1 Introduction

Polymers have an important part in everyday life, both now and in the future [1]. Because of their high chemical resistance, flexibility, and low density [2], polymers are popular in many fields, including biomedicine, industry, and agriculture. However, some polymers have drawbacks, such as poor adhesion, wettability [3], and low surface free energy, all of which are caused by low surface polar groups, making them unsuitable for some applications. As a result, we need to adjust the surface qualities using a variety of methods, including “wet chemical processing–plasma processing, ion enforcement, flame spraying, and radiation.” Plasma processing is the most environmentally friendly procedure. It does not require chemical materials, and it is a low-temperature, low-cost, non-toxic, and effective approach to surface modification since it only affects the surface properties of the polymer without affecting the bulk properties [4,5,6,7]. Plasma treatment of polymeric materials is a common method for introducing polar groups or increasing surface roughness to improve surface properties such as wettability, biocompatibility, adhesion, dyeing and printing [8, 9].

Microwave discharge (MD) is an electrical discharge caused by electromagnetic waves with a frequency ≤ 300 MHz. The most used frequency for industrial, medical, and scientific applications is 2.45 GHz and the wavelength of MD ranges from millimeters to several tens of centimeters [10, 11]. MDs are commonly used in the deposition of films and coatings on materials. The ideal merchant products for low-cost MD are a microwave oven and a regular laboratory Pyrex chamber [12, 13]. Non-thermal plasma is generated by MD which generates free radicals, energetic ions as well as numerous excited species that lead to physicochemical changes, reactions of a few nanometers on the film surface, it causes to remove the chains of hydrogen polymers and the generation of free roots. By introducing polar functional groups to the polymer surface, MD can raise the surface energy, resulting in an increase surface hydrophilic characteristic. The hydrophilic surface generated on a thin film attracts water molecules, allowing them to quickly spread across the polymer film and form a homogeneous, non-light scattering water film rather than individual water droplets [6, 14].

PVA is a semi-crystalline or linear synthetic polymer that is creamy or whitish, tasteless, odorless, non-toxic, biocompatible, thermostable, granular, or powdered. It has incredible optical qualities, a high dielectric strength, and a high charge storage capacity [15,16,17,18]. PVA is available in a variety of grades depending on viscosity and degree of hydrolysis. The degree of hydrolysis, crystal precipitation, molecular mass and moisture affect PVA’s physical properties (density, crystallinity, film formation, water solubility, molar volume, degree of polymerization, and so on) [19,20,21]. PVA is a flexible, robust, and rigid material with oxygen and scent barrier qualities. Moisture must be prevented to avoid any PVA impairment in gas permeability [22]. Table 1 lists some of the physical features of pure PVA.

Table 1 Physical properties of pure PVA
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Surface modification of polymers has gotten a lot of attention recently since it influences physical and chemical properties of materials such hydrophilicity, roughness, dispersion, wettability, and adhesive qualities [23, 24]. Several approaches were used for surface modification of various polymers, such as chemical treatment, ozone treatment, irradiation procedures, corona discharge, and plasma treatment.

Plasma treatment is the most appealing of these approaches since it alters only the surface without changing the bulk characteristics and adds new functional groups on the surface [6]. The functional groups attached to the carbon chain of the PVA matrix allow other components to be integrated or through the hydroxyl group. As a result, it is critical to adjust PVA’s properties for specific applications [25].

The present manuscript reports the use of low-cost microwave oven oxygen plasma-treated PVA film to increase its surface roughness, hydrophilicity, and adhesion as well as reducing the humidity. Improvements in these quantities were discovered to have potential applications in air-conditioner systems [26], food packaging, coatings, and building materials [27].

2 Experimental setup

2.1 Microwave plasma system

The plasma of the microwave system [25] was generated by a low-cost microwave oven, as shown in Fig. 1. The vacuum pump and oxygen gas tank were linked to a microwave oven from a household. A Pyrex bowl was inverted and put on a rubber gasket inside the microwave chamber. Following the placement of the sample within the chamber, the vacuum pump was activated to remove any remaining room air, humidity, or water layer on the sample surface. After that, needle valves controlled the amount of oxygen gas, which was detected by a pressure gauge. With the timer adjusted, the microwave oven was set to continuously generate 1000 watts of power. Plasma was produced and blasted onto the PVA film surface. This activity will generate a lot of heat. However, if the PVA film is exposed to heat for an extended period, it can be burned [25].

Fig. 1
figure 1

Schematic of microwave plasma system

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In this study, we employed a PVA film that was made using the same casting procedure as in the previous study [6, 25] In a Petri plate, a transparent mixture of 100-mL distilled water and 5 g polyvinyl alcohol was poured and left to dry for 3 days. After the solution has dried, a polyvinyl alcohol film is obtained, which is then cut into 2 × 2 cm2 pieces and exposed to microwave plasma to change some of its properties such as wettability and hydrophilicity. The duration of treatment time varies as well, with 5 s, 10 s, and 15 s being used in each cycle.

The water drop angle on the treated PVA surface was then assessed using the contact angle and drop shape analysis system DSA25 at measuring range of contact angle 1–180°. Contact angle measurement resolution 0.1°, Power supply 88–264 V, 100 W, 50–60 Hz, Interfaces RS232, IEEE1394b, KRÜSS USA. Morphological evaluations have been obtained by using scanning electron microscope (JSM-6510LV) after gold sputter coating and atomic force microscope measurement (Agilent 5100 AFM/SPM Microscope). X-ray photoelectron spectroscopy spectrum was performed at pressure 10–9 mbar with full spectrum pass energy 200 eV and narrow spectrum 50 eV) of all samples by using (Thermo Fisher Scientific, USA) with monochromatic X-ray AL K-alpha radiation (1350 eV) spot size 400 µm.

3 Results

3.1 Contact angle

The contact angle is a useful tool for determining a surface’s sensitivity to water or other liquid. By analyzing the ability of a drop of liquid to disperse over a flat surface, PVA typically has hydrophilic qualities, which are significant in a variety of applications [8]. This hydrophilic feature comes from the hydroxide group and has been reported to improve following oxygen plasma treatment [26]. Contact angle (CA) measurements were used to assess the extent of hydrophilicity for plasma-treated PVA substrates. A considerable increase in wettability was discovered because of plasma modification [26]. It was found that the wettability of a solid surface depended on the contact angle of the solution on it [26].

From image in Fig. 2a, we measure the contact angle as the angle between the polymer surface and the initial tangent of the curved water drop. The average contact angle for an untreated PVA film is 38.3°. Figure 2b, c shows the dependence of this angle on the duration of plasma exposure. It reveals that the contact angle decreases dramatically with an increase in exposure time to values of less than 15° ± 3, which indicates the presence of a profoundly hydrophilic surface [8].

Fig. 2
figure 2

Drops of water (10 mL) on the surface of three various samples. a Untreated PVA surface, b oxygen plasma-treated PVA film with flow rate 2 L/min, pressure 26 mbar and time 5 s c treated at 15 s

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As shown in Fig. 3a, the contact angles of plasma-treated PVA films is reduced by increasing the treatment time from 5, 10, 15, to 20 s at flow rates (2 L/min and 6 L/min) with a constant pressure of 26 mbar. Figure 4a shows the contact angles at two pressures (16 mbar and 26 mbar) with a fixed flow rate (6 L/min). The behavior shows decrease in the contact angle with an increase in the treatment time, from 38.3° (untreated film) to 25° at pressure of 16 mbar and to 21° at a pressure of 26 mbar, respectively.

Fig. 3
figure 3

a Contact angle, b work of adhesion of PVA film with constant flow rates 6 L/min at two pressures (16, 26 mbar) at various treatment times

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Fig. 4
figure 4

a Contact angle, b work of adhesion curves of PVA film with constant pressure 26 mbar at two flow rates (2 L/min, 6 L/min) at various treatment times

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The species generated on the surface can be directly associated with contact angle values. However, two primary processes [8] can occur during plasma treatment, the addition of new polar groups or an increase in hydroxyl groups, which cause reduced contact angles following oxygen plasma treatment, and the etching process, which causes the opposite effect (higher CA values). As a result, an increase in hydroxyl groups [28] on the polymer surface, as indicated in XPS, can explain the significant decrease in contact angle value seen on PVA films after oxygen plasma treatment. This suggests that plasma treatment of PVA thin films can improve their surface wettability.

The work of adhesion quantifies the energy needed to detach a drop from a surface when the drop is in thermodynamic equilibrium. Let us consider a drop on top of a flat surface. Let us denote the liquid, solid, gas–liquid, and solid–liquid surface tensions by \(\gamma ,\gamma_{s} { },\gamma_{l} ,{ }\gamma_{sl} ,\gamma_{sg}\) as shown in Fig. 5. The work of adhesion is calculated by knowing the adhesion force between the PVA film and droplet water which can be calculated from equation [27, 29].

$$w_{sl} = \gamma_{s} + \gamma_{sl} - \gamma_{sg}$$
(1)

When the contact angle \(\theta\) is the young equilibrium angle \(\theta_{\gamma }\), the net force on the element dl is zero. This balance of forces leads to the Young–Dupre ́equation

$$0 = \gamma_{s} + \gamma_{sl} - \gamma_{sv}$$
(2)

where \(\gamma_{s}\) is the solid surface free energy, while \(\gamma_{l}\) refers to liquid (water) surface free energy, and \(\gamma_{sl}\) denotes solid–liquid interfacial energy.

Fig. 5
figure 5

Surface tensions and contact angle at point P of the line

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Combining this equation with the Young–Dupre ́equation,

$$\gamma_{s} = \gamma_{sl} + \gamma_{l} \cos \theta_{\gamma }$$
(3)

gives equation [28],

$$w_{sl} = \gamma_{l} \left( {1 + \cos \theta_{\gamma } } \right)$$
(4)

where \(\theta_{\gamma }\) stand for the equilibrium (Young’s) contact angle between the plasma-treated PVA film and water, while \(\gamma_{l}\) symbolizes the surface tension of water on the solid surface. Figure 4b illustrates the increase in the adhesion work of the PVA film from 129.5 to 136.13 and 140.5 mN/m by increasing the processing time from 0 to 20 s with a constant pressure of 26 mbar at two different flow rates 2 L/min, 6 L/min. Figure 5b shows that the adhesion work of the PVA film is increased from 129.5 to 138.13 and 140.5 mN/m by increasing the processing time from 0 to 20 s with a constant flow rates 6 L/min at two different pressures 16 mbar and 26 mbar [28].

Under the conditions of this study, it is easy to achieve adhesion enhancement by microwave plasma treatment system by measuring the contact angles of the untreated and treated film surface and water droplet. Adhesion enhancement measurements reported that the surface energy of the polymers can be altered, and the surface oxygen content increased by exposure of oxygen (and vacuum) plasma. The surface energies can be routinely increased more than 6–10 mNm [30].

3.2 Surface morphology

The scanning electron microscope and atomic force microscopy were utilized to investigate the surface morphology and roughness of the PVA film. To begin, SEM images are obtained for two plasma treatments, as shown in Fig. 6. The majority of the articles make use of dielectric barrier discharge (DBD) and radiofrequency discharge (Rf) to demonstrate morphology on the surface of PVA film [5, 31]. Although a microwave oven can generate plasma, its usefulness for researching surface morphology is still constrained by this fact. The effect of oxygen plasma treatment was investigated for a duration of five and fifteen seconds at a constant oxygen flow rate of six liters per minute and a pressure of twenty-six millibars. As shown in Fig. 6a, untreated PVA has a smooth shape, which indicates that it has a high capacity for film formation and is homogenous. According to results of our research, increasing the length of the plasma treatment from 5 s, Fig. 6b to 15 s Fig. 6c resulted in a roughening of the treated PVA film. This leads one to believe that the morphology of the untreated sample is more refined than that of the plasma-treated samples.

Fig. 6
figure 6

Scanning electron microscope images of a untreated PVA, b oxygen plasma-treated PVA film at flow rate 6 L/min, 26 mbar and 5 s, c oxygen plasma-treated PVA film at flow rates 6 L/min, 26 mbar and 15 s

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Furthermore, AFM is used to investigate the difference of roughness between untreated and oxygen microwave plasma-treated samples. The AFM images are measured in two different areas, 5 × 5 \(\upmu {\mathrm{m}}^{2}\) and 10 × 10 \(\upmu {\mathrm{m}}^{2}\) for untreated and oxygen plasma-treated PVA films at a flow rate of 6 L/min, 26 mbar and 15 s, respectively, as shown in Fig. 7. At a measured area of 5 × 5 \(\upmu {\mathrm{m}}^{2}\), both mean roughness and the root mean square of the roughness are increased from 0.864 to 2.29 nm and 1.36 to 3.18 nm, respectively, as shown in Fig. 7a, b. The mean of roughness and the root mean of square of the roughness both increased from 0.955 to 2.61 nm and 1.42 to 3.41 nm at measured area 10 × 10 \(\upmu {\mathrm{m}}^{2},\) respectively, as shown in Fig. 7c, d, and Table 2.

Fig. 7
figure 7

Atomic force microscopy (AFM) images of untreated and oxygen plasma-treated PVA films. The plasma conditions were oxygen flow rate 6 L/min, 26 mbar and 15 s

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Table 2 AFM results of surface roughness parameters of untreated PVA and oxygen plasma-treated PVA films, the plasma conditions were oxygen flow rate 6 L\min, 1000 W, 26 mbar, 15 s
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Our findings indicate that there has been an increase in the surface of roughness due to the treatment of the PVA membrane with oxygen plasma, the split of the chains occurred, and their reorganization into highly hydrophilic parts compared to the untreated parts. For the etching process, the split of the chains leads to small fragments which are removed from the surface; all this proves the surface became more rough, uneven, and hydrophilic [31, 32]. As shown in Fig. 7b, d, the plasma-treated PVA film has an irregular surface morphology. These results in a broad contact area for the water droplets, which in turn improves the adhesiveness of the film and leads to a smaller contact angle. It is common knowledge that increasing in surface energy, in addition to the polar components, can be attributed to either a greater number of polar groups or increased surface roughness. This is the case regardless of whether the surface is smooth or rough. It is discovered that the surface roughness of the oxygen plasma-post-treated surface is significantly higher than untreated surface, which may have been the result of etching or oxygenation occurring during the oxygen plasma process. It is expected that making the surface rougher will help to make the material easier to wet and give it much more surface energy [14, 15].

3.3 Surface chemical analysis

The chemical composition of the untreated and plasma-treated surfaces of PVA thin films was investigated using X-ray photoelectron spectroscopy. XPS is widely considered as one of the most effective methods for analyzing chemical changes on polymer surfaces [33]. In addition, XPS can offer information about a sample’s architecture, morphology, and homogeneity as a function of depth. New interfaces (e.g., in semiconductors, electrodes), solid synthesis pathways, catalytic and surface processes, and electronically active materials, among other things, may benefit from such procedures. The fundamental electronic structures of the C and O elements are analyzed to obtain precise information on the chemical environment of the various atoms on the sample surface [33, 34]. The results of curve fitting into the C1s spectra of the untreated and oxygen plasma-treated PVA thin films, with oxygen flow rate of 6 L/min, 1000 W and 26 mbar, 15 s, respectively, to gain information on the existence of different functional groups, are shown in Fig. 8a, b.

Fig. 8
figure 8

XPS for untreated PVA film and treated PVA film at plasma oxygen conditions (26 mbar, 6 L/min, 15 s), a high-resolution carbon C1s spectrum of untreated PVA, b high-resolution carbon C1s spectrum of PVA-treated oxygen plasma, c high-resolution spectra of N1s for untreated PVA film, d N1s for treated PVA film, e high-resolution oxygen O1s spectrum of untreated PVA, f O1s for treated PVA film, g survey of untreated PVA film, h survey of treated PVA film

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The atomic compositions (%) of the components of the C1s spectra are reported in Table 3 based on curve fits of the C1s spectra for untreated PVA film. The C1s spectrum of PVA film is divided into several subcomponents, including the peak at 284.7 eV, with atomic compositions of 20.81%, which corresponds to C–C, C–H, and the peak at 285.9 eV, with atomic compositions of 30.15%, which corresponds to C–O–, the peak at 283.5 eV, with atomic compositions of 5.9%, which corresponds to C=O, and the peak at 288 eV. The binding energies for the C1s spectrum after oxygen plasma treatment with conditions (6 L/min, 1000 W and 26 mbar, 15 s) are as follows: the atomic compositions of C–H and C–C increased by 55.19%, the C–O– with atomic compositions of 18.88%, and C=O increased atomic compositions to 7.88%. The atomic compositions of 18.50% correspond to O=C–O [35].

Table 3 High-resolution C1s of PVA film
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Deconvolution analysis revealed an increase in oxygen-based functional groups such C=O and –O=C–O in plasma-treated films. These findings suggest that plasma exposure caused PVA polymer chains to be cleaved into shorter lengths and oxidized further. These functional groups have seen a significant increase in the number of hydrophilic groups and the wettability of the material. Because the chemical shift of the C1s photoelectron is caused by the high electronegativity of oxygen atoms, and the shift in binding energy emerged in the C1s spectrum for untreated PVA, i.e., there is a chemical shift [36].

Figure 8c, d shows the N1s high-resolution spectra of PVA-untreated film and treated oxygen plasma; our finding shows that the nitrogen composition is 100% for untreated, while an additional peak is appeared at binding energy 407 eV with nitrogen composition of 59.5% as shown in Table 4.

Table 4 High-resolution N1s of PVA film
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As shown in Fig. 8, one peak at 533.37 eV was detected at the O1s region of PVA, which belongs to the carbonyl group (C=O). After treatment plasma, O 1s showed an almost unchanged binding energy or peak [33], which can be confirmed by comparing Fig. 8e with Fig. 8f and Table 5.

Table 5 High-resolution O1s of PVA film
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These results imply that the PVA molecular chains were cut into shorter lengths and oxidized by the plasma treatments, resulting in a decrease in C-C bonds and increase in C=O [37].

Figure 8g, h illustrates the survey spectrum of untreated PVA film and oxygen plasma treatment film; our results show that the oxygen composition increased from 24.92 to 26.29%, while carbon composition increased from 70.76 to 72.24%, and the composition of nitrogen slightly decreased from 1.59 to 1.47% for untreated and treated sample and this expected [31]. Table 6 describes that our finding show that we have not significant difference in their chemical shifts which the ratio N/C is 0.02% but the O/C is increased from 0.35 to 0.369% as expected.

Table 6 Surface chemical composition obtained from XPS survey spectra
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The high concentration of O=C–O and C=O groups at the surface led to a reduction in the water contact angle of PVA film, see Fig. 8 and Table (3, 5). Also, the C=O bonds are strongly hydrophilic, so an increase in its intensity leads to obtaining an improvement of the hydrophilic surface. Therefore, increasing wettability after plasma treatment agrees with XPS results [38].

4 Conclusions

From the research that has been carried out, it is possible to conclude that oxygen plasma treatment using a low-cost microwave oven system has successfully improved the hydrophilicity and increased the adhesion of the PVA film surface. Its surface becomes uneven. The contact angle and the work of adhesion were studied by varying the treatment time. It has been found that CA is decreased from 39° to 20.8° by increasing the treatment time from 5 to 20 s. The property of hydrophilicity has attributed to the convergence of water molecules with the oxygenated functionalities presented on the surface of the PVA as the processing time increases. This indicates an improvement in surface wettability. On the contrary, the adhesion has increased from 129.5 to 137.3 mN/m. The surface morphology of PVA films has been studied. SEM images were obtained for two plasma treatments, 5 s and 15 s. The treated surface of PVA film becomes rougher. Roughness has been measured using AFM at two areas, 5 × 5 \(\upmu {\mathrm{m}}^{2}\) and 10 × 10 \(\upmu {\mathrm{m}}^{2}\) for untreated and oxygen plasma-treated PVA films at 15 s, respectively. At a measured area of 5 × 5 \(\upmu {\mathrm{m}}^{2}\), both Ra and Rq are increased from 0.864 to 2.29 nm and 1.36 to 3.18 nm, whereas Ra and Rq both increased from 0.955 to 2.61 nm and 1.42 to 3.41 nm at measured area 10 × 10 \(\upmu {\mathrm{m}}^{2},\) respectively, which may have been the result of etching or oxygenation occurring during the oxygen plasma process. It is expected that making the surface rougher will help to make the material easier to wet; this leads to decrease in the humidity and gives much more surface energy. Finally, XPS indicates an increase in surface roughness as well as an increase in hydrophilic polar functional groups ((C=O from 8.26 to 21.85%) and (O=C–O from 6.2 to 18.05%)). According to the findings, PVA-treated film can be used in food packaging, air-conditioner panels, coatings, and building materials.