Surface Modification of EPDM Rubber by Plasma Treatment

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Langmuir 2006, 22, 6109-6124

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Surface Modification of EPDM Rubber by Plasma Treatment Kai Frode Grythe and Finn Knut Hansen* Department of Chemistry, UniVersity of Oslo, P. O. Box 1033, Blindern, N-0315 Oslo, Norway ReceiVed December 23, 2005. In Final Form: April 25, 2006 The effect of argon, oxygen, and nitrogen plasma treatment of solvent cast EPDM rubber films has been investigated by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and surface energy measurements. Plasma treatment leads to changes in the surface energy from 25 to 70 mN/m. Treatment conditions influenced both the changes in surface energy and the stability, and it became more difficult to obtain good contact angle measurements after longer (> ca. 4 min) treatment times, probably because of an increasingly uneven surface structure. XPS analyses revealed that up to 20 at. % oxygen can be easily incorporated and that variations of ∼5% can be controlled by the plasma conditions. Oxygen was mainly found in hydroxyl groups, but also as carbonyl and carboxyl. XPS analyses showed more stable surfaces than expected from contact angles, probably because XPS analysis is less surface sensitive than contact angle measurements. AFM measurements revealed different surface structures with the three gases. The surface roughness increased generally with treatment time, and dramatic changes could be observed at longer times. At short times, surface energy changes were much faster than the changes in surface structure, showing that plasma treatment conditions can be utilized to tailor both surface energies and surface structure of EPDM rubber.

Introduction Solid propellant rocket motors are insulated thin walled containers loaded with a solid propellant in which the most important ingredients are an oxidizer and a polymeric binder.1 The insulation material used to protect the motor casing against the massive heat from the burning propellant is typically a particle filled and fiber reinforced EPDM.2-4 The adhesive bonding between the propellant and the casing insulation is one major area of concern in the production and storability of such motors.4-12 We have previously13,14 investigated the diffusion of curing agents in the uncured propellant and in the insulation material in order to evaluate the effect of diffusion on the adhesion process. In addition to diffusion processes, the adhesion between the propellant and the casing insulation may be controlled by the surface energies of the materials in contact. Because of the low surface energy of EPDM relative to the energy rich propellants, a mismatch of surface energies may cause adhesion problems. * Corresponding author. Tel: (+47)22855554. Fax: (+47)22855542. E-mail: [email protected]. (1) Davenas, A. Solid Rocket Propulsion Technology; Pergamon Press: Oxford, 1993. (2) Park, B. Y.; Jung, S. K.; Yun, Y. J.; Jung, B.; Won, Y. G. Int. SAMPE Symp. Exhib. 2002, 47, 1573-1578. (3) Guillot, D. G.; Harvey, A. R. EPDM rocket motor insulation. 2000-US734 2000043445, 20000113, 2000. (4) Dutra, J. C. N.; Massi, M.; Otani, C.; Dutra, R. D. C. L.; Diniz, M. F.; Urruchi, W. I.; Maciel, H. S.; Bittencourt, E. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2002, 374, 45-52. (5) Haska, S. B.; Bayramli, E.; Pekel, F.; Ozkar, S. J. Appl. Polym. Sci. 1997, 64 (12), 2355-2362. (6) Haska, S. B.; Pekel, F. Int. Annu. Conf. ICT 1995, 26th (Pyrotechnics), 49/1-49/12. (7) Sanden, R.; Wingborg, N. J. Appl. Polym. Sci. 1989, 37 (1), 167-71. (8) Schreuder-Gibson, H. L. Rubber World 1990, 203 (2), 34-44. (9) Gottlieb, L.; Bar, S. Propellants, Explos., Pyrotech. 2003, 28 (1), 12-17. (10) Giants, T. W. Case bond liner systems for solid rocket motors; Report; Mater. Sci. Lab., Aerosp. Corp., El Segundo, CA, USA. FIELD URL: 1991; p 21. (11) Hemminger, C. S. In Surface Characterization of Solid Rocket Motor HTPB Liner Bond System, 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Seattle, WA, 1997; Seattle, WA, 1997. (12) Tod, D. A.; Wylie, P. D. Adhesion ‘99, International Conference on Adhesion and AdhesiVes, 7th, Cambridge, United Kingdom, Sept. 15-17, 1999 1999, 375-379. (13) Grythe, K. F.; Hansen, F. K.; Walderhaug, H. J. Phys. Chem. B 2004, 108 (33), 12404-12412. (14) Grythe, K. F.; Hansen, F. K. Manuscript in preparation.

Thus, modification of the insulation surface may also be a necessary step in adhesion improvement. An initial study of some common EPDM based insulation materials showed that the surface is heterogeneous, consisting of both polymer and filler particles, and has a highly variable degree of roughness. Plasma treatment of these materials showed increased adhesion to the propellant and liner, but the chemical and physical heterogeneity of the material made a reproducible analysis very difficult. To overcome this problem we have in this work used a pure solvent cast EPDM film as a model material to investigate the effects of the plasma treatment process. It has been stated by Fowkes15 that the strength of an adhesive bond is directly proportional to the thermodynamic energies of adhesion, i.e., the work of adhesion. Since the work of adhesion consists of the sum of dispersive and polar (or Lifshitz-van der Waals and acid-base) contributions, maximization of these interactions are among the fundamental guidelines in the improvement of adhesion in a given system.16 The use of plasma treatment to increase the polar surface energy contributions and thus improve the adhesion properties of polymers is a commonly used technique.17-24 In a plasma the surface is exposed to ions, electrons, exited neutrals, radicals, and UV and VUV radiation.21,25 Plasma treatment has four major effects to a surfacescleaning, etching, cross linking, and chemical modificationsand all of (15) Fowkes, F. M. J. Adhes. Sci. Technol. 1987, 1 (1), 7-27. (16) Morra, M. Acid-Base BehaVior of Polymer Surfaces; Marcel Dekker: New York, 2002; Vol. 1. (17) Johansson, B.-L.; Larsson, A.; Ocklind, A.; Ohrlund, A. J. Appl. Polym. Sci. 2002, 86 (10), 2618-2625. (18) Kaminska, A.; Kaczmarek, H.; Kowalonek, J. Eur. Polym. J. 2002, 38 (9), 1915-1919. (19) Hegemann, D.; Brunner, H.; Oehr, C. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 208, 281-286. (20) Nihlstrand, A.; Hjertberg, T.; Schreiber, H. P.; Klemberg-Sapieha, J. E. J. Adhes. Sci. Technol. 1996, 10 (7), 651-675. (21) Beake, B. D.; Ling, J. S. G.; Leggett, G. J. J. Mater. Chem. 1998, 8 (12), 2845-2854. (22) Clouet, F.; Shi, M. K. J. Appl. Polym. Sci. 1992, 46 (11), 1955-66. (23) Landete-Ruiz, M. D.; Martinez-Diez, J. A.; Rodriguez-Perez, M. A.; De Saja, J. A.; Martin-Martinez, J. M. J. Adhes. Sci. Technol. 2002, 16 (8), 10731101. (24) Osterhold, M.; Armbruster, K. Prog. Org. Coat. 1998, 33 (3-4), 197201. (25) Liston, E. M.; Martinu, L.; Wertheimer, M. R. J. Adhes. Sci. Technol. 1993, 7 (10), 1091-127.

10.1021/la053471d CCC: $33.50 © 2006 American Chemical Society Published on Web 06/10/2006

6110 Langmuir, Vol. 22, No. 14, 2006

these effects may contribute to a better bonding. One of the most apparent results of plasma treatment is modified wettability of the surface, which is thought to be limited by the competition of chain scission (etching) and functional modification of the surface.21 Generation of increased oxygen functionalities improves the wettability and can be achieved in oxygen-containing plasmas or by post-plasma reactions.19 Thus, plasma treatment increases the number of Lewis acid-base sites. In an acid-base perspective, oxygen containing functionalities increase the strength of the Lewis acid-base interaction with water and other polar liquids. These groups give both an electron donor character through the electron lone pair of oxygen and an electron acceptor character through the active hydrogen linked to the electronegative oxygen atoms.16 The stability of plasma treated surfaces with time and as a function of environmental conditions is critical. All modified surfaces will be subject to aging, but by choosing the proper type of gas and the plasma treatment conditions for the selected polymer, it is possible to minimize degradation and aging effects.19 Some plasma treated surfaces may remain stable for days and weeks,24 while others are much less stable. Time is also an important factor in the analyses of plasma treated surfaces, and the results may be strongly influenced by analysis times. Hegemann19 has showed an example where the contact angle of water on polycarbonate (PC) was reduced from approximately 70° to 10° after plasma treatment. After approximately 1 week the contact angle had again increased to 30°, after 1 month to 35°, and after 1 year to 40°. Carrino et al.26 found the wettability improvement due to plasma treatment of a polypropylene surface to be reduced by 7% after 1 day, and 29% after 10 days. Paynter27 investigated the aging of polystyrene (PS) exposed to an Ar/O2 plasma, and found that the new oxygen functionalities created by the plasma, and so also the surface energy, decreased quite rapidly with time. It was found that especially CdO and O-CO-O (carbonate) groups loose their functionality over time. For nitrogen plasma treated low-density polyethylene (LLDPE) surfaces Foerch et al.28 found that the nitrogen content decreased by 25% in 3 days, but then remained constant for a month. On the other hand, the oxygen content was doubled in 3 days, before remaining approximately constant for a month. According to Liston et al.25 the storage stability of plasma treated polymer samples is not easily predicted and thus has to be specially investigated for any given type of polymer material and blend. To complicate matters, not all authors specify the time from plasma treatment to subsequent analysis, so the consequence of storage conditions cannot easily be evaluated. Kaminska et al.,18 Hegmann et al.,19 and Beake et al.21 measured contact angles within 30 min after plasma treatment of poly(ethylene terephthalate) (PET), but did not give any details of storage times before XPS analysis. Johansson et al.17 measured contact angles directly after plasma treatment, but they do not specify what “directly” means. In rubbers, as contrary to plastics, surface treatment may be more complicated because rubbers contain considerable amounts of additives that may be present in/on the surface, and also because rubber will be more prone to chain scission due to the presence of unsaturated bonds.29 It is claimed30 that there are very few (26) Carrino, L.; Polini, W.; Sorrentino, L. J. Mater. Process. Technol. 2004, 153-154, 519-525. (27) Paynter, R. W. Surf. Interface Anal. 2002, 33 (1), 14-22. (28) Foerch, R.; Izawa, J.; McIntyre, N. S.; Hunter, D. H. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1990, 46 (Plasma Polym. Plasma Interact. Polym. Mater.), 415-37. (29) Brevis, D. M. In Plasma and other pretreatments to enhance the adhesion to polymers, 12th International Colloquium on Plasma Processes, Antibes, France, 1999; Antibes, France, 1999; pp 157-161.

Grythe and Hansen

effective pretreatments for additive free EPDM, due to the low amount of double bonds. Out of the different forms of chemical and UV treatments investigated,30 only acid dichromate oxidation gave improved adhesion. However, plasma modification was not attempted. Plasma treatment of EPDM has been reported by a few investigators. A process to increase the bondability of EPDM has been described,31 and Kondyurin32 reported that plasma treatment gave increased oxygen content in the surface of EPDM. In the work of Dutra et al.4 a rocket engine insulation material (vulcanized carbon black filled EPDM) was modified by plasma, and the evolution of chemical groups as a function of plasma conditions was studied by ATR-FTIR. Wheale et al.33 showed that oxygen plasma treatment increased the O:C ratio in EPDM from 0 to 0.13 in 30 s, and Husein et al.34 used a plasma immersion ion implantation (PIII) technique to increase the total surface energy for EPDM from 25 to 75 mN/m by 30 min treatment. In the first 10 min the dispersive component increased from 22 to 31 mN/m, but then remained constant. The polar component showed an insignificant decrease in the first 10 min, but from 10 to 30 min it increased from 3 to 44 mN/m. Another 15 min treatment reduced the polar component to ca. 5 mN/m. According to Husein, a sample treated with the PIII technique was stable for 3 weeks, while a sample treated with a low pressure plasma had a polar surface energy on the same level as an untreated sample after 3 weeks. They suggest a reason for this is the high cross-linking among polymer chains from the PIII method. Termoplastic olefin elastomers (TPO), consisting of EPDM particles in a polypropylene (PP) matrix are widely used in industrial applications as automotive parts and building materials. The low surface energy of this composite material is often a problem when it comes to adhesion. Therefore techniques to improve the adhesion properties, such as grafting techniques35 and plasma and corona treatment,20,36-38 have been used. According to Friedrich et al.,39 only short exposures to O2 plasma are required to significantly increase the adhesion between polyurethane adhesives and PP, other polymers, blends, and mineral or fiber filled composites. By XPS analysis they found that only a rather small amount of oxygen uptake (1.5-5%) is sufficient to improve the adhesion strength to optimum values. The surface energy of EPDM is quite low, and different values have been reported. Husein et al.34 found a surface energy of 24.3 mN/m, but they did not give details of EPDM type and quality. For PP-EPDM blends Osterhold40 measured surface energies of 21-23 mN/m, whereas Konar et al.41 found a value as low as 18.9 mN/m and Wu42 28 mN/m. In many papers the (30) Dahm, R. H.; Brewis, D. M.; Mathieson, I.; Tegg, J. L. RubberChem 2002, International Rubber Chemicals, Compounding and Mixing Conference, 3rd, Munich, Germany, June 11-12, 2002; 2002, pp 75-83. (31) N. N. Process for improving the adhesiveness of the surface of vulcanised articles of ethylene/a-olefin/diene terpolymers. GB1553297, 1975. (32) Kondyurin, A. V. J. Appl. Polym. Sci. 1993, 48 (8), 1417-1423. (33) Wheale, S. H.; Badyal, J. P. S.; Bech, J.; Nilsson, N. H. Polymer Surfaces and Interfaces III, [Contributions to a Conference], Durham, U.K., July 1997; 1999, pp 285-297. (34) Husein, I. F.; Chan, C.; Chu, P. K. J. Mater. Sci. Lett. 2002, 21 (20), 1611-1614. (35) Hong, S.-G.; Ho, C.-A. Macromol. Mater. Eng. 2001, 286 (10), 583590. (36) Jacobasch, H. J.; Grundke, K.; Schneider, S.; Simon, F. Prog. Org. Coat. 1995, 26 (2-4), 131-43. (37) Yoon, T.-H.; McGrath, J. E. Korea Polym. J. 1998, 6 (2), 181-187. (38) Gleich, H.; Hansmann, H. Adhaesion 1991, 35 (3), 27-32. (39) Friedrich, J. F.; Unger, W.; Lippitz, A.; Gross, T.; Rohrer, P.; Saur, W.; Erdmann, J.; Gorsler, H.-V. J. Adhes. Sci. Technol. 1995, 9 (5), 575-598. (40) Osterhold, M.; Armbruster, K. Macromol. Symp. 1998, 126 (6th Dresden Polymer Discussion Surface Modification, 1997), 295-306. (41) Konar, J.; Kole, S.; Avasthi, B. N.; Bhowmick, A. K. J. Appl. Polym. Sci. 1996, 61 (3), 501-506. (42) Wu, S. Polymer interface and adhesion; Marcell Dekker Inc.: New York, 1982.

Surface Modification of EPDM Rubber

EPDM quality is not specified, e.g. whether it is a pure or filled EPDM rubber, the type and amount of diene, the content of additives, and the degree of cross-linking. This may be a reason for the differences in the reported surface energies. In this work we will therefore investigate the surface of an unfilled and unmodified EPDM surface in order to distinguish the effects on the pure EPDM rubber from that of the additives and fillers. It will also be shown that this approach gives much more reproducible and hopefully generally useful results. Experimental Section Chemicals. EPDM was obtained from Uniroyal Chemical Co., Inc., with the trade name Royalene 563. The ethylene/propylene (E/P) ratio is given as 56/44, and it contains 5% ethylidene norbornene. Tg ) -54 °C was found by differential scanning calorimetry (DSC) analysis of the material. No crystallinity was seen. Thermogravimetric analysis (TGA) showed decomposition at 440-470 °C, but a very slight weight loss was observed already at temperatures above 150 °C. Hexadecane (Fluka, g98%) was used for solvent casting. The liquids used in the contact angle measurements were diiodomethane (Aldrich, 99%), formamide (Fluka, puriss p.a. >99%), dimethyl sulfoxide (DMSO) (Fluka puriss p.a. g99.9%), 1-bromonaphthalene (purum g95%, Fluka), ethylene glycol (Prolabo), glycerol (Kebo Lab, Puriss), and water (ion exchanged followed by distillation). Sample Preparation. For the casting of homogeneous EPDM films hexadecane was found to be the best solvent, and EPDM was therefore dissolved in hexadecane to a 2% solution at room temperature by stirring for ca. 96 h. Glass slides were cleaned in chromic sulfuric acid and washed in water and ethanol. Thin films were made by deposition of a few (ca. 6) drops of solution on the glass slides, with subsequent evaporation at 110 °C for 15 h. Clear and smooth film surfaces with thickness 10-30 µm were thus achieved. A plasma chamber (Plasma Science PS0150E), with a 500W RF generator and 13.56 MHz high frequency power supply, was used for the plasma treatments. In this chamber, RF power is applied to two side electrodes and the gas flow is in the direction front to back. Insulating materials are used to reduce wall/electrode interactions, thus giving uniform plasma throughout the reaction chamber. The chamber has three mass flow controllers, allowing a gas flow in the order 5-500 sscm (standard cubic centimeter per minute). Stable plasmas were hard to achieve with gas flows below 50 sscm. Before and after each treatment the chamber was evacuated to a base pressure on 0.025 Torr. The samples were placed in the middle of the chamber. After the treatment, air was let into the chamber until atmospheric pressure was reached. All samples were stored in closed dishes at room temperature after plasma treatment. There are mainly four important factors that govern the effect of the plasma treatment: treatment time, type of gas, gas flow, and power input. We have in this work mainly studied the effect of the plasma treatment time with three different gases: oxygen, nitrogen, and argon. For each of these, initial experiments gave the conditions for the gas flow and the power input. These conditions were chosen to give maximum changes in the contact angles of water and diiodomethane for the given treatment time. It should be noticed that by changing the gas flow and the input power less efficient treatments may be obtained, which can be used to customize surface energies outside of those reported in this paper. For oxygen and nitrogen plasmas, gas flow and power input of 100 sscm and 500 W were used, while for argon plasma the gas flow and power input were 450 sscm and 250 W, respectively. Contact Angles and Surface Energy Measurements. The measurement of contact angles with various liquids is a common method to measure the wettability of a surface. A decrease in the contact angle, caused by polar groups produced on the surface by plasma treatment, usually correlates with better bonding of adhesives, and therefore the contact angle has often been used as an estimate of bonding quality.25 Contact angle measurements are very surface sensitive, since they record information about the outermost 5 Å of the sample surface.43 Advancing contact angles were measured with

Langmuir, Vol. 22, No. 14, 2006 6111 an automated contact angle goniometer (Model 200, rame´-hart instruments Co.), including an automated dispenser and the DROPimage computer program. The contact angles were measured by increasing the drop volume in steps of 1 µL and measuring the resulting angles after 1 s. For each liquid, an average of 15 measurements on each side of the drop, divided between 3 different drops, were recorded, the exact number determined by the reproducibility of the measurements. Thus, a standard deviation of (3% could be obtained. To calculate the surface energy from contact angles, several different methods are commonly used, but opinions vary widely on the suitability of several of these, and they also tend to give different results. One of the most recognized methods is the Fowkes’ theory, where the surface energy is split in a dispersive and a polar component and the geometric mean of the dispersive components is used to calculate the work of adhesion. In the so-called extended Fowkes theory44-46 the geometric mean of the polar components is added, so that the expression for the work of adhesion is Wa12 ) 2(γd2γd1)0.5 + 2(γp2γp1)0.5 ) γ1(1 + cos θ)

(1)

Here, indices 1 and 2 are used for the liquid and solid, respectively, and θ is the contact angle. Solving this equation for a set of two liquids yields a polar and a dispersive surface energy component. Instead of the geometric mean, the use of a harmonic mean for the work of adhesion has also been argued for,42 especially for polymeric surfaces. In that case, eq 1 becomes Wa12 )

4γd1γd2

+ d

γd1 + γ2

4γp1γp2 γp1 + γp2

) γ1(1 + cos θ)

(2)

These methods are simple, but it is often observed that the polar component of the surface energy depends on the choice of liquids, and therefore it is not an intrinsic property of the solid surface.47 The method is referred to as the “two-liquid” method is this paper, and a geometric mean is used in the calculations. Equation 1 can be transformed by dividing each term by 2(γd1)0.5 so that48 (1 + cos θ)γ1 2(γd1)0.5

) (γd2)0.5 + (γp2)0.5

() γp1

γd1

0.5

(3)

Then by plotting [(1 + cos θ)γ1]/[2(γd1)0.5] as a function of (γp1/γd1)0.5, we obtain a straight line with slope (γp2)0.5 and intercept (γd2)0.5. This approach is useful when doing measurements with three or more liquids as we obtain values that are an average over all liquids. This method is referred to as the “multi-liquid” method in this paper. Van Oss et al.49-56 have proposed a method where the dipole and induced dipole energies (Keesom and Debye energies) are included (43) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110 (17), 58975898. (44) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13 (8), 17411747. (45) Kaelble, D. H. J. Adhes. 1970, 2 (April), 66-81. (46) Kaelble, D. H. Phys. Chem. Adhes. 1971; p 507. (47) Dalet, P.; Papon, E.; Villenave, J. J. J. Adhes. Sci. Technol. 1999, 13 (8), 857-870. (48) Rabel, W. Farbe + Lack 1971, 77 (10), 997-1006. (49) Van Oss, C. J.; Chaudhury, M. K.; Good, R. J. AdV. Colloid Interface Sci. 1987, 28 (1), 35-64. (50) Van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. ReV. 1988, 88 (6), 927-41. (51) Van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Sep. Sci. Technol. 1989, 24 (1), 15-30. (52) Van Oss, C. J.; Giese, R. F., Jr.; Good, R. J. Langmuir 1990, 6 (11), 1711-1713. (53) Van Oss, C. J.; Good, R. J. J. Macromol. Sci., Chem. 1989, A26 (8), 1183-1203. (54) Van Oss, C. J.; Good, R. J.; Chaudhury, M. K. J. Colloid Interface Sci. 1986, 111 (2), 378-390. (55) Van Oss, C. J.; Good, R. J.; Chaudhury, M. K. Sep. Sci. Technol. 1987, 22 (1), 1-24. (56) Van Oss, C. J.; Ju, L.; Chaudhury, M. K.; Good, R. J. J. Colloid Interface Sci. 1989, 128 (2), 313-319.

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Grythe and Hansen

Table 1. Surface Energy Parameters for the Liquids Useda liquid

g

γd ) γLW

γp

γ+

γ-

water diiodomethane formamide DMSO 1-bromonaphthalene ethylene glycol glycerol

72.75 50.80 57.49 43.58 44.01 47.99 63.11

21.75 50.80 38.49 35.58 44.01 28.99 33.11

51 0 19.00 8.00 0 19 30

25.50 0 2.28 0.50 0 1.92 3.92

25.50 0 39.60 32.00 0 47 57.4

a

Values in mN/m.57

in the dispersive energy which is then called a Lifschtz-van der Waals (LW) energy, γLW. It is argued that it is now recognized that, in condensed phases, the contribution of permanent dipoles only negligibly affects the overall magnitude of the LW component.16 Then the “polar” surface energy component must be governed by the acid-base (AB) interaction, and Van Oss et al. therefore added an acid-base term, γAB, representing interactions between a Lewis acid (electron acceptor) and a Lewis base (electron donor) γAB ) 2(γ+γ-)0.5

(4)

where γ+ and γ- are the acid and base components, respectively, that are assumed to be intrinsic characteristics of the solid surfaces. The expression for the work of adhesion is then LW 0.5 + - 0.5 + 0.5 Wa12 ) 2(γLW + 2(γ) γ1(1 + cos θ) 1 γ2 ) + 2(γ1 γ2 ) 1 γ2 ) (5)

By measuring contact angles with three liquids whose LW and AB parameters are known, it is possible to determinate these parameters for the surface. This method is referred to as the “acidbase” method in this paper. Van Oss recommends that one of the three liquids used is nonpolar. Kwok et al.58,59 have shown that this acid-base approach often fails experimentally, since it shows a strong dependence on the liquids used, and it also often gives negative values for surface tensions and square roots of surface tensions. We have also observed these abnormalities in earlier work.60 The calculation tools for all the methods used above are built into the DROPimage program. Surface energy parameters for the liquids that are used in this work are given in Table 1. Surface Analysis. XPS is among the most powerful surface sensitive techniques for the characterization of polymer surfaces,25 and it is therefore a natural choice for surface analysis pre and post plasma treatment. XPS spectra were recorded on an instrument equipped with an achromatic Al KR X-ray source (ESCALAB III, Vacuum Generators). The time between plasma treatment and XPS analysis was 1-4 h, during which the samples were stored under ambient conditions in closed dishes. The pressure during the analysis was ca. 10-8 Torr. Analyses of the XPS data were done with the CasaXPS software. Survey scans in the range 0-1350 eV with step size 1 eV and high-resolution scans with step size 0.05 eV for C 1s, O 1s, and N 1s were recorded. Peak fitting of recorded spectra was done using a Shirley background. All spectra were calibrated to a C 1s value on 284 eV, and the high-resolution spectra were peak fit with GL 30 (gaussian lorenzian) peaks. To evaluate the carbonoxygen bonding, the C 1s envelope was peak fit using 4 peaks of approximate equal full width half-maximum (fwhm), with the following chemical shift: C-C, C-O (+1.5 eV), CdO (+2.9 eV), O-CdO (+4.3 eV).61 A fwhm of 1.3 was used, since this is the fwhm of the C 1s peak for an untreated film. The fwhm value was for a few of the samples allowed vary to 1.4 if better curve fitting (57) Kwok, D. Y.; Li, D.; Neumann, A. W. Langmuir 1994, 10 (4), 13231328. (58) Kwok, D. Y. Colloids Surf., A 1999, 156 (1-3), 191-200. (59) Kwok, D. Y.; Neumann, A. W. AdV. Colloid Interface Sci. 1999, 81 (3), 167-249. (60) Grythe, K. F. Study of the interface between insulation and propellant in solid propellant rocket motors. NTNU, Trondheim, 2002. (61) Briggs, D. Surface analysis of polymers by XPS and static SIMS; Cambridge University Press: Cambridge, 1998.

could be achieved. In CasaXPS the sensitivity factors 1, 2.93, 8.52, and 1.8 are used for C, O, N, and Na, respectively. Microscopy. Surface topography can be observed using light scattering or microscopy techniques such as Nomarski microscopy (NM), scanning electron microscopy (SEM), confocal scanning microscopy (CSM), or scanning probe microscopy (SPM). When it comes to observing features on a surface from close to atomic size up to tens of micrometers, the atomic force microscope (AFM) has largely replaced all electron microscopes as the instrument of choice.62 This is mostly due to the ease of sample preparation and the excellent height resolution. With AFM, spatial wavelengths can be observed down to approximately 10 Å, but this depends on the probe shape and radius as well as the surface slopes. The tip shape is therefore critical for the resolution. Typical AFM probes are pyramidal shaped with the tip of the pyramid a spherical profile of radius 30-50 nm. So-called “tapping mode” tips usually have a radius of 4-10 nm. The use of tapping mode instead of contact mode reduces the wear on the tip and the damage of the surface, and it was a natural choice for our soft surfaces. The AFM can produce a variety of topographic representations. In addition to height information, phase contrast AFM images were also recorded. In these images the phase signal from the cantilever is compared with the phase that is driving the cantilever. Differences in adhesion and elasticity can thereby be detected. Because of a small signal lag, phase imaging will also show the derivative of the topography.62 The exact surface property that is being measured may therefore not be clear, but phase imaging can give excellent results for boundaries between different phases in the material. AFM has been commonly used on plasma treated surfaces. AFM has shown that argon plasma smoothed the surface of poly(dimethylsilane) (PDMS), probably due to ablation,63 and it has also been observed that polypropylene (PP) foil is efficiently smoothened within a few seconds by oxygen glow discharge treatment.39 On the other side, it has also been reported increasing roughness after argon and oxygen plasma treatments of a semicrystalline poly(ethylene terephthalate) (PET) material.64 A Dimension 3100 AFM (Digital Instruments, Santa Barbara) was used in tapping mode at ambient conditions. In tapping mode a high frequency, small amplitude z-oscillation is applied to the probe, and topographic and phase images are recorded simultaneously. Noncoated silicon tips (Mikromasch) with a nominal tip radius of curvature less than 10 nm and a typical force constant on 4.5 N/mm were used. The scan size was 1 µm, and the samples were scanned at a frequency of 0.5-1 Hz. The images were stored as 512 × 512 data arrays. Since the AFM data is digitally stored, they can readily be treated mathematically to quantitatively determine the characteristics of a surface.65 Image data analysis was done using the Nanoscope 5.30 software. Some modifications of the images were needed to remove artifacts66,67 such as artificial curvatures, tilt, and distortion. All images were therefore subject to a third order flatten and plane fit using the Nanoscope software. In addition, noisy scan lines were also removed. There are many statistical parameters that can be used to characterize surfaces, and the most important are the root-meansquare (RMS) roughness and the power-spectral-density function (PSD).62,65 The RMS roughness is the standard deviation from the mean surface level of the image and will depend strongly on the area of the profile, the lateral resolution, and the sampling distance. Different instrumentation will also give different RMS values for the same surface due to different spatial wavelength range, and (62) Bennett, J. M.; Mattsson, L. Introduction to surface roughness and scattering, 2nd ed.; Optical Society of America: Washington, D.C, 1999. (63) Feinberg, A. W.; Brennan, A. B. Polym. Mater. Sci. Eng. 2003, 88, 574575. (64) Seo, E.-D. Macromol. Res. 2004, 12 (1), 134-140. (65) Skolnik, A. M.; Hughes, W. C.; Augustine, B. H. Chem. Educ. [Electronic Publication] 2000, 5 (1), 8-13. (66) Ghosh, A.; Rajeev, R. S.; Bhattacharya, A. K.; Bhowmick, A. K.; De, S. K.; Wolpensinger, B.; Bandyopadhyay, S. Rubber Chem. Technol. 2003, 76 (1), 220-238. (67) Fang, S. J.; Gu, X.; Nguyen, T.; VanLandinghan, M.; Karim, A. J. Appl. Phys. 2000, 82 (12).

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“bad” data points caused by scratches or dust particles can also make a great difference to the RMS value.62 PSD analysis is therefore considered the most important statistical quantity characterizing a surface.62 A PSD analysis gives a more detailed description of the roughness in the surface than an RMS value, since not only height deviations but also its lateral distribution is considered.66 It is possible to see the size of the individual features that together contribute to an RMS value. The PSD is the Fourier transform of the auto covariance function and represents a plot of the density, in spatial frequency space, of the power spectrum (power is the roughness amplitude squared). The x-axis is a function of spatial wavelength of the features contributing to the surface image, while the y-axis is a function of the roughness amplitude. Small features on the surface appear as high frequency/short wavelength peaks at the righthand side of the spectra, while larger features appear on the lefthand side of the spectra. The results from PSD analysis can be presented as 2D isotropic PSD plots, but we will only tabulate the total power (the sum of the power contained in the entire spectrum) and the equivalent RMS (the square root of the total power). A section analysis gives information regarding the size of the features on the diagonal line in the surface. In addition to RMS and PSD, also the arithmetic average of the absolute values of the surface height deviations measured from the mean plane, Ra, and the maximal vertical distance between the highest and lowest data points in the image, Rmax, are reported.

Results and Discussion Contact Angle Measurements. The results from contact angle measurements of the plasma modified EPDM are shown in Figures 1-3 for argon, nitrogen, and oxygen plasmas, respectively. Contact angles are given for the three liquidsswater, diiodomethane, and formamide. In Figures 1a-3a the contact angles are given as a function of plasma treatment time. Figures 1a and 2a shows measurements taken 1-2 h after plasma treatment, while Figure 3a shows measurements taken 5-10 min after treatment. Figures 1b-3b show the total, dispersive, and polar surface energy components, calculated by the two-liquid method, eq 1, and Figures 1c-3c the total, dispersive, polar, acidic, and basic surface energy components calculated by the acid-base method, eq 5. In all figures it is apparent that the contact angles, and thereby the surface energies, change dramatically when the samples are plasma treated, and the greater part of the change takes place almost instantaneously when the samples are exposed to plasma. In oxygen plasma the reduction in the contact angle takes more time, and after 4 min treatment the contact angles are actually increasing again. It should be noticed that the higher contact angles in Figure 3a compared to those in Figures 1a and 2a are most likely due to the measurements carried out only 5-10 min after plasma treatment. In all cases the changes in contact angle with treatment time are due mostly to increased oxygen functionality, as will be shown below. The reason for the general behavior may be that the main mechanism in all plasma gases is the creation of carbon radicals on the surface. These radicals react with oxygen when the sample is exposed to air after treatment. In the oxygen plasma, some oxygen containing groups are probably formed on the surface already during the plasma treatment, but all in all the efficiency of oxygen insertion is surprisingly lower than for the argon and nitrogen plasmas. As will also be shown below, the surface structure is also modified by the plasmas, and the resulting contact angle and surface energies may be complex functions of both surface functional groups and surface structure. Figures 4-6 show the stability in air of the different plasma treated EPDM surfaces. Contact angles are plotted as a function of the storage time after the different plasma treatments for argon,

Figure 1. Contact angles and surface energies of EPDM treated by argon plasma, as a function of the plasma treatment time: (a) contact angles, surface energies (b) from the two-liquid model and (c) from the acid-base model.

nitrogen, and oxygen plasmas, respectively. All contact angles were measured on a fresh surface, to avoid a possible surface modification due to the liquid drop from an earlier measurement. All three figures show that, although the contact angles of water were rapidly reduced by the plasma treatments, these contact angles decreased further in the time period immediately following the treatment, until they reached a minimum and relatively stable level. Both this “stable” contact angle itself and the time it took to reach this level varied with the type of plasma gas and the treatment time, but generally the “stable” level was between 20° and 30° and the time between 10 and 80 min. It seemed that

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Figure 2. Contact angles and surface energies of EPDM treated by nitrogen plasma, as a function of the plasma treatment time: (a) contact angles, surface energies (b) from the two-liquid model and (c) from the acid-base model.

generally both the level values and the relaxation times were lowered with longer plasma treatments. However, very long treatment times showed a more complex picture, and the results were much less reproducible. These effects will be discussed below, but it was generally observed that longer treatment times resulted in very heterogeneous and rough surfaces which resulted in difficulties in sessile drop stability and stronger time effects. In parts a and b of Figure 4 it is seen that for the argon plasma treated surfaces the contact angle of water levels off after ca. 20 min after both 0.5 and 2 min treatment. This value is around 35° for 0.5 min and around 30° for the 2 min treatment. The reproducibility of the measured contact angles of water during

Grythe and Hansen

Figure 3. Contact angles and surface energies of EPDM treated by oxygen plasma, as a function of the plasma treatment time: (a) contact angles, surface energies (b) from the two-liquid model and (c) from the acid-base model.

the first few minutes was not very good, but the differences became less pronounced after ca. 15 min. After 20 h the contact angle of water decreased further to 15-20° for the 0.5 min treatment, meaning that the “stable” levels obtained after ca. 20 min can decrease further at longer time intervals. Thereafter, however, the contact angle of water was quite stable for 120 h, while the contact angle of diiodomethane showed a small increase after 48 h. This may indicate that a further oxidation of the surface can take place in the time interval up to 20 h after plasma treatment and that the surface then remains stable over a longer time (several days). For the 2 min treatment the stable level of

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Figure 4. Stability of argon plasma treated EPDM to air exposure as a function of time after plasma treatment, measured by contact angles: (a) water and diiodomethane after 0.5 min plasma; (b) water and diiodomethane after 2 min plasma (two stored samples from this treatment was still stable at 30 ( 2° (water) after 7 weeks); (c) water, diiodomethane, and formamide after 4 min plasma; (d) water, diiodomethane, and formamide after 6 min plasma.

ca. 20° for water remains the same for 100 h, but after 20 h the measurements are quite unstable. However, for the 4 min treatment the contact angle of water stayed around 50° the first hour after treatment, as seen in Figure 4c. After 7 h though, the contact angle was measured to 23°. Thereafter, the measurements are unstable, rising from 25° to 40°, but the trend seems to be an increase in the contact angle of water and diiodomethane with time. Surfaces treated by argon plasma could have large variations in the contact angle over the film. These deviations were often much larger than with surfaces treated by nitrogen and oxygen plasmas, and the reproducibility was poor in some cases. These samples have not been included in the results shown here. A possible reason for the deviations is variable air flows and variable humidity over the surface after treatment resulting in variable surface reaction rates. The instability problems seem to increase with the time of plasma treatment. For samples treated for 6 min we were able to achieve stable contact angle measurements first for samples stored for 70 h. These contact angles were quite low, approximately 20° for water and 17° for diiodomethane, as shown in Figure 4d. The values after 19 h shown in this figure are considerable higher, but they were less reproducible. For all argon plasma treated surfaces the contact angles decreased with the time of treatment, and 60 h after treatment the contact angles of water were 35°, 30°, 22°, and 20° for 0.5, 2, 4, and 6 min treatment, respectively. For the 6 min treatment samples it was not possible to obtain stable readings up to 48 h after treatment.

The nitrogen treated surfaces in Figure 5 display the same decreasing trend in the contact angle of water as that of argon. For the samples treated for 0.5 min (Figure 5a) it takes an average of ca. 1 h for the contact angle to level off around 35°; however, this time could vary between 10 and 80 min in repeated experiments. We believe this quite large variation is due to variations in the rate of reaction with oxygen and humidity in the air above the samples during the short time between plasma treatment and contact angle measurements. Thereafter, the contact angles of water, diiodomethane, and formamide remained approximately constant for the measured time interval up to 5 days. For the sample treated for 2 min, shown in Figure 5b, the contact angles for water and formamide are lower, while diiodomethane has higher contact angles. The same tendency can be observed in Figure 2a. A rapid decrease in the contact angle of water was observed during the first 10 min, but the angle continued to decrease and was up to 10° lower after 1-2 days, and even lower after 5 days, as seen in Figure 5b. The same trend is also evident for the contact angle of diiodomethane, although these contact angles are generally much higher. For 6 min treatment, as shown in Figure 5c, the measured values were not stable, and the contact angles of water, diiodomethane, and formamide increased from approximately 18°, 34°, and 17° to 80°, 50°, and 60°, respectively, in 5 days. With longer treatment times, the measurement of contact angles became increasingly difficult, as during the first hour after plasma contact angles of water varied between 10° and 50°. The

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Figure 5. Stability of nitrogen plasma treated EPDM to air exposure as a function of time after plasma treatment, measured by contact angles: (a) water, formamide, and diiodomethane after 0.5 min plasma, for the first 80 min the results are an average of three samples; (b) water, formamide, and diiodomethane after 2 min plasma; (c) water, diiodomethane, and formamide after 6 min plasma.

reproducibility was generally bad, leading to somewhat random results, but there was still a clear trend in the data. The drop shape was not stable, and the drop was creeping along the surface. Strong hysteresis effects were observed, as well as the drops sticking and suddenly shifting on the surface. Very variable values could thus be obtained, as mentioned above. We believe these problems, as well as the hysteresis effect, was due to the increased surface roughness and/or heterogeneity. Now, it is well-known that an increased surface area leads to a changed measured value for the contact angle, in which direction depending on if the

Grythe and Hansen

Figure 6. Stability of oxygen plasma treated EPDM to air exposure as a function of time after plasma treatment, measured by contact angles: (a) water, formamide, and diiodomethane after 0.5 min plasma, for the first 80 min the results are an average of three samples; (b) water, formamide, and diiodomethane after 2 min plasma, the results for the first 30 min are an average of two samples; (c) water, diiodomethane, and formamide after 4 min plasma.

contact angle is below or above 90°. This is expressed by Wenzel’s relation,

cos θ* ) K cos θ

(6)

where θ* is the measured contact angle and K is the ratio between the real and the projected (flat) surface area, i.e., K g 1. From the section analysis plots, Figures 14-16, we see that the surface roughness increases considerably with increased time of plasma

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treatment, but considering the scale of these plots, we see that the surface area only increases a maximum of ca. 1% over the flat surface if the height/width ratio of a typical roughness is maximum 0.1. The error in the observed contact angle caused by geometrical roughness alone is therefore expected to be low. This is also confirmed by the (almost) independence of the contact angle of treatment time as shown in Figures 1-3. Because the contact angles in these figures are