Activation of the Internal Surfaces of Polymer Tubes with Argon Plasma Through Hollow Cathode Effect

1. Introduction

Surface modification with low-temperature plasma is one of the most popular research topics. So far, many plasma-based surface-related processes have been developed and applied in industries. On the other hand, polymers are known for their flexibility, low density, low cost, and easiness of manufacturing. However, their surface properties often cannot satisfy the demands on low gas transmission, high biocompatibility, high wettability, etc. [1]. Many studies were reportedly seeking to improve the surface chemistry or morphology of polymers. Unfortunately, the use of chemical treatments may cause the change of polymer structure [2] that affects their overall characteristics. Also, these methods are normally not environmentally friendly. Hence, suitable surface modification by cold plasma is required to achieve the desired surface properties [3,4,5,6].

Tubes made of polymers are widely applied in human daily life and in critical engineering fields (e.g. fluid transport in biomedical fields). In biomedical industries, surface activation of tubes is often required. To succeed in this attempt, the most critical challenge is to generate plasma inside the tube and subsequently modify tube's inner wall. However, so far, only minor efforts have focused on surface modification within the tube, including surface activation or functionalization. Only few papers have reported the application of cold plasma for the surface activation [7] or functional coating on tubes’ inner surface [8,9]. Li et al. [9] applied argon/oxygen plasma by discharge and reported that the plasma generated inside the tube could change the state of the tube's inner surface. Among all the approaches adopted for the modification of polymer surface, cold plasma is a well-known process to achieve the needed surface properties without the use of extra physical and chemical reactants. This is highly desirable for the modification of polymers being applied for biomedical applications, or many other uses [10]. Another significant benefit of plasma treatment is that it is considered as a green process, relying on types of discharge, treatment time, and characteristics of the applied gases [7,8,9,11].

The effects of low-pressure plasma processes on water contact angle (WCA) or surface energy of various substrates have been studied for long time. The characteristics of plasma depend on the gas input to the plasma discharge. For example, O₂, H₂, N₂, and CF₄ produce O, H, N, and F radicals, respectively [12]. Sharm et al. [13] reported that WCA was observed to change from 80° to 40° after plasma activation for 600 sec. in Ar plasma for polyimide (PI). In this study, surface activation is most likely induced by the bombardment of Ar ions. Getty and Zhao [14] reported that, if PI surface is treated with remote Ar, N2, and O2 plasma, the substrate would not be exposed to ion bombardment. The results show that the exposure of substrate to Ar and N₂ plasma only slightly reduced the WCA, while oxygen plasma caused a large decrease due to the presence of O radicals. In the latter case, the WCA of PI is reduced from 60° to 5°. Egitto and colleagues [15] found that WCA of PI was reduced from 70° to 20° after direct exposure of the substrate to He/O₂ plasma for only 50 seconds. In their conclusion, the surface energy of the polymer substrate can be significantly increased by the treatment with plasma due to the generation of O radicals, which should be beneficial to improve the wettability of hydrocarbon-based polymers [16]. Furthermore, many efforts have been made to functionalize polymer surfaces by plasma processing. The functionalized polymer surfaces by plasma may contain -COOH, -OH, -NH₂, -COOR, -COR, -CFx, -Br, -SH and thiophene-based units [12]. Great interests reported in these functionalized surfaces come from their fundamental applications in modern bio-medical applications, including interlayers to promote metal modification adhesion [17], biocompatibility [18], antimicrobial modification [19], or drug release [20].

In this study, a low-pressure cold plasma was applied to activate the inner surfaces of PET (polyethylene terephthalate), PU (Polyurethane), and PTFE (Polytetrafluoroethylene) tubes with Ar plasma, if plasma can be generated through hollow cathode discharge (HCD). Similar approach developed in this study can be applied to plasma functionalization of polymer tubes.

PET is a kind of thermoplastic polyester. It is one of the most widely used polymers currently in various industries. In life science, the surface states of PET could be tailored according to blood-polymer or cell-polymer interaction [21]. Plasma processing could be applied directly to control these interactions. Additionally, plasma can be applied to increase or decrease reactive sites aiming at immobilizing or loading bioactive components, which may include antithrombotic and anticoagulant agents [22].

PU is a polymer known for its well-accepted mechanical properties, wear resistance, flexibility, durability, and biocompatibility, along with light weight and smooth finishing [23]. Although PU has all these advantages, for certain applications where low WCA and excellent adhesion are needed, the application window of PUs is quite narrow. Specifically, its high WCA would usually induce poor adhesion [24,25]. The adhesion problem can be traced back to the fact that it has a shortage in the usable polar functional groups. Therefore, further surface activation is required for quite large number of applications [26,27]. Plasma processing was proved to be able to incorporate various polar functional groups on PU’s surface. Improved hydrophilic behaviors could be hence achieved. [28].

PTFE is a kind of thermoplastic polymer, mainly with C-F bonds. It has melting point close to 600 K, before which the mechanical properties can be maintained. Accordingly, it has been very popular in many industries. Its chemical inertness attributed to C-F bonds, however, will cause very poor adhesion of any additional substance or thin film coatings. To solve this problem, many research topics have been focused on the improvement of wettability. It has been a major technological and scientific task for the past decades. However, it is one of the most difficult polymers to be activated [29]. So far, there are many studies on the activation of PTFE by plasma. The predominant mechanism leading to hydrophilic surface is still not well understood. In some literatures, it is proposed that bond scission during treatment is required. This might be caused by ultraviolet radiation and ion bombardment rather than direct functionalization with reactive oxygen species [30].

Up to date, many plasma systems, including atmospheric pressure and low-pressure plasmas (APP and LPP), are used to treat various polymers. However, there are only few studies on the plasma modification of tubes’ inner surface, and none of these reports applied the theory of HCD and understand how it can be generated to proceed with the process. An HCD device normally consists of a cathode with hollow structure (ex: aperture, hole, tube.) [31], as illustrated in Figure 1. Compared to conventional CCP discharge, hollow cathode discharge is outstanding due to higher concentrations of electrons and ions, as well as radicals (i.e. high plasma density). These properties will contribute to phenomenon known as hollow cathode effect (HCE) [31,32].

As shown in Figure 1, HC and oscillation movement of electrons between repelling electric fields originating from the two plasma sheaths are illustrated. In general, electrons emitted from the cathode surface accelerate toward the opposite side due to electric field through sheath and are bounced back. As a result, these electrons oscillate across the narrow band between two opposite inner walls of the electrode due to the repelling fields of the opposite plasma sheaths and it can cause more frequent collisions with the gas in the glow. Ions may accelerate toward the cathode walls and hence cause severe ion bombardment that would directly result in bond breakage. Furthermore, photoemission of electrons is also possible due to UV light irradiation. This may further increase ionization rate and therefore plasma density [33]. Inside the tubes, ions may work to break up some of the bonds on the polymer’s surface. These fractured bond sites are then chemically available to form polar bonds with radicals, thereby creating an activated polymer surface.

In HCD, the thickness of plasma sheath normally decreases with the increase of power or gas pressure, which means the electric field in the plasma sheath may be increased. Thus, the electrons generated on one cathode surface would accelerate towards the negative glow and are retarded only by opposite cathode sheath. This would allow electrons to perform several ionizing collisions. In addition, the ion bombardment of positively charged ions on cathode surface would cause secondary electron emission which in ture cause more ionization. Without question, metastable atoms are also generated in the processes [34,35,36]. In sum, these HCE mechanisms depends significantly on the operating parameters, the characteristics and pressure of gas used in the hollow region, the cathode material, electrical circuit, and HC geometry. [37].

In the following, Table 1 summarizes some representative examples of surface activation on various polymers with conventional capacitively coupled plasma (CCP) and unconventional high density plasma technologies. Overall, most of the literature proposed that surface activation is related to the increase of oxygen concentration and forming C-O or, more effectively, C=O bond. In the case of PTFE, the removal of fluorine atoms, followed by oxygen bonding, was normally critical [38,39]. Furthermore, according to this table, ion and radical density and the related ion bombardment may play critical roles in activating polymer surfaces.

The purpose of this study is aimed at studying and applying low-pressure plasmas for the activation of tube's inner surfaces. When tubes are attached to the cathode (power electrode), they can be considered as hollow cathodes (HC). For HCD to be generated inside a tube, it is thought that the tube diameter should be greater than twice of the plasma sheath. Otherwise, plasma will not be initiated inside the tube. Subsequently, the tube’s inner surface will not be activated. [45]. Therefore, one of the major works in this study is to verify whether the hollow cathode effect (HCE) prevails in the modification process, with the variation of tube diameter and power input. To carry out the experiment, the plasma sheath was calculated according to the results of optical emission spectroscopy (OES). From the OES results, the temperature and density of electrons in plasma could be obtained with line ratio and modified Boltzmann's plot methods [46,47]. The plasma sheath was then estimated and finally compared with the experimental results to confirm the formation of HCD.

2. Experimental Procedures

2.1. Plasma Activation Source and Process

The schematic of the home-made RF reactor is displayed in Figure 2. The RF discharge was generated in a grounded stainless-steel vacuum chamber having internal volume of 7.5 liter, with dimensions of 35 cm (dia.) x 15 cm (height). Two parallel circular plates electrodes with a diameter of 24.5 cm were placed 60 mm apart. The powered electrode was capacitive coupled to an RF generator (13.56 MHz). The bottom electrode was grounded. The negative bias was formed naturally on the power electrode. The chamber was pumped down using a mechanical pump to an ultimate pressure of 1 ×10^-2 Torr and then filled with Ar gas. A 7-cm window enabled the observation of the discharge. The tubes (lengths of 10 and 15 cm and inner diameters of 3, 4, 5, 6, and 7 mm) were attached to a powered electrode. The tubes were cleaned by DI water in an ultrasound cleaner for 20 min, then blown dry with nitrogen. Experiments were carried out at various power inputs and tube diameters to understand how they affect plasma characteristics respectively. For the detection of excited species in plasma, an OES was employed. The optical collecting lens was attached to the chamber quartz window during deposition process. The spectrometer captures the plasma spectra inside the chamber. More detailed information could be found in literature [45]. In the experiment, PET, PU, and PTFE tubes with various diameters were used. Table 2. shows the material characteristics of the selected tubes [48,49,50].

The polymers containing aromatic rings (e.g. PET and PU) exhibited a high level of oxygen uptake. This can be driven by the reactions of ring-opening attributed to ion bombardment. Afterward, hydroxyl groups may be formed on the polymer surface and enhance hydrophilicity.

2.2. Verification of Plasma Generation Through HCE

Table 3. shows the process parameters selected for the verification of plasma generation inside the tube, while Table 4. shows the process parameters for the modification of the tubes’ internal surfaces, followed by the characterization by WCA and XPS.

Estimation of Plasma Sheath~

As mentioned in the previous sections, to modify the inner surface of a polymeric tube, a plasma needs to be generated inside the tube. It is therefore required that the tube diameter (Td) should be greater than twice plasma sheath (Ds), i.e. Td>2Ds. The plasma sheath can be estimated as in the following [51].

D_s ~ η^2/3 × λ_D

(1)

Here,

D_{s =} thickness of plasma sheath

λ_D = Debye length

η = e × (V_P -V_B) / (k × T_e)

Sheath potential (plasma potential) is given by

V_s = (k × T_e /2e) × ln (m_e/ (2.3 × m_i))

(2)

Here,

V_{s =} sheath Potential

m_e =mass of electron in kg

m_i = mass of ion in kg

k = Boltzmann’s Constant in JK^-1

T_{e =} Electron Temperature in eV

n_e = Electron density in cm^-3

The Debye length can be estimated by the following equation:

λ_D = (ε₀k_BT_e/e²n_e)^1/2 = 743 x (Te/n_e)^1/2 (cm)

(3)

where T_e is the electron temperature in eV and n_e is the electron density in cm^-3 [52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67]. For the plasmas generated in the present study, a common value of λ_D is within 2 × 10^-2 to 4 × 10^-4 cm for n_e = 1 × 10⁹–1 × 10¹³ cm⁻¹ and T_e = 1–2.5 eV. For chamber pressures between ~0.13 up to ~1333 Pa (roughly 1 mTorr to ~ 10 Torr) cold plasma’s λ_D is between 0.1% and 1% of the electrons’ collisional mean free path. [52].

The above-mentioned approach for the estimation of plasma sheath should be considered as the upper limit of its value since the n_e inside the tube is normally greater than outside the tube in the chamber.

2.3. Material Characterization

A contact angle measurement system (homemade) was used to check the WCA of the tubes’ internal surfaces before and after plasma processing, as functions of tube diameter and plasma power. The measurement of contact angle is illustrated in Figure 3. The binding energies of C, O, N, and F elements on the tubes’ inner surface was examined by X-ray photoelectron spectrometer (XPS, PHI 5000 VersaProbe III, ULVAC-PHI. Inc., Tokyo, Japan). The calibration of peak positions used the C1s peak (284.6 eV). For the calibration, gold sample surface was measured to obtain the carbon peak position. The carbon signal on the gold surface may originate from the environment. The pass energy was 15 eV. This will give a fixed resolution of 0.9 eV. Prior to analysis, the tube samples were cut open to form a small piece with dimension of 5x5 mm. The chemical binding situation of those critical elements could be determined from the spectra.

2.4. Diagnostics of Plasma Using OES

In the system, there is a quartz viewport which enables the passing through of various lights generated inside plasma and captured by OES. The collected light intensity may contain those generated inside and outside the tube. The collected OES spectra were later analyzed using abovementioned approaches. The spectrometer (AVANTES, AVASpec-2048L, Apeldoorn, the Netherland) had a spectral range within 200-1100 nm. The spectral resolution was 1.4 nm. In every OES experiment, the collecting lens was attached to clean quartz viewport to capture the emission spectra in the chamber with the focus at the tubes’ internal space during the plasma surface processing. The approaches used to obtain electron density and temperature, and the subsequent plasma sheath can be found in previous literature [45].

3. Results and Discussion

3.1. Verification of Plasma Generation Through Hollow Cathode Effect

As shown in Table 5 and Table 6, by increasing the RF power from 70 to 260 W, the electron density increases (n_e) with the decrease of electron temperature (T_e), with vacuum pressure at 0.1 and 0.4 torr. λ_D is known to be proportional to the square root of T_e, and 1/n_e [45]. Thus, λ_D decreases with the increased RF power, as seen in both Tables. Therefore, the sheath thickness would decrease with the increase of RF power, as sheath thickness is proportional to λ_D. As described previously, T_e, n_e along with λ_D provides critical information for the estimation of sheath thickness. The immediate effect of increasing RF power on plasma characteristics of HCD can be described as follows: (1) The increased excitation and ionization rates of Ar atoms in plasma were the main reason for the increase of plasma density. (2) In the HCD plasma, the plasma intensity is increased due to collision impact on Ar atoms by the oscillating electron. When RF power increases, the ionization and excitation rates are enhanced, and n_e therefore is increased. (3) For similar reasons, T_e would decrease due to the increased collision frequency of electrons. As shown in Table 5 and Table 6, it can be observed that the value of n_e increases with the decrease of T_e, when RF power increases. It is also important to know that the working pressure plays a critical role in terms of plasma characteristics because the mean free path (MFP) would decrease with the increase of working pressure. In this case, n_e will increase while T_e will decrease.

As mentioned before, the inner tubes’ diameters were varied with the change of process parameters. Tubes’ inner diameters between 3 to 7 mm were selected for the initiation study of plasma inside the tubes. If the value of sheath thickness is equal or greater than the internal diameter of tube, the HCD plasma will not be initiated inside the tube (i.e. no glow discharge). Figure 4 shows the cross-sectional images of tubes with RF power set at 150 W (Figure 4a) or 260 W (Figure 4b). According to Figure 4, the experiments with RF power set at 150 W could not generate stable plasma inside the tube when the diameter was either 3 mm or 4 mm. This is because the sheath thickness was greater than the inner radius of the tube. However, if the RF power was increased to 260 W, the tubes with inner diameter of 4 mm or bigger could generate HCD plasma inside the tubes. The results are consistent with the results shown in Table 6. With the power set at 150 W, the sheath thickness was calculated as 2.38 mm, which implies that the tubes with diameters less than 4.76 mm would not be able to generate plasma inside the tube. On the other hand, if the power is set at 260 W, the tubes with diameter less than 3.88 mm would not generate plasma inside the tubes.

The broadening of sheath thickness will inevitably result in the sudden drop of electric field along the radial direction of the tube. Eventually, there would be no plasma generation inside the tube for some samples with small diameters. Without plasma generated inside the tubes, the inner surfaces of the tubes could not be modified.

Table 5 and Table 6 show also the values of T_e, n_e, and plasma sheath thickness under different power and chamber pressure of Ar plasma. The results show that when the power increases from 70 W to 260 W, T_e decreases, and n_e increases, and the sheath thickness also decreases. According to the hollow cathode theory, if a tube with 4 mm in diameter, only one condition that can result in the generation of plasma inside the tube. This condition would be the one that applied 260 W under 0.4 torr in chamber pressure.

3.2. Water Contact Angle Analysis

The wettability (hydrophilicity) of polymers is normally reflected by WCA. Extensive work has been carried out on various polymers using different plasma systems. In one example, Koh et al. [53,54] used ion assisted process to study surface activation on various polymers in reactive environments. The results showed that the formation of polar functional groups, including C=O, (C=O)-O, C-O etc., is critical in effective surface activation of polymers.

Figure 5 shows the values of WCA, as a function of tube location, with input powers at 150 W or 260 W for PET, PU, and PTFE tubes. These tubes had a fixed internal diameter of 4 mm, and all treated in Ar plasma of HCD for 5 minutes. As seen in Figure 4, the plasma could not be generated inside the tubes with input RF power set at 150 W. The WCA on the internal surfaces are at the highest near the center region. For example, PET had a contact angle of 73.1^o, while PU has 79.6^o, and PTFE has 103.3^o. With RF power set at 260 W, HCD plasma can be generated inside these tubes, and hence the surface can be activated. Under this circumstance, the water contact angles were 38.7°, 33°, 30.2° for PET, PU, and PTFE respectively. These results can echo the previously calculated plasma sheath. The sheath thickness is 2.38 mm with input power at 150 W, and it is 1.94 mm with input power at 260 W. Figure 6 are some images of the water droplets on the samples taken from the tubes’ center regions. According to Figure 5, although there was no plasma generated in the tubes, with input power set at 150 W, certain degree of surface activation can be observed near both opening ends, particularly on PET samples. These results are thought to be due to the diffusion of the oxidative radicals, such as OH, H, and O, into the tubes, which may cause chemical oxidation. For PET, this phenomenon is much more obvious, since the role of ion bombardment is not as important as that for PTFE or PU. In the study, although Ar was used as the working gas, the ultimate vacuum was about 10 mtorr. Hence, a quite large number of water and air molecules were involved in plasma activity. This would cause the formation of various radicals with tremendous oxidative power.

Figure 7 shows the WCA values of tubes as a function of tube location, with internal diameters at 3 mm or 4 mm, under Ar plasma of HCD for PET, PU, and PTFE tubes. The experiments were carried out with a fixed power of 260 W. It was observed that the WCA of the tubes with internal diameter at 3 mm were about 85.3°, 79.8° and 104.2° at the center point. This is because no plasma was generated in 3-mm tubes. For 4-mm tubes, the plasma was generated inside the whole tubes. The internal surfaces were hence activated. The WCA of the tubes were about 38.7°, 33° and 30.2° respectively. For 3-mm tubes, at the locations near both ends, the contact angle tends to be lower. This would be due to the penetration of plasma, and further due to the diffusion of oxidative radicals, although it is not as significant as in Figure 5. Since the diameter is smaller (i.e. 3 mm), it is expected that the diffusion distance of radicals would be shorter.

The improvement of wettability, as reported by Valerio et al. [55], can be explained as the synergistic effects of plasma radicals and UV/VUV (vacuum ultraviolet) radiation. In the present study, the radicals may come from water and air molecules remaining in the chamber, since the experiments were carried out in a vacuum chamber that was pumped out only by a mechanical rotary pump. Based on this, it is possible to tell the difference between two kinds of plasma processing that may enhance different mechanisms in surface modification: (i) plasma activation with radicals, and (ii) plasma treatment with plasma species that may cause bond-scission first and the subsequent depletion of fluorine or carbon from the surface. PET activation is a typical example of the first case. For PTFE and even PU, the mechanism that causes bond breaking of C and F atoms arises from ion bombardment which is the typical result of HCE. After bond scission, some reactive species would readily interact chemically with the polymer surface forming polar surface. This result will cause a formation of unstable fragments containing carbon, fluorine and oxygen, which may increase the surface energy.

It was reported previously that, for most F-free polymers, the hydrophilic surface can be commonly obtained by plasma sustained in pure O₂ or a mixture of O₂ with an inert gas. The difference between F-free polymer and PTFE is explained as follows. For PTFE, each carbon atom in PTFE is bonded to two F atoms. The C–F bond strength is known to be one of the strongest among all chemical bonds. Therefore, the bond breakage requires high-energy ion bombardment. This may occur in HCD. Additionally, F has the highest oxidation potential compared with all other elements. Therefore, the substitution of F atoms with O or OH radicals are thermodynamically not possible. The existence of O radicals from plasma, hence, does not necessarily lead to the substitution of F on the surface of PTFE. The bombardment energy and momentum carried by energetic Ar species (particularly, Ar ions) is sufficient to break C-F bonds. As described by Carbone et al. [38], the interactions simultaneously induced by reactive O species and the ion-bombarded surface could cause the formation of volatile CxFyOz molecules, which may desorb from the PTFE surface eventually. Therefore, the net reaction may be like reactive ion etching.

With the existing of water molecules, it is possible that the additional hydrogen radicals would enable the breakage of C–F bonds and the creation of dangling bond on surface. The best results in terms of WCA were reported recently by Nguyen et al. [56]. The WCA value changed from 118^o to 4^o. In this study, they applied an inductively coupled RF discharge with power at 100 W. Several gases were used. The results showed that O₂ and Ar plasmas did not show obvious improvement since the WCA remained at 112^o and 110^o with both plasmas. By using plasma generated in Ar mixed with NH₃ or H₂O, significant reduction in F concentration near the surface was found. Accordingly, hydrophilic functional groups containing O and N were detected on the surface of PTFE. As a result, Ar ions may contribute to breaking bonds, such as C-F bond, while radicals, such as O, may form C-O and C=O bonds. This further explains the importance of HCE which may provide large amount of energetic Ar ions and radicals.

Figure 8 shows two plasma treated PTFE tubes with diameter at 4 mm, but with different tube length. It is seen that the length would not affect the modification results, since HCD plasma can be generated inside the tube no matter how long the length would be. Recently, Chen et al. [57,58] applied TAPP (transferred atmospheric pressure plasma) to treat the inner surface of PTFE tube and found that a uniform and effective surface activation with any length can be easily obtained by moving the tube along its axis. In the present study, the uniform treatment of tubes can also be realized if HCD plasma can be generated inside the tube.

3.3. Chemical Bonding Analysis of the Tubes’ Internal Surfaces

The XPS was applied to study the chemical binding status of the internal surfaces for PET, PU, and PTFE tubes, before and after HCD plasma treatment. The comparison of XPS spectra between the untreated PET and the plasma treated PET surfaces are shown in Figure 9. Table 7 summarizes the concentrations of C, O and N signals obtained from the spectra of untreated and treated PET surfaces. According to this table, plasma treatment on the internal surfaces of PET tubes leads to a decrease in C concentration from 74.4 at. % to 61.9 at. %. At the same time, the plasma causes an increase in O concentration on PET surface from 22.6 at. % to 35.6 at. %. Table 7 also shows that a small amount of nitrogen was found on the surface after plasma treatment. This is evidence of the existence of air molecules which were not pumped out thoroughly. From Table 7, it can be observed that the O/C ratio matches well with the wettability of the internal surfaces. The increase of O/C ratio after the HCD plasma treatment was accompanied by the decrease of WCA. To explain the relationship between wettability and O/C ratio, the deconvolution of C1s peak obtained from the untreated PET sample was carried out. Three Gaussian-Lorentzian curves were observed. These peaks match well with three chemical bonds, i.e. C–C(H), C–O, and O–C=O. They are located at binding energies of 285.0 (C1), 286.5 (C2), and 289.0 (C3) eV, respectively [59,60,61].

After the HCD plasma treatment, the component (C1) is observed to decrease. The intensities of components (C2) and (C3) is observed to increase, respectively. The increased concentration of (C3) components is usually attributed to the enhancement of O-C=O groups. The O-C=O group is known to have polar behavior. Thus, a higher concentration of these polar groups can result in higher surface energy with lower WCA. The O1s peak was deconvoluted into two peaks, as shown in Figure 9c and Figure 9d. The peaks at 531.9 (O1) eV and 533.5 (O2) eV are assigned to C=O and C-O bonds [59,62,63] respectively. After the HCD plasma treatment, component O1 increased significantly whereas component O2 decreased. This finding is consistent with the deconvolutions of the C1s peak when O-C=O groups contribute to the formation of C3. Accordingly, an increase of O1, which is related to C=O bond, could be expected. In sum, it is interesting enough to understand that WCA is correlated to the concentration of oxygen near the surface region as analyzed by XPS. As reported by many literatures, it shows that an increased concentration of oxygen near the surface region reflects a lower WCA, thus better wettability. Furthermore, the increase of C=O intensity stands for the restructuring of the soft segment–ether group [62,63]. This is closely related to the reactions of O radicals originating from air. The increase in C=O intensity is hence driven by the production of O radicals. In sum, the plasma treated surface is attributed to the restructuring of ether group, and the subsequent oxidation.

The XPS spectra for the untreated PU and the HCD plasma treated PU are shown in Figure 10. Table 8 summarizes the surface concentrations of C, O and N obtained from the spectra of untreated and HCD plasma treated PU inner surfaces. The PU has aromatic and aliphatic double bond hydrocarbons (–C=C, =C–H), carbonyl groups with different vibrations based on their conjugation (–N–C=O and O–C=O, respectively), ether group (C–O–C) and N-containing groups in the fingerprint bonds (–NH) [28]. According to Table 8, it is seen that C1s concentrations decreased from 87.2% to 74.7%, O1s increased from 8.8% to 20.9%, and N1s did not change much after HCD plasma treatment. Figure 10a and Figure 10b show that C-O and C=O increased significantly after HCD plasma treatment. The intensities of C=O and C-O also increased according to the perspective peaks of O1s, especially the C=O peak.

In contrast to the condition in conventional plasma treatment [28], the positive ions from the HCD plasma would break the bonds in the polymer surface, which would result in the enhancement of C=O bonds. Similarly, this can be explained with the restructuring of ether group in soft segments due to the breakage of C bonds and the reaction with O radicals, concurrently with decreasing C-O intensity. This is the result of direct effect by intensified ion bombardment in HCD plasma. This XPS results match well with the WCA measurements. It was found that low WCA was obtained from PU samples treated with HCD plasma. This implies the formation of new polar functional groups which is riched with C=O bonds [64].

The comparison of XPS spectra for the untreated PTFE and the HCD plasma treated PTFE are shown in Figure 11. Table 9 summarizes the surface concentrations of C, O, F and N obtained from the spectra of untreated and HCD treated PTFE surfaces. According to the XPS results (Table 9), it is observed that the F1S concentrations decreased from 66.7% to 41.7%, C1s increased from 33.3% to 47.4%, and O1s increased from <0.1% to 8.2% after HCD treatment. In Figure 11(a) and 11(b), after peak deconvolution, it can be found that C-OH and C-C appear after HCD plasma treatment, respectively, and C-F, C-F2 and C-F3 all have a downward trend, and O1s peak appears from noise to an obvious peak, as shown in Figure 11(d).

For HCD plasma treatment of PTFE, the mechanisms governing the surface activation are still not clear [29]. Basically, it has been known that at least a moderate concentration of oxygen functional groups formed near the PTFE surface region is essential for better wettability. This is like other polymers. Up to date, it is certain that the activation of PTFE should be attributed to the attachment of foreign materials onto the surface. In HCD plasma treatment, this kind of attachment is achievable by inducing heavy ion bombardment on the surface since HCD plasma contains high density ions. The bombardment of ions may cause bond breakage and thus, the attachment of extra atoms with high surface energy. In addition, VUV radiation may be critical to the treatment of PTFE polymers with plasma. It was reported that the reaction ratio between VUV and reactive O species may be a decisive factor for the surface wettability [65]. On this regard, HCD plasma can provide high density ion bombardment and intensified VUV irradiation.

4. Conclusions

The purpose of this study is to investigate how HCD plasma can be generated and modify the inner walls of three polymer tubes, said PET, PU, and PTFE. A hollow-cathode model was proposed and proved to be able to explain the generation of plasma discharges inside the tubes. The tubes’ inner surfaces are hence activated. It was confirmed that HCD plasma can be generated inside the tube if the inner diameter of the tube is greater than twice of the plasma sheath. This also explains the disappearance of plasma inside the tube with smaller diameters. Overall, the results show that when plasma density increases, the thickness of the sheath decreases. Accordingly, tubes with smaller diameters can be treated. When HCD is generated inside the polymer tubes, the surface becomes hydrophilic, which is attributed to the increase of carbon-oxygen bonds, particularly C=O bonds. This is not normally observed in the activation of polymers using CCP plasma. Reasonable explanation is that HCE may enhance bond breaking by significant ion bombardment, together with radical oxidation.