Carbon Nanotube Microfiber Actuators with Reduced Stress

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Carbon Nanotube Microfiber Actuators with Reduced Stress Relaxation Anne-Sophie Michardière, Cintia Mateo-Mateo, Alain Derre, Miguel A. Correa-Duarte, Nicolas Mano, and Philippe Poulin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12673 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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The Journal of Physical Chemistry

Carbon Nanotube Microfiber Actuators with Reduced Stress Relaxation

Anne-Sophie Michardière1, Cintia Mateo-Mateo2, Alain Derré1, Miguel A. Correa-Duarte2, Nicolas Mano1, Philippe Poulin*1

1 Centre de Recherche Paul Pascal, CNRS, University of Bordeaux, Avenue Schweitzer, 33600 Pessac, France.

2 Department of Physical Chemistry, Biomedical Research Center (CINBIO), and Institute of Biomedical Research of Ourense-Pontevedra-Vigo (IBI), Universidade de Vigo, 36310 Vigo, Spain

* [email protected]

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Abstract Assembling carbon nanotubes (CNTs) into robust macroscopic structures remains the main challenge to efficiently exploit their electromechanical properties in actuator applications. CNT based actuators generally suffer from creep and stress relaxation due to poor interactions between the assembled CNTs. In order to overcome this limitation, a new class of porous single wall CNT microfiber electrodes is here proposed. The present fibers are produced by a wet spinning process and generate a mechanical stress when they are stimulated at low voltage (~1V) in a liquid electrolyte. The used fabrication process enables the inclusion of small amounts of chemically cross-linked polymer such as polyvinyl alcohol within the fibers. The presence of cross-linked polymer limits the sliding of nanotubes with respect to each other, without sacrificing the porosity and electrical conductivity of the fibers. As a result, stress relaxation is greatly reduced. The fibers generate a negative stress (propensity to expand) when a positive or negative voltage is applied. This behavior suggests that the intrinsic deformation of the CNTs is likely the dominant actuation mechanism, as opposed to ionic swelling, which was recently observed in yarns made of multiwall CNTs.


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1- Introduction Carbon nanotubes (CNTs) are ideal candidates for several electrochemical and electromechanical applications1-5 due to their chemical stability, electrical conductivity, and high specific surface area. In particular, their mechanical strength combined to their response to an applied voltage makes them promising materials for novel actuator technologies6-16. It has been indeed shown in the recent years that CNT assemblies such as membranes, fibers and yarns deform in response to a low voltage applied in a liquid electrolyte. The conversion of electrical energy into mechanical energy involves different mechanisms including quantum mechanical effects, electrostatic repulsions, ionic swelling of the CNT assembly and even thermal expansion in bimorph structures5-22. The contributions of different mechanisms can explain differences in performances since nanotubes with different chiralities are expected to display

different

conductivity

and

electromechanical

responses.

Discrepancies

in

performances can arise from inconsistencies in CNT synthesis, control of which remains difficult in particular for single wall nanotubes. CNT actuators could find applications in MEMS, robotics, microsurgery, etc. Most CNT based actuators operate in capacitive regime, in contrast to conducting polymer actuators which involve red-ox chemical reactions. Capacitive phenomena are preferable to ensure long life time actuators without changes of the chemical structure of their components. Nevertheless, optimizing CNT based actuators remains challenging. It is necessary to achieve porous and highly conductive structures in which ions can migrate. The porous structure should in addition be mechanically strong in order to sustain mechanical load. High electrical conductivity is needed to ensure fast responses and efficient conversion of electrical energy. So-called bucky papers, which are films obtained by membrane filtration, are the simplest form of macroscopic assembly that can be used as actuator6, 8, 13. But the properties and performances of CNT based actuators can be improved if CNTs are assembled into yarns or fibers14,

23-25

. Improvements arise from the ordering of the CNTs and from optimized

dimensional changes along a given direction. Also, from a general point of view small actuators generally respond faster than larger structures. For example, CNT yarns obtained from forests of multiwall nanotubes (MWNTs)23-25 deform essentially in response to ionic swelling. Ionic swelling corresponds to volume changes induced by the migration of ions within the porous structure formed by the carbon nanotubes. The volume changes can be quite large for ions surrounded by great solvation shells. We note however that such ionic swelling does not necessarily involves breaking up of nanotube bundles, in particular for the case of 3 Environment ACS Paragon Plus


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MWNTs which are not assembled as bundles. The diameter of MWNT yarns increases with the applied voltage in response to the infiltration of ions. As a consequence the yarns shorten along their axis, or generate a contractile stress when they are mechanically loaded. Fiber wet spinning methods allow the fabrication of fibers from any types of CNTs including single wall nanotubes (SWNTs) and MWNTs. Regarding this method, Shin et al. recently reported microfiber actuators made of DNA functionalized MWNTs26 in which DNA can provide biofunctionality, but also implies electrochemical reactions even at low voltage. On the other hand, Viry et al.14 fabricated thermally treated composite polyvinyl alcohol (PVA)-CNT fibers with a large fraction of SWNTs to take advantage of their higher specific surface area compared to MWNTs. PVA is more stable than DNA with respect to red-ox reactions at low voltages. But raw PVA-CNT fibers are poorly conductive and non-porous before thermal treatment. The fraction of PVA can be as high as 85wt%14. In order to make these fibers conductive and porous, PVA must be removed by thermal treatments at high temperature (above 600°C). The stress generated by those fibers is greater than the stress generated by bucky-paper actuators. But, as indicated in 14, treated fibers are brittle and difficult to handle. In addition, the control of the thermal degradation of PVA is difficult and chemical impurities, which can induce uncontrolled faradic phenomena, remain after the thermal process. More critically, and regardless the involved mechanisms and macroscopic shape of their assembly, CNT based actuators suffer from creep and stress relaxation. Creep and stress relaxation occurs because CNTs slide with respect to each other when their assemblies are mechanically loaded. Indeed, cohesion of such assemblies is only due to weak van der Waals interactions and to entanglements of the CNTs. Stress relaxation is particularly strong in the first cycles of operation. It tends to diminish with time. But a very long time, of several hours, is needed before an almost stable signal can be obtained. Even after a few hours a residual stress relaxation of a few kPa/sec is still observed14. The addition of binders could be a good approach to reduce creep and improve the properties of CNT based actuators. Nevertheless, even though binders can certainly improve mechanical properties, they may also downgrade electrical properties and reduce porosity. In particular binders obstruct surface porosity if they are added after the fabrication of CNT assemblies; making these materials less efficient for electrochemical applications. Herein, we present a synthetic approach that allows the inclusion of small amounts of strong binders within CNT microfibers. Binders are introduced during the synthesis of the fiber without modifying the surface porosity of the fibers. By contrast to previous approaches14, the amount of polymer can be rationally controlled from large to very low 4 Environment ACS Paragon Plus

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weight fractions. Indeed, the fibers are spun under conditions so as to control the fractions of binders and CNTs. The used binders should be compatible with the fiber spinning process, be chemically stable and be efficient at reducing stress relaxation of the actuators. We report that PVA with subsequent cross-linking can significantly reduce the stress relaxation of CNT fiber based actuators while preserving a large porosity and high electrical conductivity. In addition, the presence of binders makes the fibers less brittle, and their manipulation easier, than fibers studied in previous work14. The actuators are tested in different electrolytes: NaCl, CaCl2 and AlCl3 with cations of different sizes. The obtained results suggest that, by contrast to MWNT yarns23-25, the actuation mechanism is presently dominated by intrinsic deformations of the CNTs rather than by ionic swelling of the structure.

2- Materials and Methods Chemicals: HiPCo SWNTs were purchased from Unidym, Sunnyvale, California (batch number R0513). Sodium dodecyl sulfate (SDS), glycerol, poly(vinylalcohol) (195K, 98% hydrolysis ratio), glutaraldehyde (25%), HCl and H2SO4 were supplied by Sigma-Aldrich. Hydrophilic polyvynilidene fluoride (PVDF) membrane (0.22 µm pore size) was supplied by Merck Millipore. Water was purified using a Milli-Q system (Millipore).

Carbon nanotube purification: Raw HiPCo SWNTs contain a large fraction, above 20wt%, of iron catalysts. The purification of CNTs is based on a thermal annealing with a subsequent HCl treatment. Usually, 250 mg of CNTs are heated in air at 200° C for 8 h. The oxidized material is magnetically stirred at room temperature in 1L of HCl (conc, 37 %, v/v). The purified CNTs are then collected by filtration under vacuum over a PVDF membrane (0.22 µm pore size) and washed several times with de-ionized water until reaching a pH up to 6-7. Thermogravimetric analyses are performed after removing the water by heating the nanotubes at 200°C during 24h showing less than 5wt% of iron impurities.

Microfiber synthesis: The CNT fibers are obtained by following a fiber wet-spinning process described in the literature27-29. In this process, CNTs (0.3 wt%) are dispersed and stabilized in water by sodium dodecyl sulfate (SDS) (0.6 wt%). Homogeneous dispersions are achieved by applying tip sonication during 45 minutes using a Branson 205A sonifier delivering 15W of acoustic energy. We note that sonication induces debundling and scission of the CNTs30-32. Comparisons with earlier experiments30 on similar materials suggest that the average CNT length should be on the order of 1µm. 5 Environment ACS Paragon Plus


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Afterwards, a controlled amount of a PVA solution (5 or 10 wt%) could be added to the CNT dispersions in order to make composite CNT-PVA fibers with a given and controlled ratio of PVA to CNT. This process differs from a more conventional, and less controlled, approach which would consist in infiltrating raw fibers with polymers. Fiber spinning is achieved by injecting the CNT dispersion in the absence (CNT fibers) or in the presence of PVA (PVACNT fibers) at 50 mL/h of flow rate into a co-flowing stream of a mixture of glycerol (45 vol%),

ethanol (45

vol%)

and acetone (10

vol%)

(GEA mixture). The co-flowing stream is

achieved by placing the GEA mixture in a rotating bath26 which rotates at 40 rpm. The dispersion is injected through a syringe needle with an internal diameter of 300 µm. The tip of the spinneret is located two and a half centimeters from the rotation axis of the GEA mixture container and tangential to it. The GEA mixture is a non-solvent medium for all the compounds initially dispersed in water. As a consequence, these compounds coagulate to form a fiber when the dispersion is injected into the rotating bath. Lastly, the obtained fibers are extracted from the coagulation bath, rinsed with water and dried vertically for one hour at room temperature. The area of the cross-section of the obtained fibers is of about 250 µm2. The process is sketched in Figure 1.

PVA crosslinking of microfibers. PVA chains are chemically cross-linked for certain fibers, named afterwards cPVA-CNT fibers. Cross linking is achieved by immersing CNT dried fibers into a solution of 50 mL of 25 mM glutaraldehyde (1 mM H2SO4) and heated at 80°C for 1 hour. Glutaraldehyde is among the most efficient covalent cross-linking agents of PVA33,34. PVA chains are cross-linked through an acetalization reaction catalyzed by acids. The excess of glutaraldehyde is removed with de-ionized water and the fibers are lastly dried at room temperature.


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Figure 1. Scheme of the fabrication of a CNT microfiber actuator. (1) Addition of aPVA solution (represented in red) to a CNT dispersion. (2) Injection of the final dispersion in a rotating bath containing a mixture of glycerol, acetone and ethanol. (3) Magnification of the CNT fiber structure. The CNT form a continuous network reinforced by chemically cross-linked PVA chains.

Characterizations: Mechanical properties of fibers were characterized with a Zwick Z2.5/TN1S tensile test instrument.

Electrical conductivity. The resistance (R) of the nanotube fibers was measured with a Keithley 2000 multimeter between two silver paint contacts. The conductivity σ (S.cm-1) of the fibers was deduced by taking into account the section area (A) of the CNT fibers and the length (L) between the contacts: σ = R x A/ L.

Electrochemical and electromechanical properties were characterized using a modified three electrodes set-up shown in Figure 2. This set-up is composed of a working electrode (WE, CNT fiber), a counter electrode (CE, Pt mesh) and a reference electrode (RE, Ag/AgCl 3M KCl). The CNT fiber is placed on a polyvinyl chloride (PVC) sample holder and electrically 7 Environment ACS Paragon Plus


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connected to a copper wire at the bottom of the PVC sample holder by using conductive carbon paint. The connection between the wire and the CNT fiber is protected with an insulating resin in order to avoid exposure to the electrolyte. The upper part of the fiber is stuck to the lever arm of a force sensor (300C-LR, Aurora Scientific Inc). The electrodes were then immersed into an aqueous electrolyte and connected to a potentiostat (CH Instruments, model CHI 1140 B). The nanotube fiber can be electrically stimulated with the potentiostat while their electromechanical response is monitored by the force sensor. In practice, the fiber is stretched under a given mechanical pre-load prior electromechanical characterization. An oscillatory square voltage is applied between the fiber and the reference electrode14. The fibers are characterized in so-called isometric mode. In this mode, the strain is kept fixed and variations of stress are deduced from variations of forces measured by the force sensor and normalized by the cross-section of the fiber. Different aqueous electrolytes made of NaCl, CaCl2 and AlCl3 (0.1

M) were used in this work. The immersed part of the fiber is about 2 cm long. Cyclic voltammetry measurements were performed from -0.4V to 0.8V vs. Ag/AgCl at 5, 7, 10 and 15 mV/s scan rates to determine the capacitance of the fibers35.

Scanning Electron Microscopy (SEM) characterization. Scanning electron micrographs were performed with a JEOL 6700F high resolution microscope at an operating voltage of 5 kV. Cross-section images were obtained by placing CNT fibers on a stainless-steel sample holder using double-side carbon tape after cutting them with liquid nitrogen.


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Figure 2. Representation of the CNT microfiber actuator setup composed by a reference electrode (Ag/AgCl), a counter electrode (Pt mesh) and a working electrode (CNT microfiber). All the electrodes are connected to a potentiostat and immersed into an aqueous electrolyte. The CNT microfiber is stimulated by a voltage applied. The top of the fiber is fixed to the lever arm of a force sensor. The length of the immersed part of the fiber is about 2 cms.

The force sensor allows the stress generated by the fiber along its main axis to be determined. As shown in Figure 3, it is taken as the difference from the mechanical load of the fiber at rest (0.0V) and the load of the fiber under stimulation at either -0.4V or +0.8V. We note that a positive stress reflects a propensity of the fiber to contract whereas a negative stress reflects a propensity of the fiber to expand.



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Figure 3. Example of electromechanical response of a CNT based microfiber stimulated by a square voltage in a liquid electrolyte. The length of the fiber is kept fixed by the arm of the force sensor. The mechanical load measured by the force sensor varies when the potential is varied. The stress generated at a given voltage is taken as the difference of load between the reference pre-load at 0V from the load at the considered potential.

3- Results and discussion This section will cover the study of three different classes of CNT microfibers which correspond to bare fibers comprised of CNTs (named CNT fibers), to fibers composed by CNTs and raw PVA (named PVA-CNT fibers) and fibers comprising CNTs and cross-linked PVA (named cPVA-CNT fibers). The work is focused on the electrochemical, mechanical and electrical properties shown by the three kinds of fibers studied.

3-1 Electrochemical properties: Typical cyclic voltammograms (CVs) of the investigated fibers are shown in Figure 4.


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Figure 4. Typical cyclic voltammograms in aqueous NaCl 0.1M electrolyte of a CNT fiber (black), PVA-CNT fiber (blue) and cPVA-CNT (red) fiber. The current is shown as a function of the voltage vs a Ag/AgCl reference electrode. The scan rate is 10 mV s-1. The CVs are recorded at room temperature.

Cyclic voltammetry reveals the absence of faradic processes when the fibers are stimulated by voltages in between -0.4V and +0.8V vs Ag/AgCl (3M) in aqueous electrolytes. These results suggest that the present materials do not contain carbonaceous species other than CNTs. The potential of zero charge (pzc), which could also be used as reference, can be determined by considering the minimum of capacitive current as a function of the potential. The pzc of the present fibers is found to be of +0.3V vs Ag/AgCl (3M). A small potential window is chosen to avoid any possible pneumatic effects coming from the generation of micro-bubbles. In the present conditions, microfiber actuators are expected to operate only via capacitive effects such as previously investigated bucky papers6. The electrochemical capacitance deduced from measurements at different scan rates (in NaCl 0.1M)35 show a capacitance of 45F/cm3 for CNT fibers, in contrast to PVA-CNT and cPVACNT fibers which exhibit a lower capacitance about 25F/cm3. This could be related to the presence of PVA inside the fibers which does not affect their electrical conductivity but reduces their capacitance. Actually fibers loaded with a fraction of 10 wt% of PVA exhibit an even lower capacitance, ∼20F/cm3, along with a lower electrical conductivity making the materials inappropriate for actuator applications. This suggests that the binder reduces the porosity of the structure and the specific surface area available for the adsorption of ions. While the BET surface area could not be measured because of the small amounts of produced 11 Environment ACS Paragon Plus


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materials, the surface area and porosity are generally directly related to the electrochemical capacitance36, 37. So, it is necessary to highlight the importance of PVA binder which must be kept sufficiently low. By reducing the amount of PVA down to 5wt% it is possible to obtain reinforced fibers with a preserved large capacitance. Therefore, these fibers can still operate as efficient electromechanical actuators. Further studies are therefore only dealing with materials containing a relative fraction of PVA to CNT of 5wt%.

3-2 Mechanical and electrical properties: The mechanical properties and electrical resistivity of the investigated fibers are given in Table 1. Young’s Modulus Strain at break Strength

Resistivity

(GPa)

(mΩ.cm)

(%)

(MPa)

CNT fibers

5.5

2.5

25

4.5

PVA-CNT

7.5

1.5

75

5

14

1

70

5

fibers

cPVA-CNT fibers

Table 1. Electrical and mechanical properties of the investigated fibers. Mechanical properties correspond to tensile deformations.

Bare CNT Microfibers composed exhibit a low Young’s modulus and tensile strength because of the weak interactions between the CNTs which are bound together by entanglements and van der Waals interactions. It is also interesting to note that the present fibers have a Young’s modulus lower than the modulus of fibers wet-spun in super acids38 or than the modulus of dried-spun fibers made by CVD39, 40. In these other processes, CNTs are significantly longer than the presently used CNTs which have been cut during sonication for their dispersion. The differences in CNT length can explain differences in mechanical properties. In comparison to fibers solely comprised of CNTs, the addition of 5 wt% of PVA binder, produces an enhancement of the Young’s modulus and tensile strength (PVA-CNT fibers). It is interesting to highlight that a further improvement of the Young’s modulus is obtained when the polymer is cross-linked (cPVA-CNT fibers). This improvement indirectly shows the 12 Environment ACS Paragon Plus

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efficiency of the cross-linking reaction. The Young’s modulus varies from 5.5 GPa for a bare CNT fiber to 14 GPa for a CNT fiber reinforced by 5wt% of cross-linked PVA. Thus, the improvement of mechanical properties is achieved while maintaining the electrical conductivity, which is one of the main requirements for CNT fiber based actuators. Also it can be noted that fibers made by the present method are less brittle than thermally treated fibers14. Regarding the electrical properties, shown in Table 1, the resistivity of a bare fiber is slightly lower than a fiber containing raw PVA. On the other hand, cross-linking of the polymer has no effect on the conductivity. These observations can be understood since the conductivity is ensured by the carbon nanotubes and not by the insulating PVA binder.

3-3 Electromechanical properties: As depicted in the scheme in Figure 2, the CNT fiber is stimulated by an applied voltage while being fixed to a force sensor, which allows the monitoring of the electrochemical properties of the microfibers. In order to carry out this characterization, a preload must be applied at the beginning of each measurement so that the fiber remains under tension. The pre-load chosen for the present experiments is set typically of about 20±3 MPa. Regardless the type of fiber, stress relaxation is observed right after the application of the preload. As shown in Figure 5, bare CNT fibers exhibit a stress relaxation on the order of 100kPa/s after application of the pre-load. This estimate is obtained by considering the decrease of stress from about 19 MPa to about 14 MPa in the first 50 seconds. Such a large stress relaxation at early times was also observed for thermally treated fibers14. For such fibers, stress relaxation was found to diminish with time. This suggests the possible presence of uncontrolled binders coming from impurities and residues of the thermal degradation of PVA. Nevertheless, these uncontrolled and unknown binders are poorly efficient. Indeed, a stabilization of the response was obtained only after a few hours14. And, even after such a long time, a still weak stress relaxation could still be detected. The present bare CNT fibers also display a reduction of stress relaxation with time. After a few minutes, stress relaxation is on the order of a few kPa/s. As shown in Figure 5a, such large stress relaxation makes difficult any electromechanical characterizations. Indeed, the stress varies when the electrical potential is cycled, but the periodic variations of stress remain small compared to the large relaxation of stress. As shown in Figure 5 b, similar trends are observed for PVA-CNT fibers. The effect of stress relaxation seems to be even more dramatic than the one observed for bare CNT fibers. Here the response to variations of potential becomes negligible. This may arise from 13 Environment ACS Paragon Plus


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the fact that raw PVA act as a lubricant between neighboring CNTs in microfibers immersed in water.


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Figure 5. Representation of the stress (in red) of a microfiber mechanically loaded at t=0 as a function of time in response to variations of a square voltage (in blue). Electromechemical response of a bare CNT fiber (a) and of a PVA-CNT microfiber containing 5wt% of raw PVA (b).

cPVA-CNT fibers display a very different behavior. As shown in Figure 6a, a stress relaxation is observed right after the application of pre-load. The initial relaxation rate is about 20kPa/s, typically decreasing from 23 MPa to 18 MPa after 250 seconds. But, it clearly levels down to 0kPa/s after a few minutes (Figure 6b). In such conditions a clear and stable electromechanical response is observed. We note that the time needed for the stabilization of the response is much shorter than the time needed for the stabilization of thermally treated fibers.



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Figure 6. Representation of the stress (in red) of a cPVA-CNT microfiber mechanically loaded at t=0 as a function of time in response to variations of a square voltage (in blue), from t=0 to t=250s (a) and from t=1800s to t=2050s (b).

The stable response allows the generated stress to be measured. For such a characterization, the voltage is set to zero after the stress relaxation has level down. The mechanical load measured at 0V serves as a reference state. We note that it is very difficult to accurately control the pre-load since it depends on the specific relaxation of each sample. Actually, the stress after relaxation was found to remain typically in a range from 13 to 18MPa for all the investigated fibers. The voltage is then cycled following a square variation between -0.4V and +0.8V and the generated stress is determined as shown in the example of Figure 3. Additionally, the electrochemical properties of the cPVA-CNT fibers containing 5 wt% of cross-linked PVA have been studied as a function of the used electrolyte. The results are given in Table 2.


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NaCl 0.1M +

rion (Na ) 1.84Å Generated stress

CaCl2 0.1M 2+

rion (Ca ) 3.09Å

AlCl3 0.1M rion (Al3+) 4.38Å

-1±0.1

-0.6±0.1

-0.30±0.05

-0.08±0.02

-0.08±0.02

-0.08±0.02

at -0.4V (MPa) Generated stress at 0.8V (MPa) Table 2. Study of the generated stress by cPVA-CNT immersed in different electrolytes: NaCl, CaCl2 or AlCl3.

The stress generated at positive voltages is very low and quasi-independent on the type of tested electrolyte. This can be understood by the fact that the behavior at positive voltage involves the migration of anions, which are here Cl- ions for the tested electrolytes. By contrast, the stress generated at negative voltages is greater and depends on the type of tested cations. It is of 1MPa for a voltage of -0.4V in the NaCl (0.1M) aqueous solution. Direct comparisons with the stress generated by other CNT actuators are not straightforward because the generated stress depends on a number of factors which include the composition of the solvent and electrolyte, the applied voltage, the Young’s modulus of the material6,8 and the applied pre-load14. Nevertheless, it is worthy to point out that the presently generated stress is greater than the stress generated by bucky-papers in similar conditions. For example, the strain of bucky-paper actuators in NaCl aqueous solutions for voltages below 0.5V is less than 0.05%6, 13. Considering that such materials have a Young’s modulus on the order of 1GPa, the generated stress should not exceed 0.5MPa in conditions comparable to the present ones. CNT based yarns and fibers display greater generated stress because of their superior mechanical properties14, 25. Even if tested in a different electrolyte, CNT yarns show a generated stress above 1MPa for a voltage of 0.5V. It is estimated from published data to be on the order of 5 MPa25. CNT fibers made by a process similar to the present one can show generated stresses above 10 MPa but at greater voltages and after being highly stretched to achieve improved performances14. The stress generated by the present fibers is greater for cations with the smallest sizes. As discussed further, this behavior differs from that observed in other types of CNT based actuators. The above results show that raw CNTs directly assembled into microfibers cannot be efficiently used as micro-actuators because of dominant stress relaxation. The binding of CNTs with cross-linked PVA provides a significant reduction of stress-relaxation and allows 17 Environment ACS Paragon Plus


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the fibers to exhibit an efficient electromechanical behavior. The stress-relaxation is inhibited by the formation of a three dimensional network of PVA interpenetrated in the CNT microfiber. This network can be visualized in the SEM (Scanning Electron Microscopy) micrographs of Figure 7. The presence of the PVA can be clearly observed in the core of the microfiber and in the magnification of the micrographs of the fiber. It is worthy to point out that even with the inclusion of the PVA through the fiber, the fiber remains highly porous and electrically conductive.

Figure7. Cross-section SEM micrograph of a nanotube fiber containing 5 wt% of PVA (a) and detailed SEM magnification images by six (b) and twelve (c) times. CNTs can be clearly observed through the fiber along with PVA in the core and at the surface of the fiber.

The response of the fibers to an applied potential provides information on the involved actuation mechanisms. As debated in the literature, mechanisms involved in CNT actuators can produce ionic swelling or intrinsic deformations of the CNTs. Indeed, CNTs can expand or contract in response to charge injection. This effect takes place in any graphitic materials. For graphite and graphene, the length of C-C bonds expands upon injection of electrons and contracts upon injection of holes21. Variations of the length bonds are due to changes of the occupation of bonding and antibonding orbitals and to electrostatic interactions associated to the formation of an electrical double layer10, 18. Nevertheless, the case of carbon nanotubes is more complicated than the case of graphene because the response depends on the electronic properties, chirality and diameter of SWNTs21, 41, 42. In addition, it can also depend on the 18 Environment ACS Paragon Plus

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nature of the counter ions present in the electrical double layer10. Some SWNTs can expand both for positive and negative charge injection, with generally a much more pronounced effect for injection of negative charges42. Here it is observed that the stress is decreased when a negative or positive potential is applied (the generated stress is negative). This means that the fibers would have the propensity to expand along their main axis upon charge injection if they were acting in conditions of free strain. The effect is clearly more pronounced when negative voltages are applied as expected theoretically for most SWNTs and graphitic materials. The present behavior differs from the one reported for twisted yarns made of MWNTs41. In twisted yarns the fibers contract when a voltage is applied. This phenomenon was ascribed to the ionic swelling of the yarn. The latter induces an increase of the section and a decrease of the fiber length. The present actuation mechanism is therefore likely due to the intrinsic deformation of the CNTs. It presumably differs from that at play in MWNT yarns because the present fibers are made of SWNTs. Lastly it is seen that the counter ions play an important role. Indeed, the stress generated in presence of monovalent ions is greater than that generated in present of di- or trivalent ions. This behavior is again different from the behavior of MWNT yarns which exhibit a more pronounced response in presence of large ions41. This observation supports the fact that the response of the present actuators is not dominated by ionic swelling. But further experiments would be necessary to provide a direct evidence of the dominant actuation mechanism. In addition, the exact role of the valence of ions remains to be elucidated.

4- Conclusion Stress relaxation and creep are limiting obstacles towards the efficient use of carbon nanotubes in actuator applications. The binding of CNTs is not straightforward since the presence of binders can have negative effects on the porosity and electrical conductivity of CNT assemblies. It has been shown in this work that wet spinning technologies allow the inclusion of small amounts of stiff and chemically stable polymers that act as efficient binders of the CNTs. Also, the small amount of binders allows sufficient porosity and conductivity to be maintained. As a result, the achieved microfiber actuators do not suffer from stress relaxation and can be used as robust actuators. This approach can be extended to any types of carbon nanotubes although herein it was used with SWNTs, taking the advantage of their large specific surface area compared to the one shown by large diameter MWNTs. Nevertheless, it was observed that fibers of SWNTs exhibit a behavior qualitatively and quantitatively different from that of MWNTs yarns. This suggests that different mechanisms 19 Environment ACS Paragon Plus


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play an important role in the actuation properties of such materials. Additionally, the present approach can be extended to other electroactive materials such as graphene-based actuators and even to other binders that are chemically compatible with the wet fiber spinning process. In a longer term, we hope that the present work could open new routes towards the development and improvement, of novel technologies for energy conversion potentially useful in micro-devices, biomedical applications and micro-robotics.


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Acknowledgments: This project was financially supported by the ANR project RATIOCELLS (n°ANR-12-BS08011-01 and the Région Aquitaine, MINECO-Spain CTM2014-58481-R, Xunta de Galicia (INBIOMED-FEDER “unha maneira de facer Europa” and EM2014/035), and Fundación Tatiana Pérez de Guzmán el Bueno. C. Mateo-Mateo acknowledges the financial support from the F.P.U. (Ministerio de Educación y Ciencia, Spain).



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(41) Carbon nanotube yarn actuators, Tissaphern Mirfakhrai, PhD Thesis 2009, University of British Columbia. (42) Verissimo-Alves, M.; Koiller, B.; Chacham, H.; Capaz, R. B. Electromechanical Effects in Carbon Nanotubes: Ab Initio and Analytical Tight-Binding Calculations. Phys. Rev. B 2003, 67, 16140(R).



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TOC Graphic


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FIGURE 1 ACS Paragon Plus Environment


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FIGURE 2 ACS Paragon Plus Environment

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14.0

Reference point at rest 0V Stress generated for +0,8V

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ACS Paragon Plus Environment


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Type 1 Bare CNTfiber Type 2 CNT fiber + 5wt% PVA Type 3 CNT fiber + 5wt% cross linked PVA

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FIGURE 7


Carbon Nanotube Microfiber Actuators with Reduced Stress

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