Investigation of the Impact of Cold Plasma Treatment on the Chemical Composition and Wettability of Medical Grade Polyvinylchloride

Edward Bormashenko*a, Irina Legchenkovaa, Shiri Navon-Veneziab, Mark Frenkela, Yelena Bormashenkoa a*Ariel University, Chemical Engineering Department, Engineering Sciences Faculty, 407000, P.O.B. 3, Ariel, Israel, fax: 972-3-9366621; E-Mail: edward@ariel.ac.il bDepartment of Molecular Biology, Faculty of Natural Sciences and the Adelson School of Medicine, Ariel University, Ariel 40700, Israel. Correspondence: Edward Bormashenko, Ariel University, Israel; E-mail: edward@ariel.ac.il Abstract Impact of the Corona, dielectric barrier discharge and low pressure radiofrequency air plasmas on the chemical composition and wettability of the medical grade polyvinylchloride was investigated. Corona plasma treatment exerted the most pronounced increase in the hydrophilization of polyvinylchloride. The specific energy of adhesion of the pristine and plasma treated PVC tubing is reported. The kinetics of hydrophobic recovery following the plasma treatment was explored. The time evolution of the apparent contact angle under the hydrophobic recovery is satisfactorily described by the exponential fitting. Energy-dispersive X-ray spectroscopy of the chemical composition of the near-surface layers of the plasma treated catheters revealed their oxidation. The effect of the hydrophobic recovery is hardly correlated with oxidation of the polymer surface, which is irreversible.

treatment creates a complex mixture of surface functionalities, which influence surface physical and chemical properties; this results in a dramatic increase in the surface energy and consequent change in the wetting behaviour of the surface [9][10][11][12]. It was demonstrated recently that the plasma treatment may result in cross-linking of polymer molecules [13]. It was reported that plasma treatment leads to the essential electrical charging of a polymer surface observed for both synthetic and biological polymers [14][15]. It is well accepted that the cold plasma treatment increases the specific surface energy of polymers, thus resulting in their pronounced hydrophilization [16][17][18][19][20][21]. At the same time the precise physico-chemical mechanism of this hydrophilization remains debatable. It was suggested that hydrophilization of organic surfaces by plasmas may be at least partially related to the re-orientation of hydrophilic moieties constituting organic molecules [22][23]. Oxidation of plasma-treated surfaces and removal of low-mass fragments present on organic surfaces also contribute to hydrophilization [24][25].
It was reported that the specific surface energy of the cold plasma-treated polymers is decreased with time and consequently hydrophilization provided by the plasma treatment is partially lost with time [26][27][28][29][30]. This process is called hydrophobic recovery [26][27][28][29][30]. It was demonstrated experimentally that hydrophobic recovery is stipulated by the re-orientation of the polar groups constituting the polymer chains, which were oriented by the plasma treatment [28]. A model for the hydrophobic recovery due to a combination of two thermodynamically nonequilibrium processes: diffusion and molecular reorientation was suggested [30]. Our paper focuses on the estimation of impact of the cold plasma treatment on the wettability and hydrophobic recovery of the medical grade Polyvinylchloride (PVC), broadly used for manufacturing of medical catheters.
Cold plasma treatment of PVC was reported by several groups [31][32][33][34], however the experimental data related to this polymer remain sparse. We report comparative study of the impact of various plasma discharges, including the Corona discharge, dielectric barrier (DBD) discharge and radiofrequency low pressure air plasma discharge on the wettability and hydrophobic recovery of the medical grade PVC.

Materials
PVC CH18 suction catheters (diameter 6 mm, length 53 cm), supplied by Unomedical ConvaTec Lim. UK, were plasma treated in our experiments. For the purposes of the goniometric measurements of wettability the catheters were flattened under = 80° to and pressure of 1kPa during 40 min, followed by the cooling to ambient conditions during 1 hour.
The de-ionized water, used for the study of wettability of catheters, was purified by a synergy UV water purification system from Millipore SAS (France) and its specific resistivity was = 18.2MΩ × cm at 25ºC.

Methods
Corona plasma device (3DT, MULTIDYNE 1000, Germantown, WI, USA) consisted of a treating head that contained two hook-shaped wire electrodes. The plasma was generated under high voltage at electrode 2 × 12 kV and a frequency of 50 Hz at atmospheric pressure conditions, using ambient air as a carrier gas. The distance between the electrode and the flattened catheter was 2±0.1cm.
The parameters of an air low pressure radiofrequency plasma discharge were: the plasma frequency was 13.56 MHz; the power was 18 W; the pressure was 2 Torr; the times of irradiation Apparent contact angles were established using the Ramé-Hart goniometer (Model 500). A typical picture of a water droplet placed on a pristine PVC catheter is shown in Figure 1. Eight measurements were taken to calculate mean apparent contact angles at ambient conditions. The apparent contact angles were taken on both sides of a droplet; the results were averaged.
Contact angle hysteresis was established with the tilted plane method. A 5 µl water droplet was placed on the PVC sample mounted on the glass slide. The slide was tilted until the drop began to move. The front and rear contact angles at which the droplet started to slip are regarded as the advancing θadv and receding θrec contact angles, correspondingly, as depicted in Figure 2.
The difference rec adv is called the contact angle hysteresis [35][36][37][38][39]. In spite of the fact that the "tilted plane experiment" was criticized as a method for the accurate establishment of the contact angle hysteresis, it is still broadly used for its estimation [39][40]. In parallel, the contact angle hysteresis was measured with needle-syringe method.
The 5 µl water droplet placed on the flattened catheter was inflated with a syringe as shown in Figure 3A. When the contact (triple line) was pinned and the contact angle was increased till a certain threshold value beyond which the triple line does move. This threshold contact angle was regarded the advancing contact angle [35][36][37][38][39]. When a droplet was deflated as depicted in Figure   3B, its volume decreased to a certain limiting value; in parallel the contact angle decreases till a threshold value , interpreted as the receding contact angle [35][36][37][38][39].
Chemical composition of the pristine and cold plasma treated PVC catheters was studied with SEM/EDS (scanning electron microscopy/energy dispersive spectrometry) carried out with SEM (MAIA3 TESCAN).

Experimental study of wetting of the pristine PVC tubing
All of investigated PVC tubing (namely pristine and plasma treated by different discharges demonstrated the partial wetting regime [35][36][37][38][39]. This means that the spreading parameter S, governing the wetting regime was negative [37][38][39]. In Eq.(1), SA   , and SL  are interfacial tensions at water/air, PVC/air and PVC/water interfaces, respectively. The equilibrium "as placed" equilibrium water contact angle was established at the non-treated PVC as = 93 ± 0.5 . This value implies the useful physico-chemical insight. Indeed, according the Young equation, defining the equilibrium (or Young) contact angle is given by Eq. 2: The as-placed equilibrium contact angle does not exhaust the characterization of wettability of a solid surface. The advancing (maximal) and receding (minimal) contact angles should be established experimentally [35][36][37][38][39][40]. Regrettably, the values of the advancing and receding contact angles depend strongly on the experimental technique used for their establishment [43,44]. Thus, we carried the measurement in parallel with the well-known the "tilted plane" and "needle-syringe" methods, illustrated with  One minute plasma treatment of the tubing by the Corona plasma discharge decreased the apparent contact angle from = 93 ± 0.5° to = 30 ± 0.5° , whereas the apparent contact angles attained by the radiofrequency plasma discharge and DBD discharge were ≅ 50 − 60° and ≅ 60 − 70° , correspondingly. Thus, we conclude that the influence exerted by the Corona discharge on the surface properties of the PVC tubing was maximal. The increase in the energy of adhesion, following the plasma treatment as established with the Dupre formula is summarized in Table 1.
It is noteworthy that the impact of the radiofrequency plasma discharge and DBD discharge on the wettability of PVC comes to saturation within first 15 s of the plasma treatment, whereas the influence of the Corona discharge grows slightly within the first minute of treatment, as shown in The kinetics of hydrophobic recovery is satisfactorily described by the exponential fitting, suggested in Ref. 28: where is the initial apparent contact angle, taken immediately after plasma treatment of PVC catheters, is the characteristic time of restoring of the contact angle, is the fitting parameter, and = + is the saturation apparent contact angle. The numerical values of the parameters of the exponential fitting for the investigated plasma discharges are supplied in Tables 2-4.
A number of common features are recognized for the hydrophobic recovery of PVC tubing exposed to various plasma discharges: i) the hydrophobic recovery following the plasma treatment is well-described by the exponential fitting for all kinds of the studied discharges; ii) the saturation contact angle is slightly lower than the initial apparent contact angle; in other words, the hydrophobic recovery following the plasma treatment of PVC is never complete.
On the other hand, the characteristic time of the hydrophobic recovery is largest for the radiofrequency plasma discharge, namely ≅ 3.0 − 6.0 days ; it is intermediate for the Corona discharge, i.e. ≅ 1.0 − 2.5 days; and it is small for the DBD discharge, namely: ≅ 0.5 − 1.5 days.
Thus, the hydrophobic recovery following the cold plasma treatment is the slowest for the radiofrequency plasma discharge, and it is rapid for the DBD plasma treatment. At this stage of our investigation we are far from the microscopic interpretation of these experimental findings, which are of a primary importance for the applications of the plasma processing of PVC, however some conclusions may be made from the energy-dispersive X-ray spectroscopy (abbreviated further EDS spectroscopy) of the plasma-treated PVC catheters, discussed in the following section.

EDS analysis of the chemical composition of the plasma treated PVC catheters.
EDS study of the chemical composition of the pristine and plasma-treated PVC catheters indicated the growth of the concentration of oxygen in the near surface layers of the plasma treated PVC catheters, as demonstrated in Figure 8.
The growth of the concentration of oxygen from 7 to 9 atomic percent was registered, as shown in Figure 8. It should be emphasized that this tendency was kept for all kind of the investigated plasma discharges. The concentration of oxygen in the surface layer of catheters grew with the time of treatment, as shown in Figure 8. These results coincide with the finding reported by the other research groups [45]. It was demonstrated that when PVC is treated with an argon plasma unsaturated bonds are created at the surface [46]. It was also shown that the pendant groups of PVC are removed by the argon plasma treatment [46]. This resulted in the formation of unsaturated bonds and cross-links in the modified layer and the outermost top layer of the polymer became oxidized after exposure to air due to a reaction between long-living radicals and oxygen [46]. It is reasonable to suggest that that the similar mechanism takes place also under the cold air plasma treatment, thus, giving rise to the change in the wettability of PVC, discussed in detail in the previous sections.
It was instructive to study the surface composition of the plasma treated PVC catheters after one week of ageing under ambient conditions. It is recognized from the data presented in Figure 9 and its comparison to the data shown in Figure 8, that the concentration of oxygen in the near-surface layers of the plasma-treated catheters decreased very slightly. This observation is quite expectable; indeed, oxidation of PVC emerging from the plasma treatment is an irreversible process [45,46].
Consider that the time of ageing was markedly larger that the characteristic time of the hydrophobic Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 November 2020 doi:10.20944/preprints202011.0590.v1 recovery, supplied in Table 1. Thus, we conclude, that the process of the hydrophobic recovery is hardly correlated with the oxidation of the polymer arising from the cold plasma treatment.

Conclusions
Cold plasma treatment is broadly used for modification of chemical composition and wettability of organic and nonorganic compounds [1][2][3][4][5][6][7][8][9]47]. Influence of the plasma treatment on the surface chemical composition and properties of the polyvinylchloride catheters was explored with various plasma sources. We conclude that the Corona, low pressure radiofrequency and DBD plasma discharges increased essentially hydrophilicity of the PVC tubing. The Corona plasma treatment demonstrated the most pronounced influence on the eventual wettability of catheters. One-minute plasma treatment of the tubing by the Corona plasma discharge decreased the apparent contact angle from = 93 ± 0.5° to = 30 ± 0.5° . The energy of water adhesion of pristine and plasma treated PVC, estimated with the Dupre formula is reported. Kinetics of the hydrophobic recovery following the plasma treatment of PVS was explored [26][27][28][29][30]. The hydrophobic recovery following the plasma treatment of PVC is never complete, in other words the saturation contact angle is slightly lower than the initial apparent contact angle inherent for the pristine PVC. It is noteworthy that the kinetics of hydrophobic recovery following the plasma treatment is well-described by the exponential fitting suggested in Ref. 28 for all kinds of the studied discharges. We established that the hydrophobic recovery following the cold plasma treatment is the slowest for the radiofrequency plasma discharge, and it is most rapid for the DBD plasma treatment of PVC. Energy-dispersive X-ray spectroscopy of the chemical composition of the near-surface layers of the catheters indicated their oxidation, emerging from the cold air plasma treatment. This oxidation was irreversible and changed very slightly under one week ageing under ambient conditions. Thus, we conclude that the phenomenon of the hydrophobic recovery is hardly correlated with oxidation of the polymer (PVC) surface.

PVC tubing
Specific Energy of Adhesion,