An Acrylonitrile−Butadiene−Lignin Renewable ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

An Acrylonitrile−Butadiene−Lignin Renewable Skin with Programmable and Switchable Electrical Conductivity for Stress/ Strain-Sensing Applications Ngoc A. Nguyen, Kelly M. Meek, Christopher C. Bowland, Sietske H. Barnes, and Amit K. Naskar* Carbon and Composites Group, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6053, United States S Supporting Information *

ABSTRACT: We report an approach for programming electrical conductivity of a bio-based leathery skin devised with a layer of 60 nm metallic nanoparticles. Lignin-based renewable shape-memory materials were made, for the first time, to program and restore the materials’ electrical conductivity after repeated deformation up to 100% strain amplitude, at a temperature 60−115 °C above the glass transition temperature (Tg) of the rubbery matrix. We crosslinked lignin macromolecules with an acrylonitrile−butadiene rubbery melt in high quantities ranging from 40 to 60 wt % and processed the resulting thermoplastics into thin films. Chemical and physical networks within the polymeric materials significantly enhanced key characteristics such as mechanical stiffness, strain fixity, and temperature-stimulated recovery of shape. The branched structures of the guaiacylpropane-dominant softwood lignin significantly improve the rubber’s Tg and produced a film with stored and recoverable elastic work density that was an order of magnitude greater than those of the neat rubber and of samples made with syringylpropane-rich hardwood lignin. The devices could exhibit switching of conductivity before and after shape recovery.

1. INTRODUCTION Common resilient polymers are not electrically conducting materials. However, they are combined with electrically conducting nanoparticles to form composites that are conductors of electricity and that retain the physical properties of the neat polymer. Mechanical deformation of polymer-based nanocomposites alters conductivity, making them useful as electrochemical sensors, biosensors, and strain sensors as well as stretchable conductors and electrodes.1−12 Specifically, smart and stretchable electronic materials whose behavior mimics that of human skin is a target for components in modern robots. These materials often possess flexing and self-healing characteristics.13−15 Also, because electrical conductivity can be varied by deforming the materials at certain strain amplitudes, stretchable conductors have been sought for strain-sensing applications.10,12,16−23 For example, in a composite of a silicone substrate and carbon nanotubes, deformation-induced controlled assembly of carbon nanotubes in the substrate offers a strain-sensing capability for detection of human movement.18 The fibrillary structure and alignment of the carbon nanotubes play key roles in the deformity and conductivity of the materials. Similarly, silver (Ag) nanowires were embedded in poly(dimethylsiloxane) to make stretchable conductors.22 In a different approach, polyurethane nanocomposites of spherical gold nanoparticles prepared by “layer-by-layer” assembly or by “vacuum-assisted flocculation”20 showed reorganization of the nanoparticles and associated changes in electrical conductivity © XXXX American Chemical Society

in the matrix during stretching. However, these materials behave like elastomers. The materials’ original dimension is recovered after unloading the applied force. Although the electrical conductivity of these elastic films do not change even after repeated stretching and subsequent elastic recovery from a specific strain amplitude, it limits the ability to tune the conductivity of the material without applying a load. Deformation usually alters self-assembly of the conductors added to soft matrices and thus alters pathways for electron transport. Still, these stretchable conductors do not offer an option for on−off switch and an ability to thermally program its electrical conductivity. We aim to address this limitation by developing a thermoresponsive electrical switch with thermomechanically programmed conductivity. Shape-memory polymers have been studied intensively due to their ability to program the shape. A temporary shape can be fixed after deformation and recover its permanent shape by exposing to external stimuli.24−26 Temperature and light are possible triggers to activate the dimensional restoration.27−29 Various shape-recoverable systems exhibiting particular restoration mechanisms of the programmed shapes have been demonstrated, such as polymer networks in the presence of nanoparticle cores,30 ionic coordination complexes,31 semiReceived: November 1, 2017 Revised: December 4, 2017

A

DOI: 10.1021/acs.macromol.7b02336 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules crystalline elastomers,32 and physical and chemical cross-linked structures.33−35 The shape of thermoresponsive shape-memory polymers can be programmed due to the presence of physical cross-links. Thermal transition is activated to switch and fix the deformed shape into a predetermined shape. The original shape is recovered by entropic stabilization of the covalent crosslinks.25 This approach is advantageous and promising to develop a programmable electrically conducting material. However, it requires inventing new synthetic materials containing complex functional groups and structures.25,29,36−40 Herein, to the best of our knowledge, we present the first report on utilization of chemically unmodified lignin, a natural product isolated from plant biomass, as a feedstock to prepare shape-memory composites. We developed a novel technique for producing acrylonitrile−butadiene−lignin (ABL)-based material41 with a high lignin content (40−60 wt %) that can acquire programmable and restorable shape and engineered electrical conductivity. The objective of this research is to understand and tune the chemistry-dependent shape-memory effects of lignin in a composite of acrylonitrile−butadiene rubber and lignin with the intent to design programmable and switchable (on−off) electrically conductive materials. The nitrile−butadiene rubber (NBR) elastomer exhibits very large extensibility (strain of several hundred percent).42 In addition, NBR recovers its original shape quickly after releasing the external stress. This suggests NBR’s potential for stretchable sensor applications. However, the NBR network characteristics including cross-link density and distribution of the cross-links are critical to trigger the shape restoration.43−46 A very low glass transition temperature (−16 °C) of NBR limits its ability to maintain the deformed shape at above the trigger temperature (Tg). Therefore, NBR exhibits very poor shape fixity and poor programming characteristics. In this study, oligomeric lignins were incorporated into the nitrile−butadiene elastomer network to offer a tunable thermomechanical response at high temperatures (much above the Tg of the host rubber matrix) for a wide range of deformation and stress/ strain-sensing applications. The formation of chemical crosslinks and hydrogen bonds between lignin and NBR enhanced the programmability of the material’s shape at high temperatures (e.g., 50 and 100 °C). Selecting an appropriate lignin type further helped adjust the shape-memory effect. Additionally, ABL substrates were coated with silver nanoparticles to make electronic skins that exhibit programmable and switchable electrical conductivity.

and NMR spectra were recorded on a Varian 500 MHz spectrometer at 23 °C. The molecular weight and molecular weight distribution of the constituents in the lignin were determined by size exclusion chromatography; a Waters 2695 Alliance HPLC pump equipped with degasser and autosampler systems, three X Polymer Laboratories Mixed-C Ultrapolystyragel columns, a Wyatt miniDAWN 3-angle ambient light-scattering detector, and a Waters 2414 refractive index detector. N,N-Dimethylformamide with 0.050 M LiBr was utilized as the eluent at a 0.5 mL/min flow rate and a temperature of 60 °C. Poly(2-vinylpyridine) standards were employed for calibration. 2.3. Thermal Analysis. A thermogravimetric analyzer (Q500, TA Instruments) was employed to investigate the thermal stability of lignins and composites of lignin and NBR41. A sample weight of approximately 15 mg, platinum pans, nitrogen atmosphere, and sample purge flow of 60 mL/min were used for measurements. The temperature was ramped to 105 °C at a ramp rate of 10 °C/min and was then then held at 105 °C for 20 min to remove the moisture before being ramped to 650 °C at 10 °C/min. A differential scanning calorimeter (Q2000, TA Instruments) was used to determine the thermal transition temperatures of the lignin−NBR41 samples. Samples with a mass of approximately 3−4 mg each were loaded in hermetic pans for measurements. Three cycles of heating and cooling, from −80 to 250 °C, at a ramp rate of 10 °C/min, were applied. The second cycle was used to determine the Tg of materials. 2.4. Fourier Transform Infrared Spectroscopy. FTIR measurements were carried out using a PerkinElmer Frontier instrument. The attenuated total reflectance method, in which the force gauge of 60 (au) was applied, was used for all the measurements. Spectrum measurement covered the range from 500 to 4000 cm−1, and the baseline was subtracted for correction. A scan speed of 1 cm/s and a resolution of 4 cm−1 were used to obtain 32 scans. 2.5. Rheological Measurements. The rheological properties of the composites were measured using the Discovery Hybrid rheometer (DHR-2, TA Instruments). A punch was used to cut uniform samples from the specimen ABL sheets. The samples measured 8 mm in diameter and about 0.4 mm thick. All the measurements were carried out in the linear regions (very low strain) and in a nitrogen atmosphere. Frequency sweeps from 100 to 0.1 rad/s at different temperatures, including 190, 210, and 230 °C, were employed to construct the master curves at a reference temperature of 190 °C. 2.6. Characterization of Shape-Memory Effect. Samples of pristine NBR41, SW-NBR41, and HW-NBR41 that were 50 mm long, 3−5 mm wide, and 2 mm thick were prepared for the shape-memory study. Three weight fractions of SW- and HW-lignins in NBR41, 40%, 50%, and 60%, were characterized. A DMA-Q800 dynamic mechanical analyzer (TA Instruments) was utilized to investigate the shapememory characteristics of the samples. The axial tension geometry was used. All measurements were carried out in a nitrogen atmosphere. The shape programming and recovery were investigated by deforming the materials at selected strain amplitudes, including 20%, 50%, and 100%. Also, two different programming temperatures, 50 and 100 °C, were selected. Each sample was loaded and attached on axial tension geometries at ambient temperature. After the oven chamber was closed, the sample was purged with nitrogen, and the temperature was ramped at 10 °C/min to the tested temperature (50 or 100 °C). An isothermal step for 2 min was applied to equilibrate the sample before it was stretched to a predetermined strain amplitude. To fix (program) the temporary shape, the temperature was quickly cooled to −30 °C with a speed of 50 °C/min. The applying force was released to a minimum value (0.001 N) before the sample temperature was raised to the deforming temperature for recovery. An additional isothermal step was applied for 30 min at the deforming temperature for further recovery. The whole process of deforming, fixing, and recovering was repeated three times. The stress and modulus induced from deformation were measured as well. Repeatable shape recovery and fixity were determined and quantified after three of programming cycles with very high strain amplitudes. The programmable and recoverable shape properties were obtained by a procedure in which selected samples were manually stretched on a preheated hot plate and were then fixed by being placed on a chilled aluminum surface. The

2. EXPERIMENTAL SECTION 2.1. Composite Synthesis. Acrylonitrile−butadiene rubber (41 mol % nitrile content) (NBR41) was purchased from Scientific Polymer. Organosolv hardwood and Kraft softwood lignins were provided by Lignol Innovations, Canada, and Domtar, North Carolina. Various SW and HW lignin fractions, from 40 to 60 wt %, were meltmixed with the NBR41 using a Brabender Plastic Corder equipped with a half-size (30 cm3) mixing chamber and high-shear twin roller blades. The rubber was loaded in the mixing chamber and mixed for 2 min at 90 rpm and 180 °C. Then lignin was added, and the mixing continued for a total of 60 min. After they were mixed, the samples were recovered and stored at room temperature. The lignin-NBR41 composites were molded into films between two Teflon sheets in a hydraulic press machine held at 190 °C for 20 min. 2.2. Lignin Characterization. Lignin functional groups were characterized and quantified by 13C NMR and 2D 1H−13C HMQC NMR spectroscopy using preparation and analysis methods previously reported.47 DMSO-d6 solvent was used to prepare the NMR samples, B

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used in this study were determined by 13C nuclear magnetic resonance (NMR) and two-dimensional (2D) 1H− 13C heteronuclear multiple quantum coherence (HMQC) NMR spectroscopy. The measured 13C NMR spectra and 2D-NMR HMQC spectroscopy data are presented in Figure 1b,c. HWlignin contained significant β-O-4′ linkages (substructure A), while this linkage was not detected in SW-lignin. Both lignins contained β-5′ and β−β linkages (substructures B and C, respectively). The measured data indicated that the guaiacyl propane unit (substructure G) was predominant in SW-lignin and was also observed at lower concentrations in HW-lignin. Additionally, the syringyl propane units (substructures S and S′) were detected within HW-lignin at concentrations higher than “G” but were not observed in SW-lignin, while phydroxyphenolpropane units (H) were observed within SWlignin but not in HW-lignin. In summary, HW contained a higher concentration of β-O-4′ and “G” units, with no “H”; SW contained a higher concentration of “G” and “H” with no observed β-O-4′ linkages. The structure of the lignin used in the ABL formulation is significant in terms of its interaction with the nitrile rubber and the resulting shape-recovery effects. The ability of a polymeric material to recover a permanent shape from a temporarily fixed deformation can be regulated by the chemical and/or physical cross-linking density of polymer molecular segments.44,53−55 After a polymeric material is deformed, it is stabilized by being quenched to a certain fixed temperature. Dynamic noncovalent bonds (such as hydrogen bonds) are necessary to stabilize the programmed (temporary) shape.25,43 Acrylonitrile−butadiene rubber (41 mol % nitrile content) (NBR41) possesses a good recoverable strain property after deformation. The elastic recovery of nitrile rubber comes from intermolecular cross-linking of nitrile groups during thermal processing.56 However, a low glass transition temperature (−16.5 °C) of pristine NBR41 results in very low shape fixity (i.e., an inability to retain the deformed shape). In this study, lignin was used to enhance the glass transition temperatures (Tg) and the potential to store mechanical work through deformation that in turn improves shape fixity. Figure 2a is a graph that represents a typical cycle of deformation, fixing, and recovery for an ABL sample (see the Experimental Section for more detailed information). The whole process and the associated deformation in networked structures are illustrated in Figure 2b. Deformation of the networks in the ABL composite involves hydrogen bonds between the hydroxyl (−OH) groups of lignin molecules and the nitrile (−CN) groups of NBR41. The exceptional shape recovery and fixity of ABL were further employed to control the electrical conductivity for sensor applications. Figure 2c illustrates programming the conductivity of the material by embedding a layer of Ag nanoparticles on the shape-memory substrate. When the composite film was stretched, percolation and interconnections between the Ag nanoparticles were decreased and broken, as illustrated by the dashed lines. Thus, electrical conductivity was absent in the stretched material. 3.2. Thermomechanical Characteristics of ABL and Its Networked Structure. We first investigate the characteristics of the ABL materials. The results of differential scanning calorimetry (DSC) (Figure 3a, vertically shifted for clear observation) exhibited a very low glass transition temperature (−16.5 °C) for pristine NBR41 and showed an increase in Tg by adding hardwood (HW)- or softwood (SW)-lignins at

specimens having programmed (temporary) shapes were put on the preheated hot plate again, and the recovery process was observed. 2.7. Programmable and Switchable Electrical Conductivity Characterization. We used the 40 wt % SW-NBR41 composite as a shape-memory substrate to prepare a programmable and switchable electrically conducting material. Silver nanoparticles (99.95% purity, 50−60 nm, SkySpring Nanomaterials, Inc.) were blade-coated on top of the SW-NBR41 substrate using polystyrene solution as a binder layer. The coating procedure is illustrated in Scheme 1.

Scheme 1. Preparation of a Programmable Electrically Conducting Material Employing Silver Nanoparticles Coated on an ABL Shape Memory Substrate: (1) Shape Memory Material, ABL with 40 wt % SW Lignin; (2) Polystyrene (PS) Coating; (3) Silver Nanoparticle (AgNP) Blade Coating; (4) Rolling; (5) Dragging; and (6) Handling of Programmable Electrical Conducting Material (Dimensions in the Illustrations Are Not to Scale)

Polystyrene (Mw = 192 kDa, Sigma-Aldrich) was dissolved in toluene (40 mg/mL) by ultrasonicating for 30 min and then stirring at 150 rpm at room temperature for an additional 30 min. The solution was brush-painted on the surface of the ABL substrate to make a binder layer. Then Ag particles were blade-coated on the wetted ABL surface. A metal roller was then rolled and dragged over the Ag nanoparticle layer to improve the contact between the bound nanoparticles. The fabricated specimen was dried and stored overnight in ambient conditions before the electrical resistance was measured. The electrically conducting characteristics of a selected sample, at different conditions, were investigated by using a Keysight-34461a model digital multimeter. The variable resistance of the material was measured during the shape programming. BenchVue software was utilized to analyze the data. The DMA-Q800 (TA Instruments) equipped with thin film tensile clamps was used to program the shape deformation, fixity, and restoration. The corresponding resistance of the sample was measured in situ during the shape-programing and recovering cycles. The ability of the Ag nanoparticle layer to “heal” after being deformed and programmed was demonstrated by the SEM images collected at different magnifications with 10 kV accelerating voltage and a working distance of 9.5 mm.

3. RESULTS AND DISCUSSION 3.1. Shape-Memory Effects and the Concept of Programmable Electrical Conductivity in ABL. Figure 1a depicts regular components of plant cells in vascular tissues including lignin, hemicellulose, and cellulose. Lignin is an important component in plants contributing to the stiffness of plant cell walls, accounting for ca. 15%−40% dry weight (depending on the plant sources) and is the second most abundant (after cellulose) in plant biomass. It is often isolated as a byproduct from the pulping industry or a biorefinery and used as a cheap feedstock for thermal energy recovery through combustion.48−52 The representative chemical structures (see Table S1 and Figure S1, Supporting Information) of lignins C

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Figure 1. (a) Components of plant cells in vascular tissues. (b) 13C NMR spectra of hardwood (HW)-lignin (top) and softwood (SW)-lignin (bottom). (c) Two-dimensional (2D) NMR heteronuclear multiple quantum coherence (HMQC) NMR spectra of (A) HW-lignin and (B) SWlignin. Assigned lignin substructures are shown in Figure S1.

selected fractions. The degree of elevation in Tgs with respect to the pristine rubber was higher with SW-lignin-based ABL compositions. For example, the Tg was elevated from 17.1 to 27.8 °C in SW-lignin-based ABL composites for corresponding increases in lignin content from 40 to 60 wt %; the elevation of Tg of HW-lignin-based ABL was 22.6 °C for a 60 wt % HWlignin loading in NBR41 (Table S2, Supporting Information). The Fox equation57 was used for the thermal analysis. The results suggest that chemical cross-links were created between lignin and NBR41 (Figures S2 and S3 and the discussion in the Supporting Information). The 2D-NMR spectroscopy data of the two lignins revealed the predominance of guaiacyl propane units (substructure G) and p-hydroxyphenolpropane units (H) in SW-lignin, whereas the HW-lignin possesses a fewer G units and no H units. The noncondensed G and H units indicate higher chemical reactivity and less steric hindrance around the phenolic hydroxyl group. The higher reactivity and reduced hindrance promote potential cross-linking reactions and the formation of hydrogen bonds between lignins and NBR41 under high-temperature shear mixing. Our earlier study showed an example of probable chemical cross-linking between unsaturated rubber and lignin containing significant G substructures.58 Unsaturated rubbers are known to create free radicals during thermal shear. The presence of double bonds (CC), a symmetric stretch at ca. 1650 cm−1 (see Figure S5b), within the nitrile butadiene rubber promotes the chemical cross-links with free radicals generated by thermally unstable linkages of lignin structural units.59,60 Since both SW- and HW-

lignin have significant G substructure, it is expected to form chemical bonds. In addition, the thermally unstable linkages in HW-lignin such as β-O-4′ bonds and abundant methoxy (−OCH3) groups are susceptible to form free radicals during high shear mixing cycles. To investigate the molecular interactions of the ABL composites, frequency-dependent rheological measurements at elevated temperatures were performed. The results illustrated in Figure 3b,c suggest that the dynamic shear storage modulus (G′) and the complex viscosity (η*) of the pristine NBR41 are altered by the addition of various HW- and SW-lignin fractions. The ABL with SW-lignin−NBR41 composition exhibited strong improvement of G′ at a reference temperature of 190 °C in comparison with the neat NBR41. A large region of frequency-independent G′ (a plateau region) was observed in the SW-lignin-based ABL composites, from 100 rad/s to a very low frequency (terminal region), 10−3 rad/s, indicating the formation of a cross-linked structure. We anticipate that the increase in G′ of the SW-lignin-based ABL was also induced by the presence of very rigid SW-lignin structure. Several orders of magnitude higher storage modulus of the pristine SW-lignin at 190 °C as compared to that of pristine HW-lignin (Figure S2b) confirms higher rigidity of the former than the latter. Obviously, the HW-lignin-based ABL composition shows a considerable decrease in G′ of the NBR41 matrix at the low frequency. The drop of G′ of the NBR41 matrix after incorporation of HWlignin is very significant at high lignin content. The slopes of G′ versus angular frequency in the terminal region of pristine D

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of the 50 wt % SW-based ABL composite. SW-lignin is very rigid even at a very high temperature (190 °C) as demonstrated by a very high melt storage modulus (over 30 MPa) and a high complex viscosity (over 4 × 105 Pa·s) as shown in Figure S2b,c. We anticipate that a very high lignin loading, 60 wt %, and a high thermal shear (90 rpm) at 180 °C during the mixing could result in depolymerization of rigid SW-lignin. The ABL made with SW-lignin was chosen for further work because of its ability to form a rigid networked structure. Additionally, the measured results of thermogravimetric analysis (TGA) (Figure 3d,e) suggest that the intermolecular interactions of SW lignin with NBR41 (including the possibility of cross-linking and formation of more hydrogen-bonded assemblies) are superior to those of HW-lignin with the NBR41. The data in Figure 3d,e (also Figure S4) show different thermal stability behaviors of the investigated samples. Pristine NBR41 indicates the highest thermal degradation temperature, whereas both SW- and HW-lignins revealed very low thermal stability. However, by incorporating these two selected lignins into NBR41, their corresponding composites with NBR41 exhibit very different thermal stability characteristics. Although HW-lignin is thermally more stable than SW-lignin, the TGA thermograms demonstrate that the thermal stability of SWlignin-based ABL is superior to that of the HW-lignin-based ABL. For example, the temperature associated with fixed mass loss (2 wt %) is slightly higher for HW-lignin (209 °C) than the SW-lignin (204 °C). However, after melt-mixing of these lignins with pristine NBR41, the ABL compositions exhibit an increase in temperature associated with 2 wt % mass loss (see the data in Figure 3d,e). This increase in specific degradation temperature is significantly higher for SW-lignin-based ABL (approximately 100 °C) than that of the HW-lignin-based ABL (ca. 40 °C). We anticipate that the increase in molecular interactions and chemical cross-links within the SW-ligninbased ABL resulted in a more stable structure. Higher reactivity of SW-lignin with rubber58 makes the product with more saturated backbone structure. On the other hand, less reactivity of rubber with HW-lignin leaves more unsaturation in the rubber and thus, more susceptibility to thermal degradation. This analysis further corroborates the rheological data (Figure 3b,c), in which significant improvement of the storage modulus and flow resistance were determined in the SW-lignin composites. The aliphatic and aromatic hydroxyl groups in SW- and HWlignins promote hydrogen bonding with nitrile (−CN) groups (denoted by the absorbance peak at around 2200 cm−1, Figure S5b) in NBR41. The hydrogen bonds were verified by the appearance of a wide Fourier transform infrared spectroscopy (FTIR) absorbance peak at about 3300 cm−1, as shown in Figure 3f.61 The measured 13C NMR data in Table S1 indicate similar total number of aliphatic hydroxyl group equivalents determined in SW-lignin and HW-lignin, 47 and 45 (per 100 aromatic unit), respectively. However, the FTIR absorbance peaks at ca. 3300 cm−1 exhibit more interactions through hydrogen bonding formed in SW-lignin and within SW-lignin− NBR41 composites in comparison to HW-lignin and HWlignin−NBR41 samples (see Figure 3f and Figure S5a). The measured 13C NMR and 2D-NMR HMQC results reveal significant domination of a highly branching aromatic structure containing rich β-O-4′ linkages (substructure A) and syringyl propane units (substructures S and S′) in HW-lignin. It is suggested that the HW-lignin has more steric hindrance effects and hence inhibits the formation of hydrogen bonds.

Figure 2. Principles of programming the shape-memory effect and tunable electrical conductivity in ABL composites. (a) Threedimensional graph of one cycle of deformation, fixing, and recovery. (b) The corresponding programmed shape recovery of ABL networks with a magnified view showing a network structure of a nitrile− butadiene elastomer (NBR) and lignin in the presence of hydrogen bonds formed by −OH and −CN groups. (c) Principle of switchable and programmable electrical conductivity of a silver nanoparticle layer assembled on a shape-memory substrate. The dashed lines indicated the breaking of silver nanoparticle percolation resulting the decrease in electrical conductivity (dimensions in the illustrations are not to scale).

NBR41 and the SW-based ABL approach zero, whereas the slopes are slightly higher for the HW-based ABL (Figure 3b). This measured data suggests less intermolecular interaction occurs within the HW-based ABL in comparison to the others. Similarly, the comparison of a complex viscosity profile with angular frequency in Figure 3c also indicates a lower degree of cross-link density and poorer intermolecular interaction in HWbased ABL than in the pristine NBR41 or the SW-based ABL. Again, the complex viscosity of ABL is strongly dependent on the lignin content and the nature of the lignin. The melt viscosity of HW- and SW-lignin segments contributed significantly to the complex viscosity of the ABL composites (see Figure S2c). Moreover, the data in Figure 3c exhibit no zero-shear viscosity, demonstrating domination of a crosslinked or networked structure in the compositions. We anticipate that these are both physically and chemically networked structures. The physical cross-links were formed by the hydrogen bonds within lignin and nitrile rubber molecules (see discussion in the next section), while the chemical crosslinks are the results of intrinsic networked structures of lignins and the entanglements and other thermally formed networks in nitrile rubber. The degree of chemical cross-linking between rubber and the lignin is low; otherwise, its molding and thermal reprocessing including rheological study of the molded specimens would have been very difficult. Nonetheless, the formation of slightly networked structure between lignins and rubber was studied using swelling of the alloys in solvents, and the results were presented earlier.58 It is noticed that the storage modulus and the complex viscosity of 60 wt % SWbased ABL slightly decreased with respect to the measured data E

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Figure 3. Characteristics of ABL compositions. (a) Differential scanning calorimetry data of pristine NBR41 and the composites with selected hardwood and softwood lignin contents, indicating increasing trends in the glass transition temperature of NBR41. (b) Frequency-dependent storage modulus (Tref = 190 °C). (c) Frequency-dependent complex viscosity (Tref = 190 °C) of the corresponding samples (refer to color code). (d, e) Thermal stability of pristine NBR41 as well as softwood (SW)- and hardwood (HW)-lignins, respectively, and their corresponding ABL composites. (f) Fourier transform infrared spectroscopy data of the corresponding samples exhibiting hydrogen-bonding formation in the presence of lignins.

3.3. Shape-Memory Characteristics of ABL. The programmable fixity and recovered shapes of ABLs are illustrated by the digital images shown in Figure 4. Figures S6−S8 and Movies S1−S4 (see Supporting Information) show some examples of qualitative shape-memory effect of ABLs. The digital images in Figure 4a revealed good recovery characteristics of a 40 wt % HW-lignin-based ABL. The sample was placed on a preheated Teflon sheet (attached to a hot plate surface at 50 °C). After that, it was axially stretched at a very large strain amplitude (see the stretched image, second from left in Figure 4a) and then quickly cooled on a chilled aluminum surface to temporarily fix the stretched shape (see the fixed, 0 s image, third from left in Figure 4a). When the fixed sample was placed back on the hot plate at 50 °C, the sample was restored to its original shape within 90 s. Similarly, two selected ABL compositions with 50 wt % HW lignin and

50 wt % SW lignin (Figure 4b,c) also demonstrated the ability to recover their initial shapes after being deformed and programmed at a very high activating temperature (100 °C). The initial shapes were recovered within 100−200 s. Figure 4d and Movie S4 demonstrate programming and fixing of different shapes. A 50 wt % SW-lignin−NBR41 strand was twisted and wrapped on a metallic cylinder that was preheated to 100 °C to get a spring shape and was then fixed on a chilled aluminum surface. The image “fixed” in Figure 4d is a programmed shape that maintained excellent temporary shape at ambient temperature. The programmed shape was stretched multiple times at ambient conditions. Every time the applied stretching force was released, the temporary shape recovered in several seconds. Precise procedures and measurements (illustrated in Figure 2a and Experimental Section) were carried out to quantify the shape-memory effect for ABL materials containing both HWF

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and SW-lignins. The data in Figure 5a,b show the strain (%) versus time and temperature of pristine NBR and of ABL compositions based on 40 wt % HW lignin, 40 wt % SW-lignin, and 50 wt % SW-lignin. The time periods displayed in the graphs correspond to three different cycles of deformation, fixity, and recovery. Pristine NBR41, 40 wt % SW-lignin-based ABL, and 40 wt % HW-lignin-based ABL samples indicated recoverable strain after being stretched to a very high strain (100% strain at 50 °C) (Figure 5a). Examples shown in Figure S9 illustrate determination of the recovered strain and strain fixity at a selected temperature and the maximum recovery speed of the pristine NBR41. The strain recovery (R) and fixity (F) were measured by using eqs 1 and 2:62,63

Figure 4. Selected programmable and recoverable softwood (SW)and hardwood (HW)-lignin−NBR41 composites. (a) Recoverable shape of HW-lignin−NBR41 (40:60 wt %) after uniaxial stretching at 50 °C. (b) Recoverable shape of HW-lignin−NBR41 (50:50 wt %) after deforming/twisting at a high temperature (100 °C). (c) Recoverable shape of SW-lignin−NBR41 (50:50 wt %) after twisting at a high temperature (100 °C). (d) Repeated recovery of a fixed/ temporary shape of SW-lignin−NBR41 (50:50 wt %) after being deformed at ambient temperature multiple times.

R=

εd − εr εd

(1)

F=

εf εd

(2)

where εd is the strain after deformation, εr is the residual strain after the recovery process, and εf is fixed strain at a selected fixed temperature after the applied force is unloaded. The corresponding stress profiles associated with the repeated

Figure 5. Representative strain fixity and recovery of selected SW- and HW-lignin-based ABLs at two investigated temperatures, 50 and 100 °C (the solid and dashed lines are the strain and temperature curves, respectively). Three cycles of strain deformation−fixity−recovery at (a) 50 °C of pristine NBR41, 40 wt % lignin containing ABL from both HW- and SW-lignin; (b) 100 °C of pristine NBR41 and 50 wt % SW-lignin ABL; and (c, d) their corresponding computed strain fixity and recovery. The deformation cycles showing of stress−strain curves at (e) 50 °C of pristine NBR41, 40 wt % lignin containing ABL from both HW- and SW-lignin, and (f) 100 °C of pristine NBR41 and 50 wt % SW-lignin ABL. G

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Macromolecules Table 1. Computed Deformation and Elastic Work Density Data Using Eqs 3−8 sample NBR41 NBR41 NBR41 40 wt % 40 wt % 40 wt % 50 wt % 50 wt % 50 wt % 60 wt % 60 wt % 40 wt % 40 wt % 40 wt % 50 wt % 50 wt % 50 wt % 60 wt % 60 wt %

HW HW HW HW HW HW HW HW SW SW SW SW SW SW SW SW

T (°C)

εd

εf

εr

εcd

εcf

σ (Pa)

G (Pa)

(W/V)max (J/m3)

(W/V)stored (J/m3)

50 50 100 50 50 100 50 50 100 50 50 50 50 100 50 50 100 50 50

0.5 1 0.2 0.5 1 0.2 0.5 1 0.2 0.5 1 0.5 1 0.2 0.5 1 0.2 0.5 1

0.412 0.866 0.16 0.494 0.995 0.193 0.498 0.996 0.196 0.497 0.997 0.483 0.979 0.172 0.494 0.991 0.196 0.492 0.762

0.028 0.055 0.04 0.049 0.129 0.01 0.084 0.144 0.046 0.085 0.224 0.026 0.075 0.015 0.076 0.152 0.01 0.058 0.106

0.459 0.896 0.154 0.430 0.771 0.188 0.384 0.748 0.147 0.382 0.634 0.462 0.860 0.182 0.394 0.736 0.188 0.418 0.808

0.374 0.769 0.115 0.424 0.767 0.181 0.382 0.745 0.143 0.380 0.632 0.445 0.841 0.155 0.388 0.728 0.184 0.410 0.593

142758 183027 45307 187083 257249 33346.9 184879 168337 18204.6 342682 371340 854361 1723600 329158 2172940 4349770 636023 5584370 8806190

98879 59690 97499 139056 99955 58507 155082 67758 40959 288467 180445 587832 589501 596388 1772313 1784095 1115909 4280074 3241127

24708 49208 3154 30827 63329 2778 27925 40668 1218 51627 80651 148548 452834 26664 335009 1040200 52983 900392 2229919

16943 37579 1813 30083 62687 2586 27679 40332 1158 50944 80095 138998 434884 19492 326366 1020702 50882 870818 1285836

Figure 6. (a) Results of the stored elastic work density as a function of strain of different investigated samples. (b) Results of maximum elastic work density as a function of strain of the corresponding samples.

stayed at very high values (about 98−99%) and remained almost unchanged after three cycles of deformation and recovery, as demonstrated in Figure 5a,c. In contrast, the pristine NBR41 revealed low strain fixity at −20 °C (approximately 86%). We hypothesize that the excellent fixity of the ABLs results from considerable improvement of the glass transition temperatures of the composites and the formation of hydrogen-bonded associations. The measured data indicate that the pristine NBR41 has poor programmability and that it lacks the ability to control shape for shape-memory applications. However, a representative sample of 50 wt % SW-lignin-based ABL exhibited excellent strain recovery and fixity (about 98%) when programmed at a higher temperature (100 °C); (i.e.,

deformation−fixing−recovery cycles for NBR41 are discussed in Figure S10. The data presented in Figure 5c showed an excellent strain recovery of ABL with 40 wt % SW-lignin, approximately 92% strain recovery after very large deformation, and 100% strain amplitude at 50 °C (approximately 67 °C above the glass transition temperature of the pristine NBR41), which was a similar characteristic of the pristine NBR41. The measured strain restoration of the samples is consistent with and agreed with the rheological data related to the highly cross-linked and physically networked (hydrogen-bonded) structures between pristine NBR41 and SW-lignin. Remarkably, the strain fixity of both ABLs with 40 wt % HW-lignin and 40 wt % SW-lignin H

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Macromolecules about 117 °C above the glass transition temperature of the pristine NBR41) (Figure 5d). The amount of stress required to achieve the prescribed deformation can be realized from Figure 5e,f. These figures show stress−strain curves associated with deformation and recovery cycles of these selected samples programmed at 50 and 100 °C. The stress and modulus of the samples are also recoverable after multiple programmed deformation−fixity−recovery cycles as shown in Figures S11−S17 and discussed in the Supporting Information. A substantial increase in required deformation stress of the rubber matrix as a function of incorporated lignin fractions (Figure S14) indicates the potential for a wide range of sensor applications relevant to motion and stress detection. At room temperature, depending on the type and amount of lignin used, these leathery materials exhibit 10−30 MPa tensile strength with a broad range of strain to failure (180−400%).41 We further quantified the correlations between the material deformation and the elastic work for shape recovery. The specific deformation required a certain stress, and the corresponding work (W) normalized by the sample volume is called the maximum elastic work density,

unloading (εcf) were defined by eqs 7 and 8 involving residual strain (εr).61 ε − εr εcd = d 1 + εr (7) εcf =

( WV )max . A selected

programming temperatures. In contrast, the values of

( WV )stored. We used Anthamatten’s

(3)

G = nKBT

(4)

⎡ (1 + ε )2 ⎛W ⎞ 1 3⎤ cd ⎜ ⎟ = G⎢ + − ⎥ ⎝ V ⎠max 2 1 + εcd 2⎦ ⎣

(5)

⎡ (1 + ε )2 ⎛W ⎞ 1 3⎤ cf ⎜ ⎟ = G⎢ + − ⎥ ⎝ V ⎠stored 2 1 + εcf 2⎦ ⎣

(6)

( WV )stored

dramatically dropped for the pristine rubber in all programming cases, indicating very poor fixity of the rubber, which was corroborated with the results discussed earlier. Indeed, the capacity of fixing and recovering the shape of rubber at a high temperature, such as 100 °C, was the lowest as demonstrated by very low maximum and stored elastic work density (3154 and 1813 J/m3, respectively). On the other hand, ABL based on SW-lignin possesses excellent restoration and fixity character-

model63 for the ideally elastic neo-Hookean solids under uniaxial stretching to quantify the elastic work density (W ) of V the shape-memory polymers at different programming temperatures. The maximum and stored elastic work density of different studied samples (including pristine NBR41 and ABLs that contain 40−60 wt % HW-lignin and SW-lignin) under selected programming temperatures and applied strains were computed using appropriately measured stress (σ) at the applied strain (Table 1 and Figure 6a,b). The samples were programmed and analyzed at three different conditions: at 50 °C and 50% strain, at 50 °C and 100% strain, and at 100 °C and 20% strain (Figures S11−S17). The corresponding elastic shear modulus (G) of polymers was determined from the true elastic stress (σ) at a corrected deformation or strain (εcd) as shown in eq 7. ⎡ 1 ⎤ σ = G⎢(1 + εcd)2 − ⎥ 1 + εcd ⎦ ⎣

(8)

The data presented in Figure 6a,b indicate significant improvement of the maximum elastic work density of an NBR41 matrix when combined with SW-lignin. The increase in the elastic stress and elastic work density of ABLs that was required to apply a certain strain, particularly for those of SWlignin−NBR41 compositions, reveals reinforcement of the rubber matrix by the lignin component. The values determined for elastic stress and the consequent work density are consistent with the improvement of storage modulus at elevated temperatures in the terminal region (Figure 3). In all cases, lignin (both HW and SW) highly contributes to the fixity of the programmed temporary shapes. The measured results shown in the last two columns of Table 1 and the data presented in Figure 6a,b exhibit similar maximum and stored elastic work density obtained from deforming ABL samples at selected

temperature (−20 °C) was used to study the fixity of the samples, and the related work density was denoted as the stored elastic work density,

εf − εr 1 + εr

istics. The

( WV )max and ( WV )stored data of ABLs with 50 wt % SW

lignin obtained by programming at 100 °C and 20% strain are nearly 17 times higher than the corresponding results for the pristine NBR41 rubber. Also, the maximum and stored work density values for the same composition are almost identical, indicating very good fixing characteristics of the material. The measured work density data corroborate our assessment on the role of cross-linked and hydrogen-bonded associations of ABLs in assisting the repeated shape restoration and fixity under different programming conditions. 3.4. Programmable and Switchable Electrical Conductivity in ABL Shape-Memory Alloy. Embedding conductive spherical particles of metals in the bulk polymers requires very high loading of particles, commonly over 16% by volume (roughly 35−40 wt %) to reach a percolating threshold.14 This enormous metallic content added into the polymer matrix results in significant changes in material properties and cost. We took a simpler route to prepare a programmable conducting material based on an ABL composition with 40 wt % SW lignin that exhibits excellent shape-memory characteristics. The methodology, which involves bonding a layer of Ag nanoparticles to the surface of an ABL specimen with polystyrene dissolved in toluene, is illustrated in Scheme 1 (see the Experimental Section for more details). The thickness of binder and Ag nanoparticle layers measured from SEM images was 8.6 ± 1.5 μm (Figure S18). The total weight fraction of Ag nanoparticles, about 1.5 ± 0.2 wt %, on the ABL was determined by thermogravimetry (Figure S19).

where n is the strand or molecular segment density in the networked structure of the material, KB is the Boltzmann constant, W is the work, and V is sample volume. The first cycles of deformation at selected programming temperatures and strains were investigated. The appropriate work density was computed by using eq S3 and eq S4, in which the corrected strain after deformation (εcd) and corrected fixed strain after I

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Macromolecules

7d or Figure 7a, first image from left to right), the temporarily shaped specimen was stretched by an external stress at ambient temperature multiple times, and the corresponding resistance data were recorded (Figure 7a and the Movie S5). An increase in resistance occurred during the application of stress (the digital images shown from left to right in Figure 7a). The initial resistance of the thin film that was programmed into a temporary shape was approximately 0.46 kΩ. However, when stress was applied axially, the resistance gradually increased to 2.86 kΩ. Prior study of deformation in shape-programmable conductive materials suggests that the changes in resistance induced by changes in applied stress can be utilized for human motion tracking.19 Interestingly, after the applied stretching force is released, the shape-programmed sample quickly recovers its initial shape within 30 s, as demonstrated in Figure 7b (from left to right) and Movie S6. The measured resistance was restored as well, to about 0.47 kΩ (Figure 7b, the digital image at the end on the right). The process of applying stress (stretching) and recovery at ambient temperature repeatedly produced similar changes in resistance values, indicating very good shape-fixing/programming characteristics. These gentle stretch and associated change in electrical resistance followed by recovery of both shape and resistance value suggest suitability of these leathery materials (with Tg close to or slightly below room temperature) as skin-like material capable to sense change in electrical resistance as a function of stretch. Use of low aspect ratio particles (spherical AgNPs) and the breaking of particle percolation and associated change in resistance could be used for detecting even small strain amplitudes (deformation). In addition, the temporarily programmed shape and resistance of the device (about 0.46 kΩ, Figure 7d) got quickly restored to its initial shape and initial electrical resistance (Figure 7c,e) after being heated on a hot

The measured data of selected samples are presented in Figure 7a−f. A preheated (100 °C) device was shape-

Figure 7. Programmable and switchable electrical conducting characteristics of silver nanoparticle coated on an ABL substrate: (a) Increasing resistance of a shape-programmed electrical conducting material during stretching at room temperature. (b) Shape and electrical conducting restoration of the shape-programmed electrical conducting material after multiple time stretching at room temperature. (c, d, e) Initial, after fixing/shape-programming, and after restoring shape and resistance. (f) Recovery of the shape-programmed electrically conducting material after placing on a hot plate at 100 °C.

programmed following the procedure discussed earlier (Figure 2 and the Experimental Section). After being fixed (see Figure

Figure 8. (a) Multiple strain deformations and recoveries of the programmable and switchable electrical conducting silver particle-coated ABL, an electronic skin. (b) The corresponding electrical resistance data when high strain amplitude (50%) was applied at 50 °C. (c, d, e) Scanning electron microscopy images of healed electrically conducting silver nanoparticle layer after stretching and programming: initial state of the electronic skin (top view); deformed and programmed electronic skin (top view, stretching direction denoted by the white arrows) and the shape-recovered electronic skin (top view). J

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Macromolecules plate at 100 °C (Figure 7f). Thus, these stretchable leathery materials could also demonstrate thermally triggered shape recovery that could also register appropriate change in electrical resistance. A representative ABL device was attached to a dynamic mechanical analyzer and axially stretched (stress was applied) to 50% strain at 50 °C. Then the device was programmed, and the strain recovery corresponding to the electrical resistance changes was measured. By stretching the sample to 50% strain, the electrical percolation was lost as indicated by a jump in resistance of 4−6 orders of magnitude. Its initial electrical resistance was restored after recovery from the strain (Figure 8a,b). The sequential resistance loss and restoration behaved more consistently after the first two cycles. We surmise that at least two deformation and thermal cycles are required to reach thermal and structural equilibrium state of the conducting nanoparticle layer on the ABL substrate. We also carried out a program involving a deformation−fixing−recovery cycle to investigate changes in the percolating network of the Ag nanoparticle layer coated on a shape-memory ABL substrate (40 wt % SW-lignin). We used scanning electronic microscopy (SEM) to observe the topography of the coated surface. The SEM images (Figures 8c−e) indicate that the initial structure of the silver nanoparticle layer completely recovers after deformation. The uniform percolated structure of the nanoparticle layer is shown in Figure 8c. Micrometer wide cracks (Figure 8d) were detected, and electrical conductivity was lost in the coating after the device had been stretched. The structure of silver nanoparticle layer was healed, and its ability to conduct electricity was restored by heating the stretched and temporarily fixed device on the hot plate for several hundred seconds, as shown by the SEM image in Figure 8e. We confirmed that the healed structure had regained its ability to conduct electricity by measuring its resistance and comparing those findings with the resistance values measured for the specimen before it was stretched. Digital images and SEM images at different magnifications of initial, stretched, and programmed, and recovered structures of the conductive silver nanoparticle layer are presented in Figures S20−S22.

the valorization of lignin, a low-value residue from biorefinery operations, are important to enable the cost-competitive production of biofuels.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02336. Tables S1−S8 and Figures S1−S22 (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI) Movie S5 (AVI) Movie S6 (AVI)

■

AUTHOR INFORMATION

Corresponding Author

*(A.K.N.) E-mail [email protected]; phone +1-865-576-0309. ORCID

Christopher C. Bowland: 0000-0002-1229-4312 Amit K. Naskar: 0000-0002-1094-0325 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy BioEnergy Technologies Office Program. Rheology experiments were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility

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REFERENCES

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4. CONCLUSIONS For the first time, lignina renewable byproduct from the pulping industry and from biorefinerieswas incorporated at very high loading into an acrylonitrile-butadiene rubber to make excellent shape-memory composites devised with programmable and switchable electrical conductivity. The presence of oligomeric lignin significantly improved the strain fixity and recovery of the matrix after multiple deformation cycles at large strain amplitudes and at a temperature above the Tg of the rubber matrix (116 °C). Extensive hydrogen bonding and moderate interfacial cross-linking between lignin and rubber enhance the maximum and stored elastic work density of the rubber. An increase of several orders of magnitude in the elastic work density was measured. The ABL compositions revealed excellent shape-programming characteristics for sensor applications. We developed and demonstrated a simple method in which the ABL substrate is coated with Ag nanoparticles to prepare a programmable and switchable electrically conducting material that can be useful for applications such as devices for detecting human motion and for monitoring stress history. We also demonstrated that the electrical conducting layer could “heal” (i.e, it was able to regain its properties as an electrical conductor after being deformed). Technologies associated with K

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