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TiO2.[6] These new materials have a hier-archical structure resulting from the asso-ciation of their elements, often induced during synthetic assembly to produce controlled structures, for example, in the form of nanotubes packed into bundles which in turn form microfibrils preferen-tially aligned parallel to the fiber axis. This arrangement follows basic principles of forming strong synthetic fibers through alignment of long extended molecules.[7,8] However, in contrast with traditional high-performance fibers which have a mono-lithic structure, CNT fibers resemble in many ways a porous staple fiber. They have yarn-like properties,[9] such as excep-tional flexibility in bending and the pos-sibility to be knitted, knotted, and woven like a textile. They combine tensile strength in excess of 1 GPa SG−1, electrical conductivity around 106 S m−1, and a BET surface area in the range of 250 m2 g−1.[10]

Their high toughness, high electrical conductivity, and large surface area make

CNT fibers attractive candidates for sensors. Among the imme-diate challenges to realize this potential is to understand how bulk fiber properties are dictated by intra and interparticle pro-cesses, both of which can provide sensing mechanisms but of different nature. It is of interest, for example, to distinguish how the presence of foreign molecules such as liquids, gases, or polymers, which can readily be hosted in the large internal porosity of the fibers, affects the intrinsic transport properties of the constituent CNTs, and the resistance between elements. Because of the complex hierarchical structure of CNTs, bun-dles, fibrils, fiber, etc., such distinction has often been difficult to make.

Lekawa-Raus et al. have shown that for aligned fibers of few-layer CNTs, electrical resistance increases linearly with temperature above ≈100 K,[11] which suggests that above room temperature longitudinal resistance is limited by internal scat-tering in the CNTs rather than the contact resistance between CNTs. This is consistent with more recent piezoresistive studies on the same material. Changes in fiber longitudinal resistance have been shown to be proportional to tensile stress, rather than strain,[12] and thus the increase in bulk fiber resist-ance associated to distortions in the intrinsic electronic struc-ture of the CNTs under stress. Work by the same group has highlighted the role of water adsorbed on the fiber surface in

A Composite Fabrication Sensor Based on Electrochemical Doping of Carbon Nanotube Yarns

Juan C. Fernández-Toribio, Agustín Íñiguez-Rábago, Joaquim Vilà, Carlos González, Álvaro Ridruejo, and Juan José Vilatela*

This work shows evidence of conventional liquid and polymer molecules doping macroscopic yarns made up of carbon nanotubes (CNT), an effect that is exploited to monitor polymer flow and thermoset curing during fabri-cation of a structural composite by vacuum infusion. The sensing mechanism is based on adsorption of liquid/polymer molecules after infiltration into the porous fibers. These molecules act as dopants that produce large changes in longitudinal fiber resistance, closely related to the low density of carriers near the Fermi level of bulk samples of CNT fibers, reminiscent of their low-dimensional constituents. A 25% decrease in fiber resistance upon exposure to electron–donor radicals formed during epoxy vinyl ester polymerization is shown as an example. At later stages of curing the matrix undergoes shrinkage and applies a compressive stress to the fibers. The resulting sharp increase in electrical resistance provides a mechanism for detection of the matrix gel point. The kinetics of resistance change during polymer ingress are related to established models for macromolecular adsorption, thus also ena-bling prediction of polymer flow. This is demonstrated for vacuum infusion of a 150 cm2 glass fiber laminate composite, with the CNT fiber yarns giving accurate prediction of macroscopic resin flow according to Darcy’s law.

DOI: 10.1002/adfm.201602949

J. C. Fernández-Toribio, Dr. J. Vilà,[+] Dr. C. González, Dr. J. J. VilatelaIMDEA Materials InstituteEric Kandel 2, Getafe, Madrid 28906, SpainE-mail: [email protected]. Íñiguez-RábagoKavli Institute of NanoscienceDelft University of TechnologyLorentzweg 1, 2628 CJ, Delft, The NetherlandsDr. C. González, Dr. A. RidruejoDepartment of Materials SciencePolytechnic University of Madrid28040 Madrid, Spain

1. Introduction

The assembly of nanobuilding blocks into macroscopic struc-tures has led to a new generation of materials that begin to efficiently exploit the properties of the nanoconstituent on a macroscopic scale. Typical examples include continuous films and fibers of carbon nanotubes (CNTs),[1–4] graphene,[5] and

[+]Present address: Faculty of Aerospace Engineering Structural Integrity and Composites, Delft University, 2629 KS, Delft, The Netherlands

Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201602949

www.afm-journal.dewww.MaterialsViews.com

http://doi.wiley.com/10.1002/adfm.201602949

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reducing fiber resistance by as much as 21% relative to the dry state.[13] Senokos et al. have measured the quantum capaci-tance of arrays of CNTs fibers, showing that even bulk cm2 electrodes exhibit a semi-metallic density of states (DOS).[14] The superposition of DOS of individual CNTs results in a bulk joint DOS with a “V” lineshape,[15] somewhat resembling that of few-layer graphene, with the minimum at the Fermi level increasing symmetrically at both negative and positive elec-trochemical potentials. As a consequence, even for bulk CNT fiber samples the relatively low density of carriers near the Fermi level implies that the transport properties of these fibers are sensitive to interaction with foreign molecules and other stimuli altering the electronic properties of the constituent CNTs.

Experimentally, the electrical conductivity of CNT fibers and of nanocarbon-based membranes has been shown to be sen-sitive to the presence of liquids.[16,17] A wide range of organic polar and non-polar liquids produce large (>10%) increases in longitudinal fiber resistance upon infiltration in the fibre.[16] Because of the weak interaction between adjacent bundles in a CNT fiber, the ingress of liquid can slightly separate adjacent bundles and produce a small swelling, detected by small-angle X-ray scattering. Thus, the changes in electrical resistance were explained based on such elastocapillary effects, whereby bulk changes in conductivity were attributed to the increased separation between bundles and the corresponding increase in contact resistance through tunnelling.[18] Similarly, reductions in fiber resistance and non-ohmic behavior were explained by a decrease in bundle separation due to electrostatic interaction between the capacitive double layer formed by polar liquids.[19] The present work demonstrates that changes in fiber resistance are not dominated by these elastocapillarity effects, but rather due to electrochemical doping of adsorbed molecules on the surface of CNTs.

The effects of liquid or gas molecules on the transport prop-erties of CNTs has been most commonly studied in field-effect transistor (FET) devices with an individual CNT as channel (CNTFET), and which have received enormous attention for the last two decades due to their potential as miniaturized sen-sors.[20] When the changes in transport properties are exclusively due to adsorption of molecules on the CNT instead of on the metal/CNT contacts, these can occur through different mecha-nisms. At low concentrations adsorbates produce a shift in the Fermi level of the CNT but without changes to its work func-tion (or electron affinity) or to the contact barriers. In contrast, extensive coverage of the adsorbates can distort the electronic structure of the CNTs.[20] When fully immersed in a liquid, par-ticularly in polar or ion-containing ones, changes in conductance are also due to electrostatic gating, whereby charges in the CNTs screen charges in the adsorbed molecules, a process widely exploited for fluidic[21] and biochemical sensing.[22] Changes in transport properties of the CNTFET channel upon exposure to gas or liquid molecules can also originate from adsorbates on the metal contact lowering its work function and therefore the interface potential barriers in the device.[23] FET devices based on networks of predominantly metallic SWNTs have shown similar characteristics as an FET based on individual CNTs, with high sensitivity of (semiconducting) CNTs to electrostatic gating.[24]

Here, we show that some of the effects measured on indi-vidual CNTs in FET architectures are in fact observable in bulk samples of CNT fibers, and demonstrate that the changes in longitudinal resistance are caused by electrochemical doping of adsorbates. The dynamic changes in resistance upon liquid infiltration are explained in terms of established adsorption kinetics of macromolecules, providing a one-to-one correspond-ence between the macroscopic flow and the changes in fiber electrical resistance. This enables prediction of polymer flow in other geometries. As an example, we demonstrate accu-rate polymer flow predictions during preparation of a 150 cm2 glass fiber composite prepared by vacuum infusion, one of the most common composite fabrication methods in industry. We also show how the formation of highly doping radicals during polymerization enables monitoring of the curing reaction and detection of key parameters, such as the matrix gel point, which is of great importance in the fabrication of pre-impregnated laminate composites (pre-pregs).

The evidence that these fibers, produced by a simple process on a kilometer/day in the laboratory[25] and larger quantities in industrial settings,[26] can retain some of the fundamental low-dimensionality properties of CNTs opens their widespread application as sensors. This is the more relevant considering the exceptional toughness of CNT fibers,[10,27] which can unlock applications in various fields where sensors based on FETs or Bragg gratings are unsuitable.

2. Results and Discussion

2.1. Adsorption Kinetics and Electrochemical Doping

The samples in this work consist of yarns of ten CNT fiber fila-ments, consisting of predominantly aligned CNTs of 2–5 layers (Figure 1a). These two features, preferred orientation and few layers, result in the formation of extended crystalline domains (bundles) which thus enhance the contribution from the intrinsic CNT properties to the bulk fiber properties. The immersion of such a yarn in conventional liquids, solutions, or low viscosity polymers results in a rapid change in resistivity. The example in Figure 1c corresponds to a commercial epoxy vinyl ester, chosen because of its widespread use in struc-tural composite manufacturing, but similar graphs for other liquids are included in the Supporting Information. The plot shows the change in four-wire longitudinal electrical resist-ance during this experiment, corresponding to the infiltration of resin (wicking) radially inward into the fiber (Figure 1b). Ingress of the polymer occurs through capillary forces arising from the large porosity of the fibers. The graph shows a rapid increase in fiber resistance that levels off after ≈30 min of immersion.

The experimental data in Figure 1c can be very accurately fit by an expression of the form

D1*R t R e t( )( ) = ∆ − ( )− β

(1)

R e t1* ( )≈ ∆ − − ⋅D

(2)



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where ΔR* corresponds to the net change in resistance after a sufficiently long time and once the system reaches equilibrium after immersion. The exponent t⋅D is clearly suggestive of a diffusion process (D is a diffusivity per unit cross section). Indeed, adsorption of polymers under conditions of low molec-

ular mobility, for example, at low temperature or in porous media, is accurately described by an expression of the form is clearly suggestive of a diffusion process (D is a diffusivity per unit cross section). Indeed, adsorption of polymers under conditions of low molecular mobility, for example at low



Figure 1. Sensor structure, model of liquid infiltration, and fitting of the experimental data according to the model proposed. a) Optical image and electron microscopy images showing the CNT fiber yarn structure across different length-scales. b) Schematic illustration of the radial infiltration of a fluid into the CNT fiber by capillary forces. Relative changes in longitudinal fiber resistance upon immersion c) in vinyl ester epoxy resin and d) in propylamine (black solid lines). The fit to the experimental data corresponds to a diffusive rate-limited adsorption process (β ≈ 0.5).

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temperature or in porous media, is accurately described by an

expression of the form t

e t( )(1 )( / )Γ

Γ≈ − τ

∞

− β

, with Γ the surface

concentration of polymer, τ a characteristic adsorption time constant, and β an exponent typically close to 0.5, depending on temperature and molecular weight of the adsorbate.[28] Equa-tion (1) is clearly similar to such expression, particularly noting that τ ≈ 1/diffusion coefficient. The factor of 0.54 observed in Figure 1c for vinyl ester with molecular weight of 804 g mol−1, is indeed in excellent agreement with these kinetics of adsorp-tion and confirms that the changes in resistance obey a rate-limited diffusion process at a surface.

Previous reports assigned such changes in fiber electrical resistance to elastocapillarity effects, whereby the ingress of liquid would produce small fiber swelling (detected by SAXS) by opening bundle junctions, which would thus decrease the probability of interbundle tunneling.[18] Instead, the results shown above indicate that the changes in resistance follow the kinetics of an adsorption process, and point to electrochemical interaction between liquids/polymers with the constituent CNTs as the main origin of the changes in bulk fiber resist-ance. Figure 1d shows an equivalent infiltration experiment but in propylamine. In this case, there is a rapid decrease in fiber resistance, which follows Equation (1) with β taking a value of 0.33, instead of the typical 0.5, as indeed propylamine has a rel-atively low molecular weight and viscosity. Amines are electron donors, increasing the Fermi level of CNTs, as has been demon-strated by XPS or Raman spectroscopy.[26] Their effects on CNT conductance depend on the type of CNTs, presence of adsorbed oxygen, and on their type of ensemble (individual, random net-work, highly aligned). Exposure of CNTFET architectures to NH3 often affects CNT/metal contacts, but can produce never-theless increases in CNTFET channel conductance.[29] It is thus not surprising that the role of propylamine as electron donor, changing the Fermi level of the CNT network by population of states in the conduction band, manifests as a decrease in fiber resistance. As further evidence of electrochemical doping in CNT fibers, we have included additional electrical measure-ments in the Supporting Information. Moisture drying-satura-tion cycles applied to the CNT yarns used in this work show that water acts as a p-dopant, producing large decreases in resist-ance upon adsorption, in agreement with previous reports.[13] Immersion in ethanol also produces a decrease in fiber resist-ance, but only for moisture-free fibers. Exposure to ethanol vapor molecules produces detectable changes in longitudinal fiber resistance under conditions of negligible capillarity. When samples saturated with water are immersed in ethanol, the net increase in resistance comes mainly from desorption of water (Supporting Information). Furthermore, infiltration of the fibers with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), an electron acceptor and a known CNT dopant, pro-duces a drop in electrical resistance and an upshift in the G and 2D Raman bands.

These results clarify the origin of the changes in electrical resistance observed when exposing CNT fibers to liquids/poly-mers. Although small transient drop in conductance due to pie-zoresistive effects during entrance into the liquid reservoir are also observed, and elastocapillarity cannot be fully neglected, it is clear that the main mechanism of chemoresistance in these

fibers is through electrochemical interaction of CNTs with foreign molecules and its effects on the axial transport prop-erties of the CNT building blocks. These molecules can act as dopants that shift the Fermi level of the CNTs, for example, through direct charge transfer or through the formation of an electric double layer, although for extensive coverage, the elec-tronic band structure of the CNTs can be severely distorted by overlap with molecular orbitals.[20] Doping of CNT fibers is perhaps most evident when immersed in conductive ionic liquids (or other electrolytes) that enable application of an electrochemical potential. In 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, for example, resistivity decreases from the point of zero charge at a rate of roughly 5% per ±100 mV applied.[14] A further study on the different pos-sible liquid/CNT interactions (charge transfer, surface poten-tial, etc.) will be reported elsewhere. As discussed in the rest of the paper, the simple electrochemical effect observed in Figure 1c enables accurate monitoring of polymer flow in large-scale structural composites produced using standard industrial methods, as well as detecting thermoset curing reactions.

2.2. Principle of CNT Fiber Sensor Operation

The discussion above sheds light into the origin of the sensing behavior of CNT fibers. In addition, by relating conductivity changes to polymer infiltration, the results suggest that the material can be used to monitor liquid or polymer flow in sce-narios of practical relevance, such as the fabrication of struc-tural laminate composites. In the immersion experiments discussed above, all the length of fiber is exposed to the fluid at the same time. The wicking process therefore corresponds to the ingress of molecules into the fiber radially inward (Figure 1b). If a longitudinal flow were also present, such that different areas along the fiber were brought in contact with the liquid at different times, Equation (2) would take the form

R tR

ll t e t1

*

( )( ) ( )=∂∆

∂⋅ − − ×D

(3)

where l(t) describes the progression of the longitudinal flow and R

l

*∂∆∂

and D are constants of the fiber/polymer pair which

can be readily obtained by calibration experiments. D is related to the diffusion coefficient of the polymer/porous material pair, but which remains constant throughout the experiment (for

constant temperature). R

l

*∂∆∂

represents the amount of resist-

ance change per unit advance of the polymer and can thus be seen as a figure of merit for sensitivity of the fibers. We obtain values for these constants by immersing CNT fibers of dif-ferent lengths in polymer. Figure 2a presents the variation of resistance during the experiment for fiber lengths of 35, 54, 74, and 97 mm. All curves are accurately fit by Equation (1). A plot of ΔR* against fiber length (Figure 2b) gives a linear relation as expected. Moreover, the fact that the linear fit in Figure 2b is close to the origin for l = 0 confirms that effects from liquid/polymer adsorption on the CNT fiber/metal con-tact resistance are negligible, which is often not the case in



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CNTFET devices.[20] Equipped with values for these constants (D shown as inset in Figure 2b), a longitudinal flow can be pre-dicted exclusively from the temporal changes of fiber resistance

by simple rearrangement of Equation (3), with CR

l

* 1

≡∂∆

∂

−

, to obtain

l t R tC

eR t

et t

( ) ( )1

( )0.13

1 6.7 10 3= ×

−= ×

−− × − × ×−D

(4)

2.3. Monitoring Polymer Flow during Laminate Composite Fabrication

Next, we demonstrate the possible application of this material as a sensor during fabrication of a structural laminate com-posite by vacuum infusion, a common out-of-autoclave indus-trial composite fabrication method. The technique consists in the liquid resin infiltration of a lay-up of dry fabric preform enclosed by a flexible bag sealed from the atmosphere. The flow of resin occurs inside the bag from the inlet, connected to the resin pot, to the outlet, connected to a vacuum pump. The experimental setup used here consisted of a lay-up of four E-glass fiber plain and balanced woven 10 × 15 cm layers, with the CNT yarn positioned in the mid-plane and aligned par-allel to the direction of resin flow (Figure 3a–c). The electrical contacts were kept outside the preform to ensure no interfer-ence with the electrical measurements. Because of the optical contrast between the glass fibers and the yellowish resin, the resin flow along the preform can be monitored with a conven-tional video camera. Such data though, corresponds to the sur-face flow, whereas the sensor detects the flow within the fabric lay-up (Figure 3b).

Figure 3d presents the prediction of flow front posi-tion l(t) solely from electrical resistance data, according to Equation (4), compared with the results extracted from optical images. The general trends are similar in both cases, indicating

the macroscopic flow is essentially controlled by the pressure gradient between the inlet and outlet following Darcy’s law. If the fabric permeability k is constant, Darcy’s law combined with the continuity equation can be integrated to obtain the flow front position as a function of time as

l tkP

t2 atm

η( ) =

(5)

where η is the resin viscosity and Patm atmospheric pres-sure. This equation yields the well-known proportionality law between the flow front position and the square root of infusion time l t t( ( ) )≈ and gives a value of k of 5 × 10−7 cm2 for the E-glass fabric preform.

Effectively, the latter expression is only valid if the perme-ability of the fabric remains constant during the fluid flow, which is the case if the fabric thickness does not vary during the test. This assumption may not be completely valid for infu-sion experiments, as there is a variation of thickness between the inlet and outlet ports due to fiber resin stress transfer, how-ever under the conditions used here this contribution is not sig-nificant and Equation (5) is appropriate. A plot of the flow front position against the square root of time, presented in Figure 3e, shows that both sets of data (optical and resistive) give a linear behavior, as expected from Equation (5). However, their dif-ferent slopes indicate that the predicted flow progresses faster than the optical images of the surface flow. These differences are likely to reflect genuine disparities in the flow across the preform, attributed to the inhomogeneous flow progress in the fabric as a consequence of its dual-scale porosity, i.e., intra and inter-glass fiber yarns.[30–32] Viscous flow along the glass fiber yarn-to-yarn channels is much faster, driven by the pressure gradient between the inlet and outlet. However, even if yarn-to-yarn channels are totally filled by the fluid, the microscopic flow inside the fabric is still driven by capillary forces. As a conse-quence, unsaturated flow occurs at the transition zone between dry and wet regions of the fabric. Most likely, the CNT sensor



Figure 2. Calibration of CNT fibers yarn sensor. a) Change in electrical resistance during immersion of different fiber lengths (35, 54, 74, and 97 mm) in epoxy vinyl ester resin (solid lines), and corresponding fit according to Equation (1) (dashed lines). b) The constant relating longitudinal flow to changes in electrical resistance is obtained from the slope of versus*R l∆ . Inset: D is fairly independent of fiber length.

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placed between adjacent glass fiber yarns measures the faster yarn-to-yarn viscous flow, while the optical flow front position can be considered as an average value more representative of the combined capillary and pressure-driven flow. The 3D dif-ferences in macro and micro flows could be measured by in situ X-ray tomography,[32] which will be the subject of a further study.

It is reassuring that the sensor outputs the expected flow front for a successful infiltration. But also importantly, the prediction based on electrical resistance changes can capture irregular polymer flow deviations from Darcy’s law in a defec-tive infusion setup, for example, in the presence of wrinkles in the vacuum bag (Supporting Information).

In addition to the flow of resin through the fabric, the sensor enabled detection of other fine features of the fabrica-tion process. Before the resin inlet is opened, when the vacuum is applied the compaction of the fabrics itself can be detected by as a small increase in electrical resistance, which is due to the compression exerted on the CNT yarn and the fabric

by the vacuum bag (Supporting Information). Similarly, the small compression produced when drawing guiding lines used for optical flow monitoring can be clearly observed as sharp changes in fiber resistance (Supporting Information). These effects are purely piezoresistive.[33]

The experiment in Figure 3 corresponds to infusion with vinyl ester alone in order to characterize the flow sensor under constant temperature and electrochemical conditions. But if a CNT fiber is immersed in the complete thermosetting polymer mixture containing epoxy vinyl ester, methyl ethyl ketone per-oxide (MEKP) catalyst, and cobalt octoate promoter (at a ratio 100:1.5:0.3), additional features of the curing process itself can be detected through changes in fiber resistance. Figure 4a shows the changes in resistance during the curing cycle under ambient conditions, for a CNT fiber yarn sensor immersed in the thermoset precursors but without longitudinal flow. There is a fast decrease in resistance upon immersion as the viscous fluid infiltrates the yarn. This drop in resistance is consistent with the presence of highly reactive radical groups formed



Figure 3. Monitoring flow during vacuum infusion using a CNT fiber yarn sensor. a) Schematic illustration of the experimental setup used. The CNT fiber yarn is placed between two glass fiber fabrics parallel to the expected resin flow direction. The (surface) flow is independently determined by a visual correlation system. c) Photograph of the vacuum infusion setup, showing electrical contacts outside the preform. d) Comparison of predicted flow based on CNT fiber sensor and surface flow from optical images showing good agreement. e) The plot of flow front position against square root of time, l(t) versus t , is linear, indicating macroscopic flow according to Darcy’s law.

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during polymerization of epoxy vinyl ester, acting as electron donors. The reduction in fiber resistance remains for 30 min, including the first stages of the thermoset cross-linking reac-tion when temperature starts to increase due to the exothermic curing reaction. Very interestingly, at 34 min there is a sharp increase in electrical resistance, which coincides with the peak in temperature and heat release rate. It also matches the time to reach gel point according to the thermoset manufacturer (30–60 min). Upon gelation the polymer network undergoes shrinkage and thus produces a large compressive deformation in the embedded CNT yarn. The corresponding increase in fiber resistance is thus mainly piezoresistive.[33] Raman spec-troscopy measurements give a small 1.5 cm−1 upshift in the 2D band for the embedded yarn compared to the original material, confirming the presence of a residual compressive strain after curing (Supporting Information).[34]

In Figure 4b, we present the results of an infusion of fabric with the complete combination of resin, hardener, and catalyst. The drop in electrical resistance is again indica-tive of dopant species and follows the same kinetics as pro-posed above. The inset in Figure 4b shows that the predicted flow again follows Darcy’s law. However, the sharp increase in electrical resistance during the curing reaction measured by a sensor embedded in the pure thermoset matrix, is not observed in the laminate composite. This effect probably can be endorsed to the presence of the stiff skeleton of the glass fabric. The unconstrained chemical shrinkage previously observed in the neat resin experiments is now affected by the presence of the E-glass fabric. As a consequence, a residual stress state, tensile in the epoxy vinyl ester resin, and com-pressive in the E-glass fabric can exist, mitigating the increase of electrical resistance of the CNT observed in the uncon-strained curing experiment.

It is possible that the gel point of thermosetting polymers in other fabric/thermoset combinations can be readily detected, but nevertheless, future work should be directed at exploiting other mechanisms of sensing to detect the different stages of the curing process.

3. Conclusion

Macroscopic fibers made up of CNTs are interesting mate-rials that combine efficient exploitation of the axial proper-ties of their constituent building blocks with a large internal area. When comprised of predominantly aligned nanotubes of few layers, bulk fibers preserve some of the low-dimen-sional properties of CNTs. Immersion of CNT fibers in con-ventional liquids/polymers leads to large (>10%) changes in their longitudinal electrical resistance. This is a consequence of electrochemical interaction between foreign molecules and CNTs, which changes their resistivity through charge transfer or the formation of a surface potential. Further elu-cidation of these mechanisms requires the use of test mol-ecules and more controlled atmospheres (gas composition, temperature, etc.), as well as simulation tools that can model fundamental properties of molecule/CNT interactions and their observation as an average on a large number of dif-ferent CNTs.

The changes in electrical resistance of macroscopic lengths of fiber (>30 cm) can be accurately described by equations related to the kinetics of macromolecular adsorption in porous media. Through calibration measurements using different fibers lengths two relevant parameters are obtained: a constant related to the diffusion coefficient of the fiber/fluid pair and a gauge factor relating longitudinal advance of a fluid to the change in electrical resistance. These parameters enable predic-tion of polymer flow along a CNT fiber yarn. This is demon-strated during fabrication of a 150 cm2 woven glass-fiber com-posite by vacuum infusion, with great accuracy.

Finally, the various changes in chemical environment during curing of a thermoset can also be monitored by their effects on the electrical resistance of CNT fibers. In the first stage the cross-linking reaction produces a large number of radicals that act as electron donors and thus produce a decrease in fiber resistance. The gel point of polymer phase can be accurately observed as a sharp increase in fiber resistance, due to the shrinkage of the polymer network resulting in a compressive



Figure 4. a) Relative electrical resistance of a CNT fiber yarn upon immersion in a mixture of vinyl ester resin, methyl ethyl ketone peroxide hardener, and Co catalyst at a ratio 100:1.5:0.3. The initial drop in resistance is due to doping by radicals formed during the polymerization curing reaction. The sharp resistance increase at 34 min is piezoresistive and corresponds to the gel point of the polymer network. b) During vacuum infusion of the complete mixture of resin, hardener, and catalyst, the flow is accurately predicted by the sensor and follows Darcy’s law, but the matrix exerts negligible compression and thus the gel point cannot be observed.

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stress on the CNT fiber yarn. The gel point is a parameter of great interest in composite fabrication as it marks the start of the transition from semi-cured to fully cured materials. Its detection by a simple electrical resistance measurement makes CNT fiber-based sensors very attractive.

The electrochemical effects reported here are more typical of miniature CNTFET devices with individual CNTs. That they are observed in bulk CNT fibers over lengths of tens of centimeters without subjecting them to any treatment is encouraging and opens the door to their application as sensors in structural com-posite fabrication. Ongoing work on coupling curing kinetic to polymer flow, as well as on strategies to achieve selectivity, will further enhance the materialization of this potential.

4. Experimental SectionMaterials and Characterization Methods: The CNT fiber sensors were

produced by the direct spinning method. It consists in continuously drawing an aerogel of CNTs directly from the gas phase during their growth by chemical vapor deposition (CVD).[2] The CVD reaction was carried out at 1250 °C in an H2 atmosphere using ferrocene, thiophene, and toluene, as Fe, S, and C sources, respectively. The ratio S/C, which controls the number CNT morphology, was chosen so as to produce CNTs with few layers (<5), as reported elsewhere.[25] All samples were spun at a rate of 20 m min−1, which corresponds roughly to a draw ratio of 6.3 and leads to preferential alignment of the CNTs parallel to each other and to the fiber axis.[35] Sensors were fabricated by manual densification of ten individual CNT fiber filaments into a single yarn that could thus be handled more easily. The yarns have a typical diameter of 40 μm and a longitudinal conductivity of 3.5 × 105 S m−1.

DERAKANE 8084 is an elastomer-modified bisphenol-A epoxy vinyl ester resin for structural applications, available from Ashland Inc. Both the MEKP hardener and Cobalt octoate catalyst were supplied by Plastiform S.A. At the recommended concentration of precursors (100:1.5:0.3), the full curing reaction takes 24 h at room temperature, with the matrix reaching its gel point after 30–60 min. Raman spectroscopy was carried out with a 530 nm laser line and a Renishaw spectrometer, using parameters that avoid artifacts from sample heating. The spectra were collected with the fiber parallel to the laser polarization direction.

Electrical Measurements: In all the cases the electrical resistance of the CNT fiber was monitored using a commercial QuantumX data acquisition system. It delivers a small DC current and measures voltage across the sample, similar to a conventional millimeter. Most measurements were carried in four-wire configuration. Selected experiments were conducted with a Biologic potentiostat to ensure adequate operation of the acquisition system.

Infiltration experiments were carried out using different setups built in-house, depending on the type of measurements. Contacts were produced by pressing indium onto the CNT fibers. Control experiments were carried out to rule out the effects of the liquid on the possible potential barrier at the fiber/metal contacts. Furthermore, the resistance of a typical samples used in this work is in the range of kΩ which is much higher than the contact resistance. For sensor calibration experiments, different lengths of fiber were attached to the same holder and immersed in the fluid of interest, thus ensuring identical conditions of infiltration.

Vacuum Infusion of Glass Fiber Laminate Composite: The laminate composite was fabricated using a standard vacuum infusion method. The preform consisted of four woven glass fabrics of ≈10 × 15 cm, which were affixed onto a flat surface and covered with a vacuum bag. The CNT fiber yarn sensor was placed between the second and third glass fiber fabric layers, parallel to the resin flow. Electrical contacts were made by attaching metal wires to the ends of the fibers using indium. The infusion process consists of the stages of: application of vacuum

and resulting compaction of the bag, opening of resin inlet, which starts the infusion process, and finally closing the outlet and inlets when the resin reaches the end of preform. The macroscopic flow of the resin was monitored by the changes in longitudinal conductivity of the fibers, and independently using a video camera placed above the infusion the setup. The progression of the fluid front across the laminate can be determined by image analysis taking advantage of the optical contrast between the glass fiber and the yellowish resin. Vertical lines separated by 4 mm were drawn on the top part of the plastic bag. They were used as spatial reference to improve the accuracy of the optical method.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThe authors are grateful to Mr. B. Mas and V. Reguero for the fabrication of sensors, to Mr. A. Monreal-Bernal for assistance with electrical measurements, and to Dr. J. P. Fernández-Blázquez for discussions on thermoset curing reactions. J.J.V. acknowledges financial support from the European Union Seventh and Eighth Framework Programs under Grant Agreement Nos. 678565 (ERC-STEM) and 310184 (CARINHYPH), from MINECO (MT2012-37552-C03-02, MAT2015-62584-ERC, RyC-2014-15115, Spain), and the Madrid regional government (S2013/MIT-3007 MAD2D project).

Received: June 14, 2016Revised: July 18, 2016

Published online:

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