Preparation and properties of nylon 6/sulfonated graphene composites by an in situ polymerization process

Abstract

Nylon 6/sulfonated graphene (NSG) composites were prepared using sulfonated graphene (SG) with strong polar sulfonic acid groups as a precursor by the in situ hydrolytic ring-opening polymerization of ε-caprolactam. SG dissolved in the water and then quickly dispersed in an ε-caprolactam melt with simple stirring in an autoclave. The generated PA6 chains were covalently grafted onto SG sheets by the condensation reaction between the active amino groups at PA6 chain terminals and the sulfonic acid groups on SG sheets. The grafted structure and SG content have a great effect on their properties. Compared with pure nylon 6 (PA6), the mechanical properties of NSG composites can be maintained and even enhanced by the use of an appropriate SG content. Research on crystallization and rheological behaviors indicate that NSG composites have a faster crystallization rate and higher flowability than pure PA6, which are beneficial for the use of rapid molding processes. Moreover, the homogeneous dispersion of SG sheets in NSG composites is conducive to the formation of consecutive thermal conductive paths or networks at a relatively low SG content, which significantly improves the thermal conductivity from 0.203 W m⁻¹ K⁻¹ for pure PA6 to 0.398 W m⁻¹ K⁻¹ for a NSG composite with only 3 wt% SG content. Such NSG composites with a simple preparation process, good mechanical properties, excellent processability and high thermal conductivity provide great promise for wider applications of PA6 materials in thermal conductive systems.

---

Introduction

Nylon 6 (PA6) is a thermoplastics polymer containing an amide repeat unit on the main chain, which serves as a very important engineering plastic due to its light weight, good mechanical properties, excellent chemical resistance, simple processing technique and relatively low cost.^1–6 With the rising demands for products in diversified markets, single pure nylon 6 resin cannot satisfy all the requirements perfectly. To improve its property and broaden its application, various nylon 6 composites modified with nanofillers have been developed in recent years, such as nylon 6/montmorillonite composites,⁷ nylon 6/carbon nanotube composites,^8–10 nylon 6/graphene composites^11–14 and so on.

Graphene, well-known for its outstanding conductivity, heat resistance and high mechanical properties, has been recognized as one of the most promising carbon materials, after fullerene and carbon nanotubes, since its discovery in 2004 by Novoselov et al.¹⁵ Incorporation of graphene into a nylon 6 matrix, which can endow nylon 6 composites with unique performances and new applications, has recently attracted considerable interest.^16–18 However, pristine graphene contains no surface functional group and has very limited dispersibility in a nylon 6 matrix, seriously restricting its potential application in the preparation of functional composites. Therefore, chemical modification is needed for graphene to achieve a homogeneous dispersion in nylon 6 matrix with optimum properties.

Most of the previous studies for the preparation of graphene-based nylon 6 composites employed graphene oxide (GO) as a precursor since there are abundant oxygen-containing functional groups on a GO sheet, such as hydroxyl, epoxy and carboxyl groups.^19–22 The presence of such functional groups does make GO a good support for uniformly dispersion in the water and polar organic solvents after sufficient ultrasonic treatment. Furthermore, the functional groups of GO can be utilized to further modify GO through some chemical reactions. For example, Xu et al. reported an effective method to prepare nylon 6/graphene composites on the basis of the carboxyl groups of GO by in situ polymerization with a simultaneous thermal reduction from GO to graphene, and the grafted graphene sheets exhibited good compatibility with PA6 matrix in composites.¹⁹ Ding et al. also prepared nylon 6/graphene composites by this method in the presence of GO and investigated the influence of the grafted structures of different lengths PA6 chains on their thermal conductivity properties.^23,24

Sulfonated graphene (SG) is another kind of functionalized graphene, which has been widely used in the field of battery, supercapacitors and dye absorbents.^25–28 The pendent strong polar sulfonic acid groups on SG sheets not only provide reactive sites for modification, but also offer good solubility in water and in some polar organic solvents.^29,30 Accordingly, it is easier to disperse SG in a mixture of water and ε-caprolactam melt with simple stirring in an autoclave than it is with GO, and there is no need for additional ultrasonic treatment, which will be more suitable for industrialization. Moreover, SG retains a conjugated sp² network structure of graphene and exhibits high conductivity and stability.²⁸ Undoubtedly, it is advantageous to prepare SG-based nylon 6 composites with good dispersibility and excellent properties. However, to our knowledge, few reports focus on SG-based nylon composites, let alone SG-based nylon 6 composites.

In the present work, we first chose SG with strong polar sulfonic acid groups as a precursor to prepare nylon 6/sulfonated graphene (NSG) composites by the in situ hydrolytic ring-opening polymerization of ε-caprolactam, and then systematically investigated their mechanical properties, thermal properties, crystallization behaviors, flowabilities, rheological behaviors and, in particular, thermal conductivities. Our results demonstrate how the grafted structure and SG content influence the NSG composite properties by comparing them with pure PA6.

Experimental

Materials

Sulfonated graphene (99%) was purchased from Graphene-Tech (China) Inc. ε-Caprolactam (99%) was provided by Yueyang Chemical Fiber Co., Ltd. All the reagents were used as received from commercial sources.

Preparation of NSG composites

NSG composites with different SG content were prepared by an in situ hydrolytic ring-opening polymerization process. The preparation route to the NSG composites is shown in Scheme 1. A typical procedure is summarized as follows. SG was dissolved in water, and then the SG aqueous solution and ε-caprolactam melt added into a 5 L autoclave under stirring. The mixture in the autoclave was deoxygenated by a repeatedly pumping vacuum and purging with high purity N₂ three times. The reaction system was heated to 180 °C for 2 h and then to 240 °C for 2 h with stirring. The inner pressure reached 0.5–1.0 MPa due to the spontaneous pressure generated by water vapor from the reaction system in the autoclave. After that, the inner pressure was gradually reduced to atmospheric pressure in about 0.5–1 h. The reaction was then allowed to proceed for a further 4 h at 250 °C under gradually reduced pressure up to −0.07 MPa. Subsequently, the agitator was stopped and the autoclave was filled with N₂. The product was drawn into long strands in a water bath and pelletized. These particles were extracted in boiled water for 24 h and then dried under vacuum at 80 °C for 12 h. Finally, five NSG composites were prepared and named NSG-0.2, NSG-0.5, NSG-1.0, NSG-2.0 and NSG-3.0 according to the feed content of SG. The NSG composite samples for the structure characterization were purified by repeated centrifugal-washing by formic acid to remove free PA6.^19,23,24 The free PA6 was also collected by precipitation into methanol and then used for the intrinsic viscosity measurements to obtain viscosity-average molecular weights. For comparison, pure nylon 6 was prepared using the same preparation process.

Scheme 1 Preparation of NSG composites by the in situ hydrolytic ring-opening polymerization of ε-caprolactam using SG as a precursor, and a photograph and structure illustration of the NSG-2.0 wafer.

Instrumentation

The morphology and microstructure of the samples were observed by atomic force microscopy (AFM, NanoMan VS, USA), scanning electron microscopy (SEM, ZEISS EVO-18, Germany) and high-resolution transmission electron microscopy (TEM, FEI Tecnai G2 F30, USA). FTIR spectra were measured with a Nicolet IS 10 Fourier transform infrared spectrometer with the attenuated total reflection (ATR) technology. X-ray photo-electron spectroscopy (XPS) was recorded on a Thermo Fisher K-Alpha 1063 spectrometer. Thermal gravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/DSC1 1100SF instrument at a heating rate of 10 °C min⁻¹ under N₂.

The wide-angle X-ray diffraction (WAXD) patterns of melt-crystallized sample films were recorded on a Philips X′ Pert Pro diffractometer with a 3 kW ceramic tube as the X-ray source (Cu Kα) and an X′ celerator detector in the 2θ range from 3 to 40° at a scanning step of 0.2°. The crystallization and melting behaviors of the samples were detected using a Netzsch DSC 204 F1 differential scanning calorimeter (DSC) at a heating and cooling rate of 10 °C min⁻¹ under N₂.

All the samples for mechanical property tests were prepared by injection molding. Tensile strength and elongation at break were measured according to GB/T 1040.2-2006. Bending strength and bending modulus were measured according to GB/T 9341-2008. The Charpy impact notched strength was measured according to GB/T 1043.1-2008. The moisture contents of the samples were regulated under standard conditions with temperature at 23 °C and relative humidity at 50% for 18 h.

The intrinsic viscosity (η_in) measurement of the samples dissolved in 85% formic acid with a concentration of 0.5 g dL⁻¹ was determined by an Ubbelohde viscometer at 25 °C. The melt flow rate (MFR) in g per 10 min of the samples was measured by a MTS ZRZ1452 melt flow rate tester at 235 °C and a 0.325 kg load according to GB/T 3682-2000. The dynamic rheological property of the samples was investigated with a TA ARES rheometer using parallel-plate geometry (25 mm diameter). A dynamic frequency sweep ranging from 0.1 to 100 rad s⁻¹ was performed at 235 °C with a fixed strain of 1.0% in order to fall in the linear viscoelasticity region. After loading, the samples were kept in equilibrium for 5 min prior to the frequency sweep. The thermal conductivity of NSG composites was measured with a Xiangyi DRL-III thermal conductivity tester by a heat flow meter method.

Results and discussion

Preparation and characterization of NSG composites

NSG composites with different SG content were prepared by the in situ hydrolytic ring-opening polymerization of ε-caprolactam in the presence of SG, as shown in Scheme 1. Due to the strong polar sulfonic acid groups on the SG sheets, they dissolve in water and then quickly dispersed in an ε-caprolactam melt with simple stirring in an autoclave. At high temperatures, ε-caprolactam was initiated by water to conduct the ring-opening polymerization. With the consumption of ε-caprolactam monomers, the PA6 chains step-propagated, and the active PA6 chains were grafted onto SG sheets by a condensation reaction between the active amino groups at the PA6 chain terminals and the sulfonic acid groups on the SG sheets. Along with continuous propagation of the PA6 chains, the viscosity of the mixture increased gradually, and the melting NSG composites with SG content lower than 1.0 wt% obviously exhibited a Weissenberg effect³¹ under stable stirring at the end of reaction, which indicated NSG composites with a high molecular weight were prepared successfully.

AFM measurements were used to observe the microstructure of NSG composites. Fig. 1 shows the AFM images of SG and NSG composites deposited by spin-casting from an aqueous solution and a formic acid solution, respectively. Note that the grafting of PA6 chains on SG sheets obviously increased the thickness of SG sheets from 1.529 nm to 9.541, 7.624, 5.837, 4.286 and 3.172 nm for NSG-0.2, NSG-0.5, NSG-1.0, NSG-2.0 and NSG-3.0 composites, respectively. The results confirm that the PA6 chains have been grafted onto SG sheets and the length of the grafted PA6 chains decreases with increasing SG content, which is similar to the results for nylon 6/graphene composites reported by Ding et al.²³

Fig. 1 AFM images of SG and NSG composites with different SG content. (a) NSG-0.2, (b) NSG-0.5, (c) NSG-1.0, (d) NSG-2.0, (e) NSG-3.0 and (f) SG.

The FTIR spectra of SG and NSG composites with different SG contents are shown in Fig. 2. For the FTIR spectrum of SG, the peak at 1622 cm⁻¹ is attributed to a C=C skeletal vibration, and the peak at 1175 cm⁻¹ is assigned to a S=O stretching vibration, which is the characteristic peak for sulfonic acid groups on SG sheets. Compared with SG, some new peaks appear in the FTIR spectrum of NSG composites, such as the peak at 1638 cm⁻¹ assigned to the C=O stretching vibration of amide groups, the peak at 1542 cm⁻¹ assigned to the N–H and C–N stretching vibration of amide groups, and the peaks at 2933 and 2860 cm⁻¹ associated with the C–H stretching vibration of methylene groups in grafted PA6 chains, which also indicate the existence of PA6 chains grafted on SG sheets.

Fig. 2 FTIR spectra of SG and NSG composites with different SG content.

To further confirm the successful grafting of PA6 chains onto SG sheets, XPS was used to characterize the SG and NSG composites with different SG content as shown in Fig. 3. The XPS spectrum of SG offers signals mainly associated with C 1s (284.8 eV), O 1s (532.1 eV), S 2s (232.3 eV) and S 2p (168.4 eV), which confirms the presence of sulfur element in SG,^32–34 but no signal of a nitrogen element is detected. For NSG composites, a strong signal of N 1s is found at 401.5 eV besides the signals of C 1s and O 1s, indicating the abundance of the nitrogen element in NSG composites due to the grafted PA6 chains on the SG sheets.³⁵ Moreover, the intensity of the N 1s signal slightly decreases with increasing SG content, which agrees well with the results of AFM.

Fig. 3 XPS spectra of SG and NSG composites with different SG content.

Fig. 4 shows TGA curves of SG, pure PA6 and NSG composites with different SG content. For the TGA curve of SG, the weight loss below 150 °C is ascribed to the release of adsorbed water, and the initial thermal decomposition temperature (T_d) is 285.4 °C due to the pyrolysis of the sulfonic acid groups on SG. However, NSG composites exhibit excellent thermal stabilities with the T_d above 380 °C similar to that of pure PA6 and no obvious weight loss is detected below 300 °C, indicating the successful grafting of PA6 chains on SG sheets and the elevated thermal stabilities after grafting. Furthermore, as shown in Fig. 4 and Table 1, the initial thermal decomposition temperature (T_d) and the final weight loss (ΔW) of NSG composites both decrease with increasing SG content, suggesting the length of grafted PA6 chains on SG sheets decreases with increasing SG content.

Fig. 4 TGA curves of SG, pure PA6 and NSG composites with different SG content.

Table 1 TGA data of NSG composites and molecular weight characterization data of PA6 chains in NSG composites

Sample	Td^a (°C)	ΔW^a (%)	ηin^b (dL g^−1)	Mη^c (×10^3)
a The initial thermal decomposition temperature (Td) and the final weight loss (ΔW) at 700 °C were measured by TGA at a rate of 10 °C min^−1 under N2.b The intrinsic viscosity (ηin) of free PA6 in NSG composites was measured at 25 °C in 85% formic acid solution by an Ubbelohde viscometer.c The viscosity-average molecular weight (Mη) of grafted PA6 was speculated according to Mark–Houwink equation.
PA6	392.6	99.6	0.711	18.43
NSG-0.2	393.1	98.4	0.578	14.32
NSG-0.5	392.5	97.7	0.467	11.04
NSG-1.0	391.8	97.1	0.365	8.17
NSG-2.0	391.2	96.2	0.278	5.86
NSG-3.0	390.4	94.8	0.224	4.51

---

To evaluate the molecular weight of the grafting PA6 chains on SG sheets, the intrinsic viscosity (η_in) of free PA6 collected from the supernate of NSG composite solutions after centrifuging was investigated. The η_in of free PA6 was obtained by an Ubbelohde viscometer, and the M_η of the grafted PA6 chains on the SG sheets was speculated according to the Mark–Houwink equation, η_in = K[M_η]^α (where K = 2.26 × 10⁻⁴ and α = 0.82 at 25 °C). This method has been widely used in previous work by other researchers to obtain M_η from the data of η_in.^19,23,24 As shown in Table 1, the M_η of the grafted PA6 chains decreases with increasing content of SG, from 18.43 × 10³ g mol⁻¹ for pure PA6 to 4.51 × 10³ g mol⁻¹ for the NSG-3.0 composite. The M_η of the grafted PA6 chains on SG sheets depends on the content of SG, actually the whole content of sulfonic acid groups on SG sheets. The sulfonic acid groups can react with the amino groups, inevitably break the stoichiometric balance between carboxyl groups and amino groups in the multi-active site reaction system, and terminate the potential propagation of active PA6 chains. This result is consistent with the aforementioned result of TGA.

Mechanical properties of NSG composites

The mechanical properties of injection molded samples of pure PA6 and NSG composites with different SG content are shown in Table 2. Compared with pure PA6, NSG composites with low SG content, such as NSG-0.2 composite, show an increase in the tensile strength, impact strength and in particular bending strength, due to the reinforcement effect of SG sheets with large specific surface areas.¹⁹ However, these mechanical properties obviously decrease when NSG composites have high SG content, particularly for NSG-3.0. The poor mechanical properties of NSG-3.0 composite are mainly attributed to the low molecular weight of the grafting PA6 chains on SG sheets. Moreover, the elongation at break of NSG composites decreases with increasing SG content, due to the high density of grafting and the inability to undergo as much reorientation and molecular “slippage”.³⁶ Therefore, the mechanical properties of NSG composites greatly depend on the content of SG, and the mechanical properties can be maintained and even enhanced by the use of an appropriate SG content.

Table 2 Mechanical properties of pure PA6 and NSG composites with different SG content

Sample	Tensile strength^a (MPa)	Elongation at break^a (%)	Bending strength^b (MPa)	Bending modulus^b (MPa)	Impact strength^c (kJ m^−2)
a Tensile strength and elongation at break were measured according to GB/T 1040.2-2006.b Bending strength and bending modulus were measured according to GB/T 9341-2008.c Charpy impact notched strength was measured according to GB/T 1043.1-2008. The standard deviations are all less than 5%.
PA6	68.0	110	72.3	2335	11.6
NSG-0.2	68.4	100	95.6	2468	11.8
NSG-0.5	67.7	80	93.7	2412	11.5
NSG-1.0	65.6	55	90.8	2427	10.7
NSG-2.0	61.2	32	87.1	2376	9.8
NSG-3.0	54.8	14	82.5	2313	8.4

---

The crystallization and melting behavior of NSG composites

In composites of crystalline polymer, the crystallization has great effect on their performances. The crystal structure of NSG composites was investigated by WAXD. Fig. 5 shows the WAXD patterns of pure PA6 and NSG composites with different SG content. The diffraction peaks of NSG composites located at 2θ = 20.2 and 23.4° are corresponding to 200 and 002/202 reflections of α-form crystals, which is the same as those of pure PA6. It can be concluded that grafting does not change the crystal structure, and SG sheets are excluded from the crystalline region and located in the amorphous region. As the content of SG increases, the two diffraction peaks become weaker, indicating the more depressed crystallization in NSG composites.^19,23,24

Fig. 5 WAXD patterns of pure PA6 and NSG composites with different SG content.

The crystallization and melting behavior of NSG composites were investigated by DSC. The DSC curves of NSG composites during the first cooling and subsequent heating process are shown in Fig. 6 and the corresponding data are listed in Table 3. It can be found that the melting point (T_m) decreases with increasing content of SG, from 222.1 °C for pure PA6 to 218.2 °C for NSG-3.0 composite, which is ascribed to the more depressed crystallization and the lower molecular weight of grafting PA6 chains caused by the increasing SG content.^19,23,24 In NSG composites, the well-dispersed SG sheets block off the interconnected matrices to form confined regions of PA6 chains. The more content of SG means the stronger confinement to PA6 chains. The confined mobility of polymer chains is a leading dynamic factor to form crystals by arrangements of polymer chains.³⁷ This phenomenon was also reported in other nanofillers according to the previous literatures.^38,39 The crystallinity (X_c) was also determined from DSC analysis with the aid of the fusion enthalpy of 230.1 J g⁻¹ for the perfectly crystalline PA6. As shown in Table 3, the X_c of NSG composites is slightly lower than that of pure PA6, and the X_c of NSG composites decreases with increasing content of SG, which is in good agreement with the result of T_m.

Fig. 6 DSC curves of pure PA6 and NSG composites with different SG content during the first cooling (a) and the subsequent heating (b) at a rate of 10 °C min⁻¹.

Table 3 Characteristic parameters for crystallization and melting behavior of pure PA6 and NSG composites with different SG content

Sample	Tm^a (°C)	Tc^b (°C)	ΔHm^a (J g^−1)	Xc^c (%)
a The melting point (Tm) and the fusion enthalpy (ΔHm) were obtained by DSC during the second heating process.b The crystallization temperature (Tc) was obtained by DSC during the cooling process.c Xc = ΔHm/ΔH^0m, ΔH^0m = 230.1 J g^−1.
PA6	222.1	173.5	53.8	23.4
NSG-0.2	221.8	182.1	53.6	23.3
NSG-0.5	221.3	181.4	53.2	23.1
NSG-1.0	220.7	180.2	52.3	22.7
NSG-2.0	219.5	178.8	50.6	22.0
NSG-3.0	218.2	177.1	49.2	21.4

---

As shown in Fig. 6, the peak crystallization temperature (T_c) of NSG composites is higher than that of pure PA6. The result indicates that SG sheets play a role of heterogeneous nucleation because of their large specific surface areas, as other type of graphene in the literature reported.⁴⁰ Accordingly, NSG composites are easy to nucleate and then crystallize at higher temperature than pure PA6 under the same cooling rate. Moreover, with the increasing content of SG, the T_c of NSG composites decreases. As the content of SG increases, the molecular weight of the grafting PA6 chains on SG sheets decreases, the length of crystallizable chains becomes shorter, crystal growth rate decreases, and thus crystallization rate and T_c value decrease. Although the T_c of NSG composites decreases, it is still higher than that of pure PA6. Undoubtedly, the fast crystallization rate of NSG composites will contribute to the use of rapid molding process.

Flow property and rheological behavior of NSG composites

Melt flow rate (MFR) is usually used to rapidly evaluate the flow property of a thermoplastic polymer. Fig. 7 shows the MFR of NSG composites with different SG content. As shown in Fig. 7, the MFR of NSG composites obviously increases with increasing SG content, from 5.0 g per 10 min for pure PA6 to 32 g per 10 min for the NSG-3.0 composite. The increase in the MFR of NSG composites is attributed to the low molecular weight of the grafted PA6 chains on SG sheets for the termination to the active chains by sulfonic acid groups on SG sheets. The result of MFR indicates that NSG composites have high flowability by the introduction of SG during in situ polymerization.

Fig. 7 MFR of NSG composites with different SG content. Error bars represent the standard deviations.

To further investigate the dynamic rheological behavior of melted NSG composites, ARES rheometer in an oscillatory shear mode was applied. Fig. 8 shows the frequency dependence of the complex viscosity (η*) for pure PA6 and NSG composites with different SG content. Note that the η* of pure PA6 and NSG composites decreases with increasing frequency, exhibiting a shear thinning behavior of pseudo-plastic fluid. It is because molecular chains orient along the flow direction and disentangle at a high shear frequency, that the relative motions between molecules become easier.^41,42 What's more, it is clear to see that the content of SG has a dramatic effect on the η* of NSG composites. The η* of NSG composites is distinctly lower than that of pure PA6, and the η* of NSG composites decreases with increasing SG content due to fewer entanglements among the shorter grafting PA6 chains. Compared with pure PA6, the η* of NSG composites with a relatively high SG content shows a low value and little or no sensitivity to frequency, which is beneficial to improve their processability and broaden their application.

Fig. 8 Changes of complex viscosity (η*) as functions of frequency for pure PA6 and NSG composites with different SG content.

Dispersion of SG in NSG composites and its influence on thermal conductivity

The dispersion of SG in NSG composites is one of the most important topics for fabricating high performance NSG composites. The morphology and microstructure of NSG composites were characterized by SEM and TEM. Fig. 9a–d show the SEM images of the freeze-fractured surface morphology of pure PA6 and NSG composite (NSG-2.0). Compared with pure PA6 (Fig. 9a and b), the NSG-2.0 composite exhibits an obvious lamellar structure, marked by arrows in Fig. 9c and d, which confirms the successful grafting of PA6 chains onto SG sheets. Fig. 9e and f show the high resolution TEM images of NSG-2.0 composite prepared by drop-casting on the copper grid. As shown in Fig. 9e and f, SG sheets are uniformly distributed in the nylon 6 matrix, and the distinct lamellar structure can also be observed.⁴³ The homogeneous dispersion of SG is due to the effective grafting, which enhances the interfacial interaction with the matrix, greatly improving their compatibility. It is also due to the in situ polymerization process, in which SG sheets with strong polar groups can remain homogeneously distributed during the polymerization of the intercalated ε-caprolactam monomers between layers. The homogeneous dispersion of SG sheets in NSG composites can benefit the formation of the consecutive thermal conductive pathways or networks and improve the thermal conductivity of NSG composites.^23,24

Fig. 9 SEM images of the freeze-fractured surfaces of pure PA6 (a and b) and the NSG-2.0 composite (c and d), and TEM images of the NSG-2.0 composite (e and f) prepared by drop-casting on the copper grid.

Fig. 10 shows the thermal conductivity (λ) and normalized thermal conductivity (λ/λ₀) of NSG composites with different SG content by the heat flow meter method, where λ₀ is the thermal conductivity of pure PA6. Compared with pure PA6, λ of the NSG composites increases markedly, and λ of the NSG-0.2 composite with only 0.2 wt% SG content increases by 51% from 0.203 to 0.307 W m⁻¹ K⁻¹. Moreover, λ of the NSG composites increases with increasing content of SG. When the SG content increases up to 3.0 wt%, λ of the NSG-3.0 composite is 0.398 W m⁻¹ K⁻¹ with an increment of 96%. The λ/λ₀ exhibits a similar increasing trend, from 1.512 to 1.961, with increasing SG content. No doubt, the excellent thermal conductivity properties of NSG composites should be attributed to the large specific surface areas of the SG sheets, and the homogeneous dispersion and interconnected structure of the SG sheets in NSG composites. Compared with previous research, λ and λ/λ₀ of the NSG composites are slightly higher than those of nylon 6/graphene composites based on GO at the same feed content.^23,24 In addition, the preparation process of the NSG composites is simple and easy to operate because SG with strong polar sulfonic acid groups can dissolve in water and then quickly disperse in melting ε-caprolactam with no need for ultrasonic treatment, which favors the large-scale industrialized production.

Fig. 10 (a) The thermal conductivity (λ) and (b) normalized thermal conductivity (λ/λ₀) of NSG composites with different SG content. Error bars represent the standard deviations.

Conclusion

In this study, a new approach was applied for preparing NSG composites using SG as a precursor by the in situ hydrolytic ring-opening polymerization of ε-caprolactam. By a condensation reaction between the active amino groups at the PA6 chain terminals and the sulfonic acid groups on SG sheets, the PA6 chains were covalently grafted onto SG sheets. The grafted structure of the NSG composites was confirmed by AFM, FTIR, XPS and TGA measurements. Compared with pure PA6, the mechanical properties of NSG composites can be maintained and even enhanced by the use of an appropriate SG content. The crystallization and melting behavior of NSG composites were investigated by DSC. The T_m and the X_c of NSG composites decrease with increasing SG content due to the more depressed crystallization and the lower molecular weight of the grafted PA6 chains caused by the increasing SG content. The T_c of NSG composites is significantly higher than that of pure PA6 because of the heterogeneous nucleation induced by the SG sheets. Moreover, the T_c of the NSG composites decreases with increasing SG content, due to the decreasing molecular weight of the grafting PA6 chains on the SG sheets and the slowing crystal growth rate. ARES rheometer measurements indicate that the η* of NSG composites is obviously lower than that of pure PA6 and decreases with increasing SG content due to fewer entanglements among the shorter grafted PA6 chains. Such crystallization and rheological behaviors are beneficial for the use of a rapid molding process. The homogeneous dispersion and interconnected structure of SG sheets in NSG composites greatly improve λ from 0.203 W m⁻¹ K⁻¹ for pure PA6 to 0.398 W m⁻¹ K⁻¹ for the NSG composite with only a 3 wt% SG content. Such NSG composites have a simple preparation process, good mechanical properties, excellent processability and high thermal conductivity, providing great promise for large-scale application in thermal conductive materials.

Acknowledgements

This research was financially supported by the National Science & Technology Support Plan Project of China (2013BAE02B03) and the Special Fund for the development of Strategic Emerging Industry of China.

Notes and references

1. M. E. Rogers and T. E. Long, Synthetic Methods in Step-Growth Polymers, John Wiley & Sons, New York, 2003.
2. J. E. Mark, Polymer Data Handbook, Oxford University Press, Oxford, 1999.
3. K. Shi, L. Ye and G. Li, RSC Adv., 2015, 5, 30160–30169.
4. C. H. Wang, F. Hu, K. Y. Yang, T. H. Hu, W. Z. Wang, R. S. Deng, Q. B. Jiang and H. L. Zhang, RSC Adv., 2015, 5, 88382–88391.
5. G. Zhang, Y. X. Zhou, Y. Kong, Z. M. Li, S. R. Long and J. Yang, RSC Adv., 2014, 4, 63006–63015.
6. M. Shabanian, N. J. Kang, D. Y. Wang, U. Wagenknecht and G. Heinrich, RSC Adv., 2013, 3, 20738–20745.
7. E. Picard, A. Vermogen, J. F. Gerard and E. Espuche, J. Membr. Sci., 2007, 292, 133–144.
8. M. V. Jose, B. W. Steinert, V. Thomas, D. R. Dean, M. A. Abdalla, G. Price and G. M. Janowski, Polymer, 2007, 48, 1096–1104.
9. A. Baji, Y. W. Mai, S. C. Wong, M. Abtahi and X. Du, Compos. Sci. Technol., 2010, 70, 1401–1409.
10. K. Saeed, S. Y. Park, S. Haider and J. B. Baek, Nanoscale Res. Lett., 2009, 4, 39–46.
11. H. H. Liu, W. W. Peng, L. C. Hou, X. C. Wang and X. X. Zhang, Compos. Sci. Technol., 2013, 81, 61–68.
12. C. Leyva-Porras, C. Ornelas-Gutierrez, M. Miki-Yoshida, Y. I. Avila-Vega, J. Macossay and J. Bonilla-Cruz, Carbon, 2014, 70, 164–172.
13. L. Gong, B. Yin, L. P. Li and M. B. Yang, Composites, Part B, 2015, 73, 49–56.
14. H. Tan and S. Y. Park, Polymer, 2015, 78, 111–119.
15. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
16. V. Singh, D. Joung, L. Zhai, S. Das, S. I. Khondaker and S. Seal, Prog. Mater. Sci., 2011, 56, 1178–1271.
17. T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose and J. H. Lee, Prog. Polym. Sci., 2010, 35, 1350–1375.
18. X. Huang, X. Qi, F. Boeya and H. Zhang, Chem. Soc. Rev., 2014, 41, 666–686.
19. Z. Xu and C. Gao, Macromolecules, 2010, 43, 6716–6723.
20. A. O. Neill, D. Bakirtzis and D. Dixon, Eur. Polym. J., 2014, 59, 353–362.
21. D. Zheng, G. S. Tang, H. B. Zhang, Z. Z. Yu, F. Yavari, N. Koratkar, S. H. Lim and M. W. Lee, Compos. Sci. Technol., 2012, 72, 284–289.
22. N. Song, J. W. Yang, P. Ding, S. F. Tang and L. Y. Shi, Composites, Part A, 2015, 73, 232–241.
23. P. Ding, S. S. Su, N. Song, S. F. Tang, Y. M. Liu and L. Y. Shi, Carbon, 2014, 66, 576–584.
24. P. Ding, S. S. Su, N. Song, S. F. Tang, Y. M. Liu and L. Y. Shi, RSC Adv., 2014, 4, 18782–18791.
25. H. Beydaghi and M. Javanbakht, Ind. Eng. Chem. Res., 2015, 54, 7028–7037.
26. C. Bora, J. Sharma and S. Dolui, J. Phys. Chem. C, 2014, 118, 29688–29694.
27. Z. Y. Xiong, T. H. Gu and X. G. Wang, Langmuir, 2014, 30, 522–532.
28. H. Li, J. C. Fan, Z. X. Shi, M. Lian, M. Tian and J. Yin, Polymer, 2015, 60, 96–106.
29. Y. C. Si and E. T. Samulski, Nano Lett., 2008, 8, 1679–1682.
30. N. Oger, Y. F. Lin, C. Labrugere, E. L. Grognec, F. Rataboul and F. X. Felpin, Carbon, 2016, 96, 342–350.
31. K. Weissenberg, Nature, 1947, 159, 310–311.
32. D. P. He, Z. K. Kou, Y. L. Xiong, K. Cheng, X. Chen, M. Pan and S. C. Mu, Carbon, 2014, 66, 312–319.
33. F. Liu, J. Sun, L. Zhu, X. Meng, C. Qi and F. S. Xiao, J. Mater. Chem., 2012, 22, 5495–5502.
34. G. Zhao, L. Jiang, Y. He, J. Li, H. Dong and X. Wang, et al., Adv. Mater., 2011, 23, 3959–3963.
35. Y. Zhang, H. K. He, C. Gao and J. Y. Wu, Langmuir, 2009, 25, 5814–5824.
36. J. M. Warakomski, Chem. Mater., 1992, 4, 1000–1004.
37. Z. H. Tang, H. L. Kang, Z. L. Shen, B. C. Guo, L. Q. Zhang and D. M. Jia, Macromolecules, 2012, 45, 3444–3451.
38. D. M. Lincoln, R. A. Vaia, Z. G. Wang and B. S. Hsiao, Polymer, 2001, 42, 1621–1631.
39. H. Meng, G. X. Sui, P. F. Fang and R. Yang, Polymer, 2008, 49, 610–620.
40. X. Q. Zhang, X. Y. Fan, H. Z. Li and C. Yan, J. Mater. Chem., 2012, 22, 24081–24091.
41. H. B. Zhang, W. G Zheng, Q. Yan, Z. G Jiang and Z. Z. Yu, Carbon, 2012, 50, 5117–5125.
42. Z. H. Xu, Y. H. Niu, L. Yang, W. Y. Xie, H. Li, Z. H. Gan and Z. G. Wang, Polymer, 2010, 5, 1730–1737.
43. A. Yu, P. Ramesh, M. E. Itkis, E. Bekyarova and R. C. Haddon, J. Phys. Chem. C, 2007, 111, 7565–7569.

---