Hindawi Journal of Nanomaterials Volume 2018, Article ID 1047985, 9 pages https://doi.org/10.1155/2018/1047985
Research Article In Situ Polymerization of Nylon 66/Reduced Graphene Oxide Nanocomposites Xiaochao Duan, Bin Yu, Tonghui Yang, Yanpeng Wu, Hao Yu , and Tao Huang State Key Lab for Modiﬁcation of Chemical Fibers & Polymer Materials, College of Materials Science & Engineering, Donghua University, No. 2999 North Renmin Road, Songjiang District, Shanghai 201620, China Correspondence should be addressed to Hao Yu; [email protected]
cn and Tao Huang; [email protected]
Received 25 March 2018; Revised 12 June 2018; Accepted 21 June 2018; Published 13 September 2018 Academic Editor: Stefano Bellucci Copyright © 2018 Xiaochao Duan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A one-step method of in situ polymerization of nylon 66/reduced graphene oxide (PA66/rGO) nanocomposites is ﬁrst proposed, simply by introducing graphene oxide (GO) into PA66 salt with the existence of ammonium hydroxide. The GO is prereduced by the ammonium hydroxide at an early stage of the polymerization process and then grafted on the PA66 chains, accompanied with the thermal reduction of GO. The PA66 chains were grafted onto the GO nanosheets through the condensation between the oxygen-containing functional groups of the GO and the terminal amino ends of the PA66 chains. The eﬀect of GO on the mechanical properties, especially tensile strength, of nanocomposites was investigated. The results revealed that the incorporation of a very small amount (about 1 wt%) of GO caused a signiﬁcant improvement in ultimate tensile strength (about 17%). The SEM of the fracture surface of composites indicated a good dispersion of rGO in the matrix. Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscope (SEM), Fourier transformed infrared spectroscopy (FTIR), and XRD patterns of rGO, which was isolated from nanocomposites, revealed that the GO nanolayers were simultaneously reduced and PA66 chains were grafted on the rGO nanosheet during the polymerization process. The rGO grafted with the PA66 chain increases its compatibility in the PA66 matrix and eﬀectively enhanced the interfacial energy of the composites.
1. Introduction Polyamide 66 (nylon 66, PA66), a sort of thermoplastic polymer, contains an amide repeat unit on the main chain, which is generally used as important engineering plastics and industrial ﬁber on account of its light weight, good mechanical properties, high abrasive resistance, excellent chemical resistance, and relatively low cost [1–5]. Along with the fast development of the modern manufacturing industry, the manufacturers and customers have a high requirement on the performance of PA66 products; however, the unmodiﬁed PA66 cannot completely meet the demand of the emerging market. To enhance its performance and expand its ﬁeld of application, various PA66 composites modiﬁed with nanoﬁllers have been developed in recent years, such as the carbon nanotube [6, 7], nanoclay [1, 8–10], and nanoparticles [11, 12]. Graphene has attracted tremendous attention because of its unique structure and remarkable chemical and physical
properties [13–17], which has been widely used in a variety of ﬁelds, such as photoelectricity , biomedicine [19, 20], sensors , electronics [22, 23], and supercapacitors [24, 25]. Graphene oxide (GO) is one of the most important derivatives of graphene, consisting of a layered structure with a hexagonal ring of carbon network and oxygen functional groups bearing on the basal planes and edges, and can be well dispersed in multiple polar mediums [15, 26, 27]. These properties make them ideal candidates for modifying agents for polymers. Up to now, there are a great number of researchers who prepare nylon/graphene or nylon/GO nanocomposites through melt blending , solvent evaporation , or in situ copolymerization [30–32], using PA6, PA12, and other classes of nylon, with some of them pushed to industrialization. However, almost no PA66/graphene composites were reported owing to the diﬃculty of in situ polymerization of PA66 and the poor dispersion of graphene in the PA66 matrix. As matrix materials, PA66 has a lot of advantages compared with PA6; for example, PA66 has better mechanical properties
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(NH3.H2O) Weak reductant
CO OH COOH
Dissolution in formic acid
Figure 1: Synthesis of PA66/rGO nanocomposites by in situ polymerization with simultaneous thermal reduction from GO to rGO.
and abrasive resistance [2, 4, 5]. Simultaneously, the poor dispersion and weak interface interaction between graphene and the polymer matrices seem to be a bottleneck of direct blending. Thus, some researchers modiﬁed the surface of graphene oxide to enhance the interface interaction between graphene and the polymer so as to improve the dispersion of graphene in the polymer matrix. Gong et al.  functionalized GO with PVA to improve the dispersion of graphene in the PA6 matrix. Wang et al.  prepared PA6/graphene using sulfonated graphene with polar sulfonic acid groups as a precursor by in situ hydrolytic ring-opening polymerization of ε-carprolactam. However, for the polymerization process of PA66, the abovementioned methods of GO modiﬁcation were invalid for improving the dispersion of GO in the PA66 matrix because of the unique polymerization conditions. Although GO are well dispersed in water, it cannot disperse homogeneously in a PA66 salt solution and need to be prereduced because the superabundant carboxylic acid groups on its surface disequilibrate the carboxyl groups and amido groups. Herein, we ﬁrst report an eﬀective one-step approach of preparing PA66/rGO nanocomposites on the basis of in situ polymerization of carboxylated GO and PA66, during which GO were simultaneously reduced into rGO. Meanwhile, the weak reductant, ammonia, was added to the system to assist the reduction of GO under mild conditions (110°C). f-rGO (rGO grafting with PA66 chains) sheets were homogenously dispersed in nanocomposites due to the high content (up to 40%) of grafted PA66 chains onto rGO sheets.
dosage of 0.25 wt% (PA66-rGO-0.25) was described as follows: graphene oxide (3.75 g) was homogeneously dispersed in 750 ml deionized water with a strong ultrasonic treatment for about 12 h, followed by the introduction of 3000 g of PA66 salt solution with a concentration of 50 wt% and 48.75 g of ammonium hydroxide into the above GO aqueous dispersion (the mass ratio of ammonium hydroxide and graphite oxide was 13 : 1). After another ultrasonic treatment and mechanical stirring for 2 h, the mixtures were put into a polymerization reactor (5 L). The air in the reactor was replaced by evacuation and nitrogen injection for at least 3 times before heat up. After that, the mixtures were heated at 110°C for 5 h with a high pressure of 1.7 MPa. Then, the temperature was raised to 190°C and maintained for 5 h with a steady pressure. Finally, the mixtures were heated at 220°C for 3 h and then at 280°C for 3 h with steady stirring. A series of PA66/rGO nanocomposites that included PA66-rGO0.50, PA66-rGO-0.75, and PA66-rGO-1 with diﬀerent GO dosages was prepared using the same technique.
2.4. Characterization. The FTIR characteristic absorption of GO and f-rGO samples was analyzed with a Nicolet-9600 Fourier-transform infrared spectrometer (FTIR) (USA). The scanning rate was 20 min−1, and the resolution was 4 cm−1. The Raman spectrum of GO and f-rGO samples was investigated by Raman spectra analysis. Raman spectra were recorded from 100 to 3200 cm−1 on a Renishaw inViaReﬂex Raman Microprobe (Britain) using a 532 nm argon ion laser. Thermogravimetric analysis (TGA) was used to characterize the grafting ratio of PA66 on the f-rGO surface. It was performed on a TG 209 F1 from Netzsch (Germany) under a nitrogen atmosphere with a ﬂow rate of 20 ml/min. The granulated samples of about 4 mg were heated from
2.1. Materials. Graphite oxide powder was obtained from The Sixth Element (Changzhou) Materials Technology Co. Ltd. The PA66 salt was purchased from BASF SE. All the reagents used in this study were acquired from commercial sources. 2.2. Polymerization of Composites of PA66 and Graphene Oxide. As illustrated in Figure 1, the eﬀective method of preparing PA66/rGO nanocomposites was based on in situ polymerization of carboxylate GO and PA66, accompanied with the thermal reduction process of GO. The typical procedure to prepare PA66/rGO nanocomposites with the GO
2.3. The Collection of f-rGO from PA66/rGO Nanocomposites. As illustrated in Figure 1, the f-rGO (rGO grafting with PA66 chains) were separated from PA66/rGO nanocomposites by ﬁltering the formic acid solution of the nanocomposites, through repeatedly washing by formic acid and ethanol, then drying at 85°C in vacuum overnight. The f-rGO-0.25, f-rGO-0.5, f-rGO-0.75, and f-rGO-1 samples were separated from PA66-rGO-0.25, PA66-rGO-0.5, PA66-rGO-0.75, and PA66-rGO-1, respectively.
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Figure 2: The FETEM and FESEM images of GO and f-rGO: (a) GO TEM image; (b) GO SEM image; (c) f-rGO-0.5 SEM image.
ambient temperature to approximately 700°C at a heating rate of 10°C/min. Diﬀerential scanning calorimetry (DSC) was performed on a Netzsch DSC 204 F1 (Germany). The dried samples under vacuum were ﬁrst heated to 290°C at a rate of 20°C/min and held for 3 min to completely remove the previous thermal history, then cooled down to room temperature and ﬁnally heated to 290°C with cooling and heating rates of 10°C/min. The X-ray diﬀraction (XRD) analysis of f-rGO and PA66/rGO nanocomposites were performed on an HD-D/max-2550VB+/PC X-ray diﬀractometer (Japan). XRD data was collected from 5° to 90°. The morphologies of the cryogenically fractured surface and f-rGO surface were observed with a ﬁeld emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan), ﬁeld emission transmission electron microscope (FETEM, JEM-2100F, JEOL, Japan), and atomic force microscope (AFM, 5500, Agilent, USA). All the samples for mechanical property tests were prepared by injection molding. Tensile strength and elongation at break were measured according to GB/T 1040.2-2006 and the strain rate was 20 mm/min. The Charpy impact notched strength was measured according to CB/T 1040.2-2008. Mechanical testing was performed on an electronic universal testing machine (20 kN/WDW3020). Impact resistance tests were performed using a Pendulum Impact Testing Machine (XJJUD-50Q/ XJJUD-50Q). 2.5. The Viscosity Number (VN). Firstly, f-rGO was removed from PA66/rGO nanocomposites by ﬁltering the formic acid solution of the nanocomposites. After that, PA66 was precipitated with deionized water from the ﬁltrate, through repeated washing by deionized water with ultrasound until the pH of the wash solution was neutral, then dried at 85°C in vacuum overnight using PA66-0.25, PA66-0.5, PA660.75, and PA66-1, respectively. The viscosity number (VN) reﬂects the molecular weight of PA66 and its nanocomposites. It was measured with an Ubbelohde viscometer at 25°C by dissolving the samples in 90% formic acid with a concentration of 5 mg/ml according to ISO 307: 2007.
3. Results and Discussion As illustrated in Figure 1, the in situ synthesis of PA66/rGO nanocomposites by a one-step process was introduced. Firstly, the GO nanosheets were dispersed in PA66 salt
20 nm Length = 1.01 휇m, Pt = 2.87 nm, and scale = 5.00 nm
(nm) 2 1.5 1 0.5 0 −0.5 −1 −1.5 −2 −2.5 0
Figure 3: The AFM images of GO.
solution with ammonium hydroxide. Then, the GO nanosheets were partially reduced after prereduction at 110°C for 5 h. With the polycondensation process of PA66, the PA66 chains were successfully grafted onto the GO surface and GO were simultaneously thermally reduced into r-GO. 3.1. Covalent Grafting of PA66 Chains onto rGO Sheets. The FETEM and FESEM images of the GO and f-rGO are shown in Figure 2. We can see that the as-prepared GO exhibits a smooth surface while the surface of f-rGO shows a rough surface, which indicates that the PA66 chains were successfully grafted onto the GO surface. We can ﬁnd that the thickness of the GO sheets was under 1.5 nm in the AFM tapping mode in Figure 3. The FTIR spectra of the GO and f-rGO sheets are shown in Figure 4. The new broad bands that emerged at 1630, 1542, and 1425 cm−1 of the f-rGO samples corresponded to the stretching vibration of the C=O, N-H, and C-N of the
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a b c d 1798
f-rGO-1 f-rGO-0.75 f-rGO-0.5
CH2 2924 3431 3750 3000 2250 1500 Wave number (cm−1)
Figure 4: The FTIR curves: (a) GO; (b) f-rGO-0.25; (c) f-rGO-0.5; (d) f-rGO-0.75; (e) f-rGO-1.
30 40 2휃 (degree)
Figure 6: The XRD pattern of GO and f-rGO.
Table 1: The XRD data of peak position, area percent, and peak width of GO and f-rGO.
90 Mass (%)
11.18 2θ ( ) Area 94.55 percent (%) Peak width 1.28
f-rGO-0.25 f-rGO-0.5 f-rGO-0.75 f-rGO-1 11.18
Temperature (°C) GO f-rGO-0.25 f-rGO-0.5
Figure 5: The TGA curves of GO and f-rGO.
amide groups’ functionality. The stronger bands at 2854 and 2924 cm−1 were due to the C-H stretching vibrations of the grafted PA66 chains. We can also ﬁnd that the f-rGO samples showed a stronger water vibration peak at 3432 and 1093 cm−1 compared with GO. This is because of the strong water-absorbing ability of the PA66 chains. All of the abovementioned characteristic peaks conﬁrmed that the PA66 chains were grafted onto the GO nanosheets. The thermal stability of the GO and f-rGO sheets are shown in Figure 5. It was very obvious that the thermal stability of f-rGO was much better than GO. We can see that GO displayed a main weight loss (about 36%) in the range of 120–320°C which was due to the removal of the oxygenic functional groups of epoxy and hydroxyl and decarboxylation of the carboxyl groups. The weight loss below 120°C was a result from the evaporation of the water that existed in the GO. On the contrary, for f-rGO samples, a wellmarked weight loss was observed in the temperature range of 320–510°C, which was attributed to the decomposition of PA66 molecules grafted onto the GO surface. Interestingly, the weight loss curve of f-rGO samples were essentially unchanged below 300°C. It kept thermal stability up to 320°C, and this indicated that the unstable functional groups
of GO material had been translated into stable bonds or decomposed during the high-temperature polycondensation. In other words, GO had been reduced to rGO during the polymerization process, which was in accordance with the EDS measurement (Figure S1). The XRD analysis was further used to characterize GO and f-rGO. The typical patterns are shown in Figure 6. The feature diﬀraction peak at 11.2° of GO was detected due to the oxygen-contained groups on the graphite sheets which increased the interlayer distance of the graphene oxide sheets. Compared with GO, it is obvious that the peak area of 11.18° diminished after thermal reduction in the polymerization process (Table 1). These results indicated the GO was partly reduced to rGO. The new peak at 22.56° demonstrated that PA66 chains were successfully grafted onto the f-rGO surface. For the f-rGO-1 sample, the peak at 22.56° became broader and the peak at approximately 10° shifted to lower 2θ angles (10.26°) which indicated that a broader interlamellar spacing was achieved due to more PA66 chains being inserted into the interlayer of GO sheets with the increase of GO contents. Raman spectroscopy is a nondestructive technique that is extensively used to obtain structural information such as defects and the disorder of graphene samples. A typical Raman spectrum would show two characteristic peaks close to 1350 and 1580 cm−1 corresponding to the D band that comes from the structural imperfections created by the attachment of oxygenated groups on the carbon basal plane and the G band that originates from the ﬁrst-order scattering from the doubly degenerated E2g phonon modes of graphite in the Brillouin zone center as well as bond stretching of sp2 carbon pairs in both rings and chains. Generally, the
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1250 1500 Raman shift (cm−1) rGO-0.75 GO rGO-0.25 rGO-1.0 rGO-0.50
Figure 7: The Raman spectrum of GO and f-rGO.
Table 2: Viscosity number (VN) of bare PA66 and PA66/rGO. Sample
VN RV (ASTM)
integrated intensity ratio of the D and G bands (I D /I G ) represents the oxidation degree and the size of sp2 ring clusters in a sp3/sp2 hybrid network of carbon atoms and it increases along with the increase of defects on the graphene nanosheets. Figure 7 shows the Raman spectra of GO and f-rGO samples. The G bands of f-rGO samples shifted to lower frequencies at 1587 cm−1 (red shift) compared to those of GO and were closer to a single-graphene graphite’s in other reports (1580 cm−1) , which means that the f-rGO structure regresses to graphite from defects. This result was consistent with another report that the G bands of single graphene were at 1587 cm−1 and displayed red shifts owing to an increase in layers. Table S1 indicated the ID/IG and the full width at half maximum (FWHM) of the D and G bands. A notable fact is that the ID/IG of f-rGO0.25 decreased obviously compared with GO, which indicated a slight restoration of the sp2 hybrid for it on the polymerization process. 3.2. Characterizations and Properties of PA66/rGO Nanocomposites. The viscosity number (VN) of bare PA66 and PA66 separated from PA66/rGO nanocomposites were obtained through the Ubbelohde viscometer method. It was obvious that the VN of PA66 decreased with the increase of the GO content, which indicated that the molecular weight of PA66 was decreased, as shown in Table 2. This is because the additional carboxyl groups on the GO sheets broke the stoichiometric ratio of carboxyl groups and ammonia groups in the reaction system. Figure 8 gives the melting endotherm graphs of neat PA66 and PA66/rGO nanocomposites during the second heating cycle. The summary of the DSC data corresponding to the curves for the heating scan are tabulated in Table 3.
The data indicated that with the increase of GO loading, the melting peaks gradually decreased in comparison to a neat PA66. The crystallinity (X c ) of the nanocomposites was slightly increased compared with the neat PA66. It was obvious that the pure PA66 in Figure 8(a) displayed only one melting peak and the PA66/rGO nanocomposites exhibited two melting peaks. According to the report of Lin et al. , the pure PA66 showed only one melting temperature (T mI ) at around 260°C, which is the temperature found for Form I or the α-type peak . The second melting temperature (T mII ) detected was at around 252°C which was attributed to Form II or the γ-type peak. Previous studies on PA6/ graphene nanocomposites had shown that the presence of graphene promoted the growth of the γ crystal phase of PA6 [29, 36–39]. It appeared that the introduction of graphene into PA66 also increased the amount of γ-form crystals and hindered the formation of perfect α-nylon crystals during the heating cycle of DSC measurements. Figure 8(b) shows the crystallization exotherm curves of the samples during the cooling cycle. Pure PA66 had a wide crystallization peak with a crystallization temperature (T c ) peak at 210.3°C. The nanocomposites with graphene had a much higher T c , which was about 17°C higher than that of the pure polymer, and also the crystallization peaks of the nanocomposites were narrower. This suggested that the graphene acted as a nucleation agent and increased the crystallization rate of the nanocomposites. The evidence for the thermal stability of PA66 and PA66/ rGO nanocomposites was oﬀered by TGA (Figure 9). For TGA curves of PA66 and PA66/rGO nanocomposites, there was no signiﬁcant diﬀerence between them, except for a slight increase of the decomposition temperature at the maximum GO loadings. Figure 10 shows the XRD patterns of bare PA66 and PA66/rGO nanocomposites. It was found that the PA66 diffraction peaks appeared at around 2θ = 20.5° and 22.9°, and reﬂections of the α1- and α2-form of the PA66 crystal  did not change positions signiﬁcantly, but the intensities were diﬀerent. More surprisingly, a new diﬀraction peak appeared at around 13.1° in PA66/rGO nanocomposites and became stronger and stronger with the increase of graphene loadings which corresponded to the γ1 form. For the PA-rGO-1 sample, a new peak appeared at around 21.8° which corresponded to the γ2 form, suggesting that the addition of graphene caused the change in the crystalline phase of PA66 from the α to γ phase because the proximity of the surface of layers results in conformation changes of chains . The crystallinity (X c ) of the nanocomposites (Table S2) was also a slightly increased compared with the neat PA66, which was in accordance with the CDSC measurement. 3.3. Graphene Dispersion in PA66/rGO Nanocomposites and Its Mechanical Properties. Figure 11 presents the yield strength and Young’s modulus of PA66 with its graphene nanocomposites being a function of rGO loading and the error bars representing the standard deviation. Clearly, incorporation of GO nanosheets resulted in a signiﬁcant enhancement of the mechanical strength and Young’s modulus. As the rGO loadings reached 1.0 wt% and 0.75 wt%,
Heating flowing (a.u.) exo down
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Heating flowing (a.u.) exo down
PA-rGO-1 PA-rGO-0.25 PA-rGO-0.5 PA-rGO-0.25 PA
250 260 Temperature (°C)
PA-rGO-1 PA-rGO-0.75 PA-rGO-0.5 PA-rGO-0.25 PA
220 230 240 Temperature (°C)
Figure 8: The DSC characterization of PA66 and PA66/rGO nanocomposites: (a) the DSC curve of the heating curve; (b) cooling curve.
Table 3: Degree of crystallinity of PA66 and PA66/rGO samples from DSC data. PA-rGO- PA-rGO- PA-rGO- PA0.25 0.5 0.75 rGO-1
T mI (°C) 261.06 ° — T mII ( C) Crystallinity (%) 33.7
261.86 251.26 34.2
260.76 250.76 34.7
261.01 250.96 38.7
259.46 249.71 35.8
PA66 PA66-rGO-0.25 PA66-rGO-0.5
Figure 10: The XRD pattern of PA66 and PA66/rGO nanocomposites.
80 Mass (%)
60 40 20
450 Temperature (°C)
300 400 500 Temperature (°C)
PA66 PA66-rGO-0.25 PA66-rGO-0.5
Figure 9: The nanocomposites.
the yield strength and modulus reached the maximum values, respectively. The yield strength (84 MPa) and Young’s modulus (660 MPa) were enhanced by 17% and 6.5%, respectively. Moreover, the addition of GO does not have an obvious inﬂuence on the impact strength of PA-rGO nanocomposites compared with PA66 samples according to Figure 11(b). These enhancements were attributed to the homogeneous dispersion of f-rGO nanosheets in the polymer matrices and the strong interfacial adhesion between them. As we can see from the cross-sectional image of the PA66/rGO in Figure 12, a smooth-fractured surface and no aggregations of the rGO
sheets could be found which indicated that the PA66 chains that grafted rGO onto the nanosheets were well dispersed in PA66/rGO composites.
4. Conclusion In summary, we prepared PA66/rGO nanocomposites by a one-step process of in situ polymerization of PA66 salt in the presence of graphene oxide. By a condensation reaction between the carboxylic acid groups on the GO surface and terminal amino ends of PA66 chains, the macromolecular chains of PA66 were eﬀectively grafted onto GO nanosheets, simultaneously accompanying the thermal reduction from GO to rGO. The grafted rGO nanosheets showed good compatibility and strong interfacial interaction with the PA66 matrix, which was the key factor for the improvement of the mechanical properties of the PA66/rGO nanocomposites. The Young’s modulus and tensile strength of the nanocomposites were improved to 660 MPa and 84 MPa, respectively, which oﬀered great promises for a wider application of the PA66 materials. The in situ condensation polymerization we brought opened a new avenue to
Yield strength (MPa)
Impact strength (kJ/m2)
Young’s modulus (MPa)
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12 8 4 0
0.25 0.50 0.75 Filler loading (%)
0.25 0.50 0.75 Filler loading (%)
Figure 11: The mechanical properties of PA66 and PA66/rGO nanocomposites: (a) tensile strength; (b) impact strength. Error bars represent the standard deviations.
Figure 12: The SEM photographs of the cross sections of PA66/rGO nanocomposites: (a) PA66-rGO-0.25; (b) PA66-rGO-0.5; (c) PA66rGO-0.75; (d) PA66-rGO-1.
fabricate graphene-based PA66 nanocomposites of condensation polymers that were scalable and eﬀective for more extensive applications.
Conflicts of Interest
The ﬁgures and tables data used to support the ﬁndings of this study are included within the article and the supplementary information ﬁle.
This work was ﬁnancially supported by the National Key Research and Development Program of China (no. 2016YFB0303000).
There are no conﬂicts to declare.
Supplementary Materials EDS of the SEM photographs of GO and f-rGO-1 (Figure S1); Raman data containing ID/IG peak ratios of graphene (Table S1); degree of crystallinity of PA66 and PA-rGO samples from XRD data through the Jade software (Table S2). (Supplementary Materials)
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