Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode

Journal Pre-proof Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode Jae-Woo Kim, Godfrey Sauti, Benjamin D. Jensen, Joseph G. Smith, Kristopher E. Wise, Russell A. Wincheski, Roberto J. Cano, Emilie J. Siochi PII:

S0008-6223(20)31134-9

DOI:

https://doi.org/10.1016/j.carbon.2020.11.055

Reference:

CARBON 15868

To appear in:

Carbon

Received Date: 14 September 2020 Revised Date:

16 November 2020

Accepted Date: 17 November 2020

Please cite this article as: J.-W. Kim, G. Sauti, B.D. Jensen, J.G. Smith, K.E. Wise, R.A. Wincheski, R.J. Cano, E.J. Siochi, Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode, Carbon, https://doi.org/10.1016/j.carbon.2020.11.055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Published by Elsevier Ltd.

Credit Author Statement JWK: Conducted experiments, analyzed data, wrote manuscript. GS: Conducted experiments, analyzed data, contributed to writing manuscript. BDJ: Analyzed data, contributed to writing manuscript. JGS: Conducted experiments, analyzed data, contributed to writing manuscript.

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KEW: Analyzed data, contributed to writing manuscript.

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RAW: Conducted experiments, analyzed data, contributed to writing manuscript.

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RJC: Analyzed data, contributed to writing manuscript.

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EJS: Conceived of project that led to this study, analyzed data, contributed to writing manuscript.

Graphical Abstract The hierarchical microstructure of carbon nanotube fibers necessitates consideration of matrix/fiber interfaces within the reinforcement to improve interfacial shear strength and prevent failure within the reinforcement.

Jae-Woo Kim, Godfrey Sauti, Benjamin D. Jensen, Joseph G. Smith, Kristopher E. Wise, Russell A. Wincheski, Roberto J. Cano, Emilie J. Siochi

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Modifying Carbon Nanotube Fibers: A Study Relating Apparent Interfacial Shear Strength and Failure Mode

Shear crack

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Shear crack

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Dry core

Well wettedout core Tab matrix

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Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode

National Institute of Aerospace, Hampton, VA 23666, USA, 2Advanced Materials and

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Russell A. Wincheski,3 Roberto J. Cano,2 Emilie J. Siochi2*

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Jae-Woo Kim,1* Godfrey Sauti,2 Benjamin D. Jensen,2 Joseph G. Smith,2 Kristopher E. Wise,2

Nondestructive Evaluation Sciences Branch, NASA Langley Research Center, Hampton, VA

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Processing Branch, NASA Langley Research Center, Hampton, VA 23681, USA,

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ABSTRACT

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23681, USA

Achieving high strength in fiber reinforced structural composites requires effective load transfer between the high-performance fiber reinforcement, e.g., carbon nanotube (CNT) yarn, and the matrix. Various processing approaches to enhance the interaction between CNT fiber and the matrix were investigated. The apparent interfacial shear strengths (IFSS) of pristine CNT yarns, post-treated CNT yarns (cross-linked, functionalized, and polymer incorporation by resistive heating), and pre-infiltrated polymer/CNT composite fibers measured using single fiber pull-out tests were used to screen the efficacy of the processing methods. Pristine CNT yarns had

*Corresponding Authors: [email protected] (Emilie J. Siochi); [email protected] (Jae-Woo Kim)

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a low apparent IFSS (< 5 MPa) due to shear failure within their dry cores. In post-treated CNT yarns which did not exhibit good IFSS, the failure surface consisted of a resin-infiltrated sheath near the surface of the yarn and a dry section within the yarn core; failure occurred in the dry core. This failure mode is unlike those observed in traditional carbon fiber reinforced composites which fail at the fiber/matrix interface. In contrast to the sword-in-sheath failure modes of posttreated CNT yarns, pre-infiltrated polymer/CNT composite fibers displayed high apparent IFSS

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(> 20 MPa). Improved wet-out of the fiber eliminated the dry-core shear failure mode.

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KEYWORDS. Carbon nanotube, interfacial shear strength, single fiber pull-out test, composite

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fiber

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1. INTRODUCTION

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Individual carbon nanotubes (CNTs) with excellent electrical [1,2], thermal [3,4], and

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mechanical [5-8] properties have recently become available in large quantities of formats such as

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yarns, tapes, and sheets. These materials are manufactured by methods including wet or dry spinning and continuous aerogel spinning in a floating catalyst chemical vapor deposition furnace [9-13]. In contrast to CNT powder which were available earlier in the evolution of CNT materials, these new structural forms of CNTs are more suitable for use as reinforcements in lightweight structural applications [14-18]. While substantial effort has been put into developing and characterizing these large format CNT materials, it is also crucial to understand their behavior when combined with a polymer matrix to form a composite. In uniaxial tension-dominated applications, such as composite overwrapped pressure vessels, the fiber dominates the mechanical response and fiber/polymer interfacial properties are less critical [16]. For broader applications in which compression, shear,

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and impact loadings are important, the fiber/matrix interaction plays a crucial role. For example, Windle et al. reported specific tensile strength of 230 MPa/(g/cm3) with 27 fiber vol.% and specific compression strength of 118 MPa/(g/cm3) with 14 vol.% of unidirectionally aligned CNT fibers in an epoxy matrix [14]. Kim et al. fabricated unidirectional CNT yarn/polymer composites using various resin infusion processes and reported tensile and short beam shear strength [17]. The composites tested in that work, which were composed of ~80 wt.% CNT in an

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epoxy matrix, retained 60 -77 % of specific tensile strength of the reinforcing yarns [~ 1

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GPa/(g/cm3) composites from 1.30 to 1.68 N/tex yarns], as expected for a property dominated by

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the reinforcing fibers. In contrast, the short beam shear strength, which is very dependent on

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fiber-matrix load transfer, was only 9.1 MPa due to weak fiber/matrix interactions, low resin

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uptake, and poor wetting. This is over an order of magnitude lower than the typical value

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measured for unidirectional IM7 carbon fiber (12k tow)/8552 composite (~ 137 MPa with ~ 67 wt.% fiber) [19]. Improving the fabrication of high strength polymer/CNT composites to yield

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materials with a high interfacial shear strength (IFSS), in addition to high tensile properties, is

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therefore crucial to the development of light-weight composite materials that can potentially minimize the mass of future spacecraft and launch vehicles. There are a variety of mechanical tests that can give insight into the interfacial properties between the fiber reinforcement and the matrix for specimens of different sizes. Coupon-scale tests include short beam shear, flexure, and double cantilever beam. For even smaller specimens, single fiber tests are available to measure interfacial properties against a polymer matrix. Single fiber tests require less material and have shorter fabrication times than coupon-scale tests, making them appropriate for screening efforts, while the coupon-scale tests can provide a more reliable, quantitative assessment of fiber/matrix interfacial properties on a larger composite scale.

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Single fiber tests were used in this work because of the small amount of CNT yarn required and an interest in screening a large number of material processing methods. Examples of single fiber tests include micro-indentation [20], single fiber fragmentation [21], microdroplet [22], and single fiber pull-out [23,24]. A few models to aid in the interpretation of single fiber tests have been summarized in the literature [25-27] and the failure modes observed during the tests can be

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instructive in understanding the improvements needed in the materials.

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Each single fiber test method has strengths and weaknesses which depend on the material system being tested. CNT fibers (or yarns) have several unique characteristics which should be

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considered when selecting a test method, such as high strain at failure, low sensitivity to

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handling and bending damage, and large diameters. In addition, CNT fibers often experience

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limited polymer infiltration, leading to a mixed wet/dry structure with distinct sheath/core

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properties. Zu et al. [22] fabricated CNT fibers with diameters of ~ 10 µm from CNT forests and

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performed microdroplet tests to characterize the fiber/matrix interactions. Sword-in-sheath failure within the fiber was observed and the average apparent IFSS between the hydroxyl group

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functionalized CNT fiber and the epoxy resin was 14.4 MPa. This IFSS value is somewhat comparable to, but lower than, the values from untreated (18.4 – 28.1 MPa) [28,29] and chemically etched (36.3 MPa) [29] carbon fibers, respectively, measured using a similar test. It is worth noting that the length of fiber in contact with the matrix can influence the measured IFSS, with the largest contact lengths providing more consistent results [30]. Similarly, the microdroplet test may require larger bead sizes to obtain consistent results for CNT yarns with low interfacial strengths. Precisely controlling and quantifying the droplet size is challenging with CNT fibers because they can absorb small amounts of the resin into the surface and some

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resin flows along the CNT fiber during cure. This may cause the contact area to be underestimated resulting in an overestimation of the IFSS. In fiber fragmentation tests, the fiber is completely surrounded by matrix, which avoids the contact area ambiguities encountered in microdroplet testing. For example, carbon fiber (TORAYCA® T-300)/epoxy composites characterized using a fragmentation test showed an

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IFSS of 89.5 MPa, which is more than double the microdroplet test results because of the greater

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measurement sensitivity [31]. However, characterizing the interface with a fragmentation test requires a matrix with a higher failure strain than the fiber. This is a challenge because CNT

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fibers typically have a higher failure strain than engineering thermoset matrices, which have

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relatively small failure strains (less than 1.7 %) [19]. The computed IFSS is also sensitive to the

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average critical fragment length which can be challenging to measure. In addition, distinguishing

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sword-in-sheath and true interfacial failure modes with CNT fibers is not straightforward with a fragmentation test. Single fiber fragmentation testing of CNT fibers spun from CNT forests were

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performed by Deng et al. [21] and Liu et al. [31]. Deng et al. reported IFSS values of 10 – 20

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MPa for pristine fibers spun from CNT arrays. Liu et al. reported that IFSSs increased to 120 MPa with various pre-densification treatments using ethanol and polymer. The IFSS increased with increasing tensile strength and modulus of the CNT fiber and upon decreasing the fiber diameters from 10.5 to 5.5 µm. They also reported that, despite its lower strength and modulus, the IFSS of multi-walled CNT (MWCNT) fibers was surprisingly 12 - 15 MPa higher than a few-walled flattened CNT fiber because of the loosely packed arrangement and defective structures of the MWCNT fiber. Given the challenges of doing fiber fragmentation testing with CNT yarns, single fiber pull-out test (SFPT) was adopted for this work because of the ease and simplicity of specimen

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preparation and characterization. The test is a good match for the yarns evaluated in this work, which have relatively large diameters (> 100 µm), good handleability, and physical damage insensitivity. In addition, SFPT is a better method for materials with low IFSS that require large embedded lengths, as well as for multiple fiber bundles such as carbon fiber tow. For example, unsized carbon fiber (T-300) bundles with embedded lengths ranging from 5.0 to 10.8 mm were tested by a fiber pull-out method and reported IFSS values in a range between 18.8 and 25 MPa,

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depending on resin selection [32].

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In the remainder of the paper, three categories of CNT yarn are discussed: pristine yarns, posttreated yarns, and pre-infiltrated polymer/CNT composite fiber. The pristine yarns, which are

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composed of 1- or 2-plies, are highly aligned, relatively high density, high tenacity and are tested

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in their as-received condition. The post-treated yarns were prepared by exposing the pristine

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yarns to a variety of treatments intended to alter their surface properties in an effort to improve

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matrix interactions. Finally, the pre-infiltrated composite fibers were prepared by first infusing a polymer into the low density, low tenacity CNT roving, which is the as-produced CNT form

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collected from the furnace with minimal processing. After the polymer infusion step, the roving was mechanically stretched and densified to form a well-consolidated composite fiber. These pre-infiltrated composite fibers differ substantially from a post-treated yarn with the same polymer because the open pore structure of the roving permits more extensive resin infusion than is possible with the much denser as-received pristine form. The improved integration of the polymer into the fiber appears to enhance the load carrying capacity of these materials. In what follows, the shear load transfer capabilities and shear failure modes of these three categories of fibers are compared and contrasted using the apparent IFSS as a basis for comparison.

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2. EXPERIMENTAL 2.1. Materials and post-treated CNT yarns. Polymer matrices used for this work were EPONTM 828 and API-60 epoxies purchased from Hexion Inc. and Applied Poleramic Inc. (currently Kaneka Aerospace LLC), respectively. EPONTM 828, which is a two-part difunctional

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bisphenol A/epichlorohydrin derived liquid epoxy resin with a fully cured resin density of 1.16

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g/cm3, was mostly used as a tabbing material. EPIKURETM W was used as the curing agent with

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a 100 to 25 mix ratio between the EPONTM 828 resin and the EPIKURETM W. API-60, a

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toughened epoxy system, was used as an infiltration resin for composite fiber fabrication

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including both high density CNT yarn and CNT roving. It was also used as a tabbing material, in

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some limited experiments, for comparison purposes. Prior CNT and carbon fiber composite work suggests that changes in the surface functional

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groups of CNT yarns may be needed to improve adhesion to the polymer matrix for further

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improvement of mechanical properties of the composites, e.g., shear strength. In this work, chemical functionalization and cross-linking of the highly densified CNT yarns were conducted by procedures suggested from previous studies including the diazonium salt reaction [33-36] and m-chloroperoxybenzoic acid (m-CPBA) treatment [37]. Phenylenediamine (PDA) was used to cross-link CNTs in highly densified CNT yarns [36]. 1-ply (Nanocomp Technologies, Inc., 4.31 tex, 1.73 N/tex tenacity, and 102.29 N/tex specific modulus) or 2-ply (baseline study, 13.60 tex, 1.69 N/tex tenacity, and 93.32 N/tex specific modulus) yarns were used to prepare the posttreated CNT yarns. The CNT yarn was immersed in 60 wt.% sulfuric acid with m-PDA (or pPDA) and sodium nitrite (NaNO2) for 48 hrs to complete cross-linking of the CNT yarn, either at

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room or elevated temperature (70 °C). For the m-CPBA functionalization, the yarn was immersed into a m-CPBA acetonitrile solution (1 mg/mL) at 40 °C for a short period of time (10 seconds). The functionalized CNT yarn was sequentially cleaned with acetonitrile and acetone to remove excess m-CPBA and then dried under vacuum. For applying a polymer coating to the CNT yarn as a post-treatment, API-60 was diluted with

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methyl ethyl ketone (MEK, Sigma-Aldrich) to yield a 2 wt.% resin concentration to improve

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infiltration when applied to the high-density yarn. The diluted resin solution was applied directly

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onto the highly densified CNT yarns while under tension, followed by application of an electrical

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resistive heating (RH) [12,38,39].

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current through the yarn to evaporate the solvent and cure the resin at elevated temperatures by

2.2. Pre-infiltrated polymer/CNT composite fiber process. Fully infiltrated API-60/CNT

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composite fibers were fabricated by resistive heating assisted infiltration of the loosely

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networked structure of CNT roving (Nanocomp Technologies, Inc., 10.52 tex, 0.67 N/tex

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tenacity, and 7.91 N/tex specific modulus). To fabricate the pre-infiltrated polymer/CNT composite fibers, a custom-built continuous stretcher was used [40]. In contrast to the highly densified CNT yarn, CNT roving was easily wetted out with the diluted resin solution (2 wt.% of API-60 in MEK). The CNT roving consists of partially aligned, loosely bound CNT networks and is shaped like a narrow flat tape of about 2 mm in width and tens of microns in thickness, depending on manufacturing conditions. The custom-built continuous stretcher for the CNT roving stretching and resistive heating process consists of two stepper motors (ARCUS Technology, DMX-A2-DRV-23) for controlling the material supply and the stretching level, with two conductive aluminum rollers located between the stepper motors. A 4-axis motion controller equipped with USB input (ARCUS Technology, PMX-4EX-SA-TB9) was used to

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drive the motors. An Agilent E3634A benchtop power supply as well as Keithley 2100 and Hewlett Packard HP3478A digital multimeters were connected to two conductive rollers (14 cm distance) to apply and monitor the voltage across and the current flowing through the CNT roving during composite processing. In-house developed LabVIEW based software was used to control the continuous stretcher as well as the resistive heating process. A total of 50 cm of the CNT roving was loaded onto one of the TeflonTM spools mounted to the stepper motors and

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gradually stretched in 5 % increments while passing back and forth between the two motors until

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the desired level of stretching was achieved. The samples included two different stretching levels

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(72 and 82 %) with (denoted as a PI-72T) and without twist (PI-72 and PI-82). For the PI-72T

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sample, the single strand of tape-like CNT roving was mechanically twisted (Z-twist) to 2.12

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revolutions/cm in ambient condition. For consistency, the twist was applied using a custom-built

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motor-driven apparatus, before it was loaded into the stretcher. The diluted API-60 resin solution was applied at the beginning of the stretching process at a base voltage of 5 V (CNT roving

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temperature ~ 30 °C). During the stretching process the applied voltage was increased stepwise

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to 15 V (maintaining temperature below 100 °C), until the stretch level reached 20 %, to evaporate the MEK solvent (b.p.: 79.64 °C) and improve infiltration by reducing the viscosity of the applied resin. Once the stretch level reached 20 % the voltage was held constant at 15 V and stretching continued until the final desired stretch level was reached. At this point, both motors were stopped and the temperature increased to 150 °C by increasing the applied voltage to a range between 20 and 24 V, depending on the resistance of the stretched roving. The stretched roving was then partially cured for 10 min while being held in this stretched condition. The temperature during the composite fiber fabrication process was monitored using an infra-red camera (FLIR® TS650sc).

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For the tensile tests and SFPT, about 7 cm of material was taken from the middle of the processed composite fiber. Typical values for the measured diameter and linear density of the composite fibers with 72 % stretch (PI-72) were 144 ± 22 µm and 15.386 g/km, respectively. The average diameter along the fiber length of the as-prepared and pristine yarns was measured using a dual axis optical micrometer (Keyence, LS-9006D). Fiber diameter values were measured on two orthogonal axes simultaneously, gathering 50,000 data points on each axis with

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the average obtained over a 4 cm fiber length on both axes. The linear density was calculated by

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measuring the weight and length of each fiber.

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To aid in keeping track of the materials used in this work and the designation we used to

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report the results, this information is summarized in Table 1. Pristine CNT yarns

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2-ply structure

single ply pristine CNT yarn from manufacturer (Nanocomp Inc.) two ply pristine CNT yarn with a minimum twist to hold the

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1-ply

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from manufacturer (Nanocomp Inc.)

Post-treated CNT yarns m-CPBA acid

pristine 2 ply yarn functionalized with a m-chloroperoxybenzoic

(functionalization) m-PDA

pristine 2-ply yarn cross-linked with a m-phenylenediamine (crosslinking)

p-PDA

pristine 2-ply yarn cross-linked with a p-phenylenediamine (crosslinking)

RH

pristine 2-ply yarn treated with API-60 resin by resistive heating Infiltration (resin coating)

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Pre-infiltrated polymer/CNT composite fibers and starting material CNT roving

loosely networked, narrow CNT ribbon from manufacturer (Nanocomp Inc.) and starting material for pre-infiltrated polymer/CNT composite fibers fabrication

PI-72

pre-infiltrated API-60/CNT composite fiber with 72 % stretch

PI-82

pre-infiltrated API-60/CNT composite fiber with 82 % stretch

PI-72T

pre-infiltrated API-60/CNT composite fiber with 72 % stretch and

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mechanical twist (2.12 revolutions/cm)

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Table 1. List of key materials studied in this work.

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2.3. Single fiber pull-out test. While there no ASTM standard has been established for single

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fiber pull-out, this work follows a typical procedure [23, 24] with necessary modifications for

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these materials. Generally, a small piece of the as-prepared CNT yarn or pristine yarn (2 - 5 cm) was embedded to the desired length (from a few hundreds of microns to 14 mm) in a silicone

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mold (Ted Pella Inc., PELCO flat embedding molds). The specimen was mounted at the center

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of the tab by making a slot in the middle of the individual mold to ensure good fiber alignment and a repeatable measurement. Warm EPONTM 828 resin (50 - 60 °C) was then poured into the mold. The as-prepared specimen/EPONTM 828 loaded in the silicone molds was cured for 1 hr at 100 °C followed by a 2 hr hold at 177 °C to fabricate SFPT specimens. The two ends of the specimen were embedded into the EPONTM 828 tab to different lengths to ensure that the pullout of the specimen always occurred on the side with the shorter embedded length. The gap between the two tabs was about 1 cm. As prepared SFPT specimens were mounted on a grid paper tab to ensure transfer to a testing machine (Instron Model 5943 single column tabletop testing system) without any physical damage such as kinks. This also guaranteed that the

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specimen could be mounted at close to 0 degrees in the load direction of the mechanical wedge action grips. SFPTs were conducted under a quasi-static tensile loading at a crosshead speed of 1 mm/min. For each material type, the force-displacement curves were obtained for at least five specimens for the data analysis of apparent IFSS, de-bond force, and frictional stress. For the pre-infiltrated composite fiber tensile tests, the gauge length and crosshead speed were 10 mm (gap between two epoxy tabs) and 0.5 mm/min, respectively. The polymer/CNT composite fiber

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was embedded to lengths of 14 mm into both of the epoxy tabs to ensure the failure occurred in

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the gauge section. Specific tensile stress (tenacity) of the composite fiber was calculated by

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dividing the measured failure force (N) by the measured linear density (g/km) of each specimen,

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to eliminate errors associated with the measurement of the sample cross-sectional area. Young’s

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modulus was calculated from the slope between 10 and 30 % of the ultimate tenacity to eliminate

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the initial lag in stress-strain behavior [12]. At least five specimens were tested to determine tensile strength and modulus. Resin content of the stretched composite fibers was in the range of

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20 and 30 wt.%, determined by measuring linear density before and after composite processing. 2.4. Characterization. A field emission-scanning electron microscopy (FE-SEM, Hitachi Model S-5200) was used to image pristine, post-treated CNT yarns, and composite fibers before and after pull-out at an acceleration voltage of 30 keV using the secondary electron detector. Cross-sectional SEM samples were prepared by a cooling cross sectional polisher (Jeol IB19520CCP) using a 5 keV acceleration voltage under an argon plasma at -30 °C to prevent amorphous carbon development on the polished surfaces. The embedded length of each sample was measured with an optical microscope (Leica) after the pull-out test. Raman spectroscopic characterization was conducted with a Horiba XploRA PLUS Raman microscope system using a 30 mW diode laser with 638 nm excitation wavelength. The laser power was reduced by 50 %

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using a neutral density filter. The other parameters for the Raman spectra collection applied on the samples are: 10 acquisitions with 5 s of exposure time for each acquisition, 100 µm slit, 300 µm hole, 10x objective lens (0.25 numerical aperture), 1800 gr/mm grading, and laser power density of 1.97 × 109 W/m2. These parameters were used to prevent sample damage during the measurement [41]. Raman data were obtained at the middle of the sample assuming the sample is

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relatively homogeneous along the axial direction. Polarized Raman spectra were obtained in the

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VV, HH, and VH configurations with a 100x magnification objective lens. 2D and 3D order

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and

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, respectively.

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parameters (S) were calculated with the following equations [42]:

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where, IVV, IVH, and IHH are G peak intensity at parallel/parallel, parallel/vertical, and

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vertical/vertical configuration, respectively.

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3. RESULTS AND DISCUSSION

3.1. Apparent IFSS of CNT yarns. Figure 1 shows representative force-displacement curves of pristine, post-treated CNT yarns, and pre-infiltrated polymer/CNT composite fibers. In the early stage of the single fiber pull-out curve, the measured force increases linearly when the CNT yarn-matrix interface remains intact, which is from the elastic behavior of the material. When the measured force reaches a critical value, here denoted by a de-bond force (Fd) in Figure 1b, the interfacial crack initiates at some point at the interface, close to the CNT yarn entry into the epoxy matrix. As shown in Figure 1b, the de-bond force varies from specimen to specimen, depending on the interfacial interactions between the embedded yarn and the matrix. After

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initiating the interfacial crack, the crack continuously propagates along the embedded length toward the opposite end of the specimen. The de-bond regions are associated with a materialdependent frictional force that continuously increases as the crack extends until the maximum pull-out force is reached with complete de-bonding of the fiber. During this step, the measured force oscillates around a steadily increasing value, which may be evidence of inhomogeneous interactions between the local CNT bundle and the matrix. After full de-bonding of the

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embedded CNT yarn, the measured force drops from the maximum force to a substantially lower

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frictional force value, denoted as Fb in Figure 1b. This frictional force is attributed to frictional

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interaction between the CNT yarn and matrix and generally differs from specimen to specimen,

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and even within the same material, due to variations in shape and diameter along the embedded

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CNT yarn. It is worth noting that a de-bond region was not observed in the force-displacement

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curves for the majority of the PI-82 fiber specimens (Figure 1b). Pull-out of the embedded fiber

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was indistinguishable from the initiation of an interfacial crack.

Figure 1. (a) Typical pull-out force-displacement curves of the 1-ply and 2-ply pristine CNT yarns, post-treated CNT yarns cross-linked by m-PDA at room temperature [m-PDA @ R.T. (2-

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ply)] and 70 °C [m-PDA @ 70 °C (2-ply)]. (b) Detailed zoom at low displacement in (a) including pre-infiltrated polymer/CNT composite fibers (PI-72 and PI-82). Here, Fb and Fb are de-bond force and frictional force, respectively. A number of methods for calculating IFSS from pull-out test results, differing in complexity and the underlying assumptions made in their development, have been reported in the literature

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[26,27]. Because the CNT yarns used in this work are more variable in their shape and are less

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brittle than the fibers that many of the more complex approaches were intended to approximate,

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the simplest and most commonly used mean-stress approach was used in this work to compute the IFSS. For the mean-stress approach, the apparent IFSS can be calculated as IFSS =

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Fmax/(πdaL), which depends only on the maximum tensile force measured during pull-out (Fmax)

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and the yarn/matrix average contact area (πdaL), where da is the average diameter and L is the

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embedded length of the yarn. It has been pointed out in the literature that apparent IFSSs should

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not be interpreted quantitatively, but rather used as semi-quantitative metric for assessing bond

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strength as ‘good’ or ‘poor’, and for determining whether various surface modifications improve or worsen the interfacial bonding [25]. So, while the apparent IFSS should not be viewed as a high-precision quantity, it can still be useful in materials development efforts as a semiquantitative tool for determining whether or not changes to a yarn/matrix system have influenced the strength of the interface. All results reported below are apparent IFSS, although it is important to note that, due to the sword-in-sheath failure mode, shear failure was often not observed at the interface between fiber and matrix. The average diameter used to compute the IFSS was measured before each test. As detailed in Section 3.2. below, the diameter after testing was reduced for some sample types

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causing the reported apparent IFSS to be somewhat underestimated. For example, in the case of the 2-ply pristine CNT yarn, the apparent IFSS was underestimated by about 11 % when the effect of diameter reduction is accounted for. Visible black residue was transferred from the embedded yarn to the tab after the pull-out test. The amount of residual CNTs in the matrix differs from specimen to specimen. In this study, the surface roughness, irregular shape and diameter variations throughout the yarn length are not considered. Higher surface contact area

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can increase the apparent IFSS.

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Apparent IFSSs of the pristine, post-treated yarns, and pre-infiltrated fibers are plotted as a function of the embedded length in Figure 2. The 2-ply pristine CNT yarn was embedded into

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the EPONTM 828 matrix to lengths between 2 and 14 mm and used as a reference material for

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evaluation of the modified CNT yarns (Figure 2c). The apparent IFSS decreases exponentially as

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the embedded length increases, in good agreement with the energy-based model [25]. The

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apparent IFSS of the 2-ply pristine CNT yarn converges to a constant value around 2 MPa at

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large embedded lengths (e.g., 2.3 ± 0.3 MPa at 10 mm embedded length).

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Figure 2. Apparent IFSS of (a) all, (b) pre-infiltrated polymer/CNT composite fibers, and (c) pristine and post-treated CNT yarns in terms of the embedded length. All post-treated CNT yarns were made with 2-ply CNT yarns. Note that the lines are only a guide to the eye and not intended to denote curve fits.

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Compared to the measured apparent IFSSs of the 2-ply CNT yarn, 1-ply, and post-treated

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yarns showed slightly higher apparent IFSSs. Figure 2c shows the apparent IFSS of 1-ply

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pristine, 1- and 2-ply m-CPBA functionalized yarns, m-PDA (and p-PDA) cross-linked 1-ply and

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2-ply yarns, and resistive heating assisted API-60 treated 2-ply CNT yarns. Changing the matrix

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from EPONTM 828 to API-60 did not improve the apparent IFSS. Although it is possible that the post-treatments improved the interfacial strength between the fiber and the matrix, the pristine

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and post-treated CNT yarns both failed within the CNT yarn and not at the yarn/matrix interface,

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which can explain the lack of improvement in apparent IFSS between the pristine and post-

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treated CNT yarns. Unless this kind of shear failure within the CNT yarn is prevented, typical fiber interfacial strengthening approaches like increasing the compatibility of the matrix resin or chemical modification of the CNT surfaces are not expected to fundamentally improve the apparent IFSS. The apparent IFSSs of the 2-ply CNT yarns with different tenacities (1.19, 1.42, and 1.69 N/tex) were measured to explore any relationship between the IFSS and yarn tenacity (not shown). No clear relationship between the IFSS and yarn tenacity was observed although it is anticipated that higher strength yarns will be denser and even more difficult to penetrate with post-treatments. Higher tenacity yarns may also have higher CNT alignment in the axial

17

direction causing the surface of the yarns to be smoother, lowering the interfacial strength, although the surfaces of the pristine and post-treated yarns do not appear to be driving yarn/matrix shear properties. The pre-infiltrated polymer/CNT composite fibers, which have full resin infiltration followed by densification and alignment of CNTs through mechanical stretching of the CNT roving,

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generally showed high apparent IFSS, in the range between 20 and 60 MPa depending on the

ro

embedded length of the composite fiber (1100 - 500 µm) into the epoxy matrix, as shown in

-p

Figure 2b. These values are comparable with those from various types of carbon fibers, which

re

range between 16.7 and 55.5 MPa with and without surface treatment [24]. As will be discussed

lP

later, the increased apparent IFSS can be attributed to eliminating the dry fiber core, which also moves the failure site to the surface of the fiber. Generally, two factors are involved in improving

na

the IFSS of fiber reinforcement in a polymer matrix; 1) increasing interfacial interactions

ur

between the fiber and the matrix by chemical functionalization and 2) increasing contact area by

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increasing surface roughness. Unlike the polymer/spun CNT composite fibers [31] which have a more regular surface shape, the PI-72 fiber showed relatively irregular cross-sectional shapes as shown in Figures 3a and b, which can improve the apparent IFSS by increasing the contact area. The PI-72 fiber showed very rough surface morphology with wrinkles along the fiber axis (Figure 3a) while the PI-72T fiber had a smooth, round shape with lower measured variation in the diameter (Figures 3d and e). It is believed that the lower surface roughness of the twisted fiber (PI-72T) is directly responsible for its decreased IFSS as compared to PI-72 fiber, in-line with that shown in previous work examining the effect of surface roughness on IFSS of carbon fiber [29]. This surface roughness may also explain the higher apparent IFSS of the PI-72 fiber compared to the more highly stretched fiber (PI-82), even though the tenacity of the PI-72 fiber

18

was lower than other fibers and pristine yarns. A similar exponential relationship between apparent IFSS and embedded length was observed with the composite fibers, which have short embedded lengths (< 1.1 mm, Figure 2b), similar as that from the 2-ply pristine CNT yarns which have long embedded lengths (> 2 mm, Figure 2c). Note that the apparent IFSS converges at longer embedded lengths (~ 1.1 mm) to values between 20 and 30 MPa for different pre-

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infiltrated composite fiber treatments and around 2 MPa for pristine CNT yarns (~ 14 mm).

ro

Although the fiber tenacities were lower in the pre-infiltrated composite fiber samples, their

-p

improved IFSSs required the use of significantly shorter embedded lengths in the composite

re

specimens than were used for the pristine and post-treated CNT yarns to ensure that pull-out, rather than tensile failure, occurred. Because the maximum possible embedded lengths differed

lP

by more than an order of magnitude between the composite fibers and the pristine and post-

na

treated yarns, it is important to make comparisons between them at similar places along their

ur

exponential curves. For example, while it may be possible to test the pristine and post-treated

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yarns at similar embedded lengths as the pre-infiltrated composite fibers, the location along each fiber’s exponential curve would be different by an order of magnitude. While the pre-infiltrated polymer/CNT composite fibers fabricated in this work show shear strength improvements of approximately an order of magnitude over pristine yarns, the specific tensile strengths of these fibers are around 1 N/tex (Figure 4), compared to pristine yarns which had a specific tensile strength of 1.69 N/tex (2-ply). Balancing the tensile and interfacial properties of these fibers will be important in the continued improvement of this material system. Also, both the untwisted (PI-72 and PI-82) and twisted (PI-72T) composite fibers experienced a catastrophic failure under tension, as shown in Figure 4 and Figures 3a – f. This behavior is very different from the pristine yarn failure, especially for the spun CNT yarns that undergo a shear

19

induced sword-in-sheath failure [43,44]. Individual CNTs in the composite fibers were pulled out from the API-60 matrix after tensile failure in both the PI-72 and PI-72T sample, retaining hairy CNTs at the fracture surfaces. However, the PI-72 fiber did not show a resin rich surface while the PI-72T fiber displayed a relatively thick resin rich surface with a twist angle of 28 degree and

Jo

ur

na

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re

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ro

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a rounded cross-sectional shape.

20

of ro -p re lP na ur Jo Figure 3. FE-SEM images of (a-c) PI-72 and (d-f) PI-72T fibers at the failure site of post tensile test. Pre-pull-out test cross-sectional SEM images of the twisted API-60/CNT composite fiber (g, h) without stretching and (i, j) with 72 % stretching (PI-72T).

21

of ro -p re lP na

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Figure 4. Typical specific stress vs. strain curves under a tensile test.

In addition to the apparent IFSS, other data computed from the pull-out curves can provide additional insight into these material systems. The maximum pull-out force increased linearly with increasing embedded length within the tested range in Figure 5a, in good agreement with previous experimental results and a theoretical model [45]. To provide a better side-by-side comparison of the modifications of yarns, the normalized maximum force was calculated by dividing the measured maximum force by the number of plies in the yarn. Generally, the normalized maximum force was slightly higher for the 1-ply pristine yarn, most of the chemically functionalized yarns, and cross-linked yarns across the range of embedded lengths that were tested, compared to the 2-ply pristine CNT yarn. Because the normalized maximum

22

force does not consider the surface contact area, differences such as those due to swelling of the yarn during post-treatment, may be contributing to differences between the pristine and the post-

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ur

na

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re

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ro

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treated samples.

Figure 5. (a) Normalized maximum force and (b) de-bond force (Fd) vs. embedded length of various CNT yarns. The normalized maximum force was calculated by the maximum failure force divided by the number of plies in the yarn. (c) Comparison of frictional stress (τf) of various 2-ply yarns with and without treatments. Note that the lines are only a guide to the eye and not intended to denote curve fits.

23

For treated yarns where the physical changes cannot be easily decoupled, however, the debond force (Fd) can provide some insight into the interfacial interactions between the treated yarn and the matrix. Figure 5b shows the de-bond forces of the pristine and modified CNT yarns to EPONTM 828 resin or API-60 (green triangle) at various embedded lengths. The de-bond forces of the 2-ply pristine CNT yarn in the EPONTM 828 resin were similar (3.37 ± 0.51 N) at all embedded lengths tested. The de-bond forces of the functionalized CNT yarns were not

of

improved, surprisingly, even though the apparent IFSSs and maximum forces increased slightly.

ro

Note also that the de-bond force did not appear to depend on the type of resin used. The 1-ply

-p

pristine CNT yarn is a chemically densified form, which has limited resin penetration during the

re

SFPT sample preparation. The change in diameter after the pull-out test was negligible, with

lP

little residue left in the matrix (<1 µm penetration depth) due to the limited resin penetration,

na

similar to what has been reported elsewhere [22]. The 2-ply pristine CNT yarn contains two individual yarns (plies) with a minimum amount of physical twist. Between the plies there are

ur

large voids that are accessible to the resin and could potentially remain with some of the CNTs in

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the matrix after pull-out. The de-bond forces of the post-treated API-60/2-ply CNT yarns and the cross-linked 2-ply CNT yarns were improved by the improved interfacial adhesion, and more likely, the improved mechanical properties of the composite outer surface of the yarn through cross-linking. A similar improvement in mechanical properties has been observed for both polymer/CNT sheet composites by resistive heating [12] and CNT yarns after chemical crosslinking [36]. After de-bonding of the embedded yarns from the matrix, the remaining measured force results from the frictional interactions between the de-bonded yarn and the matrix. This frictional stress in de-bonded regions differs between specimens, as shown in Figure 5c. The frictional

24

stress was calculated by dividing the remaining force after full de-bond by the contact area. To compute the contact area, the fiber embedded length between Fb and the end of the pull-out step was used. Even if the samples were fabricated using the same processing conditions, the frictional stress varied due to the shape and diameter variations along the embedded CNT yarn. In many cases the frictional force of the pristine CNT yarns increased beyond the initial frictional force (sometimes beyond the maximum failure force, not shown) during the fiber pull-

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out after full de-bonding as shown in the force-displacement curve of the 1-ply pristine CNT

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yarn (Figure 1a). Importantly, the de-bond force of the pre-infiltrated composite fibers improved

-p

significantly because of strong interfacial interactions, as shown in Figure 1b, compared to the

re

pristine and post-treated CNT yarns, while the friction force was noticeably lower due to the

na

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shorter embedded length and lower frictional interaction between the failure phases.

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3.2. Comparison of pull-out failures. The failure modes of the pristine, post-treated, and pre-

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infiltrated fibers were observed during and after the pull-out test. Similar failure modes were observed for pristine and post-treated CNT yarns, while the pull-out failure was distinctly different for the pre-infiltrated composite fibers, as depicted schematically in Figure 6. Similar to what has been observed previously [22], the pristine CNT yarns do not fail at the interface between the yarn and the matrix, but instead fail within the dry core of the yarn. Unlike most fiber reinforcements, which fail at the fiber/matrix interface, the shear strength of the yarn itself is lower than the shear strength of the yarn/matrix interface, due to the hierarchical structure of CNT yarn. The shear strength of the pristine CNT yarn is exceptionally low, resulting in embedded lengths as large as 14 mm being pulled out, as shown in Figure 2c. Other fiber systems require much smaller embedded lengths, often less than 1 mm, to ensure fiber pull-out

25

instead of tensile failure of the fiber in the gauge-section. Increasing the shear strength of the fibers to eliminate the fiber shear failure, which would move the shear failure to the surface of the fiber, is expected to maximize the fiber-matrix load transfer of these CNT reinforcement

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ur

na

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re

-p

ro

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materials.

Figure 6. Depiction of failure paths for (a) pristine, (b and c) post-treated CNT yarns, and (d) pre-infiltrated composite fibers embedded in a polymer matrix. The manufacture of the pristine CNT yarns used in this work includes a capillary densification step using chlorosulfonic acid, resulting in a dense material that is challenging to modify, especially by post cross-linking or infiltration with resin. A limited degree of infiltration of the polymer matrix into the pristine fiber surface is expected, and a thin outer sheath of polymerinfiltrated fiber is left behind in the tab after a pull-out test, as shown in Figure 7a. The thickness of this outer sheath can be estimated by comparing the diameter of the fiber before and after a

26

pull-out test using a laser optical micrometer. In the case of the 2-ply pristine CNT yarns, the diameter was reduced from 136.0 to 122.6 µm after the test, resulting in an estimated penetration depth of ~ 6.7 µm. The failure mode of the post-treated CNT yarns was similar to that of the pristine CNT yarns, as shown in Figures 6b and c. The pull-out failures of the post-treated CNT yarns occurred

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within the yarns, and not at the yarn/matrix interface. Remnants of the yarn surface were again

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observed in the polymer tab after pull-out. A reduction in the yarn diameter after the pull-out test

-p

was also observed (Figures 6b and c). For example, the outer sheath of the m-PDA crosslinked

re

and m-CPBA functionalized CNT yarns was estimated to be less than 3 µm thick based on

lP

diameter reductions from 152 ± 5 to 145 ± 4 µm and from 149 ± 2 to 142 ± 5 µm, respectively. Similar to the pristine CNT yarns, the post-treated fibers could be embedded to extremely large

na

lengths of 11 mm and still experience pull-out failure instead of tensile failure at gauge-section,

ur

indicating that the shear strengths were particularly low. This dry-section failure mode and low

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shear properties observed in the pristine and post-treated CNT yarns during pull-out testing is in agreement with the low shear properties observed in macroscale composites [17] made from CNT yarns. The failure mode of the pre-infiltrated polymer/CNT composite fibers was distinctly different than those of the pristine and post-processed samples (Figure 6d). Samples that were tested at embedded lengths similar to what was used for pristine and post-treated yarns all resulted in tensile failure in the gauge-section of the fiber instead of pull-out failure. The embedded length had to be reduced below 2 mm before pull-out failure could be observed, indicating that there was a substantial increase in the shear strength of the material. In addition, there was not an

27

appreciable difference in fiber diameters before and after pull-out. These results support the assertion that both the shear strength and the de-bond force of the pre-infiltrated composite fibers

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na

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re

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ro

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have increased enough to move the site of failure to the fiber/matrix interface.

28

Figure 7. Post pull-out imaging and microscopy. (a) Optical micrograph image of the epoxy tab after pull-out of the 2-ply pristine CNT yarn, which was embedded in the entire 5 mm tab cross section. The visible black residue was transferred from the embedded yarn during pull-out. (b) Optical micrograph of PI-72 fiber after the pull-out test. FE-SEM images of PI-72 fiber after pull-out at (c) the end of the embedded fiber, (d) at the center of the embedded fiber end (marked in c), (e) at the top surface of the embedded fiber, and (f) at the middle of top surface (marked in

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ro

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e) in a higher magnification.

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The morphology of pre-infiltrated polymer/CNT composite fiber is shown after pull-out in

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Figure 7. As shown in Figures 7e and f, the pulled-out composite fiber (PI-72), embedded 614

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µm into the epoxy matrix, had a relatively smooth surface, with less residual material remaining in the matrix after pull-out compared to the surface of the as-prepared fiber (Figures 3a and c).

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Some portions of the CNT composite fiber were torn off from the main body of the fiber and

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tangled at the end of the fiber (Figures 7b, c, and e). The CNTs in the composite fiber were completely wet-out by the infiltrated resin, as clearly seen in Figure 3j and Figures 7d and f, which were taken at a central section of the fiber-end in the cross-sectional view (Figure 7c) and a middle section of the fiber surface embedded into the matrix (Figure 7e). These results indicate that the shear interactions were substantially improved in the fully wetout composite fiber. The shear failure mode changes from a ring model with two phases (e.g., highly densified pristine CNT yarns, Figure 6a, and post-treated CNT yarns, Figures 6b and c) to an one cylinder (e.g., pre-infiltrated polymer/CNT composite fiber, Figure 6d) or laminar model similar to a fully infiltrated carbon fiber tow. However, unlike carbon fiber tows, the pre-

29

infiltrated polymer/CNT composite fiber is composed of a hierarchical structure of highly interconnected and branching CNT networks, which may result in even higher potential IFSS than traditional fibers [46]. The composite fibers reported in this work were not exhaustively optimized and it is expected that the IFSS could be further improved by improving interfacial interactions between the CNT tubes and the bundles with sizing or cross-linking applied before resin pre-infiltration, maximizing the contact area, and optimizing the resin content during the

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stretching and resistive heating assisted process.

-p

As discussed previously, the interfacial properties of the composite fibers were improved by

re

over an order of magnitude compared to the pristine and post-treated CNT yarns because the dry region within the fiber was eliminated and the fiber was completely infiltrated with resin. For

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further confirmation of the full infiltration of the resin in the composite fiber, the twisted

na

composite fibers with and without stretching were ion-milled to prepare a cross-sectional

ur

specimen. Figures 3g-j show cross-sectional FE-SEM images of the twisted but unstretched and

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the twisted with 72 % stretched (PI-72T) composite fibers, respectively, processed with the same resistive heating conditions. Without stretching, the thin sheets in the roving folded and stacked several layers to form micron-sized voids between the folded layers, which are difficult to infiltrate with resin. Also, a number of unfilled nanovoids were present inside the fiber. Mechanical stretching and twisting helped to remove these voids throughout the composite fiber as shown in Figures 3i and j. The PI-72T fiber became densely packed with a fairly round crosssection (Figure 3i) with a resin-rich surface (Figure 3f), indicating that the excess resin was squeezed out while stretching.

30

3.3. Additional characterization of pre-infiltrated polymer/CNT composite fibers. As shown

in Figure 8, there are very few accessible pores on the top surfaces of the 70 % stretched roving (Figure 8b) and 1-ply CNT yarn (Figure 8d), which prohibited resin penetration into the yarn. However, the resin easily penetrates into the pristine CNT roving (Figure 8a) through open pores during composite fabrication. The majority of CNT bundles in the roving, with diameters of a few tens of nanometers, were aligned to the yarn drawing (manufacturing) direction, as indicated

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by a 2D order parameter of 0.68 (Table 2) and an IG///IG⊥ [G band (1578 cm-1) intensity ratio

ro

between the parallel and the vertical polarities] of 6.56, as shown in Figure 8e. The CNT bundles

-p

form a network structure that continuously branches and merges in a way that prevents

re

separation during mechanical stretching with the diluted resin solution. Under mechanical

lP

stretching using the custom-built continuous stretcher (Figure 8b) or chemical stretching during

na

manufacturing (Figure 8d), the CNTs become highly aligned to the stretching direction and the accessible voids are reduced significantly. For the yarns stretched to 70 % without resin, the 2D

ur

order parameter and IG///IG⊥ ratio of the stretched yarn (Figures 8b and f) were 0.79 and 13.2,

Jo

respectively, which were very similar to the PI-72 fiber (0.81 for 2D order parameter and 16.0 for IG///IG⊥, Figure 8g), and which also agrees well with previous literature reports [9, 47]. The surface morphology of the PI-72 fiber shown in Figure 8c differs from those from the 70 % stretched and 1-ply CNT yarns with relatively large bundles at the top surfaces that are most likely due to the resin coating at the beginning of the stretching process. During the resistive heating assisted infiltration and cure process, the individual CNTs or bundles were coated by API-60 and also experienced high temperature due to the applied voltage (>20 V), which created surface defects by oxidation of the CNT as shown in the IG/ID decrease from 8.4 (CNT roving) to 4.4 (PI-72 fiber).

31

of ro -p re lP na ur Jo Figure 8. FE-SEM images of (a) the pristine CNT roving, (b) 70 % stretched CNT roving, (c) PI-72 fiber, and (d) 1-ply pristine CNT yarn. Polarized Raman spectra of (e) the pristine CNT roving, (f) 70 % stretched CNT roving, and (g) PI-72 fiber.

32

of ro -p re

Table 2. Physical and mechanical properties of pristine yarns and pre-infiltrated polymer/CNT

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na

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composite fibers. aData was provided from the manufacturer.

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4. CONCLUSIONS

The interfacial shear properties of pristine yarns, post-treated CNT yarns, and pre-infiltrated polymer/CNT composite fibers embedded in a polymer matrix were compared using a single fiber pull-out test. For pristine and post-treated CNT yarns, which are highly densified starting material formats, treatments had limited resin penetration depth and the shear failure occurred within the dry core of the CNT yarn which has lower shear strength than the fiber/matrix interface. While it is possible that the post-treatments were improving the yarn/matrix interface since failure was not occurring at the interface and was instead occurring within the dry core of the treated yarn, the post-treatments of the highly densified CNT yarns were ineffective and the IFSS improved only marginally (~ 2 MPa) over the pristine material. To prevent the dry section

33

failure of the yarn under the shear force, pre-infiltrated composite fibers were fabricated with the CNT roving material, which has a low-density, loosely networked structure. The CNT roving was composited with a resistive heating assisted infiltration and cure process and mechanically stretched with a custom-built continuous stretcher. The measured apparent IFSS of the preinfiltrated composite fibers was improved over the pristine CNT yarns by approximately an order of magnitude (~ 20 - 60 MPa). The pre-infiltrated composite fibers were wet-out through the

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cross section, thereby eliminating the dry-core shear failure observed in the pristine and post-

ro

treated CNT yarns. In this study, the apparent IFSS of the pre-infiltrated composite fibers

-p

appeared to be influenced more by surface roughness than the tenacity of the studied fibers. The

re

SFPT was used to screen the different processing approaches in this work and identify promising

lP

candidates. Continued work to evaluate the approaches that were screened in this work with

na

more quantitative test methods and large composite samples is currently ongoing. Additional work to improve the strength and consistency of the CNT roving and to optimize the resin

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chemistry for the pre-infiltration process is also needed before composite fabrication processes

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can be optimized. The results of this effort provide insight for designing CNT yarn (or fiber) reinforced composite processes for structural applications and for understanding composite properties, such as fracture toughness, that depend on fiber/matrix interactions. ACKNOWLEDGMENT This work was funded through the Super-lightweight Aerospace Composites project under the NASA Space Technology Mission Directorate Game Changing Development Program.

34

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[42] Zamora-Ledezma C, Blanc C, Maugey M, Zakri C, Poulin P, Anglaret E. Anisotropic thin

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films of single-wall carbon nanotubes from aligned lyotropic nematic suspensions. Nano Lett 2008; 8(12): 4103-7. [43] Fernández-Toribio JC, Alemán B, Ridruejo Á, Vilatela JJ. Tensile properties of carbon nanotube fibres described by the fibrillar crystallite model. Carbon 2018; 133: 44-52. [44] Anike JC, Belay K, Abot JL. Effect of twist on the electromechanical properties of carbon nanotube yarns. Carbon 2019; 142: 491-503. [45] Jiang KR, Penn LS. Improved analysis and experimental evaluation of the single filament pull-out test. Compos Sci Technol 1992; 45: 89-103.

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[46] Naraghi M, Filleter T, Moravsky A, Locascio M, Loutfy RO, Espinosa HD. A multiscale study of high performance double-walled nanotube-polymer fibers. ACS Nano 2010; 4: 6463-76. [47] Jolowsky C, Sweat R, Park JG, Hao A, Liang R. Microstructure evolution and selfassembling of CNT networks during mechanical stretching and mechanical properties of

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highly aligned CNT composites. Compos Sci Technol 2018; 166: 125-130.

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Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode

National Institute of Aerospace, Hampton, VA 23666, USA, 2Advanced Materials and

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Russell A. Wincheski,3 Roberto J. Cano,2 Emilie J. Siochi2*

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Jae-Woo Kim,1* Godfrey Sauti,2 Benjamin D. Jensen,2 Joseph G. Smith,2 Kristopher E. Wise,2

Nondestructive Evaluation Sciences Branch, NASA Langley Research Center, Hampton, VA

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Processing Branch, NASA Langley Research Center, Hampton, VA 23681, USA,

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ABSTRACT

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23681, USA

Achieving high strength in fiber reinforced structural composites requires effective load transfer between the high-performance fiber reinforcement, e.g., carbon nanotube (CNT) yarn, and the matrix. Various processing approaches to enhance the interaction between CNT fiber and the matrix were investigated. The apparent interfacial shear strengths (IFSS) of pristine CNT yarns, post-treated CNT yarns (cross-linked, functionalized, and polymer incorporation by resistive heating), and pre-infiltrated polymer/CNT composite fibers measured using single fiber pull-out tests were used to screen the efficacy of the processing methods. Pristine CNT yarns had

*Corresponding Authors: [email protected] (Emilie J. Siochi); [email protected] (Jae-Woo Kim)

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a low apparent IFSS (< 5 MPa) due to shear failure within their dry cores. In post-treated CNT yarns which did not exhibit good IFSS, the failure surface consisted of a resin-infiltrated sheath near the surface of the yarn and a dry section within the yarn core; failure occurred in the dry core. This failure mode is unlike those observed in traditional carbon fiber reinforced composites which fail at the fiber/matrix interface. In contrast to the sword-in-sheath failure modes of posttreated CNT yarns, pre-infiltrated polymer/CNT composite fibers displayed high apparent IFSS

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(> 20 MPa). Improved wet-out of the fiber eliminated the dry-core shear failure mode.

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KEYWORDS. Carbon nanotube, interfacial shear strength, single fiber pull-out test, composite

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fiber

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1. INTRODUCTION

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Individual carbon nanotubes (CNTs) with excellent electrical [1,2], thermal [3,4], and

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mechanical [5-8] properties have recently become available in large quantities of formats such as

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yarns, tapes, and sheets. These materials are manufactured by methods including wet or dry spinning and continuous aerogel spinning in a floating catalyst chemical vapor deposition furnace [9-13]. In contrast to CNT powder which were available earlier in the evolution of CNT materials, these new structural forms of CNTs are more suitable for use as reinforcements in lightweight structural applications [14-18]. While substantial effort has been put into developing and characterizing these large format CNT materials, it is also crucial to understand their behavior when combined with a polymer matrix to form a composite. In uniaxial tension-dominated applications, such as composite overwrapped pressure vessels, the fiber dominates the mechanical response and fiber/polymer interfacial properties are less critical [16]. For broader applications in which compression, shear,

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and impact loadings are important, the fiber/matrix interaction plays a crucial role. For example, Windle et al. reported specific tensile strength of 230 MPa/(g/cm3) with 27 fiber vol.% and specific compression strength of 118 MPa/(g/cm3) with 14 vol.% of unidirectionally aligned CNT fibers in an epoxy matrix [14]. Kim et al. fabricated unidirectional CNT yarn/polymer composites using various resin infusion processes and reported tensile and short beam shear strength [17]. The composites tested in that work, which were composed of ~80 wt.% CNT in an

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epoxy matrix, retained 60 -77 % of specific tensile strength of the reinforcing yarns [~ 1

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GPa/(g/cm3) composites from 1.30 to 1.68 N/tex yarns], as expected for a property dominated by

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the reinforcing fibers. In contrast, the short beam shear strength, which is very dependent on

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fiber-matrix load transfer, was only 9.1 MPa due to weak fiber/matrix interactions, low resin

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uptake, and poor wetting. This is over an order of magnitude lower than the typical value

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measured for unidirectional IM7 carbon fiber (12k tow)/8552 composite (~ 137 MPa with ~ 67 wt.% fiber) [19]. Improving the fabrication of high strength polymer/CNT composites to yield

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materials with a high interfacial shear strength (IFSS), in addition to high tensile properties, is

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therefore crucial to the development of light-weight composite materials that can potentially minimize the mass of future spacecraft and launch vehicles. There are a variety of mechanical tests that can give insight into the interfacial properties between the fiber reinforcement and the matrix for specimens of different sizes. Coupon-scale tests include short beam shear, flexure, and double cantilever beam. For even smaller specimens, single fiber tests are available to measure interfacial properties against a polymer matrix. Single fiber tests require less material and have shorter fabrication times than coupon-scale tests, making them appropriate for screening efforts, while the coupon-scale tests can provide a more reliable, quantitative assessment of fiber/matrix interfacial properties on a larger composite scale.

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Single fiber tests were used in this work because of the small amount of CNT yarn required and an interest in screening a large number of material processing methods. Examples of single fiber tests include micro-indentation [20], single fiber fragmentation [21], microdroplet [22], and single fiber pull-out [23,24]. A few models to aid in the interpretation of single fiber tests have been summarized in the literature [25-27] and the failure modes observed during the tests can be

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instructive in understanding the improvements needed in the materials.

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Each single fiber test method has strengths and weaknesses which depend on the material system being tested. CNT fibers (or yarns) have several unique characteristics which should be

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considered when selecting a test method, such as high strain at failure, low sensitivity to

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handling and bending damage, and large diameters. In addition, CNT fibers often experience

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limited polymer infiltration, leading to a mixed wet/dry structure with distinct sheath/core

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properties. Zu et al. [22] fabricated CNT fibers with diameters of ~ 10 µm from CNT forests and

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performed microdroplet tests to characterize the fiber/matrix interactions. Sword-in-sheath failure within the fiber was observed and the average apparent IFSS between the hydroxyl group

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functionalized CNT fiber and the epoxy resin was 14.4 MPa. This IFSS value is somewhat comparable to, but lower than, the values from untreated (18.4 – 28.1 MPa) [28,29] and chemically etched (36.3 MPa) [29] carbon fibers, respectively, measured using a similar test. It is worth noting that the length of fiber in contact with the matrix can influence the measured IFSS, with the largest contact lengths providing more consistent results [30]. Similarly, the microdroplet test may require larger bead sizes to obtain consistent results for CNT yarns with low interfacial strengths. Precisely controlling and quantifying the droplet size is challenging with CNT fibers because they can absorb small amounts of the resin into the surface and some

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resin flows along the CNT fiber during cure. This may cause the contact area to be underestimated resulting in an overestimation of the IFSS. In fiber fragmentation tests, the fiber is completely surrounded by matrix, which avoids the contact area ambiguities encountered in microdroplet testing. For example, carbon fiber (TORAYCA® T-300)/epoxy composites characterized using a fragmentation test showed an

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IFSS of 89.5 MPa, which is more than double the microdroplet test results because of the greater

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measurement sensitivity [31]. However, characterizing the interface with a fragmentation test requires a matrix with a higher failure strain than the fiber. This is a challenge because CNT

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fibers typically have a higher failure strain than engineering thermoset matrices, which have

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relatively small failure strains (less than 1.7 %) [19]. The computed IFSS is also sensitive to the

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average critical fragment length which can be challenging to measure. In addition, distinguishing

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sword-in-sheath and true interfacial failure modes with CNT fibers is not straightforward with a fragmentation test. Single fiber fragmentation testing of CNT fibers spun from CNT forests were

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performed by Deng et al. [21] and Liu et al. [31]. Deng et al. reported IFSS values of 10 – 20

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MPa for pristine fibers spun from CNT arrays. Liu et al. reported that IFSSs increased to 120 MPa with various pre-densification treatments using ethanol and polymer. The IFSS increased with increasing tensile strength and modulus of the CNT fiber and upon decreasing the fiber diameters from 10.5 to 5.5 µm. They also reported that, despite its lower strength and modulus, the IFSS of multi-walled CNT (MWCNT) fibers was surprisingly 12 - 15 MPa higher than a few-walled flattened CNT fiber because of the loosely packed arrangement and defective structures of the MWCNT fiber. Given the challenges of doing fiber fragmentation testing with CNT yarns, single fiber pull-out test (SFPT) was adopted for this work because of the ease and simplicity of specimen

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preparation and characterization. The test is a good match for the yarns evaluated in this work, which have relatively large diameters (> 100 µm), good handleability, and physical damage insensitivity. In addition, SFPT is a better method for materials with low IFSS that require large embedded lengths, as well as for multiple fiber bundles such as carbon fiber tow. For example, unsized carbon fiber (T-300) bundles with embedded lengths ranging from 5.0 to 10.8 mm were tested by a fiber pull-out method and reported IFSS values in a range between 18.8 and 25 MPa,

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depending on resin selection [32].

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In the remainder of the paper, three categories of CNT yarn are discussed: pristine yarns, posttreated yarns, and pre-infiltrated polymer/CNT composite fiber. The pristine yarns, which are

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composed of 1- or 2-plies, are highly aligned, relatively high density, high tenacity and are tested

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in their as-received condition. The post-treated yarns were prepared by exposing the pristine

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yarns to a variety of treatments intended to alter their surface properties in an effort to improve

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matrix interactions. Finally, the pre-infiltrated composite fibers were prepared by first infusing a polymer into the low density, low tenacity CNT roving, which is the as-produced CNT form

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collected from the furnace with minimal processing. After the polymer infusion step, the roving was mechanically stretched and densified to form a well-consolidated composite fiber. These pre-infiltrated composite fibers differ substantially from a post-treated yarn with the same polymer because the open pore structure of the roving permits more extensive resin infusion than is possible with the much denser as-received pristine form. The improved integration of the polymer into the fiber appears to enhance the load carrying capacity of these materials. In what follows, the shear load transfer capabilities and shear failure modes of these three categories of fibers are compared and contrasted using the apparent IFSS as a basis for comparison.

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2. EXPERIMENTAL 2.1. Materials and post-treated CNT yarns. Polymer matrices used for this work were EPONTM 828 and API-60 epoxies purchased from Hexion Inc. and Applied Poleramic Inc. (currently Kaneka Aerospace LLC), respectively. EPONTM 828, which is a two-part difunctional bisphenol A/epichlorohydrin derived liquid epoxy resin with a fully cured resin density of 1.16

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g/cm3, was mostly used as a tabbing material. EPIKURETM W was used as the curing agent with

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a 100 to 25 mix ratio between the EPONTM 828 resin and the EPIKURETM W. API-60, a

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toughened epoxy system, was used as an infiltration resin for composite fiber fabrication

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including both high density CNT yarn and CNT roving. It was also used as a tabbing material, in

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some limited experiments, for comparison purposes.

Prior CNT and carbon fiber composite work suggests that changes in the surface functional

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groups of CNT yarns may be needed to improve adhesion to the polymer matrix for further

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improvement of mechanical properties of the composites, e.g., shear strength. In this work,

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chemical functionalization and cross-linking of the highly densified CNT yarns were conducted by procedures suggested from previous studies including the diazonium salt reaction [33-36] and m-chloroperoxybenzoic acid (m-CPBA) treatment [37]. Phenylenediamine (PDA) was used to cross-link CNTs in highly densified CNT yarns [36]. 1-ply (Nanocomp Technologies, Inc., 4.31 tex, 1.73 N/tex tenacity, and 102.29 N/tex specific modulus) or 2-ply (baseline study, 13.60 tex, 1.69 N/tex tenacity, and 93.32 N/tex specific modulus) yarns were used to prepare the posttreated CNT yarns. The CNT yarn was immersed in 60 wt.% sulfuric acid with m-PDA (or pPDA) and sodium nitrite (NaNO2) for 48 hrs to complete cross-linking of the CNT yarn, either at room or elevated temperature (70 °C). For the m-CPBA functionalization, the yarn was immersed into a m-CPBA acetonitrile solution (1 mg/mL) at 40 °C for a short period of time (10

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seconds). The functionalized CNT yarn was sequentially cleaned with acetonitrile and acetone to remove excess m-CPBA and then dried under vacuum. For applying a polymer coating to the CNT yarn as a post-treatment, API-60 was diluted with methyl ethyl ketone (MEK, Sigma-Aldrich) to yield a 2 wt.% resin concentration to improve infiltration when applied to the high-density yarn. The diluted resin solution was applied directly

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onto the highly densified CNT yarns while under tension, followed by application of an electrical

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current through the yarn to evaporate the solvent and cure the resin at elevated temperatures by

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resistive heating (RH) [12,38,39].

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2.2. Pre-infiltrated polymer/CNT composite fiber process. Fully infiltrated API-60/CNT

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composite fibers were fabricated by resistive heating assisted infiltration of the loosely networked structure of CNT roving (Nanocomp Technologies, Inc., 10.52 tex, 0.67 N/tex

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tenacity, and 7.91 N/tex specific modulus). To fabricate the pre-infiltrated polymer/CNT

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composite fibers, a custom-built continuous stretcher was used [40]. In contrast to the highly

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densified CNT yarn, CNT roving was easily wetted out with the diluted resin solution (2 wt.% of API-60 in MEK). The CNT roving consists of partially aligned, loosely bound CNT networks and is shaped like a narrow flat tape of about 2 mm in width and tens of microns in thickness, depending on manufacturing conditions. The custom-built continuous stretcher for the CNT roving stretching and resistive heating process consists of two stepper motors (ARCUS Technology, DMX-A2-DRV-23) for controlling the material supply and the stretching level, with two conductive aluminum rollers located between the stepper motors. A 4-axis motion controller equipped with USB input (ARCUS Technology, PMX-4EX-SA-TB9) was used to drive the motors. An Agilent E3634A benchtop power supply as well as Keithley 2100 and Hewlett Packard HP3478A digital multimeters were connected to two conductive rollers (14 cm

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distance) to apply and monitor the voltage across and the current flowing through the CNT roving during composite processing. In-house developed LabVIEW based software was used to control the continuous stretcher as well as the resistive heating process. A total of 50 cm of the CNT roving was loaded onto one of the TeflonTM spools mounted to the stepper motors and gradually stretched in 5 % increments while passing back and forth between the two motors until the desired level of stretching was achieved. The samples included two different stretching levels

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(72 and 82 %) with (denoted as a PI-72T) and without twist (PI-72 and PI-82). For the PI-72T

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sample, the CNT roving was twisted to 2.12 revolutions/cm using a custom-built motor-driven

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apparatus, before it was loaded into the stretcher. The diluted API-60 resin solution was applied

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at the beginning of the stretching process at a base voltage of 5 V (CNT roving temperature ~ 30

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°C). During the stretching process the applied voltage was increased stepwise to 15 V

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(maintaining temperature below 100 °C), until the stretch level reached 20 %, to evaporate the MEK solvent (b.p.: 79.64 °C) and improve infiltration by reducing the viscosity of the applied

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resin. Once the stretch level reached 20 % the voltage was held constant at 15 V and stretching

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continued until the final desired stretch level was reached. At this point, both motors were stopped and the temperature increased to 150 °C by increasing the applied voltage to a range between 20 and 24 V, depending on the resistance of the stretched roving. The stretched roving was then partially cured for 10 min while being held in this stretched condition. The temperature during the composite fiber fabrication process was monitored using an infra-red camera (FLIR® TS650sc). For the tensile tests and SFPT, about 7 cm of material was taken from the middle of the processed composite fiber. Typical values for the measured diameter and linear density of the composite fibers with 72 % stretch (PI-72) were 144 ± 22 µm and 15.386 g/km, respectively.

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The average diameter along the fiber length of the as-prepared and pristine yarns was measured using a dual axis optical micrometer (Keyence, LS-9006D). Fiber diameter values were measured on two orthogonal axes simultaneously, gathering 50,000 data points on each axis with the average obtained over a 4 cm fiber length on both axes. The linear density was calculated by measuring the weight and length of each fiber.

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report the results, this information is summarized in Table 1.

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To aid in keeping track of the materials used in this work and the designation we used to

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Pristine CNT yarns

single ply pristine CNT yarn from manufacturer (Nanocomp Inc.)

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1-ply

two ply pristine CNT yarn with a minimum twist to hold the

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2-ply structure

Post-treated CNT yarns

pristine 2 ply yarn functionalized with a m-chloroperoxybenzoic

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m-CPBA acid

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from manufacturer (Nanocomp Inc.)

m-PDA

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(functionalization) pristine 2-ply yarn cross-linked with a m-phenylenediamine

(crosslinking)

p-PDA

pristine 2-ply yarn cross-linked with a p-phenylenediamine (crosslinking)

RH

pristine 2-ply yarn treated with API-60 resin by resistive heating Infiltration (resin coating)

Pre-infiltrated polymer/CNT composite fibers and starting material CNT roving

loosely networked, narrow CNT ribbon from manufacturer (Nanocomp Inc.) and starting material for pre-infiltrated polymer/CNT composite fibers fabrication

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PI-72

pre-infiltrated API-60/CNT composite fiber with 72 % stretch

PI-82

pre-infiltrated API-60/CNT composite fiber with 82 % stretch

PI-72T

pre-infiltrated API-60/CNT composite fiber with 72 % stretch and mechanical twist (2.12 revolutions/cm)

Table 1. List of key materials studied in this work.

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2.3. Single fiber pull-out test. Generally, a small piece of the as-prepared CNT yarn or

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pristine yarn (2 - 5 cm) was embedded to the desired length (from a few hundreds of microns to

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14 mm) in a silicone mold (Ted Pella Inc., PELCO flat embedding molds). The specimen was

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mounted at the center of the tab by making a slot in the middle of the individual mold to ensure

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good fiber alignment and a repeatable measurement. Warm EPONTM 828 resin (50 - 60 °C) was then poured into the mold. The as-prepared specimen/EPONTM 828 loaded in the silicone molds

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was cured for 1 hr at 100 °C followed by a 2 hr hold at 177 °C to fabricate SFPT specimens. The

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two ends of the specimen were embedded into the EPONTM 828 tab to different lengths to ensure

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that the pull-out of the specimen always occurred on the side with the shorter embedded length. The gap between the two tabs was about 1 cm. As prepared SFPT specimens were mounted on a grid paper tab to ensure transfer to a testing machine (Instron Model 5943 single column tabletop testing system) without any physical damage such as kinks. This also guaranteed that the specimen could be mounted at close to 0 degrees in the load direction of the mechanical wedge action grips. SFPTs were conducted under a quasi-static tensile loading at a crosshead speed of 1 mm/min. For each material type, the force-displacement curves were obtained for at least five specimens for the data analysis of apparent IFSS, de-bond force, and frictional stress. For the pre-infiltrated composite fiber tensile tests, the gauge length and crosshead speed were 10 mm (gap between two epoxy tabs) and 0.5 mm/min, respectively. The polymer/CNT composite fiber

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was embedded to lengths of 14 mm into both of the epoxy tabs to ensure the failure occurred in the gauge section. Specific tensile stress (tenacity) of the composite fiber was calculated by dividing the measured failure force (N) by the measured linear density (g/km) of each specimen, to eliminate errors associated with the measurement of the sample cross-sectional area. Young’s modulus was calculated from the slope between 10 and 30 % of the ultimate tenacity to eliminate the initial lag in stress-strain behavior [12]. At least five specimens were tested to determine

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tensile strength and modulus. Resin content of the stretched composite fibers was in the range of

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20 and 30 wt.%, determined by measuring linear density before and after composite processing. 2.4. Characterization. A field emission-scanning electron microscopy (FE-SEM, Hitachi

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Model S-5200) was used to image pristine, post-treated CNT yarns, and composite fibers before

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and after pull-out at an acceleration voltage of 30 keV using the secondary electron detector.

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Cross-sectional SEM samples were prepared by a cooling cross sectional polisher (Jeol IB-

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19520CCP) using a 5 keV acceleration voltage under an argon plasma at -30 °C to prevent

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amorphous carbon development on the polished surfaces. The embedded length of each sample was measured with an optical microscope (Leica) after the pull-out test. Raman spectroscopic characterization was conducted with a Horiba XploRA PLUS Raman microscope system using a 30 mW diode laser with 638 nm excitation wavelength. The laser power was reduced by 50 % using a neutral density filter. The other parameters for the Raman spectra collection applied on the samples are: 10 acquisitions with 5 s of exposure time for each acquisition, 100 µm slit, 300 µm hole, and 1800 grating. These parameters were used to prevent sample damage during the measurement. Raman data were obtained at the middle of the sample assuming the sample is relatively homogeneous along the axial direction. Polarized Raman spectra were obtained in the

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VV, HH, and VH configurations with a 100x magnification objective lens. 2D and 3D order parameters (S) were calculated with the following equations [41]:

2

=

3

and

=

, respectively.

where, IVV, IVH, and IHH are G peak intensity at parallel/parallel, parallel/vertical, and

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vertical/vertical configuration, respectively.

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3. RESULTS AND DISCUSSION

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3.1. Apparent IFSS of CNT yarns. Figure 1 shows representative force-displacement curves of pristine, post-treated CNT yarns, and pre-infiltrated polymer/CNT composite fibers. In the

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early stage of the single fiber pull-out curve, the measured force increases linearly when the CNT

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yarn-matrix interface remains intact, which is from the elastic behavior of the material. When the

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measured force reaches a critical value, here denoted by a de-bond force (Fd) in Figure 1b, the interfacial crack initiates at some point at the interface, close to the CNT yarn entry into the epoxy matrix. As shown in Figure 1b, the de-bond force varies from specimen to specimen, depending on the interfacial interactions between the embedded yarn and the matrix. After initiating the interfacial crack, the crack continuously propagates along the embedded length toward the opposite end of the specimen. The de-bond regions are associated with a materialdependent frictional force that continuously increases as the crack extends until the maximum pull-out force is reached with complete de-bonding of the fiber. During this step, the measured force oscillates around a steadily increasing value, which may be evidence of inhomogeneous interactions between the local CNT bundle and the matrix. After full de-bonding of the

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embedded CNT yarn, the measured force drops from the maximum force to a substantially lower frictional force value, denoted as Fb in Figure 1b. This frictional force is attributed to frictional interaction between the CNT yarn and matrix and generally differs from specimen to specimen, and even within the same material, due to variations in shape and diameter along the embedded CNT yarn. It is worth noting that a de-bond region was not observed in the force-displacement curves for the majority of the PI-82 fiber specimens (Figure 1b). Pull-out of the embedded fiber

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was indistinguishable from the initiation of an interfacial crack.

Figure 1. (a) Typical pull-out force-displacement curves of the 1-ply and 2-ply pristine CNT yarns, post-treated CNT yarns cross-linked by m-PDA at room temperature [m-PDA @ R.T. (2ply)] and 70 °C [m-PDA @ 70 °C (2-ply)]. (b) Detailed zoom at low displacement in (a) including pre-infiltrated polymer/CNT composite fibers (PI-72 and PI-82). A number of methods for calculating IFSS from pull-out test results, differing in complexity and the underlying assumptions made in their development, have been reported in the literature [26,27]. Because the CNT yarns used in this work are more variable in their shape and are less brittle than the fibers that many of the more complex approaches were intended to approximate,

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the simplest and most commonly used mean-stress approach was used in this work to compute the IFSS. For the mean-stress approach, the apparent IFSS can be calculated as IFSS = Fmax/(πdaL), which depends only on the maximum tensile force measured during pull-out (Fmax) and the yarn/matrix average contact area (πdaL), where da is the average diameter and L is the embedded length of the yarn. It has been pointed out in the literature that apparent IFSSs should not be interpreted quantitatively, but rather used as semi-quantitative metric for assessing bond

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strength as ‘good’ or ‘poor’, and for determining whether various surface modifications improve

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or worsen the interfacial bonding [25]. So, while the apparent IFSS should not be viewed as a

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high-precision quantity, it can still be useful in materials development efforts as a semi-

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the strength of the interface.

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quantitative tool for determining whether or not changes to a yarn/matrix system have influenced

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All results reported below are apparent IFSS, although it is important to note that, due to the

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sword-in-sheath failure mode, shear failure was often not observed at the interface between fiber

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and matrix. The average diameter used to compute the IFSS was measured before each test. As detailed in Section 3.2. below, the diameter after testing was reduced for some sample types causing the reported apparent IFSS to be somewhat underestimated. For example, in the case of the 2-ply pristine CNT yarn, the apparent IFSS was underestimated by about 11 % when the effect of diameter reduction is accounted for. Visible black residue was transferred from the embedded yarn to the tab after the pull-out test. The amount of residual CNTs in the matrix differs from specimen to specimen. In this study, the surface roughness, irregular shape and diameter variations throughout the yarn length are not considered. Higher surface contact area can increase the apparent IFSS.

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Apparent IFSSs of the pristine, post-treated yarns, and pre-infiltrated fibers are plotted as a function of the embedded length in Figure 2. The 2-ply pristine CNT yarn was embedded into the EPONTM 828 matrix to lengths between 2 and 14 mm and used as a reference material for evaluation of the modified CNT yarns (Figure 2c). The apparent IFSS decreases exponentially as the embedded length increases, in good agreement with the energy-based model [25]. The apparent IFSS of the 2-ply pristine CNT yarn converges to a constant value around 2 MPa at

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large embedded lengths (e.g., 2.3 ± 0.3 MPa at 10 mm embedded length).

Figure 2. Apparent IFSS of (a) all, (b) pre-infiltrated polymer/CNT composite fibers, and (c) pristine and post-treated CNT yarns in terms of the embedded length. All post-treated CNT yarns were made with 2-ply CNT yarns. Note that the lines are only a guide to the eye and not intended to denote curve fits. Compared to the measured apparent IFSSs of the 2-ply CNT yarn, 1-ply, and post-treated yarns showed slightly higher apparent IFSSs. Figure 2c shows the apparent IFSS of 1-ply pristine, 1- and 2-ply m-CPBA functionalized yarns, m-PDA (and p-PDA) cross-linked 1-ply and

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2-ply yarns, and resistive heating assisted API-60 treated 2-ply CNT yarns. Changing the matrix from EPONTM 828 to API-60 did not improve the apparent IFSS. Although it is possible that the post-treatments improved the interfacial strength between the fiber and the matrix, the pristine and post-treated CNT yarns both failed within the CNT yarn and not at the yarn/matrix interface, which can explain the lack of improvement in apparent IFSS between the pristine and posttreated CNT yarns. Unless this kind of shear failure within the CNT yarn is prevented, typical

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fiber interfacial strengthening approaches like increasing the compatibility of the matrix resin or

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chemical modification of the CNT surfaces are not expected to fundamentally improve the

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apparent IFSS.

The apparent IFSSs of the 2-ply CNT yarns with different tenacities (1.19, 1.42, and 1.69

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N/tex) were measured to explore any relationship between the IFSS and yarn tenacity (not

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shown). No clear relationship between the IFSS and yarn tenacity was observed although it is

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anticipated that higher strength yarns will be denser and even more difficult to penetrate with

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post-treatments. Higher tenacity yarns may also have higher CNT alignment in the axial direction causing the surface of the yarns to be smoother, lowering the interfacial strength, although the surfaces of the pristine and post-treated yarns do not appear to be driving yarn/matrix shear properties. The pre-infiltrated polymer/CNT composite fibers, which have full resin infiltration followed by densification and alignment of CNTs through mechanical stretching of the CNT roving, generally showed high apparent IFSS, in the range between 20 and 60 MPa depending on the embedded length of the composite fiber (1100 - 500 µm) into the epoxy matrix, as shown in Figure 2b. These values are comparable with those from various types of carbon fibers, which range between 16.7 and 55.5 MPa with and without surface treatment [24]. As will be discussed

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later, the increased apparent IFSS can be attributed to eliminating the dry fiber core, which also moves the failure site to the surface of the fiber. Generally, two factors are involved in improving the IFSS of fiber reinforcement in a polymer matrix; 1) increasing interfacial interactions between the fiber and the matrix by chemical functionalization and 2) increasing contact area by increasing surface roughness. Unlike the polymer/spun CNT composite fibers [31] which have a more regular surface shape, the PI-72 fiber showed relatively irregular cross-sectional shapes as

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shown in Figures 3a and b, which can improve the apparent IFSS by increasing the contact area.

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The PI-72 fiber showed very rough surface morphology with wrinkles along the fiber axis

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(Figure 3a) while the PI-72T fiber had a smooth, round shape with lower measured variation in

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the diameter (Figures 3d and e). This surface roughness may also explain the higher apparent

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IFSS of the PI-72 fiber compared to the more highly stretched fiber (PI-82) and the stretched and

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twisted fiber (PI-72T), even though the tenacity of the PI-72 fiber was lower than other fibers and pristine yarns. A similar exponential relationship between apparent IFSS and embedded

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length was observed with the composite fibers, which have short embedded lengths (< 1.1 mm,

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Figure 2b), similar as that from the 2-ply pristine CNT yarns which have long embedded lengths (> 2 mm, Figure 2c). Note that the apparent IFSS converges at longer embedded lengths (~ 1.1 mm) to values between 20 and 30 MPa for different pre-infiltrated composite fiber treatments and around 2 MPa for pristine CNT yarns (~ 14 mm). Although the fiber tenacities were lower in the pre-infiltrated composite fiber samples, their improved IFSSs required the use of significantly shorter embedded lengths in the composite specimens than were used for the pristine and post-treated CNT yarns to ensure that pull-out, rather than tensile failure, occurred. Because the maximum possible embedded lengths differed by more than an order of magnitude between the composite fibers and the pristine and post-

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treated yarns, it is important to make comparisons between them at similar places along their exponential curves. For example, while it may be possible to test the pristine and post-treated yarns at similar embedded lengths as the pre-infiltrated composite fibers, the location along each fiber’s exponential curve would be different by an order of magnitude. While the pre-infiltrated polymer/CNT composite fibers fabricated in this work show shear

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strength improvements of approximately an order of magnitude over pristine yarns, the specific

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tensile strengths of these fibers are around 1 N/tex (Figure 4), compared to pristine yarns which

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had a specific tensile strength of 1.69 N/tex (2-ply). Balancing the tensile and interfacial

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properties of these fibers will be important in the continued improvement of this material system. Also, both the untwisted (PI-72 and PI-82) and twisted (PI-72T) composite fibers experienced a

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catastrophic failure under tension, as shown in Figure 4 and Figures 3a – f. This behavior is very

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different from the pristine yarn failure, especially for the spun CNT yarns that undergo a shear

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induced sword-in-sheath failure [42,43]. Individual CNTs in the composite fibers were pulled out

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from the API-60 matrix after tensile failure in both the PI-72 and PI-72T sample, retaining hairy CNTs at the fracture surfaces. However, the PI-72 fiber did not show a resin rich surface while the PI-72T fiber displayed a relatively thick resin rich surface with a twist angle of 28 degree and a rounded cross-sectional shape.

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of ro -p re lP na ur Jo Figure 3. FE-SEM images of (a-c) PI-72 and (d-f) PI-72T fibers at the failure site of post tensile test. Pre-pull-out test cross-sectional SEM images of the twisted API-60/CNT composite fiber (g, h) without stretching and (i, j) with 72 % stretching (PI-72T).

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Figure 4. Typical specific stress vs. strain curves under a tensile test.

In addition to the apparent IFSS, other data computed from the pull-out curves can provide additional insight into these material systems. The maximum pull-out force increased linearly with increasing embedded length within the tested range in Figure 5a, in good agreement with previous experimental results and a theoretical model [44]. To provide a better side-by-side comparison of the modifications of yarns, the normalized maximum force was calculated by dividing the measured maximum force by the number of plies in the yarn. Generally, the normalized maximum force was slightly higher for the 1-ply pristine yarn, most of the chemically functionalized yarns, and cross-linked yarns across the range of embedded lengths that were tested, compared to the 2-ply pristine CNT yarn. Because the normalized maximum

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force does not consider the surface contact area, differences such as those due to swelling of the yarn during post-treatment, may be contributing to differences between the pristine and the post-

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treated samples.

Figure 5. (a) Normalized maximum force and (b) de-bond force (Fd) vs. embedded length of various CNT yarns. The normalized maximum force was calculated by the maximum failure force divided by the number of plies in the yarn. (c) Comparison of frictional stress (τf) of

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various 2-ply yarns with and without treatments. Note that the lines are only a guide to the eye and not intended to denote curve fits.

For treated yarns where the physical changes cannot be easily decoupled, however, the de-bond force (Fd) can provide some insight into the interfacial interactions between the treated yarn and

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the matrix. Figure 5b shows the de-bond forces of the pristine and modified CNT yarns to

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EPONTM 828 resin or API-60 (green triangle) at various embedded lengths. The de-bond forces

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of the 2-ply pristine CNT yarn in the EPONTM 828 resin were similar (3.37 ± 0.51 N) at all

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embedded lengths tested. The de-bond forces of the functionalized CNT yarns were not

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improved, surprisingly, even though the apparent IFSSs and maximum forces increased slightly.

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Note also that the de-bond force did not appear to depend on the type of resin used. The 1-ply pristine CNT yarn is a chemically densified form, which has limited resin penetration during the

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SFPT sample preparation. The change in diameter after the pull-out test was negligible, with

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little residue left in the matrix (<1 µm penetration depth) due to the limited resin penetration, similar to what has been reported elsewhere [22]. The 2-ply pristine CNT yarn contains two individual yarns (plies) with a minimum amount of physical twist. Between the plies there are large voids that are accessible to the resin and could potentially remain with some of the CNTs in the matrix after pull-out. The de-bond forces of the post-treated API-60/2-ply CNT yarns and the cross-linked 2-ply CNT yarns were improved by the improved interfacial adhesion, and more likely, the improved mechanical properties of the composite outer surface of the yarn through cross-linking. A similar improvement in mechanical properties has been observed for both

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polymer/CNT sheet composites by resistive heating [12] and CNT yarns after chemical crosslinking [36]. After de-bonding of the embedded yarns from the matrix, the remaining measured force results from the frictional interactions between the de-bonded yarn and the matrix. This frictional stress in de-bonded regions differs between specimens, as shown in Figure 5c. The frictional

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stress was calculated by dividing the remaining force after full de-bond by the contact area. To

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compute the contact area, the fiber embedded length between Fb and the end of the pull-out step

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was used. Even if the samples were fabricated using the same processing conditions, the

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frictional stress varied due to the shape and diameter variations along the embedded CNT yarn. In many cases the frictional force of the pristine CNT yarns increased beyond the initial

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frictional force (sometimes beyond the maximum failure force, not shown) during the fiber pull-

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out after full de-bonding as shown in the force-displacement curve of the 1-ply pristine CNT

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yarn (Figure 1a). Importantly, the de-bond force of the pre-infiltrated composite fibers improved

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significantly because of strong interfacial interactions, as shown in Figure 1b, compared to the pristine and post-treated CNT yarns, while the friction force was noticeably lower due to the shorter embedded length and lower frictional interaction between the failure phases.

3.2. Comparison of pull-out failures. The failure modes of the pristine, post-treated, and pre-

infiltrated fibers were observed during and after the pull-out test. Similar failure modes were observed for pristine and post-treated CNT yarns, while the pull-out failure was distinctly different for the pre-infiltrated composite fibers, as depicted schematically in Figure 6. Similar to what has been observed previously [22], the pristine CNT yarns do not fail at the interface

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between the yarn and the matrix, but instead fail within the dry core of the yarn. Unlike most fiber reinforcements, which fail at the fiber/matrix interface, the shear strength of the yarn itself is lower than the shear strength of the yarn/matrix interface, due to the hierarchical structure of CNT yarn. The shear strength of the pristine CNT yarn is exceptionally low, resulting in embedded lengths as large as 14 mm being pulled out, as shown in Figure 2c. Other fiber systems require much smaller embedded lengths, often less than 1 mm, to ensure fiber pull-out

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instead of tensile failure of the fiber in the gauge-section. Increasing the shear strength of the

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fibers to eliminate the fiber shear failure, which would move the shear failure to the surface of

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the fiber, is expected to maximize the fiber-matrix load transfer of these CNT reinforcement

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materials.

Figure 6. Depiction of failure paths for (a) pristine, (b and c) post-treated CNT yarns, and (d) pre-infiltrated composite fibers embedded in a polymer matrix.

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The manufacture of the pristine CNT yarns used in this work includes a capillary densification step using chlorosulfonic acid, resulting in a dense material that is challenging to modify, especially by post cross-linking or infiltration with resin. A limited degree of infiltration of the polymer matrix into the pristine fiber surface is expected, and a thin outer sheath of polymerinfiltrated fiber is left behind in the tab after a pull-out test, as shown in Figure 7a. The thickness of this outer sheath can be estimated by comparing the diameter of the fiber before and after a

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pull-out test using a laser optical micrometer. In the case of the 2-ply pristine CNT yarns, the

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diameter was reduced from 136.0 to 122.6 µm after the test, resulting in an estimated penetration

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depth of ~ 6.7 µm.

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The failure mode of the post-treated CNT yarns was similar to that of the pristine CNT yarns, as shown in Figures 6b and c. The pull-out failures of the post-treated CNT yarns occurred

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within the yarns, and not at the yarn/matrix interface. Remnants of the yarn surface were again

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observed in the polymer tab after pull-out. A reduction in the yarn diameter after the pull-out test

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was also observed (Figures 6b and c). For example, the outer sheath of the m-PDA crosslinked and m-CPBA functionalized CNT yarns was estimated to be less than 3 µm thick based on diameter reductions from 152 ± 5 to 145 ± 4 µm and from 149 ± 2 to 142 ± 5 µm, respectively. Similar to the pristine CNT yarns, the post-treated fibers could be embedded to extremely large lengths of 11 mm and still experience pull-out failure instead of tensile failure at gauge-section, indicating that the shear strengths were particularly low. This dry-section failure mode and low shear properties observed in the pristine and post-treated CNT yarns during pull-out testing is in agreement with the low shear properties observed in macroscale composites [17] made from CNT yarns.

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The failure mode of the pre-infiltrated polymer/CNT composite fibers was distinctly different than those of the pristine and post-processed samples (Figure 6d). Samples that were tested at embedded lengths similar to what was used for pristine and post-treated yarns all resulted in tensile failure in the gauge-section of the fiber instead of pull-out failure. The embedded length had to be reduced below 2 mm before pull-out failure could be observed, indicating that there was a substantial increase in the shear strength of the material. In addition, there was not an

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appreciable difference in fiber diameters before and after pull-out. These results support the

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assertion that both the shear strength and the de-bond force of the pre-infiltrated composite fibers

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have increased enough to move the site of failure to the fiber/matrix interface.

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of ro -p re lP na ur Jo Figure 7. Post pull-out imaging and microscopy. (a) Optical micrograph image of the epoxy tab after pull-out of the 2-ply pristine CNT yarn, which was embedded in the entire 5 mm tab cross section. The visible black residue was transferred from the embedded yarn during pull-out. (b) Optical micrograph of PI-72 fiber after the pull-out test. FE-SEM images of PI-72 fiber after

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pull-out at (c) the end of the embedded fiber, (d) at the center of the embedded fiber end (marked in c), (e) at the top surface of the embedded fiber, and (f) at the middle of top surface (marked in e) in a higher magnification.

The morphology of pre-infiltrated polymer/CNT composite fiber is shown after pull-out in

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Figure 7. As shown in Figures 7e and f, the pulled-out composite fiber (PI-72), embedded 614

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µm into the epoxy matrix, had a relatively smooth surface, with less residual material remaining

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in the matrix after pull-out compared to the surface of the as-prepared fiber (Figures 3a and c).

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Some portions of the CNT composite fiber were torn off from the main body of the fiber and

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tangled at the end of the fiber (Figures 7b, c, and e). The CNTs in the composite fiber were

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completely wet-out by the infiltrated resin, as clearly seen in Figure 3j and Figures 7d and f, which were taken at a central section of the fiber-end in the cross-sectional view (Figure 7c) and

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a middle section of the fiber surface embedded into the matrix (Figure 7e). These results indicate that the shear interactions were substantially improved in the fully wetout composite fiber. The shear failure mode changes from a ring model with two phases (e.g., highly densified pristine CNT yarns, Figure 6a, and post-treated CNT yarns, Figures 6b and c) to an one cylinder (e.g., pre-infiltrated polymer/CNT composite fiber, Figure 6d) or laminar model similar to a fully infiltrated carbon fiber tow. However, unlike carbon fiber tows, the preinfiltrated polymer/CNT composite fiber is composed of a hierarchical structure of highly interconnected and branching CNT networks, which may result in even higher potential IFSS than traditional fibers [45]. The composite fibers reported in this work were not exhaustively optimized and it is expected that the IFSS could be further improved by improving interfacial

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interactions between the CNT tubes and the bundles with sizing or cross-linking applied before resin pre-infiltration, maximizing the contact area, and optimizing the resin content during the stretching and resistive heating assisted process. As discussed previously, the interfacial properties of the composite fibers were improved by over an order of magnitude compared to the pristine and post-treated CNT yarns because the dry

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region within the fiber was eliminated and the fiber was completely infiltrated with resin. For

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further confirmation of the full infiltration of the resin in the composite fiber, the twisted

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composite fibers with and without stretching were ion-milled to prepare a cross-sectional

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specimen. Figures 3g-j show cross-sectional FE-SEM images of the twisted but unstretched and the twisted with 72 % stretched (PI-72T) composite fibers, respectively, processed with the same

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resistive heating conditions. Without stretching, the thin sheets in the roving folded and stacked

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several layers to form micron-sized voids between the folded layers, which are difficult to

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infiltrate with resin. Also, a number of unfilled nanovoids were present inside the fiber.

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Mechanical stretching and twisting helped to remove these voids throughout the composite fiber as shown in Figures 3i and j. The PI-72T fiber became densely packed with a fairly round crosssection (Figure 3i) with a resin-rich surface (Figure 3f), indicating that the excess resin was squeezed out while stretching.

3.3. Additional characterization of pre-infiltrated polymer/CNT composite fibers. As shown

in Figure 8, there are very few accessible pores on the top surfaces of the 70 % stretched roving (Figure 8b) and 1-ply CNT yarn (Figure 8d), which prohibited resin penetration into the yarn. However, the resin easily penetrates into the pristine CNT roving (Figure 8a) through open pores

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during composite fabrication. The majority of CNT bundles in the roving, with diameters of a few tens of nanometers, were aligned to the yarn drawing (manufacturing) direction, as indicated by a 2D order parameter of 0.68 (Table 2) and an IG///IG⊥ [G band (1578 cm-1) intensity ratio between the parallel and the vertical polarities] of 6.56, as shown in Figure 8e. The CNT bundles form a network structure that continuously branches and merges in a way that prevents separation during mechanical stretching with the diluted resin solution. Under mechanical

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stretching using the custom-built continuous stretcher (Figure 8b) or chemical stretching during

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manufacturing (Figure 8d), the CNTs become highly aligned to the stretching direction and the

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accessible voids are reduced significantly. For the yarns stretched to 70 % without resin, the 2D

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order parameter and IG///IG⊥ ratio of the stretched yarn (Figures 8b and f) were 0.79 and 13.2,

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respectively, which were very similar to the PI-72 fiber (0.81 for 2D order parameter and 16.0

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for IG///IG⊥, Figure 8g), and which also agrees well with previous literature reports [9, 46]. The surface morphology of the PI-72 fiber shown in Figure 8c differs from those from the 70 %

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stretched and 1-ply CNT yarns with relatively large bundles at the top surfaces that are most

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likely due to the resin coating at the beginning of the stretching process. During the resistive heating assisted infiltration and cure process, the individual CNTs or bundles were coated by API-60 and also experienced high temperature due to the applied voltage (>20 V), which created surface defects by oxidation of the CNT as shown in the IG/ID decrease from 8.4 (CNT roving) to 4.4 (PI-72 fiber).

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of ro -p re lP na ur Jo Figure 8. FE-SEM images of (a) the pristine CNT roving, (b) 70 % stretched CNT roving, (c) PI-72 fiber, and (d) 1-ply pristine CNT yarn. Polarized Raman spectra of (e) the pristine CNT roving, (f) 70 % stretched CNT roving, and (g) PI-72 fiber.

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Table 2. Physical and mechanical properties of pristine yarns and pre-infiltrated polymer/CNT

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composite fibers. aData was provided from the manufacturer.

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4. CONCLUSIONS

The interfacial shear properties of pristine yarns, post-treated CNT yarns, and pre-infiltrated

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polymer/CNT composite fibers embedded in a polymer matrix were compared using a single

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fiber pull-out test. For pristine and post-treated CNT yarns, which are highly densified starting material formats, treatments had limited resin penetration depth and the shear failure occurred within the dry core of the CNT yarn which has lower shear strength than the fiber/matrix interface. While it is possible that the post-treatments were improving the yarn/matrix interface since failure was not occurring at the interface and was instead occurring within the dry core of the treated yarn, the post-treatments of the highly densified CNT yarns were ineffective and the IFSS improved only marginally (~ 2 MPa) over the pristine material. To prevent the dry section failure of the yarn under the shear force, pre-infiltrated composite fibers were fabricated with the CNT roving material, which has a low-density, loosely networked structure. The CNT roving was composited with a resistive heating assisted infiltration and cure process and mechanically

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stretched with a custom-built continuous stretcher. The measured apparent IFSS of the preinfiltrated composite fibers was improved over the pristine CNT yarns by approximately an order of magnitude (~ 20 - 60 MPa). The pre-infiltrated composite fibers were wet-out through the cross section, thereby eliminating the dry-core shear failure observed in the pristine and posttreated CNT yarns. In this study, the apparent IFSS of the pre-infiltrated composite fibers appeared to be influenced more by surface roughness than the tenacity of the studied fibers. The

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SFPT was used to screen the different processing approaches in this work and identify promising

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candidates. Continued work to evaluate the approaches that were screened in this work with

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more quantitative test methods and large composite samples is currently ongoing. Additional

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work to improve the strength and consistency of the CNT roving and to optimize the resin

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chemistry for the pre-infiltration process is also needed before composite fabrication processes

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can be optimized. The results of this effort provide insight for designing CNT yarn (or fiber) reinforced composite processes for structural applications and for understanding composite

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properties, such as fracture toughness, that depend on fiber/matrix interactions. ACKNOWLEDGMENT

This work was funded through the Super-lightweight Aerospace Composites project under the NASA Space Technology Mission Directorate Game Changing Development Program.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: