Surface modification of a biomedical polyethylene terephthalate (PET) by air plasma

Applied Surface Science 255 (2009) 4446–4451

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Surface modification of a biomedical polyethylene terephthalate (PET) by air plasma Liqing Yang a, Jierong Chen a,b,*, Yafei Guo b, Zheng Zhang a a b

School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 August 2008 Received in revised form 30 October 2008 Accepted 16 November 2008 Available online 27 November 2008

In this work, low-pressure air plasma has been used to improve polyethylene terephthalate (PET) surface properties for technical applications. Surface free energy values have been estimated using contact angle value for different exposure times and different test liquids. Surface composition and morphology of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Surface topography changes related with the etching mechanism have been followed by weight loss study. The results show a considerable improvement in surface wettability and the surface free energy values even for short exposure times in the different discharge areas (discharge area, afterglow area and remote area), as observed by a remarkable decrease in contact angle values. Change of chemical composition made the polymer surfaces to be highly hydrophilic, which mainly depends on the increase in oxygen-containing groups. In addition to, the surface activation and AFM analyses show obvious changes in surface topography as a consequence of the plasma-etching mechanism. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Surface modification X-ray photoelectron spectroscopy (XPS) Contact angle Atomic force microscopy (AFM) Low-pressure RF plasma Polyethylene terephthalate film

1. Introduction Polyethylene terephthalate (PET) film is used in many technological fields for a wide variety of applications (packaging, decorative coatings, capacitors, magnetic tape, . . .) since it has some excellent bulk properties, such as very good barrier properties, crease resistance, solvent resistance, high melting point, resistance to fatigue, and high tenacity as either a film or a fiber. However, PET is sometimes an unsuitable material to use due to its low surface free energy, leading to poor wettability and poor adhesion [1,2]. It is necessary to modify their surfaces to increase the surface free energy without change in their bulk properties for many commercial applications. Several surface modification methods are employed to modify the polymer surfaces, such as chemical, thermal, mechanical and electrical treatments [3]. The glow discharge plasma treatment of polymers has been gaining popularity as a surface modification technique: it is a dry and low temperature process that is environmentally benign and easy to handle. Moreover, it is a versatile technique where a large variety of chemically active species can be created to attach specific functional groups on the polymer surface. Furthermore, this plasma active species can induce physical and chemical surface changes in polymers through several concurrent processes

* Corresponding author. E-mail address: [email protected] (J. Chen). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.048

(etching, grafting, polymerization, cross-linking). This can provide a nanometer sized surface layer that does not change the original bulk properties of the substrate [4,5]. In the present work, the glow discharge air plasma was applied to improve the intrinsic low surface free energy of a PET film. The hydrophilicity of the PET film was characterized by measuring the contact angle as a function of exposure time. In addition, the plasma treated samples have been subjected to an ageing process to determine the durability of the plasma treatment. The surface morphology and weight loss of the modified PET films were analyzed using atomic force microscopy (AFM) and electron analytic balance. The functionalization of the plasma treated PET film surfaces were characterized by X-ray photoelectron spectroscopy (XPS). 2. Experimental details 2.1. Materials The PET films (0.08 mm thick) used in this study are supplied by Shanghai Pengwei Packaging Materials Co. Ltd. (China). The main additives of the PET were that Bisphenol type epoxy resin (Heat stabilizer), BRUGGOLEN P250 (Nucleating agent), UV328 (light stabilizer), Calcium stearate (lubricant), Triphenyl phosphate (Plasticizers or Antioxidant). Films of 25 mm  50 mm in size are washed successively and ultrasonically in ethanol and distilled water for 10 min. Clean films are dried in a vacuum condition at ambient temperature (20 8C) and stored in a desiccator before use.

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Table 1 Test liquids and their surface free energy components. Surface free energy data (mJ/m2)

gL

g dL

g Lp

Distilled water, H2O Diiodomethane, CH2I2

72.8 50.8

21.8 50.8

51.0 0

All chemistry regents used are of the analytical grade. Doubly distilled water is used in all experiments.

wheregL is the experimentally determined surface tension of the liquid, u is the contact angle, gS is the surface free energy of the solid and gSL is the solid/liquid interfacial energy. In order to calculate the solid surface free energy gS, an estimate of gSL has to be obtained. In 1962 Fowkes [7] pioneered a surface free energy component approach. The total surface free energy of each phase can be split into two parts: dispersive part and non-dispersive (or polar) part. The first part results from the molecular interaction due to London forces and the second part is due to all the non-London forces:

2.2. RF discharge air plasma treatment

g i ¼ g di þ g ip

Fig. 1. Schematic structure of plasma reactor: (1) gas bottle; (2) valve; (3) mass flow meter; (4) inductance coil; (5) reaction chamber; (6) sample; (7) vacuum gauge; (8) electromagnetism valve; (9) vacuum pump; (10) RF generator; (11) grounding protection; (12) matching system.

A schematic diagram of the plasma configuration is depicted in Fig. 1. The reactor includes four parts-gas inlet, a reaction chamber, a gas exhaust, a power supply and a matching network (SY-500 W power supply and SP-matcher made in Micro-electronics Center, the Chinese Academy of Sciences). The reaction chamber is a Pyrex glass tube (4.5 cm in diameter, 100 cm long), where inductance-coupling discharge is applied. The Pyrex glass tube has a copper coil (nine turns) for the energy input Radio Frequency (RF) power (13.56 MHz frequency). The RF power is adjusted by a power controller (SP-III model). The PET films are placed at a constant distance of 15 cm (Air plasma discharge area) and 55 cm (afterglow area) and 75 cm (remote area) from the center of the copper coil and exposed to the air plasma (Fig. 2). First, the initial air in the reaction chamber is displaced with work gas (air). Afterward, the reaction chamber is evacuated to approximately 6.2 Pa, and then the air is introduced into the Pyrex glass tube with a flux of 30 cm3/min adjusted by a mass flow controller. The total pressure of the plasma reaction chamber is adjusted by the mass flow controller and kept for 5 min. Then the plasma was generated at a RF power of 60 W and the film is exposed to the plasma for 15, 30, 60, 90 and 120 s. 2.3. Surface characterization 2.3.1. Contact angle measurements and surface free energy theory Contact angle measurements on cleaned surfaces were carried out under air at 23 8C (room temperature) by the sessile drop method using a JY-82 contact angle analyser (Chengde, China). Contact angles on the surfaces were measured with two liquids of known surface free energy (pure water and diiodomethane). The volume of the drops was fixed to 2 ml. The values of the contact angle shown in this paper are the average of at least ten measured values. An experimental error is within 2.08. The surface free energy and its components of the surfaces were calculated using the Owens–Wendt geometric mean approach. The theory of the contact angle of pure liquid on the solid surface was developed nearly 200 years ago in terms of the Young equation [6]:

g L cos u ¼ g S  g SL

Fig. 2. Schematic structure of three areas in the plasma field.

(1)

(2)

After this, Owens and Wendt [8] extended the Fowkes equation and included the hydrogen bonding term. They used geometric mean to combine the dispersion force and hydrogen bonding components: qffiffiffiffiffiffiffiffiffiffiffi

qffiffiffiffiffiffiffiffiffiffiffiffi

g SL ¼ g S þ g L  2 g dS g dL  2 g Sp g Lp

(3)

From the Young Eq. (1), it follows that qffiffiffiffiffiffiffiffiffiffiffi

qffiffiffiffiffiffiffiffiffiffiffiffi

g L ð1 þ cos uÞ ¼ 2 g dS g dL þ 2 g Sp g Lp

(4)

To obtain g dS and g Sp of a thin film, the contact angle of at least two liquids with known surface tension components (gL, g dL , g Lp ) on the solid must be determined. Surface tension components of the two liquids are shown in Table 1 [9,10]. In our case, two liquids (distilled water, diiodomethane) were used to determine gs, g dS and g Sp of the PET films. Owens–Wendt geometric mean approach has been shown to provide a reasonable description of solid surface free energy components and has been applied to many interfacial systems [11]. We have chosen it to examine whether the results obtained with this theory provide a consistent description of the surface properties for a series of PET films modified by plasma. 2.3.2. X-ray photoelectron spectroscopy (XPS) The chemical composition of the PET films is investigated by a PHI-5400 X-ray photoelectron spectroscopy (XPS) (PerkinElmer, USA), which is performed in a VG Escalab 220 XL system, using nonmonochromatic Mg Ka radiation (hn = 1253.6 eV). The anode voltage was 10 kV, the anode current is 30 mA, and the background pressure in the analytical chamber is 5  108 Pa. The size of the Xray spot was 2-mm diameter, and the take-off angle of photoelectrons was 908 with respect to the sample surface. Data analysis was performed using PHI XPS software (version 2.0). 2.3.3. Atomic force microscopy (AFM) Surface roughness was assessed by AFM as part of surface characterization and also for further investigations into possible correlations between this parameter and plasma discharge conditions. The roughness value was obtained from three areas on plasma field. The AFM analyses were performed with an Electron Optics BV Quanta 200 (FEI Company, Japan) in the contact mode. For each sample, at least four areas were imaged in the height mode. From the analysis of the images, the root mean squared roughness (RMS) and different topographic profiles measured on 50 nm  50 nm images were evaluated.

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2.4. Weight-loss ratio After the plasma treatment, the samples were immediately weighed using a Mettler Toledo Classic Light AE240 analytic balance (Mettler Toledo Instruments Co., Ltd., Shanghai, China), in order to estimate the etching effects on the surface layers of polymeric films. The plasma etching effect described by weightloss ratio was calculated as following expression [12]: W ut  W pt Weight lossð%Þ ¼  100 S

Table 2 The surface free energy and its components of the PET films in the three areas (mJ/ m2). Plasma treatment

0s 15s 30s 60s 90s 120s

Discharge area

Afterglow area

Remote area

gs

g dS

g Sp

gs

g dS

g Sp

gs

g dS

g Sp

49.3 76.7 76.9 77.1 78.3 77.9

45.0 47.5 47.8 48.9 49.0 49.2

4.2 29.2 28.9 28.2 29.3 28.7

49.3 75.7 76.5 76.4 76.9 76.7

45.0 46.2 47.0 47.4 46.7 46.9

4.2 29.5 29.5 28.9 30.2 29.8

49.3 74.9 76.3 75.4 76.3 76.4

45.0 45.0 46.4 46.6 46.5 46.4

4.2 29.9 29.9 28.9 29.9 30.0

where Wut and Wpt are the weight of untreated and plasma treated samples respectively. S is the area of PET film. 3. Results and discussion 3.1. Hydrophilic analysis: contact angle and surface free energy measurements The extent of hydrophilic modification of the plasma modified PET films was investigated by contact angle measurement. Fig. 3 shows the variation in the contact angle of the PET films for different treatment times. The initial contact angle values of the untreated PET are 74.58 and 28.28 for water and diiodomethane as test liquids, respectively. Beyond 40 s, the decrease of contact angle of PET in the discharge area is more than that in the remote area after plasma treatment under the same conditions. For 120 s treatment time, the reduction in the water contact angle for PET films are from 74.58 to 18.18, 18.48 to 18.68 respectively, while the decrease of diiodomethane contact angle is smaller. However, the difference is rather small in the three areas. The decrease of contact angle suggests that the formation of hydrophilic groups on the plasma treated polymer film surfaces which may be explained as follows: the plasma creates radical species on the polymer surface, mainly through polymer chain scission or hydrogen abstraction by bombardment of plasma particles. This species can combine with oxygen in the air, thus also contributing to increase the amount of polar groups such as such as –OH, C O, COOH and COO– on the plasma treated polymer surfaces. Hence these polar groups make the plasma treated polymer surfaces become more hydrophilic compared to the untreated polymer surface [2,13,14]. Table 2 illustrate the variation in surface free energy of the PET surfaces as a function of the plasma treatment time. The surface free energy of the untreated PET film is 49.3 mJ/m2. After plasma

treatment the surface free energy increased with respect to treatment time in the three areas; the values do not change significantly for different areas and longer exposure times. This may be due to the lack of any change in the oxygen content formed on the surface, as the exposure time is increased [2]. In any way treatment times, the surface free energy of PET films is the most in the discharge area. A similar trend is observed in the increase of polar components (g Sp ) expect for 60 s treatment, as shown in Table 2. It is mainly due to the relative concentration of free radicals is more in the discharge area than ones in the other areas, thus the effective action on the PET surface is rather stronger [15]. However, there is no appreciable change in the dispersive component (g dS ). Hence the increase in surface free energy is due to the contribution of polar components (g Sp ). For instance, it increases from 4.2 mJ/m2 for untreated PET to 28.2 mJ/m2 (the minimum of all the g Sp ) for 60 s plasma treatment PET in the discharge area. Therefore, we can conclude that it also due to the incorporation of polar groups (such as CO, COO, OH, etc. [16]) onto the PET surface. The wettability and hence surface free energy of PET films are increased on account of the interaction between the hydrogen bond and dipoles in the vertical direction of the interface [17]. The properties like wettability, adhesion, printability, etc. strongly depend upon the surface free energy. 3.2. Chemical composition analysis: XPS results Surface chemical modifications induced by plasma treatment were determined by XPS analysis. In the elemental composition and ratio were summarized in Table 3, the untreated PET film has an O/C ratio equal to 0.28, while after plasma treatment in air, the O/C atomic ratio in the discharge area, afterglow area and remote area are increased to 0.44, 0.47 and 0.43 respectively. The increase in O/C atomic ratio suggests that new oxygen-containing groups are formed on the PET films after plasma treatment, and in the afterglow area it is mostly obvious. The introduction of oxygen containing polar groups in the polymer surface may be the main reason for the hydrophilic improvement. The XPS measurements also reveal that the plasma modification lead to a spot of increase in the N/C atomic ratio, and it is more in the discharge area. The N/C ratio increase from 0.01 to 0.02. Therefore, nitrogen-containing functional groups are formed on the PET surfaces after plasma treatment. Fig. 4a shows the wide scan spectra of the untreated and plasma treated PET surfaces and they mainly contain C1s, O1s Table 3 The elemental composition and ratio of the plasma modified PET film surfaces. PET film surface

C1s (at%)

O1s (at%)

N1s (at%)

O/C

N/C

Untreated

77.5

22.1

0.5

0.28

0.01

68.8 67.5 69.4

30.2 31.8 29.8

1.1 0.7 0.8

0.44 0.47 0.43

0.02 0.01 0.01

Plasma treated (90 s) Fig. 3. Variation of contact angle of PET films as a function of treatment time.

Discharge area Afterglow area Remote area

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Fig. 4. XPS wide scan spectra and C1s peak of PET film. (a) XPS wide scan spectra (b–e) C1s spectra of PET film; (b) untreated, (c) discharge area plasma treated, (d) afterglow area plasma treated and (e) remote area plasma treated.

and N1s peaks. The C1s high resolution spectra of untreated PET film is shown in Fig. 4b.The peaks may be identified at 284.6, 286.2, 288.7 and 286.8 eV, which may be assigned to C–C/C–H, C–O, O C–O and O–C–O respectively[18]. The spectra of treated films (Fig. 4c–e) also show peaks for C–C/C–H, C–O, O C–O and/ or N–CO–N, and additional peaks at 286.3 (in the discharge area), and 287.1 eV, which may be attributed to C–N and C O groups [19]. The change of content of each chemical component is given in Table 4. It shows that the component C–C decreases

significantly after plasma treatment, and at the same time, most of the oxygen containing polar groups C–O, O C–O, C O and N– CO–N increase in the surface of the plasma treated polymer surfaces. These results indicate that some of the C–C/C–H bonds in the polymer surface may be broken by the plasma treatment, and the broken C–C/C–H bonds recombine with oxygen and nitrogen atoms that are produced by oxygen and nitrogen containing groups into the molecular chain of PET film surface [2,20]. The results are consistent with the ones of Section 3.1, so

Table 4 High-resolution XPS of C1s peak deconvolution and possible groups. PET film surface

Contribution of C1s components (%) O1s (at%)

Untreated Possible groups after treatment Plasma treated (90 s)

Discharge area Afterglow area Remote area

N1s (at%)

C–C/C–H

C–O

O

74.4 C–C/C–H

7.8 C–O

11.2 O C–O/N–CO–N

6.6 C O/O–C–O

49.5 56.5 61.4

27.8 22.7 12.7

21.9 19.2 20.4

0.8 1.6 5.6

Note: In the discharge area the total components of C–O and/or C–N is 27.8%.

C–O

O–C–O

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Fig. 5. AFM 3D-topographic images of PET with different areas to air plasma: (a) untreated, (b) discharge area, (c) afterglow area and (d) remote area.

it can be concluded that the oxygen containing polar groups play an important role. 3.3. Morphological analysis: AFM results The surface topography of plasma treated PET film was investigated by atomic force microscopy (AFM). Fig. 5a–d is AFM images obtained in a systematic investigation on the topographical changes in PET films due to plasma treatment. As seen in Fig. 5a, the surface of untreated PET film was very smooth. After the plasma treatment, the size of the conical protuberances on the PET film surface has increased in the three areas as can be seen in Fig. 5b–d. However, in the remote area the PET film surface was with conical protuberances and moderate roughness. The root mean square roughness (RMS) and the absolute surface roughness (Ra) of untreated and plasma treated PET films are reported in Table 5. The roughness is increased due to the removal of top few monolayers of the polymer films, caused by the impact of plasma species on the surface. The surface roughness improves wettability and bonding strength [2].

3.4. Etching and weight loss study Bombardment by energetic particles such as electrons, ions, radicals, neutrals and excited atoms/molecules and UV–vis radiations with the surface of polymer films causes rapid removal of low molecular contaminants such as additives, processing aids, and adsorbed species resulting in etching of the surfaces [21]. This is either due to the physical removal of molecules of fragments or

Table 5 RMS and Ra values for PET films in the different discharge areas. Samples

Untreated

Discharge area

Afterglow area

Remote area

Ra (nm) RMS (nm)

2.0 2.8

14.8 17.6

13.9 15.5

10.9 12.3

Note: Power, 60 W; treatment time, 90 s; air flux, 30 cm3/min.

Fig. 6. Weight loss of plasma treated PET films as a function of treatment time in the three areas.

L. Yang et al. / Applied Surface Science 255 (2009) 4446–4451

the breaking up of bonds, chain scission, and degradation processes. The gases evolved in the reaction are pumped out. This causes loss in the weight of the film. In our studies, we found that treatment in air plasma results in loss of weight and is depicted in Fig. 6, which increases with time of exposure and in the remote area weight-loss ratio is the least. It is due to existing of a mass of reactive particles (electrons, ions, radicals, etc.) in the discharge area, while in the remote area only there are radicals. This happens because different life-spans of various reactive species, that is to say, the rate constants of electron-positive ion recombination and radical recombination are 107 and 1033 cm3/s, respectively [22]. 4. Conclusion A glow discharge plasma has been used to modify the PET film surfaces. Significant morphological and chemical changes were produced by the treatment in the different discharge areas (discharge area, afterglow area and remote area). The plasma treatment incorporated polar functional groups onto the surface of the PET film causing decrease in the contact angle and rise in surface free energy. It is due to exist of a mass of reactive particles (electrons, ions, radicals, etc.) in the discharge area, while in the afterglow area and remote area there are only radicals. AFM, XPS, and weight loss characterization studies were made. AFM and weight loss results showed increased roughness are moderate in the afterglow area and properties of the PET film surfaces are more excellent in the afterglow area and remote area. XPS analysis identified polar groups on the PET film surfaces. The above changes in PET surfaces made films more hydrophilic and surface free energy increasing suitable for industrial applications. We may be said that the low-pressure glow discharge plasma treatment is an interesting and environmentally efficient method to modify the surface of low surface free energy polymers and in the afterglow area and remote area modified properties are more excellent than the one in the discharge area. Acknowledgements This work is supported by the National Natural and Science Foundation of China under Grant 30571636 and 20877062, by the Specialized Research Fund for the Doctoral Program of Higher Education underGrant 20060698002, by the key Scientific Technique of Shaanxi Province of 13115 Innovation Engineering 2008ZDKG-78, and the key Scientific Technique Item of Shuzhou City SG0842.

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