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Polycarbonate toughening with reduced graphene oxide: Toward high toughness, strength and notch resistance
Chemical Engineering Journal 325 (2017) 474–484
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Polycarbonate toughening with reduced graphene oxide: Toward high toughness, strength and notch resistance Jianfeng Wang, Chunhai Li, Xiaomeng Zhang, Lichao Xia, Xianlong Zhang, Hong Wu ⇑, Shaoyun Guo The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Reduced graphene oxide (RGO) was
melt-processed into polycarbonate (PC). RGO can enhance the toughness, strength and notch resistance of PC greatly. The toughening mechanism was investigated. Microcrack, crack pinning, deflection and arresting were the toughening mechanism.
a r t i c l e
i n f o
Article history: Received 22 February 2017 Received in revised form 27 April 2017 Accepted 14 May 2017 Available online 15 May 2017 Keywords: Polycarbonate Reduced graphene oxide Nanocomposites Melt compounding Mechanical properties
a b s t r a c t The toughening effect of graphene sheets on polycarbonate (PC) was investigated to fabricate PC composites with excellent balanced toughness, notch resistance as well as strength. The reduced graphene oxide (RGO) was incorporated into PC matrix through melt compounding. A maximum toughening effect was observed in PC/RGO (PCG) composites with 0.03 wt% or 0.07 wt% RGO. Particularly, the tensile fracture toughness of PCG composites with 0.03 wt% RGO was enhanced by 89%. The notched impact strength and KIC of PCG composite with 0.07 wt% RGO was increased by 46% and 58%, respectively. The point of 0.1 wt% was found to be the ductile-brittle transition point in PCG composites. Meanwhile, the yield strength of these novel materials was increased by around 12% as well at loading of 0.07 wt%. Microcrack, resulting from interfacial debonding between PC and RGO as well as breakage and pulling out of graphene layer, was proposed to be the main toughening mechanism contributing to the great enhanced fracture toughness and notch resistance. Apart from the microcrack, crack pinning, crack deflection and crack arresting were also found and proposed to be toughening mechanism in notchfractured process. This work not only provides us a novel strategy to fabricate advanced PC nanocomposites but also gives us a deep understanding on the toughening role of graphene on polymers. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Polycarbonate (PC) is a very attractive polymer used in many fields ranging from disk to helmet for astronauts due to its remarkable heat resistance, toughness, optical transparency, and electrical
⇑ Corresponding author. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.cej.2017.05.090 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
properties [1–3]. However, PC is very sensitive to the notches or cracks, which greatly limits its application in many high-end fields. It is therefore of great significance to enhance the notch resistance as well as the notch-fractured toughness of PC. It is well known that the notch sensitivity of PC is due to the change in stress state at the notch from plane stress to plane strain, resulting in the change in failure mechanism from shearing to crazing [4]. Generally, rubber is considered as the most effective method to toughen
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PC [5–7], which can induce cavitation, relieve the plane strain constraint at the notch and permit the matrix to deform by shearing. However, typically, 5–20 wt% of rubber is required, causing a dramatic reduction in strength and modulus. Apart from the rubber, rigid fillers, like carbon nanotube [8], SiO2 [9] or nano-SiO2 [10] have also been studied to improve the fracture toughness of PC. However, the increment in notch resistance or fracture toughness obtained with these rigid fillers is typically less than that with rubber. Graphene, a two-dimensional material of numerous excellent properties [11–13], has attracted tremendous interest in scientific and industrial fields. Adding into polymers to improve the mechanical properties of polymers is considered one of the most promising applications of graphene [14–17]. However, compared with the role on reinforcing polymers, the toughening role of graphene on polymers has always been neglected. To the best knowledge, there was almost no report on the toughness of graphene/ polymer nanocomposites before 2009. In the past few years, some studies have reported the toughening role of graphene on polymers. For instance, Rafiee et al. found that the fracture performance of epoxy composites with 0.1 wt% of pristine graphene was better than that with carbon nanotubes [18] and the increment in fracture toughness peaked at 0.125 wt% of graphene with a 65% increment [19]. Yong et.al [20] found that, graphene nanosheet can induce microcracks in epoxy filled with low loading graphene, resulting in an impressive toughening effect on epoxy matrix. However, up to now, there are still some challenging issues considering graphene as toughener to polymers. First of all, the matrices studied are too rare and most of the studies focused on the brittle thermosetting epoxy [18–23]. More comparative studies and modeling work are necessary to investigate the toughening ability of graphene on polymers. Then, the understanding of the toughening mechanism responsible for other polymers except epoxy is still insufficient. At last, most of the polymer nanocomposites toughened by graphene were fabricated through solution, in situ polymerization or layer-by-layer strategy using solvents or dispersions. They are not practical approaches to manufacture polymer/graphene composites environment-friendly and costeffectively, placing great limitations on industrial implementation. In this work, we sought to explore the toughening effect of graphene sheets on PC matrix and fabricate advanced PC nanocomposites with excellent toughness, notch resistance as well as strength. Reduced graphene oxide (RGO) was performed to prepare PC nanocomposite via melt compounding without any solvent. The notch resistance and notch-fractured toughness were evaluated based on notched impact mode and KIC-test mode. The strength and tensile fracture toughness were explored through tensile fracture mode. Interestingly, an impressive toughening effect was observed at 0.03 wt% or 0.07 wt% of RGO loading in all three modes. Meanwhile, the strength of PC matrix was enhanced with the addition of low loading RGO. The corresponding toughening mechanism was proposed based on the fractographs of the composite samples and morphological evolution of RGO during the fracture process.
2. Experimental 2.1. Materials Bisphenol-A Polycarbonate (PC-201, Mw = 31809, a density of 1.2 g/cm3) was purchased from LG Chem (Korea) and vacuumdried overnight at 110 °C before use. Paraffin oil (CAS: 8012-951, Kelong chemical reagent CO., LTD), being mainly composed of C16-C20 n-alkanes and with a density of 0.835 g/cm3, was used as received. Reduced graphene oxide (RGO) were supplied by
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Changzhou Six element CO., Ltd. Table 1 summaries the specifications of RGO. Moreover, the other information about the RGO, including the original morphology (SEM images), Raman spectrum and XPS wide spectrum, can be seen in the Supplementary Materials (Figs. S1 and S2). The RGO were vacuum-dried overnight at 60 °C before use. 2.2. Preparation of PC/RGO composites Firstly, PC pellets and RGO were physically blended together through paraffin oil, in a solid mixer (V5D, Feich machinery, China) at 60 r/min for 10 min. The weight ratio of PC and paraffin is 1000:1. The paraffin oil can make the RGO adhere uniformly around the PC pellets in advance, which is helpful for the dispersion of RGO in the melt-processed PC in the following process. Then PC and PC/ RGO composites (PCG) with a load varying from 0.03 to 3 wt% were prepared by melt compounding on a HaPu melt mixer at 260 °C. The melt mixing was performed by adding approximately half of the polymer quantity to the mixing bowl. When the polymer matrix melted and the torque started to decrease, the remaining composites were gradually added to the mixer. For all the materials the melt compounding was performed at a screw speed of 60 rpm for 20 min. The materials obtained via melt compounding were compression molded at 260 °C under 15 MPa, which were used as specimens for different characterizations. The as-obtained composites are coded using PCG and the RGO loading. For example, PCG-0.07 means the PCG composites with 0.07 wt%RGO. 2.3. Characterization Scanning electron microscopy (SEM) images were taken using JSM-5900LV-SEM at a voltage of 5.0 kV. Prior to being analyzed, the samples were mounted on stubs and their surfaces were platinum coated. The dispersion of graphene in PC matrix was observed with a Tecnai G2F20S-TWIN transmission electron microscopy (TEM) at an accelerating voltage of 200 kV. Ultrathin sections thinner than 100 nm were cryogenically cut with a glazing knife using a microtome and collected on 300-mesh copper grids for TEM observation. X-ray diffraction (XRD) measurements were conducted using a D8 Advance (Bruker) X-ray diffractometer using Cu Ka radiation (k = 1.5405 Å) with a scanning speed of 5°/min from 5 to 60°. The scanned area was from 190 to 600 nm. Raman spectra were measured on LabRam-HR (French Horiba, 532 nm). X-ray photoelectron spectroscopy (XPS) was conducted on K-alpha (Thermo Scientific). Standard tensile test used dumbbell shaped sample was conducted at room temperature using tensile test machine (model CMT-4104) according to GB/T 1040-92. Tensile fracture toughness were evaluated based on the area under the stress-strain curve. Furthermore, due to the sensitivity of PC to the notch, the notched Izod impact test, KIC-tests as well as J-integral tests were performed to evaluate the notch resistance and notch-fractured toughness of neat PC and its RGO composites. The notched impact test was performed following GB/1943–2007 with a XJU-22 impact test machine. Before the impact testing, a single-edge V-shaped notch with a depth of 2 mm was milled in all the rectangular bulk specimens (80 10 4 mm3). The KIC-tests as well as J-integral tests were performed according to ASTM D5045-99 and ASTM E813-89 [24], respectively (detail information can be seen in Support Information Fig. S3). All the mechanical property data with standard deviation was averaged using at least six specimens (seen in the Supplementary Material, Table S2). On the other hand, Polarized optical microscopy (POM, Leica, DM2500P) were performed to collect the initiation and propagation patterns of the crack or craze in neat PC and PCG composites after being tested through
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Table 1 Specifications of pristine RGO from the manufacturer. Notation
Product name
Product process
Oxygen content
Thickness
Size
Surface area
RGO
SE1432
Chemical reduced GO
10%
5–15 nm
6 lm
150–250 m2 g1
single-edge-double-notch four-point bend (SEDN-4PB) technique (Fig. S4). The pictures collected by POM were all recorded with a Pixelink camera (PL-A662).
gT ¼
ðEg=EmÞ 1 ðEg=EmÞ þ 2
n ¼ 2ag =3 ¼ 2lg =3tg
ð4Þ ð5Þ
3. Results and discussion 3.1. Dispersion of RGO Fig. 1a and b show the digital images of tensile and notched impacted specimens of PCG composites. The composites with low filler loading (<0.1 wt%) are transparent. As the filler loading increase gradually, the composites become opaque. This phenomenon indicates that the RGO are dispersed well in the PC matrix in low loading case, which is consistent with the results of TEM, SEM and XRD as followed. TEM images (Fig. 1c, d and Fig. S6) showed that the RGO were well dispersed nearly as individual RGO in the PC matrix at low filler loading (<0.1 wt%). With the increase of RGO content, the thickness of single RGO sheets in PCG composites increased due to van der Waals attraction. The RGO sheets tended to aggregate tougher at high filler loading, even though the aggregation state was not serious when filler loading was lower than 1 wt%. SEM images (Fig. 1e–h, Figs. S7 and S8) show the similar dispersion state of RGO in PC matrix to that seen in TEM images. On the other hand, no change in the scattering profile was observed in X-ray scattering for neat PC and PCG composites (Fig. S5) even at high loading of RGO (1.0 wt%), suggesting that the aggregation in PCG-1 composites was not so serious and no significant restacking occurred upon dispersion [25]. 3.2. Mechanical properties The mechanical properties of neat PC and PCG composites are evaluated in terms of strength and toughness, respectively. Fig. 2a shows the stress-strain curves of neat PC and PCG composites. With the addition of low loading RGO, the tensile yield strength and ultimate tensile strength of PCG composites increased gradually compared with that of neat PC, and reached its peak with 1.0 wt% RGO (an increase of 36.5% compared with that of neat PC). However, for PCG-3 composites, the tensile yield strength and ultimate tensile strength decreased compared with that of PCG-1 composites. It is obvious that the loading of RGO is a very important factor on influencing the morphology of RGO and the final properties of the composites. In this work, the Halpin-Tsai model, which can be widely used [26–28] for predicting the modulus of randomly distributed filler-reinforced composites, was used to simulate the modulus of the PCG composite and understand the reinforcing ability of RGO in polymer composites in terms of their load transfer efficiency. For randomly oriented RGO in the polymer matrix, the modulus Ec and Ek of the composites were given as following:
3 1 þ gL nVc 5 1 þ 2gT Vc Ec ¼ Em þ 8 1 gL Vc 8 1 gT Vc
ð1Þ
1 þ gL nVc Ek ¼ Em 1 gL Vc
ð2Þ
gL ¼
ðEg=EmÞ 1 ðEg=EmÞ þ n
ð3Þ
where Ec and Ek represent refer to the Young’s modulus of the composites with randomly distributed RGO and the Young’s modulus of the composites with RGO aligned parallel to the surface of the sample sheet, respectively. Eg and Em are the Young’s modulus of RGO and the polymer matrix. Vc is the volume fraction of RGO in the composites, which can be calculated from the wt% of RGO in the matrix according to the literature [29]. ag, lg and tg represent the aspect ratio, length and thickness of the RGO. The Young’s modulus of the chemically reduced graphene oxide sheet had been reported to be around 0.25 TPa [30], which may approach to that of graphene and was used here. The Young’s modulus of pure PC was measured to be 1.92 GPa. The density of the PC matrix is 1.2 g/cm3, and the density of RGO is 2.25 g/cm3. The average lg and tg of RGO were about 6 lm and 10 nm, respectively. According to the Eqs. (1)(5), the Young’s modulus of the PCG composites were calculated under two hypotheses: RGO aligned parallel to the surface of sample and RGO randomly dispersed in the PC matrix. As shown in Fig. 2b, when the loading of RGO was lower than 0.3 wt%, the theoretical predictions under-predicted the experimental results. This could be attributed to the wrinkled structure of RGO can be kept well in the low RGO loading case, which is very different from the rectangular shape of the RGO assumed by the Halpin-Tsai model [29]. However, when the loading of RGO exceeded 0.7 wt%, the theoretical predictions over-predicted the experimental results, due to the aggregation of RGO. As important as the strength of PC, the fracture toughness and notch resistance of PC are also significant properties for the application of PC. As shown in Fig. 3a, the elongation at break of PCG-0.03 composites increased by 37% compared with that of neat PC. The tensile fracture toughness calculated from the area under the stress-strain curve enhanced by 89% from 17.4 MPa of neat PC to 32.8 MPa of PCG-0.03. With the increase of RGO loading, the elongation at break and tensile fracture toughness of PCG composites decreased. In particular, when the RGO loading exceeds 0.1 wt%, the elongation at break and tensile fracture toughness decreased greatly compared with those of PCG-0.03 as well as neat PC. For example, compared with neat PC, the elongation at break of PCG-1 composite decreased by 79% and the tensile fracture toughness decreased by 71%. On the other hand, as shown in Fig. 3b, the notched impact strength of neat PC was about 9.6 kJ/m2, and the notched impact strength of PCG-0.07 composites enhanced by 46% to 14.0 kJ/m2. However, as increasing the filler content, the notched impact strength of PCG composites decreased greatly and dropped to 2.9 kJ/m2 in PCG-1 composite. Furthermore, the KIC-test and J-integral test also indicated the toughening effect of RGO on PC matrix, as shown in Fig. 3c. The critical stress intensity factor KIC and crack toughness J of neat PC was 2.86 MPa m1/2 and 1.72 kJ/m2. For the composite PCG-0.03, PCG-0.07 and PCG-0.1, the KIC and J increased greatly and reached its peak in PCG-0.07, in which the KIC and J reached 4.52 MPa m1/2 and 2.84 kJ/m2, an increment of 58% and 65% compared with those of neat PC, respectively. Then, the KIC and J of PCG composite decreased greatly when the RGO loading exceeded 0.1 wt%, as similar as the tensile fracture
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Fig. 1. Digital images of a) tensile specimens and b) notched impacted specimens of PCG composites. c) and d) are TEM images of PCG-0.07 and PCG-1 composite, respectively. SEM images of e) and f) PCG-0.07 composite, g) and h) PCG-1 composite. The SEM specimens were prepared by cryo-fracture the composite sample. The yellow circles in e) and g) indicated the location of dispersed RGO in the matrix.
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Fig. 2. a) the stress-strain cures of PCG composites with different filler contents. b) Experimental Young’s modulus of the PCG composite, calculated data derived from the Halpin–Tsai model under the hypothesis that RGO randomly dispersed as 3D network throughout the polymer matrix, and calculated data derived from the Halpin–Tsai model under the hypothesis that RGO aligned parallel to the surface of the composite.
toughness and notched impact strength in PCG composites. It is obvious that as-obtained toughness in PCG composites present similar increment tendency compared with that of neat PC, as shown in Fig. 3e. When the filler loading was lower than 0.1 wt%, the addition of RGO could play a great role on toughening PC matrix. However, the toughness of the composites tended to decrease greatly when the loading was >0.1 wt%. The point of 0.1 wt% could be considered reasonably as the ductile-brittle transition point in RGO toughened polycarbonate system, as described in Fig. 3d, which was of great significance for obtaining highperformance PC composites. When considering the comprehensive mechanical properties including strength and toughness, the PCG composites can be divided into three regions according to the RGO loading in the matrix (seen in Fig. 3e): low loading region (I: loading < 0.1 wt%); moderate loading region (II: 0.1 < loading < 1.0 wt%); high loading (III: loading > 1.0 wt%). In the region II, the strength of the composites increased gradually with the increase of RGO loading despite of the decrease in toughness. In the region III, the toughness decreased dramatically compared with that in region II, and the strength of the composites started to reduce. Interestingly, however, region I was found in PCG composites, in which the toughness as well as strength of the composites increased simultaneously by adding low loading RGO. The region I may have a great significance on fabricating polymer/graphene nanocomposites with balanced and enhanced mechanical properties. In general, rubber [7,31,32] or core-shell particle [5] was considered as the most effective methods to toughen PC matrix. The addition of rubber can enhance the toughness of PC dramatically, but at a large rubber loading (>5 wt%), in which, however, the strength of matrix decreased greatly. Even though several rigid filler like CNT [8], SiO2 [9,10] and GNP/CB [33] had been used to toughen PC matrix, they required at least 10-times higher loadings of filler to realize toughening PC matrix. Fig. 4 clearly shows that compared to rubber, CNT and SiO2, RGO exhibit better performance in toughening PC, especially in toughening and reinforcing PC simultaneously (Table S3). On the other hand, We also compared the toughening as well as reinforcing effect of RGO on PC matrix in this work with other polymer composites toughened with graphene-type materials (Fig. 5 and Table S4), from which one can observe that the toughening as well as the reinforcing effect of RGO on PC in this study was superior to most of the graphenetype material toughened polymer systems.
3.3. The role of RGO on resisting notch 3.3.1. Crack or craze propagation Fig. 6 was the result obtained from SEDN-4 PB and POM test, which clearly shows the difference on crack or craze propagation between neat PC and PCG composites. Due to the great sensitivity of PC to the notch, a large sharp linear crack with a very small plastic deformation zone ahead of the crack tip was observed in subsurface damage zone of neat PC (Fig. 6a and b). The crack tip in PCG-0.07 was much smaller than that in neat PC (Fig. 6c and d). Moreover, crack deflection and a large plastic deformation zone ahead of the crack tip, surrounded by numerous line arrays resembling the craze-like features [24] was observed in sub-surface damage zone of PCG-0.07. Crack deflection and the large plastic deformation region can absorb a large amount of damage energy, prevent the propagation of crack and thus enhance the fracture toughness of PC matrix greatly. In PCG-0.1 (Fig. 6e and f), crack deflection disappeared but plastic deformation zone can be observed ahead of the crack tip in the sub-surface damage zone of PCG-0.1, even though the size of plastic deformation zone reduced greatly compared with that in PCG-0.07. The addition of 0.1 wt% graphene can also enhance the fracture toughness of PC matrix slightly. However, in PCG-1 composite (Fig. 6g and h), both crack deflection and craze-like plastic deformation zone disappeared in the sub-surface damage zone of PCG-1. So it is clear that the addition of low loading RGO can absorb much fracture energy through inducing large craze-like plastic deformation and deflecting the crack, thus preventing the propagation of crack and enhance the resistance of matrix to notch. 3.3.2. Notch sensitivity of PCG composites Fig. 7 shows the SEM images of notch-impacted fracture surface of neat PC and its RGO composites. In this work, the neat PC fractured in a brittle manner even through some shearing bands occurred on the fracture surface. In the case of PCG-0.07, larger shearing bands and plastic deformation made up most of the fracture surface, which can undoubtedly absorb more impact energy than those in neat PC. For the composite with 0.3 wt% RGO, large plastic deformation disappeared and shearing bands became smaller compared with those in neat PC, PCG-0.07 as well as PCG-0.1, indicating relatively weak ability to resist impact. Furthermore, there was no shearing band on the fracture surface of PCG composites with high loading graphene (PCG-1 and PCG-3) and the surface
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Fig. 3. a) Elongation at break and tensile fracture toughness; b) notched impact strength c) critical stress intensity factor KIC and crack toughness J of PCG composites with different filler contents. d) the increment on the three tested-toughness of PCG composites with different filler contents; e) the increment on the three tested-toughness, tensile strength and tensile modulus of PCG composites with different filler contents.
became very smooth. This is consistent with the result of notched impact test. For PCG composites with low RGO loading (region I), larger shearing bands and plastic deformation are the main fracture manner which can absorb large amounts of impact energy and significantly enhance the notched impact strength of composites. And for PCG composites with moderate RGO loading (region II), the degree of plastic deformation becomes smaller compared with that of composites in region I as well as neat PC. The smaller the plastic deformation is, the less impact energy can be absorbed. So the notched impact strength reduced greatly. The plastic deformation becomes the smallest when the filler loading reaches region III. This may be fatal flaw for absorbing impact energy.
On the other hand, it has been well documented that the notch sensitivity of PC is due to the change in stress state at the notch from plane stress to plane strain and the resulting change in failure mechanism from shearing to crazing [4]. For notched PC and rubber-toughened PC, the brittle fracture occurs from the internal crazes, which are nucleated at the tip of the local plastic zone ahead of the notch tip [6,48]. However, almost no literature has reported the type of internal craze in the PC composites toughened with inorganic rigid filler, like CNT and graphene. We adopted the theory in rubber-toughened PC [3,6,48,49] to elucidate the toughening mechanism in graphene-toughened PC system. Similar with the situation in rubber-toughened PC system, an elliptical elastic-
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Fig. 4. Comparison of the toughening effect of different fillers on PC matrix.
Fig. 5. Comparison of toughening effect of graphene-type materials on different polymers. (See above-mentioned references for further information.)
plastic deformation region occurred ahead of the notch tip (Fig. 8), which included an internal craze region (insider elliptical) and a smooth annular region surrounding the internal craze. The internal craze is the initiation of craze and the area of this region has an important correlation with the notch sensitivity of PC [50]. On the other hand, the smooth annular surrounding the internal craze is considered as the initiation of larger plastic deformation like shearing band [50] and the area of this region may also have some relationship with the notch sensitivity of PC. Herein, the total area of elastic-plastic deformation region was defined as S (solid yellow line) and the area of internal craze was defined as S1 (dotted yellow line). So the difference between S and S1 was the area of the smooth annular, called S2. The value of S, S1 and S2 was quantitatively calculated through SEM images and were summarized in Table 2. The criteria to draw the solid and dotted yellow line and how the area of S1 and S2 are defined were presented in detail in the Supplementary Materials (Fig. S14). Compared with that of neat PC, the area of S1 in PCG composites decreased gradually with increasing the loading of RGORGO, indicating that the addition of RGO can enhance the ability of PC to resist the notch. Compared with the area of S in neat PC, the counterpart in PCG composites increased firstly and then reduced with increasing RGO loading, reaching its peak in the loading region I. At the same time, the area of S2 presented the same situation similar with that of S. It is interesting that the increment on S2 of PCG composites compared with neat PC was almost identical with the increment on notched
impact strength of PCG composites compared with neat PC, especially for PCG composites with low loading RGO (region I). As described above, it is reasonable to consider the S2 region as the initiation of larger plastic deformation which can absorb more impact energy, which means that the addition of low loading RGO can enhance the ability to resist the notch. 3.3.3. Stress-induced morphological evolution of RGO Furthermore, in order to investigate the role of RGO on increasing the toughness in notched impact mode, SEM images describing the morphological evolution of RGO during the notched impact were performed, respectively. It is well known that the microunits of the deformation region deform in two methods during the impact process: high speed tensile deformation and shear deformation due to the tensile speed gradients in the deformation region [51]. Therefore, in fact, the morphology of RGO evolved in many different ways during the impact of PCG composites. Firstly, due to the layer breakage (Fig. 9b) and pulling out of graphene layer (Fig. 9e) and debonding between graphene and PC matrix, microcrack was formed around most of the RGO during the impact, which can dissipate impact energy and enhance the impact toughness. The formation of microcrack in impact mode is similar with that in tensile mode (seen in the Section 8 in the Supplementary Material), indicating the formation of microcrack during impact process is mainly the results of tensile deformation. On the other hand, during the propagation of crack, crack deflection (Fig. 9c),
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Fig. 6. Bright field POM micrographs of sub-surface damage zone ahead of the arrested crack-tip for a) and b) neat PC; c) and (d) PCG-0.07 composite; e) and f) PCG-0.1 composite; g) and h) PCG-1 composite.
crack pinning (Fig. 9a) and crack arresting (Fig. 9a) could be induced by the two-dimensional RGO when the crack tip came across the RGO. The formation of crack deflection and crack pinning and crack arresting was regarded as the results of shearing deformation during the impact process. Similar phenomenon of stress-induced RGO morphological evolution could also be found during the impact process of PCG-0.1 (Fig. S15), indicating enhanced toughness of PCG-0.1 composite compared with that of neat PC. On the other hand, we investigated the number of different mechanism types in RGO filled PC composites (Fig. S17 and Table S1). It can be observed that most of the RGO can induce microcrack during the fracture, which was the most important toughening role of RGO, followed by crack deflection, crack pinning and crack arresting. However, as the RGO loading increases, crack deflection became the main role of graphene during the impact, while crack pinning and crack arresting induced by graphene disappeared due to decrease of the distance between adjacent RGO. The corresponding morphologies of RGO in PCG composites with high RGO loading were shown in Fig. 9f–j as well as Fig. S16. Moreover, the coalescing of microcracks between adjacent RGO or adjacent graphene layers may facilitate the propagation of the primary crack, which is detrimental for resisting the impact. The morphological evolution of RGO in the cross section near the impact fracture surface and notch tip, as shown in Fig. 10, also
indicated the positive effect of low loading RGO on enhancing the impact toughness and reducing the notch sensitivity of PC matrix. In the case filled with low loading RGO, the RGO could induce microcrack, crack pinning and crack arresting (Fig. 10b). Moreover, the as-pinned crack by RGO could be terminated by adjacent RGO. This can prevent the propagation of crack at the utmost and thus enhance the impact toughness of PC matrix. On the other hand, the 2D RGO near the notch could be act as stress concentrator and reduce the impact stress, resulting in the change in stress state at the notch from plane strain to plane stress and the failure mechanism changing into shearing from crazing. This can greatly enhance the resistance of PC matrix to the notch. However, in the case of composites filled with high loading RGO, as shown in Fig. 10c, the 2D RGO could not induce effective morphological evolution like crack pinning and crack arresting that can absorb impact energy. This is the result of the agglomeration of RGO and the decreased distance between adjacent RGO, which is beneficial to the propagation of crack. So the crack pinning and crack arresting induced by RGO disappeared in the high loading case, leading to the decrease of toughness in PC matrix. Moreover, the microcracks induced by RGO will be coalesced due to the decrease of the distance between adjacent RGO. This also facilitates the propagation of the primary crack and is detrimental for resisting the impact. So the existence of RGO in PCG composites filled with high
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Fig. 7. SEM images of impacted fracture surface of a) neat PC, b) PCG-0.07, c) PCG-0.1, d) PCG-0.3, e) PCG-1 and f) PCG-3, respectively. The white arrow indicates the impact direction. In all cases scale bar are 2 mm.
Fig. 8. SEM images of impacted fracture surface near the notch tip of a) neat PC, b) PCG-0.03, c) PCG-0.07, d) PCG-0.1, e) PCG-0.3, f) PCG-0.7, g) PCG-1 and h) PCG-3, respectively. The white arrow indicates the impact direction. In all cases scale bar are 500 lm.
Table 2 Summary of the area initiation rough and smooth region in neat PC and PCG composites. Samples
Total area S (lm2)
S1 (lm2)
S2 (lm2)
Increment of S2 compared with neat PC (%)
Increment of notched impact strength compared with neat PC (%)
Neat PC PCG-0.03 PCG-0.07 PCG-0.1 PCG-0.3 PCG-0.7
2.66 105 (3%a) 2.67 105(3%) 3.19 105(3%) 2.08 105(2%) 1.45 105 (2%) 8.84 104(2%)
1.00 105(3%) 7.90 104(5%) 6.44 104(7%) 5.06 104(3%) 5.28 104(2%) 2.96 104(6%)
1.65 105(2%) 1.88 105(2%) 2.54 105(2%) 1.57 104(2%) 9.22 104(2%) 5.88 104(1%)
/ 14 54 5 44 65
/ 18 46 5 31 53
Note: aThe ratio of standard error with the averaged value.
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Fig. 9. SEM images of morphology of RGO on the impacted fracture surface of a), b) c), d) and e) are PCG-0.07 composites. f), g), h), i) and j) are PCG-1 composites, respectively. In all cases scale bar are 5 lm.
Fig. 10. SEM images of morphology in the cross section near the impact fracture surface and notch tip; a) neat PC b) PCG-0.07 and c) PCG-1 composites. In all cases scale bar are 5 lm.
loading RGO could not play the role of improving the notched impact toughness and notch sensitivity of PC matrix. The above results show that the 2D RGO under low loading can enhance the impact toughness and reduce the notch sensitivity of PC matrix. During the notched impact process, layer breakage and pulling out of graphene layers, microcrack, crack deflection, crack pinning, crack arresting induced by RGO are considered as the main toughening mechanism in PCG composites in this study (Fig. S18). This toughening mechanism is very different from other fillers toughened PC matrix (Table S3). Apart from the microcrack (nanocavities or void), crack pinning, crack arresting, layer breakage and pulling out was observed and proposed as the toughening mechanism, which was absent in other filler-toughened PC systems. 4. Conclusions Herein, reduced graphene oxide was incorporated into PC matrix through facile melt compounding, to fabricate advanced PC nanocomposites. The low loading RGO (<0.1% wt) can enhance the fracture toughness, strength and notch resistance of PC matrix greatly. Microcrack, caused by the layer breakage and pulling out of RGO, was the main toughening mechanism resulting in the greatly improved fracture toughness and notch resistance. Apart from microcrack graphene-induced crack deflection, crack pining and crack arresting during the notch-fractured process were also positive factors contributing to enhanced notch-fracture toughness and notch resistance. The crack deflection, crack pinning and crack arresting was the result of shearing deformation during the impact
process, while microcrack is the result of tensile deformation. This paper gives us a deep understanding in the toughening mechanism in polymers toughened with graphene-type materials. Moreover, these PCG composites with high notch resistance, balanced toughness and strength fabricated without any solvent will make the large-scale manufactured graphene-type materials more competitive in the nanoscience and chemical engineering part, and may open the door to downstream industrial and green application of graphene-type materials.
Acknowledgements Financial support of the National Natural Science Foundation of China (51273132, 51573118, 51227802 and 51603131), Program for New Century Excellent Talents in University (NCET-13-0392), the Sichuan Province Youth Science Fund (2015JQ0015), the Sichuan Province Science Fund (2016JY0190), the China Postdoctoral Science Foundation (2015M572473) and the Open Research Subject of Key Laboratory of automobile high performance materials and forming technology (szjj2016-088) are gratefully acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.05.090.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Polycarbonate toughening with reduced graphene oxide: Toward high toughness, strength and notch resistance Jianfeng Wang, Chunhai Li, Xiaomeng Zhang, Lichao Xia, Xianlong Zhang, Hong Wu ⇑, Shaoyun Guo The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Reduced graphene oxide (RGO) was
melt-processed into polycarbonate (PC). RGO can enhance the toughness, strength and notch resistance of PC greatly. The toughening mechanism was investigated. Microcrack, crack pinning, deflection and arresting were the toughening mechanism.
a r t i c l e
i n f o
Article history: Received 22 February 2017 Received in revised form 27 April 2017 Accepted 14 May 2017 Available online 15 May 2017 Keywords: Polycarbonate Reduced graphene oxide Nanocomposites Melt compounding Mechanical properties
a b s t r a c t The toughening effect of graphene sheets on polycarbonate (PC) was investigated to fabricate PC composites with excellent balanced toughness, notch resistance as well as strength. The reduced graphene oxide (RGO) was incorporated into PC matrix through melt compounding. A maximum toughening effect was observed in PC/RGO (PCG) composites with 0.03 wt% or 0.07 wt% RGO. Particularly, the tensile fracture toughness of PCG composites with 0.03 wt% RGO was enhanced by 89%. The notched impact strength and KIC of PCG composite with 0.07 wt% RGO was increased by 46% and 58%, respectively. The point of 0.1 wt% was found to be the ductile-brittle transition point in PCG composites. Meanwhile, the yield strength of these novel materials was increased by around 12% as well at loading of 0.07 wt%. Microcrack, resulting from interfacial debonding between PC and RGO as well as breakage and pulling out of graphene layer, was proposed to be the main toughening mechanism contributing to the great enhanced fracture toughness and notch resistance. Apart from the microcrack, crack pinning, crack deflection and crack arresting were also found and proposed to be toughening mechanism in notchfractured process. This work not only provides us a novel strategy to fabricate advanced PC nanocomposites but also gives us a deep understanding on the toughening role of graphene on polymers. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Polycarbonate (PC) is a very attractive polymer used in many fields ranging from disk to helmet for astronauts due to its remarkable heat resistance, toughness, optical transparency, and electrical
⇑ Corresponding author. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.cej.2017.05.090 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
properties [1–3]. However, PC is very sensitive to the notches or cracks, which greatly limits its application in many high-end fields. It is therefore of great significance to enhance the notch resistance as well as the notch-fractured toughness of PC. It is well known that the notch sensitivity of PC is due to the change in stress state at the notch from plane stress to plane strain, resulting in the change in failure mechanism from shearing to crazing [4]. Generally, rubber is considered as the most effective method to toughen
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PC [5–7], which can induce cavitation, relieve the plane strain constraint at the notch and permit the matrix to deform by shearing. However, typically, 5–20 wt% of rubber is required, causing a dramatic reduction in strength and modulus. Apart from the rubber, rigid fillers, like carbon nanotube [8], SiO2 [9] or nano-SiO2 [10] have also been studied to improve the fracture toughness of PC. However, the increment in notch resistance or fracture toughness obtained with these rigid fillers is typically less than that with rubber. Graphene, a two-dimensional material of numerous excellent properties [11–13], has attracted tremendous interest in scientific and industrial fields. Adding into polymers to improve the mechanical properties of polymers is considered one of the most promising applications of graphene [14–17]. However, compared with the role on reinforcing polymers, the toughening role of graphene on polymers has always been neglected. To the best knowledge, there was almost no report on the toughness of graphene/ polymer nanocomposites before 2009. In the past few years, some studies have reported the toughening role of graphene on polymers. For instance, Rafiee et al. found that the fracture performance of epoxy composites with 0.1 wt% of pristine graphene was better than that with carbon nanotubes [18] and the increment in fracture toughness peaked at 0.125 wt% of graphene with a 65% increment [19]. Yong et.al [20] found that, graphene nanosheet can induce microcracks in epoxy filled with low loading graphene, resulting in an impressive toughening effect on epoxy matrix. However, up to now, there are still some challenging issues considering graphene as toughener to polymers. First of all, the matrices studied are too rare and most of the studies focused on the brittle thermosetting epoxy [18–23]. More comparative studies and modeling work are necessary to investigate the toughening ability of graphene on polymers. Then, the understanding of the toughening mechanism responsible for other polymers except epoxy is still insufficient. At last, most of the polymer nanocomposites toughened by graphene were fabricated through solution, in situ polymerization or layer-by-layer strategy using solvents or dispersions. They are not practical approaches to manufacture polymer/graphene composites environment-friendly and costeffectively, placing great limitations on industrial implementation. In this work, we sought to explore the toughening effect of graphene sheets on PC matrix and fabricate advanced PC nanocomposites with excellent toughness, notch resistance as well as strength. Reduced graphene oxide (RGO) was performed to prepare PC nanocomposite via melt compounding without any solvent. The notch resistance and notch-fractured toughness were evaluated based on notched impact mode and KIC-test mode. The strength and tensile fracture toughness were explored through tensile fracture mode. Interestingly, an impressive toughening effect was observed at 0.03 wt% or 0.07 wt% of RGO loading in all three modes. Meanwhile, the strength of PC matrix was enhanced with the addition of low loading RGO. The corresponding toughening mechanism was proposed based on the fractographs of the composite samples and morphological evolution of RGO during the fracture process.
2. Experimental 2.1. Materials Bisphenol-A Polycarbonate (PC-201, Mw = 31809, a density of 1.2 g/cm3) was purchased from LG Chem (Korea) and vacuumdried overnight at 110 °C before use. Paraffin oil (CAS: 8012-951, Kelong chemical reagent CO., LTD), being mainly composed of C16-C20 n-alkanes and with a density of 0.835 g/cm3, was used as received. Reduced graphene oxide (RGO) were supplied by
475
Changzhou Six element CO., Ltd. Table 1 summaries the specifications of RGO. Moreover, the other information about the RGO, including the original morphology (SEM images), Raman spectrum and XPS wide spectrum, can be seen in the Supplementary Materials (Figs. S1 and S2). The RGO were vacuum-dried overnight at 60 °C before use. 2.2. Preparation of PC/RGO composites Firstly, PC pellets and RGO were physically blended together through paraffin oil, in a solid mixer (V5D, Feich machinery, China) at 60 r/min for 10 min. The weight ratio of PC and paraffin is 1000:1. The paraffin oil can make the RGO adhere uniformly around the PC pellets in advance, which is helpful for the dispersion of RGO in the melt-processed PC in the following process. Then PC and PC/ RGO composites (PCG) with a load varying from 0.03 to 3 wt% were prepared by melt compounding on a HaPu melt mixer at 260 °C. The melt mixing was performed by adding approximately half of the polymer quantity to the mixing bowl. When the polymer matrix melted and the torque started to decrease, the remaining composites were gradually added to the mixer. For all the materials the melt compounding was performed at a screw speed of 60 rpm for 20 min. The materials obtained via melt compounding were compression molded at 260 °C under 15 MPa, which were used as specimens for different characterizations. The as-obtained composites are coded using PCG and the RGO loading. For example, PCG-0.07 means the PCG composites with 0.07 wt%RGO. 2.3. Characterization Scanning electron microscopy (SEM) images were taken using JSM-5900LV-SEM at a voltage of 5.0 kV. Prior to being analyzed, the samples were mounted on stubs and their surfaces were platinum coated. The dispersion of graphene in PC matrix was observed with a Tecnai G2F20S-TWIN transmission electron microscopy (TEM) at an accelerating voltage of 200 kV. Ultrathin sections thinner than 100 nm were cryogenically cut with a glazing knife using a microtome and collected on 300-mesh copper grids for TEM observation. X-ray diffraction (XRD) measurements were conducted using a D8 Advance (Bruker) X-ray diffractometer using Cu Ka radiation (k = 1.5405 Å) with a scanning speed of 5°/min from 5 to 60°. The scanned area was from 190 to 600 nm. Raman spectra were measured on LabRam-HR (French Horiba, 532 nm). X-ray photoelectron spectroscopy (XPS) was conducted on K-alpha (Thermo Scientific). Standard tensile test used dumbbell shaped sample was conducted at room temperature using tensile test machine (model CMT-4104) according to GB/T 1040-92. Tensile fracture toughness were evaluated based on the area under the stress-strain curve. Furthermore, due to the sensitivity of PC to the notch, the notched Izod impact test, KIC-tests as well as J-integral tests were performed to evaluate the notch resistance and notch-fractured toughness of neat PC and its RGO composites. The notched impact test was performed following GB/1943–2007 with a XJU-22 impact test machine. Before the impact testing, a single-edge V-shaped notch with a depth of 2 mm was milled in all the rectangular bulk specimens (80 10 4 mm3). The KIC-tests as well as J-integral tests were performed according to ASTM D5045-99 and ASTM E813-89 [24], respectively (detail information can be seen in Support Information Fig. S3). All the mechanical property data with standard deviation was averaged using at least six specimens (seen in the Supplementary Material, Table S2). On the other hand, Polarized optical microscopy (POM, Leica, DM2500P) were performed to collect the initiation and propagation patterns of the crack or craze in neat PC and PCG composites after being tested through
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Table 1 Specifications of pristine RGO from the manufacturer. Notation
Product name
Product process
Oxygen content
Thickness
Size
Surface area
RGO
SE1432
Chemical reduced GO
10%
5–15 nm
6 lm
150–250 m2 g1
single-edge-double-notch four-point bend (SEDN-4PB) technique (Fig. S4). The pictures collected by POM were all recorded with a Pixelink camera (PL-A662).
gT ¼
ðEg=EmÞ 1 ðEg=EmÞ þ 2
n ¼ 2ag =3 ¼ 2lg =3tg
ð4Þ ð5Þ
3. Results and discussion 3.1. Dispersion of RGO Fig. 1a and b show the digital images of tensile and notched impacted specimens of PCG composites. The composites with low filler loading (<0.1 wt%) are transparent. As the filler loading increase gradually, the composites become opaque. This phenomenon indicates that the RGO are dispersed well in the PC matrix in low loading case, which is consistent with the results of TEM, SEM and XRD as followed. TEM images (Fig. 1c, d and Fig. S6) showed that the RGO were well dispersed nearly as individual RGO in the PC matrix at low filler loading (<0.1 wt%). With the increase of RGO content, the thickness of single RGO sheets in PCG composites increased due to van der Waals attraction. The RGO sheets tended to aggregate tougher at high filler loading, even though the aggregation state was not serious when filler loading was lower than 1 wt%. SEM images (Fig. 1e–h, Figs. S7 and S8) show the similar dispersion state of RGO in PC matrix to that seen in TEM images. On the other hand, no change in the scattering profile was observed in X-ray scattering for neat PC and PCG composites (Fig. S5) even at high loading of RGO (1.0 wt%), suggesting that the aggregation in PCG-1 composites was not so serious and no significant restacking occurred upon dispersion [25]. 3.2. Mechanical properties The mechanical properties of neat PC and PCG composites are evaluated in terms of strength and toughness, respectively. Fig. 2a shows the stress-strain curves of neat PC and PCG composites. With the addition of low loading RGO, the tensile yield strength and ultimate tensile strength of PCG composites increased gradually compared with that of neat PC, and reached its peak with 1.0 wt% RGO (an increase of 36.5% compared with that of neat PC). However, for PCG-3 composites, the tensile yield strength and ultimate tensile strength decreased compared with that of PCG-1 composites. It is obvious that the loading of RGO is a very important factor on influencing the morphology of RGO and the final properties of the composites. In this work, the Halpin-Tsai model, which can be widely used [26–28] for predicting the modulus of randomly distributed filler-reinforced composites, was used to simulate the modulus of the PCG composite and understand the reinforcing ability of RGO in polymer composites in terms of their load transfer efficiency. For randomly oriented RGO in the polymer matrix, the modulus Ec and Ek of the composites were given as following:
3 1 þ gL nVc 5 1 þ 2gT Vc Ec ¼ Em þ 8 1 gL Vc 8 1 gT Vc
ð1Þ
1 þ gL nVc Ek ¼ Em 1 gL Vc
ð2Þ
gL ¼
ðEg=EmÞ 1 ðEg=EmÞ þ n
ð3Þ
where Ec and Ek represent refer to the Young’s modulus of the composites with randomly distributed RGO and the Young’s modulus of the composites with RGO aligned parallel to the surface of the sample sheet, respectively. Eg and Em are the Young’s modulus of RGO and the polymer matrix. Vc is the volume fraction of RGO in the composites, which can be calculated from the wt% of RGO in the matrix according to the literature [29]. ag, lg and tg represent the aspect ratio, length and thickness of the RGO. The Young’s modulus of the chemically reduced graphene oxide sheet had been reported to be around 0.25 TPa [30], which may approach to that of graphene and was used here. The Young’s modulus of pure PC was measured to be 1.92 GPa. The density of the PC matrix is 1.2 g/cm3, and the density of RGO is 2.25 g/cm3. The average lg and tg of RGO were about 6 lm and 10 nm, respectively. According to the Eqs. (1)(5), the Young’s modulus of the PCG composites were calculated under two hypotheses: RGO aligned parallel to the surface of sample and RGO randomly dispersed in the PC matrix. As shown in Fig. 2b, when the loading of RGO was lower than 0.3 wt%, the theoretical predictions under-predicted the experimental results. This could be attributed to the wrinkled structure of RGO can be kept well in the low RGO loading case, which is very different from the rectangular shape of the RGO assumed by the Halpin-Tsai model [29]. However, when the loading of RGO exceeded 0.7 wt%, the theoretical predictions over-predicted the experimental results, due to the aggregation of RGO. As important as the strength of PC, the fracture toughness and notch resistance of PC are also significant properties for the application of PC. As shown in Fig. 3a, the elongation at break of PCG-0.03 composites increased by 37% compared with that of neat PC. The tensile fracture toughness calculated from the area under the stress-strain curve enhanced by 89% from 17.4 MPa of neat PC to 32.8 MPa of PCG-0.03. With the increase of RGO loading, the elongation at break and tensile fracture toughness of PCG composites decreased. In particular, when the RGO loading exceeds 0.1 wt%, the elongation at break and tensile fracture toughness decreased greatly compared with those of PCG-0.03 as well as neat PC. For example, compared with neat PC, the elongation at break of PCG-1 composite decreased by 79% and the tensile fracture toughness decreased by 71%. On the other hand, as shown in Fig. 3b, the notched impact strength of neat PC was about 9.6 kJ/m2, and the notched impact strength of PCG-0.07 composites enhanced by 46% to 14.0 kJ/m2. However, as increasing the filler content, the notched impact strength of PCG composites decreased greatly and dropped to 2.9 kJ/m2 in PCG-1 composite. Furthermore, the KIC-test and J-integral test also indicated the toughening effect of RGO on PC matrix, as shown in Fig. 3c. The critical stress intensity factor KIC and crack toughness J of neat PC was 2.86 MPa m1/2 and 1.72 kJ/m2. For the composite PCG-0.03, PCG-0.07 and PCG-0.1, the KIC and J increased greatly and reached its peak in PCG-0.07, in which the KIC and J reached 4.52 MPa m1/2 and 2.84 kJ/m2, an increment of 58% and 65% compared with those of neat PC, respectively. Then, the KIC and J of PCG composite decreased greatly when the RGO loading exceeded 0.1 wt%, as similar as the tensile fracture
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Fig. 1. Digital images of a) tensile specimens and b) notched impacted specimens of PCG composites. c) and d) are TEM images of PCG-0.07 and PCG-1 composite, respectively. SEM images of e) and f) PCG-0.07 composite, g) and h) PCG-1 composite. The SEM specimens were prepared by cryo-fracture the composite sample. The yellow circles in e) and g) indicated the location of dispersed RGO in the matrix.
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Fig. 2. a) the stress-strain cures of PCG composites with different filler contents. b) Experimental Young’s modulus of the PCG composite, calculated data derived from the Halpin–Tsai model under the hypothesis that RGO randomly dispersed as 3D network throughout the polymer matrix, and calculated data derived from the Halpin–Tsai model under the hypothesis that RGO aligned parallel to the surface of the composite.
toughness and notched impact strength in PCG composites. It is obvious that as-obtained toughness in PCG composites present similar increment tendency compared with that of neat PC, as shown in Fig. 3e. When the filler loading was lower than 0.1 wt%, the addition of RGO could play a great role on toughening PC matrix. However, the toughness of the composites tended to decrease greatly when the loading was >0.1 wt%. The point of 0.1 wt% could be considered reasonably as the ductile-brittle transition point in RGO toughened polycarbonate system, as described in Fig. 3d, which was of great significance for obtaining highperformance PC composites. When considering the comprehensive mechanical properties including strength and toughness, the PCG composites can be divided into three regions according to the RGO loading in the matrix (seen in Fig. 3e): low loading region (I: loading < 0.1 wt%); moderate loading region (II: 0.1 < loading < 1.0 wt%); high loading (III: loading > 1.0 wt%). In the region II, the strength of the composites increased gradually with the increase of RGO loading despite of the decrease in toughness. In the region III, the toughness decreased dramatically compared with that in region II, and the strength of the composites started to reduce. Interestingly, however, region I was found in PCG composites, in which the toughness as well as strength of the composites increased simultaneously by adding low loading RGO. The region I may have a great significance on fabricating polymer/graphene nanocomposites with balanced and enhanced mechanical properties. In general, rubber [7,31,32] or core-shell particle [5] was considered as the most effective methods to toughen PC matrix. The addition of rubber can enhance the toughness of PC dramatically, but at a large rubber loading (>5 wt%), in which, however, the strength of matrix decreased greatly. Even though several rigid filler like CNT [8], SiO2 [9,10] and GNP/CB [33] had been used to toughen PC matrix, they required at least 10-times higher loadings of filler to realize toughening PC matrix. Fig. 4 clearly shows that compared to rubber, CNT and SiO2, RGO exhibit better performance in toughening PC, especially in toughening and reinforcing PC simultaneously (Table S3). On the other hand, We also compared the toughening as well as reinforcing effect of RGO on PC matrix in this work with other polymer composites toughened with graphene-type materials (Fig. 5 and Table S4), from which one can observe that the toughening as well as the reinforcing effect of RGO on PC in this study was superior to most of the graphenetype material toughened polymer systems.
3.3. The role of RGO on resisting notch 3.3.1. Crack or craze propagation Fig. 6 was the result obtained from SEDN-4 PB and POM test, which clearly shows the difference on crack or craze propagation between neat PC and PCG composites. Due to the great sensitivity of PC to the notch, a large sharp linear crack with a very small plastic deformation zone ahead of the crack tip was observed in subsurface damage zone of neat PC (Fig. 6a and b). The crack tip in PCG-0.07 was much smaller than that in neat PC (Fig. 6c and d). Moreover, crack deflection and a large plastic deformation zone ahead of the crack tip, surrounded by numerous line arrays resembling the craze-like features [24] was observed in sub-surface damage zone of PCG-0.07. Crack deflection and the large plastic deformation region can absorb a large amount of damage energy, prevent the propagation of crack and thus enhance the fracture toughness of PC matrix greatly. In PCG-0.1 (Fig. 6e and f), crack deflection disappeared but plastic deformation zone can be observed ahead of the crack tip in the sub-surface damage zone of PCG-0.1, even though the size of plastic deformation zone reduced greatly compared with that in PCG-0.07. The addition of 0.1 wt% graphene can also enhance the fracture toughness of PC matrix slightly. However, in PCG-1 composite (Fig. 6g and h), both crack deflection and craze-like plastic deformation zone disappeared in the sub-surface damage zone of PCG-1. So it is clear that the addition of low loading RGO can absorb much fracture energy through inducing large craze-like plastic deformation and deflecting the crack, thus preventing the propagation of crack and enhance the resistance of matrix to notch. 3.3.2. Notch sensitivity of PCG composites Fig. 7 shows the SEM images of notch-impacted fracture surface of neat PC and its RGO composites. In this work, the neat PC fractured in a brittle manner even through some shearing bands occurred on the fracture surface. In the case of PCG-0.07, larger shearing bands and plastic deformation made up most of the fracture surface, which can undoubtedly absorb more impact energy than those in neat PC. For the composite with 0.3 wt% RGO, large plastic deformation disappeared and shearing bands became smaller compared with those in neat PC, PCG-0.07 as well as PCG-0.1, indicating relatively weak ability to resist impact. Furthermore, there was no shearing band on the fracture surface of PCG composites with high loading graphene (PCG-1 and PCG-3) and the surface
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Fig. 3. a) Elongation at break and tensile fracture toughness; b) notched impact strength c) critical stress intensity factor KIC and crack toughness J of PCG composites with different filler contents. d) the increment on the three tested-toughness of PCG composites with different filler contents; e) the increment on the three tested-toughness, tensile strength and tensile modulus of PCG composites with different filler contents.
became very smooth. This is consistent with the result of notched impact test. For PCG composites with low RGO loading (region I), larger shearing bands and plastic deformation are the main fracture manner which can absorb large amounts of impact energy and significantly enhance the notched impact strength of composites. And for PCG composites with moderate RGO loading (region II), the degree of plastic deformation becomes smaller compared with that of composites in region I as well as neat PC. The smaller the plastic deformation is, the less impact energy can be absorbed. So the notched impact strength reduced greatly. The plastic deformation becomes the smallest when the filler loading reaches region III. This may be fatal flaw for absorbing impact energy.
On the other hand, it has been well documented that the notch sensitivity of PC is due to the change in stress state at the notch from plane stress to plane strain and the resulting change in failure mechanism from shearing to crazing [4]. For notched PC and rubber-toughened PC, the brittle fracture occurs from the internal crazes, which are nucleated at the tip of the local plastic zone ahead of the notch tip [6,48]. However, almost no literature has reported the type of internal craze in the PC composites toughened with inorganic rigid filler, like CNT and graphene. We adopted the theory in rubber-toughened PC [3,6,48,49] to elucidate the toughening mechanism in graphene-toughened PC system. Similar with the situation in rubber-toughened PC system, an elliptical elastic-
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Fig. 4. Comparison of the toughening effect of different fillers on PC matrix.
Fig. 5. Comparison of toughening effect of graphene-type materials on different polymers. (See above-mentioned references for further information.)
plastic deformation region occurred ahead of the notch tip (Fig. 8), which included an internal craze region (insider elliptical) and a smooth annular region surrounding the internal craze. The internal craze is the initiation of craze and the area of this region has an important correlation with the notch sensitivity of PC [50]. On the other hand, the smooth annular surrounding the internal craze is considered as the initiation of larger plastic deformation like shearing band [50] and the area of this region may also have some relationship with the notch sensitivity of PC. Herein, the total area of elastic-plastic deformation region was defined as S (solid yellow line) and the area of internal craze was defined as S1 (dotted yellow line). So the difference between S and S1 was the area of the smooth annular, called S2. The value of S, S1 and S2 was quantitatively calculated through SEM images and were summarized in Table 2. The criteria to draw the solid and dotted yellow line and how the area of S1 and S2 are defined were presented in detail in the Supplementary Materials (Fig. S14). Compared with that of neat PC, the area of S1 in PCG composites decreased gradually with increasing the loading of RGORGO, indicating that the addition of RGO can enhance the ability of PC to resist the notch. Compared with the area of S in neat PC, the counterpart in PCG composites increased firstly and then reduced with increasing RGO loading, reaching its peak in the loading region I. At the same time, the area of S2 presented the same situation similar with that of S. It is interesting that the increment on S2 of PCG composites compared with neat PC was almost identical with the increment on notched
impact strength of PCG composites compared with neat PC, especially for PCG composites with low loading RGO (region I). As described above, it is reasonable to consider the S2 region as the initiation of larger plastic deformation which can absorb more impact energy, which means that the addition of low loading RGO can enhance the ability to resist the notch. 3.3.3. Stress-induced morphological evolution of RGO Furthermore, in order to investigate the role of RGO on increasing the toughness in notched impact mode, SEM images describing the morphological evolution of RGO during the notched impact were performed, respectively. It is well known that the microunits of the deformation region deform in two methods during the impact process: high speed tensile deformation and shear deformation due to the tensile speed gradients in the deformation region [51]. Therefore, in fact, the morphology of RGO evolved in many different ways during the impact of PCG composites. Firstly, due to the layer breakage (Fig. 9b) and pulling out of graphene layer (Fig. 9e) and debonding between graphene and PC matrix, microcrack was formed around most of the RGO during the impact, which can dissipate impact energy and enhance the impact toughness. The formation of microcrack in impact mode is similar with that in tensile mode (seen in the Section 8 in the Supplementary Material), indicating the formation of microcrack during impact process is mainly the results of tensile deformation. On the other hand, during the propagation of crack, crack deflection (Fig. 9c),
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Fig. 6. Bright field POM micrographs of sub-surface damage zone ahead of the arrested crack-tip for a) and b) neat PC; c) and (d) PCG-0.07 composite; e) and f) PCG-0.1 composite; g) and h) PCG-1 composite.
crack pinning (Fig. 9a) and crack arresting (Fig. 9a) could be induced by the two-dimensional RGO when the crack tip came across the RGO. The formation of crack deflection and crack pinning and crack arresting was regarded as the results of shearing deformation during the impact process. Similar phenomenon of stress-induced RGO morphological evolution could also be found during the impact process of PCG-0.1 (Fig. S15), indicating enhanced toughness of PCG-0.1 composite compared with that of neat PC. On the other hand, we investigated the number of different mechanism types in RGO filled PC composites (Fig. S17 and Table S1). It can be observed that most of the RGO can induce microcrack during the fracture, which was the most important toughening role of RGO, followed by crack deflection, crack pinning and crack arresting. However, as the RGO loading increases, crack deflection became the main role of graphene during the impact, while crack pinning and crack arresting induced by graphene disappeared due to decrease of the distance between adjacent RGO. The corresponding morphologies of RGO in PCG composites with high RGO loading were shown in Fig. 9f–j as well as Fig. S16. Moreover, the coalescing of microcracks between adjacent RGO or adjacent graphene layers may facilitate the propagation of the primary crack, which is detrimental for resisting the impact. The morphological evolution of RGO in the cross section near the impact fracture surface and notch tip, as shown in Fig. 10, also
indicated the positive effect of low loading RGO on enhancing the impact toughness and reducing the notch sensitivity of PC matrix. In the case filled with low loading RGO, the RGO could induce microcrack, crack pinning and crack arresting (Fig. 10b). Moreover, the as-pinned crack by RGO could be terminated by adjacent RGO. This can prevent the propagation of crack at the utmost and thus enhance the impact toughness of PC matrix. On the other hand, the 2D RGO near the notch could be act as stress concentrator and reduce the impact stress, resulting in the change in stress state at the notch from plane strain to plane stress and the failure mechanism changing into shearing from crazing. This can greatly enhance the resistance of PC matrix to the notch. However, in the case of composites filled with high loading RGO, as shown in Fig. 10c, the 2D RGO could not induce effective morphological evolution like crack pinning and crack arresting that can absorb impact energy. This is the result of the agglomeration of RGO and the decreased distance between adjacent RGO, which is beneficial to the propagation of crack. So the crack pinning and crack arresting induced by RGO disappeared in the high loading case, leading to the decrease of toughness in PC matrix. Moreover, the microcracks induced by RGO will be coalesced due to the decrease of the distance between adjacent RGO. This also facilitates the propagation of the primary crack and is detrimental for resisting the impact. So the existence of RGO in PCG composites filled with high
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Fig. 7. SEM images of impacted fracture surface of a) neat PC, b) PCG-0.07, c) PCG-0.1, d) PCG-0.3, e) PCG-1 and f) PCG-3, respectively. The white arrow indicates the impact direction. In all cases scale bar are 2 mm.
Fig. 8. SEM images of impacted fracture surface near the notch tip of a) neat PC, b) PCG-0.03, c) PCG-0.07, d) PCG-0.1, e) PCG-0.3, f) PCG-0.7, g) PCG-1 and h) PCG-3, respectively. The white arrow indicates the impact direction. In all cases scale bar are 500 lm.
Table 2 Summary of the area initiation rough and smooth region in neat PC and PCG composites. Samples
Total area S (lm2)
S1 (lm2)
S2 (lm2)
Increment of S2 compared with neat PC (%)
Increment of notched impact strength compared with neat PC (%)
Neat PC PCG-0.03 PCG-0.07 PCG-0.1 PCG-0.3 PCG-0.7
2.66 105 (3%a) 2.67 105(3%) 3.19 105(3%) 2.08 105(2%) 1.45 105 (2%) 8.84 104(2%)
1.00 105(3%) 7.90 104(5%) 6.44 104(7%) 5.06 104(3%) 5.28 104(2%) 2.96 104(6%)
1.65 105(2%) 1.88 105(2%) 2.54 105(2%) 1.57 104(2%) 9.22 104(2%) 5.88 104(1%)
/ 14 54 5 44 65
/ 18 46 5 31 53
Note: aThe ratio of standard error with the averaged value.
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Fig. 9. SEM images of morphology of RGO on the impacted fracture surface of a), b) c), d) and e) are PCG-0.07 composites. f), g), h), i) and j) are PCG-1 composites, respectively. In all cases scale bar are 5 lm.
Fig. 10. SEM images of morphology in the cross section near the impact fracture surface and notch tip; a) neat PC b) PCG-0.07 and c) PCG-1 composites. In all cases scale bar are 5 lm.
loading RGO could not play the role of improving the notched impact toughness and notch sensitivity of PC matrix. The above results show that the 2D RGO under low loading can enhance the impact toughness and reduce the notch sensitivity of PC matrix. During the notched impact process, layer breakage and pulling out of graphene layers, microcrack, crack deflection, crack pinning, crack arresting induced by RGO are considered as the main toughening mechanism in PCG composites in this study (Fig. S18). This toughening mechanism is very different from other fillers toughened PC matrix (Table S3). Apart from the microcrack (nanocavities or void), crack pinning, crack arresting, layer breakage and pulling out was observed and proposed as the toughening mechanism, which was absent in other filler-toughened PC systems. 4. Conclusions Herein, reduced graphene oxide was incorporated into PC matrix through facile melt compounding, to fabricate advanced PC nanocomposites. The low loading RGO (<0.1% wt) can enhance the fracture toughness, strength and notch resistance of PC matrix greatly. Microcrack, caused by the layer breakage and pulling out of RGO, was the main toughening mechanism resulting in the greatly improved fracture toughness and notch resistance. Apart from microcrack graphene-induced crack deflection, crack pining and crack arresting during the notch-fractured process were also positive factors contributing to enhanced notch-fracture toughness and notch resistance. The crack deflection, crack pinning and crack arresting was the result of shearing deformation during the impact
process, while microcrack is the result of tensile deformation. This paper gives us a deep understanding in the toughening mechanism in polymers toughened with graphene-type materials. Moreover, these PCG composites with high notch resistance, balanced toughness and strength fabricated without any solvent will make the large-scale manufactured graphene-type materials more competitive in the nanoscience and chemical engineering part, and may open the door to downstream industrial and green application of graphene-type materials.
Acknowledgements Financial support of the National Natural Science Foundation of China (51273132, 51573118, 51227802 and 51603131), Program for New Century Excellent Talents in University (NCET-13-0392), the Sichuan Province Youth Science Fund (2015JQ0015), the Sichuan Province Science Fund (2016JY0190), the China Postdoctoral Science Foundation (2015M572473) and the Open Research Subject of Key Laboratory of automobile high performance materials and forming technology (szjj2016-088) are gratefully acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.05.090.
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