Water-Soluble Cellulose Derivatives as Suitable Matrices for

Article Cite This: Biomacromolecules 2019, 20, 2786−2795

pubs.acs.org/Biomac

Water-Soluble Cellulose Derivatives as Suitable Matrices for Multifunctional Materials Mikel Rincoń -Iglesias,† Erlantz Lizundia,*,†,‡ and Senentxu Lanceros-Meń dez†,§ †

BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain Department of Graphic Design and Engineering Projects, Faculty of Engineering in Bilbao, University of the Basque Country (UPV/EHU), 48013 Bilbao, Spain § IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain Downloaded via GUILFORD COLG on July 18, 2019 at 15:58:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: This work reports on a simple and environmentally benign route to prepare freestanding magnetic films based on cellulose derivatives through the combination of cobalt ferrite (CoFe2O4) nanoparticles with methyl cellulose (MC), hydroxypropyl cellulose (HPC), and sodium carboxymethyl cellulose (NaCMC). Nanoparticles are able to “shield” hydrogen bonding interactions between polysaccharide chains and lower the viscosity of water-dissolved MC, HPC, and NaCMC, allowing an easy film fabrication. Crack-free films with homogeneously dispersed nanoparticles having concentrations up to 50 wt % are fabricated by mechanical agitation followed by doctor blade casting. All of the nanocomposite films keep a substantial level of flexibility with elongation at break exceeding 5%. Halpin− Tsai equations serve to provide further insights on the character of matrix− CoFe2O4 interfaces. Magnetization saturation increases almost linearly with cobalt ferrite concentration up to a maximum value of ∼24−27 emu g−1 for nanocomposites containing 50 wt % of nanoparticles. The dielectric response of the films demonstrates a strong dependence on both the functional groups attached to the main cellulose chain and the ferrite nanoparticle content. The renewable character of the hosting matrices, together with the fabrication methods that solely uses water as a solvent, the decrease of the viscosity with the inclusion of fillers, particularly suitable for printable materials, and the resulting magnetic performance provide novel avenues for the replacement of traditional magnetoactive composites based on petroleum-derived polymers and avoiding the use of toxic solvents.

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INTRODUCTION Biopolymer films based on polysaccharides, lipids, and proteins are being used for packaging purposes as, besides their effective food protection ability during storage, they can alleviate the alarming environmental pollution issues associated with petroleum-derived plastic.1 Furthermore, polymers based on natural resources are one of the platform materials with highest potential to develop novel multifunctional devices, thanks to their abundance, biodegradability, nontoxicity, and potential for adopting circular-economy principles.2,3 In this framework, cellulose ((C6H10O5)n), as the most widely available natural source of raw materials, emerges as an excellent choice to develop added-value materials for diverse technological applications as it combines low density and compostability4 with high strength and stiffness.5 The outstanding properties of cellulose mainly arise from the presence of β-1,4-linked glucopyranose chains with strong inter- and intramolecular hydrogen bonds.5,6 Unfortunately, such strong hydrogen bonding makes cellulose nonsoluble in many organic solvents and water, which makes its processability difficult and thus limits its potential applications.7 In this sense, many attempts have been carried out to modify cellulose to make it water soluble as this would enable its processing through commonly © 2019 American Chemical Society

available technologies, including additive manufacturing technologies.8 More precisely, cellulose derivatives have shown remarkable film-forming properties,9 making this group of materials attractive for the development of multifunctional films. Many cellulose derivatives are available via a controlled modification of the cellulose hydroxyl groups, where methyl cellulose (MC), hydroxypropyl cellulose (HPC), and sodium carboxymethyl cellulose (NaCMC) are some of the most commercially relevant ones. MC is the simplest cellulose derivative, which can be obtained by replacing the hydroxyl groups with methoxy groups. This biodegradable cellulose derivative is extensively used in food industry, cosmetics, gels, and adhesives,10,11 as a result of its lower critical solution temperature (LCST) of 29 °C, which acts as a viscosity controller (at low temperatures, a Newtonian flow is observed, whereas above 29 °C, an increased viscosity is achieved).12 Moreover, MC shows shear-thinning properties, which is beneficial for the fiber spinning process and printing Received: April 26, 2019 Revised: May 24, 2019 Published: May 31, 2019 2786

DOI: 10.1021/acs.biomac.9b00574 Biomacromolecules 2019, 20, 2786−2795

Article

Biomacromolecules technologies.8 HPC is a water-soluble cellulose ether with many applications in the fields of food, medicine, construction, and smart materials.6,13,14 It shows good film-forming capacity, is highly transparent, presents a good resistance to oil and fats, and shows an LCST of 41 °C in water.15 As, HPC has been used as a stabilizing agent to improve the dispersibility of inorganic nanoparticles into polymeric matrices; nanoparticles dispersed in HPC typically show a homogeneous distribution.16 NaCMC is an anionic cellulose derivative produced at an industrial scale, which is obtained through the introduction of carboxymethyl groups (−CH2COOH) into the 2, 3, and 6 hydroxyl groups of cellulose.17 It can be obtained through a controlled reaction of a cellulose alkali with sodium monochloroacetate to give polar carboxyl groups that make this carbohydrate soluble in water.18 Thanks to its low-cost, renewability, biodegradability/biocompatibility, high viscosity, transparency, and hydrophilicity, it has been used for applications as varied as tissue engineering, food industry, textile, paper industry, and water treatment.19 In particular, many works have been focused to develop NaCMC-based hydrogels as their numerous hydroxyl and carboxylic groups allow an efficient water binding. Importantly, despite the fact that all of these three derivatives are water soluble, they are able to keep their morphology and do not become sticky when exposed to humidity, allowing their use in industrial applications. In order to obtain hybrid materials with specific functional responses, cellulose derivatives have been reinforced with nanoparticles as varied as graphene oxide,20 bentonite,19 attapulgite,21 ZnO,22 ZnS,23 halloysite nanotubes,24 montmorillonite,25 carbon dots,26 and carbon nanotubes.27 Overall, improved thermal, barrier, mechanical, electrical, and catalytic properties were obtained. In spite of such efforts, a few works have been focused on the development of magnetic nanocomposites based on cellulose derivatives, being magnetic materials an essential component of printed electronics, sensors, actuators,28 among others. The incorporation of magnetic nanoparticles into a hosting matrix can, for instance, provide a temporally and spatially controllable material under external magnetic fields, affording a noninvasive approach to remote control. Among all of the magnetic nanoparticles currently available, iron oxides such as cobalt ferrite (CoFe2O4) display outstanding properties as they present a high saturation magnetization of 60 emu g−1 under 10 T at 293 K, small coercivity values (0.27 T), and low toxicity (cell viabilities exceeding 70% at concentrations as high as 1.8 mg mL−1 for biocompatibility studies carried using L929 cell line).29−31 These nanoparticles have shown a great potential to develop multifunctional materials when incorporated into polymeric matrices such as poly(vinylidene fluoride) (PVDF) and its copolymers.29,32,33 In this sense, the combination of the magnetic and dielectric properties of cobalt ferrite together with the inherent electroactive properties of cellulose34 may open novel avenues for the development of environmentally benign multifunctional materials that can replace the existing magnetic materials based on rare earth metals and provide a wide range of applications in the fields of energy storage, sensors, actuators,8 smart scaffolds, and biomedicine.35 Additionally, cellulosic hybrid materials emerge as a sustainable alternative to replace the existing ferroelectric materials such as PVDF and its copolymers,28,36 which apart from being based on petrochemical resources typically require

the use of hazardous aprotic solvents such as dimethyl formamide.37,38 Till date, several cellulose derivative-based nanocomposites have been reported so far.39−41 Importantly, the applicability of such materials for bioactive packaging has been mainly exploited with the aim of improving food safety,42−44 leaving room for the development of cellulose derivative-based nanocomposites for multifunctional applications. Accordingly, the aim of this work is to fabricate and characterize a series of multifunctional materials based on cellulose derivatives. Nanocomposite films based on methyl cellulose (MC), hydroxypropyl cellulose (HPC), and sodium carboxymethyl cellulose (NaCMC) as a matrix and CoFe2O4 as a magnetic filler have been prepared by mechanical agitation followed by doctor blade casting. Interestingly, the approach here used only requires the use of water as a solvent. The morphology, optical properties, mechanical behavior, and magnetic and dielectric properties of nanocomposites with concentration up to 50 wt % are investigated.

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EXPERIMENTAL SECTION

Materials. Hydroxypropyl cellulose powder with a Mw of 100.000 g mol−1 (191884-100G), methyl cellulose powder with a viscosity of 400 cP (M0262-100G), and sodium carboxymethyl cellulose with a Mw of 250.000 g mol−1 and a degree of substitution of 1.2 (419281100G) were supplied by Sigma Aldrich and were used as received (Scheme 1). Cobalt ferrite (CoFe2O4) spherical nanoparticles of 35− 55 nm size range were purchased from Nanostructured & Amorphous Materials Inc.

Scheme 1. Molecular Structure of the Cellulose Derivatives Used in This Work in Regard to Original Cellulose: Methyl Cellulose (MC), Hydroxypropyl Cellulose (HPC), and Sodium Carboxymethyl Cellulose (NaCMC)

Sample Preparation. Contrary to the in situ synthesis processes where a precursor material is typically reduced under relatively harsh conditions to yield the desired compound,45 here, we report on the fabrication of hybrid materials through simple aqueous mixing. The 40 μm-thick films of three cellulose derivative/CoFe2O4 nanocomposites with nanoparticle concentrations from 5 to 50 wt % are fabricated by a doctor blade technique (Scheme 2). The first step involves nanoparticle dispersion and cellulose derivatives dissolution in water. CoFe2O4 were dispersed using an ultrasound bath during 6 h, whereas cellulose derivatives were dissolved in distilled water with a Teflon mechanical stirrer for the same time period. To ensure a sufficiently low viscosity, the concentrations of cellulose derivatives in water were optimized to be 2.5, 10, and 4 wt/vol % for MC, HPC, and NaCMC, respectively. Then, dispersed nanoparticles were incorporated into the cellulose derivative solution and the resulting mixture was mechanically stirred for 1 h. It should be taken into account that although concentrations of 50 wt % are often too large in the nanocomposite field, the aim of this work is to show a proof of 2787


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Scheme 2. Schematic Diagram Showing the Preparation of Methyl Cellulose (MC), Hydroxypropyl Cellulose (HPC), and Sodium Carboxymethyl Cellulose (NaCMC) Composite Films through a Doctor Blade Method

concept on the suitability of water-soluble cellulose derivatives as matrices for highly magnetic active materials. Therefore, samples with a wide range of concentrations have been fabricated. Finally, the mixtures were deposited with the doctor blade technique onto clean glass substrates, allowing water evaporation at room temperature. Very importantly, drying temperature remains below the LCST of MC and HPC, avoiding undesired thermally induced phase changes that result in poor mechanical performance.15,46 Neat films were also prepared for comparison. Overall, the approach here reported results of an energetically efficient straightforward method that enables a homogeneous dispersion of CoFe2O4 nanoparticles with no need of complex grafting steps. Characterization. Fourier transform infrared (FTIR) spectroscopy measurements in attenuated total reflection (ATR) mode were performed on a Jasco FT/IR-6100 spectrometer equipped with diamond ATR optics. IR spectra were collected after 64 scans taken in the range 3800−600 cm−1 with a resolution of 2 cm−1. The morphologies of both fracture surfaces and top surfaces of the nanocomposites were evaluated using a HitachiS-4800 field-emission scanning electron microscope (FESEM) at an acceleration voltage of 5 kV. Before imaging, samples were sputtered with a 10 nm-thin gold−palladium layer. Ultraviolet−visible (UV−vis) spectroscopy measurements were performed with an Agilent Cary 60 UV/vis double beam spectrophotometer. Total transmittance experiments have been analyzed in the range of 200−800 nm with a sampling interval of 1 nm and 25 accumulations. Mechanical properties of the nanocomposites were evaluated by tensile testing. Experiments were carried out on a AGS-X universal testing machine from Shimadzu at 1 mm min−1 by films of 30 mm length, 10 mm width, and 40 μm average thickness. Young’s modulus (E) (calculated from the slope in the 0.5−1% strain region), stress and strain at yield (σy and εy, respectively), and stress and strain at break (σb and εb, respectively) were determined. Reported values represent a mean average value and standard deviation over five specimens. Magnetic hysteresis loops were measured at room temperature using a MicroSense EZ7 vibrating sample magnetometer (VSM) from −1.8 to 1.8 T. Dielectric properties were obtained by measuring the capacity, C, and the dielectric losses, tan δ, using an automatic Quadtech 1929 Precision LCR meter. Before analysis, samples were coated with gold circular electrodes (Ø 5 mm) onto both sides by magnetron sputtering (SC502 sputter coater; 0.5 V signal was applied in the 100 Hz to 1 MHz range). The real part of the dielectric constant (ε′) was extracted from the capacity (C) measurements taking into account the thickness (d) and area (A) of the samples according to30

ε′ =

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C·d A

(1)

RESULTS AND DISCUSSION Physicochemical Characterization and Nanoparticle Dispersion. Fourier transform infrared (FTIR) spectroscopy has been performed to evaluate the occurrence of chemical interactions between water-soluble cellulose and cobalt ferrite. Accordingly, Figure 1 shows the FTIR spectra in the 3800−

Figure 1. FTIR spectra in the 3800−600 cm−1 region for neat methyl cellulose (MC), hydroxypropyl cellulose (HPC), and sodium carboxymethyl cellulose (NaCMC) films (a) and FTIR spectra of MC/CoFe2O4 nanocomposite films for concentration up to 50 wt % (b).

600 cm−1 region of MC, HPC, and NaCMC nanocomposite films with a CoFe2O4 concentration up to 50 wt % (see Figure S1 for enlarged FTIR spectra in the 1800−800 cm−1 region). It is seen that pure MC displays main absorption peaks at 3458 cm−1 (O−H stretching), 2902 cm−1 (C−H stretching), 1646 cm−1 (C−O stretching from the glucose moiety), 1455−1251 cm−1 (C−H bending) and 1065 cm−1 (C−O stretching from asymmetric oxygen bridge), and 947 cm −1 (O−CH 3 stretching).47 On the other side, the neat HPC film shows a characteristic broad band at 3440 cm−1 due to the −OH groups and narrower bands centered at 2972, 1652, 1376, and 1056 cm−1 arising from the C−O−C asymmetric valence vibration, CC stretch, −CH2 vibration, and −C−OH groups, respectively.46 Finally, NaCMC presents a wide peak at 3420 cm−1 as a result of the −OH stretching and intra/ intermolecular hydrogen bonds and a weaker absorption band at 2913 cm−1 due to the C−H stretching. The narrower peaks at 1605, 1422, and 1128 cm−1 are ascribed to the asymmetrical 2788


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Figure 2. Representative field-emission scanning electron microscopy (FESEM) images of film surfaces for NaCMC/CoFe2O4 nanocomposite films with different CoFe2O4 concentrations: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 25 wt %, and (e) 50 wt %.

Figure 3. UV−vis transmittance spectroscopy for neat films (a) and MC/CoFe2O4 nanocomposite films (b). The inset shows the UV−vis transmittance of highly concentrated samples.

evident in the case of polymer nanocomposites, where aggregation effects may lead to poor properties such as inefficient stress transfer across the filler−matrix interface,50 or nonelectrically conducting materials due to the agglomeration of conductive fillers that do not form a percolated network.51 Accordingly, the morphology of the fabricated nanocomposites has been assessed by field-emission scanning electron microscopy (FESEM). Figure 2 shows representative FESEM images of film surfaces for NaCMC/CoFe2O4 nanocomposites. Neat NaCMC presents a crack-free homogeneous surface with no irregularities, highlighting the potential of the fabrication procedure used here to develop low roughness substrates. Independently of the concentration, CoFe2O4 nanoparticles are observed as bright spots randomly embedded within the darker NaCMC matrix, where the higher the concentration, the larger the bright regions in the FESEM micrographs. These observations indicate that, despite the large concentrations here prepared, nanoparticles are homogeneously dispersed within the NaCMC. FESEM images in Figures S2 and S3 indicate that MC/CoFe2O4 nanocomposite films also show good nanoparticle dispersion, although HPC/CoFe2O4 nanocomposites show a more aggregated structure at a loading of 50 wt %. The fractured edge normal to the NaCMC/CoFe2O4 nanocomposite film surfaces in Figure S4 shows that cobalt ferrite incorporation does not substantially modify the original structure of neat NaCMC. All of the samples present a relatively rough surface characteristic of a ductile fracture.52 No voids as a result of filler debonding can be found in these images, suggesting a good physical compatibility between the CoFe2O4 filler and organic hosting matrix.52 Moreover, no

and symmetrical stretching of carboxyl methyl ether and to the >CH−O−CH2 stretching, respectively.20,23 Overall, no band displacement is observed upon the addition of cobalt ferrite for all of the three hosting matrices, suggesting that no intermolecular interactions between MC−HPC−NaCMC and CoFe2O4 exists. Furthermore, no significant variations are observed as a result of the increasing filler concentration for the different cellulose derivatives. Importantly, as proven by the absence of the band corresponding to the O−H bond in ferrites (typically achieved at 894 cm−1), cobalt ferrites were not oxidized during their processing in water.48 It is worth noting that we found that nanoparticle incorporation during fabrication decreases the inherent high viscosity of water-dissolved MC, HPC, and NaCMC. This effect facilitates the fabrication of homogeneous films as it provides more appropriate flow properties to the mixtures. Such behavior may arise from the interference of the characteristic three-dimensional system of hydrogen bonds typical for cellulose derivative aqueous solutions. On the one hand, the presence of CoFe2O4 may increase the distance between adjacent cellulosic macromolecules, therefore lowering the amount of interactions between −OH groups (see Scheme S1). On the other hand, a certain amount of chains can be adsorbed onto the surface of cobalt ferrite nanoparticles, decreasing the average density of macromolecule entanglements in the solution.49 This explanation agrees well with FTIR results, where the presence of a mere physical interaction between cellulosic derivatives and CoFe2O4 is suggested. It is well established that the properties of materials are dependent on their morphological structure. This is even more 2789


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Figure 4. Representative stress−strain curves for MC/CoFe2O4, HPC/CoFe2O4, and NaCMC/CoFe2O4 nanocomposites.

Figure 5. Experimental data and fitting results of MC/CoFe2O4, HPC/CoFe2O4, and NaCMC/CoFe2O4 nanocomposites determined according to the modified Halpin−Tsai model.

of 5.4 ± 1.2 and 2.3 ± 0.9%, respectively.25,58,59 On the contrary, neat HPC shows a semiductile behavior with a modulus of 600 ± 40 MPa and an elongation at break of 12.9 ± 3.6%.46,57 Generally, Young’s modulus and tensile strength tend to increase with the addition of cobalt ferrite for HPC and NaCMC-based hybrids, whereas E does not show any improvements when CoFe2O4 nanoparticles are incorporated into MC. The increase of E is particularly marked in the case of NaCMC, where it increases from 1120 ± 140 MPa to a maximum value of 3500 ± 320 MPa for its 5 wt % counterpart. Interestingly, the addition of cobalt ferrite does not necessarily decrease the ductility of the resulting material as typically observed for polymer composites, where the presence of particles leads to premature fracture due to stress concentration effects at filler-rich regions.59 Indeed, for all three matrices, the presence of low amounts (i.e., 5 and 10 wt %) of nanoparticles results in increased elongation at break values. This may be due to decreased hydrogen bonding between MC, HPC, or NaCMC chains as a result of the “screening” effect of added nanoparticles, which decrease the chain rigidity to enhance the overall ductility (these results are in line with both the experimentally observed decrease on the viscosity of the suspensions and the FTIR results). Interestingly, prepared films can be easily folded as depicted in Figure S6, making these materials suitable for flexible electronic applications. There is a given concentration in which further increase of the nanoparticle content does not contribute to improve the mechanical performance of the resulting composite. The presence of such critical concentration where further increase of the nanoparticle content is detrimental for the mechanical properties of the material has been reported for diverse cellulose derivative composite materials.24,60 This behavior has been ascribed to nanoparticle aggregation, which may decrease the effective filler−matrix interactions and create weak points in the material, although no cavities were observed through FESEM analysis.44 Importantly, even at a loadings as high as

aggregates are observed in the cross-sectional images, underlining that inorganic nanoparticles are well intercalated through the whole thickness of the film. Optical Properties. Optical properties of the nanocomposite films were evaluated by ultraviolet−visible (UV− vis) spectroscopy in transmittance mode. Figure 3a displays the transmittance (%) of the neat films within the λ = 200− 800 nm region, whereas Figure 3b shows the corresponding transmittance for MC/CoFe2O4 films (Figure S5 provides their macroscopic appearance). Neat MC, HPC, and NaCMC films present optical transparencies to 84, 88, and 91%, respectively, according to the ASTM D1746-15 standard (Standard Test Method for Transparency of Plastic Sheeting),53 which determines the optical transparency as the transmittance (%) in the 540−560 nm range. Such high optical transmittance values make all three MC, HPC, and NaCMC potential candidates for flexible electronics, transistors, optical sensors, and touch screens.54−56 The incorporation of CoFe2O4 nanoparticles within the three matrices yields optically black films where the amount of transmitted photon through the nanocomposite films is markedly decreased, showing transmittance values of nearly 0.5% for concentrations exceeding 5 wt %.30 Mechanical Properties. The determination of tensile properties such as Young’s modulus (E), maximum tensile stress (σy), and elongation at break (εb) can be used to predict the capacity of fabricated materials to withstand applied stresses when in use. Accordingly, mechanical properties of the materials have been evaluated by uniaxial tensile tests. Figure 4 shows the representative stress−strain curves for all of the obtained compositions, whereas Table S1 summarizes the main mechanical property values. Obtained data deviation may arise from the susceptibility of the prepared materials to moisture.57 Neat MC and NaCMC present a semibrittle behavior characterized by E of 2620 ± 390 and 1120 ± 140 MPa and εb 2790


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Figure 6. Room-temperature hysteresis loops for MC/CoFe2O4 (a) and magnetization saturation (Ms) for all of the studied compositions (b).

Figure 7. Real part of the dielectric constant (a) and the tan δ dielectric loss values (b) as a function of frequency for neat polymers. Real part of the dielectric constant (c) and the tan δ dielectric loss values (d) measured at 1 kHz for all of the studied compositions.

50 wt %, the freestanding nanocomposite films prepared here are able to keep moderate mechanical flexibility, encouraging their use for diverse flexible electronic applications. Experimentally obtained data can be compared with theoretical predictions according to the modified Halpin− Tsai model61,62

Overall, experimental values remain below predicted data in the case of MC/CoFe2O4, whereas fit markedly well for the HPC/CoFe2O4 system and remain above the theoretical predictions for NaCMC/CoFe2O4 nanocomposites. These results suggest a poor MC−CoFe2O4 interface, which becomes more noticeable as the cobalt ferrite concentration increases (a larger mismatch between theoretical and experimental results is found).64,65 On the contrary, the homogeneous nanoparticle distribution together with the effective stress transfer through the HPC−CoFe2O4 interfaces results in a perfect fit between the modified Halpin−Tsai prediction and the experimental results. Finally, a strong electrostatic interaction between the negatively charge carboxylate groups of NaCMC and the positive cobalt ferrite yields a remarkable modulus enhancement.66 However, after reaching a maximum value, E decreases with further nanofiller addition, which can be attributed to the aggregation effect of cobalt ferrite. Magnetic and Dielectric Properties. The magnetic properties at room temperature of the fabricated materials have been investigated using a vibrating sample magnetometer (VSM) to explore the potential of such materials for magnetic applications. Figure 6a depicts hysteresis loops for MC/ CoFe2O4 nanocomposites in the −1.8 to 1.8 T range, whereas the magnetization saturation is shown in Figure 6b (see Table S2 for further details). The magnetization increases with the applied magnetic field until saturation with coercivity values of 0.27, 0.23, and 0.25 MC/CoFe2O4, HPC/CoFe2O4, and

Ec i 3 yji 1 + 2ρηLVCoFe2O4 zyzz ij 5 yzjijj 1 + 2ηTVCoFe2O4 zyzz = jjj zzzjjjj z + jj zzj z Em k 8 {j 1 − ηLVCoFe2O4 zz k 8 {jj 1 − ηTVCoFe2O4 zz k { k {

(2)

ηL =

Er − 1 E −1 ηT = r Er + 2ρ Er + 2

(3)

where Ec and Em are the Young’s moduli of the composite and matrix, respectively, ρ is the CoFe2O4 nanoparticle aspect ratio (set at 1 due to its spherical shape), VCoFe2O4 is its volume fraction within the nanocomposite, and Er is defined as the ratio between the Young’s modulus of the filler and the matrix. CoFe2O4 volume fractions are extracted from the weight fraction of both components and their densities, assuming a density of 5.30 g cm−3 for CoFe2O4 (obtained from the supplier) and 1.62,63 0.50, and 1.59 g cm−3,59 for MC, HPC (obtained from the supplier), and NaCMC, respectively. Figure 5 compares the predicted Young’s modulus values according to eq 3 with the experimentally obtained results. 2791


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Figure 8. Electrical conductivity of neat MC, HPC, and NaCMC films as a function of frequency (a) and obtained electrical conductivity values at 1 kHz for MC/CoFe2O4, HPC/CoFe2O4, and NaCMC/CoFe2O4 nanocomposite films as a function of cobalt ferrite nanoparticles (b).

explained in terms of energy dissipation via charge flow across the material. Neat MC and HPC display ε′ values of 12.6 ± 0.5 and 10.6 ± 0.8, respectively, whereas neat NaCMC shows an impressive ε′ of 729 ± 191. The high values of MC and HPC can be ascribed to the combined effect of the segmental motion of the main chains via the glucosidic bonds and to the presence of moisture/air bubbles in the film that increases interfacial contributions.67 Very interestingly, the high ε′ of neat NaCMC indicates that carboxymethylation increases the contribution of the orientational polarization due to the dipole moment of −CH2OCH2COONa over the −CH2OH group,68 although the main contribution of this strong increase is associated to the increased conductivity, as indicated by the high dielectric losses. It is observed that the inclusion of intermediate CoFe2O4 concentrations within the cellulosic matrices slightly lowers the dielectric response of hybrid films, whereas concentrations exceeding 25 wt % trend to increase ε′ values.69 This behavior is ascribed to the hindering effect provided by the rigid cobalt ferrite nanoparticles that restrict dipolar dynamic of the whole composite material.70 On the contrary, further addition of magnetic nanoparticles increases the overall ionic conductivity and provides new interaction regions, which enhance interfacial polarization contributions (Maxwell−Wagner contributions), leading to an overall increase in the dielectric constant.71 Dielectric losses in polymeric materials are associated with an energy transfer to phonon dissipation in the form of heat upon the application of an electric field. The dielectric losses show a similar trend to that observed for ε′, keeping values below 0.3 for MC and HPC-based nanocomposite films similar to the results previously obtained for PVDF/CoFe2O4.29 On the contrary, the dielectric losses are quite large for NaCMC-based films, indicative of a strong conductivity. Finally, Figure 8 summarizes the dependence of the electrical conductivity (σ) on cobalt ferrite concentration for all of the studied composites, which is computed as

NaCMC/CoFe2O4 nanocomposites, respectively, similar to the results previously obtained for CNC/CoFe2O4 nanocomposites.30 This coercivity arises from the ferromagnet behavior of CoFe2O4, which shows a blocked magnetic moment at room temperature.29 The shape of magnetization hysteresis curves indicates that cobalt ferrite nanoparticles remain randomly distributed within their corresponding matrices. Magnetization saturation (Ms) shows a nearly linear increase with CoFe2O4 concentration to yield maximum values of 23.3, 26.6, and 27.0 emu g−1 for MC/CoFe2O4, HPC/ CoFe2O4, and NaCMC/CoFe2O4 nanocomposites containing 50 wt % of nanoparticles, respectively (for comparison, the Ms of pure CoFe2O4 at 1.5 T of applied magnetic field is ∼53 emu g−1).29 Such saturation magnetization values remain above from those previously obtained under similar external fields for NaCNC/maghemite composites (13.9 emu g−1),45 the 8.5 emu g−1 obtained for cellulose nanocrystal/CoFe2O4 composites having 20 wt % of ferrite nanoparticles,30 or the 10 emu g−1 achieved for CMC/Fe3O4 with a filler concentration of 30 wt %.66 The small differences observed in Ms values are ascribed to nanoparticle aggregation effects that decrease the efficiency of cobalt ferrite nanoparticles to enhance the overall Ms of the nanocomposites.30 Overall, the almost linear increase of Ms with CoFe2O4 concentration for HPC and NaCMCbased nanocomposites suggests that nanoparticles remain finely dispersed within all of the three cellulose derivatives (the lower Ms value for MC/CoFe2O4 nanocomposites indicates particle aggregation).29 The dielectric behavior at room temperature of the fabricated materials has been evaluated to investigate the potential of such composites for possible sensing (e.g., capacitive) applications. It should be taken into account that due to their inherent hygroscopic behavior the dielectric properties of fabricated materials markedly depend on the moisture content and also on the presence of small air bubbles in the films (it becomes rather difficult to obtain completely solid films due to the high viscosity of dissolutions before casting and the doctor blade procedure). In this sense, the real part of the dielectric constant (ε′) and the corresponding dielectric loss values (tan δ) of neat polymers are shown in Figure 7a,b respectively, whereas the ε′ and the tan δ values measured at 1 kHz for all of the studied compositions are summarized in Figure 7c,d respectively. The continuous decrease of ε′ with the frequency for all of the three neat polymers (MC, HPC, and NaCMC) is attributed to the fact that upon frequency increase the dipoles are not able to follow the applied field and reorient in the direction of the applied field during each cycle. A particularly high dielectric loss peak is achieved at around 5 ×103 Hz for NaCMC, which is

σ = 2πf ·ε0 ·ε″

(4)

where f represents the frequency (Hz), ε0 accounts for the vacuum permittivity (8.854 × 10−12 F m−1), and ε″ (ε″ = ε′· tan δ) is the imaginary part of the permittivity. MC and HPC are basically insulating materials with σ values of 8.15 × 10−10 and 3.73 × 10−11 S cm−1, whereas NaCMC shows a conductivity of 4.43 × 10−6 S cm−1. A general conductivity increase is observed upon CoFe2O4 addition due to increased interfacial effects.72 2792


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Notes

CONCLUSIONS Here, we report on the use of methyl cellulose, hydroxypropyl cellulose, and sodium carboxymethyl cellulose as potential matrices to develop freestanding multifunctional materials based on renewable resources. Cobalt ferrite nanoparticles with concentrations up to 50 wt % are homogeneously incorporated within the three cellulose derivatives through simple mechanical stirring followed by doctor blade casting. The presence of cobalt ferrite decreases the amount of hydrogen bonds in the solution, lowering the viscosity of water-dissolved MC, HPC, and NaCMC and therefore allowing an easier film fabrication. Morphological observations reveal that nanoparticles remain homogeneously distributed within the cellulosic matrices, both on the surface and through the whole thickness of the films. Overall, mechanical tests reveal increased Young’s modulus for intermediate concentrations, whereas large amounts of CoFe2O4 yield aggregation issues that decrease the effective filler−matrix interactions and create weak points in the material. However, films with loadings as high as 50 wt % keep a substantial level of flexibility with elongation at break exceeding 5%, which is a crucial property when accounting for flexible electronics applications. Maximum magnetization values of 23.3, 26.6, and 27.0 emu g−1 are obtained for MC/CoFe2O4, HPC/CoFe2O4, and NaCMC/ CoFe2O4 nanocomposites containing 50 wt % cobalt ferrite, respectively. The dielectric constant at room temperature remains in the range of 10−40 for MC and HPC-based nanocomposites, whereas it dramatically increases for NaCMC films. The homogeneous character of the hybrid films fabricated here together with their magnetically and dielectrically active character highlights the potential of these materials to develop multifunctional materials based on renewable resources. Prepared hybrids match the properties of traditional magnetoactive composites but avoid the use of petroleum-derived matrices and toxic solvents. Finally, thanks to their shearthinning behavior, cellulose derivative/CoFe2O4 aqueous dispersions result attractive candidates for low-cost and ecofriendly multifunctional ink formulations. The potential of such materials for magnetoactive sensor applications is currently under investigation by our group.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Basque Government under the ELKARTEK, HAZITEK, and PIBA (PIBA-2018-06) programs is also acknowledged. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, EGEF, and ESF) is gratefully acknowledged.

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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.9b00574.

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REFERENCES

Material characterization and other data, SEM images of nanocomposite films, optical photographs of the films, and VSM results (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Erlantz Lizundia: 0000-0003-4013-2721 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 2793


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