Modification of the Surface Properties of Polyimide Films Using ...

Arab Journal of Nuclear Science and Applications, 46(5), (115-123) 2013

Modification of the Surface Properties of Polyimide Films Using Oxygen Plasma Exposure A. Atta Radiation Physics Department, National Center for Radiation Research and Technology (NCRRT), Nasr City, Cairo, Egypt Received 20/10/2012

Accepted 21/5/2013

ABSTRACT Surface modification of polyimide (PI) films by oxygen plasma was studied for technical applications. The effects of oxygen plasma treatments on the surface properties of PI films were investigated in terms of Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), atomic force microscopy (AFM), and contact angle measurements. The AFM results show that the rootmean-squared (RMS) roughness of pristine PI film was 2.5 nm, increasing to 8.5 nm after 5 minutes oxygen plasma treatment. The surface free energy value of untreated PI film was 42.98 mJ/m2 and increased to 64 mJ/m2 after 5 minutes of oxygen plasma. The results show a considerable improvement in surface wettabilityy by exposure times as observed by a remarkable decrease in contact angle values. Keywords: polyimide, oxygen plasma, FTIR, SEM, AFM, Surface free energy INTRODUCTION Polymers have been applied successfully in fields such as adhesion, biomaterials, protective coatings, friction and wear-resistant composites, microelectronic devices and thin film technology. Polymeric materials have been able to replace traditional engineering materials like metals and glass because of their high strength to weight ratio, resistance to corrosion, possibility of recycling and their relatively low cost. However, the low surface energy of polymers and resulting poor adhesion of additional coatings have also created numerous important technical challenges which have to be overcome by manufacturers (1). Polyimide used in the electronics industry as a material for flexible chip carriers because of its low cost and outstanding properties such as flame resistance, high upper working temperature (250– 320), high tensile strength(70–150 MPa),and high dielectric strength (22 kV/mm). For this application, the PI must be modified to overcome the poor adhesion properties by plasma processing(2-6). Pretreatment of polymers with the purpose to change adhesion properties can be classified in two main methods: chemical surface modifications and physical surface modifications. Wet chemical processes are highly efficient but result in the disadvantage of a very strong substrate roughening. Among those modification methods, plasma and ion beam treatments are being used to an increasing extent. The effects caused on the polymer surface by these techniques are the incorporation of functional groups, changes in the surface morphology, and alteration in the chain structure. Most previous plasma works on polyimide or other polymers focused on radio frequency (RF) plasma treatment (7-8). The use of plasma treatment to produce chemical and physical surface modifications of polymers has been widely reported. PI has been subjected to oxygen plasma treatments in order to make the polymer surface more reactive toward metal deposition and improve adhesion (9-12). In this work, oxygen plasma was applied to the surface of polyimide (PI) films as a treatment process to enhance the adhesion for the surface of Polyimide . The influence of oxygen plasma treatment time on characteristics of the PI film surface were investigated by Fourier-transform

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infrared spectroscopy (FT-IR), contact angle measurement, scanning electron microscope (SEM) and atomic force microscopy (AFM) for analysis of chemical functional groups, wettability , surface morphology and surface roughness respectively. EXPRIMENTAL Kapton film ( PI, DuPontTM ) of thickness 160m was prepared as flexible substrate and cut in 20 x20 mm2 for oxygen plasma exposure process. The PI films are then exposed to negyxo plasma in the reactive ion beam etching (RIE) system operating at a frequency of 13.56MHz at National Laboratory of Advanced Technology and NanoScience (INFM-TASC), Trieste, Italy. The electrodes in this machine are 20cm×12cm and their spacing is 4 cm. The top electrode is connected to RF power, whereas the bottom electrode which holds the sample, is connected to ground. The working pressure is 3.7 E-3 mbar; gas flow rate 25 sccm; and the incident power were kept constant at 100 W with a corresponding DC self-bias voltage – 275 V and the exposure time was varied from 0 to 5 minutes. The substrate holder is placed on the bottom electrode that its temperature is kept low during the process using circulating cold water. Before performing the plasma treatment, the polyimide polymeric samples were first cleaned by acetone using ultrasonic technique in order to remove any contamination on the polymeric surface. Surface treated PI films by O2 plasma were investigated by scanning electron microscope (SEM), atomic force microscopy (AFM), contact angle measurement, and Fourier-transform infrared spectroscopy (FT-IR) for analysis of surface morphology, surface roughness, wettability, and chemical functional groups, respectively. FTIR spectra of the pristine and the irradiated samples were investigated using (FTIRBeckman-4250) spectrophotometer in the range 400 cm-1 to 4000 cm-1 at NCRRT, AEA, Cairo, Egypt. The irradiated polyimide films were exposed to the atmosphere for a few days before the FTIR experiments were conducted. The contact angle in a sessile drop method was measured by a contact angle meter ( Kruss DSA 30). Each value of the contact angles was taken as an average from the five different samples fabricated under the same modification conditions. Surface free energy, i.e., the sum of the polar force and the dispersion force, was calculated by measuring the contact angles of owt different liquids (deionized water and diiodomethane). From contact angles data, the polar forces and the dispersion forces were calculated using the Owen method (13). SEM (Model JEOL, JSM-5400, Japan) at NCRRT, AEA, Cairo, Egypt, was used to investigate the surface morphology of the pristine and irradiated polyimide surfaces. Changes in surface roughness of polymer surface after oxygen plasma exposure were observed with an atomic force microscope AFM (JPK Nanowizard II, Berlin, Germany). The root mean square (rms) value of surface roughness was averaged from scanning 3 times over 100 µm2 at different places. RESULT AND DISCUSSION FTIR Results: The nature of chemical bonds of polymers can be studied through the characterization of the vibration modes determined by infrared spectroscopy (14). Figure 1 shows the FTIR spectra of the pristine and irradiated polyimide by oxygen plasma with exposure time 1 min and 5 min. The IR spectra exhibited an absorption peak at 1850 cm1 corresponding to the C=O (in phase) vibration band. On the other extreme, bands at 1770, 1505, 1370 and 1150 cm1 corresponded to C=O (out of phase),C=C, C–N, and (OC)2NC vibration bands, respectively. There is only a slight change in the intensity of the irradiated sample as compared to the pristine sample. This is due to high resistance of the polyimide sample to ionic radiation. The minor changes in the peaks of irradiated sample may be

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due to the breakage of one or two bonds in the ladder structure, but this will not change the overall structure of the polymer. From these observations, it may be concluded that the polyimide (Kapton) is highly resistant to radiation degradation.

Fig. 1 FTIR spectra of the pristine and irradiated polyimide films using oxygen plasma. SEM Analysis: Figure 2 shows the SEM images of the -untreated and oxygen plasma-treated PI films with various times ranging from 0 to 5 minutes. The surfaces of all plasma-treated PI films are rougher than that of the untreated film as shown in Figure 2. A relatively flat surface with some mountain ranges was observed for the unmodified polyimide, while rough surfaces were observed for the modified polyimide. After one minute of bombardment time, it was found that some dimples wereformed randomly over the polyimide surface, without any significant change being observed in the surface feature. At 2 and 3 minutes of exposure the polyimide surface became drastically rougher, due to the creation of distinctive valleys. When the exposure time was further increased to 4 and 5 minutes, a cone- and rill-shaped surface was developed. These results show that the polyimide surface became rougher and the surface morphology changed as the exposure time was increased, in such a way as to provide more physical interactions or mechanical interlocking with the metal layer, which thereby increased the adhesion. The resulting specific surface features are attributed to the bond breakage, ejection and rearrangement of the chains caused by the physical collision and chemical interaction of the incident energetic oxygen ions with the polyimide molecules.

Contact Angle Measurement: Figure 3 shows the contact angle of the polyimide PI film under different time of oxygen plasma treatments using two different liquid water and dioodomethane . The water contact angle decreased from 980 for the untreated PI film to 400 after 5 minutes of oxygen plasma. The diiodomethane contact angle decreased from 39.40 for the untreated PI to reach 34.10 after 2 minutes of oxygen plasma and with increasing oxygen plasma time up to 5 minutes the contact angle reached to 390. The decreases in contact angles revealed that the O2 exposure renders the PI film surface

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hydrophilic, and that optimized hydrophilic modification is attained at an O2 flow rate of 25 sccm and RF power of 100 W. The decrease of contact angle by O2 time occurred because excessive treatment of PI in oxygen plasma increased chain scission, generating a boundary layer that is detrimental to surface wettability(15). The change of contact angles for the irradiated polymers is due to the formation of hydrophilic groups rather than the change of surface roughness.

Fig.2. SEM micrographs of the pristine and irradiated polyimide films using oxygen plasma Determination of Surface Energy: A good understanding of the surface properties of a solid may be obtained relatively inexpensively from the measurement of the surface free energy. Therefore, the contact angle measurement has been used in the study of surface free energy, wettability and adhesion of low surface energy materials (16-17). The surface free energy of a solid is an important parameter, playing a vital role in the phenomena that occur at solid-liquid and solid-gas intefaces. Hence, knowledge of this parameter is useful in the studies of adsorption and wettability processes, which play important role in many industrial applications of the material. Measurement of contact angle of liquid with the solid surface permits a rapid and qualitative evaluation of surface free energy of polymer. In the present paper, analysis of the surface free energy of polyimide has been made on the basis of dispersive and

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non-dispersive components. Surface free energy (γs) and its polar (sp ) and dispersion ((sd ) components of the sample were determined from two sets of contact angles (water and diiodomethane) according to Owens-Wendt-Kaelble equation (Owens and Wendt 1969) (13).

 l (1  cos  )  2  ld



d s



 lp  .  ld p s

(1)

where, (l, lp andld are the total surface free energy, the polar component and the dispersion component of the surface free energy of the liquid, respectively. The values of the surface free energies of the test liquids obtained from the literature are given in Table 1(18) .The values of surface free energy and its components before and after the treatment are compared in figure 4. The increase in surface free energy for irradiated samples is attributed to the functionalization of the polymer surface with hydrophilic groups on the surface. Important information obtained from the surface energy measurement is that the increase in polar component indicates the formation of covalent bonds. The values of contact angles, the surface free energy and its components before and after the treatment are compared in table 2. The increase in surface free energy is attributed to the functionalization of the polymer surface with hydrophilic groups on the surface. In plasma modification, the changes in surface wettability depend on surface free energy of materials. Discharge energy equals to the product of discharge power and treatment time. Generally, different discharge system can produce different discharge energy. Thus, treatment efficiency can be correlated with the parameter “degree of treatment,” W (J/cm2), defined as the discharge energy delivered per unit area of the treated sample surface (19,20) It is expressed as follows:

W = P ×t/ A

(2)

Here, t is the treatment time (second); P is the plasma power (watt); A is the sample area (cm2) exposed to the discharge. In general, the changes of wettability are dependent on the energy transmitted to material surface by reactive species in plasma. As shown in Eq. (2), the degree of treatment W belongs to the plasma power P, and the treatment time t, if area A, is constant. From Eq. (2), we can estimate the degree of treatment W as a function of treatment time of PI surface modification as in table 2 by using oxygen plasma with RF power of 100 W and oxygen flow rate 25 sccm as optimal condition. When ‘W’ increases, this usually leads to increasing energetic species or plasma heating effect. The energetic species collide with material surface and transmit the energy to the surface. This leads to increasing the surface free energy (or surface energy) and improvement of wettability.

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Fig.3: The contact angle as a function of oxygen plasma exposure time )setuoim( for polyimidi films

Fig.4: The surface free energy as afunction of oxygen plasma expousre time (minutes) for polyimide films.

Table 1: Surface free energy and its polar and dispersion components of water and diiodomethane used to determine the surface free energy of polyimide(18). Liquid

Water Dioodomethane

Surface free energy γl(Nm) 72.1 50.8

Polar component γlp (Nm)

Dispersion component γld(Nm) 19.9 50.8

52.2 0.0

Table 2: The contact angles (), the dispersive surface free energy (sd) , the polar surface fre energy (sp) and the total surface free energy (s) for polyimide as a function of oxygen plasma expousre time (minutes). Expousre

water

dioodomethane

emix (min.) 0

98.0

39.4

1

81.7

38.6

2

69.3

34.1

3

65.0

36.9

4

45.0

38.6

5

40.0

39.0

sd

sp

s

(mJ/m2)

(mJ/m2)

(mJ/m2)

39.82

3.16

42.98

40.20

3.20

43.40

1.5

42.40

7.39

49.79

3.0

41.10

9.80

50.90

4.5

40.20

21.05

61.25

6.0

40.00

24.06

64.06

7.5

120

Degree of treatment(W) (J/cm2)x10 3 0.0


Atomic Force Microscopy Analysis: The AFM analysis also provides information on the changes in the surface morphology and roughness introduced by the plasma beam treatment. Fig. 5 shows the AFM topographic images of the unmodified polyimide films and those subjected to oxygen ion beam treatment with times of 1,3 and 5 minutes, respectively. It can be clearly seen that the surface became rougher with increasing exposure time, creating a distinctive surface structure, which was formed by the ablation process of ion bombardment. Figure 6 shows the root-mean-square (RMS) of the roughness. The RMS roughness of pristine polyimide was about 2.5 nm . The RMS roughness values of the oxygen plasma-treated polyimide at the treatment time of 1, 2, 3, 4 and 5 min were increased to 6.7, 9.8, 12.4, 19.6, and 8.5 nm, respectively . The roughness of the polyamide surfaces increases with treatment time as shown in figure 6, hence it can support the adhesion improvement. From these results, increasing the RMS roughness led to reduction in the contact angle because surface micro-roughness produces increased total surface area. Thus, the contact angle decreased with increasing RMS. This also indicates an increasing surface energy of plasma-treated PI film by using RF power of the oxygen plasma treatment system. The Surface roughness was increased by increasing plasma time because increasing plasma exposure times led to increase bombardment effect,, moreover, surface roughness increases by increasing oxygen plasma time, implying that particulate contaminants on the PI surface have been removed by treatment with O2 plasma (31-23).We concluded that the treatment time controls the oxygen atom and as a result reactive etching of the polyimide surface leads to the surface morphological changes.

Fig.5. 2D and 3D AFM images of the pristine and irradiated polyimide films using oxygen plasma

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22 20 18

roughness(nm)

16 14 12 10 8 6 4 2 0 0

1

2

3

4

5

expousre time

Fig. 6: The average surface roughness (rms) for polyamide as function of oxygen plasma exposure Time (minutes) CONCULSION In this study, surface modification of polyimide was investigated with the oxygen plasma treatment technique by varying surface treatment time. From the AFM and contact angles measurements, the surface roughness and wettability of PI were increased by O2 plasma treatment. The RMS roughness values of the –treated PI film increased by the plasma bombardment effect. The decrease in contact angles revealed that the O2 exposure renders the PI film surface hydrophilic, and that optimized hydrophilic modification is attained at an O2 flow rate of 25 sccm and an RF power of 100 W. Surface roughening as well as formation of new functional groups by the plasma treatment contributed to the decrease in the measured contact angles. These results indicate that oxygen plasma treatment is an effective method for polymer surface modification. ACKNOWLEDGEMENTS The authors wish to thank Dr. Nicolas Marquestaut for his help with AFM measurements. Similarly, we would like to thanks Dr. Gianluca Crenci, Dr. Simone DalZilio for her kind help with plasma technique ''reactive ion etching (RIE) system' and for her help during working in Lab (National Laboratory of Advanced Technology and NanoScience (INFM-TASC), SS. 14 km 163,5, Basovizza, 34012 Trieste, Italy REFERENCES (1) S. J. Park and H. Y. Lee,J. Colloid Interface Sci. 285,267 (2005). (2) Y. S. Lin, H. M. Liu, and C. W. Tsai, J. Polym. Sci., Part B 43,2023 (2005). (3) Y. S. Lin, H. M. Liu, and C. L. Chen, Surf. Coat. Technol. 200,3775 (2006). (4)Y. S. Lin, H. M. Liu, and H. T. Chen, J. Appl. Polym. Sci. 99,744 (2006) . (5) Y. S. Lin and H. M. Liu, Thin Solid Films 516,1773 (2008). (6) S. H. Kim, S. W. Na, N. E. Lee, Y. W. Nam, and Y.-H. Kim: Surf. Coat.Technol. 200,2072 (2005).

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