from 8.2 to 11.0 kBT for A+ < TEA+< TBA+< THA+ (Figure 7b and Table S7), hence with the size of the ion (Table S6 for ionic radius and Table S7 for maximal potential).59,60 In all cases, the colloidal stability with counterions (at pH = 9) is far better compared to the aggregated state (e.g., pH = 3), and therefore changes in bulk mechanical properties for the materials prepared at pH = 9 are more 1649


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Figure 8. Colloidal stability as seen by light transmittance (LT) during salt addition. (a) CNF-Me+/MeCl and (b) CNF-QA+/QACl dispersions. (c) Example for a dispersion set to pH = 9 with NaOH and subsequent addition of 170 mmol/L of NaCl. Black arrows show the concentrations evaluated for ZP and sedimentation measurements.

dominated by interfibrillar and CNF/counterion interactions, while there may only be subtle changes in the nanostructure.45,61 Moreover, the CNF-A+ dispersion has the lowest potential maximum showing stronger binding. ZP measurements are typically conducted at high dilution, which does not represent the situation at the filter surface or during film formation during drying. Therefore, to simulate more concentrated dispersions, we measure the aggregation behavior by salt addition of each specific cation using always chloride (Cl−) as anion. In general, a continuous reduction in LT is obtained until 75 mmol/L of salt addition, whereupon a plateau is reached up to 170 mmol/L for all dispersions/salt systems, except for CNF-THA (Figure 8). The aggregation is independent of the counterion, except for THA, which is very hydrophobic. Pure THACl has a solubility limit in water of around ∼0.05 mol/L (Figure S7). The aggregation comes from the screening of charges on the nanofibrils, making the attraction via van der Waals forces more significant.62 The reduction in ZP of the CNF/salt systems further illustrate this effect (Figure 9a and Table S8). In spite of the salt concentration, the potential is more negative for larger ions and specially for the QA+ ions (TEA+ and TBA+). The QA+ cations show the most negative ZP and have, in the case of very hydrophilic TEA and TBA, no solubility limits, and potentially a structure that shields interfibrillar hydrogen bonding and may contribute with minor steric repulsion.63−65 To address their differences, we placed the aggregated dispersions at 75 and 170 mmol/L in an analytical photocentrifuge to determine the sedimentation speed (Figure 9b). The smallest counterions (Li+, Na+, Cs+ and A+) show similar rates under both salt concentrations. This is a strong indication for similar nanostructures in the CNF-Me+ series. Yet, CNF-QA+ dispersions show a continuous decrease in sedimentation speed for longer alkyl chains, indicating more stable nanofibrils and bundles (Figure 9c). CNF-THA+/THACl dispersion could not be studied because of the low solubility of THACl. These results demonstrate the importance of the dispersion state on the nanostructure formation. It can be used to explain some differences in values obtained by film casting, characterized by constant concentration increase and more prone to aggregation, vs vacuum filtration, where the concentration stays constant in the supernatant and only increases at the filtration surface. While details of the nanostructures of any nanopaper are not quantitatively accessible using scattering or imaging techniques, it can at this point only be hypothesized that the lower sedimentation speeds contribute to making nanopapers with

Figure 9. Electric double layer repulsion and sedimentation for CNF/ salt systems at higher ionic strength to mimic the situation at the filter surface. (a) Zeta potential (ZP), (b) sedimentation speed, and (c) CNFQA+ samples showing better stability by lower supernatant height for larger alkyl chains (white arrows).

slightly less aggregates. Nonetheless, those do not result in higher Young’s modulus for the CNF-QA+ series, due to much more weakened interfibrillar interactions in bulk. Those weakened interactions with well-defined bulk structures yet allow, in turn, better pore forming ability and enhanced deformation under mechanical tension. CNF/Polymer Nanocomposites. In the last part, we raise the question of whether careful selection of counterions can also beneficially influence toughness in biomimetic CNF/polymer nanocomposite formed at high fractions of reinforcements. 1650


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Figure 10. Mechanical properties of bioinspired CNF/EG54DMAm46 nanocomposites formed at 1/1 w/w with Na+, A+ and TBA+ counterions. (a) Schematic representation of the bionanocomposite, (b) tensile curves, and (c) cross-section for the bionanocomposite with TBA+ counterion.

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We select CNF-Na+, CNF-A+, and CNF-TBA+ dispersions to prepare nanocomposites in combination with a water-soluble nonionic copolymer (Figure 10a). Previously, we showed that poly[(ethylene glycol methyl ether methacrylate)-co-N,Ndimethylacrylamide] at 54/46 molar ratio (Mw = 2300 kg/mol and Đ = 1.30), abbreviated EG54DMAm46, with a glass transition temperature (Tg) of 26 °C allows for maximizing stiffness, strength, and toughness in rationally designed, homogeneous CNF/copolymer nanocomposites due to maximum energy dissipation.10,11 In general, the addition of the copolymer to the nanofibrillar network systematically reduces stiffness, yield, and tensile strength (E, σy, σb) because the mechanical consistency of intercalated EG54DMAm46 is lower and provides interfibrillar slippage between CNFs (Figure 10b and Table S9). E in CNF-Na+/EG54DMAm46 displays 3.4 GPa, while CNF-A+/ EG54DMAm46 and CNF-TBA+/EG54DMAm46 have 2.8 and 0.9 GPa, respectively. A systematic toughening is observed, in fact very similar to what could be observed above for pure nanopapers. This is not surprising, because the integration of 50 wt % of polymer in this nanostructured composite only leads to a small average separation of 0.5 nm for nanofibrils of 2.5 diameter.11 For the CNF-TBA+/EG54DMAm46 nanocomposite, the film is soft and flexible with the lowest E and strength but with a superior εb of ∼60% evidencing further reduction of interfibrillar hydrogen bonding due the butyl chains and higher interfibrillar sliding. Correspondingly, mesoscale layers and fibril pull-out are even more evident than for CNF-THA+ nanopapers. (Figure 10c versus Figure 4g). Compared to other nanocomposites combining CNF with polyacrylamides, amylopectin, or hydroxyethyl cellulose of similar composition, the present materials show lower stiffness and mechanical strength.11,19,22,23,66,67 However, the results are remarkable in terms of deformability as the maximum reported elongation for a CNF/polymer nanocomposite is at ca. 30% (CNF/HEC) with low fibril content (1:9 weight ratio) and 15% for a 1:1 composition. In the future, one can take advantage of such a high deformation in bioinspired nanocomposites with high reinforcement content to implement dynamic supramolecular bonds capable of sacrificial bonding and able to provide enhanced energy-dissipation mechanisms to raise the level of frictional sliding to higher stress levels.10,68−73

CONCLUSIONS We have presented a systematic correlation between structure formation, as well as structural and mechanical response, and deformation mechanisms in CNF nanopapers modified with different monovalent counterions. Interestingly, only small amounts of the counterions (∼0.3 mmol/gCNF) are necessary to produce a significant improvement. Outstanding values of stiffness and toughness can be achieved by introducing small molecules capable of forming multiple hydrogen bonds (metals or ammonium) or larger ones with strong plasticizer effects (quaternized amines), respectively. Inelastic deformation mechanisms are promoted by reducing the counterion binding strength for larger counterions, which increases interfibrillar sliding through weakened interfibrillar interactions, as well as by providing capabilities for pore formation. Such nanopores can act as energy-dissipators during deformation, and thermoporosimetry evidences larger propensity for pore formation for larger counterions. This is cross-correlated to the colloid stability of the dispersions in solution, where better stability (degree of ionization) can be shown by lower zeta potentials and higher energy barriers in DLVO for increasing counterion size. Moreover, sedimentation analysis of CNFs with bulky quaternized ammonium ions in dispersions with high ionic strength points to lower sedimentation speeds, and, hence, to lower propensity for aggregation when using larger counterions. In a wider perspective, the thorough analysis provides insight into the overall complexity of such systems. Colloidal stability, structure formation, and structural as well as mechanical response are mutually coupled, and a better overall understanding can be achieved by combining analytics from different fields. It becomes clear that simple changes in the preparation procedures (i.e., selection of counterions, pH, vacuum filtration versus film casting) have profound impact on the final properties, and can be used to drastically alter and improve properties. We already demonstrated this for the preparation of bioinspired CNF/polymer nanocomposites, where significantly larger strainto-failures can be engineered by simple counterion exchange. On a fundamental materials level, this further rational design criterion calls for a future exploitation by, for instance, incorporating defined supramolecular interactions. Yet, on a performance benchmarking level, it also shows that detailed experimental protocols and standardized processes would be 1651


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beneficial to better compare mechanical properties from different groups.

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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/acs.biomac.7b00263. Supporting thermoporosity method, nanopaper preparation, dispersion stability of CNF-Na+ with pH, ion exchange efficiency by elemental analysis, mechanical properties, summary of Ashby plot parameters, cross sections of CNF-Na+ fracture at different humidities, summary of nanostructure characterization parameters, colloid stability of CNF dispersions at pH = 9, DLVO theory for crossed cylinders, colloid stability by salt addition and mechanical properties of bionanocomposites (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andreas Walther: 0000-0003-2170-3306 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the BMBF for funding the AquaMat research group in the framework of the NanoMatFutur program. This work was performed in part at the Center for Chemical Polymer Technology, supported by the EU and North Rhine-Westphalia (EFRE 30 00 883 02).

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