Intercalation Structure and Enhanced Thermal Oxidative Stability of

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Intercalation Structure and Enhancing Thermal Oxidative Stability of Polyamide 6/Graphene Nanocomposites Prepared through in situ Polymerization Ruiguang Li, Kaihua Shi, Lin Ye, and Guangxian Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03293 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Industrial & Engineering Chemistry Research

Intercalation Structure and Enhancing Thermal Oxidative Stability of Polyamide 6/Graphene Nano-composites Prepared through in situ Polymerization Ruiguang Li Kaihua Shi Lin Ye* Guangxian Li State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute of Sichuan University, Chengdu, China *Corresponding author. Tel.: 862885408802; E-mail address: [email protected] Abstract: Polyamide 6 (PA6)/graphene oxide (GO) nano-composites were prepared through in situ polymerization method. GO layers were partially reduced, exfoliated by intercalation of PA6 molecules with high intercalation ratio, and dispersed uniformly in the matrix without obvious aggregation. The crystalline form of PA6 transformed from γ-form to α-form by addition of GO, and the network structure centering on GO formed through intermolecular interaction between the two phases. During thermo-aging process, compared with pure PA6, the reduced viscosity and tensile strength of the composite kept a high level, and the carbonyl index increased much more slowly. In addition, the degradation temperature increased, the degradation rate decreased, and the activation energy changed slightly, indicating the enhanced thermal oxidative stability of PA6. With increasing GO content, the oxygen permeability coefficient decreased significantly, and the radical scavenging ratio increased, which was favourable for inhibiting the oxidative degradation of PA6 molecules. Keywords: Polyamide 6 (PA6); Graphene oxide (GO); In situ Polymerization; Intercalation structure; Thermal stabilizing mechanism 1. Introduction Polyamide 6 (PA6) is a prominent engineering thermoplastic and widely applied because of its superior mechanical performances, self-lubrication property, etc.1,

2

Generally, the weakest C-H bond on the methylene group adjacent to amino-group led to its oxidation reaction when exposed to heat, light, oxygen, resulting in the serious 1

ACS Paragon Plus Environment


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destruction of chemical structures and physical properties, and thus shortening the service life of PA6.3-10 Graphene, as a typical two-dimensional material with one-atom-thick planar sheet of sp2 carbon atoms arranged in a hexagonal lattice, has drawn great attention due to its special properties, like outstanding mechanical strength, electrical and thermal performances.11-15 Moreover, it also possesses a large specific surface area (about 2600 m2/g), which can hinder the diffusing of oxygen into the polymer matrix and spreading outward of the degradation products. In the meantime, it has high electron affinity to capture free radicals generated by molecular degradation.16 Therefore, graphene has a unique stabilizing effect on polymers. M. Tang et al.17 found that graphene is an excellent antioxidant for inhibiting styrene-butadiene rubber (SBR) from thermo-oxidation, attributing to its free-radical scavenging and gas barrier abilities. J. Yang et al.18 investigated the thermo-stabilizing effect of the reduced graphene oxide (rGO) on polypropylene (PP) and suggested that the degradation of the matrix was inhibited remarkably by GO and the enhanced thermo-stability of the composites resulted from the decrease of peroxy radicals concentration and oxygen permeability. As for polyamide materials, C. Li et al.19 prepared MC PA6/ grapheme-polysiloxane nano-composite via in situ polymerization and

studied

the

synergistic

reinforcing

and

friction-reducing

effect

of

grapheme-polysiloxane on PA6. But few literatures investigated the thermal oxidative stabilizing effect of graphene on PA6. Graphene oxide (GO) is an important derivative of grapheme with some oxygen functional groups on its surface, such as hydroxyl, epoxy, carboxyl, etc.20-22 These oxygen groups on GO surface make them compatible with PA6 by formation of hydrogen bonds. In this work, GO was first dispersed in caprolactam monomer by 2


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sonication, and then through in situ polymerization method, the well dispersion of GO in PA6 matrix was achieved. On the other hand, under the polymerization conditions with high temperature and high polarity, GO can be partially reduced, so as to obtain the PA6/graphene nano-composites. The layer structure, thermal oxidative stability and thermal stabilizing mechanism of the composites were investigated. 2. Experimental 2.1 Materials Caprolactam (CL) with a commercial grade product was provided by China Petroleum and Chemical Co. Ltd., 6-aminocaproic acid with analytical purity was bought from Huaxia Reagent Co., Ltd. (Chengdu, China). Graphene oxide (GO) with micron grade particle size was provided by the Sixth Element Technology Co. Ltd. (Changzhou, China). 2.2 Synthesis of PA6/GO nano-composites Firstly, a certain quantity of caprolactam (CL) monomer, 6-aminocaproic acid and grapheme oxide (GO) powders was added into a beaker and heated to 80 °C. After melting, the mixture was sonicated for 1 h, and then heated to about 180 °C for prepolymerization under N2 for 1h. Afterwards the melt was heated to 250 °C and reacted for 9h. With the proceeding of reaction, the viscosity of the melt gradually increased and the color changed from brown to black. The resulting composites were cooled to room temperature and subjected to mechanical grinding. 2.3 Measurements 2.3.1 FTIR analysis The analysis of GO structure was conducted with a Nicolet-560 Fourier-transform infrared (FTIR) spectrometer (U.S.A) under the scanning rate of 20 min-1, and the resolution of 4 cm-1. 3



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2.3.2 Raman spectra analysis The GO skeletal structures were analyzed with a RENISHAW Invia Raman microscope (UK) by using Laser excitation source at 532 nm. 2.3.3 XPS analysis The XPS analysis of GO samples was conducted with a XSAM 800 spectrometer (KRATOS Co., UK) with AlKa radiation (1486.6 eV) at a pressure of 2.0 ×10-7 Pa. 2.3.4 TGA analysis TGA analysis of GO and PA6/GO nano-composite samples was conducted with a TA2950 thermobalance from TA Co. (USA) under air atmosphere. The sample was heated from room temperature to about 800 °C under a heating rate of 10 °C/min. 2.3.5 X-ray diffraction (XRD) analysis The XRD analysis of GO samples was conducted with a Rigaku D/max ⅢB X-ray diffractometer (Japan) in range of 5° - 40° by using Cu Kα radiation (λ=0.154 nm). The accelerating voltage and current were 40 kV and 40 mA, respectively. The d-spacing of sample layers was obtained with Bragg equation: 2dsin θ = n λ

(1)

where θ is the diffraction angle ; n is the diffraction order, and λ is the wavelength. 2.3.6 Transmission electron microscopy (TEM) analysis TEM analysis of the composite samples were conducted with a JEOL JEM 100CX II TEM equipment (Japan) at the voltage of 200 kV. The tested TEM samples of the composites were prepared by thin sections under cryogenic conditions. 2.3.7 Dynamic rheological analysis Dynamic rheological analysis of the composites was conducted by using a Gemini 200 dynamic rheometer from Bohlin Co. (UK). The dynamic frequency scanning experiment was carried out with parallel plate mode over the frequency range of 4


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0.01~ 100 Hz at the temperature of 250oC and the strain value of 1%. 2.3.8 Reduced viscosity The composite samples of about 0.5 g were dissolved in formic acid (100 mL). The outflow time of the solution was tested with an Ubbelohde viscometer at 25 ºC, in accordance with ISO 307–1984. The reduced viscosity could be obtained with equation (2):

η=（

− 1） ×

(2)

where t is the outflow time of sample solution (s), t0 is the outflow time of the solvent (s) and C1 is the concentration of sample solution (g·mL-1). 2.3.9 Mechanical properties The tensile properties of the composites were tested with a 4302 material testing machine from Instron Co. (U.S.A), in accordance with ISO 527-1993. The tensile speed was 10 mm/min, while the dried dumbbell specimens were prepared. 2.3.10 In situ FTIR analysis FTIR analysis of the composites was conducted with the Nicolet 560 instrument (Nicol, American). The scanning was conducted for 32 times with the resolution of 4 cm-1 and the wavelength ranging from 400 to 4,000 cm-1. The in situ thermo-aging experiment was conducted at 150 oC for 3 h in the sample cell by collecting the spectra per 2 min. 2.3.11 O2 permeability analysis The O2 permeability analysis was conducted with gas permeation device (VAC-V1) from Labthink Instrument Co.(Jinan, China). The device included an upper chamber, where an atmospheric pressure was maintained, and a lower chamber, vacuumed before test. The sample was fixed in a window between the two chambers. The test temperature was 40 oC. The oxygen permeability coefficient (PO2) can be 5



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obtained with the following equation in accordance with GB/T1038-2000:

=

× ×

×

(3)

where ∆P/∆t is the average value of pressure change per unit time; V is the volume of low-pressure chamber; S is the testing area; T is the testing temperature; T0 is 273 K; P0 is 1.01×105 Pa; D is the sample thickness; (P1-P2) is the pressure difference between the upper chamber and lower chamber. The schematic illustration of the gas permeation measurement device was shown in Figure 1.

Figure 1. The schematic illustration of the gas permeation device

2.3.12 DPPH free radical scavenging rate 3 mL of DPPH solution (0.1 mM in methanol) was incubated with 1.0 mL of the samples in DMF with different GO concentrations (10~160 µg/mL). After 30 min of incubation, the solution was tested at 517 nm with a Alpha-1860 UV-vis spectrophotometer and termed as A1. The scavenging activity was obtained with the 6


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following equation:

Scavenging activity% = 100 −

' ' ×(( '

（4）

where A0 is the absorbance value of methanol & graphene solution in DMF (1.0 mL). The absorbance value of DPPH solution in methanol (3.0 mL) & DMF (1.0 mL) is denoted as A2. 3. Results and discussion 3.1 Synthesis and structure of PA6/GO nano-composites Based on the industrialized graphene oxide (GO) product, PA6/GO composites were synthesized by in situ polymerization through the mechanism of cationic ring-opening polymerization, with 6-aminocaproic acid as the catalyst, as illustrated in Figure 2.

Figure 2. Synthesis of PA6 and PA6/GO composites by ring-opening polymerization of caprolactam

3.1.1 Intercalation behavior The composite sample was dissolved in formic acid, centrifugated and washed to remove free PA6 molecules, and the obtained grafted GO sample was donated as g-GO-X (X was the weight percent of GO in PA6). 7



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The structural features of GO and g-GO-0.5wt% were analyzed by FTIR (Figure 3(a)). For GO sample, the absorption peaks at 3424 cm-1, 1797 cm-1, 1622 cm-1 and 1224 cm-1 were assigned to the stretching vibration of hydroxyl (O-H), carboxyl group, C=C, and C-OH, respectively. And the absorption bands at 1050 cm-1 and 871 cm-1 were assigned to stretching vibrations of epoxy groups (C-O-C). For g-GO-0.5wt% sample, the absorption peaks of -OH and C-O-C disappeared, and the absorption peaks related to PA6 molecules were found. The peak at 3301 cm-1 was assigned to N-H stretching vibration of the amide, and the bands at 2933 cm-1 and 2864 cm-1 were related to stretching vibrations of methylene (-CH2-) groups, respectively. The peak at 1639 cm-1 and 1542 cm-1 corresponded to the stretching vibration of the C=O groups of the amide carbonyl and the bending vibration of the N-H groups of the amide, respectively. The above results indicated that GO was partially reduced and PA6 chains were grafted onto GO layers. Figure 3(b) showed the Raman spectra of GO, g-GO-0.5wt% and PA6. For GO sample, two peaks at 1350 and 1585 cm-1 corresponded to the D band and G band, attributing to structure defects and the first-order scattering of the E2g vibration mode, respectively.23 For g-GO-0.5wt%, the two bands red-shifted to 1330 cm-1 and 1561 cm-1 respectively and the ID/IG value decreased from 0.99 to 0.79, indicating that there may be a interaction for GO/PA6 system, and the defect of GO was reduced. Meanwhile, the band at 2894 cm-1 attributing to the absorption of -CH2- of PA6 molecules was observed, indicating that an abundance of PA6 molecular chains were intercalated onto GO layers. Figure 3(c) presented the XPS spectra of GO and g-GO-0.5wt%, and the elements were presented in Table 1. For GO sample, two peaks of C and O were observed, and no other peak was detected. For g-GO-0.5wt% sample, besides the 8


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signals of C and O, the signal of N was found. In addition, the C/O atomic ratio of g-GO was higher than that of GO, confirming partial reduction of GO may occur.

Figure 3. FTIR spectrum (a), Raman spectra (b) and XPS C1s spectra (c) of GO and g-GO-0.5wt% Table 1. Percentage of atomic composition of GO and g-GO-0.5wt% Samples

C (%)

O (%)

N (%)

C/O ratio

GO

57.08

42.92

-

1.33:1

g-GO-0.5wt%

64.80

32.68

2.52

1.98:1

XPS peak-fitting software was used to analyze the carbon elements in different chemical environments, and the XPS C1s data were presented in Table 2. For GO sample, three peaks appeared at 284.6 eV(C=C and C-C), 286.6 eV(C-O-C and C-OH) and 288.0 eV (C=O and O=C-OH), respectively. For g-GO-0.5wt% sample, four obvious peaks were found at 284.5 eV(C=C and C-C), 288.6 eV(C=O and O=C-OH), 285.4 eV (C-N) and 287.2 eV (O=C-NH), respectively. Compared with GO sample, the 9



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C-O-C and C-OH group on g-GO-0.5wt% surface disappeared and the content of C=O and O=C-OH group significantly declined, indicating that GO was reduced, and the reduction reaction mainly occurred on epoxy and hydroxyl groups. The content of C-N and O=C-NH were 35.82% and 4.77%, respectively, which showed that PA6 molecular chains were intercalated onto GO layers. Table 2. XPS C1s data of GO and g-GO-0.5wt% C=C, C-C Samples

C-O-C, C-OH

B.E. （ev）

B.E. %

（ev）

C=O, O=C-OH B.E.

%

（ev）

C-N

O=C-NH

B.E. %

（ev）

B.E. %

%

（ev）

GO

284.6

54.98

286.6

34.37

288.0

10.65

-

-

-

-

g-GO-0.5wt%

284.5

53.95

-

-

288.6

5.46

285.4

35.82

287.2

4.77

TGA analysis was conducted. As shown in Figure 4. GO had a weight loss of 12.2% in the range of 30~150 °C, attributing to the removal of the water on the surface. A second weight loss of 37.8% at 150~280 °C was assigned to the degradation of oxygen-containing groups on GO surface.24 For g-GO sample, no significant weight loss can be observed at 30~375 °C, and the weight loss at 375~460 °C was due to the degradation of PA6 molecules grafted on GO layers.

10


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Figure 4. TGA curves of GO and g-GO with different GO content

The intercalation ratio of PA6 molecules can be obtained according to the TGA data. For g-GO, the intercalation ratio (D) of PA6 molecules on the unit mass of GO was obtained with equation (5)-(7).

) = )( × *+, − )( − ) × *, that is, ) =

D=

-./01-01

3

3 3

-01

× )(

× 100%

(5)

(6)

(7)

where WGO and Wg-GO are the thermal weight loss ratio of GO and g-GO, respectively; M0 is the weight of the added GO; M1 is the weight of PA6 grafted on GO surface. It was found that the intercalation ratio of g-GO-0.1wt%, g-GO-0.5wt% and g-GO-1.0wt% were 106.41%, 70.39% and 7.28%, respectively, which decreased with the increase of GO content. 3.1.2 Intercalation structure The XRD patterns of GO, CL, CL-GO and g-GO-0.5wt% samples were illustrated in Figure 5(a). The diffraction peak for the (001) crystal plane of GO was observed at

11



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2θ=12.25°, and the corresponding layer spacing (d001) is 0.72 nm. When the GO content was 0.05wt%, the (001) peaks of CL-GO shifted to low angles (2θ = 8.83°), and the corresponding interlayer spacing (d001 = 1.00 nm) increased, which showed that the CL molecules intercalated into the interlayers of GO. With the increase of GO content, the interlayer spacing of CL-GO changed slightly. The increase of layer spacing corresponded to the thickness of the intercalated CL molecules (∆d), as shown in Table 3. From the XRD pattern of g-GO-0.5wt%, the (001) diffraction peaks of GO can’t be observed, while new obvious diffraction peaks corresponding to α1 and α2 crystalline form of PA6 appeared,25 indicating that GO layers were exfoliated by intercalation of PA6 molecules. The XRD patterns of PA6/GO nano-composites were illustrated in Figure 5(b). For pure PA6, the peak at 2θ=21.5o corresponded to the reflection (200)/(001) of γ crystal form. For PA6/GO nano-composites, two new diffraction peaks appeared at 2θ=20.5o and 24.0o, attributing to the reflection (200) and (002)/(202) of α crystal form of PA6, respectively. The results indicated the γ crystalline form transformed to α-form by addition of GO. In addition, the characteristic diffraction peaks of GO were not found, which indicated that the GO dispersed uniformly in PA6 matrix without obvious aggregation.

12


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Figure 5. XRD patterns of (a) GO, CL-GO and g-GO-0.5wt%, (b) PA6 and PA6/GO composites with different GO content Table 3. XRD characteristics of GO and CL-GO Samples

GO

CL-0.05wt%GO

CL-0.1wt%GO

CL-0.5wt%GO

CL-1.0wt%GO

2θ (°)

12.25

8.83

8.83

8.62

8.62

d001 (nm)

0.72

1.00

1.00

1.03

1.03

∆d (nm)

--

0.28

0.28

0.31

0.31

The TEM image of the cryogenically fractured surface of PA6/0.5wt%GO nano-composite was shown in Figure 6. It can be seen that the GO layers uniformly distributed in the PA6 matrix, almost in the state of exfoliation.

Figure 6. TEM images of PA6/0.5wt%GO composite

The dynamic rheological analysis of PA6/GO nano-composites was conducted. It can be seen from in Figure 7, with increasing shear frequency, the complex viscosity of PA6 samples gradually decreased, the elastic modulus (G') and the viscous modulus (G'') increased. At low content of GO, the complex viscosity, elastic modulus and viscous 13



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modulus of PA6 samples were close to each other. But above 0.5wt% GO, the viscosity and modulus of samples increased significantly, indicating that the intermolecular entanglement in the composite was enhanced, and the network structure centering on GO gradually formed through the strong intermolecular interaction between GO and PA6 chains, as shown in Figure 7.

Figure 7. Complex viscosity, elastic modulus, viscous modulus and scheme of the crosslinking network structure of PA6/GO composites with different GO content

3.2 Thermal oxidative stability of PA6/GO nano-composites The variation of the retention of reduced viscosity of all PA6 samples with aging time at 150 oC was shown in Figure 8a. With increasing aging time, the retention of the reduced viscosity declined steadily, attributing to the molecular degradation of PA6. Compared with pure PA6, the reduced viscosity of the nano-composite kept a high level during the whole aging process. With increasing GO content, the retention of the reduced viscosity increased first and then decreased, reaching the maximum at

14


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0.5wt%GO. The results showed that addition of GO played an important role in inhibiting molecular decomposition of PA6 during thermal aging. The variation of the tensile strength and its retention of all PA6 samples with aging time at 150 oC were shown in Figure 8b. At the primary stage, the tensile strength and the retention of the tensile strength increased due to crystallization. During the aging process, the tensile strength and the retention of the tensile strength declined and showed the same trend as the reduced viscosity with GO content. The results showed that addition of GO can hinder the deterioration of mechanical performances of PA6 during aging.

Figure 8. Variation of (a) the retention of reduced viscosity, (b) the tensile strength and the retention of tensile strength of the PA6/GO composites with aging time

The in situ FTIR spectra of pure PA6 and PA6/0.5wt%GO nano-composite samples during thermal oxidative aging at 150 oC were shown in Figure 9. For pure PA6, the characteristic absorption peak at 3311 cm-1 corresponded to the N-H stretching vibration of the amide carbonyl. The bands at 2945 cm-1 and 2870 cm-1 were related to 15



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the stretching vibrations of methylene (-CH2-) groups. The peaks at 1655 cm-1 and 1539 cm-1corresponded to the stretching vibration of the C=O groups of the amide carbonyl and the bending vibration of the N-H groups of the amide, respectively.26-29 For PA6/0.5wt%GO composite sample, the characteristic peaks attributed to GO were covered by the peaks of PA6 due to very small addition of GO, and no new peaks can be observed. With aging process, the above characteristic peaks of PA6 showed a downward trend, indicating that the content of methylene and amide groups decreased gradually, due to the consumption of C-H bonds attacked by oxygen and the rupture of amide bonds. New absorption peaks appeared in the range of 1700–1800 cm-1, assigned to the stretching vibration of different carbonyl groups, due to the increase of oxygen-containing groups during aging.

Figure 9. The in situ FTIR spectra of pure PA6 and PA6/0.5 wt%GO composite

Peak separation analysis was conducted for the band of 1700–1800 cm-1 attributed to the carbonyl vibrations. The five peaks at 1716, 1733, 1749, 1772 and 1791 cm-1 were assigned to different carbonyl groups, and the carbonyl index of these 16


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peaks as a function of thermo-aging time were shown in Figure 10. With increasing aging time, the carbonyl index of each type of carbonyl group increased monotonously, indicating that the oxidation degree of PA6 was enhanced. Compared with pure PA6, the carbonyl index of PA6/0.5wt%GO nano-composite increased much more slowly, indicating that addition of GO could inhibit the thermal oxidative decomposition of PA6 remarkably.

Figure 10. The carbonyl index of pure PA6 and PA6/0.5wt%GO as a function of aging time

Figure 11 showed the TGA curves of pure PA6 and PA6/GO nano-composites, and the characteristic parameters were listed in Table S1. TG curve of PA6 showed an inverse “S” shape with one decomposition stage. Compared with pure PA6, for PA6/GO nano-composites, the initial degradation temperature (Tonset), the final degradation temperature (Tend) and the temperature at the maximum heat loss rate (Tpeak) increased, while the maximum degradation rate (Vmax) decreased significantly, indicating that addition of GO can remarkably enhance the thermo-stability of PA6.

17



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Figure 11. TGA curves of pure PA6 and PA6/GO composites with different GO content

The Friedman method was applied to conduct the kinetic analysis.30 The heating rate (β) was 5, 10, 15 oC/min. The rate equation of heterogeneous condensed phase reaction is well known to be expressed with the following formula: 45 4

= 6789

(8)

where α is the conversion fraction, t is time, k(T) is a function of temperature, and f(α) is the reaction model. Based on the Arrhenius equation k(T)=Aexp(-Ea/RT) and taking the logarithm form of Eq. 8, Eq. 9 can be obtained, the commonly used Friedman method: 45

ln ; < = lnAfα− 4

@A

B

(9)

where A is the pre-exponential factor, Ea is the activation energy, and R is a gas constant. At a constant α, the plot of ln(dα/dt) versus 1/T showed a linear relationship, which slope gives the value of Ea at this fixed α. Figure 12 showed the representative plot of Friedman method of pure PA6 and PA6/GO nano-composites. It can be seen that ln(dα/dt) had a good linear relationship with 1/T. The variation of the activation energy (Ea) of the samples with the conversion (α) was shown in Figure 13. The activation energy of all samples showed the same trend with conversion, that is, at low conversion, the activation energy remained stable. 18


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With the increase of the conversion, a turning point of the activation energy appeared and the activation energy showed an upward trend due to the effect of thermal crosslinking. With the increase of GO content, the activation energy showed the trend of first increasing, reaching the maximum at 0.5wt%GO, and then decreasing, while the turning point of the activation energy was postponed. During the aging process, the activation energy of PA6/GO nano-composites changed relatively slightly, which indicated that the molecular structure of the composites kept stable.

Figure 12. The Friedman method fitting of pure PA6 and PA6/GO composites with α (0.1- 0.8)

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Figure 13. Ea derived from Friedman method as a function of α of pure PA6 and PA6/GO composites with different GO content

3.3 Thermal oxidative stabilizing mechanism of PA6/GO nano-composites As shown in Figure 14. It can be seen that the oxygen permeability coefficients of pure PA6 was 9.23×10-15 cm3 cm cm-2 s-1 Pa-1, and with the increase of GO content, the oxygen permeability coefficient decreased gradually. When the graphene content was 1.0wt%, the oxygen permeability coefficient sharply decreased to 1.92 ×10-15 cm3 cm cm-2 s-1 Pa-1, and the oxygen barrier property was remarkably enhanced, which was favorable for inhibiting the oxidative degradation of PA6 molecular chain.

Figure 14. O2 permeability coefficients (PO2) of PA6/GO composites with different GO content

DPPH method was widely used for quantitative measurement of antioxidant scavenging ability on free-radicals.31 Figure 15 shows the radical scavenging ratio of 20


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GO and g-GO toward DPPH free radical. With increasing GO and g-GO concentration, the scavenging ratio monotonically increased, presenting enhancing radical scavenging ability. Moreover, the scavenging ratio of GO in the composites (g-GO) was higher than that of GO.

Figure 15. The radical scavenging activity of GO and g-GO toward DPPH free radical

The thermal stabilizing mechanism of GO on PA6 could be attributed to its unique layer structure, which uniformly dispersed in the matrix to form “labyrinth” structure, resulting in blocking the diffusing and penetration of oxygen in PA6 matrix. On the other hand, GO could effectively capture free radicals generating in the process of thermo-aging of PA6, and thus the molecular degradation was slowed down. The scheme of stabilizing mechanism was shown in Figure 16.

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Figure 16. Scheme for the thermal stabilizing mechanism of GO on PA6

4. Conclusions PA6/GO nano-composites were fabricated by dispersing in caprolactam (CL) melt and in situ polymerization. GO was partially reduced under high temperature and polar environment, and the reduction reaction mainly occurred on epoxy and hydroxyl groups of GO. PA6 chains were effectively grafted onto GO layers, and the grafting ratio declined with increasing GO content. Compared with GO, the interlayer spacing of CL-GO increased, while for the grafted sample (g-GO), GO layers were exfoliated by intercalation of PA6 molecules. For the composites, the γ crystalline form transformed to α-form by addition of GO, which dispersed uniformly in PA6 matrix without obvious aggregation, and the network structure centering on GO gradually formed through the intermolecular interaction for the system. During the whole thermal oxidative aging process, compared with pure PA6, the reduced viscosity and tensile strength of the composite kept a high level, and the carbonyl index increased much more slowly, indicating that the molecular oxidative degradation and deterioration of mechanical properties of PA6 were inhibited. In the meantime, the degradation

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temperature of the composites increased, the degradation rate decreased significantly, while the activation energy changed slightly, indicating that the thermal stability of PA6 was remarkably enhanced. With increasing GO content, the oxygen permeability coefficient of the composites decreased significantly, which was favorable for inhibiting the oxidative degradation of PA6 molecular chain, while the radical scavenging ratio monotonically increased, presenting enhancing radical scavenging ability of the composites. Supporting Information: TGA characteristic parameters of pure PA6 and PA6/GO nano-composites were presented in Table S1. Compared with pure PA6, for PA6/GO nano-composites, Tonset, Tend and Tpeak increased, while the maximum degradation rate (Vmax) decreased significantly. Acknowledgements This study was financially supported by the Key Natural Science Foundation of China (Grant No. 51133005). References (1) Li, L.; Yang, G. Variable ‐temperature FTIR studies on thermal stability of hydrogen bonding in nylon 6/mesoporous silica nanocomposite. Polym Int. 2009, 58, 503. (2) Hu, Z.; Chen, L.; Lin, G, P.; Luo, Y.; Wang, Y, Z. Flame retardation of glass-fibre-reinforced polyamide 6 by a novel metal salt of alkylphosphinic acid. Polym Degrad Stabil. 2011, 96, 1538. (3) Sagar, B. Autoxidation of N-alkyl-amides. Part II. N-alkyl-amide hydroperoxides and di-N-alkyl-amide peroxides. J Am Chem Soc. 1967, 428. (4) Hu, X.; Scott, G. Mechanisms of antioxidant action: the role of O-macroalkyl hydroxylamines in the photoantioxidant mechanism of HALS. Polym Degrad Stabil. 23



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(16) Wang, F. T.; Chen, L.; Tian, C, J.; Meng, Y.; Wang, Z, G.; Zhang, R, Q.; Ding, D, J. Interactions between free radicals and a graphene fragment: Physical versus chemical bonding, charge transfer, and deformation. J Comput Chem. 2011, 32, 3264. (17) Tang, M.; Xing, W.; Wu, J.; Huang, G.; Xiang, K.; Guo, L.; Li, G. Graphene as a prominent antioxidant for diolefin elastomers. J Mater Chem A. 2015, 3, 5942. (18) Yang, J.; Huang, Y.; Lv, Y.; Zhao, P.; Yang, Q.; Li, G. The intrinsic thermal-oxidative stabilization effect of chemically reduced graphene oxide on polypropylene. J Mater Chem A. 2013, 1,11184. (19) Li, C.; Xiang, M.; Zhao, X.; Ye, L. In Situ Synthesis of Monomer Casting Nylon-6/Graphene-Polysiloxane

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Caption Figure Captions Figure 1. The schematic illustration of the gas permeation device Figure 2. Synthesis of PA6 and PA6/GO composites by in situ ring-opening polymerization of caprolactam Figure 3. FTIR spectrum (a), Raman spectra (b) and XPS C1s spectra (c) of GO and g-GO-0.5wt% Figure 4. TGA curves of GO and g-GO with different GO content Figure 5. XRD patterns of (a) GO, CL-GO and g-GO-0.5wt%, (b) PA6 and PA6/GO composites with different GO content Figure 6. TEM images of PA6/0.5wt%GO composite Figure 7. Complex viscosity, elastic modulus, viscous modulus and scheme of the crosslinking network structure of PA6/GO composites with different GO content Figure 8. Variation of (a) the retention of reduced viscosity, (b) the tensile strength and the retention of tensile strength of the PA6/GO composites as a function of aging time Figure 9. The in situ FTIR spectra of pure PA6 and PA6/0.5 wt%GO composite Figure 10. The carbonyl index of pure PA6 and PA6/0.5wt%GO as a function of aging time Figure 11. TGA curves of pure PA6 and PA6/GO composites with different GO content Figure 12. The Friedman method fitting of pure PA6 and PA6/GO composites with conversion fraction (α) ranging from 0.1 to 0.8 Figure 13. Activation energy (Ea) derived from Friedman method as a function of conversion fraction (α) of pure PA6 and PA6/GO composites with different GO content Figure 14. O2 permeability coefficients (PO2) of PA6/GO composites with different GO content Figure 15. The radical scavenging activity of GO and g-GO toward DPPH free radical Figure 16. Scheme for the thermal stabilizing mechanism of GO on PA6 27



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Table Captions Table 1. Percentage of atomic composition of GO and g-GO-0.5wt% Table 2. XPS C1s data of GO and g-GO-0.5wt% Table 3. XRD characteristics of GO and CL-GO

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For Table of Contents Only

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Intercalation Structure and Enhanced Thermal Oxidative Stability of

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