2.1 Materials

PTFE film (thickness 100±5 μm) was provided by Guangdong Shengyi Technology Co., Ltd., China AAc was purchased from Sinopharm Chemical Reagent Co., Ltd., China. All of the chemicals in this work were of analytical grade and used without further purification.

2.2 Preparation of PTFE-g-PAAc film

The preparation process of PTFE-g-PAAc film is described in Fig. 1. PTFE film was pretreated with a low-temperature plasma (Changzhou Zhongkechangtai Plasma Science and Technology Co., Ltd., China) under an argon atmosphere and then put into a glass tube containing a deionized aqueous solution of AAc after placement in air for several minutes. AAc was graft-polymerized onto the PTFE film surface at 70 oC under the protection of nitrogen to prepare PTFE-g-PAAc film. After that, the PTFE-g-PAAc film was fetched out and rinsed with deionized water to remove any homopolymers and remaining monomers. Eventually, the PTFE-g-PAAc film was dried in a vacuum drying oven at 60 oC until the weight of the film became constant. The grafting density (GD) was calculated by:

Fig. 1.Full-screen View

Schematic diagram of low-temperature plasma and Ar-pretreatment-induced AAc graft polymerization onto a PTFE film surface.

where Wo and Wg are the weights (in μg) of the original PTFE and PTFE-g-PAAc films, respectively. A0 represents the area (in cm2) of the PTFE film.

2.3 ESR measurements

The low-temperature plasma-treated PTFE films under argon atmosphere were put into quartz glass tubes (Φ, 5 mm; length, 200 mm) for ESR measurements at room temperature with a Bruker EMX300 spectrometer (frequency, 9.65 GHz; power, 1 mW; field modulation frequency, 100 kHz). The direction of the PTFE films was maintained perpendicular to the ESR magnetic field. It should be noted that the PTFE films treated by the plasma were in contact with air for several minutes before ESR measurements. The spin number of free radicals was determined with the spectral double integration method [12]. Five different points of the tested sample were determined to obtain a mean value of the spin number.

2.4 Characterization

The surface topography of the PTFE samples was investigated using SEM (FEI Quanta 250, USA). PTFE films were covered with a thin layer of gold before observation, and an acceleration voltage of 10 kV was used. An atomic force microscope (AFM) (Nanoscope V, Bruker Corp., USA) equipped with a J scanner was used to investigate the surface morphologies of the PTFE films. AFM images (scale 20×20 μm2) were obtained in the tapping mode. The nominal spring constant of the silicon nitride cantilevers (NSC-11, Bruker Corp., USA) used in this study was 0.24 N/m.

The chemical structure of the PTFE films was analyzed according to Fourier transform infrared (FTIR) spectroscopy. FTIR spectra were acquired from 4000 to 650 cm-1 on a Bruker Tensor 27 FTIR spectrometer in attenuated total reflectance mode by averaging eight scans at 4-cm-1 resolution. The chemical composition of the PTFE specimens was investigated via XPS spectra. The investigations were carried out via a Kratos Axis Ultra instrument with monochromatic Al Kα radiation. XPS spectra were obtained through wide scans in the range of 1100–0 eV, and high-resolution spectra of the C1s region were collected.

The thermal stability of the PTFE films was assessed by TGA on a TG 209 F3 Tarsus (NETZSCH, Germany) instrument. PTFE specimens were heated from 20 to 800 °C under nitrogen atmosphere with a heating rate of 10 °C/min.

The surface hydrophilicity of the PTFE films was evaluated by static WCA measurements using a KSV Theta Optical Tensiometer equipped with a digital camera (KSV Instruments Ltd., Finland). A drop of water (5 µL) was laid on the tested PTFE film surface. Five various points of each specimen were measured and the WCA was the mean value of the acquired data.

2.5 Electrical properties

Εr and tanδ of the PTFE films were determined at a frequency of 10 GHz by an Agilent N5230A vector network analyzer using a split post dielectric resonator (SPDR) method.[13] The relative humidity and temperature were controlled at 50% and 23 oC, respectively.

Rs (Ω cm2) of the PTFE films was determined using a sample cell made of Plexiglas® with two platinized platinum electrodes, two electrolyte chambers, and a GW INSTEK LCR-817 meter with an accuracy of 0.05%. The prepared square-shaped PTFE films (5×5 cm2) were put into a KOH (30 wt%) deionized aqueous solution for more than 24 h and then immersed into the sample cell containing a KOH (30 wt%) deionized aqueous solution. The temperature of the sample cell, voltage, and frequency were set at 25 oC, 1 V, and 1 kHz, respectively. Rs was calculated according to

where R1 and R0 are obtained resistance (Ω) of the film immersed cell and the blank cell, respectively, and A is the sample surface area (in cm2).

3.1 ESR measurements

Figure 2 shows the ESR spectrum and the spin number of free radicals in a PTFE film formed by plasma treatment under an argon atmosphere. The characteristic ESR spectrum shown in Fig. 2(a) belongs to the overlapping signals of peroxy mid-chain radicals and peroxy end-chain radicals, both of which are products of the reaction between free radicals and oxygen in air.[14] The emergence of the peak of the spin number depicted in Fig. 2(b) indicates that an appropriate radio-frequency power is more favorable to PTFE surface activation and the formation of peroxy radicals. Figure 2(c) indicates that the spin number logically increases with the extension of plasma treatment time.

Fig. 2.Full-screen View

(a) ESR spectrum of an argon-plasma-treated PTFE film. Effects of (b) radio-frequency power and (c) treatment time on the spin number of free radicals in PTFE film.

3.2 Characterization of PTFE films

SEM pictures of the original PTFE and PTFE-g-PAAc (GD=696 μg/cm2) films are illustrated in Fig. 3. As clearly illustrated in Figs. 3(a) and 3(a'), there are a number of nanopores distributed on the unmodified PTFE film surface. Compared with Figs. 3(a) and 3(a'), it can be obviously observed from Figs. 3(b) and 3(b') that the surface nanopores have disappeared and the surface of the PTFE-g-PAAc film becomes rougher, indicating that PAAc chains are successfully immobilized on the PTFE film surface.

Fig. 3.Full-screen View

SEM images of the original PTFE [(a), ×2000; (a'), ×10000] and PTFE-g-PAAc films [(b), ×2000; (b'), ×10000] with GD=696 μg/cm2.

The surface topography of the pristine PTFE and PTFE-g-PAAc (GD=696 μg/cm2) films is visualized by AFM. As is evident in Fig. 4, the PTFE-g-PAAc film surface is much rougher [Figs. 4(b) and 4(b')] than that of the neat PTFE film [Figs. 4(a) and 4(a')]. The surface roughness of the PTFE samples computed from AFM data is listed in Table 1 and estimated in terms of three parameters: the average surface roughness (Ra), the root-mean-square roughness (Rq), and the maximum distance between the peak and valley of the roughness (Rmax).[15] As seen in Table 1, Ra, Rq, and Rmax of the PTFE-g-PAAc film are correspondingly about 2.6, 2.7, and 2.4 times larger, respectively, than those of the unmodified PTFE film. Therefore, the surface roughness of PTFE-g-PAAc film is effectively enhanced, which might be more convenient for adhesion to other materials.

Fig. 4.Full-screen View

AFM images of the pristine PTFE [(a), 2D; (a'), 3D] and PTFE-g-PAAc films [(b), 2D; (b'), 3D] with GD=696 μg/cm2

ATR-FTIR spectra of the nascent PTFE and PTFE-g-PAAc films are shown in Fig. 5(a). Compared with that of the original PTFE, a new peak at 1697 cm−1 can be seen from the spectrum of the PTFE-g-PAAc sample, which is attributed to the stretching vibrations of carboxyl group.[16,17] XPS spectra of the virgin PTFE and PTFE-g-PAAc films are illustrated in Fig. 5(b). The neat PTFE film exhibits two characteristic peaks, ascribed to C1s (289.8 eV) and F1s (687.0 eV), respectively. After graft modification, a fresh band at 532.1 eV (O1s) emerges on the curve of the PTFE-g-PAAc sample. In addition, the intensity of the F1s peak is much weaker than that of the original PTFE specimen. As listed in Table 2, the F/C atomic ratio decreases from 2.71 (neat PTFE) to 0.34 (PTFE-g-PAAc), whereas the O/C atomic ratio increases from 0.01 (neat PTFE) to 0.59 (PTFE-g-PAAc). The high-resolution C1s spectra and C1s components of the pristine PTFE and PTFE-g-PAAc films are illustrated in Figs. 5(c), 5(d), and Table 3, respectively. As we can see, Fig. 5(c) consists of a strong peak component at 292.3 eV, which represents CF2 (79.21%) and a minor peak at 284.8 eV (20.79%) ascribed to C-C. Compared to that of neat PTFE film ([Fig. 5(c)], the C1s peak of PTFE-g-PAAc film is fitted by four distinctive carbon moieties in Fig. 5(d), which are C-C (284.8 eV, 41.92%), C-O (286.1 eV, 13.89%), O=C-O (288.8 eV, 26.07%), and CF2 (291.8 eV, 18.12%). The ATR-FTIR and XPS spectra further confirm that AAc is graft-polymerized onto the PTFE film surface.

Fig. 5.Full-screen View

(a) ATR-FTIR and (b) XPS spectra of nascent PTFE and PTFE-g-PAAc (GD=696 μg/cm2) samples. C1s core-level spectra of (c) neat PTFE and (d) PTFE-g-PAAc samples [in (d), GD=696 μg/cm2].

The thermal stability of the pristine PTFE and PTFE-g-PAAc films is assessed via TGA. The TGA and DTG profiles are depicted in Fig. 6. TGA and DTG profiles of the pristine PTFE film show one decomposition step, and the initial decomposition temperature and the temperature of the maximum degradation rate are approximately 515 and 577 oC, respectively.[18] On the other hand, the thermodegradation of PTFE-g-PAAc film can be divided into two steps according to the TGA and DTG profiles. The temperatures of the maximum degradation rate are about 410 oC owing to the decomposition and carbonization of PAAc chains and 595 oC due to the degradation of the PTFE main chain. As compared with that of original PTFE film, the thermal stability of PTFE-g-PAAc film decreases very slightly, which can be ascribed to the comparatively low thermal stability of the PAAc chain. Nevertheless, the thermal stability of bulk PTFE film remained nearly invariable after AAc graft modification.

Fig. 6.Full-screen View

(a) TGA and (b) DTG profiles of the neat PTFE and PTFE-g-PAAc (GD=696 μg/cm2) films in a N2 atmosphere.

WCA measurements have been extensively used to evaluate surface hydrophilicity of membranes.[19] Figure 7 depicts the static WCA of the neat PTFE and PTFE-g-PAAc (GD=218 μg/cm2) films. Compared with the WCA (116o) of the original PTFE film, the WCA of the PTFE-g-PAAc sample decreases to 78o, indicating that the surface wettability of the PTFE-g-PAAc film is efficiently improved due to the introduction of carboxyl groups.

Fig. 7.Full-screen View

Water contact angle of the neat PTFE and PTFE-g-PAAc films (GD=218 μg/cm2).

3.3 Electrical properties

Table 4 presents both dielectric properties (εr and tanδ) at 10 GHz and Rs of the virgin PTFE and PTFE-g-PAAc (GD=218 μg/cm2) samples. Dielectric properties of polymeric materials are dependent on not only their intrinsic electrical properties, but also on their surface composition and morphology.[13] Compared to that of the original PTFE film, the εr of PTFE-g-PAAc film remains constant due to the fact that the performance of bulk PTFE film remains unchanged. On the contrary, the tanδ of the PTFE-g-PAAc film increases significantly from 0.0002 to 0.0073, since the introduction of carboxyl groups increases the polarity and moisture level of the PTFE film surface, both of which could result in energy loss. After AAc graft modification, the Rs of the PTFE-g-PAAc film is reduced slightly, from 131.89 to 110.28 Ω cm2, which is attributed to the fact that the grafted carboxyl groups, to some extent, improve the proton exchange capacity of PTFE film. However, PTFE-g-PAAc film maintains its inherent high impedance performance because the AAc is only graft-polymerized onto the PTFE film surface, not the interior of the PTFE film.