Fabrication of Polyamide 6 Nanocomposite with Improved Thermal

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Materials and Interfaces

Fabrication of polyamide 6 nanocomposite with improved thermal conductivity and mechanical properties by incorporating low content of graphene Rui Wang, Lixin Wu, Dongxian Zhuo, Jianhua Zhang, and Youdan Zheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01070 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Industrial & Engineering Chemistry Research

Fabrication of polyamide 6 nanocomposite with improved thermal conductivity and mechanical properties by incorporating low content of graphene Rui Wang a, b, Lixin Wu a,*, Dongxian Zhuo c,*, Jianhua Zhang d and Youdan Zheng a,b a

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,

Fuzhou 350000, P. R. China. b c

University of the Chinese Academy of Sciences, 100049, Beijing, P. R. China

Quanzhou Normal University, 362000, Quanzhou, P. R. China.

d

Fujian Special Equipment Inspection and Research Institute, 351100, Putian, P. R.

China ABSTRACT: A 3D graphene network was constructed in polyamide 6 (PA6) monomers through reduction self-assembly of graphene oxide (GO), and then PA6 nanocomposite with low content of graphene was fabricated through in situ polymerization. The effect of the 3D graphene network on the structure and properties of the PA6 were systematically investigated. Results show that the 3D graphene network can significantly improve the thermal conductivity of PA6. In the case of the PA6 only with 0.25 wt.% graphene, its thermal conductivity is 0.69 W/(m K), about 2.88 times of that of the pure PA6. This improvement is attributed to the more compact thermal conductive paths of the 3D graphene network and its stronger interfacial interaction with PA6 in this work compares to those of pre-synthesized free-standing 3D graphene networks, Moreover, the mechanical properties and water resistance of PA6 also have significantly improved with the incorporation of the 3D graphene network. KEYWORDS: 3D graphene network; polyamide 6; thermal conductivity; water resistance

INTRODUCTION Polymeric composite has been applied in many fields due to its lightweight, high chemical resistance, excellent insulation performance, and processability.1-3 However, the applications of the present polymeric composites cannot be extended and deepened because of some of poor correlative properties.4-6 Thereinto, the high thermal conductivity is urgent in need for the polymeric composites which apply in the fields of package, electronics, and thermal management.7 To date, in order to meet the high thermal conductivity requirements, direct

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addition of high loadings thermal conductive filler (about 20–60 vol.%) into the polymer are usually necessary, which will in turn result in the deterioration of other properties of the resultant polymeric composites such as mechanical properties, electrical insulation, water resistance, and so on.8,9 Obviously, conventional direct addition methods are hard to fully exert the improvement of thermal conductive filler. Therefore, developing new methods to fabricate the high thermal conductive polymeric composites simultaneously with improved other key properties is still a challenge and an interesting subject. Owing to its prominent thermal conductivity, graphene as one of ideal nanofillers has been incorporated to polymer. The as-prepared graphene-related polymer nanocomposites can achieve the significantly improved thermal conductivity and other properties.10-13 However, graphene is often difficult to achieve good dispersion and exfoliation in the graphene-related nanocomposites due to its large surface area and strong van der walls force, which will result in its low efficiency of improvement.14,15 Many researchers have designed and prepared series of the functionalized graphene to improve the dispersion and interfacial interactions of graphene with polymeric matrices and reduce the interface thermal resistance.16,17 However, the good dispersion usually only occurs in the polymeric composite at low graphene content, because continuously increasing loading will lead to the irreversible aggregation of graphene. Such low loading is difficult to form the high effective conductive path which is a crucial factor for preparing high thermal conductive polymer.10,18 Recently, a new technology has been introduced to enhance the modified efficiency of graphene on the thermal conductivity of polymers. In detail, the free-standing 3D graphene structures such as graphene foams, aerogels which are expected to act as the thermal conductive paths are pre-synthesized, and then the high thermal conductive grahene/polymer nanocomposites are fabricated by backfilling polymer, prepolymer or monomer into the pores of these 3D graphene structures.19-23 However, this method usually involves freeze drying or supercritical drying in the pre-synthesis of free-standing 3D graphene network, and it is costly, low-efficiency and then will be challenged for large-scale applications.24 Secondly, the large interconnected pores in these free-standing 3D graphene structures are range from several micrometers to decade micrometers, and these thermal conductive paths can be more compacted in polymer by minimizing the agglomeration of graphene sheets ACS Paragon Plus Environment

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and increasing the utilization of graphene;21, 25 Finally, the interfacial interactions between these free-standing 3D graphene structures and polymers are relatively weak since few organic groups exists on the surface of reduced graphene oxide, which will reduce the relevant performances of the as-prepared composite.10 Except that, it has also reported that high thermal conductive polymeric composites are fabricated by incorporating ultralow loading of vertically aligned graphene networks to polymers.25,26 However, such highly anisotropic materials cannot meet the requirement of some practical applications which require isotropic materials. In fact, there are two other methods to construct the 3D graphene network/polymer composites (3DGPCs): (i) assembly of graphene oxide (GO) sheets with polymers, and (ii) direct incorporating polymers, prepolymers or monomers during reduction self-assembly of GO sheets.27-29 As for the former method, the selectivity on polymers which need to cross-linking with GO limit its application. In this work, in order to increase the utilization of graphene on thermal transfer in polymer and its modified efficiency by a more convenient and energy-saving method compare to the method of pre-synthesized free-standing 3D graphene structures, the latter method-direct incorporation of monomers/prepolymer during reduction self-assembly of GO sheets was used to fabricate the graphene/PA6 (GNPA6) nanocomposites. Briefly, a 3D graphene network was constructed through reduction self-assembly of GO sheets in PA6 monomers solution (including ε-caprolactam, 6-aminocaproic acid, and a little water) owing to the existence of ascorbic acid, and PA6 monomers were simultaneously filled in the as-prepared 3D graphene network. Subsequently, the GNPA6 nanocomposites were obtained through in situ polymerization of PA6 monomers. Obviously, this method is simple and efficient-saving because it doesn’t need to pre-synthesize the free-standing 3D graphene structures. Most importantly, by this method, PA6 monomers act as spacers to minimize the agglomeration of graphene sheets, and the specific surface area of the as-prepared 3D graphene network may higher than those of free-standing 3D graphene structures,29,30 indicating that the utilization of graphene was increased which may lead to the higher properties of the resultant composites.

EXPERIMENTAL PROCEDURES Materials. Graphite powder (325 mesh) was supplied by QingdaoYanhai carbon



materials Co. Ltd (China). Sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30% aq.) and ascorbic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. ε-caprolactam and 6-aminocaproic acid were obtained from Aladdin Industrial Co. Ltd (China). All agents were used without further purification. Preparation of graphite oxide. Graphite oxide was prepared using a modified Hummer’s method from graphite powder.31 Typically, 3 g graphite, 3 g KNO3, and 200 mL of 98 wt.% H2SO4 were added to a three-necked bottle in an ice bath with vigorous stirring for 2 h. Subsequently, 18 g KMnO4 was slowly added in 15 min, and then, the mixture was reacted at 35 °C for 1.5 h; then 240 mL distilled water was slowly added to the mixture which was below 70°C. Afterward, temperature raised to 98°C and maintained for 30 min, and then the mixture was poured into 600 ml of 10 wt.% H2O2 aqueous solution. Finally, a yellow powder was obtained after repeatedly washing with distilled water and completely drying at 50 °C in a vacuum oven. Fabrication of GNPA6 nanocomposites. The as-synthesized graphite oxide was dispersed into the deionized water and treated in a bath sonicator for 1 h to form gelatinous and stable GO suspension with a variety of concentration (8, 12, 16 mg/ml). For preparing GNPA6, 75g ε-caprolactam were melted at 75 °C under nitrogen flow, then, 10ml GO suspension and 5g 6-aminocaproic acid were added into liquid ε-caprolactam

and treated by an ultrasonic tip for 10 min to form the homogeneous

brown solution. Afterward, a certain amount of ascorbic acid (weight ratio, ascorbic acid: graphene oxide=3:1) was added into the as-prepared homogeneous brown solution and stirred until it all dissolved, and then, the result solutions were poured into the hermetically sealed Teflon-lined autoclaves and placed in oven with 80 °C for 4 h to consutruct the PA6 monomers filled 3D graphene network by reduction self-assembly of GO. After that, the Teflon-lined autoclaves were opened and removed the free PA6 monomers outside the 3D graphene networks. Afterward, the as-prepared PA6 monomers filled 3D graphene networks were placed in a oven which was heated to 230 °C for 2h, then to 245 °C for 2 h and 265 °C for 2.5 h in a nitrogen atmosphere for in situ polymerization of PA6 monomers. After cooling to room temperature, the resultant products were taken out and washed in boiling deionized water for 2h several times to remove the monomers and oligomers and vacuum-dried at 100 °C for 12h. Finally, it obtained GNPA6 nanocomposites. The preparation process of GNPA6 nanocomposites are shown in Figure 1. The weight ratios of ACS Paragon Plus Environment

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graphene in resultant composites are 0.16 wt.%, 0.25 wt.% and 0.31 wt.%, respectively, after calculating their final weight, and the resultant nanocomposites were demolded and coded GN0.16PA6, GN0.25PA6 and GN0.31PA6, respectively. In contrast, graphene aerogel constructed in water was prepared through reduction self-assembly of GO by ascorbic acid. GN0.25PA6 was cut into small particles (2-5mm) and obtained GF0.25PA6 by injection molding. The rGO0.25/PA6 composite was fabricated through in situ polymerization according the previous report.32 Characterization. Fourier Transform Infrared (FTIR) spectra was recorded using a Perkin Elmer Spectrum One spectrometer (USA) from 4000 to 400 cm-1 with a resolution of 2 cm-1 (KBr pellet technique). The concentration of ε-caprolactam in water was measured by concentration meter (TBD5-CMD+MS1204p) at 70 °C. Raman Microscope (Renishaw, 514 nm) was used to determine the Raman scattering characteristics of the samples. The laser power used is 1 mW. X-ray photoelectron spectroscopy (XPS) was performed on vacuum filtered films in a system equipped with a VG CLAM II electron analyzer and PSP twin anode source. All PA6 monomers filled 3D graphene networks were prepared by removing the filled PA6 monomers by washing with water and drying. GNPA6 samples were dried after removed PA6 chains by formic acid. Differential scanning calorimetric (DSC) analysis was performed on a Mettler TAGS equipment with a heating rate of 10 °C/min in a nitrogen atmosphere. The viscosity measurements were taken in 85% formic acid solution of nylon-6 with a concentration of 5 g/L, in an Ubbelohde viscometer at 30 °C. The morphologies of the samples were investigated by Scanning electron microscopy (SEM, JSM-6700F, JEOL, Japan). Thermal gravimetric analysis (TGA) was performed on a Netzsch STA409PC simultaneous thermal analyzer. The data of TGA was collected from room temperature to 800 °C with the heating rate of 10 °C/min under nitrogen atmosphere. The thermal conductivities of GNPA6 composites were measured by TC 3000E thermal conductivity meter (Xiatech Electronics Co., Ltd, China) according to ASTM D5930, and the specimens were prepared by cutting the bulk of PA6 and its composites from different direction. Dynamic Mechanical Analysis (DMA) was performed using TA DMA Q800 apparatus from TA Instruments (USA). A single cantilever clamping mode was used. DMA tests were carried out from 25 to 200 °C with a heating rate of 3 °C·min-1 at 1 Hz. The tensile property test was performed with a constant speed of 5 mm·min-1 using a load cell of 1 kN. Specimens are dumbbell



type according to ASTM D638 and the results were averaged from five specimens. The nanoindentation was performed in a nanoindenter instrument (Hysitron Inc., Tribo Indenter 750, USA) with a three-sided pyramid Berkovich diamond indenter (the radius of the indenter probe was 50 nm). The maximum force of 8000 µN was applied at a constant rate of 1600 µN·S-1. The water absorption of the composites was determined according to ASTM D570-98.

RESULTS AND DISCUSSION The formation of PA6 monomers filled 3D graphene network and in situ polymerization of GNPA6. Generally, GO can be readily well-dispersed in water by suitable ultrasonic treatment and exhibits colors of brown or wine red at high concentration, but it is not well-dispersed in the most of organic solvents.33 Herein, it is noteworthy that the GO/PA6 monomers solution prepared by ultrasonic treatment exhibits deep transparent wine red and Tyndall phenomenon (Figure S1), and no sediments are observed upon long-term standing. This result indicates the presence of exfoliated graphene oxide sheets well disperse in the PA6 monomers solution which provides the favorable condition for the formation of 3D graphene network.

Figure 1. Schematic illustration of the fabrication of GNPA6 nanocomposites: fabrication of PA6 monomers filled 3D graphene network by self-assembly of reduced GO (a); in situ polymerization of PA6 in the resultant 3D graphene network (b). Figure 2 shows the 3D graphene network all can be constructed in water and in ACS Paragon Plus Environment

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PA6 monomers solution, indicating that the reduction self-assembly of GO occurs either in water or in PA6 monomers at the presence of ascorbic acid. With regard to graphene aerogel constructed in water, it exhibits a large volume shrinkage compare to the original GO aqueous solution, which can be found in other reports.30 However, it is noted that the volume shrinkage of PA6 monomers filled 3D graphene network shows the lower volume shrinkage at the same content of graphene. SEM was introduced to observe the microscopic morphology of 3D graphene network. As shown in Figure 3, it can be seen that the average size of pores in PA6 monomers filled 3D graphene network is 1.5 µm, which is smaller than that in graphene aerogel constructed in water. It can be further confirmed by BET analysis, and the density and specific surface area are listed in Table S1. It can be seen that the specific surface area of PA6 monomers filled 3D graphene network reaches 367 m2/g, which is 2.6 times of that of graphene aerogel constructed in water (143 m2/g) after drying. It has reported that there is the cation-π interaction exists between graphene and PA6 monomers.34 The concentration of ε-caprolactam out of PA6 monomers filled 3D graphene network was measured. Results show that the concentration of ε-caprolactam out of the PA6 monomers filled 3D graphene network is 75 wt.%, which is lower than the concentration of ε-caprolactam in the original solution (85 wt.%). As Figure 3b shows, the thickness of walls of PA6 monomers filled 3D graphene network after removing the filled PA6 monomers by washing with water and drying are thicker than that of graphene aerogel constructed in water. These results indicate it has the strong interfacial interactions between PA6 monomers and 3D graphene network. Therefore, it can be concluded that the PA6 monomers filled 3D graphene network was constructed though the reduction self-assembly of GO at the presence of ascorbic acid, and the PA6 monomers minimize the agglomeration and restack of graphene sheets in the reduction due to its strong interfacial interaction with reduced GO. Obviously, as-prepared PA6 monomers filled 3D graphene network exhibits smaller pores and large specific surface area, which benefits the infiltration of polymer.



Figure 2. Photograph of graphene aerogel constructed in water (left) and PA6 monomers filled 3D graphene network (right), and the mass fraction of GO were both 0.1 wt.%. Figure 4 shows the FTIR spectra of GO, PA6 monomers filled 3D graphene network, and GNPA6. Compared with GO, it can be seen that the new absorptions peak near at 1640, 2851, and 2932 cm-1, which are attributed to the bending vibration of the N-H bond, the stretching vibration of the C-N bond, and C-H stretching vibrations, respectively, appear in the spectra of PA6 monomers filled 3D graphene network and GNPA6. Meanwhile, the characteristic band at 1058 cm-1 attributing to C–O (epoxy group) stretching vibration is removed, indicating the molecular chain of PA6 is successfully grafted on the surface of graphene.35,36

Figure 3. SEM images of graphene aerogel constructed in water (a) and PA6 monomers filled 3D graphene network (b).


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GNPA6

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3303

PA6 monomers filled graphene network

2851 2932

1540 1639 1541

3280

2852 2932

1728 1640

GO 1390 1227 1726 1622

3420

4000

3500

3000 2500 2000 Wavenumber (cm-1)

1500

1000

Figure 4. FTIR spectra of GO, PA6 monomers filled 3D graphene network, and GNPA6. Figure 5 demonstrates the Raman spectra of GO, PA6 monomers filled 3D graphene network, and GNPA6. It can be noted that two prominent peaks at 1333 and 1591cm-1, which are corresponding to the G and D bands, respectively, showing in the spectra of GO. In general, the level of chemical modification of the graphitic carbon sample is commonly quantified by the intensity ratio of the D band and G band (ID/IG), and a relatively complete graphite structure is an important factor for graphene playing its reinforcing roles in polymer.20,36 Herein, the ID/IG ratios for GO, PA6 monomers filled 3D graphene network and GNPA6 are 1.16, 1.33 and 1.42, respectively. The increase of ID/IG ratios for PA6 monomers filled 3D graphene network and GNPA6 compared with GO indicates that the new domains of conjugated carbon atoms are formed owing to the reduction of GO, and GNPA6 have higher reduction degree due to the high temperature (250 °C) which provides a favorable condition for preparing high performance polymer composites. In addition, XPS was also used to characterize the structure of GO, PA6 monomers filled 3D graphene network and GNPA6 as shown in Figure S2, the increase of C/O ratio and abundance of N element further affirm the reduction of graphene oxide and the grafting of PA6 chains on the graphene.



ID:IG GO PA6 monomers filled 3D graphene network GNPA6

Intensity (a.u.)

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1.16

1323 1585

1.33 1.42 1326 1590

GNPA6 PA6 monomers filled 3D graphene network

1333

1591

GO 600

800

1000

1200

1400

1600

1800

2000

Raman shift (cm-1)

Figure 5. Raman spectra of GO, PA6 monomers filled 3D graphene network, and GNPA6 Based on the above analysis, the 3D graphene network filling with PA6 monomers with high specific surface area was successfully prepared through synchronous self-assembly of graphene oxide in the polymerization process of PA6, and thus it is expected this structure will play very good role in improving the thermal conductivity of as-prepared PA6 nanocomposites. The structure of GNPA6 nanocomposites. DSC was performed to study the influences of 3D graphene network on the melting and crystallization behaviors of PA6. As shown in Figure 6a, a strong peak at about 187 oC represents the crystallization temperature (Tc) of α-form crystal of PA6 was observed.37 Comparing with pure PA6, all GNPA6 nanocomposites have higher Tc values. However, the value of Tc is dependent on the content of graphene, there is an optimum content of graphene (0.25 wt.%) to get the high value of Tc. The degrees of crystallinity (χc) for PA6 and GNPA6 nanocomposites were calculated from the enthalpy evolved during crystallization based on the cooling scans.38 It can be found that the variation of χc for the PA6 with the incorporation of 3D graphene network are also following the trend of Tc as described above. The reasons behind this phenomenon can be explained by the nucleation effect of graphene and the confinements of 3D graphene network. On the one hand, graphene can act as a nucleator which promotes the crystallization of PA6. On the other hand, the 3D graphene network greatly restrains the molecular


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motion of the PA6 chains, especially in high loading graphene nanocomposites, and thus limits the crystallization of PA6.32 As considering the melting behaviors, it can be found that a weak peak at 209 °C and a strong peak at 217 °C which represents the melting temperature (Tm) of α-form crystals and γ-form crystals, respectively.39 Similar to the previous reports, only one strong peak which attributes to γ-form crystals is observed in the melting curve of all GNPA6 nanocomposite, which indicates the α-form crystals are depressed by adding graphene.18 Moreover, the incorporation of 3D graphene network results in a significant increase on Tm, as the graphene loading reaches 0.16 wt.%, the Tm reaches 224 °C, which increases by 7 °C compares to that of pure PA. However, the value of Tm for the GNPA6 inversely decreases with continuously increasing the content of graphene, which can be attributed to the decrease of the molecular weight (Mη) of GNPA6 as shown in Table S2.

Figure 6. DSC curves of PA6 and GNPA6 nanocomposites: (a) the crystallization scans; (b) is the melting scans. ACS Paragon Plus Environment


DMA tests were performed to get information about both the structure and thermo-mechanical properties of the materials. As shown in Figure 7, it can be found that the values of Tg for GNPA6 nanocomposites are significantly enhanced by incorporating 3D graphene network. Specifically, the value of Tg for GN0.25PA6 nanocomposite even increases by 12 °C compares to that of PA6. Generally, the Tg value of a material is related to the motions of polymer chains segments. Apparently, motions of PA6 chains segments are confined by the 3D graphene network. Moreover, it is known that the tan delta value reflects the interaction between filler and polymer matrix.40 The values of tan delta for all GNPA6 nanocomposites are smaller compare to that of pure PA6, which indicates that the confinement effect is strengthened with the addition of the 3D graphene network.

0.12 0.11 Tan Delta

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0.10 0.09 Tg (°C)

0.08

pure PA6 GN0.16PA6 GN0.25PA6 GN0.31PA6

0.07 0.06

30

70.7 77.5 82.4 80.3

60 90 120 Temperature (°C)

150

Figure 7. The temperature dependence of the tan delta of pure PA6 and GNPA6 nanocomposites. In addition, the confinement effects can be further confirmed by observing the dispersion of 3D graphene network in PA6 (Figure 8). Comparing with PA6, the SEM image of GN0.25PA6 shows a rough fractured surface and the graphene nanosheets are well dispersed in PA6 without agglomeration, indicating the large phase interfaces area between graphene and PA6 which exhibits the strong confinement effects for the motion of PA6 chains. Not only that, after removing PA6 chains by etching with formic acid, the PA6 chains are still grafted on to the surfaces of graphene as shown in Figure 8c, indicating the strong interfacial strength between graphene and PA6. Based on the above discussion, it can be stated that the introduction of 3D


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graphene network into the PA6 significantly changes the structure of the resultant nanocomposite. Such changes in the structure will in turn affect the macroscopic properties of PA6.

Figure 8. SEM images of the fractured surface of PA6(a), GN0.25PA6(b), and GN0.25PA6 after removing the free PA6 chains(c). Thermal conductivity of GNPA6 nanocomposites. It has been reported that graphene was introduced to improve the thermal conductivity of polymer because of its outstanding thermal conductivity.41 Herein, the thermal conductivity of PA6 and GNPA6 nanocomposites were characterized and the results are shown in Figure 9. It can be seen that all nanocomposites containing graphene have higher value of thermal conductivity than that of pure PA6, which indicates the improvement of graphene on the thermal conductivity of PA6. However, it is noted that more loading of graphene in PG via direct-mixing route is needed to achieve the same thermal conductivity as those of nanocomposites containing 3D graphene network. Furthermore, the necessary loading of graphene in GNPA6 to achieve the same enhancement of thermal conductivity is less than that in another 3D graphene network reinforcing PA6 nanocomposite (PCF-1, containing 1 wt.% graphene) from the previous report.21 The thermal conductivity of GN0.25PA6 which contains only 0.25 wt.% graphene reaches 0.69 W/(mK), which increases by 188 % compares to that of pure PA6 (0.24 W/(mK)). It is notable that the 3D graphene network in this work exhibits highly modified efficiency on the thermal conductivity of polymer compares with other graphene-releated polymeric composites as shown in Figure 10, and the value of enhancement per wt.% of GN0.25PA6 in this work touches 1152. In addition, by comparing with other polymeric composites with similar content of fillers (0.2~1 wt.%) as shown in Table S3, and it is found that the enhancement per wt.% of GNPA6



Thermal conductivity (W/(mK))

nanocomposites are the highest. 0.8

0.6

0.4

0.2

0.0

6

) 7] [1 O G PG r % wt e) 0 20] en (1 -1[ F aph

PA

A6

gr

25

%

0.

5P

6

A6

PA

A6

1P

5 .2

.3

.2

wt

O

F0

N0

PG

(1

rG

G

G

N0

6

6P

PA

.1

re

N0

G

G

Pu

Figure 9. Thermal conductivity of pure PA6, GNPA6, GF0.25PA6, rGO0.25PA6 and other graphene-based PA6 nanocomposites from the previous reports.

1600

Enhancement per wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Highly anisotropic graphene aerogel/EP [26]

1200

GN0.25PA6 in this work

Vertically aligned graphene network/EP [25]

800

PA6/graphene nanoribbons [11]

400

PA6/TCA-rGO [10]

PA6/graphene foam [21] PDMS/graphene foam

0

0

1

2

3

[23]

Epoxy/graphene [17] PMMA/graphene aerogel [19]

4

5

6

7

Filler loading (wt%) Figure 10. Comparation of enhancement per wt.% of GN0.25PA6 and graphene-based polymeric composites. In order to investigate the mechanism of improvement on thermal conductivity for the graphene-based polymer composites, the following reasons must be considered.


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Firstly, the intrinsic thermal conductivity of graphene is a key factor in the enhancement of thermal conductivity. Herein, the reduced GO in the 3D graphene network possesses high C/O and relative integrity graphite structure as shown in the results of raman spectra, which shows relatively high intrinsic thermal conductivity as discussed above, and then it plays an important role on transfer of heat to the PA6 matrix.42,43 Secondly, with regard to polymer nanocomposites, one of the most representative thermal conductive mechanisms is conductive chains mechanism or percolation theory.10,34 In detail, the percolation thresholds where the rapid increases in conductivity started were achieved with increasing the content of filler. However, the loading of graphene in polymers is often excess to reach the percolation threshold, because it inevitably contains lots of agglomeration and restack of graphene by the direct mixing with polymers, and it ultimately leads to the modified inefficiency of graphene on thermal transfer. In this work, interlaminar PA6 monomers polymerized and formed PA6, which acts as the spacer to minimize the agglomeration and restack of graphene, resulting in the high utilization of graphene nanosheets. Therefore, the 3D conductive path can be constructed with the low content of graphene (no more than 0.16 wt.%), which is obviously lower than those of other reports. In other words, the resultant 3D graphene networks exhibit higher specific surface area compared to those of pre-synthesized free-standing 3D graphene networks on the same content of graphene, which indicates the higher thermal conductive efficiency of the resultant 3D graphene networks prepared in this work. In addition, the thermal conductivity of the GF0.25PA6 composite reaches 0.62 W/(mK), which is close to that of GN0.25PA6. It demonstrates that the 3D graphene networks can be reconstructed by injection molding of GNPA6 small particles, which are attractive in industrial application. This phenomenon can be explained that the isotropic 3D network structures inside different GNPA6 particles can be connected to each other and form the interconnected 3D graphene network again. Finally, the thermal conductivity of graphene-based polymer composites is also dependent on the interfacial interaction between polymer and graphene.10 In this work, the oxygen-containing polar group on the surface of GO such as carboxyl, hydroxyl and epoxy can react with the PA6 monomers in formation of 3D graphene network and the in situ polymerization, which links the PA6 chains with 3D graphene network by chemical bonding. Therefore, chemical bonding increases the interfacial interaction, which decreases the interface thermal resistance. By all accounts, it can be proposed a mechanism: the 3D graphene network is ACS Paragon Plus Environment


constructed through reduction self-assembly of GO in the PA6 monomers. Reduced graphene oxide possess high intrinsic thermal conductivity which supports the 3D graphene network to act as the effective thermal conductive path. PA6 chains act as the spacers to minimize the restack of the reduced GO, resulting in the higher specific surface area and more compact structure of the 3D graphene networks compare to those of pre-synthesized free-standing 3D graphene networks in the same content of graphene. Moreover, the 3D graphene networks exhibit the lower interface thermal resistance with PA6 by chemical bonding. These positive factors finally contribute to achievement of the excellent thermal conductivity for the as-prepared GNPA6 nanocomposites with low content of graphene. Therefore, it can be explained the results of enhancement per wt.% as shown in Figure 10 and Table S3 using this mechanism. Comparing with those of graphene-polymer prepared by the direct addition method, GNPA6 nanocomposites on the similar show higher thermal transfer efficiency due to the effective thermal conductive 3D graphene network. Comparing with those of the 3D graphene network/polymer composites, the graphene in 3D graphene networks exhibit higher utilization and 3D graphene networks show higher specific surface area to transfer the heat, resulting in the higher enhancement per wt.% of GNPA6 composites. Mechanical properties. From aforementioned investigations of structure of GNPA6 nanocomposites, the nano-scaled dispersion and interconnected 3D graphene network may result in potential reinforcement for the effective stress transfer to achieve superior performances of nanocomposites. Typical stress-strain curves of pure PA6 and GNPA6 nanocomposites with different contents of graphene are shown in Figure 11, and the corresponding results are summarized in Table 1. It can be seen that the pure PA6 reveals a typical yield behavior and the corresponding tensile strength, Young’s modulus, and the elongation at break are 50.7 ± 1.8 MPa, 1450 ± 25.6 MPa, and 23.2 ± 2.2 %, respectively. Note that the tensile strength and Young’s modulus of PA6 are increasing with the incorporation of 3D graphene network. When the loading of graphene is 0.25 wt.%, the tensile strength and Young’s modulus increase by 50.9 % and 64.0 %, compare to those of pure PA6, respectively. However, stress yield disappear and the elongation at break gradually decreases with the increasing content of graphene, and GNPA6 nanocomposites exhibit the typical thermosetting tensile behavior at the high content


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of graphene ( ≥ 0.25 wt.%). Such phenomena also can be observed in other graphene-based nanocomposites, which are attributed to the reinforcement of graphene.42

80

60 Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 pure PA GN0.16PA6 GN0.25PA6 GN0.31PA6

20

0

0

5

10

15 20 Strain (%)

25

30

Figure 11. Typical stress–strain curves of PA6 and GNPA6 nanocomposites. Table 1. Selected tensile data for PA6 and GNPA6 nanocomposites Samples

Tensile strength

Young’s modulus

Elongation at break

(MPa)

(MPa)

(%)

Pure PA6

50.7 ± 1.8

1450 ± 26

23.2 ± 2.2

GN0.16PA6

66.5 ± 2.1

1965 ± 27

9.6 ± 0.5

GN0.25PA6

76.5 ± 2.6

2378 ± 30

3.5 ± 0.2

GN0.31PA6

68.9 ± 3.1

2569 ± 42

1.4 ± 0.2

Figure 12 shows the storage modulus (E′) as a function of temperature for PA6 and GNPA6 nanocomposites. It is noted that the incorporation of 3D graphene network results in the increase of the storage modulus in the whole range temperature compared to that of pure PA6, and the storage modulus of nanocomposites continuously increases with the increasing content of graphene. The trend of the



storage modulus is similar to that of Young’s modulus, which further confirms the reinforment of 3D graphene network.

3000 Storage modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2500 2000 1500 1000 500 0 0

pure PA6 GN0.16PA6 GN0.25PA6 GN0.31PA6

40

80 120 160 Temperature (°C)

200

Figure 12. The temperature dependence of the storage modulus for pure PA6 and GNPA6 nanocomposites. Nanoindentation is one of technique that can provide a wealth of valuable quantitative information on the relation between microstructures and mechanical properties.43 Herein, nanoindentation was introduced to evaluate the micro-structures and properties of GNPA6 nanocomposites. The typical load-displacement relations of pure PA6 and GN0.25PA6 nanocomposite are illustrated in Figure 13a, and it is clearly shown that the maximum depth for GN0.25PA6 nanocomposite remarkably decreases with the incorporation of 3D graphene network. The average hardness and the elastic modulus for pure PA6 and GNPA6 nanocomposites were calculated and depicted in Figure 13b.43 It can be shown that the hardness and modulus steadily increase with the continuous loading of graphene. With the addition of only 0.31 wt.% of graphene, the modulus and hardness of GN0.31PA6 is increased by ca. 84 % (from 1.77 to 3.26 GPa) and ca. 70 % (from 0.10 to 0.17 GPa) compare to those of pure PA6, respectively.


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Figure 13. Typical load of pure PA6 and GN0.25PA6 nanocomposite as a function of displacement in indentation measuement (a), and the modulus and hardness of PA6 and GNPA6 nanocomposites as a function of graphene loading (b). Figure 14a shows the map of an array of 8 by 8 indentations separated by 5 µm covering an area of 80×80 µm2, and the results of the modulus map are depicted in the Figure 14b. The values of modulus for the fractured surface of GN0.25PA6 nanocomposite lie in a narrow range from 2.8 to 3.2 GPa. The properties of microstructures of GN0.25PA6 nanocomposites are homogeneous, meaning that PA6 chains uniformly fill in the entire 3D graphene network and graphene disperses well in PA6 matrix.44,45



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（ Gpa）

(b)

2.800 2.850 2.900 2.950 3.000 3.050 3.100 3.150 3.200

1µm

Figure 14. Optic image of an array of 80×80 µm2 (a) and modulus contour plots constructed by indentation data (b) for GN0.25PA6 nanocomposite. In a word, the incorporation of 3D graphene network can significantly improve the mechanical properties of PA6, and the good dispersion of graphene and the strong interfacial strength between graphene and PA6 matrix which are the vitally important factors on reinforcing mechanism. Water resistance. It is well known that PA6 has a high affinity for water, and its mechanical properties and dimensional stability are often significantly affected by the absorption of water, greatly inhibiting its application on many cutting-edge fields.


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Recently, it has reported that graphene can act as a physical barrier for resisting water absorption in polymer.46,47 Therefore, it has expected that the 3D graphene networks could improve the water resistance of PA6. Figure 15 shows the weight change of PA6 and GNPA6 nanocomposites as function of immersion time in deionized water at 45 °C as function of square root time. It is observed that the equilibrium reaches at about 100 h of immersion time after a nearly linear increase for pure PA6. In the case of GNPA6 nanocomposites, the equilibrium times are prolonged and the absorbed amounts of water are significant decrease. The equilibrium water sorption of GN0.25PA6 is 6.6 wt.%, which decreases by 35.3 % compares to that of PA6 (10.2 %), and the time to equilibrium water sorption reaches to 900 h, which is 9 times of that of pure PA6. It is noted that GN0.25PA6 shows the lower diffusion rate and absorbed amount of water compared to those of rGO0.25/PA6 nanocomposite, indicating the better barrier effect of 3D graphene network compares to that of single graphene nanosheet in PA6. The water resistance of PA6-based composite is also related to the crystalline structures of PA6.48 Therefore, the improved water resistance of GNPA6 nanocomposites is attributed to the higher crystallinity of GNPA6 and the barrier effect of the integrated 3D graphene network. Blending with inorganic compounds or polymer is a common technique to improve the water resistance of PA6.49-51 However, there are still some obstacles for the use of as-prepared polymer composites. As for inorganic compounds, high loadings are usually necessary to meet the requirements of water resistance, which will in turn lead to a significant deterioration of other key properties of PA6.52 With regard to blend with polymer, it is difficult to achieve good compatibility with PA6, and thus weakens the modified efficiency.53 Compare with these traditional methods, the 3D graphene network developed herein, (i) promotes the crystallinity of PA6 and reduces the amorphous phase which results in the decrease of water uptake, (ii) exhibits large interfacial area and strong barrier effect toward water, (iii) not only improves the water resistance, but also improves other key properties of PA6 at low content of graphene. Therefore, the 3D graphene network developed herein show wider application prospect in high performance polymeric composites.



12 Weight change (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 8 6 PA6 rGO0.25/PA6 GN0.16PA6 GN0.25PA6 GN0.31PA6

4 2 0

0

10 20 Square root time (h1/2)

30

Figure 15. Weight change of PA6 and GNPA6 nanocomposites at 45oC

CONCLUSION A polyamide 6 nanocomposite (GNPA6) was fabricated with low content of graphene through reduction self-assembly of GO in the PA6 monomers, and then in situ polymerization. The fabrication is simple and efficient without pre-synthesizing free-standing 3D graphene network. Great improvements in the thermal conductivity, mechanical properties and water resistance have been achieved for the resultant GNPA6 nanocomposites. Specially, the as-prepared 3D graphene network exhibits the high efficiency on improving the thermal conductivity of PA6. The improved thermal conductivity of GNPA6 can be attributed to the compact as-prepared 3D graphene network which possesses larger specific surface area and acts as the effective thermal conductive path to transfer heat fast through the GNPA6 nanocomposites, and its strong interfacial interaction with PA6. Moreover, it is interesting to note that the constructed 3D graphene network can also act as a multifunctional modifier to obviously improve the mechanical and water resistance of PA6.

ASSOCIATED CONTENT Supporting Information Apparent density and specific surface area of graphene aerogel constructed in water and PA6 monomers filled 3D graphene network(Table S1); Molecular weight of free


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polyamide 6 (Table S2); Values of thermal conductivity and enhancement per wt.% of graphene-based polymeric composites (Table S3); Photograph of colloidal suspensions of exfoliated graphene oxide in water (a) and ε-caprolactam (b). The light beams were incident from the side to demonstrate the Tyndall effect. The content of graphene oxide were 8 mg/ml (Figure S1); XPS spectra of GO, PA6 monomers filled 3D graphene network and GNPA6 (Figure S2)

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was financially supported by the State’s Key Project of Research and Development Plan (Grant No.: 2016YFB1100900), the National Natural Science Foundation of China (Grant No. 51403212) and the Fujian-CAS STS Foundation (Grant No.: 2016T3035, 2016T3040 and 2018T3011).

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Table of Contents Graphic

Self-assembly of GO

GO/PA6 monomers solution

In situ polymerization

PA6 monomers filled 3D graphene network


High thermal conductive 3D graphene network/PA6 composite

Fabrication of Polyamide 6 Nanocomposite with Improved Thermal

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