surface free energy changes of polyethylene after … · surface free energy changes of...

Surface free energy changes of polyethylene after plasma treatment

K. Terpiłowski1 and D. Rymuszka1 1 Maria CurieSkłodowska University Faculty of Chemistry, Department of Physical Chemistry, Interfacial Phenomena,

Sq. Maria Curie-Skłodowska 3, 20-031 Lublin, Poland, [email protected]

Owing to very good physicochemical properties and low production costs polymer materials are widely used as protective coatings, microelectronic elements and in biotechnology nowadays. Unfortunately, surface properties have to be often modified for special application. Over the years, surface modification methods have been developed and include inter alia modification with gamma and roentgen radiation or with ions/electron beam. Recently a very popular method of surface modification is plasma technique which allows wettability and surface free energy changes. This technique provides an alternative to traditional methods of the surface modification by wet chemical cleaning, etching, crosslinking and functionalization [1]. It is very quick and ecological [2].

The aim of our study was determination of surface properties changes of polyethylene (PE) after low pressure plasma treatment. The contact angles were measured using the sessile droplet method. Then the surface free energy and its components were calculated using the values of the measured contact angles and applying the acidbase approach proposed by van Oss, Good and Chaudhury [3,4] as well as the contact angle hysteresis approach [5,6]. Chemical changes of the polymer surface were also reflected in surface roughness and literature spectroscopic data.

It was found that after plasma treatment wettability and surface free energy were significantly changed, which resulted in the increase of hydrophilic nature of the polymer surface. On the other hand, big changes in the surface roughness were also observed.

Keywords: polyethylene; plasma; wettability

1. Introduction

Polyethylene (PE) (Fig.1), one of the most popular polymers has a very wide application in various industrial and everyday life fields. Besides good flexibility and readiness to process, PE is characterized by high durability. The first systems using the PE pipes were installed 40 years ago and are still used. New methods of PE production allowed to obtain PE elements even with one hundred year durability and includes 40% of the global plastic production. Unfortunately, PE exhibits poor adhesion properties and low surface free energy which negatively influence on printing, painting or sticking and are related with a low amount of polar groups on its surface. One of the ways to modify surface properties of polymer materials which changes only upper layers of the surface and increases polarity as a result of polar groups addition [7] or crosslinking on the surface is plasma treatment.

Fig. 1. Polyethylene structure scheme.

2. Methods of polyethylene (PE) plasma treatment

There are many methods of plasma excitation but here only the methods used in the case of polyethylene surface modification will be described.

2.1Dielectric barrier discharge (DBD)

The dielectric barrier discharge (DBD) plasma devices are built of two electrodes, the upper electrode is connected to a high frequency power source and the lower ele ctrode is connected to the ground. The frequency of high voltage can be adjusted and it can be 10 kHz [8], 50 kHz [9], 80 kHz [10] 90 kHz [11]. The electrodes are usually round [9, 11, 12],

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

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but also the high voltage can be applied between the spiralwound stainless steel electrode fitted inside a watercooled quartz tube which is ground. Polymer materials are placed between these electrodes. The pressure in the reactor chamber is reduced to about 15 kPa and gas (from which plasma is prepared) is introduced into the chamber with the constant flow. After applying high voltage, plasma is produced. For this type of plasma any gas can be used. Some development of this method is Low Pressure Plasma (LPP). In contrast to DBD in LPP the pressure of gas in the chamber is lower and starts from 0.01 kPa [12]. Because of that the lower voltage is used for excitation of plasma.

2.2 Radio frequency plasma (RF)

The radio frequency plasma (RF) with the characteristic frequency 13.56 MHz is produced in a cylindrical stainless steel chamber, one electrode is powered and the other is grounded [14, 15]. The pressure in the chamber is about 1 to 7 x 10-3 Pa [15,16, 17] and the power is about hundreds of Watts [16,17] maximum to 1200 W [14] .

2.3 Atmospheric Pressure Plasma Jet (APPJ)

In the case of atmospheric pressure plasma jet (APPJ) the gas where the plasma to be obtained flows through the plasma jet with the constant value. The electrode is placed inside the jet. The gas pressure can be for example 590 kPa [19], the power used for plasma excitation is usually high and it can be 5 kV [19] 12 kV [20], 7 to 14 kV [21, 22]. From the end of the jet there escapes a free plasma beam which can touch the surface to be activated. Plasma can be also obtained as low temperature plasma for instance when the plasmotron with the frequency of 2860 kHz with air pressure 4 kPa and the current intensity 140 mA will be used [23]. The temperature of cold plasma is from 2000 K to 30000 K [24, 25].

2.4 Microwave electron cyclotron resonance (ECR)

The schematic diagram of the electron cyclotron resonance (ECR) is listed in Guruvenket [26] manuscript. The plasma source with two electromagnetic coils is connected to a vacuum pomp. Microwave power generated in a continuous wave magnetron source (2.45 GHz) is fed from the top through a quartz window with appropriate waveguids. The electromagnets get the magnetic mirror inside the source chamber. The gas flow is constant. The downstream plasma extends to the processing chamber where the polymer surface will be modified. The working gas pressure amounts to 10-4 kPa.

3. Methods of wettability determination of polyethylene plates treated by plasma

Wettability of plasma treated plates are determined by the sessile droplet method. The droplet volume is 1 µl [10] 2 µl [9, 12, 21, 23, 27], 8 µl [28, 29]. Its follows from our previous research that 2 µl droplets can be too small because small volume droplet is very sensitive to local heterogeneity of surface and the standard deviation can be large. It is interesting that in the case of the paper where 2 µl droplets were used, the data are listed without the standard deviation. In our research 6 µl droplets [30] are used because this is average between the representativeness of the area occupied by the drop and the influence of gravity on the droplet shape.

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Fig.1. Water contact angles measured for the untreated polyethylene samples. HDhigh density, LDlow density. As can be seen in Fig. 1 the water contact angle can differ due to different roughness (described below), fillers, low or high density. The average water contact angle for the untreated surface is about 90°. It is interesting to notice how polyethylene surface changes after the plasma treatment. Based on the scientific literature it is possible to compare oxygen/air plasma and argon plasma treatment by means of different methods. Oxygen and air plasma were compared


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because of similar effects. Using argon as reactive gas gives rather different effects so the decision was made to compare it with argon plasma treatment.

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Fig. 2. Water contact angles on the polyethylene plates treated by a) oxygen and b) argon plasmas. As can be seen in Fig. 2 a) the highest contact angle decrease is obtained for the RF plasma by Li et al. [16]. Such high contact angle decrease can be disputable. However, in the case of RF plasma, it is possible to obtain high increase of wettability Drnovska [31] used plasma glow discharge in RF and with twice as high power and obtained 41.6° decrease of water contact angle. When the LPP was used, the decrease of the contact angle was about 70°. Because of that it can be concluded that RF and LPP oxygen plasmas are the best for activation of polyethylene surface. It should be also mentioned that APPJ plasma does not require very sophisticated equipment and is characterized by the highest industrial application potential. In most cases wettability is described only by the change of water contact angle value. It seems that surface properties should be also described by changes of apparent surface free energy. In the papers by Owens–Wendt (OW) Eq.1 [32] and van Oss, Good, Chaudhury [3, 4, 8, 32] (LWAB) Eq.2 are used for calculation of apparent surface energy.

1 2 / 2 / (1)

were: liquid surface tension, contact angle, apolar component of liquid surface tension, apolar component of apparent surface free energy, polar component of solid surface tension, polar component of apparent surface free energy.

1 2 / 2 / 2 / (2)

where: apolar component of liquid surface tension, apolar component of apparent surface free energy, /

electron acceptor parameter of solid/liquid, / electron donor parameter of solid/liquid. Because the literature data is not always complete, it is not possible to use the above mentioned approaches compare the data. The third approach proposed by Neumann [33] will be used because for calculation of apparent surface free energy only advancing contact angle of one liquid is needed, in this case water.

1 2 (3)

where: apparent surface free energy, constant 0.000125.


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Fig. 3. Water contact angles on the polyethylene plates treated by a) oxygen and b) argon plasmas. As can be seen in Fig. 3 plasma treatment leads to increase of apparent free energy value. In the case of oxygen plasma the highest increase of energy value is observed by RF and LPP plasma treatment. Based on the data obtained by Li [16] it is impossible to calculate the energy due to complete spreading. The highest increase of energy is about 40 mJ/m2 for the LPP plasma. In the case of argon RF plasma the best effects were obtained using APPJ. Increase of energy leads to the increase of adhesion properties of treated material. Some researchers also calculated components of apparent surface free energy for oxygen plasma treated plates and stated that plasma activation leads to the increase of polar component of apparent surface free energy. Using the OW approach Kamińska [23] obtained the increase of polar component from 6.4 mJ/m2 to 28.89 mJ/m2. In our studies it was 3.0 mJ/m2 and after 1minute plasma treatment it was 15.4 mJ/m2. Longer activation does not much larger increase of this parameter as after 5 minutes it was 17.4 mJ/m2. It can be concluded that it is no use activating polyethylene longer than 1 minute. A similar conclusion can be drawn from Drnovska et al. [31] data using the LWAB approach the polar parameter for the untreated surface was 3.0 mJ/m2 after 1 minute activation it increase to 31.1 mJ/m2 and after 30minute activation it did not change. It should be mentioned here that after plasma activation also the dispersive parameter changed after a short time of activation it decreased. For the Drnovska [31] data it was from 25.3 mJ/m2 to 17.7 mJ/m2 for one minute treatment, longer activation leads to the increase of this parameter. After 20 minutes it was 25.3 mJ/m2. Using our data for argon plasma treatment after one minute of plasma activation the polar component of energy increased to 14.9 mJ/m2 and it is almost the same as in case of oxygen plasma. The literature reports activation of polyethylene by other kinds of plasma and these are: N2, HeO2 and NH3 plasma. However the effects obtained using these gases are not better than in case of oxygen/air or argon. In the case of NH3 plasma [31] the value of apparent surface free energy calculated from the LWAB approach did not change much but considering energy components and parameters it can be clearly seen that the dispersive component of energy decreased to 15.5 mJ/m2 after 5 min plasma treatment, the minimum is after 20 min plasma treatment and it is 2.1 mJ/m2. Using ammonia plasma leads to increase of electron acceptor parameter of from 5.7 mJ/m2 to 13.5 mJ/m2. The maximum after 20 min of plasma treatment is 25.7 mJ/m2. Further activation leads to the decrease of value of dispersive component and electron acceptor parameters. The reason for this might be change of surface structure.

4. Topography

Low density polyethylene (LDPE) treated by oxygen capacitively coupled radio frequency plasma under the RF power of 200 W leads to strong etching of the surface and significant topography changes [16]. The exposure for 1 min preserves morphology with the nodules in a nanoscale. The nanofibrils were also found after 5 min oxygen plasma treatment time. The authors presume that the nanotexture of LDPE surface can depend on the semicrystalline microstructure [16]. Drnovská et al. [31] investigated polyethylene surface treated with oxygen and ammonia plasma from 5 to 30 min. According to the SEM images they found that in the case of ammonia used as the reactive gas PE surface becomes smoother with the increase of exposure time due to the crosslinking reactions between the ammonia and polymer surface. The surface roughness changes of oxygen plasma treated samples were also detected and it can be concluded that the surface is rougher than the virgin PE. On the other hand, direct Ar+ plasma treatment of PE for 120 s results in creation of lamellar structures reflecting arrangement of molecular chains on the PE surface which makes it smoother [28]. Comparison of AFM images of LDPE, highdensity polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE) activated for 240 s by Ar plasma [29] allowed to conclude that the largest changes in surface morphology were observed in the case of HDPE whose surface is rougher. The average surface roughness (Ra) changed


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from 7.4 nm for the untreated surface to 12.0 nm for that treated by the Ar plasma sample. Probably this is due to lamellar structures formation which might be a result of its large crystallinity [34]. Very small roughness changes are observed for LDPE. A different situation is observed in the case of UHMWPE surface for which Ra decreased after modification whis is probably due to immense material ablation. It was also found that short plasma treatment does not cause significant damage due to plasma etching [10,11]. Longer exposure of UHMWPE for HeO2 plasma (25 s) leads to increase of the micro depressions. According to the AFM measurements carried out by Lommatzsch et al. [19], it was found that air and nitrogen plasma treatment increases RRMS roughness (the root mean square index) determined from a 2.5 x 2.5 m scanning area in comparison to the unmodified (RRMS=9.1nm). Higher value of this parameter was obtained for the airtreated surface and it amounts to 14.3 nm. Perni et al. [22] found that in the case of topography of UHMWPE surface does not change after cold HeO2 plasma treatment and the number of asperities is close to that on the unmodified one. Table 1. Roughness parameters of untreated and treated by oxygen and argon plasma polyethylene (PE) surface, size of the image 1.3×0.94 mm.

Sample Ra[nm] RRMS[nm] Rt[m]

untreated 732.3±51.5 988.5±108.7 16.0±3.4

O2 plasma 1 min 806.3±28.8 1018.5±40.6 11.0±2.1

O2 plasma 5 min 832.4±16.8 1050.0±35.6 10.6±1.1

O2 plasma 25 min 606.0±36.7 768.8±46.1 12.3±1.2

Ar plasma 1 min 692.0±20.1 1019.1±52.3 17.6±1.7

Ar plasma 5 min 679.8±18.9 918.9±43.4 17.1±2.1

Ar plasma 25 min 639.2±7.4 888.9±7.2 16.5±5.6

The changes of polyethylene roughness after plasma treatment are significant. The analysis of the surface roughness parameters (Tab. 1) on the polymer surfaces presented in Figs. 4 and 5 shows that plasma activation by 1 and 5 minutes leads to the increase of Ra and RRMS parameters. When Ra is 732.3±51.5 nm and RRMS is 988.5±108.7 nm for untreated surface, it increases to 806.3±28.8 nm (Ra) and 1018.5±40.6 nm (RRMS) after 1 min activation and 832.4±16.8 (Ra) and 1050.0±35.6 (RRMS) after 5 min activation with oxygen. On the other hand, activation of PE surface with argon leads, contrary to oxygen, to decrease of all roughness parameters which is probably related to the surface etching. However, in the case of 25min modification, the greatest decrease can be clearly seen for the oxygentreated samples probably due to temperature increase during the modification process which results in surface melts. The observation about the surface roughness is also reflected in the contact angle and surface free energy changes (Fig. 1).

untreated

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Fig. 4. 3D surface roughness images of untreated and 1, 5, 25 minutes treated by oxygen and argon plasma polyethylene (PE) surface.


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Fig. 5. Surface roughness profile along the X axis of untreated and 1, 5, 25 minutes treated by a) oxygen and b) argon plasmas polyethylene (PE) surface.

5. Spectroscopic description of polyethylene surface activated by plasma

The untreated LDPE film contains 96.5% C–C/C–H and 3.5% C–O groups. The XPS analysis confirmed that during plasma exposure polar groups are introduced on the polyethylene surface. The C1s envelope of the LDPE samples can be decomposed into 4 distinct peaks: a peak at 285.0 eV corresponding to C–C and C–H bonds, a peak at 286.5 eV corresponding to C–O functional groups, a peak at 287.7 eV linked to C=O and O–C–O groups and a peak at 289.1 eV attributed to O–C=O groups [27]. After argon atmospheric pressure plasma jet (APPJ) treatment, the peaks characteristic of C–O, C=O and O–C–O and O–C=O groups significantly increase. On the other hand, the ratio of the oxygen over carbon (O/C) surface concentration of the UHMWPE was doubled due to the exposure only for 2.5 s to the helium–oxygen plasma. The analysis of C1s peak of the UHMWPE XPS spectra confirms the presence of oxidized carbon species [11]. Longer treatment (25 s) leads to further increase of O/C over four times which is related to the introduction of ketone and/or acetal carbons and carboxyl groups on the surface after modification. The exposure of the polymer to the argon plasma is sufficient to form free radicals which then interact to form crosslinks and unsaturated groups with the chain scission. The plasma also removes lowmolecular weight materials and can lead to conversion to a highmolecularweight by crosslinking [26]. The presence of peaks at 1700–1737 cm1 in the IR spectra of oxygen modified polyethylene surface [26, 32] correspond to (C=O) stretching vibration. The additional band at 1646 cm-1 corresponds to COO asymmetrical stretching and the one at 1368 cm-1 (broadening of the peak) may be due to the COO- symmetrical stretching [26]. According to Lommatzsch et al. [19] introduction of O and N atoms onto the surface is possible using air plasma and which is obvious in the case of N atoms for the nitrogen plasma treatment. It was also found that a large amount of oxygen found in the nitrogen plasma treated sample can result inter alia from the reactions of the substrate surface with the ambient atmosphere after the plasma treatment, as can be also seen in the case of ammoniatreated PE [32] or oxygen/or water vapour diffusion into the nitrogen plasma jet and subsequent incorporation into the surface. The N1s XPS spectrum after air plasma treatment shows oxidized nitrogen species NO2 and NO3 which can be explained by composition of the air. The carboxyl and amino functional groups formed at the PE surface can change surface properties which enables biomolecules immobilization through covalent bonding.

6. Conclusions

Plasma activation of polymer surfaces can lead to changes of their properties. In the case of polyethylene the largest changes in surface properties were obtained using the RF and LPP plasma. However, application of a specific method of atmospheric plasma is also valuable. Effect of plasma treatment is described by its wettability changes based on the measurement of contact angles in majority for water. In our opinion for full description of wettability, contact angles at least one polar (water) and one apolar (diiodomethane) liquids should be measured. During contact angle measurement also contact angle hysteresis should be determined. The hysteresis depends on the surface roughness and energetic heterogeneity of the surface [35]. Because of this fact, hysteresis can give additional information about surface. Moreover, apparent surface free energy should be also estimated. It is important to calculate polar and dispersive components of energy to show which component changes after plasma treatment. Then except for water contact angles of other liquid (formamide, ethylene glycol) will be measured. It is possible to use the LWAB approach and determine dispersive component of energy and electrondonor and electronacceptor parameters. In the literature the droplet volume for the contact angle measurements is diverse. From our experience it results that a droplet with a small volume


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13 µl can be very sensitive to local heterogeneity (roughness, chemistry) of the surface and usually shows a high standard deviation. Some papers do not show standard deviation of the contact angle. On large droplets, 89 µl, the influence of gravity is visible. The best volume for the contact angle measurements are 57 µl droplets. The area taken by that size droplet on the surface is representative. Topography of plasma treated surfaces is investigated using SEM, AFM and optical profilometry. Taking into account the area taken even by 1 µl, it is about square millimetres. Using the AFM method, it should be remembered that one image cannot be representative for the surface so there should be taken about three images from different places and compared. Good description of roughness includes the values of three parameters Ra average roughness, Rq (RMS) square mean of the surface roughness and Rt maximum roughness height. These parameters should be listed with standard deviation. A quite new technique of roughness description is optical profilometry which allows to obtain the same parameters as those using AFM. However, it is possible to examine roughness from about one square millimetre area which corresponds to that taken by a droplet during the contact angle measurement. Nowadays the SEM technique is very popular, but it is impossible to obtain roughness parameters using it. Because of that description of SEM images in the case of plasma treatment is usually poor. The SEM method is useful in the case of coverage plasma treated surfaces by any layers and shows the differences in structure. Introduction of the functional groups using plasma gas on the surface is described using infrared spectroscopy (IR) and Xray Photoelectron Spectroscopy XPS. The IR technique is older and it is more difficult to obtain good quality data for description of the functional group number and type of changes on surface after plasma treatment. XPS gives full description of changes of surface chemistry in the case of plasma treatment. However, using IR it is also possible to obtain information about surface. The IR technique is sometimes used to select samples to be investigated by XPS. In our opinion both techniques can be used for description of surface changes, but for plasma treatment XPS is preferable. It should be mentioned that the literature lacks consistency in the designation of plasma activation process. To compare the effect besides the technique that was used for plasma treatment power used for activation, frequency of plasma source, pressure of gas from which plasma was obtained should be used. Based on the literature and experimental data, longer activation of polyethylene surface by plasma is pointless because it is not effective in further change of apparent surface free energy and its components. Air activation is important from the technological point of view. Oxygen plasma gives comparable effects as air plasma, roughness of treated surface increases and polar groups including oxygen are introduced on the surface (in the case of air, some groups including nitrogen also appear). In the case of argon plasma, surface roughness decreases. It is caused by “semi scratching” of the surface and the maximum 50few hundred Å [36, 37] layer is removed from the polymer surface. Then because of contact with air on the surface and introduced polar groups including oxygen and carbon. Using the ammonia plasma electronacceptor parameter of surface free energy increases, which is uncommon. Using plasma is recommended for the polymer whose surface will be covered by any protective or special properties layer. In any case adhesion of this layer increases.

Aknowledgement; The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Programme (contract no. POIG. 02.01.0006024/09 Centre for Functional Nanomaterials).

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