Processing-structure–property relationships of SWNT–epoxy composites prepared using ionic liquids

Accepted Manuscript Processing-Structure-Property Relationships of SWNT-Epoxy Composites Prepared using Ionic Liquids Arianna Watters, Jefferson Cuadra, Antonios Kontsos, Giuseppe Palmese PII: DOI: Reference:

S1359-835X(15)00108-6 http://dx.doi.org/10.1016/j.compositesa.2015.03.019 JCOMA 3890

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

6 January 2015 13 March 2015 14 March 2015

Please cite this article as: Watters, A., Cuadra, J., Kontsos, A., Palmese, G., Processing-Structure-Property Relationships of SWNT-Epoxy Composites Prepared using Ionic Liquids, Composites: Part A (2015), doi: http:// dx.doi.org/10.1016/j.compositesa.2015.03.019

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Processing-Structure-Property Relationships of SWNT-Epoxy Composites Prepared using Ionic Liquids Arianna Watters1, Jefferson Cuadra2, Antonios Kontsos 2, and Giuseppe Palmese3*

1. Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 2. Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 3. Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA

Corresponding Author: Giuseppe Palmese [email protected] 3141 Chestnut Street Philadelphia, PA 19104, USA +1 (215) 895-5814

Abstract

Experimentally achieved mechanical properties of nanotube-epoxy composites fail to match theoretical expectations; shortcomings are mainly attributed to poor dispersion. The elastic modulus of a well-dispersed single walled carbon nanotube (SWNT)-ionic liquid-epoxy composite was evaluated in tension and compared to predictions by a micromechanics homogenization model. The model takes into account the mechanical properties of the constituent phases in addition to SWNT aspect ratio, spatial distribution, dispersion, and agglomeration. These parameters were evaluated using information obtained via scanning and transmission electron microscopy. The Young’s modulus of the composite shows excellent agreement with the model at low

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concentrations, while discrepancies at high SWNT concentrations are possibly due to composite processing limitations. At high concentrations the uncured composite mixture is above the rheological percolation threshold. As the polymer network reaches its maximum capacity for welldispersed SWNTs, increasing volume fraction does not result in further significant reinforcing effects.

Keywords: A. Polymer-matrix composites (PMCs); B. Mechanical properties; B. Rheological properties; C. Micro-mechanics

Introduction:

Epoxies are thermosetting resins that are important in high performance structural applications [13], and their properties can be modified by the inclusion of a second phase [4, 5]. In this context, the mechanical [6, 7] and electrical [8, 9] properties of carbon nanotubes (CNT) have attracted much interest in the development of advanced composite networks, particularly in structural light weight composites since [10-12]. Carbon nanotubes have been reported to have Young’s Modulus (YM) values of up to 1 TPa [6] depending on their structure and manufacturing method. The first report of a nanotube-reinforced epoxy composite was published by Schadler et al. in 1998 [13], demonstrating a 20% increase in YM with 5 wt% inclusion of multi-walled carbon nanotubes (MWNT). In this context, much progress has been made in the past 15 years as nanocomposite preparation techniques have matured. However, there has been limited success in translating the nanoscale properties to structural composites with enhanced mechanical properties. These shortcomings in mechanical property improvements are mostly attributed to poor dispersion of the filler [5, 10, 14] and poor stress transfer from the polymer to the dispersed filler phase through their

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interface [14-17]. Other important processing considerations to overcome are tube cleavage, entanglement, distribution and orientation [18-20].

Imidazolium-based ionic liquids (IL) serve as excellent dispersants for many nanoparticles, particularly carbon nanotubes [21]. In the presence of shear forces, the ionic liquid non-covalently interacts with carbon nanotubes to disrupt the π-π bonds responsible for nanotube bundling [22, 23]. There has been much recent interest in using an IL dispersant for composite preparation, and ILs containing pyridinium [24], phosphonium [25] and imidazolium groups [26-28] have been investigated. The authors have demonstrated that ILs containing a dicyanamide anion can initiate anionic cure of epoxy resins over a wide range of concentrations [29], and more recent work by the authors [30, 31] has shown that by using an ionic liquid consisting of an imidazolium cation and dicyanamide anion, the ionic liquid can serve as both a dispersant and initiator for symthesis of a well-dispersed SWNT-IL-Epoxy composite system. This well-dispersed composite shows electrical conductivity behavior in excellent agreement with percolation theory and a critical percolation of 4.29x10-5 volume fraction SWNTs (0.005 wt%), comparable with the best dispersed systems presented in literature [32].

Nanocomposite properties are dependent on the quality of dispersion, among other factors. A large number of nanotube-epoxy composites and their corresponding mechanical property improvements have been presented in literature, reporting Young’s modulus enhancements over a wide range of nanotube concentrations. Compiled in Table 1 are Young’s modulus enhancements presented in literature for SWNT-epoxy composites, including reports up to 10 wt% SWNTs. Mechanical property enhancements are best reported as percent increase in modulus, since epoxy network, hardener and nanotube specifications vary widely among literature reports. One report by Loos et

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al. in 2008 [33] stands out, reporting a significant modulus increase just short of 500% at a SWNT content of only 0.25wt%. However, this is a result of using an epoxy matrix which is rubbery under ambient conditions. Another report by Wang et al. in 2008 [34] demonstrates a 103% increase in YM at a SWNT content of 0.3 wt% and 177% increase in modulus at a SWNT content of 10 wt%, but this report contains aligned nanotubes, whereas all other composites reported in Table 1 contain randomly oriented networks. The work by Wang et al. prepared composites by solution casting. SWNTs were pretreated with nitric acid and sonicated in ethanol before dispersion in the epoxy matrix, and in order to align the SWNTs the material was stretched with a draw ratio of 50 before it was dried, folded and stretched repeatedly for 100 times.

The work presented in this article evaluates the mechanical property improvements of welldispersed SWNT-IL-Epoxy composites. Evaluating the effect of nanotube-epoxy interfacial interactions is outside the scope of this report, which focuses solely on the structure-property relation between dispersion and mechanical property improvements. Experimental work is compared to Young’s modulus predictions by the micromechanics homogenization model which uses experimentally measured parameters to accurately predict composite behavior.

Methods

The difunctional epoxide used in this study is diglycidyl ether of bisphenol A (Epon 828) purchased from Miller Stephenson (99% purity). The ionic liquid is composed of imidazolium cations and dicyanamide anions; 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCN) and

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was purchased from Sigma-Aldrich (purity >98%). The SWNTs used in this study were produced by chemical vapor deposition, have a diameter of 1.5 nm and length ranging 1-5 µm (purity >95%), and were purchased from nano-lab.com. All materials were used as received.

SWNTs, IL and epoxy were combined at a constant IL concentration of 15 wt% with respect to epoxy and SWNT concentrations ranging from 0.01-5.0 wt%. The material was processed by a Torrey Hills 2.5x5” three roll mill ten times using a 6.5 µm gap. Liquid composites were poured into appropriate molds and followed a cure schedule of 12 hours at 80ºC and a 2 hour postcure at 120ºC. The cured composite was sanded and polished to even thickness. More details regarding composite preparation can be found in Watters et al. [32].

Composite dispersion was evaluated by electrical conductivity measurements in conjunction with percolation theory to confirm the highest quality dispersion was achieved prior to mechanical evaluation. Electrical conductivity was calculated from through-plane resistivity measurements of the bulk composite, reported in S/m. All information regarding electrical conductivity and percolation model for SWNT-Epoxy composites can be found in Watters et al. [32].

Composite mechanical properties were measured in tension in accordance with ASTM D638-03. Composite samples were prepared in a dogbone shaped mold, following Type IV sample size specified in the standard, and modified to have a 12.5 mm gage length to accommodate the strain gage extensometer (dynamic extensometer 12.5 mm GL ± 5 mm travel, part number 2620-601). The extensometer was clamped along the gage length of the sample, and composite materials were loaded into an Instron 8872. The test was carried out at a constant strain rate of 1 mm/min. The test was paused at 0.5% strain, and the extensometer was removed before resuming the test. Samples

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were tested until failure. Ultimately, the Young’s modulus was calculated from the stress-strain relation recorded by the extensometer.

Qualitative assessment of dispersion, agglomeration and nanotube length was determined by image analysis using a Zeiss Supra 50VP scanning electron microscope (SEM) and a Tecnai FEI T12 transmission electron microscope (TEM) imagery. SEM samples were prepared by mounting a fracture surface of the tensile samples onto a base, and sputter coating with platinum to a nominal thickness of 7-9 nm. TEM micrographs were prepared by microtoming composite sections <100 nm thick and placing on a 300 µm copper mesh grid

Rheological response of liquid phase composites was evaluated by TA Instruments AR-2000 Rheometer at 25˚C using a 40 mm steel plate and a gap of 1000 µm. Oscillatory frequency sweep measurements were carried out in a frequency range of 0.1-628 radians/second. Complex viscosity and elastic modulus were evaluated to determine the transition from Newtonian to solid-like behavior, commonly known as the rheological percolation threshold.

Results and Discussion

Nanocomposite Young’s modulus values were evaluated by performing tensile tests in accordance with ASTM D638-03 for the unmodified IL-Epoxy network, for SWNT-IL-Epoxy composites at low concentrations of 0.01, 0.05, and 0.1 wt% SWNTs and high concentrations of 1.0, 2.0, and 5.0 wt% SWNTs. Figure 1 shows experimentally measured modulus values of SWNT-IL-Epoxy composites over a SWNT concentration range of 0.01-5.0 wt%. The modulus of the unmodified IL-

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Epoxy network is 2.89 GPa. At low concentrations of 0.01, 0.05, and 0.1 wt% SWNTs, the composites exhibited moduli of 2.92 GPa, 2.95 GPa, and 3.14 GPa, respectively, corresponding to 1.04%, 2.08%, and 8.65% increases with respect to the unmodified network. At high concentrations of 1.0, 2.0, and 5.0 wt%, the composites exhibited moduli of 3.45 GPa, 3.46 GPa, and 3.51 GPa, respectively, corresponding to 19.4%, 19.7%, and 21.5% increases with respect to the unmodified network. SWNT-IL-Epoxy composites exhibit enhancements in Young’s modulus up to a concentration of 1.0 wt% SWNTs, above which any increment of SWNT concentration (up to 5 wt%) did not result in any further significant improvement in experimentally measured modulus values .

The distribution of nanotubes at the microscale and nanoscale was evaluated by SEM and TEM characterization, respectively. Figure 2 contains two columns of characteristic SEM (left) and TEM (right) micrographs of low concentration SWNT-IL-Epoxy composites, used to qualitatively assess dispersion. The top row shows images of 0.01 wt% SWNTs composite, the middle row contains images of 0.05 wt% SWNTs composites, and the last row contains 0.1 wt% SWNT composites. SEM and TEM images were prepared from the same composite sample. SEM images of low concentration composites reveal a generally well dispersed material, and upon closer observation via TEM imaging, a mixture of well dispersed tubes with some agglomerations spans the composite. Agglomerations appear loosely bound, in which nanotubes are clustered, but not densely packed. This is a result of the IL and polymer network infiltrating the bundles and disrupting the tube-tube attractions.

Similarly, SWNT dispersion and distribution was also evaluated for high concentration composites. Figure 3 contains two columns of characteristic SEM (left) and TEM (right) micrographs of high

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concentration SWNT-IL-Epoxy composites. The top row shows images of 1.0 wt% SWNTs composite, the middle row contains images of 2.0 wt% SWNTs composites, and the last row contains 5.0 wt% SWNT composites. Similarly to the low concentrations, the SEM and TEM images were comparably prepared from the same composite sample. SEM images of high concentration composites reveal a randomly dispersed surface, while TEM images at a higher resolution show very loosely bound nanotube agglomerations within a densely packed and well dispersed network. In addition, SWNT presence clearly increases with higher nanotube loading.

Figures 2 and 3 present a collective group of micrographs at low and high magnifications for each composite sample, providing important information about dispersion, distribution and agglomeration of the SWNT network. In order to further investigate the composite Young’s modulus at concentrations above 1.0 wt% SWNTs, image analysis was carried out for each of the composites using Image J software. A minimum of 3 images were evaluated for each composite, at low, medium and high magnifications, evaluating all features of circularity of 0-1 and surface area >10 nm2. Percent SWNT surface area was within 10% of the average for all images evaluated. Table 2 contains average agglomeration size, reported in nm2, and percent area of the image that contains SWNTs for each composite sample. Up to a SWNT concentration of 1.0 wt%, average agglomeration size remains in a narrow range between 295-366 nm2, then increasing significantly for higher nanotube concentrations of 2.0 and 5.0 wt% SWNTs. Percent surface area of TEM images containing SWNTs is shown in the last column, which increases with SWNT concentration in the composite. The average agglomeration size was not expected to show significant variability since it is determined by the gap in the three roll mill, and all composites were processed identically. However, the jump in average agglomeration size at high concentration composites (2.0

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and 5.0 wt% SWNTs), corresponds to the composites which exhibit a plateau in Young’s modulus enhancements and indicates the existence of a limitation in composite performance.

To investigate the plateau in Young’s modulus that is observed for composites containing greater than 1.0 wt% SWNTs, a micromechanics homogenization method (MHM) [35-37] coupled with an agglomeration model [37, 38] previously developed by the authors for determining effective mechanical properties of SWNT-epoxy composites was used to suggest enhancements in modulus. The MHM is based on the Mori-Tanaka approach and accounts for non-uniform dispersion and agglomeration of SWNTs in the macroscopic behavior of the composite by using model parameters of a particular experimental material system, and was used herein to estimate overall mechanical properties of the SWNT-IL-Epoxy composite. The model takes into account the mechanical properties of the constituent phases in addition to SWNT aspect ratio, spatial distribution, dispersion and agglomeration, as it is shown by the formulation derived in detail in Appendix A. The aspect ratio is evaluated in such formulation in the form of a geometrically dependent tensor (i.e. the Eshelby tensor) while the spatial distribution is considered to be random and mathematically represented by an orientation dependent tensor. In the case of agglomeration and dispersion parameters, the agglomeration is measured by the agglomerate inclusion volume occupied within the representative volume while the SWNTs are considered to be dispersed if the proportion of SWNTs in the inclusions is low with respect to the hybrid volume of matrix. These parameters were evaluated and compared using information obtained via scanning and transmission electron microscopy.

In order to directly evaluate the effects of agglomeration and dispersion on the composite properties, the MHM was implemented by using parametric indices that quantify dispersion and

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agglomeration [39]. SEM and TEM micrographs, such as those presented in Figures 2 and 3, are examples of a larger set of images that serve as representative material regions from which evaluations of spatial distribution of the SWNTs were made, providing parameters which include nanotube length, diameter, agglomeration, dispersion and spatial distribution. These experimentally-based geometric considerations are critical for an effective model development [40]. The aspect ratio parameter in the model used a value of 500. This value was chosen after evaluating the effect of the SWNT aspect ratio (AR) on the Young’s modulus in a parametric study at different concentrations using the MHM for a well dispersed system. The study showed a nonlinear effect at low AR values; however, in the range of 500 AR the effective moduli in all concentrations unveiled convergence to its highest values. The nanotube aspect ratio parameter in the model could possibly quantify SWNT imperfections (i.e. waviness, entanglement, and others) that may influence the composite overall properties due to deficiencies in the original aspect ratio. An average nanotube diameter of 1.5 nm was confirmed via TEM imaging.

In addition to filler geometry, SWNT and epoxy material properties used as parameters for the model are shown in Table 3. Uniaxial Young’s modulus of SWNT, presented as E11, is 450.47 GPa as used by Kontsos et al. [38], and E11 for the unmodified epoxy matrix is 2.89 GPa, as measured in tension. Parameters listed in Table 3 were implemented into the MHM to provide model predictions for composite modulus. The maximum modulus values determined by the MHM are: 2.8965 GPa at 0.01 wt%, 2.9190 GPa at 0.05 wt%, 3.141 GPa at 0.1 wt%, 3.55 GPa at 1.0 wt%, 4.1 GPa at 2.0 wt%, and 6.25 GPa at 5.0 wt% SWNTs. For additional information regarding the MHM please refer to Spanos et al. [37] and Kontsos et al. [38].

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The composite mechanical performance in this study were found experimentally to be significantly affected by the nanofiller dispersion, and while all other processing parameters remained unchanged, the agglomeration from one representative region to another within the same material was confirmed to vary. To account for this effect in the modeling approach, a deagglomeration index (α) was defined as the fraction of inclusions in the material region. It should be noted as the parameter α approaches 1, the more homogeneous the material region (i.e. composite) becomes. In addition, a dispersion index was defined as the ratio of SWNT concentration values within agglomerated SWNT inclusions to corresponding concentrations of SWNTs in the entire composite. Figure 4 shows computed Young’s modulus as a function of the deagglomeration index for low SWNT concentration composites with (a) 0.01 wt% SWNTs, (b) 0.05 wt% SWNTs, and 0.1wt% SWNTs, and high SWNT concentration composites with (d) 1.0 wt% SWNTs, (e) 2.0 wt% SWNTs, and (f) 5.0 wt% SWNTs. On each plot two models are shown, indicating optimal and poor dispersion. As the dispersion index approaches 1, the more heterogeneous the composite becomes which represents the poor dispersion condition. As the dispersion index approaches 0, the more homogeneous the composite becomes which represents the optimal dispersion condition (refer to the Appendix for more detailed information about the model). These dispersion conditions are shown in Figure 4, using triangles and squares, respectively, indicating a range of predicted modulus values for the SWNT-epoxy composites based on the quality of composite dispersion.

At low concentrations, the experimental modulus is in excellent agreement with model predictions. While processing low concentration composites by the three roll mill the material remains liquid with a relatively low viscosity, allowing the nanotubes to separate from the bundles and effectively disperse throughout the resin while remaining easily processable. At high concentrations, the experimental modulus is lower than the range predicted by the MHM models, and experimentally

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measured composite modulus reaches a plateau at 1.0 wt% SWNTs with negligible improvements at higher SWNT content. This can be attributed to the fact that high concentration composites exhibited a significant increase in viscosity during shear processing. Moreover, the composites reached nanotube gelation after only one or two passes through the mill, and after 10 passes the composites had the consistency of a paste [41-44].

Evaluating rheological properties of a composite during processing can provide a further understanding of structure-property relationships found after cure [45]. Rheological testing can be used to determine the concentration at which the filler particles create a three dimensional network accompanied by solid-like behavior of the liquid-phase composite before cure. Rheological properties depend on many factors including filler content, aspect ratio, dispersion state and fillerpolymer interface [46]. The variable explored in this work is the filler content. Complex viscosity (comprised of elastic and viscous responses) and rheological storage modulus (elastic response) as a function of frequency are two commonly used characteristics to evaluate rheological properties, and have been used to evaluate nanotube dispersion in various thermoplastic [47-49] and thermosetting [42-44] matrices.

The complex viscosity and storage modulus of the composite were evaluated by rheological measurements of the liquid-phase composite after three roll mill processing. Figure 5 shows the rheological response of SWNT-IL-Epoxy composites as a function of frequency at 25ºC: (a) complex viscosity, (b) storage modulus, and (c) corresponding rheological percolation threshold at a frequency of 1 radian/second. The unmodified network and low concentration composites exhibit a frequency-independent complex viscosity response typical of Newtonian behavior (Figure 5a). With higher SWNT content, the complex viscosity increases and becomes frequency-

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dependent, indicating that the SWNTs begin to interfere with the polymer network, resulting in a shear thinning response. The storage modulus of the composite gradually increases with frequency and increasing SWNT content, which is a reflection of the transition from a viscous liquid to solidlike behavior (Figure 5b). By plotting storage modulus as a function of increasing SWNT content, the composite shows a clear rheological transition to solid like behavior, which is characterized by a plateau in storage modulus with increasing nanotube content. This transition occurs between 0.11.0 wt% SWNTs concentration and it is defined as the rheological percolation threshold. It was found that the experimentally determined rheological percolation threshold corresponds to the plateau in modulus enhancements at 1.0 wt% SWNTs. Characteristic SEM and TEM images of high concentration composites in Figure 3 further support these findings, as images reveal that the SWNT filler is well dispersed and densely packed with small nanotube agglomerations. Once the matrix has become saturated with well-dispersed carbon nanotubes, those remaining in agglomerates will not be able to contribute to the randomly oriented network necessary for further modulus enhancement. However the composites will follow the trend of increasing rheological storage modulus with larger more dense agglomerates contributing to an increasingly solid-like behavior of the liquid phase composite.

Conclusions

This study used an ionic liquid dispersant/initiator for composite preparation that resulted in excellent dispersion quality. The Young’s modulus of the resultant well-dispersed SWNT-epoxy composite was evaluated and compared to a relevant micromechanics model. At low SWNT contents, this experimental study of the composite Young’s modulus shows excellent agreement with model predictions. SEM and TEM microscopy of low content nanotube composites exhibit

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excellent nanoscale dispersion of the nanotube filler which is responsible for the enhanced Young’s modulus of these composites. At 1.0 wt% SWNTs, the experimentally determined modulus reached a pleateau at ~3.5 GPa and exhibited negligible modulus enhancements for further increases of SWNT content, while the model predicts that the modulus should show greater enhancements. Processing limitations of nanotube epoxy composites could potentially be attributed to the discrepancies between experimental values and model predictions for Young’s modulus of high concentration SWNT-IL-epoxy composites. A rheological percolation threshold exists between 0.1 and 1.0 wt% SWNTs, which corresponds to the experimental plateau in modulus enhancements. The rheological transition to solid-like behavior of the uncured composite suggests that as the polymer system reaches its maximum capacity for well-dispersed SWNTs. As a result, nanotubes that remain agglomerated in composites with high filler content fail to contribute to effectively creating the desired well-dispersed randomly-oriented network of SWNTs within the composite that is necessary for mechanical property enhancements.

Acknowledgements

The authors wish to acknowledge the US Army Research Laboratory for financial support under the Army Materials Center of Excellence Program (contract #W911NF-06-2-0013). The authors acknowledge James Throckmorton and Edward Basgall and Drexel University Central Research Facilities for assistance and use of SEM and Dewight Williams and Ray Meade at the University of Pennsylvania Electron Microscopy Resource Labs for use of TEM.

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Swaminathan, S., S. Ghosh, and N. Pagano, Statistically equivalent representative volume elements for unidirectional composite microstructures: Part I-Without damage. Journal of composite materials, 2006. 40(7): p. 583-604. Shtein, M., et al., Fracture behavior of nanotube-polymer composites: Insights on surface roughness and failure mechanism. Composites Science and Technology, 2013. 87: p. 157163. Sumfleth, J., et al., A comparative study of the electrical and mechanical properties of epoxy nanocomposites reinforced by CVD- and arc-grown multi-wall carbon nanotubes. Composites Science and Technology, 2010. 70(1): p. 173-180. Schlea, M.R., et al., Network behavior of thermosetting polyimide/multiwalled carbon nanotube composites. Polymer, 2012. 53(4): p. 1020-1027. Martin-Gallego, M., et al., Comparison of filler percolation and mechanical properties in graphene and carbon nanotubes filled epoxy nanocomposites. European Polymer Journal, 2013. 49(6): p. 1347-1353. Song, Y.S. and J.R. Youn, Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon, 2005. 43(7): p. 1378-1385. Kim, P.-C.M.a.J.-K., Carbon Nanotubes for Polymer Reinforcement, T.F. Group, Editor 2011, CRC Press: FL, USA. p. 224. Huang, Y.Y., S.V. Ahir, and E.M. Terentjev, Dispersion rheology of carbon nanotubes in a polymer matrix. Physical Review B, 2006. 73(12). Kota, A.K., et al., Electrical and rheological percolation in polystyrene/MWCNT nanocomposites. Macromolecules, 2007. 40(20): p. 7400-7406. Urena-Benavides, E.E., M.J. Kayatin, and V.A. Davis, Dispersion and Rheology of Multiwalled Carbon Nanotubes in Unsaturated Polyester Resin. Macromolecules, 2013. 46(4): p. 1642-1650. Fidelus, J.D., et al., Thermo-mechanical properties of randomly oriented carbon/epoxy nanocomposites. Composites Part a-Applied Science and Manufacturing, 2005. 36(11): p. 1555-1561. Gojny, F.H., et al., Influence of nano-modification on the mechanical and electrical properties of conventional fibre-reinforced composites. Composites Part a-Applied Science and Manufacturing, 2005. 36(11): p. 1525-1535. de Villoria, R.G., et al., Mechanical properties of SWNT/epoxy composites using two different curing cycles. Composites Part B-Engineering, 2006. 37(4-5): p. 273-277. Yavari, F., et al., Synergy Derived by Combining Graphene and Carbon Nanotubes as Nanofillers in Composites. Journal of Nanoscience and Nanotechnology, 2012. 12(4): p. 3165-3169. Ashrafi, B., et al., Influence of the reaction stoichiometry on the mechanical and thermal properties of SWCNT-modified epoxy composites. Nanotechnology, 2013. 24(26). Ashrafi, B., et al., Single-walled carbon nanotube-modified epoxy thin films for continuous crack monitoring of metallic structures. Structural Health Monitoring-an International Journal, 2012. 11(5): p. 589-601. Wang, S.R., et al., Effective amino-functionalization of carbon nanotubes for reinforcing epoxy polymer composites. Nanotechnology, 2006. 17(6): p. 1551-1557. Wang, S., et al., Reinforcing polymer composites with epoxide-grafted carbon nanotubes. Nanotechnology, 2008. 19(8). Pizzutto, C.E., et al., Study of Epoxy/CNT Nanocomposites Prepared Via Dispersion in the Hardener. Materials Research-Ibero-American Journal of Materials, 2011. 14(2): p. 256263.

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Gerson, A.L., et al., Curing effects of single-wall carbon nanotube reinforcement on mechanical properties of filled epoxy adhesives. Composites Part a-Applied Science and Manufacturing, 2010. 41(6): p. 729-736. Subramanian, G. and M.J. Andrews, Preparation of SWNT-reinforced composites by a continuous mixing process. Nanotechnology, 2005. 16(6): p. 836-840. Yan, Y.H., et al., Interface molecular engineering of single-walled carbon nanotube/epoxy composites. Journal of Materials Chemistry, 2012. 22(5): p. 1928-1936. Yang, J.P., et al., Cryogenic mechanical behaviors of carbon nanotube reinforced composites based on modified epoxy by poly(ethersulfone). Composites Part BEngineering, 2012. 43(1): p. 22-26. Li, X.D., et al., Nanomechanical characterization of single-walled carbon nanotube reinforced epoxy composites. Nanotechnology, 2004. 15(11): p. 1416-1423. Farahani, R.D., et al., Manufacturing composite beams reinforced with three-dimensionally patterned-oriented carbon nanotubes through microfluidic infiltration. Materials & Design, 2012. 41: p. 214-225. Yuan, W., et al., Use of Polyimide-graft-Bisphenol A Diglyceryl Acrylate as a Reactive Noncovalent Dispersant of Single-Walled Carbon Nanotubes for Reinforcement of Cyanate Ester/Epoxy Composite. Chemistry of Materials, 2010. 22(24): p. 6542-6554. Zhao, S., et al., Improving dispersion and integration of single-walled carbon nanotubes in epoxy composites by using a reactive noncovalent dispersant. Journal of Polymer Science Part a-Polymer Chemistry, 2012. 50(21): p. 4548-4556. Camponeschi, E., et al., Properties of carbon nanotube-polymer composites aligned in a magnetic field. Carbon, 2007. 45(10): p. 2037-2046.

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Figure 1: Experimentally measured Young’s Modulus of SWNT-IL-Epoxy composite, with SWNTs concentration in the range of 0.01-5.0 wt%.

Figure 2: The left column contains characteristic low magnification SEM micrographs (a-c) of low concentration SWNT-IL-Epoxy composites at 0.01, 0.05, and 0.1 wt% SWNT content, each with a scale bar of 1 micron. The right column contains characteristic high magnification TEM micrographs (d-f) of low concentration SNWT-IL-Epoxy composites at 0.01, 0.05, and 0.1 wt% SWNT content, each with a scale bar of 100 nm. Arrows denote SWNT agglomerations.

Figure 3: The left column contains characteristic low magnification SEM micrographs (a-c) of high concentration SWNT-IL-Epoxy composites at 1.0, 2.0 and 5.0 wt% SWNT content, each with a scale bar of 1 micron. The right column contains characteristic high magnification TEM micrographs (d-f) of high concentration SNWT-IL-Epoxy composites at 1.0, 2.0, and 5.0 wt% SWNT content, each with a scale bar of 100 nm. Arrows denote SWNT agglomerations.

Figure 4: Deagglomeration and dispersion effects on SWNT composite Young’s Modulus for (a) 0.01wt%, (b) 0.05wt%, (c) 0.1wt%, (d) 1.0wt%, (e) 2.0wt%, (f) 5.0wt% SWNTs.

Figure 5: Rheological response of SWNT-IL-Epoxy composites as a function of frequency at 25ºC: (a) complex viscosity, (b) storage modulus, and (c) corresponding rheological percolation threshold at a frequency of 1 radian/second.

19

Figure 1: Experimentally measured Young’s Modulus of SWNT-IL-Epoxy composite, with SWNTs concentration in the range of 0.01-5.0 wt%.

Figure 2: The left column contains characteristic low magnification SEM micrographs (a-c) of low concentration SWNT-IL-Epoxy composites at 0.01, 0.05, and 0.1 wt% SWNT content, each with a scale bar of 1 micron. The right column contains characteristic high magnification TEM

20

micrographs (d-f) of low concentration SNWT-IL-Epoxy composites at 0.01, 0.05, and 0.1 wt% SWNT content, each with a scale bar of 100 nm. Arrows denote SWNT agglomerations.

Figure 3: The left column contains characteristic low magnification SEM micrographs (a-c) of high concentration SWNT-IL-Epoxy composites at 1.0, 2.0 and 5.0 wt% SWNT content, each with a scale bar of 1 micron. The right column contains characteristic high magnification TEM micrographs (d-f) of high concentration SNWT-IL-Epoxy composites at 1.0, 2.0, and 5.0 wt% SWNT content, each with a scale bar of 100 nm. Arrows denote SWNT agglomerations.

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Figure 4: Deagglomeration and dispersion effects on SWNT composite Young’s Modulus for (a) 0.01wt%, (b) 0.05wt%, (c) 0.1wt%, (d) 1.0wt%, (e) 2.0wt%, (f) 5.0wt% SWNTs.

22

Figure 5: Rheological response of SWNT-IL-Epoxy composites as a function of frequency at 25ºC: (a) complex viscosity, (b) storage modulus, and (c) corresponding rheological percolation threshold at a frequency of 1 radian/second.

Figure A1: Representative volume along with MHM volume parameters to describe dispersion and agglomeration.

23

Table 1: Literature reports of Young’s Modulus (YM) improvements of epoxy-SWNT composites. SWNT wt%

Details

YM matrix (MPa) 2.828

YM composite (MPa) 2.8

% change

Reference

-0.99

Fidelus 2005 [50]

2.599

2.681

3.16

Gojny 2005 [51]

1

1.08

8.00

Fidelus 2005 [50]

0.01

EPON 815/Amine

0.05

DGEBA/H137i

0.05

EPON 815/Amine

0.1

Epoxiber-20/hardener type IB-72

2.875

2.909

1.18

0.1

DGEBA/H137i

2.599

2.691

3.54

De Villoria 2006 [52] Gojny 2005 [51]

0.1

System 2000 epoxy resin/2120 epoxy hardener MY0510 Araldite as an epoxy and 4,4'-diaminodiphenylsulfone Araldite GY 251/Aradur HY 956 DGEBA/H137i

3.26

3.8

16.56

Yavari 2012 [53]

4.02

4.4

7.00

Ashrafi 2013 [54]

0.258 2.599

1.535 2.812

494.96 8.20

Loos 2008 [33] Gojny 2005 [51]

3.5

3.98

14.00

Ashrafi 2012 [55]

0.415

0.843

103.00

Wang 2008 [34]

2.44

2.52

3.28

Wang 2006 [56]

2

2.3

15.00

Wang 2008 [57]

3.5

4.3

18.60

Pizzutto 2011 [58]

0.2 0.25 0.3 0.3

0.5

105 Epoxy Resin and the 206 Slow Hardener (West System) Bisphenol A/N,Ndimethylbenzylamine DGEBA/Epicure 9470

0.5

Epoxy/amine

0.5

Araldite GY251/Aradur HY956

0.5

Hyson 9309.2/aliphatic amine

1.45

1.9

31.00

Gerson 2010 [59]

0.5

EPON 862/Epicure system

0.161

0.222

27.00

0.5 1

1.68 3.45

1.77 3.6

5.36 4.35

1

DGEBA/Triethylenetetramine DGEBA/Jeffamine D230 and Jeffamine D-2000 DGEBA/EPON W

Subramanian 2005 [60] Yan 2012 [61] Yang 2012 [62]

2.026

2.123

4.79

Zhu 2003 [7]

1

Amine hardener

4

4.4

10.00

Li 2004 [63]

1

EPON 862

2.34

2.93

25.00

Farahani 2012 [64]

1

2.6

4.7

80.00

Yuan 2010 [65]

1580 415

1720 595

8.86 43.37

Zhao 2012 [66] Wang 2008 [34]

390

480

23.08

Wang 2008 [34]

3

a-glycidyl terminated bisphenol A/3,30-dihydroxy-4,4'diaminobiphenyl DGEBA/Triethylenetetramine BP A-epichlorohydrin/modified polyamine/SWNTs aligned, parallel BP A-epichlorohydrin/modified polyamine/SWNTs aligned, perpendicular Amine hardener

3.964

2.282

-42.28

3

Amine hardener

1.925

1.105

-42.60

Camponeschi 2007 [67] Camponeschi 2007 [67]

0.3

1 1

1

24

5

5

10

10

BP A-epichlorohydrin/modified polyamine/SWNTs aligned, parallel BP A-epichlorohydrin/modified polyamine/SWNTs aligned, perpendicular BP A-epichlorohydrin/modified polyamine/SWNTs aligned, parallel BP A-epichlorohydrin/modified polyamine/SWNTs aligned, perpendicular

415

1020

145.78

Wang 2008 [34]

390

610

56.41

Wang 2008 [34]

415

1150

177.10

Wang 2008 [34]

390

625

60.26

Wang 2008 [34]

Table 2: Agglomeration and dispersion in SWNT-IL-Epoxy composites determined by image analysis.

SWNT Concentration (wt%) 0.01 0.05 0.1 1.0 2.0 5.0

Average Agglomeration Size (nm2) 321 ± 33 295 ± 30 363 ± 36 366 ± 25 891 ± 36 921 ± 66

% SWNT Area 1.4 ± 0.1 2.0 ± 0.2 2.9 ± 0.3 8.4 ± 0.8 11.4 ± 1.1 18.4 ± 1.8

Table 3: SWNT and epoxy material properties used in MHM.

SWNT Material Properties E11 (GPa) 450.47 E22 (GPa) 12.13 G12 (GPa) 27 G23 (GPa) 4.4 ν12 0.42

Epoxy Material Properties E11 (GPa) 2.89 ν12 0.35

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Appendix A. Micromechanics Homogenization Model A micromechanics homogenization model (MHM) based on the Mori-Tanaka approach was utilized to estimate the overall mechanical properties of a single walled carbon nanotube (SWNT) reinforced composite. In order to evaluate the effects of agglomeration and dispersion on the composite properties, the MHM was implemented by using parametric indices quantifying dispersion and agglomeration [1]. SEM and TEM evaluations of spatial distribution of the SWNTs provided parameters which include nanotube length, diameter, agglomeration, dispersion and spatial distribution. These experimentally-based geometric considerations are critical for effectively developing the model [2]. After defining a representative volume element, the composite was modeled as a two-phased medium having a hybrid matrix and spherical inclusions, which are comprised of reinforced composite with perfectly bonded, straight, randomly oriented and uniformly dispersed SWNTs. The total volume of the material region (VMR), shown in Figure A1, and the volume occupied by the SWNTs (VSWNT) is defined as:

V MR  V HM  V AI and V

SWNT

SWNT SWNT  Vhybrid  Vinclusions

(1)

where V HM and V AI are the volume of the hybrid matrix and agglomerate inclusion respectively SWNT

SWNT and Vhybrid and Vinclusions are the volume of SWNT in the hybrid and inclusion respectively.

Using (1) a deagglomeration index (  ) can be derived to represent the fraction of inclusions in the MR and the dispersion index (  ) by the fraction of SWNTs within inclusions over the entire volume of SWNTs:

SWNT Vinclusions V AI (1) and   V MR V SWNT As  approaches 1, the more homogeneous the composite becomeswhile as  approaches 1, for



   , the more heterogeneous. This suggests that the SWNT inclusions are agglomerations in the composite rather than well dispersed. Given the volume fraction of the SWNT ( SWNT ), the AI HM volume fractions for the agglomerates ( SWNT ) and hybrid matrix ( SWNT ) can be calculated using

the two indices:

 1  HM SWNT and SWNT   SWNT (2)  1 Using these volume fractions, the effective mechanical properties were calculated implementing the Mori-Tanaka method (MT) [3, 4] which takes into consideration essential material parameters AI SWNT 

26

for accuracy such as aspect ratio, volume fraction, shape and orientation of the constituent phases. In this particular case, the two constituents are homogenized and presumed to be linearly elastic and perfectly bonded. Introducing a uniform strain, the average strains (  ) by each constituent may be expressed as: (3)

  m m  r { r } where m and r denote the matrix and reinforcement,  is the volume fraction and averaging in all orientation. The effective stiffness tensor ( T Benveniste tensorial formulation:

eff

represents

) can be then represented using

T eff  Tm  r (Tr  Tm ) B[m I  r {B}]1

(4)

where   T eff  , I is the fourth order identity tensor, and B is an orientation dependent tensor which can be defined in terms of the stiffness tensors.The fourth order Eshelby Tensor (S) can be written as follows:

B  [ I  STm1 (Tr  Tm )]1

(5)

References: 1.

2.

3. 4.

Shi, D.L., et al., The effect of nanotube waviness and agglomeration on the elastic property of carbon nanotube-reinforced composites. Journal of Engineering Materials and Technology-Transactions of the Asme, 2004. 126(3): p. 250-257. Swaminathan, S., S. Ghosh, and N.J. Pagano, Statistically equivalent representative volume elements for unidirectional composite microstructures: Part I - Without damage. Journal of Composite Materials, 2006. 40(7): p. 583-604. Benveniste, Y., A new approach to the application of Mori-Tanaka's theory in composite materials. Mechanics of Materials, 1987. 6(2): p. 147-157. Sia Nemat-Nasser, M.H., Micromechanics: Overall Properties of Heterogeneous Materials, ed. S. Nemat-Nasser. 1999, New York: Elsevier Science & Technology Books.

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