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sulfonated graphene oxide nanocomposite membranes for the measurement of mechanical properties
Journal of Membrane Science 451 (2014) 40–45
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Nanoindentation of annealed Nafion/sulfonated graphene oxide nanocomposite membranes for the measurement of mechanical properties Sangmin Lee a, Bong Gill Choi b, Dukhyun Choi a, Ho Seok Park c,n a Department of Mechanical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea b Department of Chemical & Biomolecular Engineering (BK 21), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea c Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 31 August 2013 Accepted 20 September 2013 Available online 30 September 2013
The functionalized graphene oxides (GOs) are advanced nanofillers to improve the mechanical properties of the nanocomposite membranes, which can be further enhanced by a simple thermal annealing. Here, we report the preparation and annealing of Nafion/sulfonated GO (N/sG) nanocomposite membranes for the improvement of mechanical properties. In particular, the influence of thermal annealing on the mechanical behaviors of the N/sG composite membranes was investigated by the nanoindentation. The macro- and microscopically uniform dispersion of sGO in the polymer matrices arising from the favorable interactions between them could effectively improve the modulus of Nafion by up to 24.3% (from 0.70 GPa to 0.87 GPa at the loading of 1.0 wt% of sGO) while preserving the hardness. Both the modulus and hardness of the composite membranes were further enhanced by 27% (from 0.70 GPa to 0.89 GPa) and 6.6% (from 62 MPa to 65 MPa), respectively, after the thermal annealing at 160 1C. This annealing-induced improvement results from the reorientation of crystalline domain in the composite membranes. These findings provide a new insight to improve the mechanical behaviors of the nanocomposite membranes by means of the incorporation of the functionalized GOs and the treatment of thermal annealing. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Nanomaterials Composite membranes Nanoindentation
1. Introduction Graphene oxides (GOs) are of a pivotal importance as an amphiphilic soft material owing to the strong interfacial interactions relevant to oxygen-containing functional groups, large surface area, intrinsically excellent physical properties, and chemical tunability of properties [1–3]. Accordingly, the functionalized GOs are used as good nanofillers to modify and improve the physical properties of polymer matrices for widespread applications in fuel cells, lithium rechargeable batteries, supercapacitors, solar cells, and field effect transistors [4–7]. A variety of polymer/GOs nanocomposites, such as poly(vinyl acetate)/GOs, polyurethane/GOs, Nafion/GO, poly(butylene succinate)/GO, polyimide/GO, and poly (vinyl alcohol)/GO, have been explored so far [6–11]. Among them, Nafion/GO nanocomposites were first exploited by us as polymer
n
Corresponding author. Tel.: þ 82 31 201 3327; fax: þ82 31 204 8114. E-mail addresses: [email protected] (S. Lee), [email protected] (B.G. Choi), [email protected] (D. Choi), [email protected] (H.S. Park). 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.038
electrolyte membranes, which expand the applicative fields of GOs into the ion transporting materials [12,13]; the functional moieties of GOs acting as proton donor or acceptor sites can transport ions through hopping process, and the tethering of GOs by highly acidic moiety such as sulfonic group can increase the ionic conductivity. Although the good mechanical properties of the nanocomposite membranes are required for practical applications, they have been neither explored nor improved yet. The unique bicontinuous microstructure of Nafion, a perfluorosulfonate ionomer, consisting of a hydrophobic backbone and hydrophilic ionic channels enables to achieve a remarkable physical properties such as high ionic conductivity and good thermal, chemical, and mechanical stabilities [14,15]. The thermal annealing has been particularly shown to reorganize the crystalline structure of Nafion [16]. Considering that the structural feature in the crystalline domain of Nafion is closely related to the mechanical properties [14], the reorientation of polymer backbone is expected to change the mechanical properties. Despite our previous work about the effect of the incorporated sulfonated graphene oxide (sGO) on the transport properties of Nafion [12],
S. Lee et al. / Journal of Membrane Science 451 (2014) 40–45
the influence of sGO incorporation and thermal annealing on the mechanical behavior of the composite membranes has been unexplored yet. Herein, we investigate the deep understanding of mechanical behavior of annealed Nafion/sGO (N/sG) nanocomposite membranes by using the nanoindentation technique. Nanoindentation is a suitable methodology to measure the mechanical properties such as Young's modulus and hardness of the graphene-based nanocomposites because the nanoscale dimension of deformations measured in this method is consistent with the dimension of graphene nanofillers. Despite the extensive works about the mechanical properties of mono- and few-layer graphene by nanoindentation [17], there were very few researches for the investigation of mechanical properties of graphene-based nanocomposites [18] and most of efforts on macro scale tests [19,20].
2. Experimental section 2.1. Synthesis of Nafion/sGO nanocomposite membranes Graphite powder (o20 μM) and hydrazine solution (65 wt% in water) were purchased from Aldrich. Sulfuric acid (97%) and nitric acid (70%) aqueous solutions were obtained from Junsei Chemical Co., Ltd. Exfoliated GOs were prepared by previous report using the modified Hummers method. Using GO as the starting material [8], the sGOs were synthesized through a microwave-assisted functionalization method [21]. The as-prepared GO powder (20 mg) was reacted with a mixture of 10 mL of nitric acid (70%) and 10 mL of sulfuric acid (97%) in the reaction chamber lined with Teflon PFA and controlled with a pressure (0–200 psi). The sGOs were obtained after the mixture was then treated with a microwave radiation (CEM MDS-2100 microwave digestion system) at 50% of a total of 900 W and 20 psi of pressure for 3 min. The resulting mixture was purified by dialysis water until the filtrated solution was neutral and washed by de-ionized (DI) water (500 mL) carefully. The powder of sGO was collected by filtration and dried under vacuums at 60 1C. Nafion solution (perfluorinated resin solution, 5 wt% in lower aliphatic alcohol and water mixture, Aldrich) was dissolved in N,N′-dimethylacetamide and added by sGOs (0.5 wt% relative to Nafion). The mixture was stirred and treated under sonication for 1 h. The mixture of Nafion and sGO was slowly poured into a petri dish in an amount suitable for adjusting the thickness of the as-cast membrane into ca. 50 μm. The filled dish was placed on the leveled plate of a vacuum dry oven, and then was dried by slowly increasing the temperature from 60 1C to 120 1C for 12 h. The resulting composite membranes were boiled in 30% H2O2 for 2 h at 70 1C and then immersed in 1 M H2SO4 solution for 1 h. After washing with DI water, the composite membranes were dried at 70 1C under vacuum. The resulting composite membranes are denoted as N/sG-1 (0.1 wt% of sGO), N/sG-2 (0.5 wt% of sGO), and N/sG-3 (1.0 wt% of sGO) as a function of the sGO loading. For annealed Nafion and sGO composite membranes, the annealing was carried out in a vacuum oven. The pristine and composite membranes were heated to the temperature of 160 1C and kept for 1.5 h. And then, the oven slowly cooled down to room temperature over 5 h. 2.2. Characterization Transmission electron microscope (TEM) images were collected on a JEM-3010h TEM (300 kV) using samples prepared from cross section. In order to observe the dispersion state of sGO sheets in the composite membranes, thin sections (ca. 80 nm) were cut by
41
using ULTRACUT UCT ultramicrotome (Leica, Austria) after quenching by liquid nitrogen for freezing and slicing the samples without damage, and then the sliced samples were placed on TEM grids. Scanning electron microscope (SEM) images were obtained using a field emission scanning electron microscope (FEI Sirion model) equipped with an in-house Schottky emitter in high stability. The X-ray diffraction (XRD) data were obtained on a Rigaku D/max lllC (3 kW) with a θ/θ goniometer equipped with a CuKa radiation generator. The diffraction angle of the diffractograms was in the range of 2θ ¼ 10–401. The mechanical properties of pristine Nafion and composite membranes were measured by using nanoindenter XP of MTS™. Since the trend of the tensile strength and the elongation can be predicted from the modulus and hardness which obtained from the nanoindentation test, this method is commonly used for measuring mechanical properties of thin films up to nanometer scale thickness [22–25]. These techniques depend on the fact that the displacements recovered during unloading are largely elastic, which allows the Oliver and Pharr method to be used to determine elastic modulus (E) and hardness (H) values from an analysis of indentation load–displacement data [26–29]. The E and the H are calculated from several parameters, including load (P), displacement (h) and contact stiffness (S), which can be measured from the indentation load–displacement curve and unloading slop at the peak load. All the membranes revealed a typical behavior load– displacement curve. The H, which is typically defined as the mean pressure under the nanoindenter, can be derived from the following equation, H¼
P max AC
ð1Þ
where Pmax is obtained from the load-displacement curve, and AC is the projected contact area of the nanoindenter tip. In Fig. 2, the initial slope of the unloading curve (dP/dh) can be used for the indentation modulus (M) of the material by the following expression, S¼
dP 2 pffiffiffiffiffiffi ¼ βpffiffiffiffiM AC dh π
ð2Þ
where β is a correction factor that depends on the geometry of the nanoindenter, and a Berkovich nanoindenter tip (β ¼ 1.034) was used. Therefore, the E of the material can be determined by, 1 1 v2 1 v2i ¼ þ M E Ei
ð3Þ
where ν is Poisson's ratio for the sample, and Ei and vi are the same parameters for the nanoindenter. Based on Eqs. (1)–(3), the E and the H of the prepared samples can be measured and we relatively compared the elastic moduli for each sample due to ambiguous Poisson's ratio of the samples. The indentation results were obtained under displacement control with maximum displacement of 500 nm. The tests were performed at least 20 times for each sample.
3. Results and discussion The N/sG composite membranes were prepared through the solvent casting process, as previously reported by us [12]. All samples were uniform without any cracks, because the sGO were well dispersed in polymer matrices by means of the compatibility between them. As shown in the morphology of the composite membranes by TEM and SEM images, the sGO sheets were randomly incorporated into a polymer matrix without discernible aggregation in a macro- and microscopic manner (see Fig. 1). The good dispersion of sGO and tight binding at the interface was
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S. Lee et al. / Journal of Membrane Science 451 (2014) 40–45
Fig. 1. (a) TEM (200 nm bar) and (b) SEM (1 μm bar) images of the N/sG-1 composite membranes.
Fig. 2. FT-IR spectra of Nafion, sGO, and N/sG composite membranes.
attributed to favorable interactions between the Nafion matrices and nanofillers. In order to investigate the interfacial interactions between the sGOs and Nafion polymer, Fourier-transform infrared spectroscopy (FT-IR) spectroscopy was performed, as shown in Fig. 2. The characteristic bands of Nafion were observed at 970, 980, 1050, 1150, and 1210 cm 1, which are typical bands of Nafion polymer. The favorable interactions of sGOs with Nafion through hydrogen bonding interactions between sulfonic groups of Nafion and sGO and hydrophobic interactions between hydrophobic backbone of Nafion and aromatic domain of sGO were verified by the shift and broadening of the characteristic bands of the Nafion in N/sG. After annealing, all the membranes were protonized in 1 M H2SO4 solution at 80 1C for 2 h and then washed thoroughly with the DI water. As shown in a TEM image (Fig. S1), the annealed composite membranes still remain homogeneous without including any pinhole and crack. Fig. 3a and b shows the load–displacement curves for pristine Nafion and composite membranes before and after annealing treatment under the depth-sensing indentation experiment. P and h were measured directly from the indentation load–displacement curve, as shown in Fig. 3a, and S was determined by measuring the unloading slop at peak load. Based on Eqs. (1)–(3), we calculated the elastic moduli for each sample. Fig. 3c and d shows the modulus and hardness of pristine Nafion and composite membranes before and after the annealing treatment based on the nanoindentation results of Fig. 3a and b. Before annealing, the pristine Nafion showed 0.70 GPa of modulus, which is in a reasonable agreement with those previously reported in the literature [16]. As the density of sGO fillers increases, the modulus of the composite membranes increases, showing the similar standard
deviation. It means that sGOs were uniformly dispersed in Nafion matrices and tightly bound with them. Along with the incorporation of sGO fillers into a polymer matrix, the modulus of the membranes was enhanced up to 24.3% (i.e. the modulus for the unannealed sGO composite at the loading of 1.0 wt% was about 0.87 GPa). On the other hand, the hardness was little affected by the density of the sGO filler except for the loading of 0.1 wt%. The composite membrane at the loading of 0.1 wt% was slightly lower compared to those of others presumably due to the typical behaviors of the composite consisting of a matrix and fillers. Since it is impossible to realize perfectly uniform dispersion of the fillers into a matrix, many previous results showed the similar phenomenon, and thus, an overall tendency was shown either to increase or decrease with a marginal fluctuation as the filler contents increased [30,31]. Likewise, although the hardness of the N/sG-1 composite membrane was slightly lower compared to those of others, the overall results showed that the hardness was little affected by the density of sGO filler used in our experimental condition. As a result, the incorporation of sGO can improve the mechanical properties of the sGO composite membrane, which was caused by the strong interfacial interaction between the Nafion and the sGO, as mentioned earlier. In order to study the influence of the annealing on the mechanical property of Nafion and composite membranes, the modulus and hardness of all samples were measured after the annealing treatment. In the case of pristine Nafion, both modulus and hardness decreased from 0.70 GPa to 0.39 GPa and from 61 MPa to 33 MPa, respectively, due to the unfavorably reorganized crystalline structure of Nafion by annealing. On the other hand, the simple annealing effectively improved the mechanical properties of the composite membranes along with the incorporation of sGO. The modulus increased by 14.1% (from 0.71 GPa to 0.81 GPa), 15.8% (from 0.76 GPa to 0.88 GPa), and 2.3% (from 0.87 GPa to 0.89 GPa) for the N/sG-1, N/sG-2, and N/sG-3 membranes, respectively, after annealing at 160 1C. Furthermore, we confirmed that the modulus can be increased even at the small sGO loading of 0.1 wt% unlike the result before annealing. The hardnesses of the composite membranes were changed from 55 MPa to 61 MPa, from 61 MPa to 60 MPa, and from 61 MPa to 65 MPa, respectively. Compared to the mechanical properties of the pristine Nafion, the modulus for the annealed sGO composite at the loading of 1.0 wt% increased by 27% (from 0.7 GPa to 0.89%), and the hardness increased by 6.6% (from 61 MPa to 65 MPa), and consequently the mechanical properties of sGO composite membrane was further enhanced by the thermal annealing because the favorable interaction between sGO and Nafion can promote the reorientation of crystalline domain [32,33]. In order to make sure the effect for the thermal annealing, we measured thermal gravimetric analysis (TGA) curves of the composite membranes before and after annealing (Fig. S2).
S. Lee et al. / Journal of Membrane Science 451 (2014) 40–45
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Fig. 3. Plots of displacement vs. load of Nafion, N/sG-1, N/sG-2, and N/sG-3 (a) before and (b) after annealing measured for depth-sensing indentation experiment. (c) Modulus and (d) hardness of the pristine Nafion and N/sG composite membranes before (left bar) and after (right bar) the annealing treatment.
The decomposition temperature of crystalline backbone was enhanced after the annealing because of the promotion of crystallization by the interplay of sGO with Nafion. Meanwhile, we considered the reason why the standard deviation for measuring the modulus and the hardness became larger after the annealing. In general, the change in the crystallinity of Nafion after the annealing is very sensitive to the intrinsic and extrinsic parameters such as the molecular weight and equivalent weight of polymers, the method of membrane fabrication, the annealing temperature and time, humidity, and so on. As a result, the effect of annealing on the mechanical properties is still controversial depending on the enhancement of the crystallinity [16,34] or a detrimental impact on the crystallinity [35]. In other words, the mechanical property of Nafion membrane can decrease after the annealing due to the deformation of crystalline structure by means of the humidity-induced and thermal stress [16]. In our case, the decreased mechanical properties of the annealed Nafion represented the detrimental effect of the annealing on the crystallinity probably due to the deformation of crystalline structure by means of the thermal stress, considering our annealing process performed at the temperature of 160 1C in a vacuum oven. On the other hand, the crystallinities of the sGO composite membranes were enhanced at the same condition because the interaction between sGO and Nafion was enough strong to prevent the deformation of crystalline structure due to the nanofiller-induced promotion of structural reorientation and local ordering. We can carefully infer from such process that the crystalline structure may be partially deformed, which consequently led to the larger standard deviation. In order to find the exact reason, further studies are needed in future. Obviously, however, our results clearly showed that the mechanical properties can be not only improved by the incorporation of the sGO, but also further enhanced by a simple thermal annealing. For the further understanding of such enhancement of mechanical properties for annealed composite membranes, we investigated the crystalline structures of the Nafion and N/sG composite membranes before and after annealing with XRD
spectra as shown in Figs. S3 and 4. The broad peak at around 2θ ¼17.51 can be resolved into two domains consisting of crystalline and amorphous domains. Neither a precise decoupling nor a quantitative analysis of two domains can be done because of the superposition of the sharp crystalline peak and the broad amorphous halo [36]. Notably, the incorporation of sGO into the polymer matrix broadened the crystalline peak of Nafion, resulting from the effects of lattice distortions and/or prevention of crystallization as previously reported by us [12]. The crystalline peak of the Nafion was weakened by means of annealing at 160 1C, which is in a good agreement with previous literatures [35], while those of the composite membranes were intensified [16]. This finding indicates the restoration of crystalline domain as a consequence of the melting and re-crystallization during the annealing process [35]. Consequently, the mechanical properties of Nafion polymer could be effectively improved by the incorporation of sGO and simple annealing process. On the other hand, the mechanical properties of the N/sG composite may be different with the annealing temperature because the temperature to reach equilibrium state of reorganized composite membranes can be changed by the existence of the interplay with sGO nanofillers. Hence, further studies are needed to determine the effect of various annealing temperature in future.
4. Conclusions We demonstrated the comparative nanoindentation studies about the mechanical properties of pristine Nafion and N/sG nanocomposite membranes before and after the thermal annealing. After the incorporation of sGOs at the loading of 1.0 wt% into the polymer matrices, the modulus of the membranes was enhanced 24.3% (from 0.70 GPa to 0.87 GPa) while preserving the hardness because of the homogeneous distribution of nanofillers. Along with the incorporation of sGO, the thermal annealing at 160 1C effectively improved the mechanical properties of the membranes: the modulus for the annealed sGO composite at the
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S. Lee et al. / Journal of Membrane Science 451 (2014) 40–45
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.09. 038.
References
Fig. 4. XRD patterns of (a) N/sG-1, (b) N/sG-2, and (c) N/sG-3 before and after the annealing treatment.
loading of 1.0 wt% increased by 27% (from 0.7 GPa to 0.89%), and the hardness increased by 6.6% (from 61 MPa to 65 MPa), compared to the mechanical properties of the pristine Nafion. These results can be rationalized by the incorporation of sGO nanofillers and the enhancement of crystallinity in the composite membranes.
Acknowledgments This work was supported by a grant from Kyung Hee University in 2012 (20120595).
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Nanoindentation of annealed Nafion/sulfonated graphene oxide nanocomposite membranes for the measurement of mechanical properties Sangmin Lee a, Bong Gill Choi b, Dukhyun Choi a, Ho Seok Park c,n a Department of Mechanical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea b Department of Chemical & Biomolecular Engineering (BK 21), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea c Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 31 August 2013 Accepted 20 September 2013 Available online 30 September 2013
The functionalized graphene oxides (GOs) are advanced nanofillers to improve the mechanical properties of the nanocomposite membranes, which can be further enhanced by a simple thermal annealing. Here, we report the preparation and annealing of Nafion/sulfonated GO (N/sG) nanocomposite membranes for the improvement of mechanical properties. In particular, the influence of thermal annealing on the mechanical behaviors of the N/sG composite membranes was investigated by the nanoindentation. The macro- and microscopically uniform dispersion of sGO in the polymer matrices arising from the favorable interactions between them could effectively improve the modulus of Nafion by up to 24.3% (from 0.70 GPa to 0.87 GPa at the loading of 1.0 wt% of sGO) while preserving the hardness. Both the modulus and hardness of the composite membranes were further enhanced by 27% (from 0.70 GPa to 0.89 GPa) and 6.6% (from 62 MPa to 65 MPa), respectively, after the thermal annealing at 160 1C. This annealing-induced improvement results from the reorientation of crystalline domain in the composite membranes. These findings provide a new insight to improve the mechanical behaviors of the nanocomposite membranes by means of the incorporation of the functionalized GOs and the treatment of thermal annealing. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Nanomaterials Composite membranes Nanoindentation
1. Introduction Graphene oxides (GOs) are of a pivotal importance as an amphiphilic soft material owing to the strong interfacial interactions relevant to oxygen-containing functional groups, large surface area, intrinsically excellent physical properties, and chemical tunability of properties [1–3]. Accordingly, the functionalized GOs are used as good nanofillers to modify and improve the physical properties of polymer matrices for widespread applications in fuel cells, lithium rechargeable batteries, supercapacitors, solar cells, and field effect transistors [4–7]. A variety of polymer/GOs nanocomposites, such as poly(vinyl acetate)/GOs, polyurethane/GOs, Nafion/GO, poly(butylene succinate)/GO, polyimide/GO, and poly (vinyl alcohol)/GO, have been explored so far [6–11]. Among them, Nafion/GO nanocomposites were first exploited by us as polymer
n
Corresponding author. Tel.: þ 82 31 201 3327; fax: þ82 31 204 8114. E-mail addresses: [email protected] (S. Lee), [email protected] (B.G. Choi), [email protected] (D. Choi), [email protected] (H.S. Park). 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.038
electrolyte membranes, which expand the applicative fields of GOs into the ion transporting materials [12,13]; the functional moieties of GOs acting as proton donor or acceptor sites can transport ions through hopping process, and the tethering of GOs by highly acidic moiety such as sulfonic group can increase the ionic conductivity. Although the good mechanical properties of the nanocomposite membranes are required for practical applications, they have been neither explored nor improved yet. The unique bicontinuous microstructure of Nafion, a perfluorosulfonate ionomer, consisting of a hydrophobic backbone and hydrophilic ionic channels enables to achieve a remarkable physical properties such as high ionic conductivity and good thermal, chemical, and mechanical stabilities [14,15]. The thermal annealing has been particularly shown to reorganize the crystalline structure of Nafion [16]. Considering that the structural feature in the crystalline domain of Nafion is closely related to the mechanical properties [14], the reorientation of polymer backbone is expected to change the mechanical properties. Despite our previous work about the effect of the incorporated sulfonated graphene oxide (sGO) on the transport properties of Nafion [12],
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the influence of sGO incorporation and thermal annealing on the mechanical behavior of the composite membranes has been unexplored yet. Herein, we investigate the deep understanding of mechanical behavior of annealed Nafion/sGO (N/sG) nanocomposite membranes by using the nanoindentation technique. Nanoindentation is a suitable methodology to measure the mechanical properties such as Young's modulus and hardness of the graphene-based nanocomposites because the nanoscale dimension of deformations measured in this method is consistent with the dimension of graphene nanofillers. Despite the extensive works about the mechanical properties of mono- and few-layer graphene by nanoindentation [17], there were very few researches for the investigation of mechanical properties of graphene-based nanocomposites [18] and most of efforts on macro scale tests [19,20].
2. Experimental section 2.1. Synthesis of Nafion/sGO nanocomposite membranes Graphite powder (o20 μM) and hydrazine solution (65 wt% in water) were purchased from Aldrich. Sulfuric acid (97%) and nitric acid (70%) aqueous solutions were obtained from Junsei Chemical Co., Ltd. Exfoliated GOs were prepared by previous report using the modified Hummers method. Using GO as the starting material [8], the sGOs were synthesized through a microwave-assisted functionalization method [21]. The as-prepared GO powder (20 mg) was reacted with a mixture of 10 mL of nitric acid (70%) and 10 mL of sulfuric acid (97%) in the reaction chamber lined with Teflon PFA and controlled with a pressure (0–200 psi). The sGOs were obtained after the mixture was then treated with a microwave radiation (CEM MDS-2100 microwave digestion system) at 50% of a total of 900 W and 20 psi of pressure for 3 min. The resulting mixture was purified by dialysis water until the filtrated solution was neutral and washed by de-ionized (DI) water (500 mL) carefully. The powder of sGO was collected by filtration and dried under vacuums at 60 1C. Nafion solution (perfluorinated resin solution, 5 wt% in lower aliphatic alcohol and water mixture, Aldrich) was dissolved in N,N′-dimethylacetamide and added by sGOs (0.5 wt% relative to Nafion). The mixture was stirred and treated under sonication for 1 h. The mixture of Nafion and sGO was slowly poured into a petri dish in an amount suitable for adjusting the thickness of the as-cast membrane into ca. 50 μm. The filled dish was placed on the leveled plate of a vacuum dry oven, and then was dried by slowly increasing the temperature from 60 1C to 120 1C for 12 h. The resulting composite membranes were boiled in 30% H2O2 for 2 h at 70 1C and then immersed in 1 M H2SO4 solution for 1 h. After washing with DI water, the composite membranes were dried at 70 1C under vacuum. The resulting composite membranes are denoted as N/sG-1 (0.1 wt% of sGO), N/sG-2 (0.5 wt% of sGO), and N/sG-3 (1.0 wt% of sGO) as a function of the sGO loading. For annealed Nafion and sGO composite membranes, the annealing was carried out in a vacuum oven. The pristine and composite membranes were heated to the temperature of 160 1C and kept for 1.5 h. And then, the oven slowly cooled down to room temperature over 5 h. 2.2. Characterization Transmission electron microscope (TEM) images were collected on a JEM-3010h TEM (300 kV) using samples prepared from cross section. In order to observe the dispersion state of sGO sheets in the composite membranes, thin sections (ca. 80 nm) were cut by
41
using ULTRACUT UCT ultramicrotome (Leica, Austria) after quenching by liquid nitrogen for freezing and slicing the samples without damage, and then the sliced samples were placed on TEM grids. Scanning electron microscope (SEM) images were obtained using a field emission scanning electron microscope (FEI Sirion model) equipped with an in-house Schottky emitter in high stability. The X-ray diffraction (XRD) data were obtained on a Rigaku D/max lllC (3 kW) with a θ/θ goniometer equipped with a CuKa radiation generator. The diffraction angle of the diffractograms was in the range of 2θ ¼ 10–401. The mechanical properties of pristine Nafion and composite membranes were measured by using nanoindenter XP of MTS™. Since the trend of the tensile strength and the elongation can be predicted from the modulus and hardness which obtained from the nanoindentation test, this method is commonly used for measuring mechanical properties of thin films up to nanometer scale thickness [22–25]. These techniques depend on the fact that the displacements recovered during unloading are largely elastic, which allows the Oliver and Pharr method to be used to determine elastic modulus (E) and hardness (H) values from an analysis of indentation load–displacement data [26–29]. The E and the H are calculated from several parameters, including load (P), displacement (h) and contact stiffness (S), which can be measured from the indentation load–displacement curve and unloading slop at the peak load. All the membranes revealed a typical behavior load– displacement curve. The H, which is typically defined as the mean pressure under the nanoindenter, can be derived from the following equation, H¼
P max AC
ð1Þ
where Pmax is obtained from the load-displacement curve, and AC is the projected contact area of the nanoindenter tip. In Fig. 2, the initial slope of the unloading curve (dP/dh) can be used for the indentation modulus (M) of the material by the following expression, S¼
dP 2 pffiffiffiffiffiffi ¼ βpffiffiffiffiM AC dh π
ð2Þ
where β is a correction factor that depends on the geometry of the nanoindenter, and a Berkovich nanoindenter tip (β ¼ 1.034) was used. Therefore, the E of the material can be determined by, 1 1 v2 1 v2i ¼ þ M E Ei
ð3Þ
where ν is Poisson's ratio for the sample, and Ei and vi are the same parameters for the nanoindenter. Based on Eqs. (1)–(3), the E and the H of the prepared samples can be measured and we relatively compared the elastic moduli for each sample due to ambiguous Poisson's ratio of the samples. The indentation results were obtained under displacement control with maximum displacement of 500 nm. The tests were performed at least 20 times for each sample.
3. Results and discussion The N/sG composite membranes were prepared through the solvent casting process, as previously reported by us [12]. All samples were uniform without any cracks, because the sGO were well dispersed in polymer matrices by means of the compatibility between them. As shown in the morphology of the composite membranes by TEM and SEM images, the sGO sheets were randomly incorporated into a polymer matrix without discernible aggregation in a macro- and microscopic manner (see Fig. 1). The good dispersion of sGO and tight binding at the interface was
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Fig. 1. (a) TEM (200 nm bar) and (b) SEM (1 μm bar) images of the N/sG-1 composite membranes.
Fig. 2. FT-IR spectra of Nafion, sGO, and N/sG composite membranes.
attributed to favorable interactions between the Nafion matrices and nanofillers. In order to investigate the interfacial interactions between the sGOs and Nafion polymer, Fourier-transform infrared spectroscopy (FT-IR) spectroscopy was performed, as shown in Fig. 2. The characteristic bands of Nafion were observed at 970, 980, 1050, 1150, and 1210 cm 1, which are typical bands of Nafion polymer. The favorable interactions of sGOs with Nafion through hydrogen bonding interactions between sulfonic groups of Nafion and sGO and hydrophobic interactions between hydrophobic backbone of Nafion and aromatic domain of sGO were verified by the shift and broadening of the characteristic bands of the Nafion in N/sG. After annealing, all the membranes were protonized in 1 M H2SO4 solution at 80 1C for 2 h and then washed thoroughly with the DI water. As shown in a TEM image (Fig. S1), the annealed composite membranes still remain homogeneous without including any pinhole and crack. Fig. 3a and b shows the load–displacement curves for pristine Nafion and composite membranes before and after annealing treatment under the depth-sensing indentation experiment. P and h were measured directly from the indentation load–displacement curve, as shown in Fig. 3a, and S was determined by measuring the unloading slop at peak load. Based on Eqs. (1)–(3), we calculated the elastic moduli for each sample. Fig. 3c and d shows the modulus and hardness of pristine Nafion and composite membranes before and after the annealing treatment based on the nanoindentation results of Fig. 3a and b. Before annealing, the pristine Nafion showed 0.70 GPa of modulus, which is in a reasonable agreement with those previously reported in the literature [16]. As the density of sGO fillers increases, the modulus of the composite membranes increases, showing the similar standard
deviation. It means that sGOs were uniformly dispersed in Nafion matrices and tightly bound with them. Along with the incorporation of sGO fillers into a polymer matrix, the modulus of the membranes was enhanced up to 24.3% (i.e. the modulus for the unannealed sGO composite at the loading of 1.0 wt% was about 0.87 GPa). On the other hand, the hardness was little affected by the density of the sGO filler except for the loading of 0.1 wt%. The composite membrane at the loading of 0.1 wt% was slightly lower compared to those of others presumably due to the typical behaviors of the composite consisting of a matrix and fillers. Since it is impossible to realize perfectly uniform dispersion of the fillers into a matrix, many previous results showed the similar phenomenon, and thus, an overall tendency was shown either to increase or decrease with a marginal fluctuation as the filler contents increased [30,31]. Likewise, although the hardness of the N/sG-1 composite membrane was slightly lower compared to those of others, the overall results showed that the hardness was little affected by the density of sGO filler used in our experimental condition. As a result, the incorporation of sGO can improve the mechanical properties of the sGO composite membrane, which was caused by the strong interfacial interaction between the Nafion and the sGO, as mentioned earlier. In order to study the influence of the annealing on the mechanical property of Nafion and composite membranes, the modulus and hardness of all samples were measured after the annealing treatment. In the case of pristine Nafion, both modulus and hardness decreased from 0.70 GPa to 0.39 GPa and from 61 MPa to 33 MPa, respectively, due to the unfavorably reorganized crystalline structure of Nafion by annealing. On the other hand, the simple annealing effectively improved the mechanical properties of the composite membranes along with the incorporation of sGO. The modulus increased by 14.1% (from 0.71 GPa to 0.81 GPa), 15.8% (from 0.76 GPa to 0.88 GPa), and 2.3% (from 0.87 GPa to 0.89 GPa) for the N/sG-1, N/sG-2, and N/sG-3 membranes, respectively, after annealing at 160 1C. Furthermore, we confirmed that the modulus can be increased even at the small sGO loading of 0.1 wt% unlike the result before annealing. The hardnesses of the composite membranes were changed from 55 MPa to 61 MPa, from 61 MPa to 60 MPa, and from 61 MPa to 65 MPa, respectively. Compared to the mechanical properties of the pristine Nafion, the modulus for the annealed sGO composite at the loading of 1.0 wt% increased by 27% (from 0.7 GPa to 0.89%), and the hardness increased by 6.6% (from 61 MPa to 65 MPa), and consequently the mechanical properties of sGO composite membrane was further enhanced by the thermal annealing because the favorable interaction between sGO and Nafion can promote the reorientation of crystalline domain [32,33]. In order to make sure the effect for the thermal annealing, we measured thermal gravimetric analysis (TGA) curves of the composite membranes before and after annealing (Fig. S2).
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Fig. 3. Plots of displacement vs. load of Nafion, N/sG-1, N/sG-2, and N/sG-3 (a) before and (b) after annealing measured for depth-sensing indentation experiment. (c) Modulus and (d) hardness of the pristine Nafion and N/sG composite membranes before (left bar) and after (right bar) the annealing treatment.
The decomposition temperature of crystalline backbone was enhanced after the annealing because of the promotion of crystallization by the interplay of sGO with Nafion. Meanwhile, we considered the reason why the standard deviation for measuring the modulus and the hardness became larger after the annealing. In general, the change in the crystallinity of Nafion after the annealing is very sensitive to the intrinsic and extrinsic parameters such as the molecular weight and equivalent weight of polymers, the method of membrane fabrication, the annealing temperature and time, humidity, and so on. As a result, the effect of annealing on the mechanical properties is still controversial depending on the enhancement of the crystallinity [16,34] or a detrimental impact on the crystallinity [35]. In other words, the mechanical property of Nafion membrane can decrease after the annealing due to the deformation of crystalline structure by means of the humidity-induced and thermal stress [16]. In our case, the decreased mechanical properties of the annealed Nafion represented the detrimental effect of the annealing on the crystallinity probably due to the deformation of crystalline structure by means of the thermal stress, considering our annealing process performed at the temperature of 160 1C in a vacuum oven. On the other hand, the crystallinities of the sGO composite membranes were enhanced at the same condition because the interaction between sGO and Nafion was enough strong to prevent the deformation of crystalline structure due to the nanofiller-induced promotion of structural reorientation and local ordering. We can carefully infer from such process that the crystalline structure may be partially deformed, which consequently led to the larger standard deviation. In order to find the exact reason, further studies are needed in future. Obviously, however, our results clearly showed that the mechanical properties can be not only improved by the incorporation of the sGO, but also further enhanced by a simple thermal annealing. For the further understanding of such enhancement of mechanical properties for annealed composite membranes, we investigated the crystalline structures of the Nafion and N/sG composite membranes before and after annealing with XRD
spectra as shown in Figs. S3 and 4. The broad peak at around 2θ ¼17.51 can be resolved into two domains consisting of crystalline and amorphous domains. Neither a precise decoupling nor a quantitative analysis of two domains can be done because of the superposition of the sharp crystalline peak and the broad amorphous halo [36]. Notably, the incorporation of sGO into the polymer matrix broadened the crystalline peak of Nafion, resulting from the effects of lattice distortions and/or prevention of crystallization as previously reported by us [12]. The crystalline peak of the Nafion was weakened by means of annealing at 160 1C, which is in a good agreement with previous literatures [35], while those of the composite membranes were intensified [16]. This finding indicates the restoration of crystalline domain as a consequence of the melting and re-crystallization during the annealing process [35]. Consequently, the mechanical properties of Nafion polymer could be effectively improved by the incorporation of sGO and simple annealing process. On the other hand, the mechanical properties of the N/sG composite may be different with the annealing temperature because the temperature to reach equilibrium state of reorganized composite membranes can be changed by the existence of the interplay with sGO nanofillers. Hence, further studies are needed to determine the effect of various annealing temperature in future.
4. Conclusions We demonstrated the comparative nanoindentation studies about the mechanical properties of pristine Nafion and N/sG nanocomposite membranes before and after the thermal annealing. After the incorporation of sGOs at the loading of 1.0 wt% into the polymer matrices, the modulus of the membranes was enhanced 24.3% (from 0.70 GPa to 0.87 GPa) while preserving the hardness because of the homogeneous distribution of nanofillers. Along with the incorporation of sGO, the thermal annealing at 160 1C effectively improved the mechanical properties of the membranes: the modulus for the annealed sGO composite at the
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.09. 038.
References
Fig. 4. XRD patterns of (a) N/sG-1, (b) N/sG-2, and (c) N/sG-3 before and after the annealing treatment.
loading of 1.0 wt% increased by 27% (from 0.7 GPa to 0.89%), and the hardness increased by 6.6% (from 61 MPa to 65 MPa), compared to the mechanical properties of the pristine Nafion. These results can be rationalized by the incorporation of sGO nanofillers and the enhancement of crystallinity in the composite membranes.
Acknowledgments This work was supported by a grant from Kyung Hee University in 2012 (20120595).
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