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Revealing Chemical Heterogeneity of CNT Fiber Nanocomposites via Nanoscale Chemical Imaging Anastasiia Mikhalchan, Agnieszka M. Banas, Krzysztof Banas, Anna M. Borkowska, Michal Nowakowski, Mark B. H. Breese, Wojciech M. Kwiatek, Czeslawa Paluszkiewicz, and Tong Earn Tay Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04065 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Chemistry of Materials

Revealing Chemical Heterogeneity of CNT Fiber Nanocomposites via Nanoscale Chemical Imaging Anastasiia Mikhalchan*§, Agnieszka M. Banas‡, Krzysztof Banas‡, Anna M. Borkowska#, Michal Nowakowski#, Mark B. H. Breese‡, Wojciech M. Kwiatek#, Czeslawa Paluszkiewicz#, Tong Earn Tay§ §

Department of Mechanical Engineering, National University of Singapore, 117576, Singapore

‡

Singapore Synchrotron Light Source, National University of Singapore, 117603, Singapore

#

Institute of Nuclear Physics, Polish Academy of Sciences, 31342, Krakow, Poland

ABSTRACT: Lightweight nanocomposites reinforced with carbon nanotube (CNT) assemblies raise the prospects for a range of high-tech engineering applications. However, a correlation between their heterogeneous chemical structure and spatial organization of nanotubes should be clearly understood to maximize their performance. Here, we implement the advanced imaging capabilities of Atomic Force Microscopy combined with near-field infrared spectroscopy (AFM-IR) to analyze the intricate chemical structure of CNT fiber-reinforced thermoset nanocomposites. As an example, we unravel the chemical composition of a nano-thin polymer interphase exclusively from CNT assemblies and visualize in a two- and three-dimensional format with resolution of sub-30 nm. We furthermore introduce a contact frequency map co-localized with CNTs and surrounding polymer, which might correlate the local mechanical properties with polymer chemistry and the high anisotropy of CNTs. Nano-resolved chemical imaging offers possibilities for in-depth characterization of nextgeneration composite materials and devices based on CNT assemblies interacting with a certain chemical environment.

INTRODUCTION Carbon nanotubes (CNT) assembled in mats or films,1-5 fibers,6-13 yarns,14-16 and textiles,17-19 possess a combination of properties desirable for advanced materials, such as wearable and stretchable electronics20, thermoelectricity harvesting textiles21, electrical wires,22 and supercapacitors.23 Depending on process conditions, as well as on density and quality of CNT bundle alignment, CNT fibers and yarns synthesized via the floating catalyst chemical vapor deposition (FC-CVD) exhibit superior specific thermal and electrical conductivities.12,24 Their low density, high tensile strength and stiffness6,12,13 with 100% knot efficiency25,26 pave the way for novel composite structures with enhanced toughness and resistance to crack propagation.27-30 To a great extent, the unique properties of CNT assemblies are intimately based on their hierarchical structure. In contrast to conventional carbon fibers with a monolithic cross-section, CNT fibers appear in the yarnlike form,11-13,25,26 where bundles of adjacent single-, double- or multi-walled carbon nanotubes are aligned and packed in the axial direction. In composite processing, a monomer (or a liquid polymer) intercalates the inter-bundle pores of a CNT fiber and occupies almost half of its volume, depending on the porosity (up to 50-70%).12,13,29,31 Therefore, the polymerinfiltrated CNT fiber appears as a hierarchical nanostructured composite, albeit in a preserved fibrous form.32-34 Theoretical and experimental findings affirm the importance of the CNT fiber/polymer interface in governing

a balance among strength, stiffness, and toughness of the CNT fiber-reinforced materials.28,30,35 Although some insights on CNT fiber infiltration31,36,37 and composite processing27-30 have been provided, the evolution of chemical structure and the resultant fiber/matrix interface are yet poorly understood; and neither nanoscale FTIR recognition nor other direct chemical imaging has been implemented so far for their analysis. The hierarchical structure of CNT fiber nanocomposite is notoriously difficult to be characterized because of inherent spatial organization of reinforcing elements, i.e. CNTs and their bundles. Thus, not the dispersion of discrete CNTs in a polymer, but the polymer distribution within continuous and tight assembly of CNTs (e.g. fiber or yarn) should be defined; taking into account a fact that resin cure occurs in the close vicinity of carbon nanotubes. Virtually, the questions related to chemical molecules distribution within CNT assemblies emerge in any case when fibers or yarns are immersed in a certain chemical environment and their performance is subject to interaction with it. For example, in manufacturing polymerinsulated CNT cords for electrical wires to replace traditional metal ones, potential polymer infiltration of a CNT fiber may result in a severe deterioration of its electrical and thermal properties.22 In another example, adsorbed polymer molecules could act as dopants, and their electrochemical interaction with CNTs affecting longitudinal fiber resistance could be employed to monitor polymer flow in fabrication of composite laminates.38

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In this work, we analyze and visualize the heterogeneous chemical structure of the epoxy nanocomposite reinforced with continuous CNT fibers by means of AFM combined with near-field infrared spectroscopy (AFMIR). This case study is done on the CNT fibers preinfiltrated with amine hardener (similar to electrochemically doped CNT yarns38) to define the chemistry at the interface between infiltrated CNT fiber and surrounding epoxy matrix. We describe whether it is possible to distinguish the misbalanced polymer chemistry within the CNT assembly, probe polymer fingerprints previously invisible via conventional FTIR microspectroscopy directly from CNT bundles, and discuss applicability of nanoscale chemical imaging for further nanocomposite engineering. EXPERIMENTAL SECTION Materials and sample preparation. CNT fibers have been synthesized by the FC-CVD method, with details on the synthesis and structure published elsewhere.39 The EPICOTE epoxy-novolac resin (Polymer Technologies Pte Ltd.) was chosen as a polymer matrix mainly because of its long pot life (5 hours), high degree of hardness, good chemical resistance, and mechanical properties, according to the supplier. Pre-infiltrated CNT fibers were embedded into a resin system mixed at a recommended ratio, followed by curing for 24 hours at room temperature. The cured nanocomposites were sliced with a Leica Ultracut UCT ultramicrotome to a thickness of 300-800 nm, and fixed on polished calcium fluoride (CaF2) optical windows (Crystran, UK) for further FTIR analysis. FTIR spectra of the reference materials. Analysis of the reference materials was performed using MIRacle (PIKE) device with single-reflection ATR ZnSe crystal (inserted to the sample chamber of FTIR Bruker spectrometer IFS 66v/S). As during the experiments samples were in an intimate contact with the crystal; in order to exclude any possible contamination occurring from one sample to another, special care was paid to cleaning the face of the crystal and then performing a background scan (a single channel spectrum of the ZnSe crystal) prior to a new sample placement. FTIR spectra were collected using a liquid nitrogen-cooled MCT (mercury cadmium telluride) detector; consist of 137 co-added scans measured at a spectral resolution of 4 cm-1 in the wavenumber range 4000–520 cm-1. The scanner velocity was set to 20 kHz. Recorded interferograms were Fourier transformed with a Blackman-Harris 3-term apodization and a zero filling factor of 2. Spectral data were acquired with OPUS 6.0 software. Far-field FTIR spectroscopy and chemical imaging. FTIR chemical maps of the CNT fiber nanocomposite slice were collected in transmission mode by means of Bruker Hyperion 3000 IR microscope with Focal Plane Array (FPA) detector attached to Vertex 70v spectrometer. Background channel was measured on pure CaF2 window. IR objective (with 36x magnification) was used that provides pixel size equal 1.2 µm (with 64 by 64 pixels of FPA detector). 128 spectra were co-added in order to ensure sufficient signal-to-noise ratio. Spectral range of 3845 to 800 cm-1 was set with the spectral resolution of 4 cm-1.

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Zero filling factor was set to 2 and Blackman-Harris 3Term appodization function with phase resolution 32 and power phase correction mode was used for converting interferograms to spectra. Near-field AFM-combined IR spectroscopy (AFMIR) and chemical imaging. Thin nanocomposite slices fixed on CaF2 substrates were analyzed within the 2000900 cm-1 spectral range by means of the nanoIR2 system (Anasys Instruments, USA) with top-down illumination and a tunable infrared source (optical parametric oscillator, Ekspla, Lithuania). The samples were scanned in contact mode at a scan rate of 0.02 Hz using a gold-coated AFM tip with an apex of nominal curvature radius (R) of sub-30 nm (Anasys Instruments, USA). The cantilever had a nominal spring constant of 0.07-0.4 N/m and a resonance frequency of 13±4 kHz while in a “free-state”, with the corresponding contact resonance frequencies (when cantilever is in contact with the sample’s surface) occurring in a higher range depending on the tip-sample interactions and the sample’s mechanical properties.40-42 During the experiments, the second harmonics cantilever oscillation mode was chosen for optimizing the cantilever ringdown signal centered at its frequency of 170 kHz with a frequency window of 50 kHz. The spectral data point spacing was 4 cm-1, and for each spectrum 128 scans were co-averaged. The AFM-IR spectra were smoothed using Savitzky-Golay filter with a polynomial order set to three and averaging of every five points. For chemical imaging, the scanning was performed with 400 points in the x and 100 points in the y direction (pixel size of 2.5 by 10 nm), also co-averaged over 128 scans. Additionally to the IR absorbance images, AFM images (height signal) and contact resonance peak frequency images were simultaneously recorded for the same regions of interest. Chemical and frequency maps reconstruction. Two-dimensional (2D) chemical maps and contact frequency maps were reconstructed using the integrated nanoIR2 software (Analysis Studio v3.11, Anasys Instruments, USA) and Gwyddion open-source software for scanning probe microscopy analysis (developed by D. Necas and P. Klapetek, GNU General Public License). Three-dimensional (3D) chemical maps and RGBcomposite image were generated by ImageJ software package (developed by W. Rasband, the National Institutes of Health). The frequency deviation maps (frequency shift from the initial value centered at 170 kHz) were recorded simultaneously with the AFM-IR absorbance maps with a laser tuned at 1108 cm-1 and 1240 cm-1. To ensure that the color variations represent the material property (i.e. not caused by post-processing), the maps were reconstructed with a zero-order plainfit (no filtering has been applied), the frequency shift profiles were smoothed using Savitzky-Golay filter with a polynomial order set to three and averaging of every twenty points. RESULTS AND DISCUSSION Far-field and near-field FTIR analysis. The CNT fiber-reinforced nanocomposite was made with a phenolic epoxy resin commonly utilized in composites and coatings43 and being capable of curing with an aliphatic polyamine hardener at room temperature. FTIR spectra of all


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reference materials (as-synthesized CNT fibers, cured resin, and its initial unmixed components in a liquid state; Supporting information, S1) were collected at the ISMI beamline, Singapore Synchrotron Light Source (SSLS, NUS, Singapore) using attenuated total reflection (ATR) mode. Spectra were pre-processed by a baseline correction and Min-Max normalization before pick assignment, ensuring interpretability and accuracy of the subsequent data. FTIR analysis affirmed characteristic absorbance peaks of the primary aliphatic amine hardener (commonly used

for curing from room to moderate temperature, as in our case) and epoxy-phenolic novolac resin.44 As for chemical visualization, the spatial resolution of any conventional IR microscope is diffraction-limited.45 Even when an IR microscope (Bruker Hyperion 2000) is combined with a synchrotron radiation source, it operates at a scale comparable to the diameter of a typical CNT fiber (15-25 um), thus, constraining the detection of polymer chemistry over the nanocomposite cross-section to a pixel size of a few microns.

Figure 1. (a) Contact-mode AFM height image (scale bar is in um) with (b) the enlarged region of 10 by 10 um (scale bar is in um), and (c) fingerprint region spectra of the reference materials taken by far-field FTIR (panel A, Href and Rref spectra from hardener and cured resin, correspondingly), and near-field AFM-IR spectra taken outside (panel B, points #1, #2) and inside (panel C, points #3, #4) the CNT fiber in the nanocomposite cross-section.

Nanoscale spatial resolution is a unique attribute of AFM-IR spectroscopy.46 The method is based on a direct measurement of the material’s thermal expansion, with details published elsewhere.47 In brief, rapid laser pulses (a few nanoseconds duration, 1kHz) tuned to a corresponding absorption wavelength, cause thermal expansion, thereby resulting in an oscillation of the AFM tip placed in contact with the material. The local AFM-IR absorption spectrum can be obtained at a certain point if the oscillation amplitude is recorded as a function of wavelength. On the other hand, probing the oscillation amplitude at various positions (by changing the position of the sample with respect to a tip position) results in two-dimensional (2D) IR maps for a given frequency. The AFM-IR spectra taken at several points across the nanocomposite slice (Figure 1a, b) confirm the cured epoxy matrix enclosing the CNT fiber (points #1 and #2). The obtained near-field spectra (Figure 1c) are rather identical with conventional FTIR spectra of the reference materials in terms of the absorption peak positions and band shapes. Such an agreement is due to the fact, that the AFM-IR oscillation amplitude is directly proportional

to the absorption coefficient and the amount of absorbed light,46,47 thus, allowing straightforward comparison with the classic far-field FTIR absorption spectroscopy. The spectra collected in the interior part of the CNT fiber cross-section are slightly noisy compared with those from the surrounding matrix, and exhibit a distinctive chemistry. A near-field AFM-IR spectrum taken at point#3 testifies to a presence of amine molecules (excess amount of hardener) from the strong absorption peak at 1108 cm-1, corresponding to C-N stretching in aliphatic amines, with almost diminished signal from 1240 cm-1 band. At the same time, some traces of the epoxy resin are still detectable within the fiber’s structure, for example at point#4, where the AFM-IR spectrum shows a strong contribution from C-O asymmetric stretching of aryl-alkyl ether group at 1240 cm-1. These observed traces may indicate that upon immersing of the pre-infiltrated CNT fiber into a liquid epoxy matrix, the diffusion of components (e.g. amine hardener and epoxy molecules) is yet possible within the interconnected pores of the CNT fiber, at least, closer to its outer surface.



Spectral data normalization for chemical mapping of nanocomposites. The nanocomposite slices exhibit sophisticated (‘hairy’) topography with numerous CNT bundles pulled out during the ultramicrotome slicing operation. AFM height channel images (Figure 1a,b) and profiles (Supporting information, S2) show roughness varying from tens of nm for the ordinary epoxy matrix surroundings to the hundreds of nm for the regions with a random network of long CNT bundles. Evaluation of the spatial distribution of chemical components via far-field or near-field IR spectroscopy across such a complex topography is rather a challenging task in comparison with chemical analysis of typical spin-coated polymeric films,48 thin epoxy coatings,43 or relatively smooth microtome slices of polymer multilayers.49 Considering the significant roughness and potential chemical heterogeneity of the nanocomposite slices, a special attention should be bestowed for the data processing and normalization in order to eliminate any effect of complex background on the IR signal intensity. The chemical heterogeneity of specimens could be revealed by direct identification and mapping of the characteristic absorbance peaks of functional groups unique to each component, for example, in a phase-separated polymer blends.50 However, usually, for mapping of polymeric systems it is sufficient to normalize the spectral data via a ratio to a single chemical band (“reference” peak) which does not change during the experiment. For example, for a stoichiometric epoxy phe-

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nolic system the IR absorbance peak at 916 cm-1 relative to 1180 cm-1 could be used to evaluate the completeness of curing reaction.43 In our case study, we could not follow the single-band normalization43 to distinguish the chemical distribution of excess amount of amine hardener, which is also capable to react with surrounding matrix, within preinfiltrated CNT fibers. The fingerprint regions comprised the absorption contribution from both hardener and (un)cured resin. For example, aromatic doublet peaks in the epoxy and in-plain N-H bending vibrations in the hardener are detectable nearly at the same region of 16001450 cm-1. Therefore, to yield consistent chemical imaging only of particular chemical components, we chose to operate with 1108 cm-1 and 1240 cm-1 absorption bands and to use their intensity ratios for the chemical map reconstruction. Employing a ratio of certain chemical bands is appropriate for non-homogeneous complex specimens to eliminate any potential influence of the local properties, topography, and non-uniform thickness, and regarded as “a good practice to confirm the (data) interpretation”.51 As seen from the FTIR spectra (Figure 1c, panel A), the absorption signal at 1108 cm-1 is much stronger in hardener, but the pronounced band at 1240 cm-1 is visible only in (un)cured resin. Accordingly, each pixel of the image with a calculated ratio larger than 1 (one) represents amine molecules, and vice versa for the resin.

-1

-1

Figure 2. (a) FTIR microspectroscopy chemical map reconstructed for the 1108 cm / 1240 cm intensity ratio from the area of 64 by 64 um. Note, the single-pixel is 1.2 by 1.2 um. (b) Contact mode AFM height image of the enlarged region of 1 by1 um at the CNT fiber/matrix interface (scale bar is in um). (c) Near-field AFM-IR spectra taken at points #1 and #2 (panel A), and at points #3 and #4 (panel B), confirming the different chemical composition. AFM-IR chemical images of the enlarged region, showing -1 -1 distribution of amine molecules (d, reconstructed for the 1108 cm / 1240 cm intensity ratio) and epoxy matrix (e, reconstructed -1 -1 for the 1240 cm / 1108 cm intensity ratio).


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Nanoscale chemical heterogeneity of the CNT fiber/epoxy interface. Figure 2a displays a FTIR microspectroscopy map reconstructed from the area of 64 by 64 um of the given nanocomposite cross-section, with a color scale from blue to red showing the intensity ratio of chosen absorption bands. The position of numerous yellow and red pixels with intensity ratio >0.5 correlates well with the CNT fiber cross-section visible in the center of the map and, apparently, illustrates a higher concentration of amine molecules. However, each pixel of this chemical map corresponds to an area of ~1.44 um2, which is significantly larger than the typical dimensions of the nanocomposite structural elements (i.e. CNT bundles). By scanning at this resolution, the spectral information was simply averaged at each point, implying that absorption signals from different molecular features were overlapped across the ~1.44 um2 area. Therefore, we were able neither to define amine dopant traces nor to visualize the chemical heterogeneity arising in contact with the CNT assembly. Next, we applied AFM-IR nanospectroscopy and imaging to study the chemistry variation across the area of 1 by 1 um selected at the CNT fiber/matrix interface (nota bene, smaller than a single pixel-size of the chemical image in Figure 2a; position is marked by a blue arrow). Spectra recorded at a few points (Figure 2c) shed light on the non-homogeneous distribution of the chemicals at the sub-micron level (Figure 2d,e). In the CNT fibernanocomposites the occluded polymer spreads on a welldeveloped interfacial area, as the surface area of CNT fibers used in this study was about 60-90 m2/g,40 and could be up to 200 m2/g for the similar FC-CVD fibers.12,13,25,26 Upon infiltration, the polymer interphase with chemistry differing from the bulk-state emerges in direct contact with CNT bundles, more specifically, around each bundle,

as the polymer does not infiltrate the bundle itself.31 The scale factor in such a case is not comparable with traditional fiber-reinforced plastics. For example, in carbon fiber/epoxy composites the polymer interphase of a thickness of ~200 nm has been clearly visualized by AFMIR spectroscopy.52 Taking into account that the CNT bundles are only a few tens of nm in diameter, the polymer interphase surrounding them in the nanocomposites should be at the range of a few nm only. Nanoscale chemical imaging of pulled-out CNT bundles. With the unprecedented resolution of the AFMIR spectroscopy it is possible to elucidate chemical features of the nanocomposites which were otherwise invisible via conventional FTIR microspectroscopy and imaging. As an example, we unravel the chemical information directly from the pulled-out CNT bundles (Figure 3a). The near-field spectra taken at the positions marked in the topography image (Figure 3b, only representative region of 1300-900 cm-1 is shown) and reconstructed chemical image for the 1108 cm-1 / 1240 cm-1 intensity ratio (Figure 3d) reveal the amine molecules being primarily associated with CNT bundles, which is in fact not surprising, as the fibers have been pre-infiltrated with amine hardener. To co-localize precisely chemical information with topography and position of CNT bundles, the obtained data were reconstructed in the RGB-composite image, shown in Figure 3e, combining the topography (green color) with the distribution of amine molecules (red color) and the epoxy matrix (blue color). In addition, the chemical structure was visualized in a 3D format (Figure 3f), where x, y, and z coordinates originate from the AFM height channel, acquired simultaneously with the AFM-IR chemical probing, and a color code illustrates the certain chemistry (red color in Figure 3d-f shows the presence of amine molecules).



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Figure 3. (a) Contact mode AFM height image of 1 x 1 um area at the CNT fiber/matrix interface with four pulled-out CNT bundles (scale bar is in um). (b) Spectra taken at points #1 and #2 marked in the topography image, note only representative region -1 of 1300-900 cm is shown. (c) AFM height profile across the CNT bundle as marked by white line in (a). (d) AFM-IR chemical -1 -1 image reconstructed for the 1108 cm / 1240 cm intensity ratio. (e) RGB-composite image, showing the topography (green color) with distribution of amines (red color) and epoxy matrix (blue color). (f) Chemical image of the nanocomposite reconstructed in a 3D form. Red color shows the distribution of amine molecules across the topography.

The co-localization of different signals into a single RGB-composite image is useful for visualization of complex samples containing a component with low thermal expansion coefficient otherwise hardly measurable via near-field AFM-IR. Rosenberger et al.53 were unable to visualize individual CNTs via AFM-IR due to their small diameter and almost negligible thermal expansion when scanned nanotubes without a polymer layer underneath (experiments were done with a bottom-side illumination, previous version of nanoIR system). We assume that in our case the detectable signal originated from the thermal expansion of thin polymer interphase remaining on each CNT bundle rather from CNTs themselves, due to the known difference in their thermal expansion coefficients. It is, therefore, remarkable that the chemical fingerprints are defined with high sensitivity, and some fine traces of amine hardener are visible even on the thinnest (in this area) CNT bundle according to the AFM profile (Figure 3c). Notably, this nano-thin layer is retained on the CNT bundles even after severe mechanical deformation (pulled out from the nanocomposite by an ultramicrotome knife), which is, probably, a consequence of high affinity of CNTs to amines.54,55 The obtained results testify that it is, in

principle, possible to track the appearance and gradient of the polymer interphase along each CNT bundle. Recent tapping-mode AFM-IR with a higher repetition rate quantum cascade laser (QCL) exhibits sensitivity sufficient for tracking and visualization 4 nm-thick traces of a lubricant on a bare polymer surface47 or 5 nm-thick monolayers of a surfactant.56 In the desired future, high sensitivity of AFM-IR will help in the direct characterization of crosslinkages between adjacent CNTs to maximize their reinforcing potential.34,57,58 Co-mapping of local elastic properties. As already mentioned, the material’s thermal expansion excites the cantilever deflection of a certain amplitude and frequency. The eigenmodes of the cantilever are related to the contact resonances,51 where the peak frequencies of the cantilever oscillation depend on the elastic properties of the sample: the stiffer the sample, the higher the contact resonance frequency.46 Complementary, the contact resonance peak frequency maps were recorded simultaneously (exactly at the same time and over the same areas as AFM-IR absorbance maps shown in Figure 3) to correlate topography and chemical distribution with the local mechanical properties of the nanocomposite (Figure 4).

Figure 4. Contact resonance frequency maps recorded simultaneously with AFM-IR chemical probing at (a) 1108 cm-1 and (b) 1240 cm-1 for the same area as in Figure 3a, and (c) for the different area with CNT bundles recorded with chemical probing at 1591 cm-1. (d-f) Corresponding frequency shift profiles taken across pulled out CNT bundles at positions marked by black arrows.

The color code delineates stiffer (orange-to-red) and softer (green-to-blue) regions across the area with CNT localization (Figure 4 a,b). Surprisingly at first glance, the areas corresponding to pulled-out CNT bundles appear to be softer than the surrounding resin. The similar trend was observed for other parts of the nanocomposite while

scanning with a laser tuned at different absorption bands (not only at 1108 cm-1 and 1240 cm-1 used for the reconstruction of chemical maps, but for example, at 1591 cm-1 and 2930 cm-1) (Figure 4 c and Supporting information S3). The collected frequency maps and corresponding frequency shift profiles correlate well with each other and


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show strong consistency in representing the heterogeneous structure of the nanocomposite. It should be noted, that in these experiments the AFM tip was, in fact, probing thin polymer layer remaining on top of each bundle, not the “pure” individual CNTs fixed on a rigid substrate. Thus, the frequency shift to lower values can refer to the chemical structure of the amine-rich interphase with presumably lower stiffness formed in the close vicinity of CNT bundles during the composite processing in comparison with the cured resin. The mapping of local nanomechanics in a case when the AFM tip probes the pulled-out CNTs or their bundles in the lateral direction is potentially sensitive to high anisotropy and relatively weak radial properties of CNTs, for which the radial elastic modulus varies from 16-23 GPa59 to only 0.3-4 GPa60 – values comparable with that of polymers. Future attention should be bestowed to the accumulation of individual CNTs into bundles at the synthesis step, leading to inevitable co-existence of un-, partially-, and fully-collapsed nanotubes even within a single bundle25, which may affect their axial and radial mechanical properties. The radial characteristics of CNT bundles surrounded by a polymer could be particularly important if the structure is subject to transverse deformation. As has been shown for the CNT-fiber reinforced nanocomposites, their modulus and strength increase almost linearly with the volume fraction of CNTs, whereas the effective reinforcing potential is significantly affected by degree and “quality” of CNT alignment at the sub-micro scale.27,29 At the same time, crosslinking process and interfacial interactions taking place at the nano-scale between CNT bundles and polymer chains in the polymer-infiltrated CNT assemblies are also substantial for achieving superior mechanical properties, according to the recent multi-scale numerical simulations.35 The AFM-IR near-field infrared spectroscopy appears to be an advanced tool for the direct correlation of nanoscale chemistry and local elastic properties with CNT distribution in such complex systems as nanocomposites reinforced with CNT fibers or yarns. CONCLUSIONS Direct chemical analysis and nanoscale imaging of the hierarchical CNT fiber nanocomposites is extremely challenging because of inherent assembling of continuous CNT bundles and polymer intercalation in the interbundle pores. AFM-IR near field spectroscopy helps elucidate the chemical heterogeneity of the CNT fiber/polymer interface with the nanoscale resolution and visualize it down to the scale of reinforcing elements, i.e. CNT bundles. Significant chemical variation throughout the nanocomposite cross-section resulted from the diffusion of chemical components when the pre-infiltrated CNT fiber was incorporated in liquid epoxy matrix. According to chemical probing, nano-thin polymer interphase layer on pulled-out CNT bundles was predominantly formed by amine molecules, leastwise for the evaluated case of amine-doped CNT fibers. The straightforward correlation with local elastic properties indicated that this amine-rich interphase surrounding each bundle was less stiff than the outer cured matrix. The ability to visualize

the direct chemical evidence from nano-thin traces of the material is an important practical feature highly applicable for advanced nanocomposite engineering and analysis of a wide range of CNT assemblies doped or functionalized with external chemical molecules.

ASSOCIATED CONTENT Supporting Information. The following file containing FTIR spectra of the initial components, AFM height profiles, and an additional contact resonance frequency map (PDF file) is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Author Contributions A.M. conceived an idea and collaborative project, requested the time allocation for experiments at SSLS and INP PAS, prepared the nanocomposites, evaluated the results with support from Ag.M.B. and K.B., and wrote the first draft of the manuscript. A.M., Ag.M.B., and K.B. designed the experimental approaches. Ag.M.B., K.B., An.M.B., and M.N. performed far-field and near-field FTIR tests and primary data processing; Ag.M.B. and K.B. fulfilled pre- and postprocessing of all spectroscopy data and chemical images. M.B.H.B., W.M.K., C.P., and T.E.T. provided important materials, equipment, and administrative support. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources A.M., T.E.T. acknowledge the financial support from the NUS University Strategic Funding for the Centre for Composite Engineering and Research under R-265-000-523-646. An.M.B., M.N. acknowledge funding from Marian Smoluchowski Kraków Scientific Consortium "Mater- Energy- Future" under the KNOW grant. The part of the spectroscopy research was performed using equipment purchased in frame of the project co-funded by the Malopolska Regional Operational Programme Measure 5.1 Krakow Metropolitan Area as an important hub of the European Research Area for 20072013, project No. MRPO.05.01.00-12-013/15. The authors declare no competing financial interest.

ACKNOWLEDGMENT A.M. and T.E.T. acknowledge the Transmission Electron Microscopy Lab (Dept. of Materials Science and Engineering, NUS) for the access to the ultramicrotome, and the Materials Science group (Dept. of Mechanical Engineering, NUS) for providing CNT fibers. The authors acknowledge the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the research. The Laboratory is a National Research Infrastructure under the National Research Foundation Singapore.



(9) Zhang, X.; Li, Q.; Tu, Y.; Li, Y.; Coulter, J. Y.; Zheng, L.; Zhao,

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