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Fabrication of hierarchical polymer nanocomposites with capillary-densified aligned carbon nanotube reinforcement

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To design next-generation hierarchical polymer nanocomposites (PNCs), scalable processing techniques such as vertically aligned carbon nanotube (A-CNT) growth, solvent-mediated CNT capillary densification, and capillary-assisted polymer infusion are attractive methods to create high-density and morphology-controlled bulk nanostructured materials. In this work, multiwalled A-CNT aerospace-grade infusion epoxy PNCs with mm-tall capillary-densified CNT cell and pin patterned reinforcement are fabricated for the first time, and their process-structure relationships are analyzed as a function of A-CNT volume fraction (Vf) and pattern type via scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD). These analyses provide multi-scale information to show how CNT confinement in dense cell and pin patterns (~8% CNT Vf in the cell and pin walls) influences polymer infiltration, wetting, and the CNT-matrix morphology. SEM and XRD results reveal that CNT alignment is consistently maintained in the epoxy matrix during processing, and SEM images show how the cell and pin PNCs exhibit CNT-matrix pull-out during fracture. Raman analysis shows that the atomic-scale defect density decreases with increasing CNT Vf in the PNCs, which is due to the greater CNT contribution to the spectra compared to the amorphous epoxy matrix. XRD further quantifies how the A-CNT walls dominate the diffraction patterns at higher CNT Vf in the cell and pin PNCs. Through these analyses, this study provides new insights into both the synthesis and multi-scale structure of hierarchical CNT PNCs, supporting the development and integration of advanced nano-engineered composites using facile patterning and densification techniques.
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TWENTY-SECOND INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM22)
ABSTRACT
To design next-generation hierarchical polymer nanocomposites (PNCs), scalable processing
techniques such as vertically aligned carbon nanotube (A-CNT) growth, solvent-mediated CNT
capillary densification, and capillary-assisted polymer infusion are attractive methods to create high-
density and morphology-controlled bulk nanostructured materials. In this work, multiwalled A-CNT
aerospace-grade infusion epoxy PNCs with mm-tall capillary-densified CNT cell and pin patterned
reinforcement are fabricated for the first time, and their process-structure relationships are analyzed as
a function of A-CNT volume fraction (Vf) and pattern type via scanning electron microscopy (SEM),
Raman spectroscopy, and X-ray diffraction (XRD). These analyses provide multi-scale information to
show how CNT confinement in dense cell and pin patterns (~8% CNT Vf in the cell and pin walls)
influences polymer infiltration, wetting, and the CNT-matrix morphology. SEM and XRD results
reveal that CNT alignment is consistently maintained in the epoxy matrix during processing, and SEM
images show how the cell and pin PNCs exhibit CNT-matrix pull-out during fracture. Raman analysis
shows that the atomic-scale defect density decreases with increasing CNT Vf in the PNCs, which is due
to the greater CNT contribution to the spectra compared to the amorphous epoxy matrix. XRD further
quantifies how the A-CNT walls dominate the diffraction patterns at higher CNT Vf in the cell and pin
PNCs. Through these analyses, this study provides new insights into both the synthesis and multi-scale
structure of hierarchical CNT PNCs, supporting the development and integration of advanced nano-
engineered composites using facile patterning and densification techniques.
1 INTRODUCTION
Due to their advantaged multifunctional properties, high volume fraction (Vf) aligned carbon
nanotubes (A-CNTs) are top candidates to enhance the strength, toughness, and thermal conductivity
of polymer nanocomposites (PNCs) [1-3] and provide dense interfacial reinforcement in aerospace
composite laminates, a process termed ‘nanostitching’ [4]. While it has been challenging to create
high-density, shape-tunable, and large-area CNT structures to improve the versatility of PNCs, recent
work has enabled the simple and predictable capillary densification [5-8] of mm-scale tall CNT arrays
with high substrate adhesion [9,10]. This allows CNT PNC process-structure-property relations to be
studied systematically at a variety of CNT Vf, length scales, and pattern sizes (see Figure 1), with
CNTs spanning several orders of magnitude in height, density, and hierarchy [3-9].
These new processing capabilities enable the fabrication of PNCs reinforced with dense, patterned
A-CNT architectures that can be designed with a variety of morphologies, such as cell networks and
pins as shown in Figure 1, which are created by densifying bulk-scale or patterned A-CNT arrays,
respectively [6,7]. Solid pins and long-range cell networks can be formed by densifying A-CNT arrays
within small and large pattern sizes (s) [5-9], respectively, with the critical pattern size (scr) separating
each morphology for a given CNT height (h) (see Figure 1). Different patterns and CNT Vf in the
densified cell and pin walls are therefore achievable by tuning the processing conditions and system
parameters, such as post-growth annealing for increased CNT-substrate adhesion, variable solvent
surface tension, substrate pre-patterning to tune s, and CNT growth time to vary h. Tuning these
FABRICATION OF HIERARCHICAL POLYMER
NANOCOMPOSITES WITH CAPILLARY-DENSIFIED
ALIGNED CARBON NANOTUBE REINFORCEMENT
Ashley L. Kaiser1 and Brian L. Wardle2
1Department of Materials Science and Engineering, Massachusetts Institute of Technology,
Cambridge, MA 02139 U.S.A., kaisera@mit.edu, Web: mit.edu/~kaisera
2Department of Aeronautics and Astronautics, Massachusetts Institute of Technology,
Cambridge, MA 02139 U.S.A., wardle@mit.edu, Web: aeroastro.mit.edu/brian-wardle
Keywords: Carbon nanotubes, Polymer nanocomposites, Capillary densification, Nano-engineered
NON PEER REVIEWED
Ashley L. Kaiser and Brian L. Wardle
parameters creates nano- to mm-scale densified structures with CNT Vf up to 3040% that can be
predicted based on elastocapillary densification theory [6,7].
Figure 1: Illustrations showing an overview of the capillary densification of A-CNT arrays into (a-b)
dense cell and pin architectures with tunable height (h), wall thickness (t), pattern size (s), cell width
(w), and CNT volume fraction (Vf w/tc for cells and Vf s/tp for pins). Pins are formed from A-CNTs
densified in a pattern size smaller than the critical pattern size (i.e., s < scr). (c) Illustration of dense
CNTs as hierarchical polymer nanocomposite (PNC) reinforcement and laminate reinforcement.
However, to investigate the potential use of these newly-developed CNT architectures for next-
generation high-strength composite reinforcement in aerospace-grade polymer matrices, it is necessary
to determine how polymer matrix infusion at high A-CNT Vf (i.e., greater Vf than the as-grown array
value of 1% A-CNTs [6,7]) influences the structure, morphology, and mechanical behavior of PNCs
for a given h value [3,11,12]. Past work on bulk A-CNT PNCs highlights the benefits of using
vertically aligned CNTs for advanced composite fabrication up to CNT Vf of ~20% in an epoxy matrix
[2,11], as the alignment facilitates polymer infusion, wetting, and the controlled manufacturing of
PNC test structures [12,13]. While early results are promising, these studies also underscore the need
for greater understanding and tunability over the influential processing factors in PNC fabrication,
which ultimately govern mechanical performance: CNT height, crystallinity/quality, alignment, and
dispersion in the polymer matrix, and CNT-polymer interactions [3,13]. These factors will become
even more critical when the CNT Vf increases, as CNT-polymer interactions and the resulting
confinement and interfacial effects will dominate the synthesis, structure, and properties of these
composite materials.
TWENTY-SECOND INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM22)
To address these outstanding challenges and support the development of experimentally-validated
models to predict dense PNC behavior across length scales [2,3], this work presents an experimental
process-structure study of capillary-densified CNT PNCs using an aerospace-grade polymer matrix
(here RTM6 epoxy). The capillary densification of new mm-tall CNT cell and pin architectures
followed by polymer infiltration and curing offers a means to study both CNT-polymer interactions at
higher CNT Vf and enables the facile manufacturing of patterned structures for applications requiring
specific shapes, such as sensor networks, structural health monitoring, and location-specific composite
reinforcement, including ‘nanostitching’ between carbon fiber plies in composite laminates [4] (see
Figure 1). Combined with scanning electron microscopy (SEM) studies of CNT wetting, alignment,
and PNC fracture behavior, and additional structural quantification via Raman spectroscopy and X-ray
diffraction (XRD), this work provides new insights into both the fabrication and multi-scale structure
of hierarchical CNT composites to support the development and commercial integration of nano-
engineered hybrid materials.
2 METHODS
2.1 CNT Array Synthesis, Thermal Cementation, and Capillary Densification
In this work, multiwalled CNT (MWCNT)-polymer (RTM6 epoxy) PNCs with mm-tall dense A-
CNT cell and pin reinforcement were fabricated via the following steps, as shown in Figure 1. First,
1% Vf A-CNT arrays were synthesized via chemical vapor deposition (CVD), where the arrays were
grown via a base-growth mechanism in a 22 mm internal diameter quartz tube furnace at atmospheric
pressure and 700°C via a thermal catalytic CVD process, which uses ethylene as the carbon source and
600 ppm of water vapor added to the inert helium gas [14]. The CNTs were grown on a catalytic layer
composed of 1 nm Fe on 10 nm Al2O3 deposited via electron beam physical vapor deposition [14] on 1
cm × 1 cm SiO2/Si substrates. To create patterned CNT arrays as densified CNT pin precursors, a
subset of these substrates were mechanically scribed by hand to remove the Fe/Al2O3 layers in
rectangular grid patterns of one-dimensional pattern size s 500 µm on average. Due to the selective
removal of catalyst inside the ~40 µm-wide scribed grid lines, CNT arrays grew only in the
rectangular pattern where the catalyst remained. As will be described herein, solid CNT pins formed
from the capillary densification of these patterned A-CNTs since the scribed grids were smaller than
the critical pattern size (scr) defining pin vs. cell formation (see Figure 1) [7,9]. A subset of SiO2/Si
substrates were left unpatterned (i.e., s = 1 cm >> scr) as precursors for bulk-scale CNT cell networks
that formed from the capillary densification of bulk-scale (non-patterned) A-CNT arrays [6].
Following substrate preparation, A-CNT arrays were grown to mm-scale height (h) via the CVD
process described above. During the ~10 min CVD growth period, the growing CNTs self-assembled
into aligned arrays with h 1 mm (as measured via optical microscopy using a Carl Zeiss Axiotech 30
HD optical microscope) and were comprised of MWCNTs with an average outer diameter of ~8 nm
(3–7 walls with ~5 nm inner diameter and intrinsic CNT density of ≈1.6 g/cm3) [15], inter-CNT
spacing of ~6080 nm [16], and Vf of ~1% CNTs [15]. To increase the CNT-substrate adhesion
strength of the as-grown CNTs, a thermal cementation (annealing) step was performed directly after
the CNT growth period during the CVD process. The CNT arrays were subjected to 200 sccm of inert
helium gas for 40 min [6,10] at 800°C, which resulted in a ~10× increase in the CNT-substrate
adhesion force, ensuring that the mm-tall A-CNTs would not delaminate from the substrate during
capillary densification [6,9].
After growth and cementation processing and before capillary densification, all A-CNT array
samples were then exposed to a previously developed recipe for O2 plasma treatment to remove the
entangled CNT layer that often forms on top of the CNT array during the growth process [17], which
can cause non-uniform densification under capillary forces [7]. Then, the CNTs were densified via a
previously reported paper-soaking capillary densification technique using acetone as the solvent, as
shown in Figure 1 [18], as this gentle process leads to slower wetting and therefore less CNT array
delamination and disruption than direct immersion techniques [17]. Here, the paper acts as a
hydrodynamic damper to allow the solvent to move into the CNT forest slowly, delicately wetting the
Ashley L. Kaiser and Brian L. Wardle
entire array from top to bottom [18]. The wetted CNT arrays were then left to dry under ambient
pressure and temperature until all of the solvent had evaporated, during which time the bulk-scale and
patterned cemented CNT arrays densified into mm-tall cellular networks and solid pins, respectively.
2.2 Polymer Nanocomposite Fabrication
Once A-CNT arrays were grown, cemented, and a subset were densified to create CNT cell and
pin networks, the following steps were performed to create CNT/epoxy samples, which included CNT
cell and pin PNCs (see Figures 13), bulk PNCs fabricated from as-grown 1% Vf A-CNT arrays, and
cured neat RTM6 epoxy samples as a baseline. Fabrication of bulk CNT-epoxy PNCs via vacuum-
assisted wetting and curing was performed by first placing free-standing 1% Vf A-CNT arrays (which
were delaminated from their Si wafer growth substrate with a standard razor blade) into hollow
cylindrical Al molds, ensuring that the primary CNT axis was orthogonal to the plane of the mold,
following Ref. [14]. The A-CNT arrays were then infused with degassed RTM6 epoxy resin (Hexcel
Corp., USA [19]) at 120°C for 1 hour and then cured at 180°C for 2 hours, following the
manufacturer’s recommended cure cycle. RTM6 resin without the addition of CNTs was also cured in
the same manner to create cured neat polymer samples.
Figure 2: Representative SEM images of a CNT-polymer nanocomposite (PNC) with dense CNT cell
reinforcement in an epoxy matrix. (a) Top view of a bulk cemented A-CNT array formed into cells via
capillary densification. (b) Side views of a cell PNC showing epoxy-wetted CNTs maintaining their
alignment. (c) PNC fracture surface as prepared via room temperature hand fracture post-curing
showing CNT bridging and pull-out from the epoxy matrix.
TWENTY-SECOND INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM22)
Figure 3: Representative SEM images of a CNT-polymer nanocomposite (PNC) with dense CNT pin
reinforcement in an epoxy matrix. (a) Patterned cemented A-CNT arrays densified into vertical pins.
Top views of (b) a large-area CNT pin PNC with epoxy-wetted CNT pins and (c) side view of the
PNC fracture surface in the middle of a CNT pin, as prepared via room temperature hand fracture.
For the fabrication of CNT cell and pin PNCs, a ~2 mm-thick pool of warm degassed RTM6 resin
was placed in an Al mold, and the cell and pin arrays (still attached to their Si wafer growth substrate)
were attached to a vertical z-stage using SEM tape following Ref. [2]. The CNTs were then lowered
into the pool of uncured epoxy for the capillary-assisted wetting/infiltration of polymer into the A-
CNTs. After this, the fully wetted CNTs were removed from the pool, and the PNCs were cured
following the steps above. Once cured, the Si wafer was pulled off vertically to delaminate the CNTs
from the substrate for subsequent SEM and structural characterization, and a subset of the cell and pin
PNCs were hand-fractured at room temperature to prepare PNC fracture surfaces for SEM imaging.
2.3 Morphological Characterization: SEM
High resolution SEM was used to image and characterize the overall morphology, pin pattern sizes
(s), and geometry (cell width w, cell/pin wall thickness t, and Vf) of the capillary-densified CNT cell
and pin architectures, as well as the morphology and fracture surfaces of the PNCs created via
mechanical hand-fracture of the samples. This characterization was performed using a Zeiss Merlin
scanning electron microscope with a 5 mm working distance and 5 kV accelerating voltage [6,7].
Figures 2a and 3a present SEM images showing the morphology of the cell and pins formed via the
capillary densification of bulk-scale and patterned cemented A-CNT arrays, respectively (resulting in
an average CNT Vf of ~8% and ~5% in the cell and pin walls, respectively, as determined via Vf w/tc
for cells and Vf s/tp for pins [6,7]). Figures 2b-c and 3b-c show both top-view and side-view SEM
images of the CNT cell and pin PNCs, respectively.
Ashley L. Kaiser and Brian L. Wardle
2.4 Structural Characterization: Raman Spectroscopy and XRD
2.4.1 Raman Spectroscopy
Raman spectroscopy was performed to quantify the atomic-scale structural evolution, chemical
bonding character, and crystallinity of the A-CNT array, neat epoxy, and PNC samples in the high
CNT Vf cell and pin regions, as this technique is broadly used to analyze defect densities and disorder
in carbon-based materials [20-22]. Dominant spectral features include the Raman D-band, which is
found at ~13351350 cm-1 and represents carbon defects and structural disorder in the (002) plane and
breathing modes of sp2 atoms in rings, and the Raman G-band, which is found at ~15801600 cm-1
and represents in-plane sp2 bond stretching in rings and chains [20-22]. Epoxy Raman bands were also
evaluated at ~1190 cm-1, ~1450 cm-1, and ~1614 cm-1 corresponding to C-C stretching and C=C
aromatic chain vibrations in the cured polymer, respectively [23,24]. The intensity ratio of the D- and
G-bands (ID/IG) and the relative band shapes (e.g., breadth/size and location) were compared to
determine the relative defect density and structural order in the samples [19-24], which allows for an
investigation of the PNC structural evolution as a function of polymer infusion into higher Vf
(capillary-densified) A-CNT arrays. Raman spectra were collected using a LabRam HR800 Raman
microscope (Horiba Jobin Yvon) with 532 nm (2.33 eV) laser excitation through a 50× objective.
Several spots on each sample (for at least 3 samples of each type) were studied to ensure that
representative data were used when calculating ID/IG. The representative Raman spectra (smoothed
using a 32-point moving average) for the as-grown A-CNT arrays, neat RTM6 epoxy, bulk 1% Vf
PNCs, and CNT cell and pin PNCs are presented in Figure 4.
Figure 4: Raman spectra (as labelled from bottom to top) of aligned MWCNTs (black), the high CNT
Vf region within a cell PNC (blue) and pin PNC (red), a PNC with uniform 1% CNT Vf (purple), and
neat RTM6 epoxy (green). The spectra show the structural evolution of PNCs as the A-CNT density
(Vf) increases in the epoxy matrix via characterization of the carbon and epoxy Raman bands.
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2.4.2. X-ray Diffraction
X-ray analysis was employed to examine the multi-scale structural evolution of CNT cell and pin
PNCs as a function of CNT Vf using X-ray diffraction (XRD). In particular, this analysis was carried
out to investigate the nanometer-scale feature sizes and orientations (including alignment), inter-layer
spacing, structural strain, polymer interphase formation, nanoscale interaction effects, CNT wall-wall
and carbon crystallite spacing, and polymer chain packing and crystallization effects due to increased
CNT packing [2,11,14,25-33]. The (002), (100), and (110) carbon peaks were evaluated at positions of
2θ 25°, 43°, and 78°, respectively [14,31], and the broad epoxy peak was evaluated at 2θ ≈ 21°
[25,32,33]. A PANalytical X'Pert Pro XRD system in Bragg Brentano geometry was used to analyze
the A-CNT array, neat epoxy, and PNC samples post-curing, where the CNTs were aligned/oriented
normal to the stage so that the 2θ angle ranged from 0° (perpendicular to the CNT axis) to 90°
(parallel to the CNT axis). Cu Kα radiation was passed through a 2° anti-scattering slit with a 0.04 rad
soller slit in X'celerator mode. The XRD experiments were performed at 45 kV and 40 mA with a
scanning step interval of 0.02° (2θ). LaB6 was used as the standard material for all measurements. The
representative diffraction patterns (smoothed using a 32-point moving average) for the as-grown A-
CNT arrays, neat RTM6 epoxy, bulk 1% Vf PNCs, and CNT cell and pin PNCs are shown in Figure 5.
Figure 5: X-ray diffraction (XRD) diffraction patterns (as labelled from bottom to top) of aligned
MWCNTs (black), the high CNT Vf region within a cell PNC (blue) and pin PNC (red), a PNC with
uniform 1% CNT Vf (purple), and neat RTM6 epoxy (green), with the vertical CNT alignment normal
to the plane of the XRD stage. The diffraction patterns show the evolution of the (002), (100), and
(110) carbon peaks at 2θ 25°, 43°, and 78°, respectively, and the broad epoxy peak at 2θ 21°
as the packing density of A-CNT reinforcement increases within the PNC's epoxy matrix.
Ashley L. Kaiser and Brian L. Wardle
3 RESULTS AND DISCUSSION
3.1 SEM Analysis
SEM imaging of the capillary-densified CNTs and CNT-epoxy PNCs (see Figures 2 and 3)
provides multi-scale structural information to investigate the densified cell and pin morphologies,
including the CNT array packing and alignment [6,7], and the morphology of the PNC surfaces,
especially after fracture. Figures 2a and 3a show that the densified CNTs maintain their vertical
alignment after capillary densification and exhibit an average CNT Vf of ~8% and ~5% in the cell and
pin walls, respectively, as determined by Vf [%] w/tc for cells and Vf [%] ≈ s/tp for pins [6,7]. For the
various PNCs, SEM illustrates how CNT confinement in the PNC epoxy matrix influences polymer
infiltration, wetting, CNT dispersion/aggregation, and the CNT-matrix morphology, including the
CNT alignment in the matrix and the composite microstructure after curing [1-3]. Figures 2b and 3b
show that the polymer matrix completely fills the spaces between the CNT cells and pins and infuses
into the densified CNT arrays, effectively wetting but not swelling the CNTs in the cell and pin walls
while maintaining their alignment and dispersion within the matrix. This is consistent with the
complete A-CNT wetting demonstrated for bulk-scale CNT-epoxy PNCs studied in previous work
which had an A-CNT Vf up to ~20% [2,11], showing that the CNT cell and pin morphologies can
serve as a viable architecture for composite reinforcement when facile tuning of the CNT Vf towards
higher values is desired.
SEM imaging of the cell and pin PNCs is also used to analyze their cross-sectional fracture
surfaces, which reflect the interfacial fracture behavior during the mechanical hand fracture of the
PNCs after curing. As shown in Figures 2c and 3c, the PNC fracture behavior within the cell and pin
sections is observed to be dominated by CNT bridging and pull-out/debonding from the matrix [1-
3,23]. Here, the CNTs exhibit a pull-out caused by the interfacial debonding from the epoxy matrix
owing to the resistance to the applied load [29], where failure is governed by the relative adhesion
strength of the polymer and the A-CNTs, and the tensile stress exceeds the magnitude of the
interaction forces between the polymer layer surrounding the CNTs and the CNTs themselves. This
analysis gives a qualitative indication of the relative strength of CNT-matrix interactions and pull-out
behavior based on CNT Vf in the cell and pin walls, which is consistent with the fracture morphologies
of bulk CNT PNCs with thermoset polymer matrices and bulk CNT Vf up to ~20% [11,13,23].
3.2 Raman Spectroscopy Analysis
Raman spectroscopy is used to quantify the atomic-scale structural changes and bonding character
evolution in the CNT-epoxy PNCs as a function of Vf for samples that were not fractured after curing.
Raman spectra of the as-grown A-CNT array, neat RTM6 epoxy, bulk 1% Vf PNC, and CNT cell and
pin PNCs presented in Figure 4 shows that the atomic-scale defect density decreases with increasing
CNT Vf in the PNCs, and that the CNT structure is not significantly disrupted upon polymer infusion
and curing. Here, the D- and G-bands of the high crystallinity CNTs are well defined at ~1350 cm-1
and ~1580 cm-1 for the cell and pin PNCs, representing sp2 carbon defects and in-plane sp2 carbon
stretching [20-22]. As CNT Vf increases, these bands dominate the spectra compared to the amorphous
epoxy bands at ~1190 cm-1, ~1450 cm-1, and ~1614 cm-1, which correspond to C-C stretching and C=C
aromatic chain vibrations in the RTM6 epoxy [23,24].
The ID/IG ratios and relative peak locations (frequencies) of the bands quantify the relative defect
density and structural order in the samples, while also giving a qualitative indication of the strain on
the CNTs in the dense cell and pin PNCs. The ID/IG ratio evolves from ~1.03 to 0.99 to 0.56 for the
~5% Vf pin PNC, ~8% Vf cell PNC, and A-CNT array, respectively, showing how higher Vf results in
more structural order. Higher disorder is observed here for the 1% PNC since the polymer dominates
the Raman spectrum. Additionally, an upshift in the D-band frequency is observed from ~1340 cm-1 to
~1346 cm-1 for the A-CNT array and cell PNC samples, respectively, showing that when an increased
concentration of CNTs is added to the epoxy matrix, the epoxy intercalates with the CNT bundles and
opens them during polymer infiltration and curing, producing a compressive strain in the composite
TWENTY-SECOND INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM22)
[34,35]. This is consistent with prior work showing Raman band upshifts for ~5wt% CNT-epoxy
PNCs compared to pure CNTs [35] and a G-band downshift to lower frequencies when a tensile axial
strain was applied to suspended CNTs [34]. These results show the efficacy of the epoxy to infuse
between the CNT bundles during processing, as corroborated by the aforementioned SEM imaging
showing effective CNT wetting. Additionally, the Raman G-band is located at approximately the same
Raman shift across all cell/pin PNC and A-CNT array samples (~1580 cm-1 ± 1 cm-1 on average),
showing that the G-band is less affected by polymer infusion and curing. Collectively, these results
provide process-structure insight into the PNC structural evolution as a function of polymer infusion
into capillary densified A-CNT arrays.
3.3 X-ray Diffraction Analysis
Complementary to Raman spectroscopy, XRD analysis was employed to further investigate how
the structure of high-density, patterned A-CNT-reinforced epoxy nanocomposites evolves with CNT
Vf, polymer infiltration, and local confinement effects in the high Vf CNT cells and pins. The
representative diffraction patterns in Figure 5 show the characteristic (100) and (110) carbon peaks at
2θ 43° and 78°, respectively [14,31] for the as-grown A-CNT array, bulk 1% Vf PNC, and CNT cell
and pin PNC samples. These arise from the (100) and (110) carbon planes and have a greater
contribution to the diffraction pattern at higher CNT Vf, akin to the Raman spectroscopy results. The
characteristic asymmetry in the (100) and (110) peaks for the MWCNT diffraction pattern in Figure 5
is due to the changes in the local curvature of the nanotube, which arise from particle size broadening
(i.e., slight variations in CNT diameter along its length) and strain broadening (i.e., a distribution of
graphitic crystallite d-spacing along the CNT length) [26,27,29]. Additionally, because of the vertical
alignment of the A-CNTs in both the as-grown array and the PNCs, which also corresponds to their
orthogonal orientation to the XRD stage, the typical (002) carbon peak arising from the inter-layer
spacing of the MWCNT walls is not observed, since this d-spacing is not probed by the Bragg
Brentano XRD geometry. The absence of the (002) peak in all tested samples confirms that the CNTs
maintain their vertical alignment after densification, polymer infiltration, and PNC curing,
demonstrating the viability of this process to create oriented and high density CNT composites. These
results corroborate the CNT alignment observed in the SEM images described earlier, and they are
also consistent with prior work reporting near-zero (002) peak intensity for highly aligned CNTs
characterized in this XRD geometry [26-28].
Notable changes in the XRD peak positions and intensities are observed in Figure 5 as the CNT Vf
in the polymer increases from the neat RTM6 epoxy (Vf = 0%) to bulk PNC (Vf ~1%), pin PNC (Vf
~5%), cell PNC (Vf ~8%) and pure A-CNT array samples, which have no polymer contribution. In
addition to the aforementioned carbon peaks, the diffraction patterns also exhibit the broad epoxy peak
at 2θ 21° and a shoulder-type peak at 2θ 13° [25,32,33] for the bulk PNC and cell/pin PNC
samples, signifying the packing and crosslinking of polymer chains [31-33]. Since thermoset
polymers, such as the RTM6 epoxy studied here, exhibit a cross-linked network structure after curing,
they often show a broad, amorphous XRD peak below 25° owing to their small degree of crystallinity
[2], as the matrix takes on a largely amorphous structure during the curing reaction [25,32]. All
aforementioned XRD peaks in Figure 5 are interpreted to be a superposition of the CNT and RTM6
features. A new peak could signify the formation of a chemically distinct polymer interphase region
directly surrounding the CNTs, as this has previously been determined to form approximately 10100
nm in the vicinity of CNTs for a variety of polymers [11,30]. While it is plausible that an interphase
could form for the cell and pin PNCs based on the inter-CNT spacing achieved here after capillary
densification (< 50 nm) [6,7,16], no additional XRD peaks besides the aforementioned carbon and
epoxy peaks appear in Figure 5 when the A-CNTs are added to the polymer matrix.
Therefore, while the epoxy does not form a measurably distinct interphase for these PNC
(consistent with past work on ~20% Vf bulk CNT-epoxy PNCs [11]) relatively small changes in the
feature sizes related to polymer packing in the PNCs can still be observed to show the minor effect of
CNT confinement on polymer behavior in these materials. For all samples listed above and in Figure 5
Ashley L. Kaiser and Brian L. Wardle
(in order of increasing CNT Vf), the ~21° peak is observed to monotonically shift down in position
from ~21.5° to 21.0°, and the ~13° shoulder peak monotonically shifts down from ~13.9° to 13.6° as
the CNT Vf in the epoxy matrix increases from 0 to ~8%. This effect suggests that neat polymer
packing in the matrix may be slightly reduced (i.e., exhibiting a larger characteristic size) due to
reduced inter-CNT spacing. Finally, no significant changes in the positions of the (100) peak for all
samples in Figure 5 (~42.5° ± 0.1° on average) or of the (110) peak (~77.8° ± 0.1° on average)
indicates that the CNTs do not significantly influence the crystallite packing or curvature in these
directions, as the CNTs at higher Vf simply contribute more intensity to the diffraction pattern owing
to the superposition of CNT and epoxy features.
4 CONCLUSIONS
In summary, the fabrication and structural characterization of multiwalled A-CNT-epoxy PNCs
with novel mm-tall capillary-densified CNT cell and pin reinforcement were presented to investigate
the process-structure relationships of these materials as a function of A-CNT Vf in the polymer matrix.
These PNCs were created first via 1% Vf A-CNT chemical vapor deposition growth on Si substrates (a
subset of which were mechanically hand-scribed for patterned CNT pin formation). Then, the
patterned and bulk-scale A-CNT arrays were subjected to post-growth thermal annealing, capillary
densification up to ~8% CNT Vf, and polymer infusion and curing with an aerospace-grade epoxy
(RTM6) to create arrays of mm-tall CNT cell and pin PNCs for the first time. SEM, Raman
spectroscopy, and XRD characterization were used to determine how CNT confinement influences
polymer infiltration, wetting, atomic-scale structure, and the CNT-matrix morphology. In sum, this
analysis shows that CNT alignment is maintained during all processing steps, complete wetting of the
CNTs is achieved by the epoxy during infiltration, lower defect densities are observed for higher Vf
PNCs, and fracture behavior is dominated by CNT-matrix pull-out due to the governing CNT-polymer
matrix interactions. In the future, higher CNT Vf studies and additional property studies will be useful
to further quantify the governing process-structure-property relations and CNT-polymer confinement
effects for greater packing densities and other polymer systems, such as thermoplastics, where
polymer interphase formation is more likely to occur and scale performance. Once these relations are
known, the integration of capillary-densified CNTs may be realized for a wide range of composite
applications, such as aerospace composites laminates and other nano-engineered hybrid structures.
ACKNOWLEDGEMENTS
This work is partially supported by Airbus, ANSYS, Embraer, Lockheed Martin, Saab AB, Saertex,
and Teijin Carbon America through MIT’s Nano-Engineered Composite aerospace STructures
(NECST) Consortium, and partially supported by the National Aeronautics and Space Administration
(NASA) Space Technology Research Institute (STRI) for Ultra-Strong Composites by Computational
Design (US-COMP), grant number NNX17AJ32G. This work is also supported in part by the U. S.
Army Research Laboratory and the U. S. Army Research Office through the Institute for Soldier
Nanotechnologies under contract number W911NF-13-D-0001. A.L.K. is supported by the
Department of Defense (DoD) through the National Defense Science and Engineering Graduate
Fellowship (NDSEG) Program and gratefully acknowledges the members of necstlab for technical
support and advice. This work makes use of the MIT Materials Research Laboratory (MRL) Shared
Experimental Facilities, supported in part by the MRSEC Program of the National Science Foundation
under award number DMR-1419807, and is carried out in part through the use of MIT’s Microsystems
Technology Laboratories.
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