Vitrimer Chemistry Meets Cellulose Nanofibrils - ACS Publications

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Vitrimer Chemistry Meets Cellulose Nanofibrils: Bioinspired Nanopapers with High Water Resistance and Strong Adhesion Francisco Lossada, Jiaqi Guo, Dejin Jiao, Saskia Groeer, Elodie Bourgeat-Lami, Damien Montarnal, and Andreas Walther Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01659 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Biomacromolecules

Vitrimer Chemistry Meets Cellulose Nanofibrils: Bioinspired Nanopapers with High Water Resistance and Strong Adhesion

Francisco Lossada†,‡,§, Jiaqi Guo†,‡,§, Dejin Jiao†,‡,§, Saskia Groeer†,‡,§, Elodie Bourgeat-Lami∥, Damien Montarnal*,∥, Andreas Walther*,†,‡,§,⊥ †

Institute for Macromolecular Chemistry and ‡Freiburg Materials Research Center, University of Freiburg, Stefan-MeierStrasse 31, Freiburg 79104, Germany § Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, Freiburg 79110, Germany ⊥Freiburg

Institute for Advanced Studies, University of Freiburg, Freiburg 79104, Germany

∥Univ Lyon. Université Claude Bernard Lyon 1, CPE Lyon, CNRS, UMR 5265, Chemistry, Catalysis, Polymers and Processes, 43 Bvd du 11 Novembre 1918, F-69616 Villeurbanne, France

Correspondence to [email protected]; [email protected]

ABSTRACT Nanopapers formed from cellulose nanofibrils (CNFs) are an emerging and sustainable class of high performance materials. The diversification and improvement of the mechanical and functional property space critically depend on integration of CNFs with rationally designed, tailor-made polymers following bioinspired nanocomposite designs. Here we combine for the first time CNFs with colloidal dispersions of vitrimer nanoparticles (VP) into mechanically coherent nanopaper materials. Vitrimers are permanently crosslinked polymer networks that undergo temperature-induced bond shuffling through an associative mechanism and which allow welding and reshaping on the macroscale. The choice of low glass transition, hydrophobic vitrimers derived from fatty acids and polydimethylsiloxane (PDMS), and achieving dynamic reshuffling of crosslinks through transesterification reactions enables excellent compatibility and covalent attachment onto the CNF surfaces. The resulting films are ductile, stretchable and offer high water resistance. The success of imparting the vitrimeric polymeric behavior into the nanocomposite, as well as the curing mechanism of the vitrimer, is highlighted through thorough analysis of structural and mechanical properties. The dynamic exchange chemistry of the vitrimers enables efficient welding of two nanocomposite parts as characterized by good bonding strength during single lap shear tests. In future, we expect that the dynamic character of vitrimers becomes a promising option for the design of mechanically adaptive bioinspired nanocomposites and for shaping and reshaping such materials.

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INTRODUCTION Nature provides examples for outstanding structural materials with synergistic mechanical properties, uniquely combining stiffness, strength and toughness with lightweight properties and unique selfhealing abilities.1-5 In general, there exists a constant desire in the science and engineering world to mimic the structures and performances of such natural materials, while adding functionalities relevant to future technologies. For instance, wood presents a hierarchical design, enclosing cellulose microfibers in a matrix of lignin and hemicellulose, which allows to reach significant strength and toughness.6-8 The cellulose microfibers are composed of crystalline arrays of β-(1-4)-D-glucopyranose polymers, bonded via interchain hydrogen bonds to result in a high stiffness, represented by a Young’s modulus (E) of ca. 150 GPa, whereas the lignin and hemicellulose provide a comparably soft and lubricious matrix.9-13 Basically, the combination of crystalline and soft domains together with the complete hierarchical structure provides the remarkable mechanical behavior of wood. In recent years, the extraction and functionalization of cellulose nanofibers (CNFs) from plants has come into focus, as it allows to isolate one of nature’s stiffest, strongest and most lightweight sustainable nanomaterials, and to use it for functional materials design.14-22 On a materials scale, CNF-based films/nanopapers present excellent mechanical properties with high elastic moduli (E ≈ 6-10 GPa) and tensile strength (σb ≈ 200-300 MPa).14,16,23-26 For mechanical high-performance materials using stiff colloidal reinforcements (independent of their dimensionality), it is attractive to think in the direction of bioinspired design principles, that aim for embedding stiff reinforcements into a soft polymer phase with molecular energy dissipation mechanisms at high fractions of reinforcement.27-28 Due to its nanoscale dimensions and abundance of hydroxy groups (and other functionalities), CNFs can be easily functionalized and also potentially bonded to tailor-made polymers using non-covalent and covalent interactions.29-40 The addition of a polymeric phase allows control over the mechanical properties of the whole nanocomposite, varying its flexibility by simply adjusting the content and properties of the polymer. Previously, we showed that well-defined polymer-coated core/shell CNFs could be assembled into structurally well-defined fibrillar bioinspired nanocomposites, resulting in materials with tunable toughness as a function of the glass transition temperature (Tg) of the intercalated polymer phase and as a function of the polymer fraction.3435 Other approaches discussed the integration of latex nanoparticles, or the co-casting with polysaccharides.36-37 Furthermore, modifying the CNF surface by polymer grafting can promote thermal stability and hygro-mechanical properties in the nanocomposites.38-39 A recent review summarizes the mechanical property space.41 So far research in the direction of bioinspired nanocomposites mostly explored commercial polymers with limited control over precise tuning of molecular interactions. Although this has allowed to diversify the mechanical behavior, the most critical advances will be in reach if we implement suitable and advanced polymer chemistry tools that become increasingly available on a larger scale to tune molecular interactions. Vitrimers emerge as a highly promising new class of permanently crosslinked networks, which can be reprocessed and reworked by activation of bond shuffling mechanisms at high temperature.42-44 Originally, the exchange dynamics were discovered for transesterification reactions in presence of an excess of hydroxy groups, by catalysis with Lewis acids (zinc and tin salts) or bases (triazabicyclodecene and triphenylphosphine).42,45 More recently, vitrimer chemistries broadened and now include transalkylation, transamination, disulfide exchange or even catalyst free reactions.43-44,46-50 These exchange reactions at the molecular level allow stress relaxation on the macroscale, which, if integrated into CNF-based nanocomposites, could generate relevant new characteristics. Additionally, crosslinking strategies have been successful for other bioinspired nanocomposites.51-55 However, little work on tunable crosslinking strategies has been reported for CNF-based nanocomposites. 2 ACS Paragon Plus Environment

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Here we show tunable mechanical properties for bioinspired CNF/vitrimer-based nanocomposites obtained by inclusion and sintering of hydrophobic, low Tg poly(dimethylsiloxane) vitrimer latex nanoparticles (VP) into the CNF network.56 Sintering and bond shuffling within the vitrimer phase provides a continuous matrix phase and simultaneously ensures covalent linkages with the CNF network. A stiff-to-ductile transition occurs that depends on both the content as well as connectivity and crosslinking extent of the vitrimer phase. We also show that embedding hydrophobic vitrimers into the CNF-based nanocomposites offers high water resistance and stability. Finally, we quantify the adhesion of the CNF/vitrimer interface and demonstrate welding of two CNF/vitrimer nanocomposite films, as quantified by single lap shear test.

EXPERIMENTAL SECTION Chemicals. All reagents (highest purity available) and SDS (99 %) were purchased from Aldrich and used as received. Zinc diacetate dehydrate (98%) was purchased from Acros. Preparation of Cellulose Nanofibrils (CNFs). A suspension of TEMPO-oxidized Kraft pulp, oxidized under neutral condition, was set to pH = 9 with NaOH and homogenized in a LM10 Microfluidizer from Microfluidics applying four shear cycles (2 × 1400 bar, 2 × 1000 bar). The content of carboxyl groups is 0.44 mmol/g, the degree of polymerization as determined by viscosimetry (DPv) is 725 and the degree of crystallinity by X-ray diffraction (XRD) is 77 %.16 Vitrimer Nanoparticles of Poly(dimethylsiloxane) (VP). The VP synthesis is analogous as reported previously.56 In short, the mixture of precursors (diglycidyl ether-terminated poly(dimethylsiloxane) oligomer (PDMS-diepoxy, 800 g/mol), hydrogenated fatty acid dimer, 2,4,6,8-tetramethyl-2,4,6,8tetrakis(propyl glycidyl ether) cyclotetrasiloxane (crosslinker) and xylene) were dispersed in water/SDS solution (SDS = sodium dodecylsulfate), and were allowed to react for 12 h at 120 °C in a pressure vessel to obtain a latex of partially crosslinked vitrimer particles. The molar ratios of zinc:SDS:carboxylic acids was set to 0.05:0.10:1. As demonstrated previously, these partially crosslinked particles form upon drying a viscous liquid, referred to as “uncured vitrimer” in the text. Upon heating this viscous liquid at 120 °C for 16 h in absence of water, curing and transesterification exchanges yield fully reticulated films by sintering of the particles, to which we refer to in the manuscript as “cured vitrimers”. Preparation of nanocomposites. The CNF dispersion (pH = 8; 0.25 wt%) was added to a VP dispersion (0.25 wt%) dropwise while stirring vigorously to ensure a homogeneous combination of both components. The overall polymer content in the nanocomposites was adjusted to 35 and 50 wt%. Then 30 mL of the final suspensions were poured into a Petri dish and dried at room temperature, leading to films with a thickness range of 10 - 18 µm after drying. Following the same terminology as above, the nanocomposites prepared in this way are referred to as “uncured CNF/vitrimer nanocomposites”. After heating these materials at 120 °C for 16 h they are referred to as “cured CNF/vitrimer nanocomposites”. Preparation of bonding and welding test samples. Samples investigating the function of the vitrimer matrix as a glue layer between CNF nanopapers were prepared by depositing uncured and cured vitrimer films between two pure CNF nanopapers at 5 mm × 2 mm overlapping area. Samples were mended by hot-pressing at 3 bar for 1 h at 120 °C. After being hot-pressed, the non-cured vitrimer was cured for 24 h at 120 °C. Welding or lamination of nanocomposites consisted of hot-pressing two CNF65/VP35 nanocomposite films, with no added chemicals, at 3 bar, for 24 h at 120 °C. The corresponding numbers indicate the respective weight fractions of CNF and vitrimer. Fourier-Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded using a Vector 22 spectrometer from Bruker with a frequency range from 4000 to 500 cm-1.



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Differential Scanning Calorimetry (DSC). The DSC measurements were performed using a PerkinElmer DSC8500 with a heating rate of 10 °C/min under nitrogen purge of 80 mL/min. All samples weighted around 3-10 mg in a sealed aluminum pan. Atomic Force Microscopy (AFM). AFM was performed using a MultiMode scanning probe microscope with NanoScope V controller (Digital Instrument) operating in tapping mode at room temperature. The samples were obtained from diluted suspensions in water (ca. 0.005 wt%) onto freshly cleaved mica. Dynamic Light Scattering (DLS). DLS was used to measure the sizes of vitrimer nanoparticles in a NanoLab 3D at a fixed scattering angle of 90° and temperature of 20 °C. Very dilute solutions were run through 1.2 µm syringe filters before the measurements. Tensile tests. Tensile tests were performed on a DEBEN minitester equipped with a 200 N load cell at room temperature and different relative humidities (RHs). The samples were conditioned in 20, 55, 80 and 99% RH for at least 2 days before the measurements. Specimens had a size of 10 mm × 2 mm × 10 - 18 µm. At least 6 samples were tested for each nanocomposite at a strain rate of 1.0 mm/min. Tensile single lap shear tests used the same parameters. The adhesive area in each specimen had a size of 5 mm × 2 mm. The Young’s modulus (E) was determined from the slope of the linear region in the stress/strain curves. The yield points were determined by the intersection of tangential lines of the elastic region with permanent inelastic region. Moisture and water uptake. The moisture uptake was measured by weighing samples for CNF and nanocomposite films after conditioning at their respective RHs for 24 h, starting from 20% RH up to 99% RH. The water uptake was measured by weighing daily, for 20 days, samples of CNF and nanocomposite films soaked in water. Dynamic Mechanical Analysis (DMA). The DMA measurements were performed using a TA Q800 device with tensile loading and a strain of 0.1 %, preload of 0.01 N, force track of 150 % and frequency of 1 Hz. The specimens had a size of 20 mm × 2 mm × 10 - 18 µm. The samples were equilibrated at 110 °C for 5 min and scanned up to 210 °C at a rate of 5 °C/min. Scanning Electron Microscopy (SEM). Cross sectional fracture surfaces were observed with a FEI Scios DualBeam FIB/SEM at 5 kV. The samples were sputter-coated with an ultrathin Au layer. Transmission Electron Microscopy (TEM). The TEM imaging was performed using a FEI TALOS L120C operating at 120 kV. For sample preparation, 3 µL of 0.9 wt% VP solution was adsorbed on a hydrophilized carbon-coated copper TEM grid (EMS) for 60 s, blotted away using filter paper.

RESULTS AND DISCUSSION Preparation of CNF/Vitrimer-based Nanocomposites The introduction of vitrimers into CNF/polymer hybrid nanopapers starts with the appropriate design of the vitrimer component to fit to the needs of CNF-based bioinspired nanocomposites. The latter require waterborne processing strategies to reach very homogeneous structure formation in the nanopapers. This is needed to develop reliable structure/property relationships that are not overly dominated by flocculated or aggregated CNF suspensions.41 Since most vitrimers are however of hydrophobic nature, an appropriate colloidal route needs to be designed to deliver hydrophobic vitrimers as submicron-sized particles into the final structure, where they can lead to a homogeneous infiltration during drying (Scheme 1). Suitable, stable, very hydrophobic vitrimer latexes have recently been developed using miniemulsion polymerization.56 Similar to other vitrimer systems, these materials 4 ACS Paragon Plus Environment

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rely on epoxy/acid condensation that leads to the formation of β-hydroxyester linkages. After curing, appropriate catalysis at high temperatures enables dynamic transesterification exchanges between hydroxyl and ester groups, thus affording malleability, weldability and pressure-free sintering in the case of submicron particles. This chemistry is attractive as it can be interfaced with CNFs bearing a large fraction of hydroxy and acid groups on their surface. The synthesis of the aqueous dispersion of vitrimer nanoparticles (VP) starts from a mixture of crosslinkable fatty dimer acids and epoxy-functionalized polysiloxanes (Scheme 1b). Xylene is added as compatibilizer to also reduce the viscosity of the mixture. The most challenging part is to properly disperse the reaction mixture in aqueous medium, and to perform the epoxy/acid condensation at 120 °C while avoiding destabilization of the emulsion or ester hydrolysis. Careful choice of reaction time, surfactant nature and concentration affords stability of the emulsion and control over the size of the vitrimer particles.56 It must however be noted that a competition cannot be avoided between the first reaction of epoxy and acids into β-hydroxyesters, and their subsequent hydrolysis into acids and monoglycerols. The chosen vitrimer latex is stabilized with SDS (20 wt% oil phase, 20 mM SDS in the aqueous phase) and allowed to react for 12 h at 120 °C leading to partially crosslinked network with a gel content of 74 wt% measured by dialysis.56 It has an overall solid content of 9.2 wt% and contains nanoparticles with an average diameter z ≈ 100 nm at relatively broad dispersity, as deduced from dynamic light scattering (DLS) and transmission electron microscopy (TEM; Figure S1). 1H NMR confirms its composition to comprise approximately a molar fraction of epoxy, β-hydroxyester, carboxylic acids and mono-glycerols of 3 %, 41 %, 27 % and 30 %, respectively.56 Upon drying, the low Tg crosslinked VP form a viscoelastic liquid (η ≈ 17 Pa·s). Such low viscosity is important for the integration into CNF-based nanopapers as it ensures that VP can flow into the fibrillar nanopaper structure during final stages of film casting. Finally, during a subsequent heat treatment (also termed sintering or curing) at 120 °C for 16 h, the vitrimer undergo further completion of the esterification reactions between remaining epoxides, carboxylic acids and hydroxyls, as well as transesterification exchanges between hydroxyls and esters. Exchange reactions at the interface of individual, free-flowing vitrimer particles induce sintering of the VP into a homogeneously crosslinked matrix. We refer the reader to the comprehensive vitrimer characterization of the corresponding bulk material in our previous publication.56 For nanocomposite design, we combined aqueous VP dispersions (0.25 wt%) with TEMPO-oxidized CNF dispersions (0.25 wt%; COOH content of ca. 0.44 mmol/g, micrometer length, diameters of 2-4 nm, atomic force microscopy (AFM) in Figure S2), at different ratios and pH = 8 (Scheme 1). Macroscopically, mixing of both components leads to homogeneous dispersions without aggregation or phase separation, because both the VP, stabilized by anionic surfactants (SDS), and the TEMPOoxidized CNF, bearing carboxylate groups, are negatively charged and promote colloidal stability (photographs of dispersions in Figure S3). The colloidal stability of the mixtures allows for a welldefined homogeneous structure formation of the nanocomposite during drying, leading to macroscopically homogeneous and transparent films (Figure 1d,e).27 Due to the strong physisorption of the vitrimer components via multiple non-covalent interactions and the large CNF surface area inside the highly loaded CNF/vitrimer nanocomposites, it is not possible to obtain reliable quantitative values of the vitrimer gel content by e.g. swelling, dissolution and isolation of non-crosslinked compounds via dialysis as done for the pure vitrimer materials.56 The VP content was set to 35 and 50 wt%, corresponding to high fractions of reinforcement in the bioinspired nanocomposite design. The nanocomposites are denoted as CNF65/VP35 and CNF50/VP50, in which the corresponding numbers indicate the weight fractions.



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Scheme 1. Schematic preparation route of CNF/vitrimer nanocomposites by aqueous mixing of PDMS-based low Tg vitrimer nanoparticles (VP) with CNFs. Drying leads to deformation of the VP and homogeneous infiltration in between the CNF network. Further heating treatment promotes completion of crosslinking reactions, exchange reactions leading to sintering of the vitrimer phase, and interfacial reactions between the CNF and the vitrimer matrix.

FTIR illustrates the interactions between the vitrimer and CNF after drying. Figure 1b demonstrates the presence of the vitrimer in the nanocomposites by the appearance of the stretching signal νC=O of esters and carboxylic acids at 1735 and 1714 cm-1, respectively. Moreover, the stretching signal of C-O alkoxy (1020 - 1250 cm-1) is higher in nanocomposites when compared to the pure polymer. Additionally, the CNF -COO- signal indicates the presence of salt form -COONa on the surface of CNFs and shows a slight displacement (from 1620 to 1614 cm-1), expected to originate from the adsorption of the polymer phase to the multiple anchor points in the CNF.57 The next step consists of heating the films to 120 °C for 16 h to provide an efficient curing step within the vitrimer phase and at the CNF/vitrimer interface. Scheme 1d and Figure 1a-c illustrate the sintering within the vitrimer and bond shuffling process between vitrimer/CNF interfaces. For simplicity, we start with the pure vitrimer (Figure 1a). Three relevant changes are observed during curing. The first variation can be followed as a decrease of the signal of carboxylic acids (νC=O = 1714 cm-1) and an increase of the esters (νC=O = 1735 cm-1) arising from the completion of the vitrimer curing through esterification between mono-glycerols and carboxylic acids. A simultaneous variation of the stretching signal of C-O alkoxy (1024 - 1260 cm-1) corresponds to the consumption of -OH groups. Finally, a decrease of O-H stretching signal (3425 cm-1) is visible as free hydroxyl groups form additional covalent bonds within the vitrimer network. Similar changes are observed for the CNF/vitrimer 6 ACS Paragon Plus Environment

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nanocomposites (Figure 1b), with the exception of O-H stretching reduction (3425 cm-1) due to the significant amount of –OH groups in the CNF.

Figure 1. Selected characterization of CNF/vitrimer nanocomposites. FTIR spectra of (a) uncured and cured vitrimers (VP; colored; dashed line), (b) pure CNF (black) and CNF50/VP50 (colored; full line), with their corresponding bands and variation on the curing process (“NC” = denotes not cured/as prepared; “C” = cured at 120 °C for 16 h). (c) A change from blue to red indicates further crosslinking in VP, as well as activation of transesterification reactions. (d) Transparent and homogeneous film of CNF50/VP50 (NC) (Thickness ≈ 16 µm). The logo is used with permission from The Institute of Macromolecular Chemistry, University of Freiburg, Germany. (e) Cross section micrograph of the homogeneous layered structure of CNF65/VP35 (NC).

Mechanical Tensile Properties Next, we proceed to the evaluation of the mechanical tensile behavior of the nanocomposites, and discuss them as a function of vitrimer content together with the curing process. Standard mechanical tensile tests are conducted at 55% relative humidity (RH) at room temperature, while we later evaluate the behavior also at higher relative humidity and in water to investigate water stability for such waterborne materials. Figure 2a illustrates the tensile curves for CNF/vitrimer nanocomposites as a function of the vitrimer content and sintering. Table 1 summarizes the characteristic values. Upon incorporation of the vitrimer, the CNF/vitrimer nanopapers undergo softening and show a more ductile behavior, as seen in the decrease of the Young’s modulus (E), the yield point (sy), the tensile strength (sb), while an increase of the strain-to-failure (eb) occurs, compared to pure CNF. This smooth transition indicates a homogeneous nanocomposite structure and originates from the integration of a soft low Tg, and interfacially adhering polymer phase. The phase is well intercalated inside the interfibrillar spaces between the CNFs and thus allows for interfibrillar movement.34-35,58-60 Importantly, after curing the nanocomposites, the materials show an increase of stiffness, which is due to sintering of the vitrimer matrix, additional crosslinking, and formation of covalent linkages at the CNF/vitrimer interface. In more detail, the inclusion of 35 wt% vitrimer in CNF65/VP35 (NC, not cured/as prepared) provides enough interfibrillar motion and flexibility to promote macroscale ductility. Starting from pure CNF, E, sy and sb are reduced from 9 GPa down to 7 GPa, from 79 MPa to 24 MPa and from 210 MPa to 68 MPa, respectively. This reduction of strength in the material goes along with a doubling of eb from 8 % to 16 %. This stiff/strong-to-ductile transition of the system is caused by the proper inclusion of the soft and deformable vitrimer particles between the CNFs.34 The yield point decreases in stress as the presence of vitrimer limits the CNFs interfibrillar connections, and a good adhesion at the vitrimer/CNF interfaces and the viscoelastic character of the vitrimer phase enable and mediate the slippage of the network, leading to high ductility.61 The sintering of the vitrimer matrix in CNF65/VP35 (C, cured) and the implementation of vitrimer/CNF chemical bonds during the curing step leads to a re7 ACS Paragon Plus Environment


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strengthening and the nanocomposite reaches higher E from 7 GPa to 8 GPa, higher sy from 24 to 43 MPa and higher sb from 68 to 115 MPa, while sacrificing some maximum eb as seen in a decrease from 16 % to 11 % (Figure 2b). Note that this film has almost regained the original stiffness of the pure CNF nanopaper, but presents a significantly higher ductility. This also leads to a toughness of the film around 10 MJ/m3, being on a similar level to CNF. An increase of the vitrimer content to 50 wt% in CNF50/VP50 (NC) provides an expectedly stronger transition from stiff/strong to ductile behavior in the nanocomposites. This demonstrates that the amount of vitrimer can be used to control the mechanical performance of the nanocomposite materials and indicates indeed a well-controlled structure formation during the colloidal processing. The crosslinking effect for higher amount of vitrimer leads to similar trends in terms of stiffening and strengthening of the nanocomposites.

Figure 2. Mechanical tensile behavior for CNF/vitrimer nanocomposites with 35 and 50 wt% of vitrimer. (a) Tensile curves for pure CNF, CNF65/VP35 and CNF50/VP50. (b) Mechanical properties obtained from the stress/strain curves. The properties show the variation of Young’s modulus (E), work-of-fracture (Ut), tensile strength (σb) and strain-at-break (eb), as a function of vitrimer content and crosslinking. The arrows guide the reader from CNF/vitrimer films “as prepared/not cured (NC)” to “cured (C)”.

Table 1. Overview of mechanical tensile test values for CNF/vitrimer nanocomposites Sample

Vitrimer/VP (wt%)

E (GPa)

sy (MPa)

sb (MPa)

eb (%)

Ut (MJ/m3)

CNF CNF65/VP35 (NC) CNF65/VP35 (C) CNF50/VP50 (NC) CNF50/VP50 (C)

0 35 35 50 50

8.9 ± 0.8 6.6 ± 0.6 8±2 1.2 ± 0.2 1.9 ± 0.1

79 ± 11 24 ± 1 43 ± 1 11.1 ± 0.6 15 ± 1

210 ± 17 68 ± 4 115 ± 2 28 ± 1 44 ± 4

7.6 ± 0.9 16 ± 2 11 ± 2 19 ± 1 11 ± 2

10 ± 2 9±1 10 ± 2 4.2 ± 0.3 3.7 ± 0.5

Figure 3 shows scanning electron microscopy (SEM) images of the corresponding cross sections of CNF and CNF/vitrimer nanocomposites before and after sintering to clearly elucidate different mesostructural deformation behavior. In comparison to pure CNF, the addition of uncured vitrimer leads to the observation of pronounced pull-out of mesoscale layers (orange arrows), and originates from the ductile behavior as the vitrimer promotes motion of the fibrillar network (Figure 3b).34 The layered structure forms during the late stages of the solvent evaporation, as the increase of the colloid concentration can induce aggregation and floc formation.62-63 More importantly, the appearance of single CNFs pulled out from the structure (yellow arrows) demonstrates very effective lubrication down to the level of individual CNFs as well. These mechanisms are different to pure CNF (Figure 3a). The absence of the soft polymeric structure in pure CNF prevents extensive inelastic deformation mechanisms and provides a rather feature-less cross section. Hence, the changes in deformation behavior observed by SEM correspond fully to the changes in macroscale tensile behavior. After the 8 ACS Paragon Plus Environment

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sintering of the vitrimer (Figure 3c), the pull-out of individual CNFs is no longer visible (red arrows), strongly indicating effective crosslinking at the CNF/vitrimer interface and within the vitrimer phase.

Figure 3. Cross section SEM micrographs of fractured samples to represent the transition of their deformation mechanisms. The mechanisms are presented from stiff/strong behavior in (a) CNF to (b) CNF50/VP50 (NC) with an increase of ductility with mesoscale layer and CNF pull-out. (c) The crosslinking effect is observed by presenting mesoscale layers with strongly embedded CNFs with CNF fracture in CNF50/VP50 (C).

Next, we evaluate the thermo-mechanical behavior of the CNF/vitrimer nanocomposites via dynamic mechanical analysis (DMA) in tension (Figure 4). We focus entirely on the sample incorporating 50 wt% of vitrimer, CNF50/VP50, to have sufficient quantity of the soft phase to be traceable in DMA. Pure CNF nanopapers exhibit a very high storage modulus (E’ = 28 GPa at -100 °C) due to the high crystallinity and strong hydrogen bonding, and show a decrease as a function of the temperature due to increase of internal motion of the amorphous regions together with generation of free volume (Figure 4a).64-65 As demonstrated above by tensile testing, the materials soften by addition of vitrimers to the nanocomposite, as visible by the lower level of E’, but the decrease of E’ at high temperatures remains very moderate.36 Similarly, the values of tan δ for the nanocomposites remain very low over the whole temperature range, even before sintering the vitrimer particles, suggesting that the vitrimer phase is distributed very homogeneously within the CNF network and without large domains, i.e. being nicely infiltrated. A small peak is visible in the tan δ curves around -60 °C, which relates to the Tg of the vitrimer (Differential Scanning Calorimetry (DSC) in Figure S4).

Figure 4. Temperature-sweep in dynamic mechanical analysis (DMA) of CNF/vitrimer nanocomposites (1 Hz, 5 °C/min). Temperature dependence of the storage modulus (E’) and the damping ratio (tan δ).

Water Resistance and Behavior at High Relative Humidity One of the biggest limitations of waterborne CNF-based materials is still their susceptibility against humidity or even their full redispersion when exposed to water. In general, at low humidity, CNF presents a very stiff nanofibrillar network with multiple hydrogen bonds. At high humidity, the ingress of water molecules competes with the interfibrillar hydrogen bonds and weakens the interfibrillar 9 ACS Paragon Plus Environment


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network.63 We hypothesized that key advantages for stabilization against moisture could originate from the integration of hydrophobic vitrimer dispersions with the ability to form covalent bonds to the CNFs. To determine and quantify the hydrophobization imparted by the addition of vitrimer inside the CNF, we measured the water uptake as a function of relative humidity (Figure 5a). These data show two behavioral domains as function of humidity. At low humidity (≤55% RH), the effect of moisture is similar for all samples tested, while for higher humidity (>55% RH) clear differences can be observed. Notably, the nanocomposites show less moisture absorption than pure CNF due to the hydrophobic vitrimer structure. However, since there is no strong difference between water uptake of CNF65/VP35 before and after sintering, we also turned to the swelling of the films when placed into pure water (Figure 5b). To this end, it is known that the interactions of water with the CNFs differ as a function of liquid or gaseous state.38 When facing water as a liquid, the polymeric phase might repel water molecules and maintain the stability of the structure, however the breathability of the network will not be hindered for gas vapor. Pure CNF nanopapers present the wetting behavior of hydrophilic materials with a rapid water absorption of more than 2500 % in just one day. Prolonged exposure in water even leads to disengagement of the network and redispersion of the CNFs, as seen by a decrease of the mass of the swollen material after one week. The incorporation of the hydrophobic vitrimer within the CNF nanopapers leads to a 3 times smaller water uptake compared to pure CNF. The amount of water uptake remains relatively stable for non-cured CNF65/VP35 (NC) for prolonged times as the hydrophobic vitrimer phase provides structural integrity and limits water uptake. However, the greatest extent of water resistance is achieved after the curing of the materials, which allows to diminish the water uptake down to 90% in the cured CNF65/VP35 (C). Two effects contribute to this improvements: Firstly, the sintering of the vitrimer latex particles decreases the amount of hydrophilic moieties (hydroxyl and carboxylic acid groups) and also eliminates the hydrophilic interparticular layer prone to facilitate the interstitial migration of water within the material. Secondly, the introduction of covalent bonds at the CNF/vitrimer interface diminishes the hydrophilic surface layer and prevents the disengagement of the CNF network.66 Photographs of swelling tests further demonstrate the physical variation of the CNF65/VP35 (NC) and CNF65/VP35 (C) after 20 days of uninterrupted water uptake (Figure 5c,d). CNF65/VP35 (NC) presents a gel-like behavior, the sample can be easily damaged and it is unable to hold its own weight, whereas CNF65/VP35 (C) shows its water resistance by remaining exactly the same shape as it was before being in contact with water. Note that we also measured the contact angles of water on top of the CNF, CNF65/VP35 (NC) and CNF65/VP35 (C), which develop from a hydrophilic state on pure CNF (45.4° ± 0.1°) to an increasingly hydrophobic state as seen by the increase for CNF65/VP35 (NC) (75.7° ± 0.1°) and CNF65/VP35 (C) (79.7° ± 0.4°). However, the only minor changes between the non-cured and cured sample of less than 5° clearly indicate that the full swelling tests in water are most meaningful to elucidate the water response of the films and that contact angle measurements alone, similar to water sorption isotherms (Figure 5a), are insufficient to fully reveal the behavior.


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Figure 5. Water uptake behavior of CNF/vitrimer nanocomposites before (NC) and after the curing step (C). (a) Moisture uptake as a function of relative humidity (RH). (b) Time-dependent water absorption over 20 days of exposure in pure water. A loss in water content (mass) indicates redispersion of material. (c,d) Photographs of physical variation of dry CNF65/VP35 (NC) and CNF65/VP35 (C) before (c) and after 20 days of immersion in pure water (d).

The effect of the wettability properties of the nanocomposites on their mechanical behavior were further evaluated by tensile tests within a controlled humidity atmosphere (Figure 6a-e). Again, we represent the variation of the tensile properties as a function of the vitrimer content together with the crosslinking effect. Pure CNF nanopapers undergo a softening and an increase in ductility as humidity increases, as the water content weakens interfibrillar interactions and facilitates interfibrillar movement.63 The inclusion of flexible vitrimer in the nanocomposites structure increases the inelastic behavior and allows the materials to undergo higher deformation. After curing, all CNF/vitrimer nanocomposites (Figure 6c,e) are stiffer and stronger in every humidity scenario compared to the non-cured samples (Figure 6b,d). Table S1 summarizes all mechanical properties.

Figure 6. Mechanical tensile behavior as a function of relative humidity for CNF/vitrimer nanocomposites with 35 and 50 wt% vitrimer. Tensile curves from 20 to 99% RH and soaked in water (4 h) for (a) CNF, (b) CNF65/VP35 (NC), (c) CNF65/VP35 (C), (d) CNF50/VP50 (NC) and (e) CNF50/VP50 (C). Gradient arrows show stiff/strong-to-ductile transition obtained by plasticizing effect of water molecules in the nanocomposites. Red arrows show water resistance effect obtained only after sintering of the vitrimer. The SEM micrographs of the fracture cross sections of (f) CNF, (g) CNF65/VP35 (NC) and (h) CNF65/VP35 (C) at 80% RH.



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The most striking difference can be observed when exposing non-cured and cured nanocomposites to pure water. Figure 7 compares the samples soaked in water for 4 h, showing how strong the sintering of the vitrimer and the covalent fixation between the vitrimer and the CNF interface allows CNF65/VP35 (C) and CNF50/VP50 (C) to maintain considerable stiffness (E = 0.20 – 0.30 GPa) and tensile strength (σb = 20 – 35 MPa). Conversely, swollen pure CNF nanopapers demonstrate a loss of mechanical properties by displaying a stiffness up to 13 times lower (0.01 – 0.02 GPa) with minimum tensile strength (2 – 4 MPa).

Figure 7. Water resistance after thermo-induced exchange and crosslinking reactions of vitrimer into CNF-based nanocomposites. Tensile curves for CNF/vitrimer nanopapers soaked in water for 4 h. Red arrows represent the crosslinking of CNF65/VP35 and CNF50/VP50.

To fully understand the deformation mechanisms of nanocomposites at elevated humidity, Figure 6f-h illustrates the cross section of fracture areas after tensile tests at 80 %RH in the strongly plasticized state. CNF65/VP35 (NC) demonstrates the lubricating effect of the soft, un-sintered, vitrimer phase with the most pronounced pull-out of single CNFs in between their mesoscale layers. The sintering reduces the pull-out of CNFs and the movement of the mesoscale layers. Pristine CNF nanopapers display the lowest degree of movement, corroborating the mechanical tensile data. Although water stability in cellulose nanocomposites have been studied 38,54,67-72, to our knowledge this is one of the first attempts to obtain water resistance in CNF-based nanocomposites by activation of thermo-induced exchange and crosslinking reactions. Hence, this opens the opportunity to continue developing the design further. Interfacial Adhesion and Vitrimer Welding While we have shown above that covalent bridging between the vitrimer and the CNF occurs during the sintering, we propose next a more quantitative measure of the interfacial bonding strength due to transesterification reactions between the hydroxyl and carboxylic acid groups at the surface of the CNFs, and the ester groups present in the vitrimer phase during sintering. Note that creep tests measured previously on the cured vitrimer materials enabled to calculate relaxation times at 120 °C to about 20 h, giving some indication of the involved time scales for bond shuffling.56 We compared thus the interfacial adhesion of vitrimers sandwiched between two CNF films in three different conditions: 1) liquid, un-cured vitrimer particles (VP-NC) hot-pressed for 1 h at 120 °C, 2) fully pre-cured (C) vitrimer film hot-pressed for 1 h at 120 °C and 3) liquid, un-cured (NC) vitrimer particles hot-pressed for 1 h at 120 °C, and further cured for 24 h at 120 °C. In the first case, the VP should be able to migrate, in a limited way, within the low porous CNF surface, but the curing and sintering process should remain incomplete. In the second case, the vitrimer film is fully pre-cured and it can only bind to the CNF surface via dynamic reorganization of the network. Such reorganizations lead to deformation of the vitrimer film to match the CNF surface, reacting by interfacial transesterification reactions, and however remain incomplete as the processing time is shorter than the 12 ACS Paragon Plus Environment

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relaxation time. In the third case, the adhesion should be optimal as the VP is able to migrate shortly at the low porous CNF surfaces, and can be effectively mechanically anchored due to curing of unreacted groups and sintering of the vitrimer phase. Strong interfacial bonding between CNF and vitrimers should also be enabled by transesterification reactions. During the hot-pressing, the non-cured, liquid VP is squeezed out from the joint (cases 1 and 3), and only leaves a very small interfilm layer (ca. 1 µm), while for the pre-cured VP materials (case 2), interlayer films of well-defined thicknesses can be obtained. Due to the different thicknesses the following data are all discussed as force/displacement curves. SEM micrographs show absence of voids and tight anchorage to the CNF layers with the VP glue layer, which implies a favorable heat-induced interaction between both surfaces (Figure 8c). Note that pressing at room temperature does not provide any effective bonding for any of the tests (Figure 8d). The characteristic adhesive strengths of the joints were assessed by single lap shear experiments (Figure 8b). In all cases the samples fail at the weakest parts, which are the interstitial vitrimer layers, however, distinct differences are obvious. Figure 8b compares the force/displacement behavior of uncured and cured vitrimer films between two CNF nanopapers. Solid, crosslinked vitrimer (C, green curve, case 2) provides a reduced stiffness and lower strength (about 50%) when compared to uncured vitrimer particles (NC, blue and red curves, cases 1 and 3). This is due to the fact that uncured VPs (NC) are soft and deformable. They can thus flow, conformably adapt and interact to a greater degree with the nanopaper surfaces, therefore providing a higher bonding strength. Moreover, once the vitrimer is cured for an extended period of time (24 h), we see an extended ductility of the overall specimen (red curve), whereupon the stress is transferred efficiently into the vitrimer layer and leads to delayed failure with inelastic deformation of the vitrimer layer. This layered material presents a final displacement nearly three times higher than pure CNF. The calculated shear strengths (τb) of the joints (eq 1) are shown in Table 2 and demonstrate a good interfacial adhesion as they range between 90 – 165 MPa for VP (C) and VP (NC). A slightly higher value for VP (NC) once more relates to its better interaction with the CNF. Such strength values are obtained when two materials present a favorable adhesion and therefore allow to sustain high stresses evenly during tensile loading.73-74 Figure S5 additionally discusses the dependence of the VP (C) system for different joint thickness. 𝜏$ =

'()*

(1)

+$

where Fmax, L and b are the maximum force, length of joint and width of joint, respectively.



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Figure 8. Interfacial adhesion of vitrimer as glue layer between CNF films measured by single lap shear tests. (a) Representative scheme of the single lap shear test samples, following to the suggested reconfiguration of vitrimer into CNF anchor points through transesterification exchange reactions at high temperature. (b) Tensile curves for single lap shear tests in force/displacement behavior as function of the vitrimer curing method within CNF films. Pure CNF is shown for comparison. (c) SEM of the side view of the adhesive joint area (case 2). (d) Preparation example for single lap shear test of fully pre-cured vitrimer (C) samples, presenting effective interfacial adhesion only in presence of heat-induced exchange reactions at the components interface. When heated, the vitrimer is able to adhere smoothly with the CNF surfaces and leads to bonding (left), while the lack of heat maintains the vitrimer as non-reacting agglomerates between the CNF and bonding does not occur (right). Photographs are taken from different angles.

Table 2. Shear strength values of the joints in the interfacial adhesion tests Sample

L (mm)

b (mm)

Fmax (N)

Displacement at break (mm)

tb (MPa)

VP (NC) + HP + Curing VP (NC) + HP VP (C)

5 5 5

2 2 2

1.1 ± 0.3 1.7 ± 0.2 0.8 ± 0.2

2.25 ± 0.02 0.17 ± 0.04 0.17 ± 0.03

112 ± 34 165 ± 15 90 ± 20

Lastly we turn to a preliminary approach towards welding of vitrimer/CNF nanocomposites, by quantifying the bonding strength of two nanocomposite films after hot-pressing at small overlap area (3 bar, for 24 h at 120 °C; Figure 9).75 Due to the slow relaxation time of the transesterification reactions, a first attempt of welding at 120 °C for 1 h did not lead to a cohesive bond in the laminate. The lack of adhesion also demonstrates that potential further crosslinking of any remaining non-reacted epoxy/acid groups is not an important contributor forming adhesive bonds as this should happen on a more rapid time scale (< 1h). A partial recovery of mechanical properties for pristine CNF65/VP35 (C) is only obtained after 24 h at 120 °C. Figure 9b illustrates the tensile curves of the corresponding single lap shear test with an overlap area of 5 mm × 2 mm. The welded nanocomposite nanopaper follows a similar force/displacement curve, but only provides a force at break of ca. 1 N, while a pristine CNF65/VP35 (C) sample breaks ca. at 3 N. Thus, the laminate with a partial overlap region regains 33 % of the initial bonding strength with a comparable elastic zone. However, a clean separation on the welding area during failure suggests that the needed interfibrillar entanglements of the CNFs to reach further into the inelastic deformation regime have not been attained. This can be understood considering the low dynamics for forming CNF entanglements on a colloidal level. The overall behavior suggests yet the presence of strong interfacial exchangeable bonds at the vitrimer/CNF interface and in the 14 ACS Paragon Plus Environment

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vitrimer phase, however, it also encourages the improvement of the vitrimeric chemistry and in the lamination process to obtain better healing behavior in bioinspired nanocomposites.

Figure 9. Welding of CNF/vitrimer nanocomposite films by two hot-pressed nanocomposite films. (a) Welding example for CNF65/VP35 with no added chemicals at 120 °C for 24 h. (b) Characterization of welding strength by tensile curves for single lap shear tests. For strength comparison, the welded sample is plotted against CNF65/VP35 (C) in force/displacement behavior.

CONCLUSIONS We reported the first study integrating vitrimer chemistries in water-borne bioinspired nanocomposites based on CNFs. By increasing the vitrimer content the overall ductility increases, whereas the stiffness is regained through activation of the crosslinking and transesterification reactions. This interfacial crosslinking, carefully elucidated through mechanical tests and microscopy, allows to address one of the biggest limitations of CNF-based materials, their hydrophilicity. The addition of a hydrophobic vitrimeric phase protects CNFs against water interactions, limits the water uptake of the system to a minimum while its mechanical resistance stands out greatly. This concept has thus far only exploited one part of the vitrimer potential, which is the introduction of covalent crosslinks at the interface and a hydrophobization, while in future, we will exploit the bond-shuffling reactions to understand the potential of the materials for hot-pressing into laminates and shaping, as well as for generating switchable and adaptive properties in bioinspired nanocomposites.

ASSOCIATED CONTENT Supporting Information Supporting measurements such as DLS, AFM and DSC, photographs of homogeneous dispersions, and additional mechanical properties of tensile and single lap shear tests.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]



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*E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the BMBF in the framework of the AQUAMAT research group and the Deutscher Akademischer Austauschdienst (DAAD) for a scholarship.

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Vitrimer Chemistry Meets Cellulose Nanofibrils - ACS Publications

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