Preparation and physico-mechanical properties of amine-functionalized graphene/polyamide 6 nanocomposite fiber as a high performance material

Abstract

A facile approach which is based on the different functionalities in graphene focused on facilitating polyamide 6 (PA6) to graft onto graphene surface to form homogeneous nanocomposite in which graphene was well-distributed, leading to increasing physico-mechanical properties of the composite. Graphene was oxidized to form graphene oxide (GO), which was then reacted with amine compounds to obtain the graphene bonding with amine functional groups of –NH₂ and –(CH₂)₆NH₂. The nanocomposite of functionalized graphene grafted by PA6 was fabricated by in situ polycondensation of caprolactam (CPL) and connection of the PA6 to the functionalized graphene, and their continuous nanocomposite fibers were prepared by use of melt spinning and drawing process. The grafting PA6 chains on graphene sheets were confirmed by FTIR, TGA and AFM measurements. Replacement of the –COOH group by –NH₂ and –(CH₂)₆NH₂ in the composite of PA6 and graphene changed the grafting polymerization chemistry, thereby leading to the covalent attachment of longer graft polymer chains to the graphene. Tensile strength of the nanocomposite fibers containing the –(CH₂)₆NH₂ functional group with 0.1 wt% graphene loading was significantly increased, over twice as high as that of neat PA6.

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1. Introduction

Following the discoveries of fullerene and carbon nanotubes (CNTs) in earlier decades,¹ the emergence of graphene with its combination of extraordinary physical properties has opened up an exciting new field in the science and technology of two-dimensional (2D) nanomaterials.^2–4 As an atomically thick 2D sheet composed of sp² carbon atoms arranged in a honeycomb structure, graphene has been viewed as the building block of all graphitic forms including 0D fullerene, 1D CNT and 3D graphite.⁵ Compared to CNTs, graphene is being deeply investigated due to its unique and excellent electrical,⁶ mechanical,⁷ and optical properties that stimulate burgeoning interest for its applications in nano-electronics, chemistry, biology, and nanocomposite material science.^8–14 Regarding to the preparation of high performance graphene-based nanocomposite, the challenge is clearly the attainment of a fine dispersion of the graphene in the polymer matrix. Thus, it is necessary to modify the surface of graphene by functionalization in order to make it get excellent interfacial adhesion with polymer matrices. Currently, the modification of graphene with polymers has been achieved via a variety of ways.¹⁵ Graphene oxide (GO) itself is one of the modified graphenes, and more suitable for covalent functionalizations than graphene, because a large number of chemical reactions can be taken place on its surface due to the abundance of different oxygenated groups.¹⁶ Indeed, using functionalized graphene, composites have been produced in aprotic solvents with hydrophobic polymers such as polystyrene (PS),¹⁷ polyurethane (PU),¹⁸ or poly methyl methacrylate (PMMA).¹⁹

As far as the preparation of nanocomposites based on polyamide 6 (PA6) and graphene is concerned, some works have been reported as follows. Steurer et al.²⁰ reported on the preparation of nanocomposites based on thermally reduced graphite oxide and PA6 by melt blending, which showed improved stiffness and lower percolation threshold with respect to the nanocomposites containing conventional carbon nanoparticles. Melt blending approach was also applied by Fukushima et al.²¹ to prepare PA6 composites containing exfoliated graphite flakes. Indeed, the addition of small amounts of the above filler showed a marked improvement in thermal and electrical conductivity of the composites. In zhang's paper,²² GO was firstly prepared using a modified Hummers and Offeman's method, then caprolactam (CPL) was fixed onto the GO sheets coupling by 4,4′-methylenebis, and then PA6 was grafted to the GO surface by in situ anionic ring-opening polymerization. When incorporation of 1.0 wt% of GO, the tensile strength and Young's modulus of PA6–GO nanocomposite films have been enhanced about 88.0% (95.9 ± 0.4 MPa) and 66.5% (1795.7 ± 65.3 MPa) in comparison with that of neat PA6, respectively. Xu²³ prepared graphene reinforced PA6 composites by in situ polymerization of CPL directly with GO. The tensile strength and Young's modulus of the fibers prepared from composites were 123 MPa and 722 MPa with the graphene loading of 0.1 wt%, respectively. Although melt copolymerization of caprolactam with GO in situ is an efficient method to improve the dispersity of graphene in PA6, existence of carboxyl will block the increase of the molecular weight of PA6. When incorporation of 1.0 wt% of GO, the molecular weight of PA6 is decreased from 20636 to 9840.²¹ Moreover, as an atomically thick 2D sheet composed of sp² carbon atoms,^2–4 stereo-hindrance effect in GO will hinder the ends of PA6 chains to attack the surface groups of GO, but using modified GO in polymer-based composite could enhance reinforcing efficiency of GO sheets, thus increase the mechanical properties of polymer.²⁴

It was reported²⁵ that, small chain primary amine grafted to GO made the GO derivatives disperse more easily in organic compounds. Furthermore, according to Gao's report,²⁶ existence of amine group can help to facilitate propagation of PA6 molecular chains. And the higher content of extended polymer chains can improve the dispersion of graphene on the substrate surface, overcoming the intermolecular van der Waals force between the graphene layers.²⁷ So we tried to select NH₃ and H₂N(CH₂)₆NH₂ to modify GO to form GO–NH₂, GO–(CH₂)₆NH₂, then the amine groups reacted with CPL or PA6 to connect to GO sheets, and nanocomposite fibers were prepared through melt spinning and drawing. It was found that tensile strength of the fibers was dramatically increased with small amount of functionalized GO loading.

2. Experimental

2.1 Materials

Graphite powder (20 μm), concentrated H₂SO₄, SOCl₂, K₂MnO₄, N,N-dimethylformamide (DMF), NH₃, 1,6-hexanediamine (H₂N(CH₂)₆NH₂), CPL, and 6-aminocaproic acid (H₂N(CH₂)₅COOH) were AR and purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2 Synthesis of graphene oxide

GO was fabricated from graphite powder (GP) by oxidation with K₂MnO₄ in concentrated H₂SO₄, followed by hydrolysis, washing and centrifugation, according to the modified Hummers' method.^28,29 This procedure has been confirmed by AFM to yield GO sheets with majority of monolayer dispersed structures.

2.3 Synthesis of amine-functionalized graphene oxide (GO–NH₂ and GO–(CH₂)₆NH₂)

GO–Cl was prepared by reacting GO with SOCl₂. As shown in Scheme 1, 50 mg GO was first sonicated in 20 mL of anhydrous DMF to form a homogeneous suspension, to which 50 mL SOCl₂ was added dropwise at 0 °C. GO–Cl was prepared by stirring at 0 °C for 2 h, and keeping stirring for 36 h at 70 °C. The obtained GO–Cl was collected by filtration through a PTFE membrane and then re-dispersed in 60 mL of anhydrous DMF. The GO–NH₂ and GO–(CH₂)₆NH₂ were synthesized by adding NH₃ and NH₂(CH₂)₆NH₂ into the dispersion of GO–Cl in DMF for about 4 h with magnetic stirring at 0 °C, respectively, and then the GO–NH₂ and GO–(CH₂)₆NH₂ were obtained by bringing the above mixtures to room temperature, filtering through a PTFE membrane, washing with water, and drying at 30 °C in vacuum for 24 h.

Scheme 1 Synthesis routes of PNG–COOH, PNG–NH₂ or PNG–(CH₂)₆NH₂.

2.4 Synthesis of PA6–Graphene (NG) nanocomposites

For example, a typical procedure to prepare PA6/GO–NH₂ nanocomposite with 0.1 wt% GO–NH₂ was depicted as follows: GO–NH₂ (10 mg) and CPL (9 g) were loaded into a 50 mL three-neck round-bottom flask and the mixture was sonicated at 80 °C for 2 h, then 6-aminocaproic acid (1 g) was added. The mixture was heated at 260 °C for 6 h with a steady stirring under N₂ atmosphere. The color of the mixture changed gradually from white to black, and the viscosity was also gradually enhanced during the reaction. After cooling to room temperature, the solid PA6–GO–NH₂ nanocomposite were crushed into small particles and washed four times in fresh boiling water each for 1.5 h to remove monomers and oligomers completely. The obtained black pieces of the nanocomposites were dried at 110 °C in vacuum for 24 h. In the end, we obtained the nanocomposites noted as 0.01 NG–NH₂, 0.1 NG–NH₂, 0.5 NG–NH₂, 1.0 NG–NH₂ corresponding to the PA6 and graphene nanocomposites containing GO–NH₂ 0.01 wt%, 0.1 wt%, 0.5 wt% and 1.0 wt%. Similar preparing procedure and notations were used for PA6/GO–(CH₂)₆NH₂ and PA6–GO nanocomposites. For convenience, the nanocomposites containing 0.1 wt% functionalized graphene were selected as main samples to study in this paper. Through repeating centrifugal-washing by formic acid to remove free PA6, purified PA6–grafted graphene (PNG) brushes were obtained, which was noted similar to NG, such as 0.1 PNG–(CH₂)₆NH₂. The free PA6 was also collected by precipitation into methanol.

2.5 Melt-spinning for NG nanocomposite fibers

The nanocomposite fibers were fabricated using a self-made apparatus (Fig. 1a). The melt obtained by heating the composite particles at 260 °C was extruded by nitrogen pressure through spinning hole to form long continuous as-spun fibers after cooling in the ambient atmosphere, and afterwards the fibers were drawn 4 times through heater at 120 °C and wound on the roller. Neat PA6 and different functionalized graphene–PA6 composite fibers are shown in Fig. 1b.

Fig. 1 (a) Melt spinning apparatus used for preparing nanocomposite fibers. (b) Photographs of neat PA6 and 0.1 NG fibers.

2.6 Characterization

Elemental analyses were determined using a Vario EL III elemental analyzer. Atomic force microscope (AFM) observation of GO sheets and PA6–graphene sheets were recorded using a Vecco Digital Instrument Multimode V scanning probe microscope, in which samples prepared by spin-coating sample solutions onto freshly exfoliated silicon substrates. Scanning electron microscopy (SEM) images were taken on a Hitachi S-4700 field-emission SEM system. X-ray diffraction (XRD) analysis was performed on an X'Pert-Pro MPD diffractometer with a Cu Kα radiation source at room temperature. Raman spectra were collected on a Jobin-Yvon Raman spectroscope equipped with a 514 nm laser source (HR800, france). Fourier-transform infrared (FT-IR) spectra were recorded on a ProStar LC240 (Varian, USA). Differential scanning calorimetric (DSC) analysis was performed on TA Q200 equipment. In DSC analysis, dried samples under vacuum were heated to 250 °C and cooled slowly to room temperature in the first cycle to remove thermal history. The data were collected using a heating rate of 20 °C min⁻¹ in a nitrogen atmosphere in the second cycle. Thermal gravimetric analysis (TGA) was carried out on a Perkin-Elmer Pyris 6 TGA instrument at a heating rate of 20 °C min⁻¹ under nitrogen flow. The data of the mechanical properties were collected under a draw speed of 5 cm min⁻¹ at room temperature from electronic yarn strength tester HD021N (Nantong Hongda, China). Intrinsic viscosity of PA6 contained in NG nanocomposites was measured at 25 °C in 98% sulfuric acid solution with the PA6 concentration of 4 g L⁻¹ by Ubbelohde viscometer.

3. Results and discussion

3.1 Synthesis and characterization of PA6−grephene sheets

In our experiments, PNG nanocomposites, i.e. NG nanocomposites removing free PA6, with a range of graphene loadings were synthesized by in situ ring-opening polymerization of CPL in the presence of GO, GO–NH₂, GO–(CH₂)₆NH₂ with 6-aminocaproic acid as a initiator. Amine or carboxyl end groups taken by the PA6 molecular chains of the PNG nanocomposites depend on the functionalized groups on graphene (Scheme 1).

The FTIR spectrum of the functionalized GO is shown in Fig. 2a. The spectrum of GP has no clear peaks, while as an oxide of GP, GO shows spectrum with some peaks: the broad peak at 1600 cm⁻¹ assigning to C=C double bonds located near the functionalized sites and the peak at 1740 cm⁻¹ corresponding to the C=O stretch of the carboxylic acid groups, etc. For the spectra of GO–NH₂, GO–(CH₂)₆NH₂, the band at 1630 cm⁻¹ corresponds to the stretching vibration of the C=O group of the amide functionality in –CONH–. In the spectra of PNG–(CH₂)₆NH₂, the characteristic absorptions of amide I (at 1644 cm⁻¹) and amide II (at 1550 cm⁻¹) were clearly observed, and the bands at 1250 cm⁻¹ and 3330 cm⁻¹ are assigned to C–N and N–H bond stretching vibrations, respectively. From the results of FTIR of GO–NH₂, GO–(CH₂)₆NH₂ and PNG–(CH₂)₆NH₂, it can be concluded that the amine groups or PA6 chains have been incorporated into the sheets of GO. The Raman spectra in Fig. 2b show the characteristic peaks located at 1339 and 1598 cm⁻¹ corresponding to the D and G bands of graphene sheets, respectively. Relative to the G band of GO (1598 cm⁻¹), the G band of the NG nanocomposites (1591 cm⁻¹) shifts toward lower wavelength which close to the value of graphite, bearing out the thermal reduction of GO during condensation polymerization.³⁰

Fig. 2 (a) FTIR spectra of GP, GO, GO–NH₂, GO–(CH₂)₆NH₂ and 0.1 PNG–(CH₂)₆NH₂. (b) Raman spectra of NG–(CH₂)₆NH₂ nanocomposites and GO.

The good dispersion of GO or functionalized GO in CPL melt is a premise for better grafting onto graphene for CPL. It is clear that GO–NH₂ and GO–(CH₂)₆NH₂ are well dispersed in CPL as shown in Fig. 3a3 and a4, while there are some sediment precipitate on the bottom of the bottle for GO dispersion in CPL in the same situation as shown in Fig. 3a2, suggesting that amine-treated GO containing amide bond bears a strong chemical similarity to CPL.

Fig. 3 (a) Photographs of (1) caprolactam melt and (2)–(4) dispersions of caprolactam containing 0.1 wt% loading of GO, GO–NH₂, and GO–(CH₂)₆NH₂ at 80 °C, respectively. (b) Optimized structures of (1) GO sheet, (2) GO–NH₂ sheet, and (3) GO–(CH₂)₆NH₂ sheet.

Fig. 3b shows schematic diagram of GO, GO–NH₂ and GO–(CH₂)₆NH₂ sheets. It must be emphasized that among these three functionalized graphenes, the functional group –(CH₂)₆NH₂ on GO is the longest (Fig. 3b3), which can get more space to contact with end groups on PA6 chains, leading to more CPL or PA6 chains grafting onto the surface of graphene sheet. On the other hand, as stated above, amine groups ended graphene can favour PA6 chain propagation, and because GO–(CH₂)₆NH₂ has longer molecular chain plus good dispersity in CPL melt, longer PA6 chains grafted graphene and higher conversion of CPL to PA6 are expected.

As shown in Table 1, the intrinsic viscosity (η_in) and the viscosity-average molecular weight (M_η) of NG–COOH is lowest, NG–NH₂ takes second place, while NG–(CH₂)₆–NH₂ has the highest molecular weight. Then we roughly estimate the grafting ratio of PA6 grafted onto graphene. Through repeating centrifugal-washing by formic acid to remove free PA6 in the composite, the PNG was collected. The grafting ratio of PA6 grafted onto GO sheets is the lowest (465.0%), while PA6 grafted onto GO-(CH₂)₆NH₂ sheets is the highest (726.4%). On the other hand, elemental analysis of the PNGs is shown in Fig. 4. It is found that the contents of nitrogen (N) in PNG–NH₂ and PNG–(CH₂)₆NH₂ are distinctly higher than PNG–COOH, indicating the contents of PA6 in PNG–(CH₂)₆NH₂ and PNG–NH₂ are higher than PNG–COOH, so we can infer that, more or longer PA6 chains were grafted to GO–NH₂ and GO–(CH₂)₆NH₂ sheets.

Table 1 Molecular weight of free PA6 of 0.1 NG composites and grafting ratio of PA6 in graphene sheet

Samples	PA6	NG–COOH	NG–NH2	NG–(CH2)6–NH2
a Intrinsic viscosity of free PA6 extracted from NG nanocomposites, which was measured at 25 °C in concentrated sulfuric acid solution by a Ubbelohde viscometer.b The viscosity-average molecular weight (Mη) was calculated using the Mark–Houwink equation, ηin = K[Mη]^α, where K = 2.26 × 10^−4, α = 0.82.^23c The grafting ratio of polymer was calculated by [m(PNG) − m(graphene)]/m(graphene) × 100%, where m(graphene) = m(NG) × 0.1%.^31
[η]^a	0.80	0.71	0.78	0.81
Mη^b	21 369	18 475	20 717	21 693
Grafting ratio (%)^c	–	465.0	599.3	726.4

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Fig. 4 Pie charts of element content of N, H, C in PA6 and 0.1 PNGs.

AFM is a direct method to characterize the morphologies and the thickness of the interface layer grafted from the graphene surface. Fig. 5 shows the tapping mode AFM images of GO (Fig. 5a) and different PNG sheets (Fig. 5b–d) with different functional groups on graphene. The thickness of GO is about 0.8 nm, which agrees with other researchers' reports,^22,23 suggesting the complete exfoliation of graphite oxide into individual GO sheet. After grafting PA6 chains onto the GO surface, the thickness of PNG–COOH (GO grafted PA6 chains) (Fig. 5b) reaches about 8 nm. In the contrast, as shown in Fig. 5c, the thickness of PNG–NH₂ (GO–NH₂ sheets grafted PA6 chains) increases to about 12 nm, suggesting that there are longer grafted PA6 chains on GO–NH₂ than those on GO–COOH. Similarly, in Fig. 5d, the thickness of PNG–(CH₂)₆NH₂ (GO–(CH₂)₆NH₂ grafted PA6 chains) raises to about 14 nm, suggesting longer and more grafting chains with high chain density due to the effect of the functional groups –(CH₂)₆NH₂ as stated above. So as shown in Scheme 2, PNG–NH₂ and PNG–(CH₂)₆NH₂ have longer chains than PNG–COOH, and in addition, PNG–(CH₂)₆NH₂ has denser chains.

Fig. 5 (a) AFM height image of GO on silicon wafer deposited from aqueous solution. (b)–(d) AFM height images of 0.1 PNG–COOH, 0.1 PNG–NH₂ and 0.1 PNG–(CH₂)₆NH₂ on silicon wafer spinning-cast from the solution of formic acid, respectively.

Scheme 2 Structural illustration of three types of PA6 grafted graphene sheets.

The overall quantity of the grafted PA6 on the graphene sheets can be obtained from TGA. The weight loss curves for GO, PNGs, and neat PA6 are presented in Fig. 6. The weight loss of GO almost increases with temperature proportionally and slowly. However, there are still several stages on the curve. The first stage starts below 100 °C due to the volatilization of stored water in its π-stacked structure. The relatively large weight loss stage was observed around 260 °C, which is attributed to the pyrolysis of unstable oxygen-containing functional groups. In contrast, PA6 grafted GO nanocomposites appear to be effective in enhancing thermal stability. Although the starting temperature of the maximum weight loss stage of the PNG–COOH is a little bit lower than that of neat PA6, it can be considered that the weight loss of this stage is due to the decomposition of grafted PA6 chains. Combining Fig. 6b, estimated from the TGA curve, the weight loss of PNG–COOH is about 78.1 wt% at 800 °C, while the weight loss of PNG–NH₂ and PNG–(CH₂)₆NH₂ increases to 82.7 wt% and 85.2 wt%, respectively, presenting a higher grafting quantity of PA6 chains on the GO surface, which agrees with the above results.

Fig. 6 (a) TGA curves and (b) weight loss percentages of the pristine GO, PNG sheets and PA6.

3.2 Melting and crystallization behavior of NG nanocomposites

As shown in Fig. 7a, it is found that the neat PA6 and all the NG nanocomposites have only one exothermic peaks and the incorporation of graphene sheets leads to a considerable increase in crystallization temperature (T_c), implying that the well dispersed graphene sheets act as nucleation agents during crystallization of PA6. T_c of NG follows the order NG–(CH₂)₆NH₂ > NG–NH₂ > NG–COOH, indicating under the same conditions, longer grafting chains cause chain movement to be difficult, so result in higher T_c. As shown in Fig. 7b, all samples contain two principal diffraction peaks, α₁ (I₁) at 20° and α₂ (I₂) at 24° corresponding to the (200) and (002/202) reflections of α-form crystals of PA6, respectively.^32,33 It is found that the intensity of the peaks at 20° almost does not change, while the intensity of the peaks at 24° become gradually weak in the order of PA6, NG–COOH, NG–NH₂, and NG–(CH₂)₆NH₂, indicating that more and longer grafted PA6 chains disturb the ordered arrangement of hydrogen-bonded sheets in PA6 matrix to prevent the growth of α₂ crystal. It is well-known that PA6 has two crystal forms: more thermodynamically stable α-form crystal with hydrogen bonds between antiparallel chains and γ-form crystal with hydrogen bonds between parallel chains. As shown in Fig. 7c, neat PA6 shows one main peak at about 214.5 °C ( T_m1) and a shoulder at about 203 °C (T_m2), indicating the co-existence of α and γ crystal structures of PA6. The main peaks (T_m1) of all the NG nanocomposites are sharper than neat PA6 and move towards higher temperatures, reflecting existence of more complete α-form crystal. Among them, T_m1(218.8 °C) for NG–COOH is slightly lower than the two others, and T_m2 disappears, suggesting the depressed γ–form crystallization of PA6 and the graphene composite. However, a slight shoulder for NG–NH₂ shows up again, and the shoulder for NG–(CH₂)₆NH₂ becomes clear, indicating γ-form crystallization behavior. For the PNG as shown in Fig. 7d, although the shoulders disappear, the main peaks remain sharper and higher than neat PA6, which basically agrees with the situations in Fig. 7c. PNG–COOH has obviously lower melting temperature, which answers for that the length of the polymer chains get shortened with introduction of GO for the termination to the active chains by carboxyl acids on GO sheets during the in situ polymerization for NG nanocomposites. The results seem contradictory in contrast with that shown in Fig. 7b, indicating that 2D nanosheets have comprehensive influences on the crystallization, aggregation or assembly behaviors of polymer chains. In this case, PA6 grafted graphene having amine end group may have low crystallinity but high crystal perfection.

Fig. 7 (a) and (c) DSC curves of neat PA6 and 0.1 NGs. (b) XRD patterns of neat PA6 and 0.1 NGs. (d) DSC hearting curves of neat PA6 and 0.1 PNGs.

3.3 Tensile properties of NG nanocomposite fibers

The nanocomposite fibers were fabricated using an apparatus of melt spinning assembly, and the fiber diameter from about 100 μm of the as-spun fibers to about 30 μm of the drawn fibers. The mechanical properties of the nanocomposite fibers are compared in Fig. 8. The fibers prepared using NG–NH₂ and NG–(CH₂)₆NH₂ have higher tensile strength than the fibers prepared from neat PA6 and NG–COOH. With the functionalized graphene content increases, the tensile strength first increases then decreases. The highest tensile strength of the NG–(CH₂)₆NH₂ nanocomposite fibers was over twice higher than the neat PA6 fiber with 0.1 wt% loading. Besides the well-known reinforcement of graphene itself to PA6 fibers, longer PA6 chains grafted to the graphene than the polymer chains grafted to GO, and good distribution of the graphene in PA6 matrix are also important reasons for the increase of the fiber strength. However, when the content of graphene is increased to 0.5%, the tensile strength is decreased, which suggests to be due to the agglomerates of large amounts of graphene in nanocomposite fibers.

Fig. 8 Tensile strength of NG nanocomposite fibers with different loadings of graphene.

Meanwhile, the cross sections of the nanocomposite fibers may further explain the improvement of mechanical properties. The cross sections were prepared by cutting the fibers in liquid nitrogen to give an intact surface fracture as shown in Fig. 9. The bright regions in these images are attributed to the graphene as a result from charge accumulation. It is apparent that the graphene are homogeneously distributed in the PA6 matrix without aggregation for all the nanocomposites prepared using functionalized graphene. On the other hand, the cross-section of neat PA6 fiber as shown in Fig. 9a looks smooth, while the other three cross sections display unevenness at different degrees, especially for nanocomposite fibers of NG–(CH₂)₆NH₂ as shown in Fig. 9d, which may result from the strong interaction of PA6 and graphene.

Fig. 9 SEM images of cross-sectional structure of (a) neat PA6 fiber and (b)–(d) nanocomposite fibers of 0.1 NGs with the following functionalities of –COOH, –NH₂ and –(CH₂)₆NH₂, respectively.

4. Conclusions

In this work, we synthesized PA6-graphene nanocomposites using different functionalised graphene oxide, the PA6 chains were effectively grafted onto the graphene sheets, accompanying with the reduction from GO to graphene. GO–NH₂ and GO–(CH₂)₆NH₂ are well dispersed in CPL, which is helpful to in-situ polymerization of CPL on the graphene, while there are some sediment precipitates for GO dispersion in CPL in the same situation. Amine groups ended graphene can favour chain propagation, and because GO–(CH₂)₆NH₂ has longer molecular chain plus good dispersity in CPL melt, longer chain PA6 grafted graphene and higher conversion of CPL to PA6 were achieved. The continuous nanocomposite fiber was fabricated using melt spinning and hot drawing process. NG–(CH₂)₆NH₂ performances the highest tensile strength 344.2 MPa, twice higher than that of neat PA6 fiber. Except the well-known reinforcement of graphene itself to PA6 fibers, longer PA6 chains grafted to amine functionalized graphene, particularly GO–(CH₂)₆NH₂, than the polymer chain to GO, and good distribution of graphene in PA6 matrix are also important reasons. Comparison of the nanocomposites prepared using GO, GO–NH₂ and GO–(CH₂)₆NH₂ indicates that the nature of the functional groups affects the grafting chemistry of PA6 to the graphene and thereby the PA6-graphene interaction, composite morphology, and mechanical properties.

Acknowledgements

This research was supported by Natural Science Fund of Jiangsu Province (BK2012623) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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