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The role of reduced graphene oxide on chemical, mechanical and barrier properties of natural rubber composites
Composites Science and Technology 102 (2014) 74–81
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
The role of reduced graphene oxide on chemical, mechanical and barrier properties of natural rubber composites Ning Yan a, Giovanna Buonocore a, Marino Lavorgna a,⇑, Saulius Kaciulis a, Santosh Kiran Balijepalli b, Yanhu Zhan c, Hesheng Xia c,⇑, Luigi Ambrosio a a b c
Institute of Polymers, Composites and Biomaterials, National Research Council, P.le Fermi, 1-80055 Portici, NA, Italy Institute for the Study of Nanostructured Materials, National Research Council, PO Box 10, 00015 Monterotondo Stazione, Rome, Italy State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 20 May 2014 Received in revised form 11 July 2014 Accepted 17 July 2014 Available online 27 July 2014 Keywords: A. Polymer–matrix composites (PMCs) B. Mechanical properties B. Transport properties D. Transmission electron microscopy (TEM)
a b s t r a c t Natural rubber (NR)-reduced graphene oxide (rGO) composites were produced via latex mixing and cocoagulation approach followed by static hot-press and twin roll mixing process. Due to the process, a fine control of filler dispersion was obtained and the composites exhibited a three-dimensional rGO network or alternatively a homogeneous dispersion of single rGO platelets. The effect of rGO dispersion on chemical crosslink structure, and their influence on mechanical and barrier properties was thoroughly investigated. Small angle X-ray scattering (SAXS) and solid-state 13C NMR analysis showed that rGO platelets affect the vulcanization process of natural rubber and that the crosslinking sulphur polysulphidic species present in pristine natural rubber decrease with the rGO content. In fact, at rGO content higher than 6 phr, the crosslinking species consist mainly of monosulphidic species which attain a consequent increment of intrinsic crosslinking density. However, the composites with rGO segregated network exhibit both barrier to oxygen and water vapour permeation and mechanical properties improved with respect to pristine rubber and composites with the homogeneous dispersion of single rGO platelets. The results confirm that the morphology of filler has a prominent key role in determining the natural rubber composites properties. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In the polymer composites field, the potentials of novel fillers to impart valuable properties to the host polymer have been widely explored. In this respect graphene (GE), a single-atom-thick sheet of graphite, exhibit extraordinary physical properties such as ultrahigh mechanical stiffness [1,2], impervious property [3], thermal [4] and electronic conductivity [5,6]. Since the reduced graphene oxide (rGO) can be obtained at relatively low-costs by chemical reduction of graphene oxide (GO) [7,8], it has been regarded as the most promising and attractive filler for polymer composites. Many investigations have been engaged in harnessing the advantages of rGO in polymer materials [9–12]. The multifunctional property enhancement observed in several rGO/polymer composites surely will expand the role of polymers and composites in new application sectors. However, despite significant advances made
⇑ Corresponding authors. Tel.: +39 0817758838 (M. Lavorgna), +86 02885460535 (H. Xia). E-mail addresses: [email protected] (M. Lavorgna), [email protected] (H. Xia). http://dx.doi.org/10.1016/j.compscitech.2014.07.021 0266-3538/Ó 2014 Elsevier Ltd. All rights reserved.
for thermoplastic and thermosets polymer/rGO composites, there are only a few studies involving the use of rGO in rubber materials [13–15]. In a previous paper, some of the authors of the present contribution developed an ultrasonically assisted latex mixing and in situ reduction process (ULMR) to prepare rGO/NR composites. It was demonstrated that ULMR followed by a two-roll mixing process produces a very fine dispersion and exfoliation of rGO in NR matrix and contribute to a significant increase in the tensile strength [16]. Recently, Zhan et al. [17] have proposed an approach based on coagulation mixing of GO platelets with natural rubber latexes (NRL) to prepare, by in situ reduction, a coagulate rGO composite, affording a ‘segregated’ three-dimensional network of rGO. The authors concluded that at higher filler content the segregated network hinders the crosslinking reaction of natural rubber giving rise to poor mechanical properties. Although, the crosslinked network of vulcanized NR matrix plays a key role in attaining high mechanical strength, it is of great significance to explore the relation between rGO network morphology and vulcanization reaction as well as to investigate their reciprocal influence on mechanical and barrier properties.
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In this work, the effect of the segregated and not-segregated rGO morphologies on the vulcanization process, gas permeability and mechanical properties of rGO/NR composites has been investigated. X-ray photoemission spectroscopy (XPS), small angle X-ray scattering (SAXS) and solid-state 13C NMR techniques were employed to investigate the effect of rGO morphology on the chemical structures of the crosslinks during the vulcanization of NR. The oxygen and water vapour permeability of rGO/NR composites with both the segregated and not-segregated morphologies were also systematically analyzed alongside with the tensile properties. The results reveal both an intrinsic correlation between filler morphology and crosslink chemical structure, and the prominent role of filler morphology on physical properties of natural rubber composites.
vapour permeability were performed using the infrared sensor technique by means of a PermatranW3/31 (Mocon, Germany). Samples with a surface area of 5 cm2 were tested at 25 and 38 °C. The oxygen permeability was measured using a standard permeabilimeter (Extrasolution, MULTIPERM), at 38 °C and 0% (or 50%) Relative Humidity (RH). Water vapour sorption kinetics was obtained by using a gravimetric microbalance (Hiden Isochema Ltd, IGA002, UK). The experiments were run at 38 °C increasing the water activity into the reactor from 0 to 0.5 at constant rate. Sample weight was monitored by the instrument software.
2. Materials and methods
TEM analysis was employed to investigate the dispersion of rGO platelets in the NR composites. Fig. 1(a) shows the non-uniform distribution of rGO platelets in the composite containing 4 phr of rGO platelets. The submicrometer NR latex particles are covered by rGO platelets deposited by co-coagulation procedure, establishing a segregated network morphology, which is preserved by static hot pressing. Micrograph reported in Fig. 1(c) relates to the latex co-coagulated rGO/NR flocculate after the two-roll mixing process. The segregated network created by latex co-coagulation process is destroyed by the milling process, resulting in a uniform distribution of slightly orientated rGO platelets (3 nm) in NR matrix. Fig. 1(b) is the micrograph of NR composite containing 8 phr of rGO platelets, where are clearly observed some latex particles wrapped by an rGO segregated network thicker than that observed for the sample with 4 phr of rGO (see Fig. 1(a)). This is likely to be ascribed to the restacking of rGO platelets. After milling process, as is shown Fig. 1(d), the segregated network is destroyed as expected, but thicker platelets and reduced orientation are observed compared to those shown in Fig. 1(c), despite the intense shear forces generated by the milling process.
2.1. Materials The materials used in this work were detailed in our prior report [16]. Chemical reagents including vulcanization agent sulphur, accelerator N-cyclohexyl-2-benzothiazolesulfenamide (CBS) and 2-mercaptobenzothiazole (MB), antioxidant (4010NA), activator zinc oxide and stearic acid are all commercially available. 2.2. Graphene rubber composite preparation Graphite oxide powder was obtained through oxidation of natural flake graphite according to the Hummers method [18], and was exfoliated in deionized water (DI) by ultrasonication to get graphene oxide (GO) aqueous suspension. For the preparation of rGO/NR composites, the as-prepared GO aqueous suspension was dispersed into NR latex by ultrasonic irradiation, and then in-situ reduced by hydrazine hydrate under ultrasonic irradiation to obtain the rGO/NR latex. Subsequently, crosslinking agent sulphur and other additives were added into the rGO/NR latex and the obtained latex was coagulated and filtrated. The filtrated solids fraction was thoroughly washed and dried in vacuum oven, and then directly hot pressed and in-situ vulcanized at 150 °C to obtain the crosslinked rGO/NR composites with segregated network structure. Samples with not-segregated network were prepared by submitting the coagulated and dried rubber mixture to two-roll mixing process. rGO/NR composites at rGO loadings of 0.5, 1.0, 2.0, 4.0, 6.0 and 8.0 phr (parts per hundred parts of rubber), are coded as NRLG-x for segregated, and NRLG-TR-x for not-segregated samples, respectively; x represents the rGO graphene loading as parts per hundred parts of rubber. 2.3. Characterization Transmission Electron Microscopy (TEM) analysis was performed using a FEI TecnaiG2 F20 S-TWIN transmission electron microscope, operating at an accelerating voltage of 200 kV. X-ray photoemission (XPS) spectra was collected by using an Escalab 250Xi (Thermo Fisher Scientific, UK) spectrometer. Small Angle X-ray Scattering (SAXS) analyses were performed using an Anton Paar SAXS, Cu Ka X-Rays with 1.5418 Å wavelength were generated by a Philips PW3830 sealed tube source (40 kV, 50 mA) and slit collimated. The crosslink density of the vulcanized samples was determined using a reported toluene swelling method [16]. NMR measurements were performed on a Bruker MSL300 spectrometer (75 MHz for 13C) with a standard Bruker CP/MAS probe head at room temperature. Uniaxial tensile testing was performed at room temperature with universal testing machine (Instron 3360) according the ASTM D-412. The measurements of water
3. Results and discussion 3.1. Morphological characterization
3.2. Chemical characterization 3.2.1. X-ray photoelectron spectroscopy XPS investigation was undertaken to analyze quantitatively the chemical composition of the samples surface: pristine NR and rGO/ NR composites with segregated and not-segregated morphology. The chemical composition of the samples, determined by using XPS quantification procedure, is reported in Table 1. In Fig. 2 are reported the selected XPS C 1s spectra for pristine rubber and NR/rGO composites with 4 phr of rGO with segregated and notsegregated morphology. The results presented in Fig. 2(a) reveal a significant difference in C 1s spectra: the pristine NR is characterized by wider C 1s peak centred at BE = 285.0 eV, whereas, in the samples containing rGO, this peak is narrower and it is centred at BE = 284.6 eV (graphitic value). The deconvolutions of C 1s spectra for each sample are shown in Fig. 2 (b)–(d) respectively, in which the first peak C1 centred at BE = 284.6 eV is assigned to graphitic bond arising from rGO [19,20], and this can also be clarified by the C1 value shown in Table 1. The C2 at BE = 285.0 eV corresponds to aliphatic carbon and the third component C3 at BE 286 eV can be attributed to CAO and/or CAN bonds [19]. Moreover, some of these samples show C4 at BE 288 eV, attributed to the ketone group, which may persist from the reduction of graphene oxide [16]. Moreover, the chemical bond of sulphur is different in pristine NR and rGO/NR composites: two chemical species of sulphur at BE = 161.7, S1 and at 163.8 eV, S2 are identified in pristine NR, whereas only the chemical state of sulphur at 163.8 eV is identified for rGO/NR composites, as plotted in Fig. 2(e) and (f). The first
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Fig. 1. TEM images of rGO/NR composites consisting of (a) 4 phr (NRLG-4), (b) 8 phr (NRLG-8) rGO prepared by self-assembly in latex and static hot pressing, (c) 4 phr (NRLG-TR-4) and (d) 8 phr (NRLG-TR-8) rGO prepared by self-assembly in latex followed by two-roll mixing.
Table 1 XPS quantification of surface chemical composition. Samples
C1
C2
C3
C4
N 1s
O 1s
S1
S2
Pristine NR
BE, eV at.%
– –
285.0 70.0
285.8 19.4
– –
400.0 2.3
532.7 6.6
161.7 0.8
163.8 0.4
NRLG–1
BE, eV at.% BE, eV at.%
284.6 27.2 284.6 40.8
285.0 34.2 285.0 19.7
286.2 16.4 286.2 14.5
288.0 6.0 287.8 5.2
399.2 0.9 399.2 0.7
532.0 13.0 532.0 11.6
– – – –
163.8 1.4 163.8 1.1
BE, eV at.% BE, eV at.%
284.5 53.1 284.5 47.6
285.0 29.8 285.0 27.8
286.1 2.3 286.1 4.8
– – – –
400.0 1.2 400.0 1.0
532.1 12.6 532.2 18.4
– – – –
163.8 0.6 163.8 0.3
BE, eV at.% BE, eV at.%
284.6 57.3 284.6 49.6
285.2 26.0 285.2 29.7
286.7 5.0 286.5 5.2
– – – –
399.9 0.8 400.1 0.55
532.0 9.2 532.0 15.8
– – – –
164.0 0.4 163.8 0.1
NRLG-TR-1 NRLG-4 NRLG-TR-4 NRLG-6 NRLG-TR-6
chemical state is attributed to polysulfide [21] or clusters of sulphur, while the second one is characteristic for thiolic CAS bond [22]. For the rGO/NR composites, the polysulphides species disappear with the addition of rGO, and the percentage of S2 atoms assigned to thiolic CAS bond decreases with increasing rGO content. This confirms that the rGO interferes with the pathways of vulcanization reaction. Moreover, there is a greater percentage (i.e. 0.6 at.% for the sample with 4 phr of rGO) of S2 atoms assigned to CAS bond in the segregated sample, compared to the ones in not-segregated sample (0.3 at.%). This suggests that mainly in the rGO segregated network, the sulphur species react with carbon atoms of rGO platelets, resulting preferentially in the formation of monosulphidic SAC species connecting graphene platelets and rubber macromolecules.
3.2.2. Small angle X-ray scattering and nuclear magnetic resonance SAXS investigation of NR composites is of great importance for probing the crosslinked structure of natural rubber. The representative SAXS profiles in terms of I(q) as a function of scattering vector q for different NR composites are shown in Fig. 3(a) and (b) alongside with the spectrum of pristine rubber. A remarkable difference in scattering signals over the q range of 0.1–2.0 nm1 can be observed depending on the content of graphene filler. It is well known that the scattering intensity in SAXS curve reflects the interference arising from the spatial correlation of fluctuations in the electron density of the material [23]. The crosslink sites where natural rubber macromolecular chains are attached each other through polysulphidic species (ACASxACA), give rise to a bump in the SAXS curve as shown in Fig. 3(a) and (b). The higher is the
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Fig. 2. C 1s (a) and S 2p (e) spectra of the selected samples: pristine NR, and rGO/NR composites (4 phr of rGO) with segregated and not-segregated morphologies, and peak fitting of C 1s spectra of (b) pristine NR, (c) rGO/NR composites with segregated morphologies, (d) rGO/NR composites with not-segregated morphologies, and (f) peak fitting of S 2p spectrum for the pristine NR with the two chemical states of sulphur, S1 at 161.7 (S2p3/2A Polysulfide or sulphur clusters) and S2 at 163.8 Ev (S2p3/2B thiolic CAS bond).
Fig. 3. Small angle X-ray scattering profiles of the uncured NR without curatives, rGO/NR composites with segregated morphologies (a) and rGO/NR composites with notsegregated morphologies (b), and the insets are the corresponding Lorentz plots of NR vulcanized composites. (c) Schematic representation of the networks of pristine natural rubber (left) and rGO/NR composite with not-segregated morphology (right). (d) Selected 13C NMR spectra of pristine NR and rGO/NR composites.
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number of sulphur atoms involved in the crosslinking species, the higher becomes the local increase of electron density and consequently the higher is the intensity of the scattering peak in the SAXS spectrum. In order to have a clear identification of scattering feature position, the Lorentz plots, i.e. the I(q)q2 vs. q scattering vector, is frequently used to obtain the structural characteristic (scattering centre) correlation distance. The corresponding Lorentz plots for natural rubber and NR composites are presented as insets in Fig. 3(a) and (b) respectively. The mean distance, d between the scattering centres can be obtained from the Eq. (1) [24]:
d ¼ 2p=qmax
ð1Þ qmax, I(qmax)q2max
The values of and the calculated d are reported in Table 2. The SAXS spectrum of uncrosslinked sample, which is produced without the sulphur agents, does not exhibit any diffraction feature. This confirms that in the spectra of vulcanized samples, both pristine and rubber composites, the diffraction hump is ascribed to the correlation between polysulphidic ACASxACA clusters. Comparing the values in Table 2, it appears that the position of the maximum scattering intensity, qmax, systematically shifts to higher values as the rGO content increases, while maximum scattering intensity I(qmax)q2max exhibits a decreasing trend for the composites both with segregated morphology and not-segregated morphology. It is worth mentioning that the segregated rGO/NR composites have higher qmax values than those of the not-segregated samples at the same rGO content. That is to say, the segregated rGO morphology affords to decrease the distance, d between scattering sulphur centres. Moreover, the scattering signal associated to the separation distance between crosslinking centres disappears at the rGO content of 8 phr for the segregated sample, whereas it is not detected for the not-segregated rGO/NR sample as soon as the rGO content attains to 6 phr. This is attributed to a change of the nature of crosslinks formed during vulcanization with respect to rGO content, i.e. from polysulphidic clusters at low rGO content to di-sulphur or tri-sulphur clusters or monosulphidic crosslinks at higher rGO content (see Fig. 3(c)). In detail, the rGO platelets both in the segregated and not-segregated morphology interfere with the vulcanization process promoting the formation of small sulphur clusters such as AS3A, AS2A or monosulphidic as the rGO content increases. In this view, the crosslinking density of the rubber matrix increases with the rGO content, since smaller sulphur clusters or monosulphidic species form. This result is consistent with that reported in the literature, which suggests that the addition of filler in NR generally leads to an increase of crosslink density [25,26]. However, in our contribution the use
of SAXS has provided more insights into the role of filler on the vulcanization of NR composites. Crosslinking density measurements by the toluene swelling method performed for the not-segregated samples show that increasing the rGO content to 2, 4 and 6 phr increases the crosslink density of pristine NR (0.96 104 mol cm3) to 1.73, 1.88 and 2.08 104 mol cm3 respectively. Unfortunately due to the effective barrier effect of filler network, it was not possible to measure the crosslinking density of the segregated NR/rGO composites by toluene swelling method. However, since the rGO filler content shows more prominent effect than filler morphology on the chemical structure of vulcanized network of NR/rGO composites, the crosslink density of segregated NR/rGO composite is supposed to have the same increasing trend with the rGO content as that of the not-segregated sample. In Fig. 3(d) are reported 13C NMR spectra. The assignment of NMR peaks corresponding to the several carbon atoms of the vulcanized structure was performed according to the paper of Mori et al. [27]. In detail, the peaks centred at 37 and 51 ppm, assigned to A1 cis polysulphidic species show significant differences among the selected pristine NR, segregated and not-segregated samples. When rGO is added in the composite, the polysulphidic clusters mainly present in pristine NR split into smaller polysulphidic species with a limited number of sulphur atoms, i.e. CASxAC with x 8. In this case more sulfide linkages are formed with a consequent increase of the peak intensity related to polysulphidic linkages, ACASA as detected in NMR. On the other hand, the increase in the peak intensity of polysulphidic links brings any variation in the peak intensity of A1 cis monosulphidic links centred at 45 ppm and trans-C1 structure at 40 ppm, whereas a substantial increment is observed for the B1 cis monosulphidic centred at 57 ppm. These results indicate that rGO platelets both in the segregated and not-segregated morphology affect the vulcanization process, promoting the formation of sulphur crosslinking species with a limited number of sulphur atoms (x = 1, 2, 3, . . .) in place of polysulphidic clusters with large number of sulphur atoms (x = 8, 7, 6, . . .). On the basis of results obtained by XPS, SAXS and 13C NMR, it can be concluded that rGO platelets and their morphology have significant influence on the crosslinking network formed during vulcanization process. The chemical structure of the vulcanized network depends mainly on the rGO content, whereas the morphology has only a moderate effect. However, physical properties of rGO/NR composites strongly depend on the matrix properties and filler morphology. Therefore, it is of great interest to tailor the rGO morphology for enhancing specific properties and balancing structural modification of polymeric matrix. Given this fact, a comparative investigation in terms of physical properties between segregated and not-segregated composites was carried out. 3.3. Gas permeability and water vapour absorption
Table 2 The data of the maximum scattering intensities and the corresponding scattering vectors, and the calculated mean distance between the scattering centres of the rGO/NR composites. qmax (nm1)
I (qmax)q2max arb. units
d (nm)
NRLG-0 NRLG-0.5 NRLG-2 NRLG-6 NRLG-8 Uncured NRLG-4
1.51 1.31 1.34 1.41 N/A N/A
3.71 4.29 3.62 1.66 N/A N/A
4.16 4.79 4.69 4.45 N/A N/A
NRLG-TR-0 NRLG-TR-0.5 NRLG-TR-2 NRLG-TR-6 NRLG-TR-8 Uncured NRLG-TR-4
1.44 1.29 1.32 N/A N/A N/A
3.21 2.29 3.06 N/A N/A N/A
4.39 4.87 4.76 N/A N/A N/A
The oxygen permeability at 0% and 50% RH of pristine NR and rGO/NR composites with segregated and not-segregated morphologies were measured at 38 °C and the results are reported in Fig. 4(a) and (b), respectively. The oxygen permeability decreases with increasing the content of rGO for both the segregated and not-segregated films. In details, at dry condition (i.e. 0% RH), the oxygen permeability of composites with segregated morphology drops from 5 107 for the pristine NR to 3.2 107 cm3 cm cm2 s1 atm1 with incorporating only 1 phr of rGO platelets. Moreover, by increasing the rGO loading, the oxygen permeability further decreases up to 2 107 cm3 cm cm2 s1 atm1 when 6 phr of rGO is added. The addition of impervious rGO platelets to the pristine rubber not only decreases the available area for penetrant flux, but also increases the diffusion length in the rubber matrix. In addition, it should be noted that the two different morphologies have different effect, i.e. the films with segregated mor-
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Fig. 4. Gas permeability of rGO/NR film as a function of rGO content: oxygen permeability measured at 38 °C: (a) 0% RH, (b) 50% RH, and (c) water vapour permeability of rGO/NR film measured at 38 °C. (d) Water vapour sorption isotherms for rGO/NR composites with both the segregated (NRLG-2) and not-segregated morphologies (NRLG-TR-2).
phology display oxygen permeability markedly lower than that of not-segregated specimens at all the investigated loadings. This is ascribed to the formation of a percolating and interconnected rGO, resulting in extremely ‘‘tortuous pathway’’ for the gases diffusion. At 50% RH, the oxygen permeability of segregated and not-segregated rGO/NR films exhibits a similar decreasing trend to that measured at 0% RH, with respect to rGO content. However, the oxygen permeability is almost unchanged compared to that of the film with the same rGO loading and morphology at 0% RH. To investigate further the effect of the rGO morphology on the permeability of the film, the water vapour permeability (WVP) of pristine rubber and rGO/NR composites was measured at 38 °C, and the obtained results are shown in Fig. 4(c). The water vapour permeability decreases as the rGO content increases, and the films with segregated morphology show water vapour permeability lower than that of not-segregated samples. Both the chemical structure of rubber matrix and the filler morphologies affect the gas barrier properties of NR composites. Compared to the pristine NR, rGO/NR composites have a higher crosslink density as well as impervious rGO platelets. This brings about to both a decrease in fractional free volume of polymer matrix and an increase in the diffusion length of gas penetrant, resulting in enhanced gases barrier properties [28]. However, given the fact that the chemical structure of segregated and not-segregated rGO/NR composites are similar, the different behaviour in gas barrier properties between these two samples is assumed to be mainly ascribed to filler morphologies. The detailed effect of rGO morphology on the gas permeability of film has been successfully interpreted in our previous publication [13], which has provided a reasonable physical interpretation of the mass transport
behaviour for the samples displaying segregated and not-segregated morphology. The permeability of a penetrant within polymeric film is controlled by both kinetic and thermodynamic factors, corresponding to the penetrant diffusivity and solubility, respectively. In general, a decrease of permeability can result from a decrease of diffusivity and/or solubility. In order to confirm further the influence of rGO morphology on the mass transport of rubber matrix, the water vapour sorption kinetics were performed using a gravimetric method to characterize the water vapour solubility and/or diffusivity in rGO/NR composites. The water vapour sorption kinetics for selected NR composites, is shown in Fig. 4(d). The kinetic curves consist of double steps: a first abrupt increase of water vapour uptake at relative low absorption time, subsequently followed by a steady increase. The first step, usually correlated to the diffusivity coefficient of water sorption, is somewhat comparable for both the morphologies. It is worth noting that segregated rGO/NR composite has a lower water vapour uptake compared to that of the not-segregated film at the same water activity all over the investigated time range. The main reason can be ascribed to rGO platelets which are not-fully chemically reduced and present some hydrophilic groups. This leads to an increase in water uptake for the not-segregated film. On the other hand, the welldefined rGO segregated network, achieved with co-coagulating rGO aqueous suspension with NR latex, creates a stiffer network with reduced exposed hydrophilic surface, resulting in a lower level of water vapour sorption. Taking into account these results, the differences in water permeability can be explained in terms of water solubility which results to be higher for the composites with not-segregated morphology due to a higher hydrophilic surface of rGO platelets.
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The measurements of mechanical properties show that segregated rGO network not only enhance greatly the tensile strength of rGO/ NR composite, but also decrease the strain at break. Permeability analysis suggests that rGO/NR films with segregated network have significantly advantages in terms of oxygen and/or water vapour barrier properties compared to that of the not-segregated composites. The results confirm that the morphology of the filler has a prominent key role in determining the NR composites properties and it is the main parameter to be controlled for tailoring the properties in a multifunctional material. Acknowledgements
Fig. 5. Representative stress–strain curves for the pristine NR and rGO/NR composites with segregated morphologies and rGO/NR composites with notsegregated morphologies.
3.4. Mechanical properties In Fig. 5, is compared the stress–strain behaviour of pristine NR and rGO/NR composites with segregated and not-segregated morphologies. The composites with much low rGO concentration exhibit a significant increased tensile strength compared with pristine NR, while the strain at break of the composites is lower than that of pristine NR. These results are in good agreement with those reported in the previous work [29]. The differences in the mechanical properties between pristine NR and rGO/NR composites are ascribed to the differences in terms of crosslink density, straininduced crystallization, and filler morphology. The addition of rGO platelets results in an increase of rubber crosslink density, as well as an increase of crystallization during stretching, both of which, in general, can increase the tensile strength, while at the same time decrease strain at break. Moreover, a larger increase in tensile strength was observed in segregated sample NRLG-2 compared with the not-segregated sample NRLG-TR-2. In fact rGO content in the two composites is the same, and a fine dispersion of rGO in NRLG-TR-2 is supposed to have a more obviously strain amplification effect due to the higher surface [14], with a consequent to a higher contribution to the mechanical properties. However, mechanical results do not support this thesis. Also, the crosslinking structure of rubber matrix is similar for segregated and not-segregated composites. These results confirm that the different mechanical properties of NRLG-TR-2 and NRLG-2 are mainly ruled by the morphology of the rGO. Segregated networks constituted by the strongly interacting rGO platelets act as skeleton in the host polymer matrix, thus leading to a significant increase of the stiffness and a decrease of the strain at break of composite.
4. Conclusions In summary, we have produced rGO/NR composites consisting of tailored geometrical arrangement of rGO platelets by adopting a facile latex co-coagulation procedure in combination with hotpress or two-roll milling treatment. Segregated dispersion of rGO in composites, as prepared directly by hot-pressing after latex co-coagulation, was successfully observed by TEM, whereas a uniformly distribution of rGO platelets appears over the cross-section of two-roll milled samples. XPS, SAXS and NMR analyses confirm that the rGO platelets affect the vulcanization process of natural rubber. In particular, polysulphidic sulphur species are more prevalent in the samples with segregated morphology and the presence of these species is reduced by increasing the rGO content.
This work was carried out in the framework of the international Multifunctional Polymeric and Biomaterials, MPB Research Centre established between the Department of Chemical Science and Materials Technology of Italy’s National Research Council (CNR) and Sichuan University, Chengdu, China supported by the Programme of Introducing Talents of Discipline to Universities (B13040). The authors would like to thank Mrs A. Aldi for technical support in the gas permeability measurements. References [1] Lee C, Wei XD, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321:385–8. [2] Zhao X, Zhang QH, Chen DJ, Lu P. Enhanced mechanical properties of graphenebased poly(vinyl alcohol) composites. Macromolecules 2010;43:2357–63. [3] Bunch JS, Verbridge SS, Alden JS, Van der Zand AM, Parpia JM, Craighead HG, et al. Impermeable atomic membranes from graphene sheets. Nano Lett 2008;8:2458–62. [4] Balandin AA, Ghosh S, Ba WZ, Caliz I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [5] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless dirac fermions in graphene. Nature 2005;438:197–200. [6] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282–6. [7] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39:228–40. [8] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [9] Liang J, Wang Y, Huang Y, Ma Y, Liu Z, Cai J, et al. Electromagnetic interference shielding of graphene/epoxy composites. Carbon 2009;47:922–5. [10] Wang X, Jin J, Song M. Cyanate ester resin/graphene composite: curing dynamics and network formation. Eur Polym J 2012;48:1034–41. [11] Kim H, Miura Y, Macosko CW. Graphene/polyurethane composites for improved gas barrier and electrical conductivity. Chem Mater 2010;22:3441–50. [12] Steurer P, Wissert R, Thomann R, Muelhaupt R. Functionalized graphenes and thermoplastic composites based upon expanded graphite oxide. Macromol Rapid Commun 2009;30:316–27. [13] Scherillo G, Lavorgna M, Buonocore GG, Zhan YH, Xia HS, Mensitieri G, et al. Tailoring assembly of reduced graphene oxide nanosheets to control gas barrier properties of natural rubber composites. ACS Appl Mater Interfaces 2014;6:2230–4. [14] Yan N, Xia HS, Zhan YH, Fei GX. New insights into fatigue crack growth in graphene-filled natural rubber composites by microfocus hard X-ray beamline radiation. Macromol Mater Eng 2013;298:38–44. [15] Potts JR, Shankar OM, Du L, Ruoff RS. Processing-morphology-property relationships and composite theory analysis of reduced graphene oxide/ natural rubber composites. Macromolecules 2012;45:6045–55. [16] Zhan YH, Wu JK, Xia HS, Yan N, Fei GX, Yuan GP. Dispersion and exfoliation of graphene in rubber by an ultrasonically-assisted latex mixing and in situ reduction process. Macromol Mater Eng 2011;296:590–602. [17] Zhan YH, Lavorgna M, Buonocore GG, Xia HS. Enhancing electrical conductivity of rubber composites by constructing interconnected network of selfassembled graphene with latex mixing. J Mater Chem 2012;22:10464–8. [18] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. pp. 1339-1339. [19] Kaciulis S. Surface spectroscopy of carbon: from diamond to nitride films. Surf Interface Anal 2012;44:1155–61. [20] Kaciulis S, Mezzi A, Calvani P, Trucchi DM. Electron spectroscopy of the main allotropes of carbon. Surf Interface Anal 2014. published on line, doi: 10.1002/ sia.5382. [21] Gusmano G, Montanari R, Kaciulis S, Montesperelli G, Denk R. Gold corrosion’’: red stains on a gold Austrian Ducat. J Appl Phys A 2004;79:205–11.
N. Yan et al. / Composites Science and Technology 102 (2014) 74–81 [22] Mazzaglia A, Scolaro L, Mezzi A, Kaciulis S, Padeletti G. Supramolecular colloidal systems of gold nanoparticles/amphiphilic cyclodextrin: a FE-SEM and XPS investigation of nanostructures assembled onto solid surface. J Phys Chem C 2009;113:12772–7. [23] Guinier A, Fournet G. Small Angle Scattering of X-rays. New York: John Wiley; 1955. [24] Salgueiro W, Somoza A, Torriani I, Marzocca AJ. Cure temperature influence on natural rubber-A small angle X-ray scattering study. J Polym Sci Part B: Polym Phys 2007;45:2966–71. [25] Arroyo M, Lopez-Manchado MA, Herrero B. Organo-montmorillonite as substitute of carbon black in natural rubber compounds. Polymer 2003;44:2447–53.
81
[26] Bhattacharyya S, Sinturel C, Bahloul O, Saboungi ML, Thomas S, Salvetat JP. Improving reinforcement of natural rubber by networking of activated carbon nanotubes. Carbon 2008;46:1037–45. [27] Mori M, Koenig JL. Solid state C-13 NMR studies of vulcanized elastomers: TBBS accelerated sulphur vulcanization of carbon black filled natural rubber. Rubber Chem Technol 1995;68:551–62. [28] Lin HQ, Van Wagner E, Swinnea JS, Freeman BD, Pas SJ, Hill AJ, et al. Transport and structural characteristics of crosslinked poly(ethylene oxide) rubbers. J Membrane Sci 2006;276:145–61. [29] Stanier DC, Patil AJ, Sriwong C, Rahatekar SS, Ciambella J. The reinforcement effect of exfoliated graphene oxide nanoplatelets on the mechanical and viscoelastic properties of natural rubber. Compos Sci Technol 2014;95:59–66.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
The role of reduced graphene oxide on chemical, mechanical and barrier properties of natural rubber composites Ning Yan a, Giovanna Buonocore a, Marino Lavorgna a,⇑, Saulius Kaciulis a, Santosh Kiran Balijepalli b, Yanhu Zhan c, Hesheng Xia c,⇑, Luigi Ambrosio a a b c
Institute of Polymers, Composites and Biomaterials, National Research Council, P.le Fermi, 1-80055 Portici, NA, Italy Institute for the Study of Nanostructured Materials, National Research Council, PO Box 10, 00015 Monterotondo Stazione, Rome, Italy State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 20 May 2014 Received in revised form 11 July 2014 Accepted 17 July 2014 Available online 27 July 2014 Keywords: A. Polymer–matrix composites (PMCs) B. Mechanical properties B. Transport properties D. Transmission electron microscopy (TEM)
a b s t r a c t Natural rubber (NR)-reduced graphene oxide (rGO) composites were produced via latex mixing and cocoagulation approach followed by static hot-press and twin roll mixing process. Due to the process, a fine control of filler dispersion was obtained and the composites exhibited a three-dimensional rGO network or alternatively a homogeneous dispersion of single rGO platelets. The effect of rGO dispersion on chemical crosslink structure, and their influence on mechanical and barrier properties was thoroughly investigated. Small angle X-ray scattering (SAXS) and solid-state 13C NMR analysis showed that rGO platelets affect the vulcanization process of natural rubber and that the crosslinking sulphur polysulphidic species present in pristine natural rubber decrease with the rGO content. In fact, at rGO content higher than 6 phr, the crosslinking species consist mainly of monosulphidic species which attain a consequent increment of intrinsic crosslinking density. However, the composites with rGO segregated network exhibit both barrier to oxygen and water vapour permeation and mechanical properties improved with respect to pristine rubber and composites with the homogeneous dispersion of single rGO platelets. The results confirm that the morphology of filler has a prominent key role in determining the natural rubber composites properties. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In the polymer composites field, the potentials of novel fillers to impart valuable properties to the host polymer have been widely explored. In this respect graphene (GE), a single-atom-thick sheet of graphite, exhibit extraordinary physical properties such as ultrahigh mechanical stiffness [1,2], impervious property [3], thermal [4] and electronic conductivity [5,6]. Since the reduced graphene oxide (rGO) can be obtained at relatively low-costs by chemical reduction of graphene oxide (GO) [7,8], it has been regarded as the most promising and attractive filler for polymer composites. Many investigations have been engaged in harnessing the advantages of rGO in polymer materials [9–12]. The multifunctional property enhancement observed in several rGO/polymer composites surely will expand the role of polymers and composites in new application sectors. However, despite significant advances made
⇑ Corresponding authors. Tel.: +39 0817758838 (M. Lavorgna), +86 02885460535 (H. Xia). E-mail addresses: [email protected] (M. Lavorgna), [email protected] (H. Xia). http://dx.doi.org/10.1016/j.compscitech.2014.07.021 0266-3538/Ó 2014 Elsevier Ltd. All rights reserved.
for thermoplastic and thermosets polymer/rGO composites, there are only a few studies involving the use of rGO in rubber materials [13–15]. In a previous paper, some of the authors of the present contribution developed an ultrasonically assisted latex mixing and in situ reduction process (ULMR) to prepare rGO/NR composites. It was demonstrated that ULMR followed by a two-roll mixing process produces a very fine dispersion and exfoliation of rGO in NR matrix and contribute to a significant increase in the tensile strength [16]. Recently, Zhan et al. [17] have proposed an approach based on coagulation mixing of GO platelets with natural rubber latexes (NRL) to prepare, by in situ reduction, a coagulate rGO composite, affording a ‘segregated’ three-dimensional network of rGO. The authors concluded that at higher filler content the segregated network hinders the crosslinking reaction of natural rubber giving rise to poor mechanical properties. Although, the crosslinked network of vulcanized NR matrix plays a key role in attaining high mechanical strength, it is of great significance to explore the relation between rGO network morphology and vulcanization reaction as well as to investigate their reciprocal influence on mechanical and barrier properties.
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In this work, the effect of the segregated and not-segregated rGO morphologies on the vulcanization process, gas permeability and mechanical properties of rGO/NR composites has been investigated. X-ray photoemission spectroscopy (XPS), small angle X-ray scattering (SAXS) and solid-state 13C NMR techniques were employed to investigate the effect of rGO morphology on the chemical structures of the crosslinks during the vulcanization of NR. The oxygen and water vapour permeability of rGO/NR composites with both the segregated and not-segregated morphologies were also systematically analyzed alongside with the tensile properties. The results reveal both an intrinsic correlation between filler morphology and crosslink chemical structure, and the prominent role of filler morphology on physical properties of natural rubber composites.
vapour permeability were performed using the infrared sensor technique by means of a PermatranW3/31 (Mocon, Germany). Samples with a surface area of 5 cm2 were tested at 25 and 38 °C. The oxygen permeability was measured using a standard permeabilimeter (Extrasolution, MULTIPERM), at 38 °C and 0% (or 50%) Relative Humidity (RH). Water vapour sorption kinetics was obtained by using a gravimetric microbalance (Hiden Isochema Ltd, IGA002, UK). The experiments were run at 38 °C increasing the water activity into the reactor from 0 to 0.5 at constant rate. Sample weight was monitored by the instrument software.
2. Materials and methods
TEM analysis was employed to investigate the dispersion of rGO platelets in the NR composites. Fig. 1(a) shows the non-uniform distribution of rGO platelets in the composite containing 4 phr of rGO platelets. The submicrometer NR latex particles are covered by rGO platelets deposited by co-coagulation procedure, establishing a segregated network morphology, which is preserved by static hot pressing. Micrograph reported in Fig. 1(c) relates to the latex co-coagulated rGO/NR flocculate after the two-roll mixing process. The segregated network created by latex co-coagulation process is destroyed by the milling process, resulting in a uniform distribution of slightly orientated rGO platelets (3 nm) in NR matrix. Fig. 1(b) is the micrograph of NR composite containing 8 phr of rGO platelets, where are clearly observed some latex particles wrapped by an rGO segregated network thicker than that observed for the sample with 4 phr of rGO (see Fig. 1(a)). This is likely to be ascribed to the restacking of rGO platelets. After milling process, as is shown Fig. 1(d), the segregated network is destroyed as expected, but thicker platelets and reduced orientation are observed compared to those shown in Fig. 1(c), despite the intense shear forces generated by the milling process.
2.1. Materials The materials used in this work were detailed in our prior report [16]. Chemical reagents including vulcanization agent sulphur, accelerator N-cyclohexyl-2-benzothiazolesulfenamide (CBS) and 2-mercaptobenzothiazole (MB), antioxidant (4010NA), activator zinc oxide and stearic acid are all commercially available. 2.2. Graphene rubber composite preparation Graphite oxide powder was obtained through oxidation of natural flake graphite according to the Hummers method [18], and was exfoliated in deionized water (DI) by ultrasonication to get graphene oxide (GO) aqueous suspension. For the preparation of rGO/NR composites, the as-prepared GO aqueous suspension was dispersed into NR latex by ultrasonic irradiation, and then in-situ reduced by hydrazine hydrate under ultrasonic irradiation to obtain the rGO/NR latex. Subsequently, crosslinking agent sulphur and other additives were added into the rGO/NR latex and the obtained latex was coagulated and filtrated. The filtrated solids fraction was thoroughly washed and dried in vacuum oven, and then directly hot pressed and in-situ vulcanized at 150 °C to obtain the crosslinked rGO/NR composites with segregated network structure. Samples with not-segregated network were prepared by submitting the coagulated and dried rubber mixture to two-roll mixing process. rGO/NR composites at rGO loadings of 0.5, 1.0, 2.0, 4.0, 6.0 and 8.0 phr (parts per hundred parts of rubber), are coded as NRLG-x for segregated, and NRLG-TR-x for not-segregated samples, respectively; x represents the rGO graphene loading as parts per hundred parts of rubber. 2.3. Characterization Transmission Electron Microscopy (TEM) analysis was performed using a FEI TecnaiG2 F20 S-TWIN transmission electron microscope, operating at an accelerating voltage of 200 kV. X-ray photoemission (XPS) spectra was collected by using an Escalab 250Xi (Thermo Fisher Scientific, UK) spectrometer. Small Angle X-ray Scattering (SAXS) analyses were performed using an Anton Paar SAXS, Cu Ka X-Rays with 1.5418 Å wavelength were generated by a Philips PW3830 sealed tube source (40 kV, 50 mA) and slit collimated. The crosslink density of the vulcanized samples was determined using a reported toluene swelling method [16]. NMR measurements were performed on a Bruker MSL300 spectrometer (75 MHz for 13C) with a standard Bruker CP/MAS probe head at room temperature. Uniaxial tensile testing was performed at room temperature with universal testing machine (Instron 3360) according the ASTM D-412. The measurements of water
3. Results and discussion 3.1. Morphological characterization
3.2. Chemical characterization 3.2.1. X-ray photoelectron spectroscopy XPS investigation was undertaken to analyze quantitatively the chemical composition of the samples surface: pristine NR and rGO/ NR composites with segregated and not-segregated morphology. The chemical composition of the samples, determined by using XPS quantification procedure, is reported in Table 1. In Fig. 2 are reported the selected XPS C 1s spectra for pristine rubber and NR/rGO composites with 4 phr of rGO with segregated and notsegregated morphology. The results presented in Fig. 2(a) reveal a significant difference in C 1s spectra: the pristine NR is characterized by wider C 1s peak centred at BE = 285.0 eV, whereas, in the samples containing rGO, this peak is narrower and it is centred at BE = 284.6 eV (graphitic value). The deconvolutions of C 1s spectra for each sample are shown in Fig. 2 (b)–(d) respectively, in which the first peak C1 centred at BE = 284.6 eV is assigned to graphitic bond arising from rGO [19,20], and this can also be clarified by the C1 value shown in Table 1. The C2 at BE = 285.0 eV corresponds to aliphatic carbon and the third component C3 at BE 286 eV can be attributed to CAO and/or CAN bonds [19]. Moreover, some of these samples show C4 at BE 288 eV, attributed to the ketone group, which may persist from the reduction of graphene oxide [16]. Moreover, the chemical bond of sulphur is different in pristine NR and rGO/NR composites: two chemical species of sulphur at BE = 161.7, S1 and at 163.8 eV, S2 are identified in pristine NR, whereas only the chemical state of sulphur at 163.8 eV is identified for rGO/NR composites, as plotted in Fig. 2(e) and (f). The first
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Fig. 1. TEM images of rGO/NR composites consisting of (a) 4 phr (NRLG-4), (b) 8 phr (NRLG-8) rGO prepared by self-assembly in latex and static hot pressing, (c) 4 phr (NRLG-TR-4) and (d) 8 phr (NRLG-TR-8) rGO prepared by self-assembly in latex followed by two-roll mixing.
Table 1 XPS quantification of surface chemical composition. Samples
C1
C2
C3
C4
N 1s
O 1s
S1
S2
Pristine NR
BE, eV at.%
– –
285.0 70.0
285.8 19.4
– –
400.0 2.3
532.7 6.6
161.7 0.8
163.8 0.4
NRLG–1
BE, eV at.% BE, eV at.%
284.6 27.2 284.6 40.8
285.0 34.2 285.0 19.7
286.2 16.4 286.2 14.5
288.0 6.0 287.8 5.2
399.2 0.9 399.2 0.7
532.0 13.0 532.0 11.6
– – – –
163.8 1.4 163.8 1.1
BE, eV at.% BE, eV at.%
284.5 53.1 284.5 47.6
285.0 29.8 285.0 27.8
286.1 2.3 286.1 4.8
– – – –
400.0 1.2 400.0 1.0
532.1 12.6 532.2 18.4
– – – –
163.8 0.6 163.8 0.3
BE, eV at.% BE, eV at.%
284.6 57.3 284.6 49.6
285.2 26.0 285.2 29.7
286.7 5.0 286.5 5.2
– – – –
399.9 0.8 400.1 0.55
532.0 9.2 532.0 15.8
– – – –
164.0 0.4 163.8 0.1
NRLG-TR-1 NRLG-4 NRLG-TR-4 NRLG-6 NRLG-TR-6
chemical state is attributed to polysulfide [21] or clusters of sulphur, while the second one is characteristic for thiolic CAS bond [22]. For the rGO/NR composites, the polysulphides species disappear with the addition of rGO, and the percentage of S2 atoms assigned to thiolic CAS bond decreases with increasing rGO content. This confirms that the rGO interferes with the pathways of vulcanization reaction. Moreover, there is a greater percentage (i.e. 0.6 at.% for the sample with 4 phr of rGO) of S2 atoms assigned to CAS bond in the segregated sample, compared to the ones in not-segregated sample (0.3 at.%). This suggests that mainly in the rGO segregated network, the sulphur species react with carbon atoms of rGO platelets, resulting preferentially in the formation of monosulphidic SAC species connecting graphene platelets and rubber macromolecules.
3.2.2. Small angle X-ray scattering and nuclear magnetic resonance SAXS investigation of NR composites is of great importance for probing the crosslinked structure of natural rubber. The representative SAXS profiles in terms of I(q) as a function of scattering vector q for different NR composites are shown in Fig. 3(a) and (b) alongside with the spectrum of pristine rubber. A remarkable difference in scattering signals over the q range of 0.1–2.0 nm1 can be observed depending on the content of graphene filler. It is well known that the scattering intensity in SAXS curve reflects the interference arising from the spatial correlation of fluctuations in the electron density of the material [23]. The crosslink sites where natural rubber macromolecular chains are attached each other through polysulphidic species (ACASxACA), give rise to a bump in the SAXS curve as shown in Fig. 3(a) and (b). The higher is the
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Fig. 2. C 1s (a) and S 2p (e) spectra of the selected samples: pristine NR, and rGO/NR composites (4 phr of rGO) with segregated and not-segregated morphologies, and peak fitting of C 1s spectra of (b) pristine NR, (c) rGO/NR composites with segregated morphologies, (d) rGO/NR composites with not-segregated morphologies, and (f) peak fitting of S 2p spectrum for the pristine NR with the two chemical states of sulphur, S1 at 161.7 (S2p3/2A Polysulfide or sulphur clusters) and S2 at 163.8 Ev (S2p3/2B thiolic CAS bond).
Fig. 3. Small angle X-ray scattering profiles of the uncured NR without curatives, rGO/NR composites with segregated morphologies (a) and rGO/NR composites with notsegregated morphologies (b), and the insets are the corresponding Lorentz plots of NR vulcanized composites. (c) Schematic representation of the networks of pristine natural rubber (left) and rGO/NR composite with not-segregated morphology (right). (d) Selected 13C NMR spectra of pristine NR and rGO/NR composites.
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number of sulphur atoms involved in the crosslinking species, the higher becomes the local increase of electron density and consequently the higher is the intensity of the scattering peak in the SAXS spectrum. In order to have a clear identification of scattering feature position, the Lorentz plots, i.e. the I(q)q2 vs. q scattering vector, is frequently used to obtain the structural characteristic (scattering centre) correlation distance. The corresponding Lorentz plots for natural rubber and NR composites are presented as insets in Fig. 3(a) and (b) respectively. The mean distance, d between the scattering centres can be obtained from the Eq. (1) [24]:
d ¼ 2p=qmax
ð1Þ qmax, I(qmax)q2max
The values of and the calculated d are reported in Table 2. The SAXS spectrum of uncrosslinked sample, which is produced without the sulphur agents, does not exhibit any diffraction feature. This confirms that in the spectra of vulcanized samples, both pristine and rubber composites, the diffraction hump is ascribed to the correlation between polysulphidic ACASxACA clusters. Comparing the values in Table 2, it appears that the position of the maximum scattering intensity, qmax, systematically shifts to higher values as the rGO content increases, while maximum scattering intensity I(qmax)q2max exhibits a decreasing trend for the composites both with segregated morphology and not-segregated morphology. It is worth mentioning that the segregated rGO/NR composites have higher qmax values than those of the not-segregated samples at the same rGO content. That is to say, the segregated rGO morphology affords to decrease the distance, d between scattering sulphur centres. Moreover, the scattering signal associated to the separation distance between crosslinking centres disappears at the rGO content of 8 phr for the segregated sample, whereas it is not detected for the not-segregated rGO/NR sample as soon as the rGO content attains to 6 phr. This is attributed to a change of the nature of crosslinks formed during vulcanization with respect to rGO content, i.e. from polysulphidic clusters at low rGO content to di-sulphur or tri-sulphur clusters or monosulphidic crosslinks at higher rGO content (see Fig. 3(c)). In detail, the rGO platelets both in the segregated and not-segregated morphology interfere with the vulcanization process promoting the formation of small sulphur clusters such as AS3A, AS2A or monosulphidic as the rGO content increases. In this view, the crosslinking density of the rubber matrix increases with the rGO content, since smaller sulphur clusters or monosulphidic species form. This result is consistent with that reported in the literature, which suggests that the addition of filler in NR generally leads to an increase of crosslink density [25,26]. However, in our contribution the use
of SAXS has provided more insights into the role of filler on the vulcanization of NR composites. Crosslinking density measurements by the toluene swelling method performed for the not-segregated samples show that increasing the rGO content to 2, 4 and 6 phr increases the crosslink density of pristine NR (0.96 104 mol cm3) to 1.73, 1.88 and 2.08 104 mol cm3 respectively. Unfortunately due to the effective barrier effect of filler network, it was not possible to measure the crosslinking density of the segregated NR/rGO composites by toluene swelling method. However, since the rGO filler content shows more prominent effect than filler morphology on the chemical structure of vulcanized network of NR/rGO composites, the crosslink density of segregated NR/rGO composite is supposed to have the same increasing trend with the rGO content as that of the not-segregated sample. In Fig. 3(d) are reported 13C NMR spectra. The assignment of NMR peaks corresponding to the several carbon atoms of the vulcanized structure was performed according to the paper of Mori et al. [27]. In detail, the peaks centred at 37 and 51 ppm, assigned to A1 cis polysulphidic species show significant differences among the selected pristine NR, segregated and not-segregated samples. When rGO is added in the composite, the polysulphidic clusters mainly present in pristine NR split into smaller polysulphidic species with a limited number of sulphur atoms, i.e. CASxAC with x 8. In this case more sulfide linkages are formed with a consequent increase of the peak intensity related to polysulphidic linkages, ACASA as detected in NMR. On the other hand, the increase in the peak intensity of polysulphidic links brings any variation in the peak intensity of A1 cis monosulphidic links centred at 45 ppm and trans-C1 structure at 40 ppm, whereas a substantial increment is observed for the B1 cis monosulphidic centred at 57 ppm. These results indicate that rGO platelets both in the segregated and not-segregated morphology affect the vulcanization process, promoting the formation of sulphur crosslinking species with a limited number of sulphur atoms (x = 1, 2, 3, . . .) in place of polysulphidic clusters with large number of sulphur atoms (x = 8, 7, 6, . . .). On the basis of results obtained by XPS, SAXS and 13C NMR, it can be concluded that rGO platelets and their morphology have significant influence on the crosslinking network formed during vulcanization process. The chemical structure of the vulcanized network depends mainly on the rGO content, whereas the morphology has only a moderate effect. However, physical properties of rGO/NR composites strongly depend on the matrix properties and filler morphology. Therefore, it is of great interest to tailor the rGO morphology for enhancing specific properties and balancing structural modification of polymeric matrix. Given this fact, a comparative investigation in terms of physical properties between segregated and not-segregated composites was carried out. 3.3. Gas permeability and water vapour absorption
Table 2 The data of the maximum scattering intensities and the corresponding scattering vectors, and the calculated mean distance between the scattering centres of the rGO/NR composites. qmax (nm1)
I (qmax)q2max arb. units
d (nm)
NRLG-0 NRLG-0.5 NRLG-2 NRLG-6 NRLG-8 Uncured NRLG-4
1.51 1.31 1.34 1.41 N/A N/A
3.71 4.29 3.62 1.66 N/A N/A
4.16 4.79 4.69 4.45 N/A N/A
NRLG-TR-0 NRLG-TR-0.5 NRLG-TR-2 NRLG-TR-6 NRLG-TR-8 Uncured NRLG-TR-4
1.44 1.29 1.32 N/A N/A N/A
3.21 2.29 3.06 N/A N/A N/A
4.39 4.87 4.76 N/A N/A N/A
The oxygen permeability at 0% and 50% RH of pristine NR and rGO/NR composites with segregated and not-segregated morphologies were measured at 38 °C and the results are reported in Fig. 4(a) and (b), respectively. The oxygen permeability decreases with increasing the content of rGO for both the segregated and not-segregated films. In details, at dry condition (i.e. 0% RH), the oxygen permeability of composites with segregated morphology drops from 5 107 for the pristine NR to 3.2 107 cm3 cm cm2 s1 atm1 with incorporating only 1 phr of rGO platelets. Moreover, by increasing the rGO loading, the oxygen permeability further decreases up to 2 107 cm3 cm cm2 s1 atm1 when 6 phr of rGO is added. The addition of impervious rGO platelets to the pristine rubber not only decreases the available area for penetrant flux, but also increases the diffusion length in the rubber matrix. In addition, it should be noted that the two different morphologies have different effect, i.e. the films with segregated mor-
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Fig. 4. Gas permeability of rGO/NR film as a function of rGO content: oxygen permeability measured at 38 °C: (a) 0% RH, (b) 50% RH, and (c) water vapour permeability of rGO/NR film measured at 38 °C. (d) Water vapour sorption isotherms for rGO/NR composites with both the segregated (NRLG-2) and not-segregated morphologies (NRLG-TR-2).
phology display oxygen permeability markedly lower than that of not-segregated specimens at all the investigated loadings. This is ascribed to the formation of a percolating and interconnected rGO, resulting in extremely ‘‘tortuous pathway’’ for the gases diffusion. At 50% RH, the oxygen permeability of segregated and not-segregated rGO/NR films exhibits a similar decreasing trend to that measured at 0% RH, with respect to rGO content. However, the oxygen permeability is almost unchanged compared to that of the film with the same rGO loading and morphology at 0% RH. To investigate further the effect of the rGO morphology on the permeability of the film, the water vapour permeability (WVP) of pristine rubber and rGO/NR composites was measured at 38 °C, and the obtained results are shown in Fig. 4(c). The water vapour permeability decreases as the rGO content increases, and the films with segregated morphology show water vapour permeability lower than that of not-segregated samples. Both the chemical structure of rubber matrix and the filler morphologies affect the gas barrier properties of NR composites. Compared to the pristine NR, rGO/NR composites have a higher crosslink density as well as impervious rGO platelets. This brings about to both a decrease in fractional free volume of polymer matrix and an increase in the diffusion length of gas penetrant, resulting in enhanced gases barrier properties [28]. However, given the fact that the chemical structure of segregated and not-segregated rGO/NR composites are similar, the different behaviour in gas barrier properties between these two samples is assumed to be mainly ascribed to filler morphologies. The detailed effect of rGO morphology on the gas permeability of film has been successfully interpreted in our previous publication [13], which has provided a reasonable physical interpretation of the mass transport
behaviour for the samples displaying segregated and not-segregated morphology. The permeability of a penetrant within polymeric film is controlled by both kinetic and thermodynamic factors, corresponding to the penetrant diffusivity and solubility, respectively. In general, a decrease of permeability can result from a decrease of diffusivity and/or solubility. In order to confirm further the influence of rGO morphology on the mass transport of rubber matrix, the water vapour sorption kinetics were performed using a gravimetric method to characterize the water vapour solubility and/or diffusivity in rGO/NR composites. The water vapour sorption kinetics for selected NR composites, is shown in Fig. 4(d). The kinetic curves consist of double steps: a first abrupt increase of water vapour uptake at relative low absorption time, subsequently followed by a steady increase. The first step, usually correlated to the diffusivity coefficient of water sorption, is somewhat comparable for both the morphologies. It is worth noting that segregated rGO/NR composite has a lower water vapour uptake compared to that of the not-segregated film at the same water activity all over the investigated time range. The main reason can be ascribed to rGO platelets which are not-fully chemically reduced and present some hydrophilic groups. This leads to an increase in water uptake for the not-segregated film. On the other hand, the welldefined rGO segregated network, achieved with co-coagulating rGO aqueous suspension with NR latex, creates a stiffer network with reduced exposed hydrophilic surface, resulting in a lower level of water vapour sorption. Taking into account these results, the differences in water permeability can be explained in terms of water solubility which results to be higher for the composites with not-segregated morphology due to a higher hydrophilic surface of rGO platelets.
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The measurements of mechanical properties show that segregated rGO network not only enhance greatly the tensile strength of rGO/ NR composite, but also decrease the strain at break. Permeability analysis suggests that rGO/NR films with segregated network have significantly advantages in terms of oxygen and/or water vapour barrier properties compared to that of the not-segregated composites. The results confirm that the morphology of the filler has a prominent key role in determining the NR composites properties and it is the main parameter to be controlled for tailoring the properties in a multifunctional material. Acknowledgements
Fig. 5. Representative stress–strain curves for the pristine NR and rGO/NR composites with segregated morphologies and rGO/NR composites with notsegregated morphologies.
3.4. Mechanical properties In Fig. 5, is compared the stress–strain behaviour of pristine NR and rGO/NR composites with segregated and not-segregated morphologies. The composites with much low rGO concentration exhibit a significant increased tensile strength compared with pristine NR, while the strain at break of the composites is lower than that of pristine NR. These results are in good agreement with those reported in the previous work [29]. The differences in the mechanical properties between pristine NR and rGO/NR composites are ascribed to the differences in terms of crosslink density, straininduced crystallization, and filler morphology. The addition of rGO platelets results in an increase of rubber crosslink density, as well as an increase of crystallization during stretching, both of which, in general, can increase the tensile strength, while at the same time decrease strain at break. Moreover, a larger increase in tensile strength was observed in segregated sample NRLG-2 compared with the not-segregated sample NRLG-TR-2. In fact rGO content in the two composites is the same, and a fine dispersion of rGO in NRLG-TR-2 is supposed to have a more obviously strain amplification effect due to the higher surface [14], with a consequent to a higher contribution to the mechanical properties. However, mechanical results do not support this thesis. Also, the crosslinking structure of rubber matrix is similar for segregated and not-segregated composites. These results confirm that the different mechanical properties of NRLG-TR-2 and NRLG-2 are mainly ruled by the morphology of the rGO. Segregated networks constituted by the strongly interacting rGO platelets act as skeleton in the host polymer matrix, thus leading to a significant increase of the stiffness and a decrease of the strain at break of composite.
4. Conclusions In summary, we have produced rGO/NR composites consisting of tailored geometrical arrangement of rGO platelets by adopting a facile latex co-coagulation procedure in combination with hotpress or two-roll milling treatment. Segregated dispersion of rGO in composites, as prepared directly by hot-pressing after latex co-coagulation, was successfully observed by TEM, whereas a uniformly distribution of rGO platelets appears over the cross-section of two-roll milled samples. XPS, SAXS and NMR analyses confirm that the rGO platelets affect the vulcanization process of natural rubber. In particular, polysulphidic sulphur species are more prevalent in the samples with segregated morphology and the presence of these species is reduced by increasing the rGO content.
This work was carried out in the framework of the international Multifunctional Polymeric and Biomaterials, MPB Research Centre established between the Department of Chemical Science and Materials Technology of Italy’s National Research Council (CNR) and Sichuan University, Chengdu, China supported by the Programme of Introducing Talents of Discipline to Universities (B13040). The authors would like to thank Mrs A. Aldi for technical support in the gas permeability measurements. References [1] Lee C, Wei XD, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321:385–8. [2] Zhao X, Zhang QH, Chen DJ, Lu P. Enhanced mechanical properties of graphenebased poly(vinyl alcohol) composites. Macromolecules 2010;43:2357–63. [3] Bunch JS, Verbridge SS, Alden JS, Van der Zand AM, Parpia JM, Craighead HG, et al. Impermeable atomic membranes from graphene sheets. Nano Lett 2008;8:2458–62. [4] Balandin AA, Ghosh S, Ba WZ, Caliz I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [5] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless dirac fermions in graphene. Nature 2005;438:197–200. [6] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282–6. [7] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39:228–40. [8] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [9] Liang J, Wang Y, Huang Y, Ma Y, Liu Z, Cai J, et al. Electromagnetic interference shielding of graphene/epoxy composites. Carbon 2009;47:922–5. [10] Wang X, Jin J, Song M. Cyanate ester resin/graphene composite: curing dynamics and network formation. Eur Polym J 2012;48:1034–41. [11] Kim H, Miura Y, Macosko CW. Graphene/polyurethane composites for improved gas barrier and electrical conductivity. Chem Mater 2010;22:3441–50. [12] Steurer P, Wissert R, Thomann R, Muelhaupt R. Functionalized graphenes and thermoplastic composites based upon expanded graphite oxide. Macromol Rapid Commun 2009;30:316–27. [13] Scherillo G, Lavorgna M, Buonocore GG, Zhan YH, Xia HS, Mensitieri G, et al. Tailoring assembly of reduced graphene oxide nanosheets to control gas barrier properties of natural rubber composites. ACS Appl Mater Interfaces 2014;6:2230–4. [14] Yan N, Xia HS, Zhan YH, Fei GX. New insights into fatigue crack growth in graphene-filled natural rubber composites by microfocus hard X-ray beamline radiation. Macromol Mater Eng 2013;298:38–44. [15] Potts JR, Shankar OM, Du L, Ruoff RS. Processing-morphology-property relationships and composite theory analysis of reduced graphene oxide/ natural rubber composites. Macromolecules 2012;45:6045–55. [16] Zhan YH, Wu JK, Xia HS, Yan N, Fei GX, Yuan GP. Dispersion and exfoliation of graphene in rubber by an ultrasonically-assisted latex mixing and in situ reduction process. Macromol Mater Eng 2011;296:590–602. [17] Zhan YH, Lavorgna M, Buonocore GG, Xia HS. Enhancing electrical conductivity of rubber composites by constructing interconnected network of selfassembled graphene with latex mixing. J Mater Chem 2012;22:10464–8. [18] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. pp. 1339-1339. [19] Kaciulis S. Surface spectroscopy of carbon: from diamond to nitride films. Surf Interface Anal 2012;44:1155–61. [20] Kaciulis S, Mezzi A, Calvani P, Trucchi DM. Electron spectroscopy of the main allotropes of carbon. Surf Interface Anal 2014. published on line, doi: 10.1002/ sia.5382. [21] Gusmano G, Montanari R, Kaciulis S, Montesperelli G, Denk R. Gold corrosion’’: red stains on a gold Austrian Ducat. J Appl Phys A 2004;79:205–11.
N. Yan et al. / Composites Science and Technology 102 (2014) 74–81 [22] Mazzaglia A, Scolaro L, Mezzi A, Kaciulis S, Padeletti G. Supramolecular colloidal systems of gold nanoparticles/amphiphilic cyclodextrin: a FE-SEM and XPS investigation of nanostructures assembled onto solid surface. J Phys Chem C 2009;113:12772–7. [23] Guinier A, Fournet G. Small Angle Scattering of X-rays. New York: John Wiley; 1955. [24] Salgueiro W, Somoza A, Torriani I, Marzocca AJ. Cure temperature influence on natural rubber-A small angle X-ray scattering study. J Polym Sci Part B: Polym Phys 2007;45:2966–71. [25] Arroyo M, Lopez-Manchado MA, Herrero B. Organo-montmorillonite as substitute of carbon black in natural rubber compounds. Polymer 2003;44:2447–53.
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[26] Bhattacharyya S, Sinturel C, Bahloul O, Saboungi ML, Thomas S, Salvetat JP. Improving reinforcement of natural rubber by networking of activated carbon nanotubes. Carbon 2008;46:1037–45. [27] Mori M, Koenig JL. Solid state C-13 NMR studies of vulcanized elastomers: TBBS accelerated sulphur vulcanization of carbon black filled natural rubber. Rubber Chem Technol 1995;68:551–62. [28] Lin HQ, Van Wagner E, Swinnea JS, Freeman BD, Pas SJ, Hill AJ, et al. Transport and structural characteristics of crosslinked poly(ethylene oxide) rubbers. J Membrane Sci 2006;276:145–61. [29] Stanier DC, Patil AJ, Sriwong C, Rahatekar SS, Ciambella J. The reinforcement effect of exfoliated graphene oxide nanoplatelets on the mechanical and viscoelastic properties of natural rubber. Compos Sci Technol 2014;95:59–66.