Contribution of surface energy and roughness to the wettability of polyamide 6 and polypropylene film in the plasma-induced process

Abstract

Plasma-induced etching and chemical vapor coating processes are well-known technologies that modify the wetting properties of polymeric surfaces by engineering the surface roughness and surface energy. The contributing effect of plasma treatment for O₂ etching and hexamethyldisiloxane (HMDSO) vapor coating on the wetting properties was investigated for polyamide 6 (PA6) and polypropylene (PP) substrates. The surface energy components of PA6 and PP were analyzed by the Owens–Wendt model, and were associated with the wettability of water and diiodomethane. With the introduction of roughness by the O₂ etching process without HMDSO coating, the wettability of PA6 substrate was enhanced while that of PP was deteriorated when they were observed after 20 days aging. When the surface was etched for 7 min or longer with the subsequent coating with HMDSO, both PA6 and PP lost hydrophilic property, giving water contact angle of 180°. The wettability was examined for the varied treatment conditions and as a function of average nano-pillar length. This study helps better understand the interactions between the surface energy and roughness of polymeric materials, and their influence on surface wettability.

Keywords

surface energy contact angle plasma etching plasma-enhanced chemical vapor deposition wettability

Plasma technology is often utilized to effectively engineer polymeric materials in the uppermost layers of the surface at nano-scale depths, not significantly interfering with the bulk properties of substrate materials.^1–5 The plasma process is particularly useful in adjusting the wettability of polymer surfaces, from hydrophilic to hydrophobic,^1,4 and is attracting growing attention because its dry process is regarded as a more environmentally favored process as opposed to other conventional wet processes.⁶

Plasma-induced wettability modification can be achieved mainly by the manipulation of surface geometry and/or the functionalization of surface chemistry. Surface geometry can be changed by the plasma etching process, which introduces certain surface patterns or roughness.^5,7–9 Depending on the intrinsic chemistry of the material, the wettability can be either enhanced or deteriorated by the plasma etching.

Plasma etching induces chain scission at the polymer surface, breaking the chemical bonds and removing a weak boundary layer of polymer chains, producing nano-textured surfaces.^10–12 The plasma etching process by itself has often been used to fabricate superhydrophilic surfaces,^6,13,14 and the enhanced wettability is generally attributed to the combined effect of surface oxidation and increased surface roughness.¹³ However, the enhanced wettability obtained by O₂ plasma etching is not permanently maintained compared to that initially attained; a hydrophobic recovery is often encountered with time, through the reorientation of polar surface groups toward the bulk.^13,15 Additionally, a polymer with higher chain mobility and lower crystallinity was reported to experience faster hydrophobic recovery by the easier rearrangement or diffusion of hydrophilic oxidized groups into the polymer bulk.¹⁵ When the hydrophobic recovery was associated with the etched roughness, the recovery was significantly delayed for highly roughened surfaces, although the etched polymeric surfaces eventually recovered the hydrophobicity of the initial chemical composition.¹³ Moreover, certain etched surfaces, after the hydrophobic recovery, achieved a higher level of hydrophobicity than the initial flat and smooth surface, due to the combined contribution of nano-scale roughness and the intrinsic hydrophobic chemical composition of polymer.¹³

Not only the surface geometry but also the surface chemistry can be modified by the plasma process to achieve certain wetting characteristics; i.e. the functionalization of surface can occur via oxygen plasma etching toward hydrophilic property or via plasma-enhanced chemical vapor deposition (PECVD) toward hydrophobic property.^16–19 PECVD or plasma polymerization takes place at the material surface with a wide range of monomer precursors, adding new functionalities or manipulating the surface free energies.^{10,12,20–22} The PECVD technique is commonly used together with the plasma etching process in creating superhydrophobic surfaces.^{10,12,22–25}

Surface energy, composed of dispersive and polar components of solid–vapor interaction, can be estimated by the Owens–Wendt’s geometric mean method,²⁶ which measures the contact angles of liquids having different surface tensions. The surface energy and its dispersive and polar components are associated mainly with the chemistry of the surface. Efforts have been made to estimate the dispersive and polar components of surface energy for roughened surfaces with nano-scale features.^14,27 However, this approach is controversial because in this case the surface roughness additionally contributes to the wetting parameter, where the assumption of the Owens–Wendt model is not strictly met.²⁶ The roughened surface would rather follow the Wenzel or the Cassie–Baxter models,^25,28 where the roughness factors are considered.

From previous studies,^13,16,23 surface engineering of polymeric materials for the modification of wetting characteristics from superhydrophilic to superhydrophobic has been attempted by controlling the surface energy and/or surface roughness; however, the quantification of the respective contributions made by the surface energy and surface roughness has seldom been investigated.

This study explored plasma treatment conditions for various oxygen plasma etching durations and plasma-polymerized hexamethyldisiloxane (ppHMDSO) coatings. The contributing effects of the respective treatments on the wettability was investigated for polyamide 6 (PA6) and polypropylene (PP) substrates, whose intrinsic surface energies are different. The surface wetting properties were measured in the static contact angle of water and diiodomethane (DI). The wettability of the O₂ etched surface was examined in terms of its hydrophobic recovery with time.

Experimental section

Materials

Cast polypropylene (PP) of thickness 100 µm and polyamide 6 film (PA6) of thickness 25 µm were obtained from Filmax (Korea) and Hyosung (Korea), respectively. Deionized water and DI (ACS grade 99%, Alfa Aesar, USA) were used to examine the surface energy components and the wettability of PA6 and PP substrates. Dispersive and polar components of the surface energy for water and DI are presented in Table 1.²⁹
Table 1.
Dispersive ( $γ_{L}^{d}$ ) and polar ( $γ_{L}^{p}$ ) components of surface energy of water and DI.²⁹

Solution Surface tension (mJ/m²)

γ_L $γ_{L}^{d}$ $γ_{L}^{p}$

Water 72.8 21.8 51.0

Diiodomethane (DI) 50.8 50.4 0.4

Plasma treatment

The plasma treatment for a film surface was conducted using custom-built plasma equipment for the following processes: (1) reactive etching in the presence of oxygen to form nano-scale roughness at the surface; and (2) PECVD to achieve nano-layer deposition of ppHMDSO. The single process, either etching or deposition, was applied separately to evaluate their individual contributions to surface wettability. The etching process followed by plasma coating process was carried out to investigate the overall effect on the wettability.

For the etching process, PA6 or PP specimens (8 cm × 8 cm) were placed on a stainless steel cathode plate, and the reactor was evacuated to base pressure lower than 2.67 Pa. After the reactor reached a stable pressure 2.67 Pa, ion etching was started with a radiofrequency glow discharge of oxygen gas and at a bias voltage of − 400 V.²³ The duration of oxygen plasma etching was varied for 1, 3, 5, 7, 10, and 15 min to form nano-pillars of various heights at the surface. The length of nano-pillars was roughly estimated by the Image J program.

A thin layer of ppHMDSO chemical coating was made on a substrate by the PECVD process, to lower the surface energy. For the PECVD process, the reactor was evacuated to base pressure of 0.13 Pa, and HMDSO precursor gas was supplied at a pressure of 1.3 Pa with bias voltage of − 400 V.²³ The duration of coating was varied from 10 to 90 s with 10 s intervals to find the optimum coating time. The coating was formed at a rate of 10 nm thickness per 10 s.^23,30 The plasma-treated specimens were placed in a Petri dish and stored/aged in a room environment of 23 ± 3℃ and 73 ± 5% relative humidity.

Material characterization

Scanning electron microscopy

The morphological change of PA6 and PP film surfaces with the plasma treatment was observed by field-emission scanning electron microscopy (FE-SEM, SUPRA 55VP, Carl Zeiss, Germany). The specimen was sputter-coated with Pt of 15 nm thickness from 300 s of coating prior to observation. The operation voltage was adjusted in the range 5–10 kV.

Differential scanning calorimetry

Differential scanning calorimetry (DSC-Q1000, TA Instruments, UK) of the polymeric films was conducted to estimate the crystallinity of pristine PA6 and PP films. The temperature was elevated at a heating rate of 10℃/min from 21℃ up to a temperature about 30℃ above the melting temperature of the polymer.

X-ray photoelectron spectroscopy

Chemical compositions of plasma-treated film surfaces were measured using X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos, Manchester, UK) using an instrument equipped with a monochromatic Al K_α X-ray source. Spectra were recorded at normal emission using an analyzer pass energy of 20 eV and X-ray power of 10 mA and 15 kV. Peak analysis was performed using Casa XPS software (Kratos, Manchester, UK).

Wetting properties

Contact angle measurement

The static contact angle of water droplets was measured at room temperature using an optical tensiometer (Attension® Theta Lite, BiolinScientific, Sweden). A film specimen was fixed on a glass slide using an adhesive tape for the measurement. For the static contact angle measurement, 4 µL drops of deionized water were placed at five different locations of the surface. The contact angle was recorded within one second upon dropping, and an average of five different measurements made from five different spots on a film specimen was used for the analysis. The static contact angle of DI was measured by the same method.

Estimation of film surface energy

The surface energy of PA6 and PP film surfaces was estimated by the Owens–Wendt method,^26,31,32 measuring the contact angles of sessile liquid drops of deionized water and DI, whose surface tensions are known from the literature (Table 1). With known surface tension components of $γ_{L}^{d}$ , $γ_{L}^{p}$ , γ_L of water and DI (Table 1), and the measured static contact angles of the respective liquid, θ for PA6 and PP surfaces, the surface free energy components of dispersive and polar components were estimated using
$\frac{γ_{L} (1 + \cos θ)}{2} = (γ_{S}^{d} γ_{L}^{d}) 1 / 2 + (γ_{S}^{p} γ_{L}^{p}) 1 / 2$
(1)
where θ is the static contact angle of liquid on the solid surface, $γ_{L}$ the surface tension of liquid, $γ_{L}^{d}$ the dispersive component of surface tension of liquid, $γ_{L}^{p}$ the polar component of surface tension of liquid $γ_{S}^{d}$ the dispersive component of surface energy of solid, and $γ_{S}^{p}$ the polar component of surface energy of solid. The surface energy of the solid surface, γ_S, was then calculated by
$γ_{S} = γ_{S}^{d} + γ_{S}^{p}$
(2)

The film surfaces coated with HMDSO were analyzed for the changed surface energy with its dispersive and polar components. The surfaces treated with etching, and by etching and coating, were not analyzed for its surface energy components, because the introduced surface roughness would have an influence on the contact angle, and the resulting estimates would no longer represent the accurate surface energy.

Contact angle changes with plasma treatment

The change of wetting property, represented by the contact angle change (CA_c_hange) of the treated specimen compared to that of pristine surface, was obtained by
${CA}_{change} (%) = \frac{{CA}_{treated} - {CA}_{pristine}}{{CA}_{pristine}} \times 100$
(3)
where CA_treated is the contact angle of treated film and CA_pristine is the contact angle of the pristine surface.

Results and discussion

Surface free energy of PA6 and PP films

The surface energy components of pristine PA6 and PP surfaces were found to be distinct (Table 2), with the PP surface exhibiting little polar components as opposed to the PA6 surface. After 40 s of PECVD treatment with HMDSO, the surface energy components of PA6 and PP surfaces converged to those of HMDSO.
Table 2.
Surface energy components of pristine and HMDSO-coated PA6 and PP films

Film Contact angle (°)
Surface free energy (mJ/m²)

Water DI γ_S $γ_{S}^{d}$ $γ_{S}^{p}$

PA6, pristine 61° 28° 52.8 41.3 11.5

PP, pristine 84° 49° 36.6 33.3 3.3

PA6, HMDSO 40 s 96° 65° 26.8 25.0 1.8

PP, HMDSO 40 s 96° 65° 26.3 24.7 1.6

Morphological changes with plasma etching

In the O₂ plasma process, reactive oxygen species, primarily oxygen radicals (O·) and singlet oxygen (O), react with a polymer to create the etched surface with nano-pillars and hairs, producing water vapor, CO, and CO₂.^13,33 Figure 1 shows the formation of nano-pillars on PA6 and PP films as a result of plasma etching for various treatment durations. All images were obtained after 20–25 days aging. The PA6 surface formed the nano-scale features more quickly than the PP surface. Both PA6 and PP formed nano-pillars at 7 min or longer etching durations, with PA6 forming higher aspect ratio pillars than PP. Generally, it is understood that the rate of etching is slower for the high crystalline polymers.³⁴ The relatively lower etching rate of PP over PA6 may partly result from a slightly higher crystallinity of PP (32%) than PA6 (27%), as was analyzed by DSC, though the difference in crystallinity was marginal. The etching rate may also be influenced by the chemical nature of polymers, such that reactivity of PA6 that has polar groups is higher than that of PP.
Figure 1.
Surface morphology of PA6 (left) and PP (right) films treated by O₂ plasma with varied etching duration and HMDSO coating (×80,000 magnifications): (a) pristine PA6, (b) pristine PP, (c) no etching, HMDSO 40 s coated PA6, (d) no etching, HMDSO 40 s coated PP, (e) 1 min etched PA6, (f) 1 min etched PP, (g) 3 min etched PA6, (h) 3 min etched PP, (i) 5 min etched PA6, (j) 5 min etched PP, (k) 7 min etched PA6, (l) 7 min etched PP, (m) 10 min etched PA6, (n) 10 min etched PP, (o) 15 min etched PA6, (p) 15 min etched PP, (q) 7 min etched and HMDSO 40 s coated PA6, (r) 7 min etched and HMDSO 40 s coated PP.

Wetting properties of O₂ etched surfaces and hydrophobic recovery

It has been reported that a surface treated with oxygen plasma generally improves its wettability due to the surface oxidation and the increased roughness,^6,13,35,36 and that the resulting hydrophilic surface experiences, to somewhat extent, a gradual hydrophobic recovery with time (aging). In Figure 2, the effect of O₂ plasma etching on surface wettability was investigated by measuring the contact angle of water with various aging times (1 day to 30 days). PA6 increased wettability with O₂ etching, while PP did not. The chemistry of PA6, with amide bonds in its intrinsic structure, would have allowed more oxidation than PP upon O₂ plasma treatment, leading to the increased surface energy. In addition, with the introduction of surface roughness, PA6 can further enhance the wettability according to the Wenzel theory.²⁸
Figure 2.
Static water contact angle of O₂ plasma etched film surfaces with varied etching duration and aging time: (a) aging effect of O₂ etching on PA6 surface, (b) aging effect of O₂ etching on PP surface, (c) effect of etching duration on PA6 and PP after 20 and 30 days of aging.

From Figure 2(a), PA6 went through a dramatic hydrophobic recovery upon aging. One day after plasma etching, the static water contact angle (CA) decreased from its pristine CA of 61°, and the extent of decrease appeared larger in a longer etching duration, due to the length of exposure of substrate to oxygen radical (O·) and singlet oxygen (O).^13,33 The PA6 surface with 15 min etching and 1 day aging showed a complete wetting with water, then went through a significant hydrophobic recovery throughout 20–30 days aging. Upon 20 days aging or longer, all the PA6 in this study showed a stabilized hydrophobic recovery.

For the PP substrate, whose original water CA was 84°, the wettability was enhanced after etching for 1 or 3 min duration, when it was measured after 1 day aging. Etching duration longer than 3 min produced higher CA values than for its pristine surface. The PP substrate experienced the hydrophobic recovery more quickly, where water CA appeared constant after 10 days aging.

Earlier studies on the hydrophilic plasma modification and its hydrophobic recovery showed a sharp deterioration of hydrophilicity during the initial 12 hours of aging and a rather gradual hydrophobic recovery at a later period.^35,36 The phenomenon of hydrophobic recovery is explained by the lowered concentration of hydrophilic groups at the surface due to the reorientation of these groups towards the bulk phase,³⁶ and/or migration of low molecular weight hydrophobic species from the bulk to the surface.³⁵ Though the hydrophobic recovery at the very initial stage (during the first 24 h) was not measured in this study, it was observed that PP was stabilized rather quickly, while PA6 took a slower hydrophobic recovery over 20 days of aging.

Hydrophobic recovery is mainly affected by the intrinsic chemistry of a substrate. For a substrate whose surface energy is not as low such as PA6, hydrophobic recovery could take place more slowly because the polar groups introduced by O₂ plasma would be rather stable on the substrate; on the other hand, a substrate that has intrinsically very low surface energy may favor the return to its original low surface energy state, undergoing faster recovery.

Knowing that the etched specimens in this experiment were stabilized after about 20 days aging, further analysis for the etched specimens was conducted after 20–30 days of aging. Figure 2(c) shows the change of wettability with the plasma etching duration when CAs were measured after 20 and 30 days aging. With 1 min etching, the contact angle was drastically lowered when compared to the pristine surface. For further experiments, etching conditions of 1, 7, and 15 min were selected to examine the wettability of surfaces having different roughness levels.

Effect of PECVD coating on wettability

To determine the optimal coating duration of HMDSO, PECVD was carried out on flat, smooth PA6 and PP film surfaces for various coating times. The CAs on a coated sample did not change with the aging time; CA values shown in Figure 3 were the ones after 20–25 days aging upon PECVD treatment. From Figure 3, CA increased significantly at the initial 10 s coating then remained about the same. To have the secured coating with small variations, the coating duration of 40 ± 5 s was used for further experiment. As the coating condition was adjusted to produce about 10 nm thickness per 10 s,^23,30 the 40 s treatment created a coating layer of about 40 nm.
Figure 3.
Water contact angle on ppHMDSO coated, smooth substrate with the varied coating duration.

Chemical composition of the plasma treated surface

Relative intensities in atomic percentage of pristine and plasma-treated films were analyzed by XPS (Table 3) after 20 days aging. For PA6, the O/C ratio increased after the O₂ etching process, while PP did not show a significant increase in O/C ratio after the same process. The O/C ratio of 0.09 from the pristine PP surface seems to come from the impurities at the surface. Considering that all the specimens were measured after 20 days aging, the possible hydrophobic recovery would have occurred during that time, reaching a stabilized surface chemistry. Thus, little change of O/C ratio for PP by a single process of O₂ etching is probably due to hydrophobic recovery having occurred during 20 days aging, and this is well associated with the results in Figure 2, where CA of etched PP surfaces (for 7 and 15 min etched PP) did not show a significantly lowered hydrophobicity.
Table 3.
Relative atomic intensities from the XPS scanning of PA6 and PP after 20 days of aging

Film Ratio Pristine O₂ etch 1 min O₂ etch 7 min O₂ etch 15 min HMDSO 40 s O₂ etch 7 min + HMDSO

PA6 O/C 0.22 0.39 0.40 0.35 0.79 0.75

Si/C 0.00 0.02 0.02 0.01 0.40 0.40

PP O/C 0.09 0.16 0.11 0.10 1.00 0.94

Si/C 0.00 0.00 0.00 0.01 0.51 0.47

Compared to the pristine samples, surfaces treated with HMDSO coating showed an increased ratio of O/C and Si/C. Though O/C ratio significantly increased after 40 s coating of HMDSO, the methyl group of HMDSO will cover the outer surface of the polymer,³⁷ thus the hydrophobicity would considerably increase both for PA6 and PP (Figure 3) regardless of higher O/C ratio.

Contribution of O₂ etching and HMDSO coating to wettability

The wetting properties of PA6 and PP surfaces were investigated for the individual and combined plasma processes of O₂ etching and HMDSO coating, and CA as a function of nano-pillar length is given in Figure 4. Oxygen-etched specimens were subject to test only after 20–25 days aging where the hydrophobic recovery was stabilized. It should be noted that the length of nano-pillars is only a rough estimation from SEM images, and the lengths from the HMDSO-coated specimens were hardly able to measure because of the considerable charging in SEM analysis with higher magnifications. Thus, the pillar lengths were measured from the specimens with O₂ etching only, and the same pillar lengths were assumed for the specimens with O₂ etching + HMDSO coating.
Figure 4.
Static contact angle of water and DI on PA6 and PP surfaces as a function of the average nano-pillar lengths formed by the varied etching duration of 0, 1, 7, and 15 min: (a) water on PA6, (b) water on PP, (c) DI on PA6, (d) DI on PP.

In general, O₂ etching on PA6 lowered the surface hydrophobicity, decreasing the contact angle of water and DI. However, the same etching process hardly affected the wettability of PP. Moreover, the etching increased the hydrophobicity of PP against water when O₂ etching duration was 7 min or longer. This result is noteworthy because O₂ etching generally enhances the hydrophilicity or wettability by adding polar groups to the substrate surface, and the result was opposite for PP in this study. Little change of O/C ratio with O₂ etching process in XPS scanning (Table 3) provides evidence of little change in chemical composition of PP after the hydrophobic recovery. The increase in hydrophobicity with longer O₂ etching is probably due to the longer nano-pillar formation that reduced the contact area between PP substrate and liquid drop by the increased roughness, where the Cassie–Baxter theory applies.²⁵

The etched surfaces on PA6 and PP substrates seem to tend toward the Wenzel or Cassie–Baxter model that considered the roughness factor,^25,28 where either hydrophilic or hydrophobic characteristic is reinforced by the introduction of surface roughness, depending on the intrinsic surface energy of a flat surface. According to the Wenzel model,²⁸ the wettability of a smooth substrate whose water contact angle is less than 90° can be enhanced with the introduction of roughness, while that of smooth substrate whose contact angle is greater than 90° can be undermined by the introduction of roughness. The pristine PP contact angle with water was 84°. Though being smaller than 90°, CA at the smooth PP was close to 90°, and its wettability was diminished with the roughness formation. On the other hand, PA6 enhanced its wettability with the introduction of roughness, from its pristine 61° contact angle.

When HMDSO coating was added to this etched surface, the hydrophobicity was reinforced both for PA6 and PP substrates up to the point where the water did not wet the surface at all, resulting in 180° static contact angle. This result consolidates the Cassie–Baxter theory that the lowered surface energy combined with the small-scale roughness amplifies the hydrophobic property by keeping the air pockets between the rough features that hold the water drop above.²⁵ Often the lotus leaf is regarded as the biomimetic model of a superhydrophobic surface, and its hierarchical binary roughness structure in micro- and nano-level roughness is considered as one of the main factors creating the superhydrophobic surface.³³ From the result of this study, the nano-scale roughness itself was found to give the superhydrophobic nature when combined with low surface energy. The HMDSO coating process itself increased the hydrophobicity of the surfaces, but the water CA on the resulting surface did not go over 150°, which is often the criteria for superhydrophobicity.³⁸ When HMDSO coating was made on PA6 and PP substrates, wettability of both substrates became very similar.

From the results in Figure 4(c) and (d), the wettability of DI was greater on the PA6 surface than on PP when substrates were not coated with HMDSO, due to the higher surface energy of PA6 than PP. The wetting of DI on both substrates became similar when the surface was coated with HMDSO without etching. The introduction of roughness combined with HMDSO coating affected the wettability, and the wettability was dependent on the pillar length. For example, PA6 with an average pillar length of about 111 nm (at 7 min etching and HMDSO coating) produced a CA of 97°, and PP with an average pillar length of 90 nm (at 15 min etching and HMDSO coating) produced a CA of 90°.

The results show that the wettability is highly dependent of the surface energy of the coating material and the roughness factor, regardless of the polymer substrate kind. In other words, when HMDSO was coated on the substrate, the chemistry that determines the surface energy was the same for PA6 and PP. Thus the wettability of either polymer substrate would become similar if the roughness factor, or the geometric feature of nano-pillar, is similar.

Without HMDSO coating, the roughness formation may contribute to either hydrophilicity or hydrophobicity depending on the substrate’s chemistry and polar component of surface energy. Roughness introduced to PA6 made the surface more hydrophilic, while that introduced to PP made the surface more hydrophobic. PP produced a certain degree of repellency (∼120° CA) against water only by etching process. This substrate-dependent characteristic can be applied to realize a fabric material with asymmetric wetting properties; a nylon fabric with one side treated by etching only, and the other side treated both by etching and chemical coating, would produce surfaces with two extreme wetting properties.

The extent of added hydrophobicity by the combined etching and coating processes was different depending on the type of substrates and liquids. To analyze the contributions of the individual processes of etching and coating, the contact angle percentage change from the untreated pristine substrate was calculated using equation (3). The results are shown in Table 4.
Table 4.
Contact angle percentage change from the pristine film

Film Treatment Contact angle changes from pristine (%)

Contributing factor Water DI

PA6 Etch 1 min + HMDSO Etching −37 23

Coating 58 127

Etching + coating 92 119

Etch 7 min + HMDSO Etching −32 −100

Coating 58 127

Etching + coating 197 241

Etch 15 min + HMDSO Etching −50 −100

Coating 58 127

Etching + coating 197 296

PP Etch 1 min + HMDSO Etching −25 −4

Coating 14 33

Etching + coating 30 38

Etch 7 min + HMDSO Etching 33 4

Coating 14 33

Etching + coating 113 69

Etch 15 min + HMDSO Etching 28 10

Coating 14 33

Etching + coating 113 83

Note: Contribution ratio was calculated from contact angle values by equation (3).

Positive percentage change in contact angle means that the substrate was modified to more hydrophobic and repellent to the test liquid. Negative percentage change occurred when the O₂ etching modified the surface to make it more hydrophilic, and this phenomenon appeared for the PA6 substrate. The increased hydrophilicity of PA6 came from the dual effect of (1) oxidation of surface or introduction of oxygen-containing groups, which increased the surface energy of PA6, and (2) introduction of roughness to the substrate, whose contact angle at the smooth surface is smaller than 90°, which enhanced the wettability according to the Wenzel model.

For those substrates, HMDSO coating to the O₂ etched surfaces dramatically changed the surface property toward being hydrophobic, by masking the oxidized surfaces and modifying it to a lower surface energy. For the PP substrate, the etching did not significantly affect the surface toward being hydrophilic; on the contrary, etching for 7 min or longer appeared to add hydrophobicity a little.

Conclusions

The contribution of O₂ plasma etching and ppHMDSO vapor coating to the surface wettability of PA6 and PP was investigated. The surface energy components, estimated by the Owens–Wendt model,²⁶ showed that PA6 was greater in polar component of surface energy than PP. HMDSO coating on either substrate made the surface energy lower, giving more hydrophobic chemistry. The O₂ etching rate was faster for PA6 than PP probably due to the relatively greater polarity and slightly lower crystallinity of PA6 over PP. The hydrophobic recovery after O₂ etching occurred faster in PP substrate than in PA6.

The difference in surface energy components between PA6 and PP produced different wettability for water and DI. By the etching process only, the PA6 substrate developed better wettability, while PP substrate somewhat deteriorated the wettability. When a substrate was treated both for etching and PECVD, the wettability was dependent of the surface energy of the coating material and the geometric character of nano-scale features such as pillar lengths.

Solution	Surface tension (mJ/m²)
Water	72.8	21.8	51.0
Diiodomethane (DI)	50.8	50.4	0.4

Film	Contact angle (°)	Surface free energy (mJ/m²)
PA6, pristine	61°	28°	52.8	41.3	11.5
PP, pristine	84°	49°	36.6	33.3	3.3
PA6, HMDSO 40 s	96°	65°	26.8	25.0	1.8
PP, HMDSO 40 s	96°	65°	26.3	24.7	1.6

Film	Ratio	Pristine	O₂ etch 1 min	O₂ etch 7 min	O₂ etch 15 min	HMDSO 40 s	O₂ etch 7 min + HMDSO
PA6	O/C	0.22	0.39	0.40	0.35	0.79	0.75
Si/C	0.00	0.02	0.02	0.01	0.40	0.40
PP	O/C	0.09	0.16	0.11	0.10	1.00	0.94
Si/C	0.00	0.00	0.00	0.01	0.51	0.47

Film	Treatment	Contact angle changes from pristine (%)
PA6	Etch 1 min + HMDSO	Etching	−37	23
Coating	58	127
Etching + coating	92	119
Etch 7 min + HMDSO	Etching	−32	−100
Coating	58	127
Etching + coating	197	241
Etch 15 min + HMDSO	Etching	−50	−100
Coating	58	127
Etching + coating	197	296
PP	Etch 1 min + HMDSO	Etching	−25	−4
Coating	14	33
Etching + coating	30	38
Etch 7 min + HMDSO	Etching	33	4
Coating	14	33
Etching + coating	113	69
Etch 15 min + HMDSO	Etching	28	10
Coating	14	33
Etching + coating	113	83

Footnotes

Funding

This work was partially supported by the SRC/ERC program of MOST/KOSEF (grant number R11-2005-065), the National Research Foundation of Korea (grant number 2011-0014765), and the BK21 Plus project of the National Research Foundation of Korea Grant funded by the Korean Government.

Acknowledgment

The plasma process was performed at the Computational Science Center of Institute of Multidisciplinary Convergence of Matter, Korea Institute of Science and Technology.

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Solution	Surface tension (mJ/m²)
Solution	γ_L	$γ_{L}^{d}$	$γ_{L}^{p}$
Water	72.8	21.8	51.0
Diiodomethane (DI)	50.8	50.4	0.4

Film	Contact angle (°)		Surface free energy (mJ/m²)
Film	Water	DI	γ_S	$γ_{S}^{d}$	$γ_{S}^{p}$
PA6, pristine	61°	28°	52.8	41.3	11.5
PP, pristine	84°	49°	36.6	33.3	3.3
PA6, HMDSO 40 s	96°	65°	26.8	25.0	1.8
PP, HMDSO 40 s	96°	65°	26.3	24.7	1.6

Film	Treatment	Contact angle changes from pristine (%)
Film	Treatment	Contributing factor	Water	DI
PA6	Etch 1 min + HMDSO	Etching	−37	23
		Coating	58	127
		Etching + coating	92	119
	Etch 7 min + HMDSO	Etching	−32	−100
		Coating	58	127
		Etching + coating	197	241
	Etch 15 min + HMDSO	Etching	−50	−100
		Coating	58	127
		Etching + coating	197	296
PP	Etch 1 min + HMDSO	Etching	−25	−4
		Coating	14	33
		Etching + coating	30	38
	Etch 7 min + HMDSO	Etching	33	4
		Coating	14	33
		Etching + coating	113	69
	Etch 15 min + HMDSO	Etching	28	10
		Coating	14	33
		Etching + coating	113	83

Contribution of surface energy and roughness to the wettability of polyamide 6 and polypropylene film in the plasma-induced process

Abstract

Keywords

Experimental section

Materials

Plasma treatment

Material characterization

Scanning electron microscopy

Differential scanning calorimetry

X-ray photoelectron spectroscopy

Wetting properties

Contact angle measurement

Estimation of film surface energy

Contact angle changes with plasma treatment

Results and discussion

Surface free energy of PA6 and PP films

Morphological changes with plasma etching

Wetting properties of O2 etched surfaces and hydrophobic recovery

Effect of PECVD coating on wettability

Chemical composition of the plasma treated surface

Contribution of O2 etching and HMDSO coating to wettability

Conclusions

Footnotes

Funding

Acknowledgment

References

Wetting properties of O₂ etched surfaces and hydrophobic recovery

Contribution of O₂ etching and HMDSO coating to wettability