Study of Radio Frequency Plasma Treatment of PVDF Film Using Ar, O2 and (Ar + O2) Gases for Improved Polypyrrole Adhesion

Improvement of the binding of polypyrrole with PVDF (polyvinylidene fluoride) thin film using low pressure plasma was studied. The effects of various plasma gases i.e., Ar, O2 and Ar + O2 gases on surface roughness, surface chemistry and hydrophilicity were noted. The topographical change of the PVDF film was observed by means of scanning electron microscopy and chemical changes by X-ray photoelectron spectroscopy, with adhesion of polypyrrole (PPy) by abrasion tests and sheet resistance measurements. Results showed that the increase in roughness and surface functionalization by oxygen functional groups contributed to improved adhesion and Ar + O2 plasma gave better adhesion.


Introduction
PVDF (polyvinylidene fluoride) is a semi-crystalline polymer in which each monomer has two dipole moments, one due to CF 2 and the other due to CH 2 . The most common and most studied crystal structures are α and β phases [1,2]. The piezo-electric and pyro-electric properties of PVDF mainly OPEN ACCESS depend on the β-phase content and its growth can be induced by several techniques, the most common being the mechanical stretching of the α-phase films at a suitable temperature [3]. Mechanical strains due to the bending, vibration or compression of the thin-film structure can be the source of the energy generation [4,5]. A PVDF sensor was integrated into an automotive seat cover fabric to obtain information about the car occupant [1]. Hutchison et al. [6] used platinum coating in between PVDF and polypyrrole (PPy) to get a stable connection in their actuator device. Choi et al. [7] used conductive fabric and PVDF to monitor cardiorespiratory signals employing the piezoelectric property of PVDF. The construction of a PVDF based energy harvesting device require electrodes on both sides of the PVDF film, these electrodes are connected to an electronic circuit to harvest electricity during motion. Different metal coatings can be used as electrode materials [8] and are brittle and uncomfortable to the skin. PPy coatings can be used instead of metals for this purpose due to ease of coating, comfort, and conformity [9]. Bhat et al. [10] synthesized a PVDF PPy composite film by electrochemical polymerization and Mansouri et al. [11] coated PPy onto PVDF microfiltration membranes by an in situ chemical polymerization method.
One of the perceived problems for PPy coating is poor adhesion due to low surface energy and the chemical nature of PVDF. Plasma can be used for the surface modification to improve subsequent coating deposition and bonding [12,13]. Exposing PVDF films to argon (Ar), hydrogen (H 2 ), and oxygen (O 2 ) remote plasma, introduced various functional groups [14]. Substitution of carbonyl or hydroxyl groups with fluorine atoms (de-fluorination) is central to the improvement of adhesion [14][15][16][17]. Duca et al. [17] investigated radio frequency (RF) Ar plasma treatment on PVDF. The water contact angle and fluorine to carbon (F/C) ratio was decreased, while surface roughness increased after the argon plasma treatment. Ar and O 2 plasma treatments [18] incorporated oxygen onto the surfaces of PVDF. Our previous investigation [19] on a PET surface suggests that the improvement of adhesion of PPy was mainly due to incorporation of oxygen based functional groups on the surface after plasma treatment.
In this work, results of our investigations on PVDF under RF plasma for the improvement of adhesion of PPy are reported using Ar, O 2 and (Ar + O 2 ) gases. The surface morphology, wettability, chemical composition, electrical resistivity and abrasion resistance of PVDF were studied before and after each plasma treatment.

Materials
The substrate was a PVDF thin film of 50 micron thickness. Films were immersed in acetone, ultrasonicated for 5 min, then washed with deionized water and dried at room temperature. Substrate materials were then conditioned under standard atmospheric pressure at 65% ± 2% relative humidity and 21 ± 1 °C for at least 24 h prior to further processing. Monomer (pyrrole), oxidant (ferric chloride FeCl 3 · 6H 2 O) and dopant (para-toluene sulphonic acid mono hydrate, pTSA) reagent quality were purchased (Sigma-Aldrich, Melbourne, Australia).

Plasma Treatment
Plasma treatment of both PVDF thin films was performed in a custom built 13.5 MHz inductively coupled reactor [20]. The base pressure 1 × 10 −3 mbar was achieved by use of a rotary pump. Ar plasma was used for the pre-treatment of the samples (100 W, 8 × 10 −2 mbar, 30 s) to activate and clean the surface. Plasma power was kept at 100 W for all the gases. The gas pressure was set at 8 × 10 −2 mbar for Ar and 8 × 10 −2 mbar for both O 2 and Ar + O 2 samples. In case of Ar + O 2 , the composition of both gases was kept at 1:1 by volume. The plasma treatment time was fixed at 120 s for all the samples.

Contact Angle Measurement
Water contact angle (WCA) measurements were performed using a CAM100 WCA tester (KSV Instruments, Helsinki, Finland) and distilled water. The films were mounted on glass slides prior to measurement using double side adhesive tape. At least six measurements of each PVDF sample condition were taken to get an averaged WCA measurement. Attention Theta v4.1.9.8 (Biolin scientific, Stockholm, Sweden) software was used to calculate WCA from the captured images after 0.80 s of water droplet contact time. All the measurements were carried out within 10 min of plasma treatment.

Scanning Electron Microscopy
A Supra 55 VP (Carl Zeiss, Oberkochen, Germany) scanning electron microscope (SEM) was used to study the surface morphology of the PVDF thin film surface before and after plasma treatment. All the samples were gold coated before viewing in the SEM.

X-ray Photo Spectroscopy (XPS)
Spectra were obtained using a K-α X-ray photoelectron spectrometer (Thermo Fischer Scientific, Waltham, MA, USA) with monochromatic X-rays focused to a 400 µm spot size. Excessive charging of the samples was minimised using a flood gun. The binding energies of the samples were accurately established by charge shift; the binding energy peak of the F1s to 688.65 eV [14]. Survey spectra were obtained at pass energy of 100 eV while high resolution peak scans were performed at 20 eV pass energy. X-ray photoelectron spectroscopy (XPS) measurements were done within one week of plasma treatment.

Abrasion Resistance and Sheet Resistance
Circular PPy coated PVDF thin film samples were cut using a GSM (grams per square meter) cutter with 38 mm diameter and were abraded against a reference wool fabric under 9 kPa head weight in a Martindale abrasion tester (I.D.M instruments, Melbourne, Australia) for 0, 200 and 2000 abrasion cycles. Micrographs of the samples before and after abrasion were taken under the same lighting and exposure by a microscope (Olympus DP-71, Tokyo, Japan). Sheet resistance was measured using a SP4 four point probe head (Signatone, Tomkins Court Gilroy, CA, USA) and source meter (Keithley 2400, Solon, OH, USA) and employing the same pressure for all the samples [21]. Sheet resistance values were obtained from six different locations of each sample and later average values were calculated.

Contact Angle Measurement
Contact angle measurements show an improvement in the PVDF surface wettability after the plasma treatment. The average WCA value of the PVDF control sample was 84.55° and it reduced with plasma treatment irrespective of the gas used .WCA reduced to 68.36, 63.2 and 46.65 for Ar, O 2 and Ar + O 2 plasma samples respectively as shown in Figure 1. WCA is directly related to the surface energy, it is also dependent on surface morphology and chemical functional groups present on the surface.

Scanning Electron Microscopy
Changes in the surface morphology are normally caused by ion bombardment and UV radiation in the plasma and depend on plasma gas. The SEM images in Figure 2a-e show the surface of the PVDF film before and after plasma treatment. The grainy features on the PVDF film surface were carried onward from the film manufacturing process.
There is no visible etching by Ar or O 2 plasma whereas (Ar + O 2 ) plasma treatment significantly altered the surface morphology (Figure 2d). A rougher surface was displayed as shown in higher resolution image (Figure 2e).

XPS Measurement
XPS measurements were done for control, Ar, O 2 and (Ar + O 2 ) plasma treated PVDF samples. Carbon, fluorine and oxygen were detected in all the samples as shown in the overall XPS scan in Figure 3 and Table 1. Plasma treatment lowered the fluorine content, however oxygen was increased. There is a large difference in bonding energy between C-F and C-H bonds. The bond energy for F-F bonds is only 1.6 eV, that for H-F bonds 5.85 eV, and that for C-F bonds 4.42 eV [14]. High resolution C1s scans were performed to study the presence of different functional groups on the PVDF surface. C1s peaks were curve fitted to calculate the relative concentration of present functional groups. Three sub peaks C1s peak A, C1s peak B and C1s peak C at 286.4, 291.4 and 288 eV respectively were found. Peak A is assigned to CH 2 -CF 2 , CHF-CH 2 -CHF and O-CH 2 groups. Peak B is assigned to CF 2 -CH 2 , while peak C is assigned to CH 2 -CHF-CH 2 , CH 2 -CHF-CHF, O-CH 2 -CF 2 [22]. O 1s peak at 533.7 eV is assigned to C-O [23], while F1s peak at 688.65 eV to C-F bond.  The untreated PVDF surface showed two main peaks at 291.4 eV (due to CF 2 ) and 286.4 eV (due to CH 2 ) components [11,24]. Ar plasma was the most effective in lowering the C1s peak B from 16.8 to 1.89, while O 2 and (Ar + O 2 ) resulted in moderate levels of de-fluorination ( Table 1). The fluorine to carbon ratio (F/C) decreased from 0.54 to 0.06 for Ar and 0.34, 0.29 for O 2 and (Ar + O 2 ) respectively. Ar plasma also increased the surface oxygen content from 2.16% to 9.15%, however two other plasma gases achieved similar results for oxygen incorporation. The oxygen to carbon ratio (O/C) rose to 0.13 for O 2 and (Ar + O 2 ) plasma from 0.03 for the control sample. The O/C value was 0.11 for Ar plasma. It is interesting to note the incorporation of oxygen based functional groups into the argon plasma. The oxygen may arise from the following sources; (i) an impurity in Ar gas [25]; (ii) exposure to air after taking out of the plasma chamber before XPS measurement [17].
The C1s spectra of PVDF films before and after RF plasma show that plasma treatment causes de-fluorination and the incorporation of oxygen into the polymer surface ( Figure 4). The C1s scans and overall composition (Table 1)

Abrasion Resistance and Sheet Resistance
PPy coated PVDF film samples abraded by the Martindale abrasion tester were photographed by a microscope (Olympus DP71, Tokyo, Japan) after 0, 200 and 2000 cycles of abrasion ( Figure 5). The magnification was set at 10 times for all the images. Some of the coating of the control sample was missing even before the abrasion test due to weak interfacial bonding and nearly all the coating faded from the control sample after 200 and 2000 cycles of abrasion leaving the surface non-conductive.
Plasma treated samples clearly showed better abrasion resistance. Although the Ar plasma sample showed some flaking off after 2000 cycles, it was still significantly better than the control sample. On the other hand the O 2 and (Ar + O 2 ) plasma treated samples had strong binding as there was only a small visible coating loss. The effectiveness of (Ar + O 2 ) plasma treatments is clearly evident, as it showed better coating fastness than use of these gases alone. This is due to de-fluorination and incorporation of oxygen based functional groups during plasma treatment [14]. The results showed that Ar plasma resulted in de-fluorination even before abrasion tests. We do not have evidence whether the surface morphology had any variation along with the de-flourination. An AFM investigation of this would be worthwhile in a future study.  The sheet resistance data of PPy coated PVDF samples in Figure 6 and photographs of abraded samples in Figure 5 agree with each other and all confirm the order of plasma gas effectiveness as (Ar + O 2 ) > O 2 > Ar. Contact angle measurements show an improvement in the PVDF surface wettability after the plasma treatment. The decrease of the F/C ratio and water contact angle prove that both chemical and physical effects play an important role in the adhesion.
It is interesting to note that control, Ar and O 2 plasma treated PVDF samples had a grainy surface morphology which did not lower the R s value as much as the (Ar + O 2 ) plasma treated surface having a flat yet nano-sized surface roughness, a similar trend was observed for the bonding strength. Both XPS and surface imaging indicates that incorporation of oxygen functional groups and nano sized surface roughness are important for coating fastness. This was confirmed by the connection of abrasion resistance imaging, sheet resistance measurement and the chemical composition data in Table 1. Similar results were obtained in our previous work on polyester fabric [19].
The treatment of PVDF using oxygen, argon and hydrogen plasmas were reported in a similar study by Park et al. [26], where they reported using remote plasma treatment in which the sample was situated in the after-glow area and the authors focussed on the individual effects of the above mentioned plasma gases. Whereas, in our experiments the sample was placed inside the plasma source and the effects of argon (inert gas), oxygen (reactive gas) and the combination argon + oxygen plasma were used to enable a better understanding of the role of argon and oxygen plasma and the effect of the combination. Overall, results showed that the most effective treatment was the argon + oxygen combination, followed by oxygen and argon. Our results agree with that of Park et al. [26] in the contact angle data for oxygen and argon plasma, that is, argon plasma treatment was more effective than oxygen in hydrophilicity manifesting as a decrease in contact angle. Similarly, oxygen plasma was not very effective in defluorination, supporting the earlier study by Park et al. [26]. In this study we were able to use the advantages of both argon plasma (effective on defluorination) and oxygen plasma (effective on dehydration and introduction of oxygen functional group) to achieve improved bonding. In other words, the improved bonding of the combination may be attributed to the combined effect. Also the argon plasma produced more reactive sites on the substrate surface, which would enhance the binding of functional groups.

Conclusions
We have studied the improvement of binding of polypyrrole with a PVDF thin film using low pressure plasma. Ar, O 2 and Ar + O 2 gases were used in the plasma treatment. The abrasion data suggest adequate strength of bonding with the PVDF surface after plasma treatment. The plasma induced increase in hydrophilicity and surface functionalization with oxygen based groups contributed to the improved coating adhesion and fastness. These results clearly show the effectiveness of Ar + O 2 plasma in lowering the resistivity and improving the binding strength of the PPy coating.