Poly(vinylidene fluoride)/poly(acrylonitrile) – based superior hydrophobic piezoelectric solid derived by aligned carbon nanotubes in electrospinning: fabrication, phase conversion and surface energy

Abstract

Multifunctional materials have attracted considerable interests from both fundamental and practical aspects such as field – effect transistors, electric protection, transducers and biosensors. Herein, we demonstrate the first superior hydrophobic piezoelectric surface based on the polymer blend of polyvinylidene fluoride (PVDF)–polyacrilonitrile (PAN) assisted with functionalized multiwalled nanotubes (MWNTs), obtained by a modified electrospinning method. Typically, β-phase polyvinylidene fluoride (PVDF) is considered as an excellent piezoelectric and pyroelectric material. However, polar β-phase of PVDF exhibits a naturally high hydrophilicity. It is a well-known fact that the wettability of the surface is dominated by two major factors: surface composition and surface roughness. The significant conversions derived by the incorporation of MWNTs, from a nonpolar α-phase to a highly polar β-phase of PVDF, were confirmed by FTIR. Moreover, the effects of MWNTs on the improvement of the roughness and the hydrophobicity of the polymer blend were evaluated by atomic force microscopy (AFM) and contact angle (CA) measurements. The molar free energy of wetting of the polymer nanocomposite decreases with increasing wt% of MWNTs. All molar free energy values for the wetting of PVDF–PAN/MWNTs were negative, which is a characteristic of a non-wettable film. The combination of surface roughness and low-surface-energy modification in the nanostructured composites leads to high hydrophobicity. In particular, the fabrication of superior hydrophobic surfaces not only has fundamental importance but also various possible functional applications in micro- and nano-materials and devices.

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1. Introduction

Poly-piezoelectric materials exhibit unique electronic performance comparable to those of silicon-based materials. Organic field-effect transistors (OFETs) are of great interest for applications in disposable electronic devices such as radio-frequency identification tags, sensor applications as well as in flexible-display devices.^1,2 Superior hydrophobic surfaces, especially superior hydrophobic piezoelectric solids, have attracted considerable interest from both fundamental and practical aspects such as field-effect transistors, electric protection, transducers and biosensors.³ The hydrophobicity provides less interference from water, which is a major reason for the failure of electrical devices, especially for piezoelectric polymers. The highly hydrophobic surface could make an organic semiconductor material more stable and lead to wide-spread applications of this material. Generally, there are two basic strategies to increase the contact angle and formation of a more hydrophobic surface. One is to increase the surface roughness, which is known as the geometrical micro/nanostructure method. Another is the modification of surface composition to lower the surface energy.^4,5 Current methods to prepare superior hydrophobic surface are based on the post modification of the surfaces.^6,7 In recent years, many hydrophobic materials have been developed using various components, such as polypropylene surfaces,⁸ poly(methyl methacrylate) and polystyrene,⁹ polyurethane and poly(vinyl chloride),¹⁰ with different techniques. It was found that electrospinning is an attractive method to alter the wetting behaviour of the polymer surface.^11,12 A superior hydrophobic piezoelectric solid for wide-spread applications has barely been reported.

The importance of polyvinylidene fluoride (PVDF) is due to its piezoelectric, pyroelectric properties and its resistance to creep, fatigue, chemical attack, high mechanical and impact strength.^13–15 Thus, it has been extensively studied for a broad range of applications, including, but not limited to, transducers,¹⁶ non-volatile memories,¹⁷ and electrical energy storage.^18,19 As a typical semi-crystalline polymer, PVDF exists in five crystalline forms (α, β, γ, δ and ε-phases).²⁰ Among the five polymorphs, β-phased PVDF (TTTT) exhibits outstanding piezoelectric and pyroelectric performance due to its highly polar structure compared to the nonpolar α-phase (TGTG′), which is a primary formation of PVDF. Variety of approaches have been developed to achieve high β-phase formation PVDF in the last 30 years such as applying an extremely high electric field to the α-phase of PVDF,²¹ optimized crystallization process from solution,²² from the melt,²³ surface epitaxy,²⁴ using a supercritical carbon dioxide method,²⁵ and electrospinning method.^26–29

For superior hydrophobic PVDF films, there are several methods that have been developed to date.^30,31 However, superior hydrophobic piezoelectric PVDF coating methods, which contain high β-phase content and increased contact angle, have barely been reported. Currently, some researchers indicated that the induction of nanomaterials could increase the surface roughness and also drive the α-phase of PVDF to β-crystal formation during a chain alignment caused by the electrostatic interaction between the methylene and charged nanoparticles.^32–36 The zigzag carbon atoms on the carbon nanotubes (CNTs) surface could induce β-phase formation during the crystallization of PVDF^37–39 and the π conjugated structure of CNTs will attract the F⁻ to CNTs surface. On the another hand, CNTs could create a rough surface at the mirco/nanometer level owing to their rigid cylindrical nanostructures with a diameter ranging from about 1 nm to dozens of nanometers and length ranging from hundreds of nanometers to micrometers.⁴⁰ Moreover, aligned CNTs could derive the aligned polymer structure of polyacrylonitrile due to the electrostatic interaction between the CH₂ and charged nanoparticle in solution processing.

The good compatibility of PVDF with other polymers, attributed to the existence of quasi-hydrogen bonding between them, provides a new approach to fabricate a composite structure by a copolymerization process, which could avoid the compromise made between the functionalization of CNTs and structural integrity in CNT–polymer composites. Polyacrylonitrile (PAN) is an ideal precursor for this purpose because it not only acts as a precursor for carbonaceous materials,^41–45 but the –CN groups of PAN could also build the intrachain and interchain interactions in the PVDF matrix via secondary bonding.^46,47

In this study, carbon nanotubes were aligned in a polymer matrix by electrical field and mechanical force obtained during an electrospinning process. Aligned CNTs were treated to nucleate and induce the PVDF chain on the CNTs surface via an electron–dipole interaction with CF₂ dipoles. PAN was blended in this composite to increase the compatibility of PVDF with CNTs (Scheme 1). Furthermore, PAN could contribute to the piezoelectric nature of the composite.^48,49 There is no report on the preparation of PVDF/PAN with MWNTs nanocomposites. In this study, we hypothesize that the combination of PVDF/PAN blends with MWNTs, which has a high surface area to volume ratio, will effectively improve the piezoelectric and hydrophobic properties of polymer nanocomposites. It is believed that the nanostructure of the nanofibers and also their lower density contribute to the very large fraction of air in the surface, which is essential for superior hydrophobicity.⁵⁰ The morphology, roughness, and hydrophobic properties of the resulting polymer nanocomposites (PVDF–PAN/MWNTs) were examined. Fourier transform infrared (FTIR) spectroscopy technique allowed the detection of the properties of the composites prepared as a function of MWNTs content.

Scheme 1 Schematic representation of the PVDF–PAN interaction with MWNTs.

2. Experimental section

PVDF with an average molecular weight of 275 × 10³ g mol⁻¹ and PAN with a molecular weight of 150 × 10³ g mol⁻¹ were obtained from Sigma Aldrich Co.; dimethylformamide (DMF) was obtained from VWR International LLC. Functionalized multi-walled carbon nanotubes (MWNTs–COOH) with a diameter of 10 nm, micron length of 10–30 and 1.9–2.1 wt% content of –COOH were supplied by Nanostructured & Amorphous Materials, Inc., USA.

The polymer blends were prepared by dissolving PVDF and PAN with polymer at a weight percentage ratio of 50 : 50 in DMF at a polymer blend/solvent weight ratio of 20/80 with stirring for 2 hours at 70 °C. MWNTs–COOH at different weights were dispersed in the PVDF–PAN solution via 30 min sonication followed by continuous stirring for 6 hours. The PVDF–PAN/MWNTs solutions were then placed in a plastic syringe fitted with a needle having a tip-diameter of 200 μm, and electrospun at 15 kV. A syringe pump was used to feed the polymer solution into the needle tip at a flow rate of 2.5 mL h⁻¹, and the distance between the needle tip and collecting plate was 10 cm (Scheme 2). The fiber was deposited on an Al sheet on the grounded electrodes, both as a flat sheet and on a rotating drum. The electrospinning process was performed at 25 °C and under a humidity of 65%. All the samples were dried in vacuum at room temperature overnight prior to characterization. For the characterization of the samples, Fourier transform infrared spectroscopy, FTIR (Varian 3100, FTIR), was carried out at room temperature. Each spectrum was recorded from 4000 to 400 cm⁻¹ using 32 scans at a resolution of 4 cm⁻¹. The surface morphology of the PVDF–PAN/MWNTs was studied using an atomic force microscope (Agilent Technology, Model N 9610 A). Surface images were obtained in the non-contact mode at different scan areas. The contact angle values of the samples were measured by the sessile drop method using a contact angle goniometer (Kyowa Interface Co. Ltd). The sessile drop was formed on the surface by depositing a droplet of deionized water slowly and steadily onto the membrane surface using a microsyringe. The contact angle was measured at room temperature. The value reported is the average of three measurements. The morphology of the composite was characterized by scanning electron microscopy (SEM) (JSM-6510GS from JEOL) operating at an accelerating voltage of 20 kV.

Scheme 2 Schematic diagram of the interaction between polymer blend (PVDF/PAN) and CNTs in the electrospinning. (A) The orientation of polymer blend (in black color) and MWNTs (in red color) under an electric field, (B) the β-phase formation instead of the α-phase and (C) the β-crystal formation in PVDF for polymer nanocomposites (in blue color).

3. Results and discussion

Polyvinylidene fluoride (PVDF)–polyacrylonitrile (PAN) with functionalized multiwalled nanotubes (MWNTs) nanocomposites were characterized by multiple techniques including spectroscopy (FTIR), microscopy (AFM) and contact angle measurements to correlate its surface energy with its morphology and surface tension. During the blending, the PVDF chains are very mobile and can effectively wrap over CNTs.⁵¹ Their fluorine groups are strongly attracted by the delocalized π-electron clouds on the CNTs and can thus establish donor–acceptor complexes. In these complexes, the mobility of PVDF chains is limited due to the strong interaction between delocalized π-electron clouds on CNTs and electrophilic fluorine groups on the PVDF chains.⁵² The FTIR spectra of the polymer nanocomposites were analyzed to confirm the interaction between polymer blends and MWNTs. To perform the quantitative measurement with FTIR, the high resolution transmission mode was used under the same sample conditions. The FTIR system was stabilized with liquid nitrogen for 30 min. The peaks at 1180 cm⁻¹ of C–F bond and at 2214 cm⁻¹ of C–N bond were use as standards. Fig. S1† shows the FTIR spectra of PVDF–PAN/MWNTs. The FTIR spectra of MWNTs showed major peaks located at 2880, 2361–2364, 1700 and 1560 cm⁻¹. The peaks at 2880 and 2361–2364 cm⁻¹ are attributed to H–C stretch modes of H–C=O in the carboxyl group and O–H stretch from strongly hydrogen-bonded –COOH, respectively, while the peaks at 1700 and 1560 correspond to the carbonyl groups of COOH and the C=C stretch of the COOH in the MWNTs, respectively. The characteristic peak at 2214 cm⁻¹ is due to the stretching vibration of cyano group (–CN), that at 1454 cm⁻¹ is for (–CH₃) and that at 1373 cm⁻¹ is for (–CH₂), which can be observed in PAN. In addition, the spectra showed a strong absorption band at 1140–1180 cm⁻¹ (–CF₂ bending) and 1411–1419 cm⁻¹ (–CH₂ stretching mode). The inset of Fig. S1† demonstrates a small shift in the CN and CF₂ peaks following the MWNT embedding compared with the PVDF/PAN blend. The CN band, originally appearing at 2214 cm⁻¹, shifted slightly to 2227 cm⁻¹ for the PVDF/PAN/MWNT composite; however, it was evident that the C–F peak for PVDF/PAN/MWNT (1156 cm⁻¹) considerably shifted compared to that for the PVDF/PAN blend (1140 cm⁻¹).

As shown in Fig. 1(A), the characteristic peaks of the α-phase (non-polar phase) were obtained at 615, 765, and 790 cm⁻¹, while the characteristic peaks of the β-phase (polar phase) were observed at 510, 840 and 1270 cm⁻¹. The characteristic peak of the γ phase was observed at 1233 cm⁻¹. The γ-phase can be obtained from strongly polar solvents such as DMF. In electrospinning, piezoelectric (β and γ) phases could still be induced via dipolar/hydrogen interactions between the local polar structure in the crystalline PAN and PVDF.⁵³ These results were similar to that reported for PVDF/nylon 11 blends.⁵⁴ The crystal structure of PVDF could be identified clearly from the FTIR results using the following equations.⁵⁵

andA_α and A_β correspond to the absorption bands of α-phase and β-phase, respectively. The results are summarized in Fig. 1(B). The opposite trends of these two crystal phases were observed with increasing MWNTs concentration. The intensity of β-phase became stronger, while the bands of α-phase became weaker, suggesting the progressive conversion of α-phase to β-phase. Similar observations had been reported.^56,57 It is well known that the specific surface area of MWNTs is higher than that of PVDF. With a higher specific surface area, MWNTs can act as nucleating agents in the initial crystallization process of PVDF, which leads to a high degree of crystallinity.⁵⁸ However, the mechanism for the increase in the electromechanical coupling is complicated. This mechanism is influenced by fluctuations in the electric field and anisotropy. The increase in the high polar β-phase content will change hydrophobic properties. In another hand, the change on volume fraction could lead opposite impact on the mechanical and electrical properties of materials respectively. It was reported that this could result in a negative effect on the overall electromechanical coupling.⁵⁹

Fig. 1 (A) FTIR spectra of PVDF–PAN and PVDF–PAN/MWNTs nanocomposites (B) plot of α, β-formation as a function of wt% of MWNTs in PVDF–PAN/MWNTs nanocomposites.

Under an external electric field in electrospinning, the conductive MWNTs can produce inductive charges on the surface, thus leading to a greater Coulomb force during the electrospinning process. This causes the PVDF chains to crystallize partially on the MWNTs surface in the β-phase, but localized amorphous microstructures still exist (Scheme 2). With the electrostatic interactions of functional groups on the MWNTs (which then act as nucleating agents) with the polar-CF₂, the PVDF chain will have the zig–zag (TTTT conformation) of the β-phase, instead of the coiled α-phase (TGTG conformation). This is consistent with the results of β-crystal formation in the PVDF/nanoclays composite.^37,60,61

AFM is based on the interaction forces (short- or long-range, attractive or repulsive) that exist between atoms and molecules, and these forces are present on all materials. It provides quantitative, three-dimensional images and surface measurements with a spatial resolution of a few micrometres down to a few angstroms. The non-contact mode (NC-AFM) is considered as a more effective method than contact mode (C-AFM) for imaging small pores such as those in ultrafiltration and nanofiltration membranes. Because the diameter of the cantilever tip apex is greater than the pore diameter, when the tip is passed over the small pore, the tip cannot penetrate into the pore and there is not a great change in cantilever deflection.

Fig. 2(A–F) present selective AFM images of surface topography and a three-dimensional surface for PVDF–PAN/MWNTs composites. The full size images are presented in Fig. S2.† It was observed that the fibres of PVDF–PAN/MWNTs have nonwoven structures with fibre diameters of about 400 nm and a pore size diameter of 480 nm. Functional groups in PAN of this nanocomposite lead to a strong interfacial bonding between the nanotubes and surrounding polymer chains to ensure the stability of the structure of the surface during electrospinning, resulting in the infusible characters of PAN in the polymer nanocomposites. Roughness parameters were obtained with the AFM analysis software. The average roughness (R_a) for the image is defined as the arithmetic average of the absolute values of the surface height deviations measured from the center plane. The root mean square roughness (R_q) represents the standard deviation from the mean surface plane. R_a and R_q appear to be most helpful and consistent in characterizing surface topography of the spun nanofiber.

Fig. 2 (A–F): AFM images of surface topography and three-dimensional surface for PVDF–PAN/MWNTs nanocomposites with (A) 0 wt% (B) 1.22 wt% (C) 5.58 wt% (D) 5.58 wt% (E) 7.99 wt% and (F) 7.99 wt%.

As shown in Fig. 3(A), the pure PVDF–PAN presents a rough surface (R_a = 185 nm and R_q = 223.1 nm) and the roughness of PVDF–PAN/MWNTs increases as the wt% of MWNTs increases. The PVDF–PAN/MWNTs exhibited the roughest surface (R_a = 259 nm and R_q = 286.5 nm) with the increase in MWNTs content. Similar results were found where the roughness increased due to the combined effect of the electrospinning process and the added MWNTs as nanocomposites.^62,63 Electrospun nanocomposites are sometimes necessary to achieve superhydrophobic properties because the polymer is not intrinsically hydrophobic enough or does not make enough roughness to achieve a water repellent behaviour.⁶²

Fig. 3 (A) The roughness of PVDF–PAN/MWNTs nanocomposites as a function of wt% of MWNTs. (B) Plot of R_a as a function with R_q of the PVDF–PAN/MWNTs nanocomposites.

The R_a values for all the composites were always smaller than the R_q values. Fig. 3(B) plots R_a values as a function of the R_q values of the PVDF–PAN/MWNTs nanocomposites. The relationship between these two parameters is linear. A possible explanation for this relationship is that the z_i coordinates of populations distributions exhibited some mathematical regularity, however, which is not dependent on the range of the z variable in composites. The SEM images of the PVDF/PAN/MWNT–COOH composite are presented in Fig. S3.† The functionalization of MWNTs increases the compatibility with PVDF/PAN and improves the dispersion of MWNT in the polymer nanocomposite. The fibers of PVDF–PAN/CNTs have a nonwoven structure. They are interconnected with a large number in different sizes. The fibers of PVDF/PAN/CNTs become more interconnected as the wt% of CNTs in the composite increases. The interconnected network morphology is expected to participate at molecular level interactions between C–F (in PVDF) and –CN (in PAN). This type of molecular interaction was considered to induce the phase mixing between PVDF and PAN.^64,65

The contact angle of the surfaces was measured using sessile drop observation. Fig. 4(A–C) show a water droplet formed on the electrospun PVDF–PAN/MWNTs surface. The droplet fell onto the surface of PVDF–PAN/MWNT, formed a bead and rolled off, which means that the surface was self-cleaning. Fig. 4(D) presents the effect of MWNTs on the advancing and receding contact angles of PVDF–PAN/MWNTs nanocomposites. The advancing contact angle increased from 83.25° to 117.68° and the receding contact angles increased from 88.19° to 111.70°, a high water contact angle, which indicates a superior hydrophobicity. It was observed that the hydrophobic properties increased by increasing the wt% content of MWNTs. The improvement in hydrophobic properties is due to the high ratio of surface area to volume of MWNTs, roughness, low surface energy of fluorinated polymers and electrospinning method.^66–68 Compared to the types of composites prepared via the solvent evaporation method, the electrospinning method provided a considerably higher contact angle because of the high surface area of the formed fibers, which ranges from the nanometer to submicron scale.⁶⁸

Fig. 4 Photograph of (A) water droplet on the non-wettable surface, (B) graphical drawing of water droplet on a nano-roughened surface and (C) contact angle of PVDF–PAN/MWNTs surface. (D) The advancing and receding contact angles of PVDF–PAN/MWNTs surface.

Contact angle hysteresis (CAH), θ_H, is typically defined as the difference between the advancing contact angle, θ_a, and receding contact angle, θ_r. It was found that the hysteresis increased when the roughness was increasing. This effect can arise from molecular interactions between the liquid and solid or from surface anomalies such as roughness or heterogeneities. The typical parameter used to characterize a solid surface wettability is the contact angle, which represents the angle formed between the liquid–solid and the liquid–vapor interfaces. In the equations to estimate the free energies, the wetting/dewetting behavior was assumed as an adsorption/desorption process. The surface free energy of wetting, Δg_a, is calculated as follows: ^69,70

Δg_a = (1/3)(RT/A)ln[(1 − cos θ_a)²(2 + cos θ_a)/4] (3)

For polymers, the molar surface area A can be calculated assuming that the surface is molecularly smooth and each molecular segment acts as an adsorption site. The area per site A is taken as the 2/3 root of the volume of a single molecular segment V_site:

A_site = (V_site)^2/3 = (M/ρN)^2/3 (4)

where ρ is the polymer density, M is molecular weight of the polymer repeat unit, and N is Avogadro's number. Multiplying eqn (2) by Avogadro's number gives the molar surface area A,

A = (M/ρ)^2/3N^1/3 (5)

The surface free energy of dewetting can be calculated as follows:

Δg_r = (RT/A)ln(θ_a/θ_r) (6)

The corresponding molar free energies, ΔG_i, can be determined from molar surface areas,

ΔG_i = AΔg_i (7)

where i = a for an advancing contact line and i = r for a receding contact line, Δg_a is the surface free energy of wetting, Δg_r is the surface free energy of dewetting, A is the average molar area of the polymer surface, M is the molecular weight of the polymer repeat unit, θ_a is the advancing contact angle, θ_r is the receding contact angle, θ_H is the contact angle hysteresis (CAH), ΔG_a is the molar free energy of wetting and ΔG_r is the molar free energy of dewetting. Table 1 lists the surface and molar wetting free energies of the various water/polymer–nanocomposites combinations. Δg_a unit is energy per area and is the change in the surface free energy of the solid due to wetting. Δg_a quantifies the strength of the interactions that drive spreading and lead to a liquid–solid bond.

Table 1 Contact angles, wetting/dewetting, free energies for PVDF–PAN/MWNTs composites

MWNT (wt%)	θa (deg)	θr (deg)	Ra (nm)	A (m^2 mol^−1)	Δga (J m^−2)	Δgr (J m^−2)	ΔGa (J mol^−1)	ΔGr (J mol^−1)	ΔG hysteresis (J mol^−1)
0	83.25	88.19	185.0	10.11 × 10^4	−7.23 × 10^−3	−1.41 × 10^−3	−730.44	−142.55	−5.82 × 10^−3
1.22	106.36	105.27	205.7	9.78 × 10^4	−12.67 × 10^−3	0.26 × 10^−3	−1240.2	25.45	−12.93 × 10^−3
5.58	114.8	107.96	224.3	9.35 × 10^4	−17.73 × 10^−3	1.62 × 10^−3	−1658.8	151.61	−19.35 × 10^−3
7.99	117.68	111.71	259.0	9.07 × 10^4	−20.05 × 10^−3	1.42 × 10^−3	−1817.2	128.71	−21.47 × 10^−3

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All wetting free energies were negative, as expected from a spontaneous process, which means that the film is non-wettable and Δg_a decreases exponentially with an increase in θ_a. In contrast to Δg_a, Δg_r value is a measure of the energy required to initiate recession of the contact line. In this regard, dewetting energy is defined as a measurement of adhesion, which means the energy per area required to rupture a bond. Surface free energies of wetting and dewetting for the PVDF–PAN/MWNTs are plotted against MWNTs content in Fig. 5(A). Wetting free energies of the polymer nanocomposite decreases exponentially with increasing the wt% of MWNTs. Surface free energy of dewetting, Δg_a, for PVDF–PAN was negative, while surface free energies of dewetting for PVDF–PAN/MWNTs were positive. Similar to Δg_a values, the magnitude of the free dewetting energies increased with increasing MWNTs concentration. In other words, the energy to create the bond between the liquid and solid was equal to the energy to rupture it. However, dewetting free energies were positive with a contact angle of >90°. Values of measured contact angle can be strongly affected by adding MWNTs, which could have a significant influence on the roughness of the measured surface. As the amount of MWNTs increased, the wettability of the films decreased due to the reduced surface energy and the enhanced roughness of the nanocomposite.

Fig. 5 (A) Surface free energies of wetting/dewetting for the PVDF–PAN/MWNTs nanocomposite; (B) molar free energies of wetting, ΔG_a, and molar free energies of dewetting, ΔG_r, for the PVDF–PAN/MWNTs nanocomposites; (C) the hysteresis molar free energies of PVDF–PAN/MWNTs nanocomposites; (D) the hysteresis molar energies, ΔG_H, of PVDF–PAN/MWNTs nanocomposites as a function of the roughness.

All wetting free energies were negative, as expected from a spontaneous process, which means that the film is non-wettable and Δg_a decreases exponentially with an increase in θ_a. In contrast to Δg_a, the Δg_r value is a measure of the energy required to initiate recession of the contact line. In this regard, dewetting energy is defined as a measurement of adhesion, which means the energy per area required to rupture a bond. Surface free energies of wetting and dewetting for the PVDF–PAN/MWNTs are plotted against MWNTs content in Fig. 5(A). Wetting free energies of the polymer nanocomposite decreases exponentially with increasing wt% of MWNTs. Wetting free energies of the polymer nanocomposite decrease exponentially with increasing wt% of MWNTs. These energies have an exponential trend, where they decrease with increasing wt% of MWNTs. However, the content effect of carbon nanomaterials to surface roughness is complex and still under investigation.^71,72

The surface free energy of dewetting, Δg_a, of PVDF–PAN was piezonegative, while the surface free energies of dewetting for PVDF–PAN/MWNTs were positive. Similar to Δg_a values, the magnitude of the free dewetting energies increased with increasing MWNTs concentration. In other words, the energy to create the bond between the liquid and solid was equal to the energy to rupture it. However, dewetting free energies were positive when the contact angle was >90°. Values of the measured contact angle can be strongly affected by the addition of MWNTs, which could have a significant influence on the roughness of the measured surface. As the amount of MWNTs increased, the wettability of the films decreased due to the reduced surface energy and the enhanced roughness of the nanocomposite.

Molar free energies of wetting, ΔG_a, and molar free energies of dewetting, ΔG_r, for the PVDF–PAN/MWNTs are plotted against MWNTs content in Fig. 5(B). All molar free energies of wetting of PVDF–PAN/MWNTs were negative, which indicate the non-wettability of the film. The molar free energy of wetting of the polymer nanocomposite decreases with increasing wt% of MWNTs. However, the molar free energy of dewetting of PVDF–PAN was negative, while the molar free energies of dewetting for the PVDF–PAN/MWNTs were positive. The hysteresis molar free energy is the energy to create the bond between the liquid and solid and the energy to rupture it. The hysteresis molar energies, ΔG_H, were negative and increased with increasing wt% of MWNTs (Fig. 5(C)). On the other hand, it was observed that the roughness influences the contact angle. As shown in Fig. 5(D), the hysteresis molar energies ΔG_H increases with increasing roughness, which means that the film is non-wettable (hydrophobic). Therefore, high hydrophobicity indicates not only a high roughness and high contact angle, but also a low hysteresis of the contact angle. The low hysteresis of the contact angle of the high hydrophobic surface is responsible for the self-cleaning properties, which means that a water droplet can easily roll off the surface and remove dust from the surface. The high hydrophobicity (non-wettable) properties of the film can be useful for applications such as self-cleaning, anti-corrosion, and anti-icing coating in the aerospace industry and biofouling protecting.

4. Conclusion

Highly hydrophobic PVDF–PAN/MWNTs nanocomposites with a wide composition range of MWNTs have been successfully prepared via an electrospinning method. FTIR results indicate that MWNTs act as nucleation agent during crystallization and slightly increase the β-phase crystal and decrease the α-phase in the PVDF/PAN/MWNTs nanocomposites. Because PVDF is considered to be a piezoelectric solid, the phase transformation from the nonpolar α-phase to the highly polar β-phase could change the PVDF piezoelectric properties. It was evident that the contact angle and surface roughness increased with increasing wt% of MWNTs, which reduced the surface energy of the film and the film was non-wettable. It can be concluded that the incorporation of MWNTs into PVDF–PAN nanocomposites plays a very important role in their morphological and surface properties and the combination of surface roughness and low-surface-energy modification, which leads to high hydrophobicity (non-wettable properties). The non-wettable properties of the film can be useful for applications such as self-cleaning, anti-corrosion, and anti-icing coating in the aerospace industry and biofouling protection.

Acknowledgements

Dr Salem Aqeel acknowledges the Fulbright Scholar Program Advanced Research Award for a postdoctoral research fellow grant. This grant was hosted at Dr Zeng’s research lab at chemistry department at Oakland University. Dr Zhe Wang acknowledges NIH under Grant 2G12MD007595-06 for support. Authors also thank the NSF MRI Scanning probe microscope major instrument grant and Carolyn Le for editing the study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11584a

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