This document summarizes research on using carbon nanotubes (CNTs) to improve the impact resistance of epoxy composites. The researchers studied how individual CNTs and CNT clusters contribute to mechanical properties. Well-dispersed individual CNTs improved properties more than clustered CNTs. Adding polyamide particles to carry CNTs improved dispersion and properties more than CNTs alone. Interlayering the epoxy composite with a CNT/polyamide film further improved the toughness of carbon fiber composites.

1. Kevin L. White1 , Peng Li2, Hung-Jue Sue2, and Atsushi Takahara1
1 Kyushu University, Fukuoka, Japan; 2 Texas A&M University, College Station, Texas, USA
Separating the Roles of Individual and Clustered Carbon Nanotubes
in Performance of Polymer Nanocomposites
Background
• High crosslink density epoxies are the most widely used matrix material in high performance carbon fiber
reinforced composites (CFRCs). Unfortunately, epoxies are inherently brittle and prone to initiate
delamination failure during low velocity impact
 Delamination failure is the most significant and detrimental failure mode in CFRCs – load bearing
capability of composite may be severely reduced without visible damage
 Residual strength after impact is strongly correlated with the Mode-II strain energy release rate of the
composite (GIIc)
• Carbon nanotubes show excellent potential as reinforcing materials; unclear how individual particles
contribute to macroscale response
• How do we control particle structure and organization to achieve desired set of properties?
• Can properties be tuned/tailored to achieve broad application space?
• Do bulk properties translate to improved performance in hierarchical systems (e.g., CFRCs)?
Experimental
Epoxy system (DGEBF + DETDA); Tg ≈ 150°C
Multi-walled carbon nanotubes (MWCNTs)
(5-10 wall; OD = 10-12 nm)
Polyamide-12 (PA) spherical particles
(D = 10 μm)
Interlayered CFRC2
Electrical conductivity
10
-5
10
-4
10
-3
10
-2
10
-1
10
-10
10
-8
10
-6
10
-4
10
-2
10
0
Filler Phase
P-MWCNT
PA(20%)/P-MWCNT
ElectricalConductivity,[Sm
-1
]
Volume Fraction, 
F-MWCNT
PA(20%)/F-MWCNT
PA(20%)/O-MWCNT
P-XDCNT
-100 -50 0 50 100 150 200
10
-1
Tan
Temperature (C)
Neat Epoxy (862/W)
Epoxy/P-MWCNT(1%)
Epoxy/PA(20%)
Epoxy/PA(20%)/P-MWCNT(1%)
Epoxy/PA(20%)/F-MWCNT(1%)
Dynamic mechanical analysis
Bulk Properties1
Fracture cross-section (Ep/PA/MWCNT)
4P-ENF Results
References
1. White KL, Sue HJ (2011), Electrical conductivity and fracture behavior of epoxy/polyamide-12/multi-walled carbon nanotube composites, Polymer Engineering & Science, 51(11):2245-2253
2. White KL, Sue HJ (2012), Delamination toughness of fiber-reinforced composites containing and carbon nanotube/polyamide-12 epoxy thin film interlayer, Polymer, 53(1):37-42
3. White KL, Li P, Sumi Y, Sue HJ (2014), Rheology of disentangled multi-walled carbon nanotubes dispersed in uncured epoxy fluid, Journal of Physical Chemistry B, 118(1):362-371
4. White KL, Shuai M, Zhang X, et al. (2011), Electrical conductivity of well-exfoliated single-walled carbon nanotubes, Carbon, 49(15):5124-5131
5. Sun L, Warren GL, Davis D, Sue HJ (2011), Nylon toughened epoxy/SWCNT composites, Journal of Materials Science 46(1):207-214
6. Warren GL, Sun L; Hadjiev VG, et al. (2009) B-staged epoxy/single-walled carbon nanotube nanocomposite thin films for composite reinforcement, Journal of Applied Polymer Science, 112(1) 290-298
7. Sun L, Warren GL, Reilly JY, et al. (2008) Mechanical properties of surface functionalized SWCNT/epoxy composites, Carbon, 46(2):320-328
Conclusions
• To design effective nano-structured systems, need to identify relationship between nano-level
structural variables and desired macro-property on appropriate length scales
• Rheology used as molecular-scale probe to understand contribution of individual particles to bulk
flow behavior of uncured epoxy liquid. At low frequencies (slow deformation), clustering behavior
dominates response; at high frequencies, able to probe internal modes of motion for individual
MWCNTs, even in clustered systems
• Mechanical properties of epoxy can be tailored by modifying the chemical and dispersion state of
the MWCNTs - clustered MWCNTs show unique combination of properties that are attributed to
“stress shielding” mechanism - network of MWCNTs relieves internal stresses
• Interlayering used to fabricate “hierarchical composite”; scalable approach to enhance damage
tolerance of fiber-reinforced composite with out-of-autoclave processing
• Improvement in fracture toughness due to addition of MWCNT phase attributed to ability of
MWCNT network to facilitate stress redistribution within resin-rich region; shields internal defects
from growing to critical size for unstable fracture
• Further work anticipated to more clearly bridge between length scales and aide in the development
of predictive multi-scale models for polymer nanocomposites.
Mechanical properties (Uniaxial Tension / SEN-3PB)
• Relaxation behavior is sensitive to molecular-scale structure
and interactions – probe of time-dependent response of fluid
• NB-functionalized MWCNTs were prepared to study dynamics
associated with isolated nanotubes in suspension. The NB-
MWCNTs are individually dispersed and stable in epoxy (TEM
image on right).
• (Top Left) Linear viscoelastic response of suspensions; data
vertically shifted for clarity. Curves fit using Yamakawa model
for infinitely rigid fibers (solid lines), and hybrid model
incorporating high-frequency Zimm modes to account for
internal flexibility. Results show that low-frequency behavior
of untreated MWCNTs dominated by presence of clusters.
For low concentration of disentangled MWCNTs, follow
predictions for individual rod-like molecules well.
• Steady shear response of NB-MWCNT suspension (1 wt%)
shown in top right. Steady shear viscosity (filled) and dynamic
viscosity (unfilled) plotted as functions of shear rate and
angular frequency, respectively. Schematic of network
structure for each region of behavior included above.
• Weak shear thickening behavior at high shear rate attributed
to shear-induced buckling (mechanism shown on bottom
right).
Particle-assisted dispersion w/ PA (non-covalent)
E/P-MWCNT (1wt%) E/PA/MWCNT (20/1%)
Functionalization (covalent)
Rheology of Disentangled MWCNTs3
Epoxy/NB-MWCNT (1.1 wt.%)
Shear Thickening Mechanism
Sample E [GPa] σT [MPa] εB [%] Gic [J/m2]
Neat Epoxy 2.9 ± 0.1 63 ± 3.1 3.2 ± 0.3 81 ± 0.2
Ep/P-MWCNT (1wt.%) 2.9 ± 0.1 76 ± 1.0 6.4 ± 1.3 190 ±75.1
Ep/SA-MWCNT (1%) 3.1 ± 0.3 83 ± 8.2 4.4 ± 0.2 106 ± 7.7
Ep/PA(20%) 2.8 ± 0.1 69 ± 2.3 8.4 ± 2.2 248 ±25.2
Ep/PA/P-MWCNT 2.5 ± 0.1 76 ± 1.7
10.6 ±
1.5 439 ±52.5
Ep/PA/SA-MWCNT 3.0 ± 0.1 75 ± 0.3 7.0 ± 0.4
Interlayering nanocomposite film with CFRC
Fabrication
Fracture surface