- Email: [email protected]
polycarbonate nanocomposite
Polymer Testing 47 (2015) 87e91
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
The nanomechanical behavior of a graphite nanoplatelet/polycarbonate nanocomposite Nannan Tian a, Tian Liu b, Wei-Hong Zhong b, David F. Bahr a, * a b
School of Materials Engineering, Purdue University, West Lafayette, IN 47906-2045, USA School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2015 Accepted 18 August 2015 Available online 28 August 2015
Quasi-static nanoindentation has been used to characterize the mechanical properties of polycarbonate reinforced with graphite nanoplatelets (GNPs). Poor dispersion or low quality interfacial interactions of GNPs in a polymer matrix can significantly decrease the relative improvement in the material's mechanical strength and stiffness. In this study, the surfaces of GNPs were modified to achieve better dispersion and interfacial interaction between fillers and matrix. The GNP/PC nanocomposite has a heterogeneous microstructure, and the original mechanical properties between filler and matrix have large differences. Using a spatially sensitive probe method leads to measured values of modulus and hardness that correlate with the indentation sampled volume. A grid indentation procedure was performed with variable sampling volumes to provide a statistical measurement of modulus and hardness for the nanocomposite materials. The surface treatment leads to a significant increase in both stiffness and hardness for GNP reinforced composites. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanoindentation Nanocomposites Graphite nanoplatelets
1. Introduction Quasi-static (QS) nanoindentation is a common method used to assess the mechanical properties of many materials at small scales. The Oliver and Pharr [1]analysis method is widely used to extract elastic and plastic mechanical properties of both homogenous bulk materials and thin films. Using the initial unloading slope and knowing, through calibration, the contact area of the indenter, the elastic modulus and hardness of specimens can be determined. While hardness in crystalline materials may vary as a function of indentation volume (the “indentation size effect”), the elastic properties are usually assumed to be invariant with the volume of material sampled for homogenous materials. However, materials that have multiple purpose applications (such as optimizing electrical behavior and mechanical performance in nanocomposites), often have heterogeneous microstructures and may contain multiple phases. When it comes to these materials, for example thin films [2], polymer composites [3] and cementatious materials [4], the measured mechanical properties are often found to be a function of indentation volume sampled due to sampling different
* Corresponding author. School of Materials Engineering, 701 W. Stadium Ave, West Lafayette, IN 47907-2045, USA. E-mail address: [email protected] (D.F. Bahr). http://dx.doi.org/10.1016/j.polymertesting.2015.08.007 0142-9418/© 2015 Elsevier Ltd. All rights reserved.
volumes of the reinforcement and matrix phase. To address the mechanics of nanoindentation on polymer composite or thin film, several researchers have reported studies about appropriate choice of indentation sizes (length scales) [5e7]. Normally, the applied indentation depths (h) and the sizes of individual phases in the microstructure (D) have been used to estimate the determination of composite properties (h << D) or individual phase properties (h >> D) [5]. Using grids or arrays of localized indentations, effectively statistical nanoindentation techniques, it is possible to provide an accurate measurement of elastic modulus for multiple phases of composite and constituent materials [8e10]. For example, Wu et al. [11] and Olek et al. [12] estimated the properties of multi-walled carbon nanotube polymer composites by nanoindentation, while Rahman [13] and Das et al. [14] reported the indentation measured properties on graphitic nanoplatelet (GNP)/polymer composites by nanoindentation using the conventional pyramidal indentation geometry. One of the most common observations from these studies was that the indentation measured hardness and elastic modulus increase with increasing filler loads, but that agglomeration of nanoscale reinforcements becomes an issue leading to poor mechanical properties for high concentrations of filler. The previous studies were restricted to analyzing the indentation hardness and modulus of these “particles/individual phase” [5], and some
88
N. Tian et al. / Polymer Testing 47 (2015) 87e91
have focused on the influence of indentation loads on the indentation modulus distribution by simulation [15,16]. For example, Ulm et al. [16] simulated the frequency distribution plots of grid indentation for any microstructure with two phases in concrete materials from shallow indents (D/h ¼ 14) to deep indents (D/ h ¼ 2). This simulation work resulted in good agreement with a previous study [17], and confirmed that indentations to shallow depths are able to sample clearly separated phases in the statistical studies, while the deeper indentations tend to present one “peak” in the properties distribution, representing the overall composite behavior. Graphitic nanoplateles (GNPs) have recently become an attractive candidate among carbon-based fillers for composites due to their relatively high mechanical and electrical properties [18e20]. The GNP reinforcement could possible improve the material properties in multiple ways [21,22]. In this study, the effects of indentation sizes and reinforcement loads on determination of localized distribution of polymer-based nanocomposites with nano-sized reinforcements have been studied experimentally by using the grid nanoindentation method. Fig. 1. The multi-step load function used for the nanoindentation experiments.
2. Materials and methods 2.1. Material preparation In this work, nanographitic polymer composites were formed from commercially available material systems. The matrix, polycarbonate (PC, Lexan 101), was provided by SABIC Innovation Plastics Company. The elastic modulus of Lexan 101 PC has been reported as 2.34 GPa [23]. Graphitic nanoplatelets (GNPs) were purchased from XG Science Inc. and were pre-exfoliated; as provided they have an average diameter of ~5 mm and thickness of ~5 nm. Soy protein isolate (SPI) was donated by ADM Co. and contained 90.9% dry basis protein. Chloroform, obtained from Fisher Scientific, was used as the solvent for both PC and GNP modification. Trifluoroethanol (TFE) was applied as the SPI denaturing agent and was purchased from SigmaeAldrich. The surface treatment of GNPs was accomplished by dispersing GNPs and SPI in equal amounts by weight in chloroform and processed for 15 min with a high power probe Ultrasonicator (Branson 450), and then heated in an oil-bath at 65 C for 4 h. Thereafter, TFE was added to the mixture. The composite membranes were prepared by mixing the PC-chloroform solution with GNP-chloroform solution using bath-sonication, then casting onto a glass substrate at ambient conditions. The thickness of the composite films was around 40 mm. The overall process is described further in Ref. [24]. 2.2. Mechanical testing methods The evaluation of the mechanical properties of these nanocomposites was carried out using a Hysitron Triboindenter 950 system (Hysitron Inc., Minneapolis, USA); nanoindentation was performed with a three-sided Berkovich diamond indenter (tip radius z800 nm). A multi-step QS nanoindentation, each step comprising a 1-s loading segment, a 5-s hold, and a 1-s unloading segment, was used to evaluate the hardness and modulus of the material. A total of three peak loads were chosen to evaluate the properties at three different depths for each indentation. The loading schedule is shown in Fig. 1. Because of the heterogeneous microstructure with the potential presence of spatially isolated agglomerated GNPs, a large grid of indents was performed on the samples. A 15X15 grid indentation matrix with 10 mm spacing between each indent was performed for three peak loads (200 mN, 1500 mN and 5000 mN) on each sample. The grid of 225 indents covers an area of 150 150 mm, which
might include several phases of GNP reinforcement (5 mm in diameter and 5 nm in thickness). The coverage area should provide sufficient data for statistical data analysis for properties frequency distributions. Prior to the nanoindentation tests, the film samples were carefully glued onto a magnetic pad, and the machine compliance was also taken into account for calibration. The geometry of the tip was calibrated using a fused quartz standard sample in order to obtain the appropriate tip area function. The mechanical response (hardness and reduced modulus) of the nanocomposites can be determined using:
H¼
Pmax A
Er ¼
pffiffiffiffi dP 1 p pffiffiffiffi dh 2 A
where A is the projected contact area, which can be calculated from the indenter geometry, Pmax is maximum load applied by the indenter, dP/dh is the slope of the partial unloading curve in the loadedisplacement curve, and h is the contact depth. Nanoindentation is a valid analysis method for estimating mechanical properties for thin film and bulk materials at low loads (shallow depths) or high load (deep depths) [25,26]. For the low loads (shallow depths), indentations need to large enough to overcome the surface roughness and any strain issues caused by rounding of the indentation probe, as well as shallow enough to measure the mechanical properties of the reinforcement phase rather than the properties of the whole composite. In this study, the Berkovich tip has an approximately 800 nm tip radius, which corresponds to a 50 nm effective indentation contact depth based on the self-similarity (i.e. indentations deeper than 50 nm should be iso-strain). For all the specimens, the surface RMS roughness is about 8 nm over an area of 30 30 mm. 3. Results To determine the effects of sampling volume on nanoindentation results, nanoindentation was performed on all samples with three levels of peak load. Fig. 2(aed) shows the frequency distribution plots for reduced modulus Er determined from the nanoindentation tests for 0.1wt% GNP/PC, 1wt% GNP/PC, 0.1wt% treated GNP/PC and 1wt% treated GNP/PC samples, respectively. To determine the effects of indentation sampling volumes, the frequency distributions at various indentation depths, corresponding
N. Tian et al. / Polymer Testing 47 (2015) 87e91
89
Fig. 2. Indentation reduced modulus (GPa) values of GNP/PC nanocomposite system at three different maximum loads for each material system; (a) pristine specimen with 0.1wt% of GNP fillers, (b) pristine specimen with 1wt% of GNP fillers, (c) surface modified specimen with 0.1wt% of GNP fillers, (d) surface modified specimen with 1wt% of GNP fillers.
to three peak loads, were plotted for all the samples. The concentration of the frequency distributions increased with increasing peak load (larger indentation depths) for 0.1wt% GNP/PC and 1wt% GNP/PC samples, while the measured value of indentation-reduced modulus increased with increased level of peak load. After the surface treatment on GNP fillers, the PC/GNP composites tend to form a more uniform structure due to the better-dispersed fillers. In Fig. 2 (c,d), the frequency distribution peaks have similar behavior between 0.1wt% GNP and 1wt% GNP. The shallowest indentations in Fig. 2 (a) represent the minimum indentation depths (h) in a range from 110 nm to 90 nm for the pristine specimen with 0.1 wt% of GNP filler reinforcement. The diameter of GNP fillers (D) is in the range of 5 mm. The indentation depth is ten times smaller than the characteristic size of fillers, h << D. These shallow indentation tests should be able to provide lateral position sensitivity of the various phases, and indeed show a broad distribution of properties. The black columns represent maximum indentation depths, h, from 1100 nm to 900 nm, which have a higher probability for each indentation to sample part of the volume that consists of both phases. These deep indents appear as one sharp peak instead of the broader distribution that would occur from two overlapping peaks. In addition, the reinforcementematrix interface might also affect the experimental values of reduced modulus at larger indentation depths. The reduced modulus for the 1wt% GNP reinforced specimens has a higher variation over the entire range of indentation loads than for the 0.1wt% GNP reinforced specimens. Fig. 3 (a,b) summarizes the mean reduced modulus and the standard deviation of reduced modulus values of each sample on various levels of sampling volume. The white columns represent 0.1 wt% of GNP fillers, and the grey columns represent 1 wt% of GNP fillers. The Halpin-Tsai equation [27,28] can be utilized to estimate the elastic modulus of composites materials.
. Ef Em 1 E 1 þ xhVf ¼ h¼ . Em 1 hVf Ef Em þ x where E is the elastic modulus of the composite, Em is the elastic modulus of the matrix, Vf is the volume fraction of reinforcement, x is factor related with reinforcement, and Ef is the elastic modulus of reinforcement. x is an adjustable parameter that depends on the shape and the loading of the reinforcement within the composite, and also takes into consideration the interaction between reinforcement and matrix. Based on the rule of mixture principle by the Halpin-Tsai equation [29], the modulus and hardness values should increase with increasing concentration of treated GNP filler, while the pristine GNP reinforcement appears to relatively lower the mechanical performance in composites. In all cases, the modulus is higher than one would expect for the neat polymer. However, for the pristine (untreated) samples (Fig. 3 (a)), the mean indentation reduced modulus of nanocomposites decreases with increased level of filler loading, whereas the mean indentation reduced modulus of nanocomposites with surface modification increases with increased level of filler loading. This is probably due to two mechanisms. First, the local mechanical behavior could be highly affected by dispersion; the agglomerated effect might decrease the mechanical properties of the composites by the non-uniform dispersion and pore generation. Second, at low concentrations (below 1wt%), GNP can work as a nucleating agent, causing weaker interfacial interactions between PC matrix and GNP fillers [30]. However, the GNP with surface modification has shown a reinforcement effect on the PC matrix, which is revealed by the increased mean indentation reduced modulus of the 1 wt% GNP/PC
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Fig. 3. The indentation reduced modulus with a range of indentation depths for (a) pristine samples (b) treated samples.
Table 1 Properties inferred from quasi-static nanoindentation correlated with various sampling volume and fillers loading and the resulting statistical significance relative to largest sampling volume and smaller fillers loading. Fillers loadings (wt%)
Surface modification (Y/N)
Indentation depth (nm)
0.1 0.1 0.1 1 1 1 0.1 0.1 0.1 1 1 1
N N N N N N Y Y Y Y Y Y
1028 564 90 1083 601 102 1079.5 601.6 102 1042.9 564.2 96.2
± ± ± ± ± ± ± ± ± ± ± ±
19 17 11 47 32 7 27.8 22.4 7.3 14.5 10 4.8
nanocomposites compared to 0.1 wt% GNP/PC nanocomposites (Fig. 3 (b)). The composite becomes stronger and stiffer with the addition of homogeneously reinforced filler, which probably improves the interfacial interaction with hydrogen bonding. To determine if the sampling volumes and surface modification on reinforcements would correlate with analysis data, a student T-test analysis was preformed as statistical reliability measurement, shown in Table 1. Fig. 3 also shows that the surface treatment is crucial to stiffening at all length scales of measurement. In all cases of the untreated GNP, the higher fraction (1%) GNP exhibits a lower modulus than the lower (0.1%) GNP sample. For the treated surface, the
Indentation modulus (GPa)/t values
Indentation hardenss [2]/t values
3.27 ± 0.06 3.28 ± 0.09/1.2 3.499 ± 0.22/14 2.89 ± 0.12/þ41.5 3.03 ± 0.21/þ16.6 3.32 ± 0.25/3.1 3.06 ± 0.15 3.01 ± 0.08/4.2 3.23 ± 0.14/12.5 3.21 ± 0.09/13.1 3.29 ± 0.08/20.2 3.47 ± 0.17/27.4
124.6 ± 4.3 128.6 ± 6.9/7.4 144.3 ± 14.2/19.9 113 ± 9.7/þ16.4 115.9 ± 13.2/þ9.4 130.2 ± 10.6/7.4 113.4 ± 5.5 113.7 ± 8/0.48 128.1 ± 9.6/19.9 121.7 ± 3.4/19.2 128.7 ± 4.5/32.2 136.2 þ 6.7/38.9
higher GNP is always stiffer than the lower GNP sample at all length scales; suggesting the composite properties in theses cases can be described by conventional models. The deviations of indentation modulus for surface modified GNP composites in Fig. 3 (b) are relative lower than that for pristine GNP composites in Fig. 3 (a) for most of the cases, which is the result of improved reinforcement dispersion. In all cases, the hardness (H) and modulus of these materials vary proportionally and are consistent, independent of surface treatment. Fig. 4 shows that the H/E ratio of all samples exhibit a constant value of about 0.0375 at depths that sample the overall composite properties (greater than 1000 nm indentation depth).
Fig. 4. Indentation reduced modulus (a) and H/E ratio (b) from nanoindentation at sample volumes in which the indenter penetrated more than 1000 nm.
N. Tian et al. / Polymer Testing 47 (2015) 87e91
This suggests that the mechanisms responsible for load transfer between the matrix and reinforcing phase are the limiting factor in this material's ability to carry mechanical loads, influencing both the elastic and plastic deformation in these solids. 4. Conclusions The modulus of GNP reinforced polymer nanocomposites is particularly sensitive to the surface treatment of GNPs and their subsequent chemical interaction with the matrix. After a surface treatment using soy protein isolate on the GNP reinforcing phase, the hardness and modulus of 1 wt% GNP improved by 7% over the pristine GNP reinforced composite. In this study, the effects of sampling volume on the testing results have been determined. For material systems with high modulus fillers reinforced in a relatively low modulus polymer matrix, larger sampling volumes result in a lower measured modulus and hardness and less distinction between phases. References [1] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564e1583. [2] C.A. Botero, E. Jimenez-Pique, T. Kulkarni, G. Fargas, V.K. Sarin, L. Llanes, Crosssectional nanoindentation and nanoscratch of compositionally graded mullite films, Surf. Coatings Technol. 206 (2011) 1927e1931. [3] M.M. Shokrieh, M.R. Hosseinkhani, M.R. Naimi-Jamal, H. Tourani, Nanoindentation and nanoscratch investigations on graphene-based nanocomposites, Polym. Test. 32 (2013) 45e51. [4] W.B. Ashraf, J. Olek, N. Tian, Statistical nanoindentation and SEM/EDS study of the microstructure of carbonated Ca-silicates mineral systems: case of wollastonite (CaSiO3), J. Cement Concrete Compos. (2015) under review. [5] G. Constantinides, K.S.R. Chandran, F.J. Ulm, K.J. Van Vliet, Grid indentation analysis of composite microstructure and mechanics: principles and validation, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process 430 (2006) 189e202. [6] C. Auvray, G. Arnold, G. Armand, Experimental study of elastic properties of different constituents of partially saturated argillite using nano-indentation tests, Eng. Geol. 191 (2015) 61e70. [7] V. Magnenet, A. Giraud, C. Auvray, About the effect of relative humidity on the indentation response of meuse/haute-marne argillite, Acta Geotech. 6 (2011) 155e166. [8] M. Vandamme, F.J. Ulm, P. Fonollosa, Nanogranular packing of C-S-H at substochiometric conditions, Cem. Concr. Res. 40 (2010) 14e26. [9] M.L. Oyen, C.C. Ko, Indentation variability of natural nanocomposite materials, J. Mater. Res. 23 (2008) 760e767. [10] M. Qu, F. Deng, S.M. Kalkhoran, A. Gouldstone, A. Robisson, K.J. Van Vliet, Nanoscale visualization and multiscale mechanical implications of bound rubber interphases in rubber-carbon black nanocomposites, Soft Matter 7 (2011) 1066e1077.
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[11] D.C. Wu, L. Shen, J.E. Low, S.Y. Wong, X. Li, W.C. Tjiu, Y. Liu, C. Bin He, Multiwalled carbon nanotube/polyimide composite film fabricated through electrophoretic deposition, Polymer 51 (2010) 2155e2160. [12] M. Olek, K. Kempa, M. Giersig, Multiwall carbon nanotubes-based composites e mechanical characterization using the nanoindentation technique, Int. J. Mater. Res. 97 (2006) 1235e1238. [13] R. Rahman, The role of graphene in enhancing the stiffness of polymeric material: a molecular modeling approach, J. Appl. Phys. 113 (2013). [14] B. Das, K.E. Prasad, U. Ramamurty, C.N.R. Rao, Nano-indentation studies on polymer matrix composites reinforced by few-layer graphene, Nanotechnology 20 (2009), http://dx.doi.org/10.1088/0957-4484/20/12/125705, 125705 (5pp). [15] M.L. Oyen, C.C. Ko, Indentation variability of natural nanocomposite materials, J. Mater. Res. 23 (2008) 760e767. [16] F.J. Ulm, M. Vandamme, H.M. Jennings, J. Vanzo, M. Bentivegna, K.J. Krakowiak, G. Constantinides, C.P. Bobko, K.J. Van Vliet, Does microstructure matter for statistical nanoindentation techniques? Cem. Concr. Compos. 32 (2010) 92e99. [17] H. Buckle, J.W. Westbrook, H. Conrad, The science of hardness testing and its applications, Am. Soc. Metals Metal Park OH (1973) 453e491. [18] G. Venugopal, K. Krishnamoorthy, R. Mohan, S.J. Kim, An investigation of the electrical transport properties of graphene-oxide thin films, Mater. Chem. Phys. 132 (2012) 29e33. [19] A. Chutia, R. Sahnoun, R.C. Deka, Z.G. Zhu, H. Tsuboi, H. Takaba, A. Miyamoto, Local electronic and electrical properties of functionalized graphene nano flakes, Phys. B-Condensed Matter 406 (2011) 1665e1672. [20] T. Kuila, P. Khanra, A.K. Mishra, N.H. Kim, J.H. Lee, Functionalized-graphene/ ethylene vinyl acetate co-polymer composites for improved mechanical and thermal properties, Polym. Test. 31 (2012) 282e289. [21] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Graphene-based polymer nanocomposites, Polymer 52 (2011) 5e25. [22] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282e286. [23] Lexan Resin 101, Americas: Commercial, Saudi Basic Industries Corporation (SABIC), 2015. [24] T. Liu, Y. Wang, A. Eyler, Y. Chang, D. Wang, W.H. Zhong, Facilitating protein denaturation in organic solvent and the contribution to the promoting dispersion of graphite nanoplatelets in a polymer, eXPRESS Polym. Lett. 9 (2015) 686e694. [25] N.X. Randall, R. Christoph, S. Droz, C. JuliaSchmutz, Localised micro-hardness measurements with a combined scanning force microscope/nanoindentation system, Thin Solid Films 290 (1996) 348e354. [26] N.X. Randall, M. Vandamme, F.J. Ulm, Nanoindentation analysis as a twodimensional tool for mapping the mechanical properties of complex surfaces, J. Mater. Res. 24 (2009) 679e690. [27] Q. Lin, S.A. Cohen, L. Gignac, B. Herbst, D. Klaus, E. Simonyi, Low dielectric constant nanocomposite thin films based on silica nanoparticle and organic thermosets, J. Polym. Sci. B Polym. Phys. 45 (2007) 1482e1493. [28] A.M. Díez-Pascual, M.A. Gomez-Fatou, F. Ania, A. Flores, Nanoindentation in polymer composites, Prog. Mater. Sci. 67 (2015) 1e94. [29] J.C. Halpin, J.L. Kardos, Halpin-Tsai equations e review, Polym. Eng. Sci. 16 (1976) 344e352. [30] S. Lee, J.Y. Hong, J. Jang, The effect of graphene nanofiller on the crystallization behavior and mechanical properties of poly(vinyl alcohol),, Polym. Int. 62 (2013) 901e908.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
The nanomechanical behavior of a graphite nanoplatelet/polycarbonate nanocomposite Nannan Tian a, Tian Liu b, Wei-Hong Zhong b, David F. Bahr a, * a b
School of Materials Engineering, Purdue University, West Lafayette, IN 47906-2045, USA School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2015 Accepted 18 August 2015 Available online 28 August 2015
Quasi-static nanoindentation has been used to characterize the mechanical properties of polycarbonate reinforced with graphite nanoplatelets (GNPs). Poor dispersion or low quality interfacial interactions of GNPs in a polymer matrix can significantly decrease the relative improvement in the material's mechanical strength and stiffness. In this study, the surfaces of GNPs were modified to achieve better dispersion and interfacial interaction between fillers and matrix. The GNP/PC nanocomposite has a heterogeneous microstructure, and the original mechanical properties between filler and matrix have large differences. Using a spatially sensitive probe method leads to measured values of modulus and hardness that correlate with the indentation sampled volume. A grid indentation procedure was performed with variable sampling volumes to provide a statistical measurement of modulus and hardness for the nanocomposite materials. The surface treatment leads to a significant increase in both stiffness and hardness for GNP reinforced composites. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanoindentation Nanocomposites Graphite nanoplatelets
1. Introduction Quasi-static (QS) nanoindentation is a common method used to assess the mechanical properties of many materials at small scales. The Oliver and Pharr [1]analysis method is widely used to extract elastic and plastic mechanical properties of both homogenous bulk materials and thin films. Using the initial unloading slope and knowing, through calibration, the contact area of the indenter, the elastic modulus and hardness of specimens can be determined. While hardness in crystalline materials may vary as a function of indentation volume (the “indentation size effect”), the elastic properties are usually assumed to be invariant with the volume of material sampled for homogenous materials. However, materials that have multiple purpose applications (such as optimizing electrical behavior and mechanical performance in nanocomposites), often have heterogeneous microstructures and may contain multiple phases. When it comes to these materials, for example thin films [2], polymer composites [3] and cementatious materials [4], the measured mechanical properties are often found to be a function of indentation volume sampled due to sampling different
* Corresponding author. School of Materials Engineering, 701 W. Stadium Ave, West Lafayette, IN 47907-2045, USA. E-mail address: [email protected] (D.F. Bahr). http://dx.doi.org/10.1016/j.polymertesting.2015.08.007 0142-9418/© 2015 Elsevier Ltd. All rights reserved.
volumes of the reinforcement and matrix phase. To address the mechanics of nanoindentation on polymer composite or thin film, several researchers have reported studies about appropriate choice of indentation sizes (length scales) [5e7]. Normally, the applied indentation depths (h) and the sizes of individual phases in the microstructure (D) have been used to estimate the determination of composite properties (h << D) or individual phase properties (h >> D) [5]. Using grids or arrays of localized indentations, effectively statistical nanoindentation techniques, it is possible to provide an accurate measurement of elastic modulus for multiple phases of composite and constituent materials [8e10]. For example, Wu et al. [11] and Olek et al. [12] estimated the properties of multi-walled carbon nanotube polymer composites by nanoindentation, while Rahman [13] and Das et al. [14] reported the indentation measured properties on graphitic nanoplatelet (GNP)/polymer composites by nanoindentation using the conventional pyramidal indentation geometry. One of the most common observations from these studies was that the indentation measured hardness and elastic modulus increase with increasing filler loads, but that agglomeration of nanoscale reinforcements becomes an issue leading to poor mechanical properties for high concentrations of filler. The previous studies were restricted to analyzing the indentation hardness and modulus of these “particles/individual phase” [5], and some
88
N. Tian et al. / Polymer Testing 47 (2015) 87e91
have focused on the influence of indentation loads on the indentation modulus distribution by simulation [15,16]. For example, Ulm et al. [16] simulated the frequency distribution plots of grid indentation for any microstructure with two phases in concrete materials from shallow indents (D/h ¼ 14) to deep indents (D/ h ¼ 2). This simulation work resulted in good agreement with a previous study [17], and confirmed that indentations to shallow depths are able to sample clearly separated phases in the statistical studies, while the deeper indentations tend to present one “peak” in the properties distribution, representing the overall composite behavior. Graphitic nanoplateles (GNPs) have recently become an attractive candidate among carbon-based fillers for composites due to their relatively high mechanical and electrical properties [18e20]. The GNP reinforcement could possible improve the material properties in multiple ways [21,22]. In this study, the effects of indentation sizes and reinforcement loads on determination of localized distribution of polymer-based nanocomposites with nano-sized reinforcements have been studied experimentally by using the grid nanoindentation method. Fig. 1. The multi-step load function used for the nanoindentation experiments.
2. Materials and methods 2.1. Material preparation In this work, nanographitic polymer composites were formed from commercially available material systems. The matrix, polycarbonate (PC, Lexan 101), was provided by SABIC Innovation Plastics Company. The elastic modulus of Lexan 101 PC has been reported as 2.34 GPa [23]. Graphitic nanoplatelets (GNPs) were purchased from XG Science Inc. and were pre-exfoliated; as provided they have an average diameter of ~5 mm and thickness of ~5 nm. Soy protein isolate (SPI) was donated by ADM Co. and contained 90.9% dry basis protein. Chloroform, obtained from Fisher Scientific, was used as the solvent for both PC and GNP modification. Trifluoroethanol (TFE) was applied as the SPI denaturing agent and was purchased from SigmaeAldrich. The surface treatment of GNPs was accomplished by dispersing GNPs and SPI in equal amounts by weight in chloroform and processed for 15 min with a high power probe Ultrasonicator (Branson 450), and then heated in an oil-bath at 65 C for 4 h. Thereafter, TFE was added to the mixture. The composite membranes were prepared by mixing the PC-chloroform solution with GNP-chloroform solution using bath-sonication, then casting onto a glass substrate at ambient conditions. The thickness of the composite films was around 40 mm. The overall process is described further in Ref. [24]. 2.2. Mechanical testing methods The evaluation of the mechanical properties of these nanocomposites was carried out using a Hysitron Triboindenter 950 system (Hysitron Inc., Minneapolis, USA); nanoindentation was performed with a three-sided Berkovich diamond indenter (tip radius z800 nm). A multi-step QS nanoindentation, each step comprising a 1-s loading segment, a 5-s hold, and a 1-s unloading segment, was used to evaluate the hardness and modulus of the material. A total of three peak loads were chosen to evaluate the properties at three different depths for each indentation. The loading schedule is shown in Fig. 1. Because of the heterogeneous microstructure with the potential presence of spatially isolated agglomerated GNPs, a large grid of indents was performed on the samples. A 15X15 grid indentation matrix with 10 mm spacing between each indent was performed for three peak loads (200 mN, 1500 mN and 5000 mN) on each sample. The grid of 225 indents covers an area of 150 150 mm, which
might include several phases of GNP reinforcement (5 mm in diameter and 5 nm in thickness). The coverage area should provide sufficient data for statistical data analysis for properties frequency distributions. Prior to the nanoindentation tests, the film samples were carefully glued onto a magnetic pad, and the machine compliance was also taken into account for calibration. The geometry of the tip was calibrated using a fused quartz standard sample in order to obtain the appropriate tip area function. The mechanical response (hardness and reduced modulus) of the nanocomposites can be determined using:
H¼
Pmax A
Er ¼
pffiffiffiffi dP 1 p pffiffiffiffi dh 2 A
where A is the projected contact area, which can be calculated from the indenter geometry, Pmax is maximum load applied by the indenter, dP/dh is the slope of the partial unloading curve in the loadedisplacement curve, and h is the contact depth. Nanoindentation is a valid analysis method for estimating mechanical properties for thin film and bulk materials at low loads (shallow depths) or high load (deep depths) [25,26]. For the low loads (shallow depths), indentations need to large enough to overcome the surface roughness and any strain issues caused by rounding of the indentation probe, as well as shallow enough to measure the mechanical properties of the reinforcement phase rather than the properties of the whole composite. In this study, the Berkovich tip has an approximately 800 nm tip radius, which corresponds to a 50 nm effective indentation contact depth based on the self-similarity (i.e. indentations deeper than 50 nm should be iso-strain). For all the specimens, the surface RMS roughness is about 8 nm over an area of 30 30 mm. 3. Results To determine the effects of sampling volume on nanoindentation results, nanoindentation was performed on all samples with three levels of peak load. Fig. 2(aed) shows the frequency distribution plots for reduced modulus Er determined from the nanoindentation tests for 0.1wt% GNP/PC, 1wt% GNP/PC, 0.1wt% treated GNP/PC and 1wt% treated GNP/PC samples, respectively. To determine the effects of indentation sampling volumes, the frequency distributions at various indentation depths, corresponding
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Fig. 2. Indentation reduced modulus (GPa) values of GNP/PC nanocomposite system at three different maximum loads for each material system; (a) pristine specimen with 0.1wt% of GNP fillers, (b) pristine specimen with 1wt% of GNP fillers, (c) surface modified specimen with 0.1wt% of GNP fillers, (d) surface modified specimen with 1wt% of GNP fillers.
to three peak loads, were plotted for all the samples. The concentration of the frequency distributions increased with increasing peak load (larger indentation depths) for 0.1wt% GNP/PC and 1wt% GNP/PC samples, while the measured value of indentation-reduced modulus increased with increased level of peak load. After the surface treatment on GNP fillers, the PC/GNP composites tend to form a more uniform structure due to the better-dispersed fillers. In Fig. 2 (c,d), the frequency distribution peaks have similar behavior between 0.1wt% GNP and 1wt% GNP. The shallowest indentations in Fig. 2 (a) represent the minimum indentation depths (h) in a range from 110 nm to 90 nm for the pristine specimen with 0.1 wt% of GNP filler reinforcement. The diameter of GNP fillers (D) is in the range of 5 mm. The indentation depth is ten times smaller than the characteristic size of fillers, h << D. These shallow indentation tests should be able to provide lateral position sensitivity of the various phases, and indeed show a broad distribution of properties. The black columns represent maximum indentation depths, h, from 1100 nm to 900 nm, which have a higher probability for each indentation to sample part of the volume that consists of both phases. These deep indents appear as one sharp peak instead of the broader distribution that would occur from two overlapping peaks. In addition, the reinforcementematrix interface might also affect the experimental values of reduced modulus at larger indentation depths. The reduced modulus for the 1wt% GNP reinforced specimens has a higher variation over the entire range of indentation loads than for the 0.1wt% GNP reinforced specimens. Fig. 3 (a,b) summarizes the mean reduced modulus and the standard deviation of reduced modulus values of each sample on various levels of sampling volume. The white columns represent 0.1 wt% of GNP fillers, and the grey columns represent 1 wt% of GNP fillers. The Halpin-Tsai equation [27,28] can be utilized to estimate the elastic modulus of composites materials.
. Ef Em 1 E 1 þ xhVf ¼ h¼ . Em 1 hVf Ef Em þ x where E is the elastic modulus of the composite, Em is the elastic modulus of the matrix, Vf is the volume fraction of reinforcement, x is factor related with reinforcement, and Ef is the elastic modulus of reinforcement. x is an adjustable parameter that depends on the shape and the loading of the reinforcement within the composite, and also takes into consideration the interaction between reinforcement and matrix. Based on the rule of mixture principle by the Halpin-Tsai equation [29], the modulus and hardness values should increase with increasing concentration of treated GNP filler, while the pristine GNP reinforcement appears to relatively lower the mechanical performance in composites. In all cases, the modulus is higher than one would expect for the neat polymer. However, for the pristine (untreated) samples (Fig. 3 (a)), the mean indentation reduced modulus of nanocomposites decreases with increased level of filler loading, whereas the mean indentation reduced modulus of nanocomposites with surface modification increases with increased level of filler loading. This is probably due to two mechanisms. First, the local mechanical behavior could be highly affected by dispersion; the agglomerated effect might decrease the mechanical properties of the composites by the non-uniform dispersion and pore generation. Second, at low concentrations (below 1wt%), GNP can work as a nucleating agent, causing weaker interfacial interactions between PC matrix and GNP fillers [30]. However, the GNP with surface modification has shown a reinforcement effect on the PC matrix, which is revealed by the increased mean indentation reduced modulus of the 1 wt% GNP/PC
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Fig. 3. The indentation reduced modulus with a range of indentation depths for (a) pristine samples (b) treated samples.
Table 1 Properties inferred from quasi-static nanoindentation correlated with various sampling volume and fillers loading and the resulting statistical significance relative to largest sampling volume and smaller fillers loading. Fillers loadings (wt%)
Surface modification (Y/N)
Indentation depth (nm)
0.1 0.1 0.1 1 1 1 0.1 0.1 0.1 1 1 1
N N N N N N Y Y Y Y Y Y
1028 564 90 1083 601 102 1079.5 601.6 102 1042.9 564.2 96.2
± ± ± ± ± ± ± ± ± ± ± ±
19 17 11 47 32 7 27.8 22.4 7.3 14.5 10 4.8
nanocomposites compared to 0.1 wt% GNP/PC nanocomposites (Fig. 3 (b)). The composite becomes stronger and stiffer with the addition of homogeneously reinforced filler, which probably improves the interfacial interaction with hydrogen bonding. To determine if the sampling volumes and surface modification on reinforcements would correlate with analysis data, a student T-test analysis was preformed as statistical reliability measurement, shown in Table 1. Fig. 3 also shows that the surface treatment is crucial to stiffening at all length scales of measurement. In all cases of the untreated GNP, the higher fraction (1%) GNP exhibits a lower modulus than the lower (0.1%) GNP sample. For the treated surface, the
Indentation modulus (GPa)/t values
Indentation hardenss [2]/t values
3.27 ± 0.06 3.28 ± 0.09/1.2 3.499 ± 0.22/14 2.89 ± 0.12/þ41.5 3.03 ± 0.21/þ16.6 3.32 ± 0.25/3.1 3.06 ± 0.15 3.01 ± 0.08/4.2 3.23 ± 0.14/12.5 3.21 ± 0.09/13.1 3.29 ± 0.08/20.2 3.47 ± 0.17/27.4
124.6 ± 4.3 128.6 ± 6.9/7.4 144.3 ± 14.2/19.9 113 ± 9.7/þ16.4 115.9 ± 13.2/þ9.4 130.2 ± 10.6/7.4 113.4 ± 5.5 113.7 ± 8/0.48 128.1 ± 9.6/19.9 121.7 ± 3.4/19.2 128.7 ± 4.5/32.2 136.2 þ 6.7/38.9
higher GNP is always stiffer than the lower GNP sample at all length scales; suggesting the composite properties in theses cases can be described by conventional models. The deviations of indentation modulus for surface modified GNP composites in Fig. 3 (b) are relative lower than that for pristine GNP composites in Fig. 3 (a) for most of the cases, which is the result of improved reinforcement dispersion. In all cases, the hardness (H) and modulus of these materials vary proportionally and are consistent, independent of surface treatment. Fig. 4 shows that the H/E ratio of all samples exhibit a constant value of about 0.0375 at depths that sample the overall composite properties (greater than 1000 nm indentation depth).
Fig. 4. Indentation reduced modulus (a) and H/E ratio (b) from nanoindentation at sample volumes in which the indenter penetrated more than 1000 nm.
N. Tian et al. / Polymer Testing 47 (2015) 87e91
This suggests that the mechanisms responsible for load transfer between the matrix and reinforcing phase are the limiting factor in this material's ability to carry mechanical loads, influencing both the elastic and plastic deformation in these solids. 4. Conclusions The modulus of GNP reinforced polymer nanocomposites is particularly sensitive to the surface treatment of GNPs and their subsequent chemical interaction with the matrix. After a surface treatment using soy protein isolate on the GNP reinforcing phase, the hardness and modulus of 1 wt% GNP improved by 7% over the pristine GNP reinforced composite. In this study, the effects of sampling volume on the testing results have been determined. For material systems with high modulus fillers reinforced in a relatively low modulus polymer matrix, larger sampling volumes result in a lower measured modulus and hardness and less distinction between phases. References [1] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564e1583. [2] C.A. Botero, E. Jimenez-Pique, T. Kulkarni, G. Fargas, V.K. Sarin, L. Llanes, Crosssectional nanoindentation and nanoscratch of compositionally graded mullite films, Surf. Coatings Technol. 206 (2011) 1927e1931. [3] M.M. Shokrieh, M.R. Hosseinkhani, M.R. Naimi-Jamal, H. Tourani, Nanoindentation and nanoscratch investigations on graphene-based nanocomposites, Polym. Test. 32 (2013) 45e51. [4] W.B. Ashraf, J. Olek, N. Tian, Statistical nanoindentation and SEM/EDS study of the microstructure of carbonated Ca-silicates mineral systems: case of wollastonite (CaSiO3), J. Cement Concrete Compos. (2015) under review. [5] G. Constantinides, K.S.R. Chandran, F.J. Ulm, K.J. Van Vliet, Grid indentation analysis of composite microstructure and mechanics: principles and validation, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process 430 (2006) 189e202. [6] C. Auvray, G. Arnold, G. Armand, Experimental study of elastic properties of different constituents of partially saturated argillite using nano-indentation tests, Eng. Geol. 191 (2015) 61e70. [7] V. Magnenet, A. Giraud, C. Auvray, About the effect of relative humidity on the indentation response of meuse/haute-marne argillite, Acta Geotech. 6 (2011) 155e166. [8] M. Vandamme, F.J. Ulm, P. Fonollosa, Nanogranular packing of C-S-H at substochiometric conditions, Cem. Concr. Res. 40 (2010) 14e26. [9] M.L. Oyen, C.C. Ko, Indentation variability of natural nanocomposite materials, J. Mater. Res. 23 (2008) 760e767. [10] M. Qu, F. Deng, S.M. Kalkhoran, A. Gouldstone, A. Robisson, K.J. Van Vliet, Nanoscale visualization and multiscale mechanical implications of bound rubber interphases in rubber-carbon black nanocomposites, Soft Matter 7 (2011) 1066e1077.
91
[11] D.C. Wu, L. Shen, J.E. Low, S.Y. Wong, X. Li, W.C. Tjiu, Y. Liu, C. Bin He, Multiwalled carbon nanotube/polyimide composite film fabricated through electrophoretic deposition, Polymer 51 (2010) 2155e2160. [12] M. Olek, K. Kempa, M. Giersig, Multiwall carbon nanotubes-based composites e mechanical characterization using the nanoindentation technique, Int. J. Mater. Res. 97 (2006) 1235e1238. [13] R. Rahman, The role of graphene in enhancing the stiffness of polymeric material: a molecular modeling approach, J. Appl. Phys. 113 (2013). [14] B. Das, K.E. Prasad, U. Ramamurty, C.N.R. Rao, Nano-indentation studies on polymer matrix composites reinforced by few-layer graphene, Nanotechnology 20 (2009), http://dx.doi.org/10.1088/0957-4484/20/12/125705, 125705 (5pp). [15] M.L. Oyen, C.C. Ko, Indentation variability of natural nanocomposite materials, J. Mater. Res. 23 (2008) 760e767. [16] F.J. Ulm, M. Vandamme, H.M. Jennings, J. Vanzo, M. Bentivegna, K.J. Krakowiak, G. Constantinides, C.P. Bobko, K.J. Van Vliet, Does microstructure matter for statistical nanoindentation techniques? Cem. Concr. Compos. 32 (2010) 92e99. [17] H. Buckle, J.W. Westbrook, H. Conrad, The science of hardness testing and its applications, Am. Soc. Metals Metal Park OH (1973) 453e491. [18] G. Venugopal, K. Krishnamoorthy, R. Mohan, S.J. Kim, An investigation of the electrical transport properties of graphene-oxide thin films, Mater. Chem. Phys. 132 (2012) 29e33. [19] A. Chutia, R. Sahnoun, R.C. Deka, Z.G. Zhu, H. Tsuboi, H. Takaba, A. Miyamoto, Local electronic and electrical properties of functionalized graphene nano flakes, Phys. B-Condensed Matter 406 (2011) 1665e1672. [20] T. Kuila, P. Khanra, A.K. Mishra, N.H. Kim, J.H. Lee, Functionalized-graphene/ ethylene vinyl acetate co-polymer composites for improved mechanical and thermal properties, Polym. Test. 31 (2012) 282e289. [21] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Graphene-based polymer nanocomposites, Polymer 52 (2011) 5e25. [22] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282e286. [23] Lexan Resin 101, Americas: Commercial, Saudi Basic Industries Corporation (SABIC), 2015. [24] T. Liu, Y. Wang, A. Eyler, Y. Chang, D. Wang, W.H. Zhong, Facilitating protein denaturation in organic solvent and the contribution to the promoting dispersion of graphite nanoplatelets in a polymer, eXPRESS Polym. Lett. 9 (2015) 686e694. [25] N.X. Randall, R. Christoph, S. Droz, C. JuliaSchmutz, Localised micro-hardness measurements with a combined scanning force microscope/nanoindentation system, Thin Solid Films 290 (1996) 348e354. [26] N.X. Randall, M. Vandamme, F.J. Ulm, Nanoindentation analysis as a twodimensional tool for mapping the mechanical properties of complex surfaces, J. Mater. Res. 24 (2009) 679e690. [27] Q. Lin, S.A. Cohen, L. Gignac, B. Herbst, D. Klaus, E. Simonyi, Low dielectric constant nanocomposite thin films based on silica nanoparticle and organic thermosets, J. Polym. Sci. B Polym. Phys. 45 (2007) 1482e1493. [28] A.M. Díez-Pascual, M.A. Gomez-Fatou, F. Ania, A. Flores, Nanoindentation in polymer composites, Prog. Mater. Sci. 67 (2015) 1e94. [29] J.C. Halpin, J.L. Kardos, Halpin-Tsai equations e review, Polym. Eng. Sci. 16 (1976) 344e352. [30] S. Lee, J.Y. Hong, J. Jang, The effect of graphene nanofiller on the crystallization behavior and mechanical properties of poly(vinyl alcohol),, Polym. Int. 62 (2013) 901e908.