Nylon 6,6/graphene nanoplatelet composite films obtained from a new solvent

Abstract

Solution processing of aliphatic polyamides (nylon) is quite challenging due to the fact that only a few solvents, such as formic acid and cresol, dissolve nylon. In general, polyamide 6,6 (nylon 6,6) is dissolved in formic acid to produce porous membranes or electrospun fibers. Herein, we propose for the first time a mixture of trifluoroacetic acid (TFA) and acetone that dissolves nylon 6,6, resulting in crystalline and non-porous films. Furthermore the same mixture of solvents was proved to be an excellent co-solvent for graphene nanoplatelets that were homogeneously dispersed in the solutions to produce reinforced nylon 6,6 polymer composites. The dispersion of the nanoplates in the polymer solution was found to be very stable over time. The mechanical and electrical properties of the developed composites were studied as a function of filler content. The Young's modulus of the nylon 6,6 films increases more than two times upon the addition of the nanoplates. Furthermore, the flexible composites exhibit semiconducting behavior, with their electrical conductivity reaching 10⁻² S cm⁻¹ for 20 wt% graphene nanoplatelet concentration.

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1. Introduction

Nylon 6,6 is a typical engineering thermoplastic material, often used in applications in which high rigidity and mechanical strength or good stability under heat is required. Nylon 6,6 composites are usually fabricated by extrusion^1–4 or using a hydraulic press.^5,6 It can be dissolved in formic acid, the typical solvent for most polyamides, including the similar nylon 6. However, dissolution of nylon 6,6 in formic acid results in porous membranes instead of compact films [ref. 7 and references therein]. As such, fabrication of non-porous nylon 6,6 films and composites from formic acid solutions is not feasible. A new solvent is thus required that can dissolve nylon 6,6 in order to produce good quality, non-porous films and composites that can expand the solution-processed nylon applications.

Baier and Zisman studied wetting of polyamide films including nylon 6,6 solvent cast from formic acid and dichloroacetic acid solutions.⁸ These acids produce uniform but porous polyamide films, highly suitable for biomedical scaffolds or filters.⁹ As shown in the detailed study of Giller et al.,¹⁰ the crystalline state, and hence final morphology of polyamides, are affected by the type of solvent and the evaporation rate of the solvent. Moreover, the nylon morphology is also affected by solvent/non-solvent combinations.¹¹ In fact, various forms of porous nylon films or pellets have been produced by solvent/non-solvent combinations, such as formic acid–water. It is hence evident, the finding of a new solvent is important since it makes possible to expand the current use of different polymeric materials.¹²

In the present work, we report an effective protocol to prepare nylon 6,6 films using trifluoroacetic acid (TFA) and acetone blend as solvent. TFA is a strong acid, with low boiling point (72 °C), that is easily miscible with many organic solvents, such as acetone.¹³ Specifically, the 1 : 1 mixture of TFA and acetone forms a very stable solution via strong hydrogen bonding between the ketone oxygen and the TFA.¹³ TFA has been shown to provide good dispersion of organic compounds that tend to aggregate due to van der Waals interactions or hydrogen bonding and has successfully been used as a co-solvent in dispersion of carbon nanotubes and poly(methyl methacrylate) (PMMA) composite fabrication.¹⁴

In addition, we have used graphene nanoplatelets as a filler to the nylon 6,6 non-porous films, in order to enhance their mechanical and electrical properties. The incorporation of graphene-based nanofillers, not only in nylon, but in different polymer matrices is widely used in order to enhance the polymer performance,^15–18 creating hence a new class of materials suitable for a wide range of applications, from nanoelectronics and biomedical engineering to textile industry.^19,20

Graphene nanoplatelets are short stacked, platelet-shaped graphene sheets. Their surface is identical to the surface of carbon nanotubes, but they have planar geometry. Their main advantage is that they combine the 2D planar geometry that leads to the laminar properties of layered silicates with the physical properties of graphene.²¹ However, due to their generally poor affinity with most polymers, their homogeneous distribution in the matrices and the prevention of agglomerates still presents a challenge. Various methods have been developed in order to improve the dispersion of graphenes in polymers, such as the functionalization of graphene surface prior to mixing with the polymer^4,22 or in situ polymerization of the polymer in the presence of graphenes.²³

In the work presented here, the composites were made by dispersing graphene nanoplatelets in TFA–acetone mixture containing dissolved nylon 6,6. The solutions were drop casted into Petri dishes resulting in compact and homogeneous films after solvent evaporation. The resulting composite films were highly crystalline and exhibited enhanced mechanical properties, as well as good electrical conductivity upon the addition of the graphene nanoplatelets. The electrical conduction mechanism was also discussed within a theoretical framework.

2. Experimental

2.1 Materials

Graphene nanoplatelets (or GnP) were purchased from Strem Chemicals. According to the company specifications, GnPs have average thickness 6–8 nm and width 5 μm. The oxygen content is less than 1%. Nylon 6,6 was obtained from Sigma-Aldrich (molecular weight 120 000 with a degree of polymerization of 531; density 1.14 g mL⁻¹). Reagent grade solvents, formic acid, trifluoroacetic acid (TFA) and acetone were purchased from Sigma Aldrich and used as received. The relative viscosity of the solution was measured to be 188 mPa s.

2.2 Sample preparation

Composite films with different GnP loading ratios were prepared, using identical conditions. The solution used to dissolve nylon 6,6 was a TFA and acetone mixture, at a 1 : 1 volume ratio. This mixing volume ratio has been shown to form very stable complex via the hydrogen bonds that form between the TFA and the ketone oxygen.¹³ Initially, nylon 6,6 pellets were dissolved in TFA–acetone solution to produce 7 wt% polymer in solution, at room temperature. After the nylon 6,6 was completely dissolved, graphene nanoplatelets were added in the solution at different weight fractions, namely between 0 wt% and 20 wt% with respect to nylon 6,6. The GnP were dispersed in the nylon-solvent mixture by sonication at 40 Hz for 3 h resulting in homogeneous solutions. They were then allowed to rest for 24 h and sonicated for 1 h at 59 Hz. The solutions were subsequently poured in polystyrene Petri dishes and the solvent was allowed to evaporate. The thickness of the resultant films was homogeneous and of the order of 100–150 μm. FTIR measurements of the dry samples, prior to any further investigation indicated that there was no residual TFA in the films. The possibility of interaction between the TFA–acetone solvent and the Petri dishes has been investigated, and there is no interaction. The results are not presented here, since they are beyond the scope of this work.

2.3 Sample characterization

The cross-section morphology of the films was studied by scanning electron microscopy (SEM, JEOL JSM-6490LA), after they had been coated by a thin gold layer. The crystal structure was studied by X-ray diffraction (XRD) using a Rigaku SmartLab X-ray diffractometer, equipped with a 9 kW Cu Kα (λ = 1.542 Å) rotating anode, operating at 40 kV and 150 mA. A Göbel mirror was used to convert the divergent X-ray beam into a parallel beam and to suppress the Cu Kβ radiation (λ = 1.392 Å). The diffraction patterns were collected at room temperature, over an angular range of 4° to 35°, with a step size of 0.05° and scan speed of 1.2° min⁻¹.

μRaman spectra were collected at ambient conditions using a Horiba Jobin Yvon LabRAM HR800 μRaman spectrometer, equipped with a microscope. A 632.8 nm excitation line, in backscattering geometry through a 50× objective lens, was used to excite the specimens, at low power of ∼0.25 mW. The experimental set-up consists of a grating 600 lines per mm with spectral resolution of approximately 1 cm⁻¹.

Thermal studies were carried out using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The degradation temperature of the materials was evaluated by TGA. During TGA measurements, samples were heated from 30 °C to 600 °C at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere set at a flow rate of 50 mL min⁻¹. Differential Scanning Calorimeter (DSC) measurements were carried out using a Perkin-Elmer Diamond calorimeter, calibrated with an indium standard. The baseline was recorded by a preliminary scan from 0 °C up to 280 °C at a rate of 10 °C min⁻¹. Thereafter, about 12 mg of nanocomposite films were placed in aluminum sample pans. The samples were first heated from 30 °C to 280 °C at a rate of 10 °C min⁻¹ and subsequently cooled down to 0 °C at a rate of 10 °C min⁻¹ to remove thermal history. A third scan from 0 °C to 280 °C at a rate of 10 °C min⁻¹ was performed for the melting point measurements. For each GnP weight fraction the measurements were repeated on three samples and the average melting mechanical characteristics of the films were measured with an Instron dual column tabletop universal testing System 3365 with 5 mm min⁻¹ cross-head speed. The tensile measurements were conducted on five different specimens for each film according to ASTM D 882 Standard Test Methods for Tensile Properties of Thin Plastic Sheeting.

Finally, the electrical transport properties of the nylon 6,6/GnP nanocomposites were studied using a Karl Suss RA150 Probe Station by the 2 probe method. The electrical conductivity in the specimens was found to be isotropic, due to the random orientation of the graphene nanoplatelets in the polymer matrix.²⁴ Therefore, in order to enhance the electrode–film contact, two gold electrodes were sputtered, 2 mm apart and were biased from −20 V to +20 V.

3. Results and discussion

3.1 Nylon 6,6 films from formic acid vs. TFA–acetone solutions

As is well known, formic acid is the most common solvent for polyamides, including nylon 6,6. However, the resulting films are porous, and as such, they can only be used as membranes [for example see ref. 7 and references therein]. Here, we have prepared nylon 6,6 films using drop casting from solutions of either formic acid or the new solvents combination proposed in this work of TFA–acetone. In this way, the morphology and mechanical properties of the prepared films could be directly compared. In Fig. 1(a) and (b) the cross section of a film casted from a formic acid solution is shown. The morphology across the thickness of the film is shown to be non-continuous, highly porous, and bead-like. On the contrary, films casted from TFA–acetone solution present a non-porous, continuous cross-sectional surface, as shown in Fig. 1(c) and (d). The ripples shown in Fig. 1(c) are due to the film cut.

Fig. 1 SEM cross sectional images of nylon 6,6 films casted from (a) formic acid solution, where the porous structure is evident. In (b) the same film is shown in higher magnification. Nylon 6,6 films casted from TFA–acetone solution are shown in (c), and in higher magnification in (d), where the continuous morphology is shown. In (e) the FTIR spectra of the TFA–acetone solvent and the dry nylon 6,6 films are shown, ensuring that all solvent is evaporated before any characterization. Finally, in (f) the load-extension curves for the nylon 6,6 films prepared by the two different solvents, as indicated in the legend, display the clear difference in the mechanical properties of the two films.

Mechanical characterization was also performed, even though the nylon 6,6 films casted from formic acid solution were very fragile. Before mechanical characterization, all samples were tested by FTIR to ensure the complete evaporation of the solvent. As shown in Fig. 1(e), the main absorbance peaks of the solvent are not present in the spectrum of the nylon film, indicating the complete evaporation of the solvent. The mechanical properties of the nylon 6,6 films casted from the two solvents are shown in Fig. 1(f) and the Young's modulus, tensile strength and elongation values at break are summarized in Table 1. From Fig. 1(f) it is evident that the nylon 6,6 films casted from formic acid are very brittle. This would limit their use in applications where mechanical strength is required. Comparatively, nylon 6,6 films casted from TFA and acetone solution exhibit enhanced mechanical properties, showing a substantial increase in the Young's modulus, tensile strength and elongation values. These values can be further enhanced with the addition of GnPs, as will be discussed later.

Table 1 The mechanical properties of nylon 6,6 films, as measured by Instron for the films prepared with different solvents

Mechanical property	Formic acid	TFA + acetone
Young's modulus (MPa)	28	350
Tensile strength (MPa)	0.01	22
Tensile strain (%)	1	40

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3.2 Morphology of the nylon 6,6/GnP composite films

In Fig. 2(a), representative photographs of the composite films with different GnP concentrations are shown. The SEM images of the cross section of the nylon 6,6/GnP films are depicted for GnP concentrations of 0.1 wt%, 1.0 wt%, 5.0 wt%, 10.0 wt%, in Fig. 2(b)–(e) respectively. Evidently, as the amount of GnP is increased, the GnP density seen in the cross section images increases. The distribution and separation of the GnP seem to be fairly good, especially for low GnP loadings, namely 0.1 wt% and 1.0 wt% (see arrows in the SEM images). Furthermore, the nylon 6,6 matrix seems dense, smooth and no porosity is noticed in contrast to standard nylon structures obtained from formic acid solvent.⁷ For the higher GnP contents, 5.0 wt% and 10.0 wt%, some multilayered GnP may be seen, probably due to their high concentration in the initial solution, that inhibits the forces developing during sonication to separate the nanoplatelets, overcoming the strong adhesive van der Waals forces that keep them together.

Fig. 2 (a) Photographs of the nylon 6,6/GnP films with GnP content from 0 to 10 wt%. SEM images of the cross section of the nylon 6,6/GnP films for (b) 0.1 wt%, (c) 1.0 wt%, (d) 5.0 wt% and (e) 10.0 wt% GnP. The white arrows in (b) and (c) indicate edges of the GnP in the polymer matrix. The green, thicker arrows indicate possible GnP multilayers.

3.3 Thermal properties (TGA)

The thermal degradation of the composites is shown in Fig. 3. The degradation of nylon 6,6 takes place between 350 °C and 500 °C, as reported by earlier studies, and only about 2% remains at 600 °C.^25,26 The weight loss of composites is gradual and the remaining material at 600 °C increases with the GnP loading. It is seen that all films do not decompose at the same temperature, but as GnP concentration increases (except for 0.1 wt% GnP), the decomposing temperature slightly shifts to higher values.

Fig. 3 TGA thermographs for neat nylon 6,6 film and the different composites studied.

3.4 Crystallographic investigation

The crystalline structure of the nylon 6,6/GnP composites was investigated by XRD. For comparison, the nylon 6,6 pellets, as purchased from the manufacturer, were also investigated by XRD. In Fig. 4(a), the diffraction pattern of the nylon 6,6 pellets is presented, along with the nylon 6,6 film obtained by casting from the new solvent. The diffractograms consist of an amorphous part, characterized by a broad halo, and a crystalline part, characterized by sharper peaks, confirming the semicrystalline nature of nylon 6,6.²⁷ Two peaks are prominent in all films, approximately at 20° and 24°, corresponding to (100) and (010)/(110) doublet, respectively, of the α-phase of triclinic nylon 6,6.⁷ The (100) peak is due to intrasheet scattering, i.e. scattering within the adjacent polymer chains forming a polymer sheet, while the (010)/(110) peaks are due to intersheets scattering, i.e. scattering between different polymer sheets, connected by hydrogen bonds.²⁷ Following the treatment with the TFA–acetone solvent, the intensity of (100) peak increases, while the (010)/(110) peaks decreases, indicating the change in the internal structure of the material upon re-crystallization. Thus, we can safely assume that TFA–acetone solvent attacks the hydrogen bonds between the polyamide sheets, disrupting thus the stacking order (as schematically depicted in Fig. 5), but not necessarily the length of the polymeric chains. Consequently, after re-crystallization, the lamellae become thinner, resulting in the decrease of the (010)/(110) peak. It is possible that the crystallization takes place across the (100) plane, causing the increase of the (100) peak intensity. Peak fitting analysis revealed that the initial crystallinity of nylon 6,6 pellets is approximately 70%, while that of the TFA–acetone treated films is decreased to approximately 46%. When a crystalline polymer is dissolved in a solvent, the polymer chains rearrange themselves during drying as the solvent evaporates and the degree of crystallization achieved depends on the initial conditions of the solution, evaporation temperature etc.²⁸ A decrease in the degree of crystallization is expected due to the difficulty of the entangled polymer chains to orient themselves and form large crystallites similar to melting–solidification transitions, like extrusion. This has also been shown for microcrystalline cellulose, where its dissolution in TFA results in a lowered crystallinity due to the breaking of the H-bonds.²⁹ However, the present value is much higher than the ∼30% crystallinity value reported in the literature for solvent processed nylons.⁷

Fig. 4 X-ray diffractograms of (a) a nylon 6,6 pellet as received from the manufacturer and the nylon 6,6 film prepared in TFA–acetone solvent and (b) nylon 6,6/GnP composites, for different GnP weight fractions.

Fig. 5 Disruption of H-bonds of nylon 6,6 by TFA–acetone. TFA–acetone solvent attacks the hydrogen bonds between the polyamide sheets, disrupting thus the stacking order, but not necessarily the length of the polymeric chains.

Upon the addition of GnP in the nylon 6,6 matrix, a new peak appears at 26.3° (corresponding to d-spacing 3.38 Å, as calculated using the Bragg law), as seen in Fig. 4(b), that can be attributed to GnP (graphite (002) peak at 26.51°, d-spacing 3.36 Å [PDF card number 00-023-0064]). Peak fitting analysis, performed using the fundamental parameter (FP) method implemented in PDXL 2.1 software from Rigaku, revealed that the initial crystallinity of TFA–acetone treated films is decreased to approximately 38% for 0.1 wt% GnP, but it increases with the addition of GnP reaching 52% for 10.0% wt GnP.

Furthermore, from the peak fitting analysis, the integrated intensity of the peaks was calculated. The integrated intensity of the (100) peak was found to decrease with the addition of the GnPs, whereas that of the (010)/(110) peaks remained almost unaltered (a small increasing tendency). The ratio of the relative integrated intensity of the (100) and (010) nylon 6,6 peaks decreases from 1.76 for the pure nylon 6,6 film to 0.43 for the film containing 10.0 wt% GnP. Since the (100) peak is the one showing the most important changes by the introduction of the GnPs in the composite, we can safely assume the GnP addition is disrupting the intrasheet bonds between the different crystalline polymer sheets. On the other hand, since the (010)/(110) remains practically unaltered we assume that the stacks of sheets comprising the lamellae do not change, indicating that the addition of GnPs does not greatly affect the intersheet, H-bonds upon re-crystallization. This is further supported by the DSC results, as shown in the ESI,† where two melting peaks appear for the processed nylon film. Upon the addition of the GnPs the first peak merges into the higher temperature melting peak, that remains practically unaffected at approximately 261 °C, and is generally attributed to the melting of the thickened lamellae, indicating that the crystal thickening, and thus the intersheet H-bonds are not affected by the GnP addition.

Finally, by using the well-known Scherrer's formula, one can calculate the size of the D₀₀₂ crystallite of the GnP. Hence, dividing the thickness by the d-spacing calculated from the diffraction pattern presented in Fig. 3, one can estimate the number of the GnP layers present in the nylon 6,6 matrix.³⁰ In the case of the nylon 6,6/GnP composite films presented here, the number of GnP layers was found to be around 2, confirming good degree of dispersion of the GnP during processing.

3.5 Raman studies of nylon 6,6/GnP composite films

Micro-Raman is a powerful technique that has been used to characterize graphites, as well as nylons. In Fig. 6 the Raman spectra of the nanocomposites are depicted. Spectrum a, depicts the spectrum of the nylon 6,6 film obtained from the new solvent. The peak at 1636 cm⁻¹ is due to the amide I group, while the peak at 1296 cm⁻¹ is assigned to CH₂ twisting mode and it is only present in nylon 6,6 Raman spectrum, while it is absent in other forms of polyamides, such as nylon 6.³¹ The peak at 1445 cm⁻¹ and the band centered at approximately 2908 cm⁻¹ are assigned to CH₂ bending and stretching modes, respectively. Finally, the N–H stretching of the amide A is seen at 3300 cm⁻¹.

Fig. 6 μRaman spectra for nylon 6,6/GnP nanocomposites nylon 6,6 film, GnPs, 1.0 wt%, 5.0 wt% and 10 wt% GnP weight fraction.

Raman spectra of the composite films are also presented in the same figure. Along with the nylon 6,6 peaks, all graphene fingerprint peaks are present at 1345 cm⁻¹ (D peak), 1585 cm⁻¹ (G peak) and 2691 cm⁻¹ (2D peak).³² In accordance with the TGA results, their intensity scales up with the GnP weight fraction. The D peak being indicative for the presence of defects in the graphene layer, the intensity ratio of the D and G peaks (I_D/I_G) can serve as an indication of defects, with higher amount of defects resulting in a higher ratio. In the case of our pristine GnP, I_D/I_G equals 0.15, however for the nylon 6,6/GnP films the ratio increases up to 0.88 for 5.0% GnP indicating higher number of defects possibly due to structure of the composites as shown in Fig. 1, where it is seen that the GnP are not flat, but instead they fold and wrinkle. These wrinkles are expected to give rise to a stronger D peak.

3.6 Mechanical performance

The tensile modulus, strength and strain of the nanocomposites are shown in Fig. 7. The increase of GnP content in the nylon 6,6 matrix results in significant increase of the Young modulus. More specifically, it is seen that the Young's modulus of pure nylon 6,6 is about 350 MPa. Increasing the GnP weight fraction results in the increase of the Young's modulus by 130% at 10.0% GnP content, demonstrating the reinforcing ability of the GnP as filler to nylon 6,6 films. On the other hand, the tensile strength of the nanocomposites behaves in a different way. It increases for low GnP content of 1.0% wt about 57%, however for higher weight fractions it gradually decreases, without nevertheless reaching values lower than that of the pure nylon 6,6 films. This behavior can be attributed to the sensitivity of the composites' tensile strength to the agglomeration of the nanofillers.³³ Hence, it is possible that the aforementioned drop in the values of the tensile strength, with respect to the 1% wt filler concentration composites, is due to the multilayer GnP present in the heavier loaded nanocomposites, as already assumed by the careful study of Fig. 2. Similarly, the measured tensile strain at break follows the behavior of the strength, reaching a value of about 65% for 0.1 wt% nanofillers loading (40% is the value for the pure nylon 6,6) and decreasing thereafter as the GnP content exceeds 1.0% wt, to values lower that the ones of the pure nylon 6,6 films. To the best of our knowledge it is the first time that mechanical testing has been presented for solvent casted nylon 6,6 films and no literature data for comparison is available at the moment. However our results on the reinforcing ability of GnPs on the nylon 66 films are similar, or better, than other solvent processed thermoplastic materials, like PMMA,³⁴ PVA³⁵ or nylon 6.³⁶

Fig. 7 (a) Young's modulus (left axis) and the tensile strength (right axis) and (b) the tensile strain (elongation) of the nanocomposites as a function of the weight fraction of the GnP.

3.7 Electrical conductivity studies of the nanocomposites

Electric transport measurements were conducted as a function of the GnP content in the composite films, as presented in Fig. 8. Upon adding the conductive nanofiller, a substantial increase in the conductivity is observed.

Fig. 8 I–V curves for nylon 6,6/GnP nanocomposites for different GnP content. Electrical conductivity is calculated at 1 V, where the I–V is ohmic for all volume fractions.

In Fig. 8 the I–V curves for different GnP weight fractions are shown. For low GnP content, the current–voltage characteristics are linear, signifying constant resistivity for the applied electric fields. However, as the GnP content increases the I–V curves show predominantly non-ohmic behavior. More specifically, the I–V characteristics are linear for low enough applied voltages, however, at higher voltages, where the electric field increases, the I–V curves bend towards the current axis, meaning that at higher applied voltages, the resistivity depends on the applied voltage.

As the GnP content increases further, the I–V curves regain their ohmic behavior, and for 20.0 wt% GnP weight fraction there is no deviation from ohmic state for applied voltages up to 20 V. In addition, all the curves are symmetric for the reverse bias. The nonlinear I–V dependence has been observed for 2D graphitic–polymer composites in the past.^37–41 Oskouyi et al.⁴¹ have recently developed a theoretical model based on electron tunneling to describe the conductive behavior of 2D nanofillers in insulating matrices. One can visualize the GnP inside the nanocomposite, as being coated by the nylon matrix. The thickness of the nylon between two neighboring GnP depends on the GnP weight fraction. Evidently, for higher GnP content, the nylon thickness between adjacent GnP is thinner. It is known, that this kind of tunneling exhibits transport behavior depending on the applied voltage,⁴² resulting in the dependence of the percolation behavior of the composite on the level of the applied voltage. According to Simmons,⁴² for low applied voltage, the tunneling resistance is proportional to the insulator thickness, and is constant, while for higher applied voltages the tunneling resistance depends on the applied voltage, resulting in non-ohmic I–V behavior. On the other hand, He and Tjong employed the combination of Zener tunneling effect and percolation theory to describe the nonlinear conductivity of carbon based polymer composites.^40,43

The electrical conductivity was calculated as a function of the volume fraction of the filler for the ohmic region and is shown in Fig. 9. The conductivity increases from 3.2 × 10⁻⁸ S cm⁻¹ for 1.0 wt% GnP content to 2.1 × 10⁻² S cm⁻¹ for 20.0 wt% GnP content. No percolation is evident from our data, since the conductivity does not exhibit an S-shaped curve, but continues to increase as the GnP content in the composite increases. An interesting point here, is the conductive nature of nylon 6,6. As it is seen in Fig. 9, the conductivity of the nylon 6,6 film is of the order of 10⁻⁸ S cm⁻¹, a surprising value since nylon 6,6 is a well-known insulator. We believe that this value is related to the solvent processing, and further investigation is underway. It is possible that the percolation threshold is hindered by the appearance of conductivity in the matrix. However, the absence of percolation is in accordance with epoxy/GnP nanocomposites, prepared with the sonication method. GnP/epoxy films were prepared with two different methods, namely three-roll milling and sonication.³⁰ The films prepared by sonication showed lower electrical conductivity and no percolation, compared with those prepared by three-roll milling. Possibly, sonication hinders the GnP from forming a percolation path due to the shear forces developed during sonication.

Fig. 9 The electrical conductivity as a function of GnP volume fraction. Electrical conductivity is calculated at 1 V, where the I–V is ohmic for all volume fractions.

4. Conclusion

Non-porous nylon 6,6 films were produced from a new solvent, namely TFA–acetone, for nylon 6,6, for the first time. The fabrication procedure followed a simple direct mixing method and resulted in non-porous films. A detailed analysis is presented based on their crystallinity, mechanical and electrical properties. The physical properties of the films were reinforced by adding GnPs as fillers. The GnPs were shown to disperse well in the polymer matrix, as indicated by XRD and Raman analyses. The Young's modulus significantly increases for all the GnP loadings whereas the increase of the tensile strength becomes considerable at low loading of 1.0 wt%. The casted films exhibited both ohmic and non-ohmic transport behavior, depending on the filler concentration and applied voltage. The electrical conductivity was calculated for the ohmic part of the I–V curves and was found to increase as a function of increasing GnP particle concentration. The present solvent-based approach for nylon 6,6/graphene nanoplatelet composites fabrication is simple, repeatable and produces non-porous nylon films compared to the most common formic acid method. We believe that such a simple technique to produce nylon 6,6 films, opens up new solvent-based routes for nylon films fabrication, with tailored functionalities.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23647a

‡ Present address: Center for Micro-BioRobotics, Istituto Italiano di Tecnologia, Viale Rinaldo Piaggio 34, 56025 Pontedera, Italy.

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