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fluoroelastomer nanocomposites
Polymer 55 (2014) 3818e3824
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Allyl-Functionalization enhanced thermally stable graphene/ fluoroelastomer nanocomposites Junhua Wei, Jingjing Qiu* Department of Mechanical Engineering, Texas Tech University, 2500 Broadway, Lubbock, TX 79409-1021, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2014 Received in revised form 13 June 2014 Accepted 18 June 2014 Available online 25 June 2014
Development of new elastomers with novel functionality has continued since their discovery in order to meet industrial and defense needs in harsh environments. The recent advance of carbon nanomaterials inspired innovative material design strategies and enable more effective production of high-performance elastomers. In this paper, the free radical initiated crosslinking reaction in graphene/fluoroelastomer nanocomposites was studied and the effects of chemical functionalization of graphene nanosheets were analyzed. It indicated that graphene oxide (GO) enhanced fluoroelastomer nanocomposites demonstrated poor high-temperature stability due to the pyrolysis at around 200 C. In contrast, reduced graphene oxide (RGO) enhanced fluoroelastomer exhibited good thermal stability, but RGO didn't participate in the crosslinking, resulting in very limited improvement in mechanical properties. In this paper, reduced allyl functionalized graphene was studied for the first time to enhance free radical initiated elastomers. The reduced allyl functionalization of graphene was demonstrated to impart superior thermal stability and enhanced mechanical properties to the elastomer matrices. The study of vulcanization kinetics provided insights that the allyl functional groups participated in and accelerated the crosslinking. These results indicated a scalable method to incorporate the advantages of graphene into polymer matrices through free radical reaction. The discovery is very promising to be used in the industry to fabricate gaskets, o-rings, and membranes for high temperature applications. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Graphene Elastomers Vulcanization kinetics
1. Introduction In the last decade, the applications of polymers have been widespread in the biology, electronic, petrol, automobile, and aviation industry. Some new applications require polymer to demonstrate versatile novel functionalities, such as electronic, thermal, barrier, flame retardant, and high mechanical properties [1e6]. In order to achieve these requirements, graphene, a twodimension one-atom-layer carbon nanoparticle, has been intensively studied as the next generation reinforcing material for polymers due to its large aspect ratio, high strength and stiffness, and sublime electrical and thermal properties [7e12]. As the interface between graphene and the polymer matrix significantly affect the performance of the nanocomposite, various mechanisms have been proposed to improve the interface, such as pep stacking [13], electrostatic interaction [14], Van Der Waals force [15], and covalent bonding [16,17]. Among all of these methods, further
* Corresponding author. Tel.: þ1(806)834 8076. E-mail address: [email protected] (J. Qiu). http://dx.doi.org/10.1016/j.polymer.2014.06.063 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
chemical functionalization based on graphene oxide (GO) becomes the most popular one because the nanocomposites connected by covalent bonding show more stable interface and GO is the cheapest source of graphene. Generally, GO was synthesized by the simple Hammond's or similar method [18]. A variety of oxygencontaining groups (epoxy, ketone, hydroxide, and carboxyl) on the GO allow for further and specific functionalization [19e21]. Till now, GO is still the most popular graphene derivative in the preparation of graphene/polymer nanocomposite [21]. However, it is well documented that oxygen-containing groups on the GO are not stable above 200 C, which presents a significant challenge for GO/polymer nanocomposites designed for high temperature applications [22e24]. Although a few studies have used chemical reduction method to remove the oxygen-containing groups on the graphene to limit its pyrolysis [25e27], rare of them have seriously investigated the influence of the reduction process on those bonding groups between graphene and polymer. Although the reaction initiation conditions are different, the mechanism for most crosslinking/polymerization for polymer is free radical reaction through double bond (such like eC]Ce) [28,29]. In this case, attaching double bonds onto graphene can
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significantly improve the reactive sites between graphene and most polymers because GO intrinsically only contains double bond in the allyl alcohol groups, which are not thermally stable and in limit quantity [30,31]. Therefore, an effective method to produce thermally stable graphene with a large quantity of double bonds is in highly demand. As the vulcanization of rubber usually require a pre-cure at ~170 C (to initiate the crosslinking) and a post-cure at ~230 C (to produce fully crosslinked polymer matrix) [32,33], the thermal stability of the nanomaterial incorporated into rubber is as important as the chemical bonding and aspect ratio. Although graphene and GO have been used to enhance the mechanical properties of many kinds of rubbers before, most of these studies focused on the homo-dispersion of graphene into rubbers instead of the reaction between graphene and rubbers [34e36]. Our previous research demonstrated that GO influences the curing behavior because the alcohol allyl groups take part in the crosslinking reaction [37]. However, the increment of tensile strength is rather limited probably because the pyrolysis of GO broke the covalent bond. In this paper, thermally stable graphene with a large quantity of double bonds was prepared by allyl functionalization. Subsequently, thermally stable graphene was incorporated into bromide fluoroelastomers (FKM, a fluorocarbon based synthetic rubber) during vulcanization (crosslinking process), which is a free radical reaction. The reaction between graphene and FKM was investigated by the vulcanization kinetics. The mechanical properties of the cured nanocomposite were examined by the tensile tests. This research presented a simple, effective, and scalable method to covalently incorporate graphene into the polymer with free radical polymerization or crosslinking reaction for higher temperature applications. 2. Experimental section 2.1. Sample preparation 2.1.1. Materials FC 2260, a brominated fluoroelastomer (FKM) di-polymer of vinylidene fluoride and hexafluoroporpylene was purchased from 3 M (fluorine content: 65.9%; specific gravity 1.8; Mooney Viscosity (ML1þ10@121 C): ~60). VAROX™ DBPH-50 (2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane) was used as peroxide curative. N-(3Dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDAC), allylamine, hydrazine, fuming nitric acid, methyl ethyl ketone (MEK), sodium chlorate, and calcium hydroxide was purchased from Sigma Aldrich. 2.1.2. Preparation of functionalized graphene The modified Brodie's method was used to prepare graphene oxide [38]. Briefly, the mixture of 10 g of graphite, 85 g of sodium chlorate and 160 ml of fuming nitric acid was stirred at room temperature for 12 h followed by 6 h of extra stirring at 60 C. The resultant was rinsed by diluted hydrogen chloride acid and then by DI water. The neutralized graphite oxide solution was tuned to pH 10.5 by ammonia solution and then ultrasonicated for 1 h. The graphene oxide powder (GO) was achieved by oven dried at 70 C overnight. The GO powder was dispersed in pH 10.5 ammonia solution by ultrosonication to form a 1 mg/ml graphene oxide solution. The GO solution was mixed with hydrazine (1 ml of hydrazine for 3 mg of GO), stirred at 80 C for 12 h and then oven dried to obtain reduced graphene oxide (RGO). 1 L of 1 mg/ml graphene oxide solution was mixed with 50 M of EDAC by stirring for 10 min [39,40]. After that, the mixture was added with 100 ml allylamine and stirred overnight. The resultant was vacuum filtrated through a
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0.2-mm thick membrane twice to remove the residues of allylamine and EDAC. It was then dried at 70 C overnight to obtain the allylfunctionalized graphene oxide powder (AGO). The AGO was further reduced by the similar chemical reduction process to obtain reduced-allyl-functionalized graphene oxide powder (RAGO). The synthesis process of AGO and RAGO is illustrated in Fig. 1. The hydrazine was used as the reduction agency to reduce the epoxy (CeOeC), ketone (eC]O) and hydroxyl (CeOH) groups on the graphene. It was reported that carboxyl on graphene can not be reduced by hydrazine [41,42]. As the allylamine is attached onto graphene through the carboxyl group, the allyl groups were supposed to be remained after the reduction. 2.1.3. Preparation of nanocomposite 350 g FKM was mixed with 5.25 g (1.5 wt%) graphene nanoparticles (RGO or RAGO), 8.75 g (2.5 phr) peroxide and 10.5 g (3 phr) Ca(OH)2 by an open twin-roll miller (Oregon Rubber Mill Co., Ltd., USA) at 80 C with a friction ratio of 1:1.2 and nip gap of ~1 mm. The mixed pastes were compression molded and cured by a hot press machine according to ASTM D3182. The optimal curing time of each sample from rheometer test was used to determine the pre-curing time for hot-press curing. The hot-press cure was carried out at 177 C, 5 MPa followed by a post-cure at 235 C for 24 h. For comparison, a FKM sample without graphene was cured and named as the control. 2.2. Material characterization The Fourier-Transform Infrared (FT-IR) spectrum was tested on Nicolet iS10 with a transmission mold. The X-ray photoelectron spectroscopy (XPS) was measured on a PHI5000 Versa Probe using a 100 mm size beam with a 45 take-off angle. The X-ray source is a monochromatic Al Ka with an excitation energy of 1486.7 eV. All the spectra were corrected by C 1s at 284.8 eV and analyzed by MultiPak (Physical Electronics USA). The morphologies of the graphene were investigated by Transmission Electron Microscopy (TEM) on a Hitachi 8100. The Powder XRD patterns of the samples were recorded with a Siemen AXS D5005 X-ray Diffractmeter using Cu Ka radiation. The thermal stabilities of functionalized graphenes were first investigated by Thermogravimetric Analysis (TGA) in nitrogen atmosphere. The heating temperature was ramped to 177 C and held for 20 min to investigate the weight loss during the vulcanization since the pre-curing process requires high pressure and sealant for better morphology of the cured elastomer. The chemical structure of functionalized graphene after vulcanization was also examined by FTIR spectra. The vulcanization kinetics of functionalized graphene enhanced FKM nanocomposites were studied by an oscillating disc rheometer (ODR, Akron Rubber Development Laboratory, INC.) at 165 C, 177 C, and 190 C according to ASTM D2084. The equilibrium swelling tests were performed at room temperature in MEK for 7 days according to ASTM D6814. The mechanical test was carried out on a Universal Tensile Tester (AGS-X, SHIMADZU) at 25 C, 75 C, 125 C, and 175 C with a crosshead speed of 250 mm/min according to ASTM D412. The interface between graphene and elastomer was investigated by the morphology of the rupture cross section of the nanocomposite by Scanning Electron Microscopy (SEM) on a Hitachi S4300. The storage modulus (E0 ) and damping (tan d) were obtained by dynamic mechanical analysis (DMA) Q800. 2.3. Vulcanization kinetics Rheometer tests were used to determine the energy required for vulcanization. Analysis of rheology results is based on the
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Fig. 1. Reaction to synthesize AGO (1) and RAGO (2) from GO.
assumption that crosslinking density is proportional to the stiffness of the rubber. Therefore, the degree of curing (a) can be calculated from the rheometer test [43]:
a¼
ðFt F0 Þ ðF∞ F0 Þ
(1)
where F0, Ft, and F∞ are the torque values at time zero, curing time t, and the end of the vulcanization process. In this case, the kinetics of the cure reaction can be related to time and temperature as:
da ¼ KðTÞf ðaÞ dt
(2)
where da/dt is the curing rate, t is the curing time, T is the curing temperature, K is the specific rate constant at T and f(a) is the
function corresponding to the phenomenological kinetic model. The f(a) can be described by the SestakeBerggren equation as [44]:
f ðaÞ ¼ am ð1 aÞn
(3)
So, the kinetic model of the vulcanization can be described as [45]:
da ¼ KðTÞam ð1 aÞn dt
(4)
Meanwhile, the K(T) is related to the activation energy by the Arrhenius equation:
ln KðTÞ ¼ ln K0
Ea RT
Fig. 2. The FT-IR spectra (a), XPS survey spectra (b) and C1s spectra of GO (c), RGO (d), AGO (e), and RAGO (f).
(5)
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where K0 is a pre-exponential factor, Ea is the activation energy and the R is the gas constant. 3. Results and discussion 3.1. Characterization of graphene The FTIR and XPS spectra of the GO, RGO, AGO, and RAGO were compared in Fig. 2. The aromatics of graphene are confirmed by the peak at around 1560 cm1. The peaks at around 1360 cm1, 1690 cm1 and 3400 cm1 indicate that GO and AGO contained oxygen-containing groups [46]. In comparison, these oxygencontaining peaks are not present on the spectra of RGO and RAGO [47]. The attachment of allylamine on AGO and RAGO was confirmed by the peaks at 820 cm1 and 1610 cm1, which represent the primary amine groups and the stretching of amine groups, respectively [48e50]. The XPS spectra also confirmed the attachment of allyl functional groups on graphene. The Fig. 2(b) presents the survey spectra of GO, RGO, AGO, and RAGO. The peak at ~399 eV indicates the nitrogen, which is only present in AGO and RAGO. The structure changes between different functionalized graphene were further validated by the C1s XPS spectra. The decrease of signals at ca. 286.1 eV (CeO) and 289.2 eV ((C]O)eOH) proved the reduction after hydrazine treatment. The signals at ca. 285.4 eV (CeN) and 287.6 eV (C(O)eN) pointed out the attachment of allylamine on the graphene [51e53]. Both AGO and RAGO showed weakened signals at ca. 289.2 eV in contrast with GO, which indicates that the (C]O) OH structure was involved in the reaction to covalently bond the allylamine to graphene. All of these results proved the successful allyl functionalization of graphene through carboxyl groups. Furthermore, the morphologies of the RGO and RAGO samples were investigated by TEM, as shown in Fig. S1. The transparency of the graphene indicated that RGO and RAGO were in mono or few layers. 3.2. Thermal stability of functionalized graphene Previous studies indicated that the oxygen groups on the GO tended to pyrolysis around 200 C [38]. The pre-curing temperature for FKM with peroxide curing system is 177 C which is close to 200 C. Therefore, the thermal stability of functionalized graphene was investigated before it was integrated to enhance elastomers. Fig. 3(a) shows the results of thermal de-oxygenation. At the vulcanization temperature, significant weight loss (~15%) was present in GO and AGO while very little weight loss (~1%) was observed in RGO and RAGO. The TGA results indicated that GO and
Fig. 4. The curing curves of FKM nanocomposites with different functionalized graphene at 177 C.
AGO are not a suitable nanofillers for high temperature utility. The pyrolysis of the oxygen groups on the GO and AGO not only distorts the vulcanization reaction, but also damages the structure of graphene. In contrast, RGO and RAGO demonstrated better thermal stability at the vulcanization temperature of 177 C. As shown in Fig 3(b), even after 24 h of post-curing heating at 235 C in the air, the allyl group on graphene in the RAGO sample didn't pyrolysis. Those results proved that RGO and RAGO allow for a higher service temperature and are more suitable to enhance FKM matrices.
3.3. Vulcanization of FKM nanocomposites To illustrate the influence of the functionalized graphene on the vulcanization kinetics in FKM, the rheology curves of all three samples at 177 C were exhibited in Fig. 4. The presence of RAGO in FKM compound provided the highest values of maximum torque, as well as the lowest values of scorch time and the optimum cure time. This result suggested that the vulcanization of FKM had been noticeably accelerated. To specifically analyze the vulcanization kinetics, some important parameters including Ts2 (scorch time, the beginning of the curing), Tc90 (the optimum cure time), and cure rate index (CRI ¼ 100/Tc90Ts2), and torque value (DS ¼ MHML, the difference between the maximum and minimum torques, The ML is the minimum torque value in the curing curves after the
Fig. 3. (a)The deoxygenation of GO, RGO, AGO, and RAGO investigated by TGA; (b) the chemical structure of RAGO after heated at 235 C for 24 h in the air.
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Table 1 Vulcanization characteristics of different FKM samples obtained from rheometer test.
Table 2 Kinetics parameters of the vulcanization reaction of different FKM samples obtained from rheometer testing.
Material
Tc/ C
Ts2/min
Tc90/min
CRI/min1
DS/dNm
Material
Tc/ C
K
m
n
Ea/kJ/mol
Control
165 177 190 165 177 190 165 177 190
4.83 2.82 1.80 5.45 3.42 2.22 4.78 2.78 2.10
20.49 9.33 4.87 22.37 10.46 5.47 18.09 8.38 4.81
6.39 15.36 32.57 5.91 14.20 30.77 7.51 17.86 36.90
11.93 10.33 9.21 10.85 8.68 7.46 12.37 10.59 9.40
Control
165 177 190 165 177 190 165 177 190
0.19 0.51 1.09 0.20 0.44 1.21 0.69 0.99 1.59
1.20 1.21 1.24 1.33 1.22 1.36 2.30 1.64 1.35
0.26 0.34 0.37 0.33 0.33 0.61 0.68 0.58 0.73
116.20
RGO/FKM
RAGO/FKM
sharp peak around zero; the MH is the maximum torque value in the curing curves after the ML.) was determined from the curing curves and shown in Table 1. The Ts2 is the time to rise 2 dNm torque above minimum torque. It can be used to characterize the induction period where the radical is produced from the peroxide thermal decomposition and then abstract Br-atom from FKM or add them to radical traps [54e56]. Neat graphene nanoparticles retarded these reactions by hindering radical diffusion as the RGO/FKM showed the longest Ts2. After allyl functionalization, the Ts2 of nanocomposite decreased as the RAGO/FKM demonstrated the shortest Ts2. This result verified that the allyl groups on graphene were involved in the vulcanization by acceleration of radicals' diffusion in the presence of RAGO. Similarly, the value of Tc90 significantly decreased after allyl functionalization, which indicates the allyl groups on the graphene remarkably accelerated the crosslinking reaction. The CRI was calculated from Ts2 and Tc90, which represents the vulcanization rate. It was found that RAGO/FKM presents higher CRI values than control sample and RGO/FKM at each curing temperature. Therefore, all the results indicated that the allyl groups on graphene significantly accelerated the vulcanization reaction by providing plenty of reactive sites. The DS is generally assumed to represent the quality of interaction at filler-matrix interphase [57]. It is found that DS decreased in the presence of the neat graphene (RGO) probably because the flexible planar structure of graphene is difficult to adhere to. On the contrary, the RAGO/FKM presented the highest DS as a result of the strong covalent bonding between allyl functionalized graphene and FKM. 3.4. Vulcanization kinetics The vulcanization kinetics of the nanocomposites at different temperatures was analyzed by the rheometer tests. The values of k,
RGO/FKM
RAGO/FKM
120.97
56.59
Table 3 The volume swell and crosslink density results of different samples. Sample
Control
RGO/FKM
RAGO/FKM
Swelling (Q)/% Crosslink density/mol/cm3 104
485.7 1.34
452.4 1.47
400.1 1.81
n, and m at different temperatures were calculated through linear multiple regression analysis of the experimental data from Fig. 4 by curve fitting tool in Matlab. A plot of lnK versus 1/T of each sample provided the linear slope, which is used to calculate the activation energy Ea during the vulcanization according to Eq. (5). In Fig. 5(a), the rheology results of the control sample at different temperatures are presented. As the curing temperature increased, the CRI were significantly increased with slightly decreased Ts2 and noticeably reduced Tc90, as shown in Table 1. Similarly, the DS also decreased with the increment of curing temperature. The plots of conversion rate (da/dt) versus the degree of conversion (a) of the control sample were summarized in Fig. 5(b). Correlations between the experimental and theoretical curves can be easily observed. It was found that the shapes of the conversion curves were temperature-dependent. As the temperature increased, both the peak height of the conversion curve and the peak position were increased. The statistics of the kinetic parameters were reported in Table 2. It is obvious that the reaction order, the sum of n and m, changed with different curing conditions, such as curing temperature, filler type, and curative level. The Ea was calculated from the slope of the Arrhenius plot of lnK versus 1/T in Fig. 5(c) and the values were presented in Table 2. By adding RGO in FKM, the Ea increased because of the hindrance effect. By allyl functionalization, RAGO dramatically decreased the Ea of the nanocomposite from 120.97 kJ/mol to 56.59 kJ/mol, which indicated that the allyl groups on the graphene accelerated the vulcanization by decreasing the Ea. This acceleration effect of the
Fig. 5. (a) Rheometer curves of control sample at three different temperatures; (b) derivative of a as a function of the a from rheometer curves of control sample at different curing temperature; (c) the plot of lnK versus 1/T used to estimate the Ea.
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Fig. 6. The tensile test of cured FKM nanocomposite at 25 C (a); (b) the summary of the tensile strength vs. temperature.
allyl groups was also observed in the curing behaviors by increasing the torque and decreasing the scorch/optimum curing time. 3.5. Swelling properties The effects of the functionalized graphene on the FKM nanocomposite systems were firstly studied by the swelling tests of FKM nanocomposites. The swelling test measured the uncrosslinked part of the FKM composites extracted by MEK. Therefore, the percentage degree of swelling is inversely proportional to the degree of swelling, as shown in Eq. (1). As presented in Table 3, the swelling values (Q) for both RGO/FKM and RAGO/FKM are lower than the Q of the control, which indicates that the graphene in the composite was an efficient barrier to the liquid and prevented the extraction of FKM. Furthermore, the RAGO/FKM demonstrates the lowest Q, which suggests that RAGO/FKM has the highest crosslink density. This conclusion is verified by the Flory-Rehner equation as seen in the supplement information [58,59]. The presence of the neat graphene in the FKM composite does not significantly affect the crosslink density of the FKM composite until the allyl groups are attached to the graphene, as shown in Table 3. The RAGO/FKM composite presents 35% more crosslink density compared to that of the control. This increment mainly originates from the better interaction between RAGO and FKM, the intrinsic solvent barrier property of graphene, and more importantly, a stable crosslinked network formed by the allyl group on graphene reacted with FKM during vulcanization.
3.6. Mechanical properties Fig. 6 and Fig. S4 shows the tensile test results of cured FKM composites. The mechanical properties of FKM were improved by adding RAGO as the M200 and M300 (the stresses at 200% and 300% strains, respectively) were increased by over 64% and 61% at 25 C. Moreover, RAGO/FKM demonstrated more improvement in high-temperature tensile tests, as its tensile strength is increased by 70.4% at 175 C, 45.6% at 125 C, and 26.3% at 75 C when compared to that of the control. The high modulus/strength and large aspect ratio of graphene might be the main reason of the improved modulus for the graphene filled FKM nanocomposites. The enhanced bonding between allyl functionalized graphene and the FKM matrix contribute to a higher tensile strength as the elongation was kept by good interaction as the RAGO-FKM interphase (Fig. S6).
3.7. Vulcanization mechanism The peroxide crosslinking is basically a free radical process. Firstly, the peroxide provides free radicals after thermal decomposition, as illustrated in Fig. S7(1). The free radicals then abstract Bromine atoms from the polymer chain, and the resultant polymeric radicals form crosslinks directly or through radical traps: RAGO (radical traps contain electron-rich hydrocarbon radical [60,61]). During the vulcanization, RAGO serves as a crosslinking bridge with multiple reactive sites to bond more electron-poor
Fig. 7. The summarized reaction of proposed crosslinking mechanism of brominated FKM with/without RAGO with peroxide curing.
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FKMs and electron-rich crosslinkers together, which significantly promotes and expedites the vulcanization process, increases the curing rate, and improves the curing degree, as illustrated in Fig. S7(4e6). Since the RAGO radical traps are covalently bonded with the whole FKM network (Fig. 7) after vulcanization, the allyl functionalization method provides an effective method to integrate the advantages of graphene nanosheets into the RAGO/FKM nanocomposites. As a summary, by adding electron-rich allyl groups onto graphene, the RAGO/FKM nanocomposite system demonstrate not only improve the interaction between FKM and graphene and crosslink density, but also accelerate the vulcanization rates. The tensile test further verified that the as-synthesized RAGO/FKM provided extraordinary thermal stability and superior mechanical properties at all service temperature. 4. Conclusion In this paper, thermally stable graphene was achieved with allyl functionalization and was incorporated into FKM by a radical trap enhanced free radical reaction. The rheometer test, equilibrium swelling, and tensile test indicated that allyl functionalization of graphene significantly improved the performance of FKM nanocomposites. The vulcanization kinetics analysis demonstrated that the activation energy was reduced by half after covalently bonding allyl functionalized graphene. This work provides an effective functionalization method to incorporate graphene into polymers through a free radical reaction. This research will facilitate broad applications of graphene in engineering materials. It will inspire more innovative material design strategies with the aid of nanotechnology and enable more effective production of high-performance polymers to meet the industrial and defense needs in harsh environments. Acknowledgment The authors would like to acknowledge the supporting from ACS PRF#52308-DNI10 grant. The authors also appreciate the help of Mike Kumbalek from Oregon Rubber Mills Co. at Corvallis for the two roll mill mixing process. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.06.063. References [1] Hussain F, Hojjati M, Okamoto M, Gorga RE. J Compos Mater 2006;40(17): 1511e75. [2] Usuki A, Kawasumi M, Kojima Y, Okada A, Kurauchi T, Kamigaito O. J Mater Res 1993;8(5):1174e8. [3] LeBaron PC, Pinnavaia TJ. Chem Mater 2001;13(10):3760e5. [4] Endo M, Noguchi T, Ito M, Takeuchi K, Hayashi T, Kim YA, et al. Adv Funct Mater 2008;18(22):3551. [5] Deng F, Ito M, Noguchi T, Wang LF, Ueki H, Niihara K, et al. Acs Nano 2011;5(5):3858e66. [6] Huang JH, Yang Y, Chu CW. Abstr Pap Am Chem Soc 2012;244. [7] Cano M, Khan U, Sainsbury T, O'Neill A, Wang ZM, McGovern IT, et al. Carbon 2013;52:363e71. [8] Biswas S, Fukushima H, Drzal LT. Compos Part a-Applied Sci Manuf 2011;42(4):371e5. [9] Zhang X, Hikal WM, Zhang Y, Bhattacharia SK, Li L, Panditrao S, et al. Appl Phys Lett 2013;102(14). [10] Kurapati R, Raichur AM. Chem Commun 2012;48(48):6013e5. [11] Zaman I, Phan TT, Kuan HC, Meng QS, La LTB, Luong L, et al. Polymer 2011;52(7):1603e11.
[12] Araby S, Zhang LQ, Kuan HC, Dai JB, Majewski P, Ma J. Polymer 2013;54(14): 3663e70. [13] Shen B, Zhai WT, Chen C, Lu DD, Wang J, Zheng WG. Acs Appl Mater Interfaces 2011;3(8):3103e9. [14] Pham VH, Dang TT, Hur SH, Kim EJ, Chung JS. Acs Appl Mater Interfaces 2012;4(5):2630e6. [15] Hassan M, Walter M, Moseler M. Phys Chem Chem Phys 2014;16(1):33e7. [16] Jing Y, Tang H, Yu GJ, Wu PY. Polym Chem 2013;4(8):2598e607. [17] Shen B, Zhai WT, Tao MM, Lu DD, Zheng WG. Compos Sci Technol 2013;77: 87e94. [18] Park S, Ruoff RS. Nat Nanotechnol 2009;4(4):217e24. [19] Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chem Soc Rev 2010;39(1):228e40. [20] Chen D, Feng HB, Li JH. Chem Rev 2012;112(11):6027e53. [21] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Nature 2006;442(7100):282e6. [22] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Nature 2007;448(7152):457e60. [23] Jeong HK, Lee YP, Jin MH, Kim ES, Bae JJ, Lee YH. Chem Phys Lett 2009;470(4e6):255e8. [24] Gao XF, Jiang DE, Zhao YL, Nagase S, Zhang SB, Chen ZF. J Comput Theor Nanosci 2011;8(12):2406e22. [25] Chua CK, Pumera M. Chem Soc Rev 2014;43(1):291e312. [26] Mathkar A, Tozier D, Cox P, Ong PJ, Galande C, Balakrishnan K, et al. J Phys Chem Lett 2012;3(8):986e91. [27] Thomas HR, Day SP, Woodruff WE, Valles C, Young RJ, Kinloch IA, et al. Chem Mater 2013;25(18):3580e8. [28] Bo ZS. Abstr Pap Am Chem Soc 2013;245. [29] Yu ZN, Wan XH, Zhang HL, Chen XF, Zhou QF. Chem Commun 2003;(8): 974e5. [30] Collins WR, Lewandowski W, Schmois E, Walish J, Swager TM. Angew Chemie-International Ed 2011;50(38):8848e52. [31] Solpan D, Guven O. Polym Degrad Stab 1998;60(2e3):367e70. [32] Maciejewska M, Walkiewicz F, Zaborski M. Przemysl Chem 2013;92(9): 1627e9. [33] Sarkhel G, Choudhury A. J Appl Polym Sci 2010;115(1):376e84. [34] Mao YY, Wen SP, Chen YL, Zhang FZ, Panine P, Chan TW, et al. Sci Reports 2013;3. [35] Li FY, Yan N, Zhan YH, Fei GX, Xia HS. J Appl Polym Sci 2013;129(4):2342e51. [36] Wu JR, Huang GS, Li H, Wu SD, Liu YF, Zheng J. Polymer 2013;54(7):1930e7. [37] Wei J, jacob S, Qiu J. Compos Sci Technol 2014;92(24):126e33. [38] Wei JH, Qiu JJ, Li L, Ren LQ, Zhang XW, Chaudhuri J, et al. Nanotechnology 2012;23(33). [39] Zhang LM, Wang ZL, Lu ZX, Shen H, Huang J, Zhao QH, et al. J Mater Chem B 2013;1(6):749e55. [40] Satti A, Larpent P, Gun'ko Y. Carbon 2010;48(12):3376e81. [41] Park S, Hu YC, Hwang JO, Lee ES, Casabianca LB, Cai WW, et al. Nat Commun 2012;3. [42] Hashim A. Advances in nanocomposite technology. InTech; 2011. [43] Ding R, Leonov AI. J Appl Polym Sci 1996;61(3):455e63. [44] Sestak J, berggren G. Thermochim Acta 1971;3(1):1e12. [45] Kader MA, Nah C. Polymer 2004;45(7):2237e47. [46] Wei J, Qiu J, Ren L, Zhang K, Wang S, Weeks B. Sci Adv Mater 2014;6(5): 1052e9. [47] Acik M, Lee G, Mattevi C, Pirkle A, Wallace RM, Chhowalla M, et al. J Phys Chem C 2011;115(40):19761e81. [48] van Os MT, Menges B, Foerch R, Vancso GJ, Knoll W. Chem Mater 1999;11(11): 3252e7. [49] Abbas A, Vercaigne-Marko D, Supiot P, Bocquet B, Vivien C, Guillochon D. Colloids Surf B-Biointerfaces 2009;73(2):315e24. [50] Krishnamurthy V, Kamel IL, Wei Y. J Polym Sci Part a-Polymer Chem 1989;27(4):1211e24. [51] Ouyang Y, Cai X, Shi QS, Liu LL, Wan DL, Tan SZ, et al. Colloids Surfaces BBiointerfaces 2013;107:107e14. [52] Shan CS, Yang HF, Han DX, Zhang QX, Ivaska A, Niu L. Langmuir 2009;25(20): 12030e3. [53] Some S, Ho SM, Dua P, Hwang E, Shin YH, Yoo H, et al. Acs Nano 2012;6(8): 7151e61. [54] Banks RE. Preparation, properties and industrial applications of organofuorine compounds. Ellis Horwood Ltd.; 1982. [55] Uschold RE. Polym J 1985;17(1):253e63. [56] Maiti M, Bhowmick AK. J Appl Polym Sci 2009;111(2):1094e104. [57] Lopez-Manchado MA, Arroyo M, Herrero B, Biagiotti J. J Appl Polym Sci 2003;89(1):1e15. [58] Allahbakhsh A, Mazinani S, Kalaee MR, Sharif F. Thermochim Acta 2013;563: 22e32. [59] Maiti M, Bhowmick AK. J Appl Polym Sci 2007;105(2):435e45. [60] Banks RE, Smart BE, Tatlow JC. Organofuorine chemistry: principles and commercial applications. Springer; 1994. [61] Logothetis AL. Prog Polym Sci 1989;14(2):251e96.
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Allyl-Functionalization enhanced thermally stable graphene/ fluoroelastomer nanocomposites Junhua Wei, Jingjing Qiu* Department of Mechanical Engineering, Texas Tech University, 2500 Broadway, Lubbock, TX 79409-1021, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2014 Received in revised form 13 June 2014 Accepted 18 June 2014 Available online 25 June 2014
Development of new elastomers with novel functionality has continued since their discovery in order to meet industrial and defense needs in harsh environments. The recent advance of carbon nanomaterials inspired innovative material design strategies and enable more effective production of high-performance elastomers. In this paper, the free radical initiated crosslinking reaction in graphene/fluoroelastomer nanocomposites was studied and the effects of chemical functionalization of graphene nanosheets were analyzed. It indicated that graphene oxide (GO) enhanced fluoroelastomer nanocomposites demonstrated poor high-temperature stability due to the pyrolysis at around 200 C. In contrast, reduced graphene oxide (RGO) enhanced fluoroelastomer exhibited good thermal stability, but RGO didn't participate in the crosslinking, resulting in very limited improvement in mechanical properties. In this paper, reduced allyl functionalized graphene was studied for the first time to enhance free radical initiated elastomers. The reduced allyl functionalization of graphene was demonstrated to impart superior thermal stability and enhanced mechanical properties to the elastomer matrices. The study of vulcanization kinetics provided insights that the allyl functional groups participated in and accelerated the crosslinking. These results indicated a scalable method to incorporate the advantages of graphene into polymer matrices through free radical reaction. The discovery is very promising to be used in the industry to fabricate gaskets, o-rings, and membranes for high temperature applications. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Graphene Elastomers Vulcanization kinetics
1. Introduction In the last decade, the applications of polymers have been widespread in the biology, electronic, petrol, automobile, and aviation industry. Some new applications require polymer to demonstrate versatile novel functionalities, such as electronic, thermal, barrier, flame retardant, and high mechanical properties [1e6]. In order to achieve these requirements, graphene, a twodimension one-atom-layer carbon nanoparticle, has been intensively studied as the next generation reinforcing material for polymers due to its large aspect ratio, high strength and stiffness, and sublime electrical and thermal properties [7e12]. As the interface between graphene and the polymer matrix significantly affect the performance of the nanocomposite, various mechanisms have been proposed to improve the interface, such as pep stacking [13], electrostatic interaction [14], Van Der Waals force [15], and covalent bonding [16,17]. Among all of these methods, further
* Corresponding author. Tel.: þ1(806)834 8076. E-mail address: [email protected] (J. Qiu). http://dx.doi.org/10.1016/j.polymer.2014.06.063 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
chemical functionalization based on graphene oxide (GO) becomes the most popular one because the nanocomposites connected by covalent bonding show more stable interface and GO is the cheapest source of graphene. Generally, GO was synthesized by the simple Hammond's or similar method [18]. A variety of oxygencontaining groups (epoxy, ketone, hydroxide, and carboxyl) on the GO allow for further and specific functionalization [19e21]. Till now, GO is still the most popular graphene derivative in the preparation of graphene/polymer nanocomposite [21]. However, it is well documented that oxygen-containing groups on the GO are not stable above 200 C, which presents a significant challenge for GO/polymer nanocomposites designed for high temperature applications [22e24]. Although a few studies have used chemical reduction method to remove the oxygen-containing groups on the graphene to limit its pyrolysis [25e27], rare of them have seriously investigated the influence of the reduction process on those bonding groups between graphene and polymer. Although the reaction initiation conditions are different, the mechanism for most crosslinking/polymerization for polymer is free radical reaction through double bond (such like eC]Ce) [28,29]. In this case, attaching double bonds onto graphene can
J. Wei, J. Qiu / Polymer 55 (2014) 3818e3824
significantly improve the reactive sites between graphene and most polymers because GO intrinsically only contains double bond in the allyl alcohol groups, which are not thermally stable and in limit quantity [30,31]. Therefore, an effective method to produce thermally stable graphene with a large quantity of double bonds is in highly demand. As the vulcanization of rubber usually require a pre-cure at ~170 C (to initiate the crosslinking) and a post-cure at ~230 C (to produce fully crosslinked polymer matrix) [32,33], the thermal stability of the nanomaterial incorporated into rubber is as important as the chemical bonding and aspect ratio. Although graphene and GO have been used to enhance the mechanical properties of many kinds of rubbers before, most of these studies focused on the homo-dispersion of graphene into rubbers instead of the reaction between graphene and rubbers [34e36]. Our previous research demonstrated that GO influences the curing behavior because the alcohol allyl groups take part in the crosslinking reaction [37]. However, the increment of tensile strength is rather limited probably because the pyrolysis of GO broke the covalent bond. In this paper, thermally stable graphene with a large quantity of double bonds was prepared by allyl functionalization. Subsequently, thermally stable graphene was incorporated into bromide fluoroelastomers (FKM, a fluorocarbon based synthetic rubber) during vulcanization (crosslinking process), which is a free radical reaction. The reaction between graphene and FKM was investigated by the vulcanization kinetics. The mechanical properties of the cured nanocomposite were examined by the tensile tests. This research presented a simple, effective, and scalable method to covalently incorporate graphene into the polymer with free radical polymerization or crosslinking reaction for higher temperature applications. 2. Experimental section 2.1. Sample preparation 2.1.1. Materials FC 2260, a brominated fluoroelastomer (FKM) di-polymer of vinylidene fluoride and hexafluoroporpylene was purchased from 3 M (fluorine content: 65.9%; specific gravity 1.8; Mooney Viscosity (ML1þ10@121 C): ~60). VAROX™ DBPH-50 (2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane) was used as peroxide curative. N-(3Dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDAC), allylamine, hydrazine, fuming nitric acid, methyl ethyl ketone (MEK), sodium chlorate, and calcium hydroxide was purchased from Sigma Aldrich. 2.1.2. Preparation of functionalized graphene The modified Brodie's method was used to prepare graphene oxide [38]. Briefly, the mixture of 10 g of graphite, 85 g of sodium chlorate and 160 ml of fuming nitric acid was stirred at room temperature for 12 h followed by 6 h of extra stirring at 60 C. The resultant was rinsed by diluted hydrogen chloride acid and then by DI water. The neutralized graphite oxide solution was tuned to pH 10.5 by ammonia solution and then ultrasonicated for 1 h. The graphene oxide powder (GO) was achieved by oven dried at 70 C overnight. The GO powder was dispersed in pH 10.5 ammonia solution by ultrosonication to form a 1 mg/ml graphene oxide solution. The GO solution was mixed with hydrazine (1 ml of hydrazine for 3 mg of GO), stirred at 80 C for 12 h and then oven dried to obtain reduced graphene oxide (RGO). 1 L of 1 mg/ml graphene oxide solution was mixed with 50 M of EDAC by stirring for 10 min [39,40]. After that, the mixture was added with 100 ml allylamine and stirred overnight. The resultant was vacuum filtrated through a
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0.2-mm thick membrane twice to remove the residues of allylamine and EDAC. It was then dried at 70 C overnight to obtain the allylfunctionalized graphene oxide powder (AGO). The AGO was further reduced by the similar chemical reduction process to obtain reduced-allyl-functionalized graphene oxide powder (RAGO). The synthesis process of AGO and RAGO is illustrated in Fig. 1. The hydrazine was used as the reduction agency to reduce the epoxy (CeOeC), ketone (eC]O) and hydroxyl (CeOH) groups on the graphene. It was reported that carboxyl on graphene can not be reduced by hydrazine [41,42]. As the allylamine is attached onto graphene through the carboxyl group, the allyl groups were supposed to be remained after the reduction. 2.1.3. Preparation of nanocomposite 350 g FKM was mixed with 5.25 g (1.5 wt%) graphene nanoparticles (RGO or RAGO), 8.75 g (2.5 phr) peroxide and 10.5 g (3 phr) Ca(OH)2 by an open twin-roll miller (Oregon Rubber Mill Co., Ltd., USA) at 80 C with a friction ratio of 1:1.2 and nip gap of ~1 mm. The mixed pastes were compression molded and cured by a hot press machine according to ASTM D3182. The optimal curing time of each sample from rheometer test was used to determine the pre-curing time for hot-press curing. The hot-press cure was carried out at 177 C, 5 MPa followed by a post-cure at 235 C for 24 h. For comparison, a FKM sample without graphene was cured and named as the control. 2.2. Material characterization The Fourier-Transform Infrared (FT-IR) spectrum was tested on Nicolet iS10 with a transmission mold. The X-ray photoelectron spectroscopy (XPS) was measured on a PHI5000 Versa Probe using a 100 mm size beam with a 45 take-off angle. The X-ray source is a monochromatic Al Ka with an excitation energy of 1486.7 eV. All the spectra were corrected by C 1s at 284.8 eV and analyzed by MultiPak (Physical Electronics USA). The morphologies of the graphene were investigated by Transmission Electron Microscopy (TEM) on a Hitachi 8100. The Powder XRD patterns of the samples were recorded with a Siemen AXS D5005 X-ray Diffractmeter using Cu Ka radiation. The thermal stabilities of functionalized graphenes were first investigated by Thermogravimetric Analysis (TGA) in nitrogen atmosphere. The heating temperature was ramped to 177 C and held for 20 min to investigate the weight loss during the vulcanization since the pre-curing process requires high pressure and sealant for better morphology of the cured elastomer. The chemical structure of functionalized graphene after vulcanization was also examined by FTIR spectra. The vulcanization kinetics of functionalized graphene enhanced FKM nanocomposites were studied by an oscillating disc rheometer (ODR, Akron Rubber Development Laboratory, INC.) at 165 C, 177 C, and 190 C according to ASTM D2084. The equilibrium swelling tests were performed at room temperature in MEK for 7 days according to ASTM D6814. The mechanical test was carried out on a Universal Tensile Tester (AGS-X, SHIMADZU) at 25 C, 75 C, 125 C, and 175 C with a crosshead speed of 250 mm/min according to ASTM D412. The interface between graphene and elastomer was investigated by the morphology of the rupture cross section of the nanocomposite by Scanning Electron Microscopy (SEM) on a Hitachi S4300. The storage modulus (E0 ) and damping (tan d) were obtained by dynamic mechanical analysis (DMA) Q800. 2.3. Vulcanization kinetics Rheometer tests were used to determine the energy required for vulcanization. Analysis of rheology results is based on the
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Fig. 1. Reaction to synthesize AGO (1) and RAGO (2) from GO.
assumption that crosslinking density is proportional to the stiffness of the rubber. Therefore, the degree of curing (a) can be calculated from the rheometer test [43]:
a¼
ðFt F0 Þ ðF∞ F0 Þ
(1)
where F0, Ft, and F∞ are the torque values at time zero, curing time t, and the end of the vulcanization process. In this case, the kinetics of the cure reaction can be related to time and temperature as:
da ¼ KðTÞf ðaÞ dt
(2)
where da/dt is the curing rate, t is the curing time, T is the curing temperature, K is the specific rate constant at T and f(a) is the
function corresponding to the phenomenological kinetic model. The f(a) can be described by the SestakeBerggren equation as [44]:
f ðaÞ ¼ am ð1 aÞn
(3)
So, the kinetic model of the vulcanization can be described as [45]:
da ¼ KðTÞam ð1 aÞn dt
(4)
Meanwhile, the K(T) is related to the activation energy by the Arrhenius equation:
ln KðTÞ ¼ ln K0
Ea RT
Fig. 2. The FT-IR spectra (a), XPS survey spectra (b) and C1s spectra of GO (c), RGO (d), AGO (e), and RAGO (f).
(5)
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where K0 is a pre-exponential factor, Ea is the activation energy and the R is the gas constant. 3. Results and discussion 3.1. Characterization of graphene The FTIR and XPS spectra of the GO, RGO, AGO, and RAGO were compared in Fig. 2. The aromatics of graphene are confirmed by the peak at around 1560 cm1. The peaks at around 1360 cm1, 1690 cm1 and 3400 cm1 indicate that GO and AGO contained oxygen-containing groups [46]. In comparison, these oxygencontaining peaks are not present on the spectra of RGO and RAGO [47]. The attachment of allylamine on AGO and RAGO was confirmed by the peaks at 820 cm1 and 1610 cm1, which represent the primary amine groups and the stretching of amine groups, respectively [48e50]. The XPS spectra also confirmed the attachment of allyl functional groups on graphene. The Fig. 2(b) presents the survey spectra of GO, RGO, AGO, and RAGO. The peak at ~399 eV indicates the nitrogen, which is only present in AGO and RAGO. The structure changes between different functionalized graphene were further validated by the C1s XPS spectra. The decrease of signals at ca. 286.1 eV (CeO) and 289.2 eV ((C]O)eOH) proved the reduction after hydrazine treatment. The signals at ca. 285.4 eV (CeN) and 287.6 eV (C(O)eN) pointed out the attachment of allylamine on the graphene [51e53]. Both AGO and RAGO showed weakened signals at ca. 289.2 eV in contrast with GO, which indicates that the (C]O) OH structure was involved in the reaction to covalently bond the allylamine to graphene. All of these results proved the successful allyl functionalization of graphene through carboxyl groups. Furthermore, the morphologies of the RGO and RAGO samples were investigated by TEM, as shown in Fig. S1. The transparency of the graphene indicated that RGO and RAGO were in mono or few layers. 3.2. Thermal stability of functionalized graphene Previous studies indicated that the oxygen groups on the GO tended to pyrolysis around 200 C [38]. The pre-curing temperature for FKM with peroxide curing system is 177 C which is close to 200 C. Therefore, the thermal stability of functionalized graphene was investigated before it was integrated to enhance elastomers. Fig. 3(a) shows the results of thermal de-oxygenation. At the vulcanization temperature, significant weight loss (~15%) was present in GO and AGO while very little weight loss (~1%) was observed in RGO and RAGO. The TGA results indicated that GO and
Fig. 4. The curing curves of FKM nanocomposites with different functionalized graphene at 177 C.
AGO are not a suitable nanofillers for high temperature utility. The pyrolysis of the oxygen groups on the GO and AGO not only distorts the vulcanization reaction, but also damages the structure of graphene. In contrast, RGO and RAGO demonstrated better thermal stability at the vulcanization temperature of 177 C. As shown in Fig 3(b), even after 24 h of post-curing heating at 235 C in the air, the allyl group on graphene in the RAGO sample didn't pyrolysis. Those results proved that RGO and RAGO allow for a higher service temperature and are more suitable to enhance FKM matrices.
3.3. Vulcanization of FKM nanocomposites To illustrate the influence of the functionalized graphene on the vulcanization kinetics in FKM, the rheology curves of all three samples at 177 C were exhibited in Fig. 4. The presence of RAGO in FKM compound provided the highest values of maximum torque, as well as the lowest values of scorch time and the optimum cure time. This result suggested that the vulcanization of FKM had been noticeably accelerated. To specifically analyze the vulcanization kinetics, some important parameters including Ts2 (scorch time, the beginning of the curing), Tc90 (the optimum cure time), and cure rate index (CRI ¼ 100/Tc90Ts2), and torque value (DS ¼ MHML, the difference between the maximum and minimum torques, The ML is the minimum torque value in the curing curves after the
Fig. 3. (a)The deoxygenation of GO, RGO, AGO, and RAGO investigated by TGA; (b) the chemical structure of RAGO after heated at 235 C for 24 h in the air.
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Table 1 Vulcanization characteristics of different FKM samples obtained from rheometer test.
Table 2 Kinetics parameters of the vulcanization reaction of different FKM samples obtained from rheometer testing.
Material
Tc/ C
Ts2/min
Tc90/min
CRI/min1
DS/dNm
Material
Tc/ C
K
m
n
Ea/kJ/mol
Control
165 177 190 165 177 190 165 177 190
4.83 2.82 1.80 5.45 3.42 2.22 4.78 2.78 2.10
20.49 9.33 4.87 22.37 10.46 5.47 18.09 8.38 4.81
6.39 15.36 32.57 5.91 14.20 30.77 7.51 17.86 36.90
11.93 10.33 9.21 10.85 8.68 7.46 12.37 10.59 9.40
Control
165 177 190 165 177 190 165 177 190
0.19 0.51 1.09 0.20 0.44 1.21 0.69 0.99 1.59
1.20 1.21 1.24 1.33 1.22 1.36 2.30 1.64 1.35
0.26 0.34 0.37 0.33 0.33 0.61 0.68 0.58 0.73
116.20
RGO/FKM
RAGO/FKM
sharp peak around zero; the MH is the maximum torque value in the curing curves after the ML.) was determined from the curing curves and shown in Table 1. The Ts2 is the time to rise 2 dNm torque above minimum torque. It can be used to characterize the induction period where the radical is produced from the peroxide thermal decomposition and then abstract Br-atom from FKM or add them to radical traps [54e56]. Neat graphene nanoparticles retarded these reactions by hindering radical diffusion as the RGO/FKM showed the longest Ts2. After allyl functionalization, the Ts2 of nanocomposite decreased as the RAGO/FKM demonstrated the shortest Ts2. This result verified that the allyl groups on graphene were involved in the vulcanization by acceleration of radicals' diffusion in the presence of RAGO. Similarly, the value of Tc90 significantly decreased after allyl functionalization, which indicates the allyl groups on the graphene remarkably accelerated the crosslinking reaction. The CRI was calculated from Ts2 and Tc90, which represents the vulcanization rate. It was found that RAGO/FKM presents higher CRI values than control sample and RGO/FKM at each curing temperature. Therefore, all the results indicated that the allyl groups on graphene significantly accelerated the vulcanization reaction by providing plenty of reactive sites. The DS is generally assumed to represent the quality of interaction at filler-matrix interphase [57]. It is found that DS decreased in the presence of the neat graphene (RGO) probably because the flexible planar structure of graphene is difficult to adhere to. On the contrary, the RAGO/FKM presented the highest DS as a result of the strong covalent bonding between allyl functionalized graphene and FKM. 3.4. Vulcanization kinetics The vulcanization kinetics of the nanocomposites at different temperatures was analyzed by the rheometer tests. The values of k,
RGO/FKM
RAGO/FKM
120.97
56.59
Table 3 The volume swell and crosslink density results of different samples. Sample
Control
RGO/FKM
RAGO/FKM
Swelling (Q)/% Crosslink density/mol/cm3 104
485.7 1.34
452.4 1.47
400.1 1.81
n, and m at different temperatures were calculated through linear multiple regression analysis of the experimental data from Fig. 4 by curve fitting tool in Matlab. A plot of lnK versus 1/T of each sample provided the linear slope, which is used to calculate the activation energy Ea during the vulcanization according to Eq. (5). In Fig. 5(a), the rheology results of the control sample at different temperatures are presented. As the curing temperature increased, the CRI were significantly increased with slightly decreased Ts2 and noticeably reduced Tc90, as shown in Table 1. Similarly, the DS also decreased with the increment of curing temperature. The plots of conversion rate (da/dt) versus the degree of conversion (a) of the control sample were summarized in Fig. 5(b). Correlations between the experimental and theoretical curves can be easily observed. It was found that the shapes of the conversion curves were temperature-dependent. As the temperature increased, both the peak height of the conversion curve and the peak position were increased. The statistics of the kinetic parameters were reported in Table 2. It is obvious that the reaction order, the sum of n and m, changed with different curing conditions, such as curing temperature, filler type, and curative level. The Ea was calculated from the slope of the Arrhenius plot of lnK versus 1/T in Fig. 5(c) and the values were presented in Table 2. By adding RGO in FKM, the Ea increased because of the hindrance effect. By allyl functionalization, RAGO dramatically decreased the Ea of the nanocomposite from 120.97 kJ/mol to 56.59 kJ/mol, which indicated that the allyl groups on the graphene accelerated the vulcanization by decreasing the Ea. This acceleration effect of the
Fig. 5. (a) Rheometer curves of control sample at three different temperatures; (b) derivative of a as a function of the a from rheometer curves of control sample at different curing temperature; (c) the plot of lnK versus 1/T used to estimate the Ea.
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Fig. 6. The tensile test of cured FKM nanocomposite at 25 C (a); (b) the summary of the tensile strength vs. temperature.
allyl groups was also observed in the curing behaviors by increasing the torque and decreasing the scorch/optimum curing time. 3.5. Swelling properties The effects of the functionalized graphene on the FKM nanocomposite systems were firstly studied by the swelling tests of FKM nanocomposites. The swelling test measured the uncrosslinked part of the FKM composites extracted by MEK. Therefore, the percentage degree of swelling is inversely proportional to the degree of swelling, as shown in Eq. (1). As presented in Table 3, the swelling values (Q) for both RGO/FKM and RAGO/FKM are lower than the Q of the control, which indicates that the graphene in the composite was an efficient barrier to the liquid and prevented the extraction of FKM. Furthermore, the RAGO/FKM demonstrates the lowest Q, which suggests that RAGO/FKM has the highest crosslink density. This conclusion is verified by the Flory-Rehner equation as seen in the supplement information [58,59]. The presence of the neat graphene in the FKM composite does not significantly affect the crosslink density of the FKM composite until the allyl groups are attached to the graphene, as shown in Table 3. The RAGO/FKM composite presents 35% more crosslink density compared to that of the control. This increment mainly originates from the better interaction between RAGO and FKM, the intrinsic solvent barrier property of graphene, and more importantly, a stable crosslinked network formed by the allyl group on graphene reacted with FKM during vulcanization.
3.6. Mechanical properties Fig. 6 and Fig. S4 shows the tensile test results of cured FKM composites. The mechanical properties of FKM were improved by adding RAGO as the M200 and M300 (the stresses at 200% and 300% strains, respectively) were increased by over 64% and 61% at 25 C. Moreover, RAGO/FKM demonstrated more improvement in high-temperature tensile tests, as its tensile strength is increased by 70.4% at 175 C, 45.6% at 125 C, and 26.3% at 75 C when compared to that of the control. The high modulus/strength and large aspect ratio of graphene might be the main reason of the improved modulus for the graphene filled FKM nanocomposites. The enhanced bonding between allyl functionalized graphene and the FKM matrix contribute to a higher tensile strength as the elongation was kept by good interaction as the RAGO-FKM interphase (Fig. S6).
3.7. Vulcanization mechanism The peroxide crosslinking is basically a free radical process. Firstly, the peroxide provides free radicals after thermal decomposition, as illustrated in Fig. S7(1). The free radicals then abstract Bromine atoms from the polymer chain, and the resultant polymeric radicals form crosslinks directly or through radical traps: RAGO (radical traps contain electron-rich hydrocarbon radical [60,61]). During the vulcanization, RAGO serves as a crosslinking bridge with multiple reactive sites to bond more electron-poor
Fig. 7. The summarized reaction of proposed crosslinking mechanism of brominated FKM with/without RAGO with peroxide curing.
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FKMs and electron-rich crosslinkers together, which significantly promotes and expedites the vulcanization process, increases the curing rate, and improves the curing degree, as illustrated in Fig. S7(4e6). Since the RAGO radical traps are covalently bonded with the whole FKM network (Fig. 7) after vulcanization, the allyl functionalization method provides an effective method to integrate the advantages of graphene nanosheets into the RAGO/FKM nanocomposites. As a summary, by adding electron-rich allyl groups onto graphene, the RAGO/FKM nanocomposite system demonstrate not only improve the interaction between FKM and graphene and crosslink density, but also accelerate the vulcanization rates. The tensile test further verified that the as-synthesized RAGO/FKM provided extraordinary thermal stability and superior mechanical properties at all service temperature. 4. Conclusion In this paper, thermally stable graphene was achieved with allyl functionalization and was incorporated into FKM by a radical trap enhanced free radical reaction. The rheometer test, equilibrium swelling, and tensile test indicated that allyl functionalization of graphene significantly improved the performance of FKM nanocomposites. The vulcanization kinetics analysis demonstrated that the activation energy was reduced by half after covalently bonding allyl functionalized graphene. This work provides an effective functionalization method to incorporate graphene into polymers through a free radical reaction. This research will facilitate broad applications of graphene in engineering materials. It will inspire more innovative material design strategies with the aid of nanotechnology and enable more effective production of high-performance polymers to meet the industrial and defense needs in harsh environments. Acknowledgment The authors would like to acknowledge the supporting from ACS PRF#52308-DNI10 grant. The authors also appreciate the help of Mike Kumbalek from Oregon Rubber Mills Co. at Corvallis for the two roll mill mixing process. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.06.063. References [1] Hussain F, Hojjati M, Okamoto M, Gorga RE. J Compos Mater 2006;40(17): 1511e75. [2] Usuki A, Kawasumi M, Kojima Y, Okada A, Kurauchi T, Kamigaito O. J Mater Res 1993;8(5):1174e8. [3] LeBaron PC, Pinnavaia TJ. Chem Mater 2001;13(10):3760e5. [4] Endo M, Noguchi T, Ito M, Takeuchi K, Hayashi T, Kim YA, et al. Adv Funct Mater 2008;18(22):3551. [5] Deng F, Ito M, Noguchi T, Wang LF, Ueki H, Niihara K, et al. Acs Nano 2011;5(5):3858e66. [6] Huang JH, Yang Y, Chu CW. Abstr Pap Am Chem Soc 2012;244. [7] Cano M, Khan U, Sainsbury T, O'Neill A, Wang ZM, McGovern IT, et al. Carbon 2013;52:363e71. [8] Biswas S, Fukushima H, Drzal LT. Compos Part a-Applied Sci Manuf 2011;42(4):371e5. [9] Zhang X, Hikal WM, Zhang Y, Bhattacharia SK, Li L, Panditrao S, et al. Appl Phys Lett 2013;102(14). [10] Kurapati R, Raichur AM. Chem Commun 2012;48(48):6013e5. [11] Zaman I, Phan TT, Kuan HC, Meng QS, La LTB, Luong L, et al. Polymer 2011;52(7):1603e11.
[12] Araby S, Zhang LQ, Kuan HC, Dai JB, Majewski P, Ma J. Polymer 2013;54(14): 3663e70. [13] Shen B, Zhai WT, Chen C, Lu DD, Wang J, Zheng WG. Acs Appl Mater Interfaces 2011;3(8):3103e9. [14] Pham VH, Dang TT, Hur SH, Kim EJ, Chung JS. Acs Appl Mater Interfaces 2012;4(5):2630e6. [15] Hassan M, Walter M, Moseler M. Phys Chem Chem Phys 2014;16(1):33e7. [16] Jing Y, Tang H, Yu GJ, Wu PY. Polym Chem 2013;4(8):2598e607. [17] Shen B, Zhai WT, Tao MM, Lu DD, Zheng WG. Compos Sci Technol 2013;77: 87e94. [18] Park S, Ruoff RS. Nat Nanotechnol 2009;4(4):217e24. [19] Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chem Soc Rev 2010;39(1):228e40. [20] Chen D, Feng HB, Li JH. Chem Rev 2012;112(11):6027e53. [21] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Nature 2006;442(7100):282e6. [22] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Nature 2007;448(7152):457e60. [23] Jeong HK, Lee YP, Jin MH, Kim ES, Bae JJ, Lee YH. Chem Phys Lett 2009;470(4e6):255e8. [24] Gao XF, Jiang DE, Zhao YL, Nagase S, Zhang SB, Chen ZF. J Comput Theor Nanosci 2011;8(12):2406e22. [25] Chua CK, Pumera M. Chem Soc Rev 2014;43(1):291e312. [26] Mathkar A, Tozier D, Cox P, Ong PJ, Galande C, Balakrishnan K, et al. J Phys Chem Lett 2012;3(8):986e91. [27] Thomas HR, Day SP, Woodruff WE, Valles C, Young RJ, Kinloch IA, et al. Chem Mater 2013;25(18):3580e8. [28] Bo ZS. Abstr Pap Am Chem Soc 2013;245. [29] Yu ZN, Wan XH, Zhang HL, Chen XF, Zhou QF. Chem Commun 2003;(8): 974e5. [30] Collins WR, Lewandowski W, Schmois E, Walish J, Swager TM. Angew Chemie-International Ed 2011;50(38):8848e52. [31] Solpan D, Guven O. Polym Degrad Stab 1998;60(2e3):367e70. [32] Maciejewska M, Walkiewicz F, Zaborski M. Przemysl Chem 2013;92(9): 1627e9. [33] Sarkhel G, Choudhury A. J Appl Polym Sci 2010;115(1):376e84. [34] Mao YY, Wen SP, Chen YL, Zhang FZ, Panine P, Chan TW, et al. Sci Reports 2013;3. [35] Li FY, Yan N, Zhan YH, Fei GX, Xia HS. J Appl Polym Sci 2013;129(4):2342e51. [36] Wu JR, Huang GS, Li H, Wu SD, Liu YF, Zheng J. Polymer 2013;54(7):1930e7. [37] Wei J, jacob S, Qiu J. Compos Sci Technol 2014;92(24):126e33. [38] Wei JH, Qiu JJ, Li L, Ren LQ, Zhang XW, Chaudhuri J, et al. Nanotechnology 2012;23(33). [39] Zhang LM, Wang ZL, Lu ZX, Shen H, Huang J, Zhao QH, et al. J Mater Chem B 2013;1(6):749e55. [40] Satti A, Larpent P, Gun'ko Y. Carbon 2010;48(12):3376e81. [41] Park S, Hu YC, Hwang JO, Lee ES, Casabianca LB, Cai WW, et al. Nat Commun 2012;3. [42] Hashim A. Advances in nanocomposite technology. InTech; 2011. [43] Ding R, Leonov AI. J Appl Polym Sci 1996;61(3):455e63. [44] Sestak J, berggren G. Thermochim Acta 1971;3(1):1e12. [45] Kader MA, Nah C. Polymer 2004;45(7):2237e47. [46] Wei J, Qiu J, Ren L, Zhang K, Wang S, Weeks B. Sci Adv Mater 2014;6(5): 1052e9. [47] Acik M, Lee G, Mattevi C, Pirkle A, Wallace RM, Chhowalla M, et al. J Phys Chem C 2011;115(40):19761e81. [48] van Os MT, Menges B, Foerch R, Vancso GJ, Knoll W. Chem Mater 1999;11(11): 3252e7. [49] Abbas A, Vercaigne-Marko D, Supiot P, Bocquet B, Vivien C, Guillochon D. Colloids Surf B-Biointerfaces 2009;73(2):315e24. [50] Krishnamurthy V, Kamel IL, Wei Y. J Polym Sci Part a-Polymer Chem 1989;27(4):1211e24. [51] Ouyang Y, Cai X, Shi QS, Liu LL, Wan DL, Tan SZ, et al. Colloids Surfaces BBiointerfaces 2013;107:107e14. [52] Shan CS, Yang HF, Han DX, Zhang QX, Ivaska A, Niu L. Langmuir 2009;25(20): 12030e3. [53] Some S, Ho SM, Dua P, Hwang E, Shin YH, Yoo H, et al. Acs Nano 2012;6(8): 7151e61. [54] Banks RE. Preparation, properties and industrial applications of organofuorine compounds. Ellis Horwood Ltd.; 1982. [55] Uschold RE. Polym J 1985;17(1):253e63. [56] Maiti M, Bhowmick AK. J Appl Polym Sci 2009;111(2):1094e104. [57] Lopez-Manchado MA, Arroyo M, Herrero B, Biagiotti J. J Appl Polym Sci 2003;89(1):1e15. [58] Allahbakhsh A, Mazinani S, Kalaee MR, Sharif F. Thermochim Acta 2013;563: 22e32. [59] Maiti M, Bhowmick AK. J Appl Polym Sci 2007;105(2):435e45. [60] Banks RE, Smart BE, Tatlow JC. Organofuorine chemistry: principles and commercial applications. Springer; 1994. [61] Logothetis AL. Prog Polym Sci 1989;14(2):251e96.