Tailoring Interfacial Properties by Controlling Carbon Nanotube Coating Thickness on Glass Fibers Using Electrophoretic Deposition.

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Tailoring interfacial properties by controlling carbon nanotube coating thickness on glass fibers using electrophoretic deposition Sandeep Tamrakar, Qi An, Erik T Thostenson, Andrew N. Rider, Bazle Z. (Gama) Haque, and John W. Gillespie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10903 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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ACS Applied Materials & Interfaces

Tailoring Interfacial Properties by Controlling Carbon Nanotube Coating Thickness on Glass Fibers using Electrophoretic Deposition Sandeep Tamrakar1, 2, Qi An1, 3, Erik T. Thostenson1, 4, Andrew N. Rider5, Bazle Z. (Gama) Haque1, 4, John W. Gillespie Jr.1, 2, 3, 4 * 1

2

Center for Composite Materials (UD-CCM) Department of Civil & Environmental Engineering 3 Department of Materials Science & Engineering 4 Department of Mechanical Engineering University of Delaware, Newark, DE 19716 5

Defence Science and Technology Organization Fisherman’s Bend, Victoria 3207, Australia

Abstract Electrophoretic deposition (EPD) method was used to deposit polyethyleneimine (PEI) functionalized multiwall carbon nanotube (CNT) films onto the surface of individual S-2 glass fibers. By varying the processing parameters of EPD following Hamaker’s equation, the thickness of CNT film was controlled over a wide range from 200 nm to 2 µm. The films exhibited low electrical resistance, providing evidence of coating uniformity and consolidation. The effect of the CNT coating on fiber matrix interfacial properties was investigated through microdroplet experiments. Changes in interfacial properties due to application of CNT coatings onto the fiber surface with and without a CNT-modified matrix were studied. Glass fiber with a 2 µm thick CNT coating and the unmodified epoxy matrix showed the highest increase (58%) in interfacial shear strength (IFSS) compared to the baseline. The increase in the IFSS was proportional to CNT film thickness. Failure analysis of the microdroplet specimens indicated higher IFSS was related to fracture morphologies with higher levels of surface roughness. EPD enables the thickness of the CNT coating to be adjusted, facilitating control of fiber/matrix 1 ACS Paragon Plus Environment


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interfacial resistivity. The electrical sensitivity provides the opportunity to fabricate a new class of sizing with tailored interfacial properties and the ability to detect damage initiation.

Keywords: Electrophoretic deposition, multiwall carbon nanotubes, microdroplet, interface, glass fiber

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1. Introduction The fiber-matrix interface governs the load transfer efficiency and plays a critical role in determining the strength, toughness and energy absorption capability of a composite material. The role of film formers and silane coupling agents on the properties of interface has been well documented in the literature.1–3 Sizing formulations with different chemical reactivity and surface morphology have a significant impact on the fiber-matrix adhesion and in its energy absorbing capability.4 Furthermore, changes in the interfacial properties at the microscale correspond to the changes in bulk properties of composites.5 Being able to tailor the interface at nano to microscale and understanding the mechanisms involved in the interface region during loading can improve the performance of composites. Recently carbon nanotubes (CNTs) have gained much attention in altering the interfacial properties of composites. Many studies have shown an increase in IFSS with the amount of CNTs deposited on the surface of the fiber.6–9 This phenomenon can be further explored by controlling the thickness of the CNT coating on the fiber to obtain the desired interfacial properties. Being able to control the CNT loading depends on the process adopted to deposit the CNTs. The chemical vapor deposition (CVD) approach is the most commonly used technique to produce CNTs. In general, the extent of decomposition of hydrocarbons and the purity of CNT produced by CVD is associated with higher processing temperature (600 - 1000°C).10,11 Several attempts have been made to significantly lower the processing temperature for CVD of CNT,12,13 with temperatures as low as 120°C in some cases for metallic substrates.14 The lower temperature range (350 - 490°C) reported for CVD using glass (or an equivalent surface)12 as



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substrates is still high enough for the tensile properties of glass fibers to start degrading and may not yield defect-free CNT coated fibers.15,16 Harsh deposition conditions and the degree of complexity required during the processing have motivated numerous researchers to explore viable alternative methods to deposit CNTs on structural components.6,17 Grafting CNTs onto glass fibers can be achieved by different techniques such as dip coating,18–20 spray coating,21,22 and electrophoretic deposition (EPD) of CNTs.23,24 Although all these approaches are relatively easy to execute and highly scalable, controlling the uniformity and the thickness of the deposited CNT through dip coating and spray coating techniques has been an issue.6,19,25 EPD is a cost effective and versatile technique to coat particulates onto substrates with complex shapes or surface textures.23,24 In this method, charged particles, which are homogeneously dispersed in a solvent such as water, are allowed to migrate to a relevant electrode under an applied electric field. Once the charged particles reach the electrode, they coagulate to form a deposit.26 Many studies have focused on enhancing the interfacial properties of composite materials by applying a thick coating of CNT using EPD method onto carbon fibers or fabric.17,27,28 Recently, some researchers have adopted the EPD approach to coat CNTs on non-conductive substrates like glass fibers.7,19 EPD of polyethyleneimine (PEI) functionalized CNTs treated using ozone onto E-glass fabric showed significant improvement in the in-plane shear strength and piezoresistivity of the CNT-coated glass/epoxy laminates.7 Zhang et al.19 demonstrated the applicability of using EPD to deposit CNTs onto glass fibers for in situ damage sensing in a single fiber fragmentation test. CNT coated single glass fibers exhibited electrical properties in the semiconducting range. No degradation in the tensile strength of the CNT coated fibers was observed, while the IFSS increased by 30%.


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Very few studies have attempted to coat a thin layer of CNT onto a single glass fiber such that the interfacial region is sufficiently conductive to provide a piezo-resistive response without compromising the interfacial properties. Although, some researchers have made an attempt to deposit a very thin layer of CNTs onto the surface of glass fibers,21,29,30 no comprehensive study has been carried out to control the thickness and uniformity of the coated film using EPD onto individual glass fibers, although film thickness as a function of field strength was examined for CNTs deposited onto glass fabric.7 In this study, an approach to control the deposited CNT film onto individual glass fibers using EPD has been carried out by investigating the kinetics of the EPD process. Hamaker’s law31 was used to estimate the particle deposition rate. The electric field strength, deposition time and the CNT dispersion concentration were manipulated to investigate the interrelationship between these variables and ultimately provide control over the quantity and homogeneity of the coating.23,24,32 The changes in interfacial properties of EPD fiber and epoxy were investigated using the microdroplet test method.

2. Experimental Work 2.1.

Materials

Multiwall carbon nanotubes (CM-95, Hanwa Nanotech, Korea) having diameters between 10 and 20 nm were treated using an ultrasonicated-ozonolysis (USO) method.7,17 USO treated CNTs were functionalized with polyethyleneimine (H(NHCH2CH2)58NH2, MW 25000, Sigma-Aldrich, USA) in a sonicator as described in the previous work.7,17 The pH of the PEI-functionalized CNTs was adjusted using glacial-acetic acid (Sigma-Aldrich, USA) to a pH around 6,7 which 5 ACS Paragon Plus Environment


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resulted in a zeta potential of the CNTs of around +40 mV.33 Aqueous dispersions were prepared for EPD with CNT concentrations of 0.5 and 1.0 g/L. PEI-functionalized CNTs were deposited onto the surface of S-2 glass fibers (Owens Corning Corporation) with γ- glycidoxypropyltrimethoxysilane sizing and epoxy film former. Epoxy resin DER 353 (Dow Chemical Company) was mixed with bis (p-aminocyclohexyl) methane (PACM20) curing agent (Air Products and Chemicals, Inc.) at stoichiometric ratio of 100:28 (weight ratio) to prepare the droplets.

2.2.

Electrophoretic Deposition

EPD was carried out at room temperature in a custom made apparatus with stainless steel electrodes. S-2 glass fibers were attached with a slight pretension onto one of the two parallel electrodes placed at a fixed gap of 6.7 mm. The entire unit was then immersed vertically in the CNT dispersion and cathodic deposition was carried out under a constant DC electric field. After the deposition, the fibers were carefully removed from the CNT dispersion and allowed to dry at room temperature. An expression derived by Hamaker31 to estimate the quantity of particles deposited over time has been used to study the kinetics of the EPD. Hamaker’s law states that the deposit yield, w (kg) is linearly proportional to time, field strength

(V/m), concentration of particles cs (kg/m3) and

electrode surface area S (m2) as shown by Eq. (1) =

∙ ∙

(1)


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In Eq. (1), electrophoretic mobility µ (m2/(V s)) is a constant related to the properties of the suspension, which is given by Eq. (2)

=

(2)

Where, ε is the dielectric constant of the liquid, ε0 (C2/J m) is the permittivity of free space, (V) is the zeta potential of the particles, and η (Pa s) is the viscosity of the liquid. Following Hamaker, many researchers have studied EPD and observed a linear rate of deposition initially which then starts to plateau.7,34,35 Based on this observation, modifications have been made to the Hamaker’s equation by incorporating the rate of change in the particle concentration of the solution with time or by considering the decrease in the velocity of the particles as a function of deposition time in an attempt to capture the non-linearity in the rate of deposition for non-conducting particles.35–38 Despite all these modifications, the actual process of deposition using electrophoresis is very complicated and still not well understood and most of the parameters cannot be quantified accurately.23,24 Therefore, in this study, the simple linear model developed by Hamaker31 has been used as a framework to investigate the interdependence of the processing parameters. EPD was carried out by varying the processing parameters as presented in Table 1. To study the interrelation between the field strength and the deposition time, each of these parameters is reduced individually keeping the CNT concentration constant. For example, in EPD-10.5-15m, only the field strength is reduced by 30% from EPD-15-15m while keeping the rest of the parameters constant. Then for EPD-15-10.5m, only the deposition time is reduced by 30% from 15 min. Similarly, for the last two cases, each of these parameters is reduced by 50% from EPD15-15m. 7 ACS Paragon Plus Environment


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Table 1 Notation

EPD-1-15-15m EPD-15-15m EPD-10.5-15m EPD-15-10.5m EPD-7.5-15m EPD-15-7.5m

2.3.

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Processing parameters in EPD

CNT Concentration (g/L) 1

0.5

Field Strength (V/cm) 15 15 10.5 15 7.5 15

Deposition Time (min) 15 15 15 10.5 15 7.5

Characterization of CNT Coating

The surface morphology of the CNT coating was characterized using confocal microscopy and scanning electron microscopy (SEM). The thickness of CNT deposition was determined by etching the EPD fiber through-the-thickness using a focused ion beam (FIB) (Zeiss Auriga 60). The uniformity of the coating along the length was quantified by measuring the electrical resistance along the length of the EPD fiber (4 mm). To calculate the electrical resistance a sub femtoamp source meter (Keithley 6430) was used to source a constant voltage of 50 V and measure the resulting current. Both ends of the EPD fiber were attached to the electrodes using conductive silver paste.


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2.4.

Microdroplet Tests

Microdroplet specimens were prepared by laying a single fiber on a tray and depositing a droplet of resin onto the glass fiber using a carbon fiber applicator, which has a single carbon fiber attached to the end of a small diameter rod in order to form a monofilament carbon brush. Carbon fiber was selected for the deposition of droplets due to its high stiffness and small diameter. Different combinations of virgin S-glass fibers and EPD glass fibers with neat epoxy and epoxy modified with CNTs were used for the preparation of the microdroplet specimens (Table 2). The three-roll mill method was adopted to homogenously disperse untreated CNTs (0.25% by wt.) in the epoxy resin. A three roll mill (EXAKT-80E; EXAKT Technologies) was used to mix the CNTs with epoxy by progressively decreasing the gap width.39 For the gap width of 50 µm, 20 µm and 10 µm, the mixture was passed through at least twice. For the gap setting of 5 µm, the mixture was passed through 20 times keeping the apron roll speed at 100 RPM. The specifics of this procedure and the resulting CNT dispersion are based on a previous study, where CNTs were dispersed in a similar epoxy resin using calendaring approach.39 The CNT reinforced epoxy was then mixed with PACM-20 curing agent for the preparation of droplets. The microdroplet specimens were then allowed to gel at room temperature for five hours, followed by curing at 80°C and 150°C for two hours each. Cured specimens were placed on a specimen holder, which were attached to the load cell. Embedded length ( and fiber gage length (Figure 1) was measured under an optical microscope prior to testing, while the diameter of the fiber in the region where the droplet was sheared off was measured after the test. Details regarding the specimen preparation procedure and the test setup can be found elsewhere.40 At least 10 valid microdroplet tests for each type of treatment were used for further calculations. The nominal diameter of the S-2 glass fibers considered in this study was 10 µm. The embedded 9 ACS Paragon Plus Environment


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length of the droplet ranged from 80 to 150 µm. Gage lengths between 1 mm and 2 mm were maintained. During the microdroplet test, one end of the fiber was attached to the force sensor and the droplet was then sheared off using a set of knives attached to a motorized actuator (See Figure 1). A gap of 5 µm between the fiber and the knife edge on each side was maintained. A pan balance (Mettler Toledo PB303-S) with a capacity of 300 g and an accuracy of 0.001 g was used as a load cell. A motorized actuator (Newport VP-25AA) with a resolution of 0.1 µm was used to load the specimen at the rate of 1 µm/s. A typical load-displacement curve is shown in Figure 2. From the load-displacement response and the geometry of the specimen, the average interfacial shear strength (IFSS) was calculated using Eq. (3).41 =

(3)

Where, refers to IFSS, F is the tensile force in the fiber at complete debonding, is the diameter of the fiber and is the embedded length of the microdroplet. The total energy absorbed by the interface up to complete debonding and during frictional sliding can be calculated using Eq. (4) and (5), respectively (See Figure 2). The immediate portion of the curve after complete debonding where dynamic sliding occurs, just prior to frictional sliding in Figure 2 has been omitted from further data reduction. The specific energy absorbed by the interface up to complete debonding and during frictional sliding was calculated using Eq. (6) and (7), respectively. =

!"#$

!%

(4)


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=

!&'(

!)*%+,%

-

(5)

(6)

.

(7)

= =

Figure 1 Schematic representation of microdroplet test



0.30 Complete debonding 0.25 0.20

Force (N)

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0.15 Frictional Sliding

0.10 0.05

Edeb

Efs

0.00 0

50

100

150

200

250

300

350

Displacement (µ µm)

Figure 2 Typical Force displacement response of a microdroplet experiment (The grey portion in the plot has been omitted from further data reduction)

Table 2 Notation SG/EP SG-1-15-15m SG-15-15m SG-15-10.5m SG-10.5-15m SG-CNT/EP-CNT SG/EP-CNT

Notations for various microdroplet specimens Fiber Virgin 1 g/L CNT, 15 V/cm 15 min 0.5 g/L CNT, 15 V/cm, 15 min 0.5 g/L CNT, 15 V/cm 10.5 min 0.5 g/L CNT, 10.5 V/cm, 15 min 1 g/L CNT, 10V, 15 min Virgin

Resin

Unmodified

Epoxy + 0.25% CNT Epoxy + 0.25% CNT

3. Results and Discussion


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3.1.

CNT coating and the kinetics of EPD

The confocal images EPD-15-7.5m, EPD-15-10.5m and EPD-15-15m (Figure 3a, c, e) show the process of CNT film development from 7.5 min to 15 min, with a 0.5 g/L CNT dispersion subjected to a field strength of 15 V/cm. The EPD-10.5-15m and EPD-15-10.5m, EPD-15-15m (Figure 3c, d, e) images showed randomly oriented CNTs with uniform thickness around the fiber. When either the electric field or the deposition time was decreased by 50% from EPD-1515m, the CNT deposition was very thin but non-uniform, showing early stage of the film development (Figure 3a) or agglomerates of CNTs (Figure 3b) attached onto the glass fiber surface. From visual inspection, EPD-15-10.5m produced the thinnest uniform CNT coating. Figure 4 shows SEM images of the EPD fibers, which have been cross-sectioned using FIB, showing the carbon nanotube film thickness and morphology. A very uniform PEI-CNT coating on the surface of the glass fiber was observed (Figure 4a). A PEI-CNT film thickness of about 200 nm was measured for EPD-15-7.5m. When the deposition time was increased from 7.5 min to 10.5 min for EPD-15-10.5m, the CNT thickness doubled. With the deposition time doubled to 15 min for EPD-15-15m, the thickness of CNT layer tripled to around 600 nm. Figure 5 shows the electrical resistance measured along the length of the EPD fibers for different cases. As expected, the electrical resistance increased as the deposition time or electric field strength was decreased, which indicates a lower quantity of CNTs. EPD fibers for case EPD-1510.5m (0.5 g/L CNT, 15 V/cm and 10.5 min) exhibited the most homogenous PEI-CNT coating (Figure 5b), which is consistent with the visual inspection for the same treatments (Figure 3c). To obtain a thinner coating, decreasing the deposition time instead of field strength leads to better film uniformity. It should, however, be noted that the variability in the electrical resistance measurement does not necessarily reflect the overall quality of the coating along the length, since 13 ACS Paragon Plus Environment


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even the smallest of local discontinuity at any point within the length of the fiber can significantly affect the overall resistance measurement. The interrelation between the two parameters (deposition time and field strength) is shown in Figure 6. The average electrical resistance has been normalized with respect to the resistance of the EPD fibers for EPD-15-15m (0.5 g/L CNT, 15 V/cm and 15 min). The abscissa represents the factor by which the parameters were reduced from EPD-15-15m. Decreasing either the deposition time or the field strength by a factor of 0.7 (for example, reducing the deposition time from 15 min to 10.5 min) had the same effect on electrical resistance measurement. During EPD, the aqueous solution near the cathode is alkaline due to electrolysis. As the positively charged CNTs approach the cathode it is hypothesized that the PEI deprotonates as a result of increase in the pH of the solution in this region.42,43 At this moment, the PEI-CNTs would lose their repulsive charge and the dispersion would destabilize, leading to precipitation onto the glass fiber.16,44–46 At a lower voltage, the electrolytic reactions at the cathode are slower, so initiation of film formation due to precipitation of the destabilized dispersion will occur at a slower rate. The movement of the particles is then governed by the Brownian motion44 and the electric field has minimal effect on their motion, which results in the formation of agglomerates before the CNT film formation on the substrate can occur (Figure 3b). CNTs adhere to the substrate due to chemical link between PEI functionalization of CNT and the silane on the fiber surface.7,16 Comparably, at higher voltage, the deposition rates are faster and such agglomerates are less likely to occur. The sites where initial deposition occur are more favorable for film growth as they become conductive pathways and develop a potential where CNTs will preferentially be absorbed. This can be seen in the early stage of the deposition process for EPD15-7.5m (Figure 3a), which eventually forms a thin yet very homogeneous CNT film at a longer 14 ACS Paragon Plus Environment

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deposition time of 10.5 min (Figure 3c). The lesser degree of uniformity of the coating observed from electrical resistance measurement when the electric field strength during EPD was lowered (Figure 5a) compared to its counterpart (Figure 5b) can be attributed to the formation of agglomerates during the initial stage of the deposition process.

(a)

(b)

(c)

(d)



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(e) Figure 3 Confocal images of S-glass fibers after electrophoretic deposition of CNT (Scale bars are 10 µm in size) (a) EPD-15-7.5m, (b) EPD-7.5-15m, (c) EPD-1510.5m, (d) EPD-10.5-15m, (e) EPD-15-15m

(a)

(b)


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(c)

(d)

Figure 4 FIB images of EPD fiber (a) CNT packing of EPD-15-15m, (b) EPD15-7.5m with CNT thickness ~ 200 nm, (c) EPD-15-10.5m with CNT thickness ~ 400 nm, (d) EPD-15-15m with CNT thickness ~ 600 nm

1.0E12

1.0E12

CNT: 0.5 g/L Deposition time: 15 mins

1.0E11

1.0E11

1.0E10

1.0E10

Resistance (Ω Ω)

Ω) Resistance (Ω

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1.0E9 1.0E8

1.0E9 1.0E8 1.0E7

1.0E7 1.0E6 5.0

CNT: 0.5% by wt. Voltage across electrodes: 10 V

7.5

10.0

12.5

15.0

1.0E6 5.0

7.5

Field Strength (V/cm)

(a)

10.0

12.5

15.0

Deposition Time (min)

(b)

Figure 5 Electrical resistance measurement of EPD fibers – CNT concentration 0.5 g/L (a) At different field strength with constant deposition time of 15 mins, (b) At different deposition times with constant field strength of 15 V/cm



1000

Normalized Resistance

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Electrode Voltage Deposition Time

100

10

1 0.5

0.6

0.7

0.8

0.9

1.0

Ratio of Parameters

Figure 6 Interdependence of processing parameters – Electrode voltage and deposition time in EPD

3.2.

Influence of PEI-CNT coating on IFSS

The response of IFSS to the different EPD coating parameters and modification of the resin matrix with CNTs is shown in Figure 7. The EPD treatment which produced a 2 µm thick PEICNT coating (SG-1-15-15m) on the fiber surface led to a 58% increase in IFSS (SG-1-15-15m) (Table 3). The IFSS decreased in proportion to the PEI-CNT film thickness from 75.5 MPa for the 2 µm thick PEI-CNT coating (SG-1-15-15m) to 57.7 MPa for 600 nm thick PEI-CNT coating (SG-15-15m). The IFSS for the 400 nm thick PEI-CNT coating (SG-15-10.5m) and unmodified epoxy was similar to the untreated baseline sample (SG/EP). No significant changes in the specific energy absorbed up to debonding were detected due to the application of PEI-CNT coating onto the fiber surface. During the microdroplet experiment, the crack initiates in the interfacial region where the knife edges come into contact with the resin droplet due to the localized stress concentration (Figure


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8). The crack typically propagates near the matrix-fiber interface in the direction of the applied tensile load before complete debonding occurs. The thickness of the PEI-CNT coating can play a major role in arresting the crack propagation. The PEI-CNT film can help to provide a modulus gradient between the stiff fiber and low modulus matrix, leading to a reduction in localized stress concentrations. Simulation work by Romanov et al.47,48 demonstrated a shift in stress concentration from fiber matrix interface to resin rich regions due to the presence of CNTs in the interface. In the previous studies examining PEI-CNT treated glass,16 it was shown that higher shear strength resulted from a shift away from the matrix glass interface and towards the PEICNT rich interphase region. The overall trends are consistent with the previously-reported increases in shear strength.16 The shift in locus of fracture could explain the 58% increase in IFSS for SG-1-15-15m with the 2 µm thick PEI-CNT treatment compared to the baseline. The resistance to crack propagation decreases as the PEI-CNT film thickness decreases and the IFSS for the PEI-CNT treatments approaches the baseline (SG/EP) strength when the PEI-CNT film thickness is about 400 nm (SG-15-10.5m specimen). Figure 9a and c show high magnification SEM images in the region near the initial debond on the fiber surface for samples with the thick PEI-CNT coatings (SG-1-15-15m and SG-CNT/EPCNT). The images show areas where CNT pull-out has occurred throughout the coating thickness, which may indicate poor resin wetting and diffusion to the PEI-CNT coating and glass fiber interface. The incidence of CNT pull-out appears to be higher for the thick PEI-CNT film and CNT-modified matrix (SG-CNT/EP-CNT). It may be expected that the addition of 0.25 wt.% CNTs would increase the viscosity of the resin and further reduce the ability to penetrate the PEI-CNT coating, particularly given the drops are formed at room temperature. It should be noted that in previous studies the use of a very low viscosity resin system cured at 130°C led to 19 ACS Paragon Plus Environment


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very good wetting of the PEI-CNT treated fabrics and CNT pull-out was not typically observed.7 The failure modes for the thicker CNT coatings (EPD-1-15-15m), therefore, may indicate that the IFSS results are affected to some extent by the level of resin wetting. However, given there is a significant increase in strength for the thick PEI-CNT film in which the resin was not modified (EPD-1-15-15m), compared to the same film with the CNT-modified resin, there clearly is some resin penetration into the coating. The fracture surface for the microdroplet sample with the thinner PEI-CNT film (SG-15-10.5m) shows good CNT wetting by the epoxy resin (Figure 9b), which supports the previous conclusion, that some resin wetting into the thicker films will have occurred, but penetration to the glass interface would be reduced as the PEI-CNT film thickness increases. The lower incidence of CNT pull-out is more similar to the previous composite samples which used the resin system with lower viscosity. The glass fiber failure surface exhibited a thin resin layer with a rough surface texture when the fibers were treated with the PEI-CNT coating in comparison to the relatively smooth surface for the baseline samples (SG/EP) (Figure 10). The presence of CNTs within the thin resin layer indicate that fracture did not proceed at the glass-coating interface, suggesting a strong chemical bond between the sized glass fiber and the PEI-functionalized CNTs had formed. Amine functional groups in the PEI could potentially react with the epoxide group of silane coupling agent and the epoxy resin, yielding a ring opening reaction, which is then followed by crosslinking reaction with other epoxide groups.49 Similar reactions occur between amine groups of curing agent and PEI with epoxide group of resin. This strong covalent bonds formed between the fiber-PEI-CNT and CNT-PEI-matrix interfaces is a critical component of the load transfer process at the fiber/matrix interface. 20 ACS Paragon Plus Environment

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The addition of untreated CNTs to the epoxy resin used to form the microdroplet led to different levels of improvement in the IFSS depending on whether or not the glass fiber was pretreated with the PEI-CNT film. The sample with the PEI-CNT film and CNT-modified resin (SGCNT/EP-CNT) only showed a 10% improvement above the baseline strength, compared to the sample in which only the CNT-modified resin was used (SG/EP-CNT), a 35% increase in IFSS was measured. As indicated above, the higher viscosity associated with the CNT-modified resin is expected to have compromised the ability of the resin to penetrate the PEI-CNT coating and bond to the glass fiber. The improvement by the CNT-modified resin sample without the PEICNT fiber coating suggests that if the resin viscosity was lowered and the CNTs used to modify the epoxy were also functionalized then even higher IFSS may be possible. In recent work using plasma treatment of CNT-modified quartz fibers and toughened epoxy, increases up to 80% of the in-plane shear strength were measured when good resin wetting was achieved and CNTs occupied the interstitial fiber regions.50 The significant improvements in the IFSS and specific energy up to debond observed for the SG/EP-CNT sample compared to the baseline may be attributed to the improved toughness of the interface resulting from CNT pullout due to the limited bonding between the matrix and untreated CNTs and the increased shear area provided by the CNT which arrests the crack growth. The fracture surface shows a very rough surface with traces of epoxy and CNTs suggesting a cohesive failure (Figure 10).



80

IFSS (MPa)

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60

40

20

0 SG/EP

SG-1-15-15m

SG-15-15m

SG-15-10.5m

SG-CNT/EP-CNT

SG/EP-CNT

Figure 7 Interfacial shear strength of different EPD fiber and epoxy system

Table 3

Interfacial shear strength and energy absorption of various EPD fiber and epoxy system

SG/EP SG-1-15-15m SG-15-15m SG-15-10.5m SG-CNT/EP-CNT SG/EP-CNT

IFSS (MPa)

Increase in IFSS (%)

47.8 (8) 75.5 (16) 57.7 (12) 48.0 (15) 52.5 (14) 64.4 (11)

̶ 58 21 0.4 10 35

Specific Energy: Debonding (J/m2) 760 (45) 770 (44) 730 (52) 700 (30) 770 (51) 1,100 (25)

Specific Energy: Sliding (kJ/m3) 9,300 (32) 15,000 (40) 18,000 (49) 14,000 (40) 12,000 (37) 5,200 (44)

Note: Values in parentheses represent coefficient of variation (%).


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(a)

(b)

Figure 8 Typical failure modes (a) SG-1-15-15m, (b) SG-15-10.5m

(a)

(b)

(c)



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Figure 9 Wettability between CNT and epoxy (a) SG-1-15-15m, (b) SG-15-10.5m, (c) SG-CNT/EP-CNT.

(a)

(b)

(c)

(d)

Figure 10 Fracture surface morphology of microdroplet specimens: (a) SG/EP, (b) SG-15-15m, (c) SG-15-10.5m, (d) SG/EP-CNT.

4. Conclusions


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This study demonstrated the ability of electrophoretic deposition method to control the thickness of PEI-CNT coatings down to submicron length scale. Hamaker’s equation was used to adjust the processing parameters to control coating thickness. The PEI-CNT coatings were found to have a very uniform thickness around the fiber circumference. Decreasing the deposition time proved to be the most efficient way to yield thinner and more homogenous PEI-CNT coatings, as opposed to decreasing the field strength. The effect on the interfacial properties resulting from the application of the PEI-CNT coatings onto the fiber and incorporation of unmodified CNTs into the resin matrix was studied using the microdroplet. Increasing the thickness of PEI-CNT coating resulted in increased IFSS. The combination of glass fiber with the 2 µm thick PEI-CNT coating and unmodified epoxy resin showed the highest improvement in IFSS with a 58% increase. The increase in IFSS for the microdroplet specimens with EPD treated PEI-CNT fibers was attributed to the resistance to the crack propagation around the fiber prior to complete debonding and the increase in shear area due to the presence of CNTs. The higher viscosity of the resin required to form the microdroplets at room temperature compromised the full extent of improvement that may be offered by the PEI-CNT coatings by reducing the level of resin wetting through the film, particularly as the film thickness increased. The EPD treated fibers exhibited good electrical conductivity, which has the potential for the CNT-modified fibers to be used as distributed sensors in the study of damage initiation and evolution at the fiber-matrix interface.



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Acknowledgements Sandeep Tamrakar, Bazle Z. Haque and John W. Gillespie, Jr. would like to acknowledge research sponsorship by the Army Research Laboratory, which was accomplished under Cooperative Agreement Number W911NF-12-2-0022. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein. Qi An and Erik T. Thostenson gratefully acknowledge funding from the US National Science Foundation (Grant #: 1254540 – Dr. Mary Toney, Program Director).

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338x190mm (96 x 96 DPI)

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Polymer Coating of Carbon Nanotube Fibers for Electric Microcables.

Controlling carbon-nanotube-phospholipid solubility by curvature-dependent self-assembly.

Effect of carbon nanotube functionalization on mechanical and thermal properties of cross-linked epoxy-carbon nanotube nanocomposites: role of strengthening the interfacial interactions.

Controlling the physicochemical state of carbon on graphene using focused electron-beam-induced deposition.

Electrophoretic Deposition of Carbon Nanotubes over TiO2 Nanotubes: Evaluation of Surface Properties and Biocompatibility.

Using in-situ polymerization of conductive polymers to enhance the electrical properties of solution-processed carbon nanotube films and fibers.

Al composite coating on NiTi.

Improvement of Interaction in a Composite Structure by Using a Sol-Gel Functional Coating on Carbon Fibers.

halloysite nanotube coatings prepared by electrophoretic deposition on titanium substrate.

Molecular tailoring of interfacial failure.

Morphology dependent field emission of acid-spun carbon nanotube fibers.

carbon nanotube nano composite fibers--a review.

Controlling water flow inside carbon nanotube with lipid membranes.

polymer composites.

Insights into carbon nanotube nucleation: cap formation governed by catalyst interfacial step flow.

Macroscopic single-walled-carbon-nanotube fiber self-assembled by dip-coating method.

Effects of Al interlayer coating and thermal treatment on electron emission characteristics of carbon nanotubes deposited by electrophoretic method.

Electrophoretic Deposition of Carbon Nanotubes on 3-Amino-Propyl-Triethoxysilane (APTES) Surface Functionalized Silicon Substrates.

Interfacial and biological properties of the gradient coating on polyamide substrate for bone substitute.

Stretchable Wire-Shaped Asymmetric Supercapacitors Based on Pristine and MnO2 Coated Carbon Nanotube Fibers.

Quantum Chemical Simulation of Carbon Nanotube Nucleation on Al2O3 Catalysts via CH4 Chemical Vapor Deposition.

Nanosoldering carbon nanotube junctions by local chemical vapor deposition for improved device performance.

Impact of the atomic layer deposition precursors diffusion on solid-state carbon nanotube based supercapacitors performances.

Interfacial free energy controlling glass-forming ability of Cu-Zr alloys.