Crystallization and mechanical properties of functionalized single-walled carbon nanotubes/polyvinylidene fluoride composites

Abstract

Single-walled carbon nanotubes were purified and functionalized by nitric acid and octadecylamine. Raman and Fourier transform infrared spectroscopy were used to characterize the functionalization of the single-walled carbon nanotubes. Polyvinylidene flouride nanocomposites containing 1 wt% purified or functionalized single-walled carbon nanotubes were prepared by solution blending and injection molding. The dispersion of different carbon nanotubes in dimethylformamide and in polyvinylidene flouride has been investigated. Mechanical properties show that adding single-walled carbon nanotubes could improve the tensile properties when the dispersion is good.

Keywords

Polyvinylidene flouride single-walled carbon nanotubes dispersion crystallization mechanical properties

Introduction

Polyvinylidene flouride (PVDF) is a kind of piezoelectric polymer, which possess high flexibility, low density, low thermal conductivity and high chemical corrosion resistance. Due to its unique properties, it has attracted more and more attention, and nowadays PVDF are used in a wide application range like sensors, audio devices, actuators, artificial muscles, super capacitors, batteries and integrated micro-electromechanical systems.¹^–⁴ As a semi-crystalline polymer, PVDF has at least four different crystalline phases, which are α, β, γ and δ, respectively. In these crystalline phases, the α phase is the most common crystalline phase and β phase provides the highest piezoelectric properties.^2,5,6

Carbon nanotubes (CNTs) possess superior mechanical, thermal and electrical properties, which enable them to be an excellent filler material for polymeric composites.⁷^–¹⁰ However, with the high aspect ratio, CNTs tend to agglomerate into bundles. Consequently, the dispersion of CNTs in polymer matrix has been a real challenge. In order to obtain good nanotube dispersion, many methods have been tried, among which modification of the CNTs and ultrasonic dispersion are widely used.^9,11^–¹³

In order to enhance the application potential, many researchers have intensively studied the electrical properties of CNT-based PVDF composites,¹⁴^–¹⁷ and the effect of CNTs on the crystallization of PVDF has been studied¹⁸^–²² aiming at improving the β phase crystallization. But most studies are using multiple-walled carbon nanotubes (MWNTs) and it is much easier to disperse MWNTs than single-walled carbon nanotubes (SWNTs) into polymers.²³^–²⁵ A study by Owens et al.²⁶ showed that the PVDF embedded with SWNTs perform improved hardness and electrical conductivity. Adding SWNTs into PVDF is desirable to improve not just the thermal and electrical properties but also mechanical properties. Unfortunately, however, the influence of SWNTs on both thermal and mechanical properties of PVDF has rarely been reported. The influence of surface functionalization of SWNTs on the dispersion, crystallization as well as the mechanical properties of PVDF composites is not yet clear.

In this study, SWNTs was purified, treated with nitric acid and modified by octadecylamine (ODA). After functionalization, the different SWNTs were characterized by Raman and Fourier transform infrared (FTIR) spectroscopy. Nitric acid has been reported to aggressively attack the nanotube side, while hydrochloride acid is passive.²⁷ ODA modification has been reported to be able to improve the solubility of CNTs in many organic solvents (such as benzene, tetrahydrofuran),²⁸ and improved interface and mechanical properties were reported by adding ODA-modified CNTs into poly(methyl methacrylate) polymers.²⁹

One wt% of different SWNTs blends with polymer PVDF were prepared by using dimethylformamide (DMF) solution blending and injection molding method. The dispersion state of nanotubes in PVDF matrix was observed by light optical microscopy (LOM) and scanning electron microscope (SEM). Thermal and mechanical properties of SWNTs/PVDF composites were investigated. The effect of surface modification of SWNTs on the composite properties was also comparatively discussed.

Materials and methods

Materials

The nanotube composites were produced using Carbolex Grade A SWNTs and a commercial Sigma-aldrich 427144-100G Poly(vinylidene fluoride) supplied by Aldrich Chemistry, Inc.

Purification of SWNTs

The impurities of SWNTs mainly consist of amorphous carbon, metal catalyst and graphite. The nanotubes were purified by first annealing in the air at 275°C for 1 h and then refluxing in 6 M hydrochloric acid for 6 h to remove the impurities. The suspension was washed with de-ionized water 5–10 times and filtered through 0.45 µm PVDF-membrane filter. Then the SWNTs were dried at 100°C for 24 h.

Nitric acid functionalized SWNTs

500 mg of purified SWNTs were refluxed in 50 ml 5 M nitric acid for 1 h. The purified SWNTs were cut shorter by the nitric acid reflux and leave the SWNTs open-ended, with the ends being decorated with oxygenated functionalities such as carboxylic acids. Then the powders were dried at 100°C for 24 h.

ODA-functionalized SWNTs

230 mg of HNO₃ treated SWNTs and 2.0 g of ODA was mixed and kept at 125°C for 6 days, during which the melted ODA reacted with the SWNTs. Hereafter, 800 ml ethanol was added to dissolve the remaining ODA and filtered through 0.45 µm PVDF-membrane filter. The filtrate was washed by ethanol in ultrasonic bath and filtered again two times to make sure that all ODA was removed. ODA react directly with shortened SWNT-COOH to form an octadecylammonium after the modification. Finally, the CNTs were dried at 100°C for 1–2 days.

Fabrication of SWNT/PVDF composites

The as-prepared CNTs were ultrasonically dispersed in N, N-dimethylformamide (DMF) for 30 min using an 80 W ultrasonic rod generator in order to form a stable suspension, during the ultrasonication an ice bath was used to keep the suspension cold. At the same time, PVDF was also dissolved in DMF by mechanically stirring at 60°C for 2 h. The CNT/DMF suspension was added to the PVDF solution, and the solution was vigorous stirred for 30 min at the speed of 5000–6000 r/min. Then, the mixture was dried in a large tray to enlarge the surface at 60°C for 8 h to remove the solvent, and after drying, the PVDF/CNT mixture became a thin layer. Subsequently, dog bone test specimens (ISO 527-2) were obtained by injection molding (Thermo HAAKE Minijet II) at 260°C with a mold temperature of 90°C and an injection pressure of 95 MPa.

Characterization

A Raman spectrometer (Renishaw Invia Raman Microscope) was used to investigate the structural changes of CNTs by functionalization, using He-Ne laser (632.8 nm) focusing through ×50 objective lens on the sample.

FTIR spectra were used to evaluate the functional group on the surface of SWNTs. The samples were ultrasonically dispersed in iso-octane and the solution was placed on the ATR-crystal (ZnSe). After removal of the iso-octane, evacuation of the sample was done, with a good contact of SWNTs on the crystal.

The dispersion of CNTs in PVDF matrix was characterized by using LOM on the composite film which is ground from the injection molded dog bone samples to obtain the same thickness.

A SEM (Zeiss 1540 XB) was also used to examine the dispersion of SWNTs in the composite materials. The dog bone composite samples were dipped in liquid nitrogen for 5 min and then broken. The fracture surfaces of the samples were coated with gold for evaluation.

The X-ray diffraction (XRD) analysis was carried out by using a Pan analytical X’pert diffractometer with X’pert Accelerator and thin film detector (Philips Ltd. Netherland) with CuKα radiation at 40 kV and 40 mA.

Thermogravimetric analysis (TGA) using Netzsch STA 409 Instrument was performed to study the decomposition behavior. The sample around 13.5 mg was heated from room temperature to 800°C with the heating rate of 10°C/min.

The melting point and enthalpy of melting of the SWNT-reinforced PVDF composites were measured with TA Differential scanning calorimetric (DSC) Q2000 instrument, the weight of the sample is about 27 mg. The measurements were performed under a nitrogen atmosphere. The samples were heated from room temperature to 260°C, then cooled to room temperature with the heating and cooling rate of 10°C/min.

The tensile properties of the polymer composites were measured by a Zwick100 tensile testing machine. The strain rate was 5 mm/min. The reported data of tensile strength and Young’s modulus were the average values of three samples.

Results and discussion

Functionalization of SWNTs

Raman spectra

The Raman spectra of purified and functionalized SWNTs are shown in Figure 1.

The feature at ∼1300 cm⁻¹ is due to disordered sp2 carbon “D band” of nanotube graphitic components indicating the presence of impurities. The feature at ∼1580 cm⁻¹ is close to the observed peak for well-ordered graphite, and it is often called the tangential mode G band. The ratio of the D band to the G band (I_D/I_G) is widely used to evaluate the degree of modification of nanotubes through covalent bonds formation. After HNO₃ treatment and the ODA modification, the I_D/I_G peak intensity ratios of CNTs increased compared to the purified CNTs, which reflects that the chemical modifications cause slight damage to the structure of the SWNTs.

FTIR spectra

Figure 2 illustrates the FTIR spectra of SWNTs. After nitric acid treatment, a new peak at around 1736 cm⁻¹ is shown, which corresponds to the vibration of C=O group of the COOH groups.

The peak at 1560 cm⁻¹ and 1450 cm⁻¹ of ODA-modified SWNTs are attributed to the carboxylate ions³⁰ indicating the formation of carboxylate ion due to a proton transfer when carboxylic acid combines with the amine groups. There is still a peak of C=O present in the curve due to the not bonded COOH groups left on the ODA-modified SWNTs.

Dispersion of SWNTs

The dispersion state of SWNTs in DMF after 10 min sonication and 4 weeks quiescent keeping is shown in Figure 3 and the purified SWNTs and HNO₃ functionalized SWNTs were very well and stably dispersed; however, ODA-modified SWNTs dispersed poorly. The darkness of the solutions with SWNTs is a visual indicator of the solubility.³¹ The stability of ODA functionalized SWNTs was not improved in DMF. DMF is a high polar solvent and has strong hydro-bonding energy. After modification by ODA, SWNTs get less polar and weaker hydro bonds, which made it dissimilar with DMF, which is analyzed by Hansen solubility parameters in our previous work.³²

Figures 4 and 5 exhibit the LOM and SEM images of various CNT/PVDF composites. The dispersion of purified SWNT and HNO₃ modified SWNT were homogenous in the PVDF and no obvious agglomerations of CNTs were observed as shown in Figures 4(a) and (b) and 5(a) and (b), while the ODA modified SWNTs were dispersed poorly in the PVDF composites and lots of large agglomerates are seen in Figures 4(c) and 5(c).

The dispersion of CNTs in PVDF polymer matrix was largely influenced by the dispersion of CNTs in DMF.

Microstructure of PVDF composites

In Figure 6, the XRD patterns of PVDF and PVDF composites are presented. For the neat PVDF, the peak at 17.7°, 18.3°, 19.9° and 26.8° are the reflections of the plane (1 0 0), (0 2 0), (1 1 0) and (0 2 1) of α phase PVDF and the peak at just above 20° is the indication of β phase.¹⁵ In our study, the α phase is the dominant phase in PVDF, the crystal formation of β phase is insignificant by adding SWNTs. By adding SWNTs, the main peak position of the PVDF composites at 17.7°, 18.3°and 19.9° does not change compared with the PVDF, while the relative intensity of peaks at 17.7°, 18.3° decreases for the HNO₃ and ODA-modified SWNT/PVDF composites.

Thermal properties

The thermogravimetric analyzer (TGA) curves in Figure 7 show that the addition of SWNTs decreased the onset temperature for weight loss upon heating. The weight loss is due to the decomposition of the polymer matrix, and with the addition of the SWNT the thermal stability of the PVDF is reduced. In Table 1 is listed the decomposition temperature to obtain a weight loss at 10% and 30% of the neat PVDF and PVDF composites, to obtain the weight loss at 10%, all the PVDF composites show a weight loss just below 430°C, whereas higher temperature is required for the neat PVDF(459.6°C); comparing the decomposition temperature at 30% weight loss, all the SWNT-filled composites decomposed again at lower temperature than neat PVDF, and composite filled with ODA modified SWNTs decomposed first at 444°C, HNO₃ modified SWNTs filled composites decomposed at 445.2°C. The thermal stability declined with SWNTs and Xu et al. also observed the thermal stability decreased with the increasing SWNT content.³³ The decreased thermal stability might be due to the defects on the SWNTs surface and the impurities such as the residue metallic catalysts.

Table 1.

Thermogravimetric results for the decomposition temperature of PVDF and PVDF composites at the different weight loss

Composite samples	T–10%^a (°C)	T–30%^b (°C)
PVDF	455.6	465.2
1 wt% purified SWNT/PVDF	429.9	446.8
1 wt% HNO₃ treated SWNT/PVDF	427.8	445.2
1 wt% ODA modified SWNT/PVDF	429.3	444.0

PVDF: polyvinylidene fluoride; SWNTs: single-walled carbon nanotubes; ODA: octadecylamine.

Weight loss at 10%.

Weight loss at 30%.

The melting temperatures (T_m) during the heating, the crystallization temperatures (T_c) during the cooling and the associated enthalpies are summarized in Table 2. According to the results obtained, lower melting points (T_m) of all the composites with SWNTs are observed, the purified SWNTs and HNO₃ treated SWNTs affected the melting point much more than the ODA functionalized SWNTs. During cooling, the T_c values of the composites containing purified SWNT and HNO₃ functionalized SWNT increased significantly, while ODA functionalized SWNTs did not significantly improve the T_c of the PVDF.

Table 2.

DSC results for PVDF composites with various SWNTs

Composite samples	T_m (°C)	$Δ Hm$ (J/g)	χ _c ^a (%)	T_c (°C)	$Δ Hc$ (J/g)	χ _c ^b (%)
PVDF	171.73	55.2	53.1	135.31	53.5	51.5
1 wt% purified SWNT/PVDF	170.77	57.8	55.6	144.58	55.8	53.7
1 wt% HNO₃ treated SWNT/PVDF	170.6	58.6	56.4	143.8	59	56.8
1 wt% ODA modified SWNT/PVDF	171.35	54.9	52.8	137.51	53.1	51.1

DSC: differential scanning calorimetric; PVDF: polyvinylidene fluoride; SWNTs: single-walled carbon nanotubes; ODA: octadecylamine.

Crystallinity based on $Δ Hm$ .

Crystallinity based on $Δ Hc$ .

Note: The degree of crystallinity (χ_c) is calculated by:

χ_{c} = \frac{Δ H_{m} or Δ H_{c}}{Δ H_{0} \times 99 %} \times 100 %

where $Δ H_{0}$ is the heat of fusion of 100% crystalline PVDF, which is 105 J/g.

Crystallization is often nucleated by the presence of fiber in crystallizing thermoplastics. The large surface area of SWNT is expected to enhance the crystallization of the polymer matrix.³³ The composites containing 1 wt% of purified SWNTs and HNO₃-treated SWNTs exhibited higher crystallinity (based both on the measured ${Δ H}_{m} and {Δ H}_{c}$ values) in this case, the SWNTs as a nucleation agent for PVDF introduce the formation of more crystallites. With ODA functionalized-SWNTs, the degree of crystallinity becomes lower than the neat PVDF.

Mechanical properties

The achieved mechanical properties in terms of Young’s modulus, ultimate tensile strength, toughness, fracture strain and their respective standard deviations of PVDF and the composites with 1 wt% nanotubes are listed in Table 3.

Table 3.

Mechanical properties from tensile testing at room temperature for PVDF and SWNT/PVDF composites

Composite type	Young’s modulus (GPa)	Ultimate tensile strength (MPa)	Toughness (J/m³)	Fracture strain (%)
Neat PVDF	1.14 ± 0.10	41.84 ± 0.85	12.34 ± 2.94	34.48 ± 7.51
1 wt% purified SWNT/PVDF	1.26 ± 0.03	41.28 ± 0.97	11.73 ± 1.05	32.34 ± 2.79
1 wt% HNO₃-SWNT/PVDF	1.18 ± 0.004	41.07 ± 0.09	10.81 ± 1.81	29.76 ± 4.92
1 wt% ODA-SWNT/PVDF	1.17 ± 0.003	40.70 ± 0.17	10.37 ± 1.40	28.92 ± 3.98

PVDF: polyvinylidene fluoride; SWNTs: single-walled carbon nanotubes; ODA: octadecylamine.

The Young’s modulus was calculated according to ISO527-1. The neat PVDF samples, that were fabricated using the same procedure as the composites with DMF as a solvent and molded by injection molding, have a Young’s modulus of 1.14 ± 0.10 GPa and strength of 41.84 ± 0.85 MPa. All the SWNTs filled PVDF composites lead to higher Young’s modulus than the original PVDF. The purified SWNTs-filled PVDF composites exhibit the greatest improvement in the Young’s modulus. However, the measured Young’s modulus does not come close to the Young’s modulus predicted by the rule of mixture^34,35 in any of the samples. The poor mechanical properties could be due to poor debundling of the CNTs.

Toughness and fracture strain are deteriorated by adding CNTs in PVDF. The composites with ODA functionalized nanotubes have the lowest toughness and fracture strain at 10.37 ± 1.40 J/m³ and 28.92 ± 3.98%. The ODA functionalized SWNTs are poorly wetted by PVDF, and form lots of agglomerates. The poor physical affinity also results in an interface with low strength, reducing the effect of the reinforcing SWNTs.

Many other researchers have found that the surface functionalization of SWNTs could improve the mechanical properties of composites.³⁶^–³⁹ The filler dispersion, aspect ratio and orientation are critical factors, which affect the mechanical properties of the composites; the interface between the polymer and fillers also plays very important role in reinforcing composites. Several researchers have observed that the mechanical properties of the nanocomposites are affected by the quality of the dispersion of the CNTs in polymer.^40,41

In our study, the properties of the composites are influenced by the modification mainly, which indicate that the dispersion state of the filler in matrix affect the mechanical properties of composites. The tensile properties are related to the crystallinity as well, the crystallinity of the purified and HNO₃ functionalized nanotubes-filled PVDF are improved while the crystallinity decreased for the ODA functionalized SWNTs-filled PVDF composites, this in some way produce an effect on the mechanical properties of the composites.

Although the HNO₃ functionalized SWNTs dispersed uniformly in DMF and PVDF, the crystallinity also improved, no significant improvement in mechanical properties has been observed. The improvement of Young’s modulus is due to stress transfer from PVDF to SWNTs during the tensile testing. The undesirable lower ultimate tensile strength is due to the breakdown of the fiber-matrix interface under strain. Further study is needed to be taken to investigate the reason for it in order to obtain better mechanical properties.

Conclusion

SWNTs functionalized by HNO₃ and ODA introduce the functional groups on the surface and lead to an increase of Raman I_D/I_G band intensity ratio. Purified and HNO₃ functionalized SWNTs dispersed well in DMF and PVDF, while ODA functionalized dispersed poorly. The dominant phase of PVDF and composites is α phase of PVDF. With SWNTs reduced thermal stability of PVDF was observed, especially the HNO₃ functionalized SWNTs. The melting temperature of the composite became lower by SWNTs, the crystallinity was improved by adding purified SWNTs and HNO₃ functionalized SWNTs and slightly reduced by adding ODA functionalized SWNTs. The better mechanical properties are dependent on a good dispersion and crystallinity. The Young’s modulus of PVDF was improved by adding SWNTs; the purified SWNT/PVDF shows the largest Young’s modulus. ODA functionalized SWNTs composites exhibit the poorest mechanical properties mainly due to the agglomeration of the SWNTs in PVDF.

Footnotes

Funding

This work was funded by the Chinese Scholarship Council and the Erasmus program.

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