CHANGES IN WETTABILITY OF POLYCARBONATE AND POLYPROPYLENE PRETREATED WITH OXYGEN AND ARGON PLASMA

CHANGES IN WETTABILITY OF POLYCARBONATE AND POLYPROPYLENE PRETREATED WITH OXYGEN AND ARGON PLASMA Konrad Terpilowski, Diana Rymuszka, Lucyna Holysz, Emil Chibowski Department of Physical Chemistry Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, Lublin, Poland The effect of oxygen and argon plasma treatment on wetting and energetic properties of polycarbonate (PC) and polypropylene (PP) was carried by means of apparent advancing and receding contact angles measurements for two probe liquids (water and diiodomethane). The total apparent surface free energy and its components of the treated polymer plates were evaluated from measured contact angles and applying contact angle hysteresis (CAH) model proposed by Chibowski, and Owens and Wendt (O-W) approaches. The topography and surface roughness were determined using an optical profilometer. Also IR-ATR spectra were taken. It was found that the oxygen plasma treatment of PC and PP plates caused significant chemical and morphological changes of the surfaces, which are reflected in smaller contact angles of the probe liquids due to appearance of polar interactions. After the plasma treatment the apolar component of the surface free energy remained almost the same while the polar component increased significantly. The plasma activation leads also to an increase in the surface roughness of PC and PP and sharp patterns appear on the modified surfaces. 1. INTRODUCTION The polymeric materials are used in intensively increasing amounts. Due to the dwindling resources of oil and other fossil fuels which are the raw material for the production of polymers, to prolong use it is necessary to protect their surface and more rationally use of natural resources. Even in newly manufactured stuff made of polymers the recycled polymers are added to a certain amount, which may worsen their properties. Many polymeric materials inherently have a low surface free energy that results in poor adhesion or even complete adhesion failure. One of the most interesting methods used to increase their surface energy and improve adhesive properties is plasma activation of their surfaces [1 4]. Surface activation with plasma is especially effective when nonpolar materials have to be treated, such as plastics that consist of long-chain polymers. Such nonpolar surfaces are show low adhesion strengths to any coating substance. Because plasma treatment selectively modifies the surface and their surface free energy, the polymeric materials can be processed easily and new material combinations can be produced, e. g. an adhesive bonding process. The plasma process results in a physical and/or chemical modification of the first few molecular layers of the surface, while maintaining the properties of the bulk phase [1]. Nowadays, the most popular plasma used for this purpose is that obtained from Ar, He, O 2, N 2, CO 2 and air [3, 5 16]. Zhang et al. [8] studied the influence of Ar/O 2 plasma activation and chromic acid etching of polycarbonate (PC) surface on the adhesion coating to the surface. They found that both processes lead to increase of surface energy and wettability but the final effect of the plasma activation was more efficient than that of chromic acid 4-155

etching. The XPS data showed the decrease in C 1s spectrum which was probably caused by the decrease in the number of C C and C H groups and the increase in O 1s spectrum which is caused by the polar groups, such as carboxylic acids ( COOH) and hydroxyl groups on the PC surface after plasma treatment [8]. Kravets et al. examined polypropylene track membrane treated with various types of plasma: air, nitrogen or oxygen. They assumed that the action of plasma formed by non-polymeryzing gases resulted in etching of the membrane surface and it changed the structure and chemical composition of the surface. In case of air or oxygen plasma used the changes in the surface relief were bigger and the treatment with these gases caused formation of oxygen containg functional groups. The increase of the surface roughness and its hydrophilization improve the surface wettability [15]. Akbar et al. characterized polypropylene samples treated under dense argon plasma. The XRD investidation showed an increase in the crystallity of the sample after the plasma treatment [16]. Seo [17] also used argon and oxygen plasma for modification of polypropylene surface. No significant changes in the IR spectra after argon plasma treatment were observed, while the increased bands of hydroxyl and carbonyl were seen in the case of oxygen plasma treatement. In our previous study [18] we investigated the apparent surface free energy changes of polypropylene and polycarbonate which were due to cause a contacting with different solid surface the melted polymer which solidified. The aim of this study is to determine the wetting and energetic properties of the same polymers after oxygen and argon plasma treatment. In recent years various laboratory techniques have been developed, so we have decided to combine many different techniques to characterize plasma activation process especially, in the case of commercially used polymers. 2. EXPERIMENTAL 2.1. Materials Commercial samples of polycarbonate (Organika S.A., Sarzyna, Poland) and polypropylene (PKN, Płock, Poland) 20x25 mm size were used for the experiments. The probe liquids for contact angle measurement were water from Milli-Q System and diiodomethane (99%, Aldrich). 2.2. Oxygen and argon plasma treatment Plasma activation was performed in low pressure plasma system Pico from Diener Electronic, Germany. The polymer plates were placed on the sample stage and the system was evaluated to a pressure 0.2 mbar and the gas flow (oxygen or argon) was set at 22 sccm (standard cubic centimeters per minute). The plates were treated by plasma power 160V (400V max) for 1 or 5 minutes. In order to remove the gaseous products the chamber was purged with air for 10 s. To open the chamber it was necessary to get inside atmospheric pressure. Immediately after the plasma treatment, the plates surface were investigated by measurements of water and diiodomethane contact angle and IR-ATR spectroscopy (Nicolet 8700 Tchermo, USA). The topography of the surface was determined using an optical profilometer (Contour GT, Veeco). 2.3. Contact angle measurements 4-156

Advancing and receding contact angle, deg Advancing and receding contact angle, deg For the contact angle measurements by the sessile drop method the Digidrop GBX Contact Angle Meter (France) equipped with the video-camera system and computer software was used. The advancing contact angles of water and diiodomethane were measured after very gentle settling 6 μl droplets on the surface. Then, after sucking 2 μl from the droplet into the syringe, the receding contact angles were measured. The contact angles of probe liquids were measured at 20±1 o C in a closed chamber. The contact angles of 15 droplets were measured on both sides of 2D contour of the droplet. 2.4. Determination of apparent surface free energy Having measured the apparent advancing a and receding r contact angles of probe liquids and their surface tensions L, the apparent total surface free energy S has been calculated from the contact angle hysteresis using following equation [19, 20]: 2 L(1 cos a ) S (1) 2 cos r cos a The values of total surface free energy obtained from equation (1) were compared with those calculated from the Owens and Wendt approach [21]. Using the O-W approach first from the advancing contact angles the apolar and polar p S components were calculated, and then the total surface free energy. d d 1/ 2 p p 1/ 2 L ( 1 cos ) 2( S L) 2( S L) (2) This approach is often use for apparent surface free energy determination of polymers [9 12]. 3. RESULTS AND DISCUSSION 3.1. Contact angles As mentioned above, to determine changes in wetting properties of both polymers (PC and PP) after plasma treatment the advancing and receding contact angles of water and diiodomethane droplets were measured. d S 100 80 A) PC W- a W- r DM- a DM- r 100 B) PP 80 W- a W- r DM- a DM- r 60 60 40 40 20 20 0 Untreated 0 Bare Fig. 1. Apparent advancing ( a ) and receding ( r ) contact angles of water (W) and diiodomethane (DM) on PC (A) and PP (B) plates depending on the time of oxygen or argon plasma treatment. 4-157

The apparent advancing and receding contact angles on bare polycarbonate and polypropylene surface (untreated), and after 1 and 5 min oxygen or argon plasma treatment are plotted in Fig. 1. As can be seen, reproducibility of the contact angle values is satisfactory (vertical bars show standard deviations). The surface modification with plasma, irrespective of its character, decreases contact angles of probe liquids. This effect is more visible if oxygen was applied as the reacting gas. The advancing water contact angle on PC drastically decreases from 81.8±3.1 (orginal surface) to 15.7±1.9 after the surface treated with O 2 plasma for 1 min (Fig. 1A). However, the extension of the surface activation time up to 5 min resulted in a decrease contact angle value to 20.1±3.3. In the case of applying the noble gas, the water advancing contact angle after one-minute activation was 35.4±2.6 and five-minute activation caused further decrease in the water contact angles to 14.3±2.2. As can be seen in Fig. 1A for the apolar liquid, i.e. diiodomethane, the changes in wettability on the plasma activated surfaces are much smaller. In the case of PP (Fig. 1B) the contact angle of water changes from 99.1±2.7 for untreated surface to 48.2±2.9 if the surface was activated with the oxygen plasma for 1 min. When argon was used, the advancing water contact angle was 53.0±2.1. Longer activation time of polypropylene with oxygen plasma caused water contact angle to be 28.1±1.3. However, longer activation time with argon does not cased significant changes in the contact angles. The contact angle changes of diiodomethane are much smaller, and on the untreated surface it is 47.0±1.6, while the smallest contact angle on the surface activated with argon plasma for 5 min is 31.4±1.1. The contact angle value of water on untreated PC surface is the same as obtained by Bismarck et al. [22] and is slightly higher than measured by Vijayalakshmi et al. [12]. Probably it is because of commercial materials used in our studies, which are characterized by higher surface roughness. 3.2. Topography and surface roughness To get better insight into the surfaces structure in Figs. 2 and 3 are presented the images obtained using the profilometer (the size of surfaces is 1.3 0.94 mm 2 ) together with the roughness parameters, average roughness R a, root mean square R RMS and total height of the roughness profile R t. Polycarbonate PC B) plasma C) plasma A) untreated R a = 12.7 nm R RMS=20.1 nm R t=0.72 µm R a = 20.9 nm R RMS=34.6 nm R t=0.67 µm D) plasma R a = 17.1 nm R RMS=23.5 nm R t=0.64 µm E) plasma R a = 20.7nm R a = 20.5 nm 4-158

R RMS=36.7 nm R t=0.78 µm R RMS=36.8 nm R t=1.7 µm Fig. 2. 3D images (1.3 0.94 mm 2 ) from optical profilometer of untreated PC surface and treated with O 2 or Ar plasma for 1 and 5 min. The analysis of the surface roughness parameters on the polymer surfaces presented in Fig. 2 show that in the case of PC surface activation with plasma results in increased of surface roughness. When activation with plasma lasted one minute, the average roughness increased to about 20 nm irrespective of the used gas (Figs. 2B) and D). However, longer activation with these gases results in some differences. The average roughness in the case of surface activated with oxygen for 5 min decreased by about 3 nm. The effect is more visible, when there are differences in R RMS are considered the difference is over 10 nm. It may be due to local temperature increase during the surface activation. The surface becomes hot and melts slightly. This effect is predominant despite the surface continuous bombardment by the plasma excited oxygen. The results show that the roughness of the surface increases which reflects in the contact angle changes (Fig. 1A). The surface becomes more hydrophilic and the liquid droplets penetrate more easily the surface roughness. This effect is enhanced because of increased polar interaction of surface. However, if PC surface is treated with oxygen plasma for 5 min, its roughness decreases (Fig. 2C and 2D) in comparison to sample activated with plasma for 1 min. For these samples average roughness of the surfaces is R a =20.9 nm and 17.1 nm, respectively, and they are more rough than the untreated one where R a =12.7 nm. The plasma process results in physical and/or chemical modification of the first few molecular layers of the surface, while maintaining the properties of the bulk. Decrease in the surface roughness is caused by melting of the surface during modification with plasma. It should be remembered that the temperature of PC softening is 145 C [23]. The analysis of IR-ATR spectra obtained for the PC samples does not reveal distinct changes between them. The energetic changes on the PC surface are caused by small number of OH groups appearing on the PC surface. Similar as polycarbonate, the polypropylene surface becomes hydrophilic due to activation with plasma. However, there is no significant difference depending on the type of plasma (Fig. 3). Polypropylene B) plasma C) plasma A) untreated R a = 100 nm R RMS =164 nm R t =4.7 µm R a = 124 nm R RMS =201 nm R t =5.5 µm D) plasma R a = 233 nm R RMS =332 nm R t =7.6 µm E) plasma R a = 129 nm R RMS =207 nm 4-159 R a = 120 nm R RMS =182 nm

Surface free energy, mj/m 2 Surface free energy, mj/m 2 R t =6.9 µm R t =4.6 µm Fig. 3. 3D images (1.3 0.94 mm 2 ) from optical profilometer of untreated PP surface and treated with O 2 or Ar plasma for 1 and 5 min. The polypropylene surface activated with oxygen plasma, for the longer time has its surface roughness larger as is seen in Figs. 3A and B. While R a is 100 nm for the untreated surface, it increases to 124 nm after 1 min activation, and after 5 min activation with oxygen plasma the roughness increases up to 233 nm. However, in the case of surface activation with argon, five-minute activation does not cause any changes in the roughness. According our previous results [18] it is worth to mentioned that the average surface roughness obtained from AFM technique is almost the same as those obtained in this paper by the profilometer technique. The R RMS is different between those techniques, in case of AFM the R RMS parameter is counted for a very small part of surface unlike with profilometer technique. 3.3. Surface free energy estimations From the contact angle values alone it is difficult to conclude about kind and strengths of the surface interactions. More information can be obtained by calculating the surface free energy of the studied surfaces. Having measured the contact angles (Fig. 1), the apparent surface free energy of modified by plasma PC surface was calculated both from contact angle hysteresis (CAH) model [19,20] and Owens- Wendt [21] approach. Comparison of the values obtained from these two approaches delivers interesting information about energetic changes occurring on the surface. The calculated values of the components and total surface free energy are plotted in Fig. 4. It is clearly seen that activation of PC by plasma causes the increase in the apparent surface free energy calculated from both approaches. 80 d A) s (CAH) s PC s (O-W) s p 80 d B) s (CAH) s PP s (O-W) s p 60 60 40 40 20 20 0 Untreated 0 Untreated Fig. 4. Changes in the apparent surface free energy and its components of untreated PC (A) and PP (B) surfaces and treated with O 2 or Ar plasma for 1 and 5 min and determined from the CAH and Owens-Wendt approaches. The surface free energy of the untreated PC surface is 44.0 ± 2.1mJ/m 2 (CAH) and 47.4± 0.8 mj/m 2 (O-W). For the surface activated with plasma it is more than 10 mj/m 2 larger, irrespective of the theoretical approach used. As follows from Fig. 4A p for the PC untreated surface the polar interaction s is 1.0±0.4 mj/m 2 while for the surface activated with argon plasma for 5 min it increases up to 20.1±0.5 mj/m 2. It is worth mention that extension of time of the surface activation with oxygen plasma does not affect the change of this polar parameter. As for the apparent surface free energy of the PP (Fig. 4B), its significantly 4-160

Intensity Intensity increases for the surface activated with plasma. The apparent surface free energy of unmodified plates is 34.8±1.6mJ/m 2 (CAH) and 36.6±0.4mJ/m 2 (O-W). Independently of the type of used gas the shorter activation causes increase in the apparent surface free energy by 5 6 mj/m 2 larger than 5 min treatment. 3.4. IR-ATR and XPS analysis The IR-ATR spectra obtained for polycarbonate are presented in Fig. 5. 0.25 0.20 0.15 A) 0 min PC 0.40 0.35 0.30 0.25 B) 0 min PP 0.20 0.10 0.15 0.05 0.10 0.05 0.00 0.00 500 600 700 800 900 2800 2900 3000 3100 1100 1200 1300 1400 1500 1600 1700 1800 Wavenumber, cm -1 Wavenumber, cm -1 Fig. 5. The most interesting parts of IR spectra for untreated PC (A) and PP (B) surfaces and treated with O 2 or Ar plasma for 1 and 5 min. The spectrum (Fig. 5A) shows distinct bands originating from not flat deformation vibrations Ar-H ( Ar H ) as well as torsion bends of the ring ( Ar ) in the range 750 900 cm 1. The unmodified surfaces are characterized by the highest intensity of the bands, then the surfaces modified for 1 and 5 min with oxygen plasma, and then the surfaces subjected to argon treatment for 5 min and 1 min exhibit the lowest intensity. A similar tendency is observed in the case of bands for flat deformation vibration of the methyl groups ( CH ) (1350 1430 cm 1 ) as well as symmetric ( s 3 Ar ) and asymmetric ( as Ar ) ones of the ring (in the range about 1500 1600 cm 1 ). The IR spectrum also shows the band in the characteristic range about 1700 1800 cm 1 corresponding to the vibrations of the carbonyl group ( C O ) (not presented here). The highest intensity band is seen for the PC surface modified with plasma O 2 for one minute. In the case of stretching symmetric ( s CH 3 ) vibrations, the least intensive band is observed for activation with oxygen plasma for 5 min and the lowest one in the case of 1-minute modification with argon. The interesting tendency observed during these studies is evident band characteristic for asymmetric vibrations ( as ) (2850 3000 cm 1 ). These bands intensity decreases in the following order: activation with 5-min argon plasma, 5- minute oxygen plasma, 1-minute oxygen and argon plasmas. The unmodified PC surface is characterized by the lowest intensity of bands. Vijayalakshmi et al., who analyzed PC IR spectra of the air plasma surface modification, obtained very similar data. However, they found also the unsaturated C=C bonds on polycarbonate treated surface. The difference probably can be explained by kind of the used gas [12]. In the case of polypropylene (Fig. 5B), all bands, flat deformation ( ) and stretching symmetric ( s ) and asymmetric ( as ) obtained for individual alkyl groups their largest CH 3 4-161

intensity appearance for the unmodified surfaces (marked with black line). On the other hand, the least intensive bands are found for surface after 1-minute modification with argon in the range about 1350 1380 cm 1 for flat deformation vibrations of alkyl groups ( ) and ( ). CH 3 CH 2 Due to activation of PP surface with plasma the spectra show the bands at about 1730 cm 1 originating from the carbonyl groups, being result of introducing of oxygen atoms on its surface. In turn, for the stretching symmetric ( s ) and asymmetric ( as H C ) vibrations observed in the range 2800 3000 cm 1, prolongation of the activation time with oxygen plasma to 5 min causes that the band for this type of modification is characterized by the smallest intensity. Our results are very comparable to those obtained by Garbassi et al. [24], and the most interesting is that after plasma modification C=O groups on the PP surface were also found by these authors. From the literature XPS data result that in the case of polycarbonate surface [25] C1s spectrum (fitted peak) is represented by 5 Gauss distributions corresponding to various bands. 284.5 ev is the aromatic C H bond, 285 ev is the aliphatic CH, C C bond, 286.24 ev is the aromatic C-O bond, 290.44 ev is the C=O bond and 291.67 ev is - *. The plasma induces a significant reduction of carboxylic carbon and increasing intensity at about 288.5 ev. The - * shake up satellite is reduced and a change in the aromatic system is indicated. The O1s signal shows a reduction of signal intensity with the singly bonded oxygen at 533.97 ev, decreasing strongly during etching. After a long treatment time, not shown here, the amount of oxygen can be reduced to 20% of the original [13, 25]. The analysis of the XPS results obtained for polycarbonate surface activation with atmospheric plasma by Sharma et al. [26] showed that when the surface was subjected to the action of plasma for 300 s an increase in the O:C ratio was observed, which resulted in formation of polar groups on the surface and thus its hydrophilization. However elongation of the time to 600 s caused decrease of the O:C ratio, which indicates exceeding of the optimal time of the surface activation with plasma of this type. During this activation cracking of oxygen group bonds and formation of covalent bonds between polymer chain atoms may occur. On the polypropylene surface activated with oxygen plasma the oxygen atoms appear which are not naturally present on the PP surface [27]. The oxygen content introduced on the polypropylene surface due to its activation with oxygen plasma for 5 minutes increases from 1.4% to 33%. The introduced oxygen atoms during PP modification with plasma cause increase in wettability of this polymer. Moreover, due to plasma treatment the carbon content decreases significantly to 67% in comparison to that of the unmodified PP surface, for which the content of C atoms is about 99% [28]. In the case of PP surface modification with Ar plasma the increase of oxygen on the surface activated is also observed from a few percent even up to 42.9%. Moreover, the O:C ratio increases from the value a few hundredth to even 0.79 [1]. Generally, the activation of the polymer surfaces with plasma causes significant increase in their hydrophilicity. In the case of polycarbonate, the surface topography changes are comparable irrespective of the used gas. However, as follows from the XPS analysis, it s on the surface of PC activation with oxygen plasma results in the increase of the oxygen atoms content and simultaneously significant increase of the surface roughness occurs resulting in the hydrophilic properties appearance. The increased number of oxygen atoms causes increase of the electron-donor parameter of the apparent surface free energy. Furthermore, from the IR analysis, results that the 4-162 C H

surface structure is also changed and the bands originating from the groups containing oxygen were greatly intensified after modification with oxygen plasma. This is confirmed by the increased hydrophilicity of activated polycarbonate surface. In the case of argon plasma the decrease in intensity of XPS signal, characteristic for oxygen atom is observed. Therefore it can be concluded that mechanical etching of the surface leads to its increasing coarseness and appearance of free electron pairs on the surface. In consequence the polar component of the apparent surface free energy is significant. This effect is more visible for longer exposition time of the PC surface to argon plasma. 4. CONCLUSIONS Activation of polypropylene surface with oxygen plasma causes the increase of oxygen amount on the surface which is confirmed by the XPS analysis and the IR spectra show which the bands in the range characteristic for carbonyl group present in the structure. The appearance of functional hydrophilic group on the surface causes sharp increase of the polar interactions. Moreover, significant increase in the surface coarseness also appears in the increased hydrophilic properties of the surface. Activation for 1 min with argon plasma results in the appearance of polar groups and free electron pairs on the polypropylene surface and hence the increase of polar component of the surface free energy, that is, the increase of hydrophilic properties of the modified polymer. It is interesting that prolongation of activation time to 5 min causes remarkable decrease in the apparent surface free energy and surface coarseness Plasma activated gas particles strike the surface and change it surface properties. This effect can be called as a semi mechanical surface etching. The best hydrophilization effects are achieved by surface modification with argon plasma for one minute only. However, no distinct differences were found for polypropylene with regard to gas or activation time. The two approaches applied to determine the surface apparent free energy exhibit the same tendency of the energy changes as well similar values. As follows from the calculation, the increasing polar component of the surface free energy interactions is responsible for the wettability changes of studied polymer surfaces. References 1. Williams T. S., Yu H., Hicks R. F., Atmospheric pressure plasma activation of polymers and composites for adhesive bonding: a critical review, Rev. Adhesion Adhesives, 2013, 1, 46 87. 2. Hegemann D., Brunner H., Oehr C., Plasma treatment of polymers for surface and adhesion improvement, Nucl. Instrum. Methods Phys. Res. Sect. B. 2003, 208, 281 86. 3. Lommatzsch U., Pasedag D., Baalman A., Ellinghorst G., Wagner H.-E., Atmospheric pressure plasma jet treatment of polyethylene surface for adhesion improvement, Plasma Process. Polymer. 2007, 4, S1041 45. 4. Awaja F., Gilbert M., Kelly G., Fox B., Pigram P. J., Adhesion of polymers, Prog. Pol. Sci. 2009, 34, 948 68. 5. Zhang S., Awaja F., James N., McKenzie D. W., Ruys A. J., Autohesion of plasma treated semi-crystalline PEEK: Comparative study of argon, nitrogen and oxygen treatments, Colloids Surf. A. 2011, 374, 88 95. 6. Weikart Ch. M., Miyama M., Yasuda H. K., Surface modification of conventional polymers by depositing plasma polymers of trimethylsilane and of trimethylsilane + O 2, J. Colloid Interface Sci. 1999, 211, 18 27. 4-163

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