Structural Polymer-Based Carbon Nanotube Composite Fibers: Understanding the Processing–Structure–Performance Relationship

Abstract

Among the many potential applications of carbon nanotubes (CNT), its usage to strengthen polymers has been paid considerable attention due to the exceptional stiffness, excellent strength, and the low density of CNT. This has provided numerous opportunities for the invention of new material systems for applications requiring high strength and high modulus. Precise control over processing factors, including preserving intact CNT structure, uniform dispersion of CNT within the polymer matrix, effective filler–matrix interfacial interactions, and alignment/orientation of polymer chains/CNT, contribute to the composite fibers’ superior properties. For this reason, fabrication methods play an important role in determining the composite fibers’ microstructure and ultimate mechanical behavior. The current state-of-the-art polymer/CNT high-performance composite fibers, especially in regards to processing–structure–performance, are reviewed in this contribution. Future needs for material by design approaches for processing these nano-composite systems are also discussed.

Full text

Materials 2013, 6, 2543-2577; doi:10.3390/ma6062543
materials
ISSN 1996-1944
www.mdpi.com/journal/materials
Review
Structural Polymer-Based Carbon Nanotube Composite Fibers:
Understanding the Processing–Structure–Performance
Relationship
Kenan Song, Yiying Zhang, Jiangsha Meng, Emily C. Green, Navid Tajaddod, Heng Li and
Marilyn L. Minus *
Department of Mechanical and Industrial Engineering, Northeastern University, 334 Snell Engineering
Center, 360 Huntington Avenue, Boston, MA 02115, USA; E-Mails: song.k@husky.neu.edu (K.S.);
zhang.yiyi@husky.neu.edu (Y.Z.); meng.ji@husky.neu.edu (J.M.); green.e@husky.neu.edu (E.C.G.);
tajaddod.n@husky.neu.edu (N.T.); li.heng@husky.neu.edu (H.L.)
* Author to whom correspondence should be addressed; E-Mail: m.minus@neu.edu;
Tel.: +1-617-373-2608; Fax: +1-617-373-2921.
Received: 2 May 2013; in revised form: 21 May 2013 / Accepted: 6 June 2013 /
Published: 20 June 2013
Abstract: Among the many potential applications of carbon nanotubes (CNT), its usage to
strengthen polymers has been paid considerable attention due to the exceptional stiffness,
excellent strength, and the low density of CNT. This has provided numerous opportunities
for the invention of new material systems for applications requiring high strength and high
modulus. Precise control over processing factors, including preserving intact CNT
structure, uniform dispersion of CNT within the polymer matrix, effective filler–matrix
interfacial interactions, and alignment/orientation of polymer chains/CNT, contribute to the
composite fibers’ superior properties. For this reason, fabrication methods play an
important role in determining the composite fibers’ microstructure and ultimate mechanical
behavior. The current state-of-the-art polymer/CNT high-performance composite fibers,
especially in regards to processing–structure–performance, are reviewed in this
contribution. Future needs for material by design approaches for processing these
nano-composite systems are also discussed.
Keywords: carbon nanotubes; polymer; mechanical properties; preparation; synthesis;
dispersion; interphase; alignment; applications
OPEN ACCESS

Materials 2013, 6 2544
1. Introduction
Since the birth of polymer science in the 1930s, these materials have dominated the market in terms
of their versatility for product applications. These materials have been utilized in the form of films,
fibers, sheets, and coatings. Today, most of the synthetic polymer fibers in use span applications such
as clothing, carpets, ropes, and reinforcement materials. Some of these fibers include polyamides such
as nylon, polyesters [e.g., polyethylene terephthalate (PET) and polybutylene terephthalate (PBT)],
polyolefins [e.g., polypropylene (PP) or polyethylene (PE)], vinyl polymers [e.g., poly(vinyl alcohol)
(PVA) and poly(vinyl chloride) (PVC)], elastomers (e.g., polyurethane (PU) and spandex), and acrylic
fibers (e.g., polyacrylonitrile (PAN)) [1]. In addition, high-performance polymer-based fibers with
high stiffness and/or tenacity include Dyneema® and Spectra® (i.e., ultra-high molecular weight
polyethylene (UHMWPE)-based fibers), Twaron® and Kevlar®, and Zylon® fibers (i.e., aromatic-based
polymers such as poly(p-phenyleneterephthalamide) (PPTA) and poly(p-phenylenebenzobisoxazole)
(PBO)) [2]. Also included is PAN, which is the dominant precursor fiber for the carbon fiber industry.
The typical properties for these materials are listed in Table 1.
Table 1. Typical mechanical properties’ values for commercially available polymer fibers
used for textile and high-performance applications.
Classification of Fibers Fiber type Strength
(GPa)
Modulus
(GPa)
Strain
(%) References
Textile Fibers
Polyamide 0.91 9.57 37 [3]
Polyesters 0.7 to 0.9 12 to 17 7 to 37 [4]
Vinyl Fibers 0.3 to 0.66 ~4.5 to 7.5 <40 [4,5]
Elastomers 0.004 to 0.009 0.01 to 0.03 >500 [5]
High-Performance
Fibers
Spectra® 2.5 to 3.6 97 to 133 2.8 to 4.5 [5]
Dyneema® 3.4 113 3.5 [5]
Kevlar® 1.44 to 3.6 62 to 190 1 to 4.4 [3]
Zylon® 4.2 280 2.5 [3]
M5 up to 9 330 to 350 1.2 to 1.5 [3,4]
PAN-based
carbon fibers 2.5 to 3.8 227 to 405 0.8 to 1.76 [3,4]
Despite the significant amount of progress made towards producing high-performance fibers from
polymer materials, the mechanical properties still remain only a fraction of the expected theoretical
values for these materials. Several technological developments in recent years have been used to improve
the high-performance properties of polymer-based fibers. For example, manufacturing processes by
sea/island composite-spinning technology have progressed to produce commercial nano-fibers
with ~700 nm uniform diameters and high tenacity [6,7]. Commercial products, including functional
sportswear (i.e., golf gloves), inner wear, skin-care products, filters, and precision grinding cloths (i.e.,
polishing cloths), taking advantage of these polyester nano-fibers have been developed due to the large
surface area, high adsorption, good dispersion, and filtration effects belonging to these thin fibers [6].
Other opportunities to produce high-performance polymer materials include using fillers to produce
composites. Carbon fiber and glass fiber composites were first produced in the 1960s and 1970s,

Materials 2013, 6 2545
leading to disruptive technological evolutions in the field of material science [8–11]. The first polymer
composites (i.e., fiber glass) revolutionized the boating industry, and later in the 1960s, the advent of
carbon fibers ushered in many disruptive technologies for producing polymer composites and
increasing their applications. Since that time, carbon-fiber-reinforced polymer composites (CFRP)
have remained a major standard for polymer-based materials in high-performance applications.
Currently, the carbon fiber (CF) market is dominated by the U.S. and Japan, where production is
expected to increase to 80,000 tons by 2016 [12]. The use of CF materials has increased at an average
annual rate of ~12% for the last 23 years, and this has been attributed to the development of new
energy technologies (e.g., wind energy) and industrial applications requiring lightweight
materials [13]. The one major hindrance of CF usage has been the high cost, and for that
reason ~47% of the CF usage is within the aerospace sector. Pseudo one-dimensional fibers such as
aluminum, glass, boron, silicon carbide, and carbon nano-fibers (CNF) have also been used over the
years as fillers in composites. Typical composite stiffness and strength properties range from 230 to
725 GPa and 1.5 to 4.8 GPa, respectively [14].
The recognition of multi-wall carbon nanotubes (MWNT) in 1991 [15] and single-wall carbon
nanotubes (SWNT) in 1993 [16] brought about a new influx of research in lightweight
high-performance reinforced polymers. As compared to the conventional carbon fibers, Young’s
modulus and tensile strength of these tubular graphitic materials have subsequently been measured to
be ~1 TPa [17–20] and ~10 to 150 GPa [21–24], respectively. Therefore, composites incorporating
carbon nanotubes (CNT) have received a great deal of attention in both academia and industry for their
potential replacement of carbon fibers in polymer-based reinforced materials. Several reviews have
already focused on summarizing the property enhancement of polymers by CNT [25–32]. CNT have
been heralded as a game changer for producing next-generation high-performance materials that would
trump the properties of current CFRPs. However, one major hangup has also been the cost of these
materials at small-scale production levels. One potential route toward reducing the cost of the
CNT-based composites is through using small quantities of CNT to reinforce the polymer for
high-performance applications.
A brief listing of the typical property improvements in polymer/CNT fibers is provided in Table 2. To
date, the best polymer/CNT fibers have tensile strengths ranging from ~0.1 to 5 GPa and modulus values
from ~5 to 200 GPa. However, in terms of repeatability for fabricating these composites, the typical
tensile strength values range from 0.5 to 2 GPa [33]. Structural composite fibers are of great importance
for several industrial uses including, but not limited to, automotive, aerospace, consumer products,
transportation, and construction [34]. Additionally, due to the unique combination of properties, usage of
CNT in polymer composites not only improves strength and modulus but can also result in enhancements
in chemical resistance, thermal conductivity, electrical conductivity, and dimensional stability [34]. For
this reason, fundamental understanding for producing new high-performance materials is necessary. This
contribution will focus mainly on the relationship between polymer/CNT fiber fabrication methods and
the micro-structural development during processing. These fundamental issues are significant and need
to be addressed for material design toward commercialization of polymer-based CNT composite fibers
meant for high-performance technologies.

Materials 2013, 6 2546
Table 2. Summary of the typically reported mechanical properties for polymer-based
carbon nanotube composite fibers.
References Sample
(polymer + wt % CNT)
Mechanical properties
Elastic modulus
[GPa] Strength [GPa] Strain [%] Toughness
[35,36]
Poly(vinyl alcohol) (PVA)
+ >60 wt % SWNT 9 to 15 0.15 ~3 –
PVA + ~60 wt % SWNT 80 1.8 >100 570 J·g−1
[37] PVA + ~60 wt % SWNT 40 0.3 >400 600 J·g−1
[38] Commercial PVA fiber 40 1.6 7 –
PVA + >60 wt % SWNT 78 1.8 ~40 120 ± 152 J·g−1
[39] PVA + 2–31 wt % SWNT Up to 244 Up to 2.9 ~3–10 –
[40]
PVA 21.8 ± 3.0 1.2 ± 0.3 11.4 ± 1.7 55.8 ± 12.3 J·g−1
PVA + 10 wt % SWNT 36.3 ± 1.3 2.5 ± 0.1 10.7 ± 0.7 101.4 ± 11.4 J·g−1
PVA + 10 wt % SWNT 119.1 ± 8.6 4.4 ± 0.5 9.7 ± 1.1 171.6 ± 30.4 J·g−1
[41] PVA ~13 ~0.4 ~15 –
PVA + 1 wt % SWNT ~17.5 ~1.2 ~17.5 –
[42]
PVA 45 ± 7 1.0 ± 0.1 5.3 ± 0.3 22 ± 4 J·g−1
PVA + 1 wt % SWNT 60 ± 6 1.4 ± 0.1 4.9 ± 0.5 29 ± 6 J·g−1
PVA 48 ± 3 1.6 ± 0.1 6.5 ± 1.4 40 ± 6 J·g−1
PVA + 1 wt % SWNT 71 ± 6 2.6 ± 0.2 6.2 ± 0.7 59 ± 7 J·g−1
[43]
Polyacrylonitrile (PAN) 22.1 ± 1.2 0.90 ± 0.18 7.4 ± 0.8 35 ± 9 MPa
PAN + 0.5 wt % SWNT 25.5 ± 0.8 1.06 ± 0.14 7.2 ± 0.6 41 ± 8 MPa
PAN + 1 wt % SWNT 28.7 ± 2.7 1.07 ± 0.14 6.8 ± 0.8 39 ± 8 MPa
[44,45] Carbonized PAN 302 ± 32 2.0 ± 0.4 0.68 ± 0.04 –
Carbonized PAN + 1 wt % SWNT 450 ± 49 3.2 ± 0.4 0.72 ± 0.05 –
[46]
Poly(p-phenylenebenzobisoxazole)
(PBO) 138 ± 20 2.6 ± 0.3 2.0 ± 0.2 −
PBO+>60 wt % SWNT 167 ± 15 4.2 ± 0.5 (~50%
increase) 2.8 ± 0.3 −
[47]
Polypropylene (PP) 6.3 0.71 18.9 7.93 dN/tex
PP + 0.5 wt % SWNT 9.3 0.84 19.1 9.37 dN/tex
PP + 1 wt % SWNT 9.8 1.03 26.6 11.5 dN/tex
[48]
Nylon 6 0.44 0.045 − −
Nylon 6 + 0.1 wt % SWNT 0.54 0.086 − −
Nylon 6 + 0.2 wt % SWNT 0.66 0.093 − −
Nylon 6 + 0.5 wt % SWNT 0.84 0.083 − −
Nylon 6 + 1.0 wt % SWNT 1.15 0.083 − −
Nylon 6 + 1.5 wt % SWNT 1.2 0.075 − −
[49]
Ultra-high molecular weight
polyethylene (UHMWPE) 2.42 ± 0.40 0.11 ± 0.002 402.0 ± 20.1 361.8 ± 22.9 MPa
UHMWPE + 5 wt % MWNT 2.62 ± 0.32 0.13 ± 0.004 540.4 ± 104.7 593.2 ± 114.5 MPa
UHMWPE 122.6 ± 1.9 3.51 ± 0.13 4.03 ± 0.15 76.7 ± 7.5 MPa
UHMWPE + 5 wt % MWNT 136.8 ± 3.8 4.17 ± 0.04 4.65 ± 0.35 110.6 ± 10.5 MPa

Materials 2013, 6 2547
2. General Fabrication Procedures for Polymer/CNT Fibers
In general, when discussing polymer/CNT composites, two major classes come to mind. First, the CNT
nano-fillers are dispersed within a polymer at a specified concentration, and the entire mixture is fabricated
into a composite. Secondly, as-grown CNT are processed into fibers or films, and this macroscopic CNT
material is then embedded into a polymer matrix [50]. This review paper will focus on the first class of
polymer/CNT composite materials to explore their processing–structure–property relationships.
The four major fiber-spinning methods (Figure 1) used for polymer/CNT composites from both
the solution and melt include dry-spinning [51,52], wet-spinning [53], dry-jet wet spinning (e.g.,
gel-spinning [54]), and electro-spinning [55,56]. An ancient solid-state spinning approach has been
used for fabricating 100% CNT fibers from both forests and aerogels [57–60]. Regardless of the
processing technique, in order to develop high-quality fibers many parameters need to
be well controlled. In general, all spinning procedures involve (i) fiber formation;
(ii) coagulation/gelation/solidification; and (iii) drawing/alignment. For all of these processes, the even
dispersion of the CNT within the polymer solution or melt is very important. However, in terms of
achieving excellent axial mechanical properties, alignment and orientation of the polymer chains and
the CNT in the composite is necessary. Fiber alignment is accomplished in post-processing such as
drawing/annealing and is key to increasing crystallinity, tensile strength, and stiffness [61].
Figure 1. Schematics representing the various fiber processing methods (a) dry-spinning;
(b) wet-spinning; (c) dry-jet wet or gel-spinning; and (d) post-processing by hot-stage drawing.

Materials 2013, 6 2548
Alignment in polymer/CNT composite fibers is dependent on the polymer chain conformation,
CNT morphology, and dispersion in the matrix. In terms of polymer conformation, for linear flexible
polymers achieving high orientation and extension requires uncoiling and disentanglement of the
chains. Very stiff polymer rod-like chains (e.g., aromatic polymers) are able to self-assemble and form
aligned structures during processing. In this way, the fiber microstructure can be very different and this
translates to the composite morphology. The CNT morphology and dispersions are also important,
since these have influences on the polymer structural development. The processing of these composites
has a direct effect on the ultimate structure–property relationship for polymer/CNT fibers.
3. Micro-Structural Development in Polymer/CNT Fibers
The overall picture of mechanical performance for polymer/CNT fibers produced at the research level
shows a broad range of properties (Figure 2). These fibers were produced using several fabrication
methods. As mentioned, the discovery of CNT ushered in a large amount of research efforts focused on
utilizing these nano-materials to make polymer composite fibers to capture these exceptional properties
(i.e., 1 TPa in tensile modulus and 10 to 150 GPa [21–24] in tensile strength). However, this realization
has been lacking in spite of there being an additional host of nano-carbon materials produced since their
discovery (i.e., SWNT, MWNT, and vapor-grown carbon nano-fibers (VGCNF), as well as layered
graphitic materials). This disappointment has led to a decline in the hype surrounding such composites
and a shift of research focus to some of the other unique features of these materials such as their
electrical [57,62–93], thermal [57,68,73,76,86,94–103], and optical properties [104–109]. This rise and
fall of interest in polymer nano-composite research is analogous to what happened between 1832 and
1939 when polymers were originally discovered but not well understood [110,111]. As a result, these
materials (polymers) went underutilized significantly for over a century. In order to avoid a similar delay,
it is important to persist in the fundamental understanding of these systems and how to process
nano-composites with tailored micro-structure for mechanical performance.
Figure 2. Summary of Young’s modulus, tensile strength, and strain-to-failure properties
for various polymer/CNT fibers produced at the research scale which exhibit properties
similar to high-performance commercial fibers [35,36,38–47,112–116] (Note: □/■ symbols
represent tensile strength/modulus properties for high-performance fibers, and ∆/▲
symbols represent tensile strength/modulus properties for textile-grade fibers).

Materials 2013, 6 2549
The inherent properties of CNT assume that the structure is well preserved (i.e., large-aspect-ratio
and without defects). Going further, the first step toward effective reinforcement of polymers using
nano-fillers is to achieve a uniform dispersion of the fillers within the hosting matrix, and this is also
related to the as-synthesized nano-carbon structure. Secondly, effective interfacial interaction and
stress transfer between CNT and polymer is essential for improved mechanical properties of the fiber
composite. Finally, similar to polymer molecules, the excellent intrinsic mechanical properties of CNT
can be fully exploited only if an ideal uniaxial orientation is achieved. Therefore, during the fabrication
of polymer/CNT fibers, four key areas need to be addressed and understood in order to successfully
control the micro-structural development in these composites. These are: (i) CNT pristine structure;
(ii) CNT dispersion; (iii) polymer–CNT interfacial interaction; and (iv) orientation of the filler and
matrix molecules (Figure 3). This review will highlight some key papers that have focused on these
areas as a means to tailor the composite structure and advance the mechanical performance of the
polymer nano-composite.
Figure 3. Four major factors affecting the micro-structural development in polymer/CNT
composite fiber during processing.
A further analysis of the published literature also shows an interesting trend, whereby the percent
increase in mechanical properties for polymer composite fibers is related to the inherent polymer
structure (Figure 4). It is already known that the polymer chain conformation plays a role in the
structural development of the fiber, which translates to the composite material, as well. Composite
fibers fabricated using aromatic polymer matrices exhibit significantly lower percent increase in
mechanical properties as compared to flexible polymer matrices. For rod-like (e.g., aromatic) polymer
chains in high-performance fibers, the addition of CNT fillers improves the composite properties
mainly due to reinforcement. However, for the more flexible polymer in textile-grade fibers the
improvement in mechanical performance is much more pronounced and may be due to factors beyond
reinforcement. Such factors include polymer chain orientation, as well as polymer crystallization due
to the presence of the CNT in the matrix. This contribution will also outline some of the studies
highlighting such phenomena.

Materials 2013, 6 2550
Figure 4. Average percent increase comparison between control fibers (no fillers) and
composite fibers for both the Young’s modulus and tensile strength properties. The graph
compares the differences in percent increase for the high-performance and textile-grade
polymeric fiber materials [35,36,38–47,112–116].
3.1. CNT Structure and Dispersion
Structural control of CNT graphitic materials is mostly influenced by the synthesis processes, which
determines their aspect ratio, morphology and dimensions, crystalline structure, purity, and properties.
Presently, a large amount of CNT can be prepared by electric arc discharge [117,118], laser
ablation [119] and chemical vapor deposition (CVD) [120] methods. CVD is the most dominant
method of large volume CNT production and typically uses fluidized bed reactors that enable uniform
gas diffusion and heat transfer to metal catalyst nano-particles [121]. A typical as-synthesized batch of
CNT has large tube variations in terms of the type (i.e., metallic vs. semiconducting), purity, and
structure uniformity (i.e., length, elongated or coiled conformation, tube or bundle diameter, aspect
ratio, morphology consistency, and crystallinity). All of these factors influence their electric or thermal
conductivity, and mechanical properties [17–22]. Contaminants generated during synthesis also
influence the final mechanical properties and often require costly thermal annealing and chemical
treatment for their removal. These steps can introduce defects in CNT sidewalls and shorten CNT
length. Obtaining highly purified and uniform CNT batches is a major challenge that has a significant
impact on their use for applications in polymer/CNT fiber composites. The variation in the CNT from
batch-to-batch results in significant property variation [122], which ultimately leads to disparities in
the properties of the subsequent composite materials [30–32].
CNT are characterized by tubes with different wall numbers, for example, SWNT, double-wall
carbon nanotubes (DWNT), few-wall carbon nanotubes (FWNT), and MWNT. As-produced and
purified nanotubes also possess surface defects and are not geometrically identical (i.e., chirality,
diameter, length). For this reason, the actual mechanical strength, as well as other properties,
significantly differs from the theoretical predictions. As-synthesized CNT batches are normally
randomly oriented and entangled with one another (Figure 5). This is especially the case for SWNT,
DWNT, and FWNT. Due to the high specific area, >103 m
2/g for these nanotubes [123] small

Materials 2013, 6 2551
attractive forces (i.e., ~0.5 eV) are able to form single CNT contacts that result in bundle
formation [124] leading to aggregation. This makes the process of dispersion and separation into
individual tubes for uniform distribution into the polymer matrix a major challenge.
Figure 5. Schematic representing the disentanglement, exfoliation, and length scission
processes for SWNT bundles during dispersion.
Disentanglement and exfoliation processes including centrifugation, homogenization, stirring, and
especially ultrasonic dispersion typically result in a sacrifice of the structural integrity and length
preservation of the nanotubes (Figure 5). However, both the dispersion in terms of tube bundle size
and length are very important to produce polymer composites which fully utilize the CNT fillers.
Studies of dispersion effects on exfoliation and length reduction of CNT have been conducted and
reported by several research groups [125–130]. While the reduction of the CNT length which occurs
during dispersion is not always desirable for producing high-performance materials, improving
exfoliation is desired and has a significant positive impact on processing [131].
Common dispersing methods that have been utilized are generally the modification on SWNT
surfaces by covalent [132,133] or non-covalent treatments [134,135]. Non-covalent dispersants are
categorized into small molecules and polymers. Exfoliating SWNT bundles through a polymer-wrapping
process has been shown to aid the formation of interphase regions in the composite. A very high
degree of exfoliation of SWNT can be obtained with low SWNT concentration (<20 mg/L) in organic
amide solvents such as N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP) and
N,N-dimethylacetamide (DMAc) [136–138]. Without chemical treatment, the formation of true
solutions can only be obtained at very low concentrations of SWNT (typically below 20 mg/L) [136]. In
this case, a minimal enthalpy of mixing close to zero is achieved, indicating athermal
solubility [136,139]. Various dispersing methods for SWNT materials are outlined in Figure 6.

Materials 2013, 6 2552
Figure 6. Currently used methods for SWNT dispersion towards fabrication of
polymer/CNT nano-composites.
Covalent functionalization of SWNT has been used to significantly improve the nanotube solubility
and chemical compatibility for reinforcing various composite materials. In this process, strong acids or
other strong oxidizing agents are used to treat SWNT to create open sites (i.e., break bonds in the
graphitic structure) and subsequently attach various functional groups to the open-end and/or defect
sites [132] (Figure 7a). Typical methods for covalent functionalization on sidewall include fluorination,
ozonolysis, organic functionalization, osmylation, and azomethineylides [132]. Some example additions
are aryl (diazonium [140]), fluorine (fluorination [141]), alkyl (radical chemistry [142], Billups
reaction [143]), cyclopropane (Bingel reaction [144], dichlorocarbene [145]), pyrrolidine (Prato
reaction [146]), and aziridine (nitrene [147]) (Figure 7a). Covalent functionalization has been shown to
greatly improve CNT dispersion in polymer matrix [148–151], and play a critical role in both thermal
and electrical properties of CNT/polymer composites [152–154]. Chemical functionalization increases
the inter-tube contacts (i.e., useful for building up a conductive network) and provides more possibilities
to bond the nanotubes to a matrix due to reactive chemical groups. On the other hand, covalent surface
treatments can destroy tube structure, resulting in shortening of nanotubes [155,156], creation of defects
in the graphitic structure of CNT walls [133,156,157], and in some cases, unzipping of the
tube structure. Consequently, chemical functionalization will decrease the mechanical properties of
CNT [158]. Non-covalent dispersing methods have also been developed to exfoliate SWNT
bundles into individual tubes in different solvents using various anionic, cationic, nonionic
surfactants [134,159] (Figure 7b), or polymers [135,160]. The adherence of chemical moieties or
polymer molecular wrapping on SWNT surface occurs due to the non-covalent supramolecular
interactions, including hydrophobic–hydrophobic interactions, van der Waals forces, π–π interactions,
hydrogen bond linkage, and electrostatic attraction [161]. These non-covalent interactions eliminate
the chemical modification of the graphitic structure (thus preserving mechanical, electrical, and optical

Materials 2013, 6 2553
characteristics of the nanotube), and enable the CNT to have improved interactions/solubility
in more solvents.
Figure 7. (a) Schematic representation of SWNT functionalization by covalent bonding;
(b) Schematic of SWNT surface modification by small-molecule surfactants.
Dispersion and structure preservation of the nanotube are important to the overall mechanical
performance of the composite. As mentioned, in addition to detangling and exfoliation, nanotube
length is also sacrificed during dispersion. In the CNT composite, the CNT length is generally on the
order of 500 nm to 1 μm. Below a critical length, the CNT cannot transfer its stiffness or strength
properties to the polymer matrix, and this results in premature failure of the composite. For this reason,
it is recognized that preserving CNT length and structural perfection during composite preparation is

Materials 2013, 6 2554
also desirable. In an ideal binary bulk composite, the same length registry for the matrix and filler
materials is assumed, and this results in an overestimation of the mechanical contributions. To
demonstrate the importance of the length contribution of the CNT an example utilizing a composite
materials mechanics viewpoint is discussed. An approximate estimation of the CNT’s strengthening
mechanism as a function of aspect ratio is determined by using a modified rule-of-mixture (ROM)
approach [14,162,163]. Equations (1) to (3) are derived from the ROM approach [14,162,163] and
used for this analysis.
(1)
() (2)
() (3)
where A and is a factor given by Equations (4) and (5):
(4)
(5)
For Equations from (1) to (5) the parameters are defined as follows. Ec, ECNT, and Ep are the modulus
of composite, CNT and polymer matrix, respectively. l, Di, and D are the length, interior and exterior
diameters of CNT, respectively. τ is the shear strength at interphase between polymer and CNT, Vf is the
CNT filler loading in the composites, and lc is the critical length above which strength of CNT can be
transferred efficiently to the polymer matrix. The SWNT modulus, strength, and interfacial
shear strength are taken to be 1 TPa, 50 GPa, and 100 MPa (i.e., based on computational
predictions) [20,164,165], respectively. To demonstrate the importance of the length contribution in the
composite, Figure 8 is plotted by using polymer matrix modulus values ranging from 1 to 100 GPa, and
strength values ranging from 0.01 to 5 GPa. These values correspond to the typical properties reported
for polymers used in CNT composite processing [35,36,38–47,112–116]. The modulus and strength
increase with respect to aspect ratio is shown in Figure 8. It can be seen that both stiffness and strength of
the fibers scale with aspect ratio. A similar trend has also been reported for composite films [26].
It is clear that the dispersion of the CNT in terms of exfoliation, distribution, and length
preservation are all-important aspects affecting the development of the composite microstructure. Each
factor is dependent on the other and finding the right balance remains a challenge. Although several
methods for dispersion have been discussed, it is important to recognize that without good polymer
nanotube interaction, even well-dispersed CNT may not provide effective reinforcement of the matrix.
To improve polymer-CNT interactions, interfacial development is necessary. The following
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

Materials 2013, 6 2555
Section 3.2 discusses some of the mechanisms for the development of interfacial structures in the
polymer composite fibers.
Figure 8. (a) Modulus/loading ratio; (b) strength/loading ratio as a function of SWNT
aspect ratio for composite fibers with full alignment. Ec and σc are the composite modulus
and tensile strength, l and D are the length and diameter of the CNT, Vf is the volume of
CNT in the composite fibers.
3.2. Interfacial Development in Polymer/CNT Fibers
The interphase can be defined as the three-dimensional boundary between the fiber and the matrix. It
has been recognized that control over the interphase (i.e., communication between the matrix and
nano-filler) regions will have significant effects on the macroscopic performance of the
material [166]. To do this will require deeper fundamental understanding of the nano-composite system
in terms of morphology formation during processing. The interfacial interaction occurs through several
mechanisms: (i) mechanical coupling, micro-mechanical interlocking and polymer chain-CNT
entanglement; (ii) physical interaction, including van der Waals forces, electrostatic forces, or epitaxial
crystal formation; and (iii) chemical interactions. As mentioned in the previous section, these chemical
interactions include covalent bonding and physical bonding such as surfactant-assisted dispersion of
CNT [133], plasma polymerization [167], and polymer wrapping [168,169].
Several studies have focused on understanding the strength of the interface for polymer/CNT materials.
For PVA/CNT composites, it was found that the shearing resulted in fracture of the matrix before the
breakage of the interphase polymer [170]. The shear stress was determined to be around 40 MPa, which is
in reasonable agreement with predicted values of ~50 MPa [170]. Other computational works have also
been carried out to predict the interfacial shear stress (IFSS). Polymer systems such as polystyrene
(PS) [171], epoxy [172], poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV),
and poly(phenylacetylene) (PPA) [173] have been calculated using molecular dynamics, where the
calculated IFSS was dependent on both the polymer and CNT. In such cases the IFSS values ranged
from 18 to 186 MPa.
Apart from the calculations and simulations, direct measurements have also been reported.
The techniques and devices for these measurements include scanning electron microscopy
(SEM) [20], transmission electron microscopy (TEM) [174], atomic force microscopy

Materials 2013, 6 2556
(AFM) [164,175], and scanning probe microscopy (SPM) [176]. These reported values range from
0.02 to 500 MPa [39,116,126,165,174,176]. The larger IFSS values are consistent with composites
where covalent bonding is present at the interphase (i.e., functionalized CNT). Values of 0.5 GPa
estimated by Wagner et al. [174], and 0.35 GPa measured by Cooper et al. [176] were measured
utilizing the AFM and were attributed to covalent bonding between CNT and polymer. To date, the
majority of interphase measurements and predictions have focused on either pristine CNT or
functionalized CNT embedded in amorphous polymer melts. Less is known about the interfacial
mechanical properties of crystalline polymer at the CNT interphase, especially in cases where the
polymer is able to form ordered phases along the CNT length.
Several recent papers have highlighted the importance of crystalline interphase formation in these
composites [39,42,43,177–179]. It has been observed that CNT can nucleate and template the growth
of ordered polymer crystals in several polymer systems including PE [180–185], nylon
6,6 [182], PVA [186], PAN [187], poly(butylene terephthalate) (PBT) [188–190], isotactic
polypropylene (iPP) [191], poly(L-lactide) (PLLA) [191], poly(e-caprolactone) (PCL) [192], and
polyethylene-b-poly(ethylene oxide) (PE-b-PEO) block copolymer [193]. One of the dominant
reinforcement mechanisms in polymer/CNT composites has been suggested to be the presence of
ordered polymer interfacial coating structure near CNT [194]. This ordered structure is able to form
due to the ability of CNT to interact specifically with the polymer matrix. Ordered or crystalline
polymer structure in polymer nano-composites is mechanically stronger than amorphous structure due
to the presence of fewer defects or less disordered regions. Therefore, it is important to study
CNT-induced polymer crystallization to control these mechanisms during the formation of the
interphase in the polymer/CNT composites.
On a molecular level, a decreased interpenetration/entanglement of chains near a solid interface
cause chain configuration, the configuration–change energies, and repeat unit-surface interaction
energies to change [195]. In addition, changes in reaction kinetics and interfacial mobility (i.e., due to
crosslink density) can also affect the system [195]. Glass transition, polymer diffusion, nanotube
diffusion, crystalline structure, crystallization kinetics, and properties can also be altered [195]. This
phenomenon is not seen with other commonly used micro-scale fillers [195]. Additional work has
shown that the interphase polymer morphology is completely different from the bulk polymer in the
composite, and this translates to high modulus and tensile strength values (i.e., modulus between 5 and
400 GPa and strength >1 GPa). Examination of these interphase regions by microscopy shows that
they display crystalline perfection [42–44,178].
As previously mentioned, several works have also shown the ability of the nanotube to nucleate
polymer crystal growth at the interphase [182,196], as well as template crystal growth and orientation
in polymers [42,43,177,178,181,185,197]. This templating effect of CNT in polymer composites has
been proven to have an effective contribution toward the stress transfer mechanism of load between the
polymer matrix and filler [42,179] (Figure 9). In such cases where templated interphase structure was
found to be present at the interphase, the mechanical properties for the composite were significantly
increased. It is also interesting that the overall crystallinity value for the composite as compared to the
control fibers is relatively the same. This implies that while a portion of the matrix polymer forms a
highly ordered interphase structure the bulk-matrix remains semi-crystalline and relatively disordered.
It is also worth mentioning that the increase in mechanical properties does not follow rule-of-mixture

Materials 2013, 6 2557
predictions. This is due to the contribution from the interphase polymer, which is often unaccounted
for. Several recent works have attempted to include this contribution for better understanding of the
composite micro-structural contribution to the bulk properties [40,61]. It is also important to note that
in some CNT-polymer systems where CNT templating is found, the crystallinity is typically much
higher in the composite versus the control system. In such cases, the effect of templating alone is
difficult to assess. Here, the focus is on two systems, which display similar crystallinity in order to
understand the role of the template-oriented polymer interphase contribution.
It has also been recognized that in cases where the interphase regions are not template or oriented
(i.e., showing chain disorder), the mechanical enhancement is not that significant [198]. Interfacial
stress transfer is a critical component/parameter controlling the performance of the composite.
Complete stress/load transfer from the polymer to the nano-filler is achievable if there is strong
adhesion. Based on these high-resolution transmission electron microscopy (HR-TEM) studies, better
chain packing was also shown to exist at the interphase [42,43,181].
Figure 9. Comparison of mechanical property improvement for (a) poly(vinyl alcohol)
PVA/CNT and (b) polyacrylonitrile (PAN)/CNT fibers, which display SWNT-templated
polymer extended-chain crystallization and orientation [42,178].
Recently shear crystallization studies in hybrid polymer/SWNT dispersion were used to induce
oriented polymer crystallization in the presence of the SWNT. These studies were specifically focused
on developing a procedure for producing ordered interphase structure on the CNT. Figure 10 shows a
HR-TEM image for a PAN-SWNT interphase, where the polymer extended-chain morphology has
been templated by the nanotube [187]. These fundamental crystallization studies provide good insight
toward the morphological capabilities of the polymer influenced by this mechanism. In terms of
processing polymer/CNT composite materials, these crystallization processes may even be
incorporated into fabrication procedures.

Materials 2013, 6 2558
Figure 10. (a) Scanning electron micrograph (SEM) of PAN tubular coating on SWNT.
High-resolution transmission electron micrograph (HR-TEM) of tubular coated
PAN/SWNT samples; (b) at the onset of electron beam exposure; (c and d1) show an area
of the PAN/SWNT sample where the PAN lattice of ~0.52 nm is observed; and (d2) a
schematic highlighting the PAN lattice observations in (d1) [187].
The fundamental understanding of the interfacial relationship between the polymer and CNT is still
being developed. However, it is recognized that this micro-structural development is necessary to
improve stress-transfer mechanisms in the composite. Polymer crystalline interphase structures have very
interesting behavior in terms of composite failure mechanisms [40,199] and properties [42–45]. What is
lacking is the fundamental understanding for consistently producing such interphase structures during
processing of the composite. Most of the studies highlighted have recognized the existence of this
micro-structural development only after the composite is being characterized. It is important that such
interfacial structures are pursued during composite preparation/fabrication, and this requires that
material design approaches be incorporated into such steps. To do this will require better
understanding for the development of the polymer interface, which is dependent on many processing
factors. However, understanding this rather complex problem may have tremendous implications
toward producing polymer/CNT composite fibers that can finally begin to consistently approach the
long heralded predicted values.
3.3. Orientation and Alignment Effects
The importance of inducing extended-chain polymer crystal growth and orientation in these
nano-composite materials is understood when looking at the theoretical modulus calculations for
various flexible polymer systems (i.e., the modulus calculations are based on the polymer being
in the extended-chain conformation) [200,201]. Thermoplastic polymers like PE, cellulose,
poly(tetrafluoroethylene) (PTFE), and PVA all possess Young’s modulus ranging from 100 to 250 GPa
along the axial direction [202]. Similarly, the predicted tensile strength of a perfect or fully extended
polymer fiber is potentially on the order of ~30 GPa [203]. To date, the majority of thermoplastic
polymeric materials possess low crystallinity ~30% to 40% and poor orientation leading to low
mechanical properties (i.e., stiffness and strength <10 GPa and ~0.5 GPa, respectively). As outlined,
the introduction of CNT as a templating material for extended-chain crystallization and orientation has
provided a new route to tailor polymer morphology in a fiber [178,197]. Several studies have shown

Materials 2013, 6 2559
the ability of these nano-carbon fillers to modify and influence the morphology of the polymer in the
composite [42,178,204]. These nucleation, crystallization, and orientation effects are especially
observed in composites with low nano-carbon loading (<1 wt %), and have a significant impact on the
overall structure and properties of the composite material [42,43].
Alignment of CNT or CNT ropes is another important factor in determining the mechanical properties
of composites containing them. According to the continuum mechanics calculations, the moduli of both
SWNT filler and polymer chains along the axial direction drop abruptly for only slight mis-orientation
with respect to the fiber axis (Figure 11). For SWNTmaterials, this effect is less pronounced as the
SWNT bundle diameter decreases [205] (Figure 11). The effect of orientation on modulus properties for
anisotropic composites can be taken into account using Equations (6) and (7) [205].
(6)
(7)
where, the values of and are given by Equations (8) and (9) [206]:
(8)
(9)
The parameters used for the orientation analysis (Figure 11) are provided in Table 3. What is
immediately obvious is that in the polymer/CNT composite fiber, the full alignment of the polymer
chain and the CNT is paramount. This is not an easy task. To date, only a handful of polymer-based
high-performance fibers exists (i.e., Kevlar®, Spectra®, Zylon®), and this is due to the high chain
alignment in the micro-structure either afforded by the inherent polymer conformational structure (i.e.,
rod-like molecules—Kevlar® and Zylon®) or special processing of low concentration polymer
solutions to reduce chain entanglement (i.e., gel-spinning of polyethylene—Spectra®). However, in
more recent work, the similarities between polymers and CNT, CNT templating effects, CNT liquid
crystalline nature, and the ability of nano-carbons materials to lubricate polymers during alignment
have been recognized. These factors all have significant implications toward greatly improving
polymer chain alignment during processing of the composite.




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12
1221
2
21
12
121
cos)
2
111
(cos)
2
2
1
(
11
EGEEEEGEESWNT
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2
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dI

Materials 2013, 6 2560
Figure 11.Graph showing the effect of mis-orientation on the effective Young’s modulus
for both (a) SWNT fillers and (b) various linear polymers.
Table 3. List of parameters used for Equations (6) and (7) for orientation analyses
corresponding to the Young’s modulus contribution along the axial direction [202,207,208].
Parameters E1 (GPa) E2 (GPa) ν G12 (GPa)
SWNT
20 nm bundle 1000 15 0.17 0.7
9 nm bundle 1000 15 0.17 2.3
<4.5 nm bundle 1 000 15 0.17 6
Polymers
Poly(vinyl alcohol) (PVA) 255 9 0.338 1
Polyethylene (PE) 240 4.3 0.46 1
Poly(tetra fluoroethylene) (PTFE) 156 5 0.46 1
Polypropylene (PP) 42 2.9 0.45 1
By comparing the structure, properties, phase behavior, rheology, processing, and applications
between SWNT and rigid-rod polymers, SWNT are considered as polymeric materials [209,210]. As
mentioned, the similarity between CNT (especially SWNT) and polymers will allow the polymer
chains to interact with SWNT more readily and nucleate on SWNT surfaces due to epitaxy. For this
reason, SWNT are potentially able to align the chains parallel to the axis direction and template
polymer crystallization with extended-chain conformation. For polymeric materials extensional force
(usually conducted through shear flows in melt or solution) is required for inducing the extended-chain
crystallization and the subsequent growing of the bundle-like fibrils or shish-kebab
structures [211–213]. This shearing mechanism is also needed to grow fibrillar (extended-chain)
crystals in polymer/CNT hybrid systems [42,43,178,181]. The processing of extended-chain polymer
crystals in CNT systems is difficult and not as typical as the observation of folded-chain crystal
structures in these composites. However, a few previous works have shown that SWNT
can induce nucleation of extended-chain crystallization and template the alignment of polymer
chains in PE [181], PBT [214], poly(ethylene terephthalate) (PET) [177], PAN [43–45], and
PVA [42,178] systems. The presence of CNT is considered to largely contribute to the polymer

Materials 2013, 6 2561
nucleus size in the hybrid system, which suppresses the energy barrier for fibrillar crystallization by
providing sufficient heterogeneous nucleation sites due to epitaxial interaction [85].
Under quiescent conditions, the final crystalline structure and morphology are determined by the
filler characteristics (i.e., concentration, composition, filler size, and shape) and by the interaction
between the filler and the polymer matrix. In the presence of the shear flow, the influencing effects
extend to shear rate, shear duration, and the interaction between shear and fillers [213]. In a
polymer/nano-particles hybrid system, the introduction of nano-fillers and polymers into shear flow
has been shown to create a synergistic effect for promoting crystallization, due to the changes in the
local stress levels and orientation of chains surrounding the nano-particles upon the application of
shear [213,215,216]. For this reason, the rod-like CNT can greatly induce anisotropic nucleation sites
at the interphase and promote the subsequent crystal growth in the flow direction.
Under appropriate shear flow at a specific crystallization temperature, PE and PAN have been shown
to crystallize into extended-chain shish directly on SWNT [181,187] surface, followed by nucleation of
folded-chain lamellae. Based on the small-angle X-ray scattering (SAXS) analysis for the pure PBT
system and PBT/SWNT composites, it was shown the very low SWNT loading (0.2 wt %) can largely
template the morphology of crystallization during flow, providing a method to obtain a highly desirable
fiber-like morphology [214]. Patil et al. have concluded that within the sheared PE/CNT nano-composite
system, the presence of CNT significantly promote the polymer chain orientation, the length increase,
and the stability of the hybrid shish-kebab structures, due to CNT templating chain alignment as
compared to the sheared pure PE system [217–219]. Wide-angle X-ray diffraction (WAXD) studies on
drawn PET/SWNT composite showed that oriented crystallization of PET was induced by aligned
SWNT in a randomized PET melt [177]. This orientation of the PET survived even after
re-melting [177]. No orientation was observed in the re-melting process in the neat PET system,
indicating the templating role of SWNT upon shear for polymer crystallization [177]. These studies
demonstrate the synergistic effects of the presence of SWNT and shear flow on promoting polymer
extended-chain crystallization at the interphase in the nano-composites.
In addition to templating, the use of rigid nano-carbons in polymer matrices may also enable
increased polymer chain alignment during processing [61]. Improvement in chain alignment has been
reported where an orientation factor (f) increase from 0.5 to 0.8 was found. This subsequently led to a
drastic increase in the mechanical performance of the composite as compared to the control fiber
(Figure 12). This work demonstrates the ability to use unique nano-fillers to act as a lubricant during
drawing to facilitate polymer chain extension and orientation.
Several studies have shown that the polymer chains form preferential alignment in the presence of
CNT, and this is not the case in their absence [61,177,178,181,214]. What is needed at this point is the
understanding of how to take advantage of such a phenomenon during processing of the composite.
The unique similarities between the CNT and polymer [210] may afford opportunities to develop new
special processing techniques that can take advantage of such parallels to produce high-performance
polymer/CNT fibers with well-controlled micro-structures.

Materials 2013, 6 2562
Figure 12. Bar chart comparing modulus, strain, toughness and tensile strength (1 is the
control fiber and 2 is the composite fiber) for drawn control and composite
PVA/SWNT fibers [61].
4. Prospects and Challenges for Processing Polymer/CNT Composites with Controlled
Structural Development
This contribution has outlined studies that recognize that tailoring interfacial properties in materials
has a direct influence on the overall performance of the system. This is especially true for composite
materials, which are composed of two or more dissimilar materials. Without good interaction between
the components of the system, the contribution from each is diminished. To date, the introduction of
nano-materials and their use in composite systems have shown that these filler materials can have
tremendous impact on the matrix components even without any optimization. However, the majority of
these improvements have so far been incremental. Taking full advantage of the CNT material requires
more design as it pertains to the interaction between the filler and the matrix, dispersion processes, and
alignment of this hybrid system during fiber spinning. For this reason, future-processing approaches of
polymer/CNT materials should incorporate some modeling/computational aspects in order to predict
what kind of effects these parameters may actually have on the polymer and nano-filler. This task is a
challenge in that to understand such a procedure, a truly multi-scale approach is necessary to envision
all steps from the atomic/nano-scale (i.e., polymer chain and CNT), to the macro-sale spinning
procedure. In addition, the complexity of modeling polymer solutions and melts in the presence of
these nano-carbons as they form solid fibers renders it a difficult task.
Experimentally, as highlighted in this contribution for many polymer/carbon nanotube composites, it
has been demonstrated that the polymer is able to have some direct interaction with the nanotubes. For
specific systems, the polymer has a high affinity for wetting, wrapping, or even crystallizing in and
around the nanotubes. What is attractive about polymer crystallization in the presence of the CNT is the
ability to create ordered polymer structures in the vicinity of CNT, with implications for a wide-range of
applications. Thus far, these phenomena have mainly been observed. Therefore, fundamental studies
are necessary to truly understand the inherent ability of CNT to nucleate and template polymer

Materials 2013, 6 2563
crystallization, and its effects on the ordered conformation characteristics of polymers. Such processes
are influenced by the polymer molecular architecture, chemical make-up, conformational capabilities,
as well as nanotube diameter, type (i.e., MWNT, SWNT, FWNT), graphitic perfection, and chirality.
The determination and control of these parameters are required to induce the crystallization process,
whether processing fibers from the melt or solution.
It is clear that both the experimental and modeling challenges are very important for design
processing fabrication approaches that can truly make the most of the polymer/CNT hybrid system in
terms of structural development and ultimate properties. Going forward in this field will require such
approaches to achieve the dream that began almost two decades ago.
5. Conclusions
This review summarizes studies on the various parameters that affect the strengthening mechanisms
in polymer/CNT fiber composite systems as a function of processing. CNT containing polymeric fibers
have exhibited improved mechanical and physical properties such as tensile strength, Young’s
modulus, strain-to-failure, toughness, and resistance to molecule changes from both solvent and heat
treatments. Experimental factors influencing composite processing include CNT structure, dispersion,
interfacial interaction, and alignment/orientation of polymer chains and CNT. The combination of
these factors needs to be well controlled in order to optimize the resultant mechanical properties of the
bulk composite fiber. An understanding of these factors is complex and a great challenge in the field of
nano-composite processing. However, increasing fundamental experimental insight coupled with
computational and “materials by design” approaches will lead to more efficient use of CNT in
composites and better optimization of fabrication procedures.
Acknowledgements
Several of the authors’ works highlighted in this contribution were supported by start-up funds at
Northeastern University and funding from the Air Force Office of Scientific Research
(AFOSR) (FA9550-11-1-0153).
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