Effects of cold SF6 plasma treatment on a-C:H, polypropylene and polystyrene

https://doi.org/10.1016/j.surfcoat.2020.125398Get rights and content

Highlights

  • Cold SF6 plasma treatment increases hydrophobicity of diverse polymers.

  • Surface F and O cause changes in θ.

  • Ageing for 10 days barely changes θ.

Abstract

The effects of cold SF6 plasma treatment on amorphous hydrogenated carbon (FA), polypropylene (PP) and polystyrene (PS) were investigated as functions of gas pressure and applied power. An anticipated increase in hydrophobicity was confirmed by the greater water contact angles, θ, observed after all the treatments. Under the best conditions θ was increased by 50.8°, 57.2° and 21°, respectively. A rise and fall in θ was observed as the pressure of SF6 was increased, this trend being most consistent for FA. Although the plasma treatments caused some changes in surface roughness, measured using profilometry, there were no clear correlations between this parameter and θ. As revealed by Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS), the treated surfaces were fluorinated. As the degree of fluorination under optimal conditions was 2.2 at.%, 10.4 at.% and 36.3 at.% for the FA, PP and PS, respectively, this factor was not alone responsible for the observed increases in θ. Sulfur was attached to the surface of all the treated samples. The relative surface carbon content was reduced by the treatments. The main causes of the changes in θ upon treatment were the induced compositional and structural changes. Ageing for ten days caused a typical decrease in θ of ~10°, probably caused by rotation of hydrophobic surface groups into the surface.

Introduction

Cold plasma treatment for the modification of polymer surfaces is well established [1,2]. Such treatments modify surface properties, such as chemical composition and structure, roughness and, consequently, surface contact angle, without changing the bulk of the material. Plasma treatment is fast, cheap, versatile, uses small quantities of gas, and has a low environmental imprint [3]. Amongst others, plasmas of nitrogen [4,5], oxygen [6,7] and of fluorinated gases [[8], [9], [10]] have been used to treat polymers. Plasmas of fluorinated gases are of special interest, for example, to increase surface hydrophobicity. Poly(ethylene terephthalate) (PET) and cotton fibers have been subjected to SF6 plasmas, which produced efficient implantation of fluorine atoms and thus an increase in water repellence [11]. Water droplet roll-off angles decreased with increasing radiofrequency (RF) power for treatments at 2 mbar for 1 min [11]. Analyses of untreated and treated PET by XPS revealed the presence of single bondC-CFx and CF2 functionalities after treatment. Fluorine atoms attack carbon atoms of both benzene and ethylene. Polypropylene fabrics were fluorinated using CF4 plasmas [12]. Fluorinated functionalities such as CFx (x = 1 to 3) and single bondC-CF3 were detected via deconvolution of the C1s XPS spectra of the treated material. Further studies of the fluorination of PET have been reported [[13], [14], [15], [16]]. Barni et al. [13] confirmed that for system pressures of <75 mTorr cold SF6 plasma treatment of PET fibers increased hydrophilicity via surface etching and surface activations. For CF4 plasmas Wen et al. [14] reported great differences in the water contact angle presented by two sides of a treated PET sample. One side was hydrophilic (water contact angle, θ ~7°), the other hydrophobic (θ ~108°). The difference was attributed to the uneven distribution of CF4 in the plasma. In another investigation [15], cold CF4 plasma treatment of PET caused an increase in θ from 68° to 89°. Subsequent ageing saw θ fall to 58° after 210 days; this may be caused by oxidation or reorientation of surface groups or both. Resnik et al. [16] suggested that full dissociation of SF6 in cold plasmas below about 750 mTorr at 200 W produces high concentrations of fluorine radicals, which cause an intensive surface fluorination to ~46 at.%, and the presence of CFx (x = 1 to 3) functional groups on PET. The treatment caused surface oxygen concentrations to fall from 26 to 10 at.%. Contact angles of 76° and 105°, respectively, were observed for the untreated and treated surfaces.

Rangel et al. [17] investigated the effects of SF6 and Ar plasma immersion ion implantation on the hydrophobicity of poly(tetrafluoroethylene) (PTFE), polyurethane and silicone surfaces. The main mechanism responsible for the observed increase in hydrophobicity of silicone produced by the SF6 treatments was surface fluorination, with F substituting Si and O. Preferential etching of Si by SFx radicals probably also plays a major role in surface modification. Surface roughnesses also tend to increase with increasing treatment time. Surface treatments in SF6 plasmas of other polymers, such as corn starch [18,19], natural rubber [20] and cellulose [21], have also been reported. Using O2 and SF6 plasma treatment of polypropylene (PP) housed in a specially-designed mask, Mangindaan et al. [22] produced a wettability gradient with water contact angles controllable in the range 20° to 135°. The same research group proposed a model of the process based on the dissociation and ionization of SF6 followed by diffusion-adsorption-reaction processes, assuming fluorine radicals as the main reactive species [23]. Additional polymers fluorinated in cold plasmas include cotton [24], polystyrene (PS) [25], polyethylene [26], Parylene-C [27] and Poly(vinyl)chloride [28].

Here, the effects of SF6 pressure (0 to 100 mTorr) and applied power (0 to 100 W) on the surface contact angle, structure and composition, roughness and morphology of plasma-treated a-C:H, PP and PS are reported. These materials were chosen to reveal possible differences in response to identical plasma treatments of amorphous hydrogenated carbon, and the polymers polypropylene, [CH2-CH(CH3)]n and polystyrene, [C6H5CH:CH2]n. Thus the effects of the treatments on amorphous ramified plasma polymer, simple long-chain polymer and long-chain polymer containing aromatic rings were studied.

Section snippets

Experimental

Treatments were carried out in a radiofrequency plasma reactor composed of a glass vacuum chamber fitted with horizontal, planar, circular, parallel stainless-steel electrodes; the upper one connected to the RF power supply (TOKYO HY-POWER, RF – 300) and the lower one grounded. A mass flow controller (MKS, 247) was used to feed gas to the chamber, which was continuously evacuated by a rotary-vane pump (EDWARDS, E2M 18). Pressure was monitored using a Pirani gauge (AGILENT, PCG – 750). The

Results and discussion

Fig. 2 shows the surface contact angle, θ, and roughness, Ra, for amorphous hydrogenated carbon, polypropylene, and polystyrene as a function of the SF6 pressure for applied powers of between 0 and 100 W. At low pressures surface fluorination is expected to be limited by the supply of SF6. As the supply of SF6 increases, fluorination and hence contact angles tend to increase. Beyond a pressure threshold, which depends on the applied power, fluorination is optimized, and the contact angle

Conclusions

All treatments with SF6 plasma caused an increase of hydrophobicity of the polymers studied. The magnitude of this increase depends on the pressure and the applied power. Choice of the plasma pressure alone at fixed applied power ensures large increases in θ. In contrast, contact angle-plasma feed pressure graphs have similar forms independently of the plasma power. Differences in the way each of the three substrates responded to the same treatment conditions indicate the influence of

Author contributions statement

Milena Kowalczuk Manosso Amorim: Conceptualization; Methodology; Investigation treatments and diverse analyses (profilometry, goniometry, infrared spectroscopy, etc.); Visualization; Writing review

Elidiane Cipriano Rangel: Investigation - EDS and SEM measurements and analysis; Visualization; Writing - review

Richard Landers: Investigation - XPS analyses, data presentation and interpretation; Visualization; Writing-review

Steven F. Durrant: Conceptualization; Methodology; Funding acquisition;

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank FAPESP (2017/15853-0) and CNPq for financial support. One of us, MKMA, gratefully acknowledges support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES). This study was also financed in part by CAPES - Finance code 001.

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