Langmuir 2004, 20, 10481-10489
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Plasma Surface Modification of Organic Materials: Comparison between Polyethylene Films and Octadecyltrichlorosilane Self-Assembled Monolayers M. Tatoulian,‡ O. Bouloussa,† F. Morie`re,† F. Arefi-Khonsari,‡ J. Amouroux,‡ and F. Rondelez*,† Laboratoire de Physico Chimie Curie, Institut Curie, 11 rue Pierre et Marie Curie, 75005 Paris, France, and Laboratoire de Ge´ nie des Proce´ de´ s Plasma et Traitements de Surface, Universite´ Pierre et Marie Curie, ENSCP, 11 rue Pierre et Marie Curie, 75005 Paris, France Received October 8, 2003. In Final Form: July 22, 2004 Low-pressure low-frequency NH3 plasmas have been used for the surface modification of bulk polyethylene films and of octadecyltrichlorosilane (OTS) self-assembled monolayers deposited on oxidized silicon wafers. The incorporation of nitrogen-containing groups by the plasma treatment has been followed by contact angle measurements and by X-ray photoelectron spectroscopy. The surface degradation of the OTS monolayers due to plasma etching has been measured separately by optical ellipsometry with subnanometric accuracy. Our data show clear evidence for the existence of an optimum treatment time, yielding a high density of NH2 functional groups without significant variation of the structural features of the organic material. Self-assembled monolayers appear as excellent model systems to characterize the effects of plasma discharges on polyolefins. In particular, they allow testing the influence of molecular orientation, packing density, and crystallinity on the final results.
1. Introduction Plasma treatments are well established to modify the surface properties of polymer films. Depending on the gas plasma and experimental treatment conditions (type of electrical discharge, power density, duration, etc.), they are used for (i) surface cleaning and etching, (ii) surface functionalization, and (iii) cross-linking. Long treatment times lead to film ablation and to formation of low molecular weight oxidized fragments. Shorter treatment times are selected to introduce polar functional groups at the material surface. In that case, the choice of the gas determines the nature of the chemical groups: for example, amino groups are generally introduced by N2 or NH3 plasmas. Finally, cross-linking is predominant in noble gas plasma such as helium or argon. In polymer films, these different effects are difficult to separate and analyze, due to their inherent structural and chemical complexity. Their structure is generally amorphous, with random orientation of the chains and random distribution of the various groups at the extreme surface. Their surface chemical composition is also illdefined. Indeed, plastic films contain significant amounts of processing aids and functional additives such as lubricants, antioxidants, and light-protective agents. Such low molecular weight compounds can exodiffuse during the plasma treatment and modify the surface in a noncontrolled way. In addition, they contain specific groups (phtalic acid esters, phenolic antioxidants, waxes, fatty acid esters, etc.) that interfere with the analysis of the chemical modifications introduced by the plasma. Finally, the large molecular weight polydispersity of the polymer chains is also a problem, and the presence of * To whom correspondence should be addressed. E-mail:
[email protected]. † Laboratoire de Physico Chimie Curie, Institut Curie. ‡ Laboratoire de Ge ´ nie des Proce´de´s Plasma et Traitements de Surface, Universite´ Pierre et Marie Curie, ENSCP.
remnants of the initial monomers and solvents can never be excluded. Self-assembled monolayers (SAMs) on oxidized silicon wafers are devoid of these problems. Discovered by Zisman1 in the mid-1940s, they are obtained by immersing a hydrophilic substrate in a dilute solution of organic molecules. As shown by Sagiv,2 the organic molecules adsorb spontaneously on the substrate and form a closepacked monolayer. The thermodynamic conditions necessary to obtain reproducible results have been defined by one of us.3,4 In such case, the film is of nanometric thickness, with all molecules in an unique orientation relative to the solid substrate. The chemical composition is perfectly defined since there are no additives in the organic solution. The organic molecule can also be chosen to mimic the polymer of interest. For instance, the deposition of n-octadecyltrichlorosilane (OTS) provides a straightforward model for polyethylene (PE) since its alkyl chains contain mostly methylene groups. Similarly, hexatriacontane crystals (C36H74) have been used in the past5 to study the effect of argon and oxygen radio frequency (rf) plasmas on high-density polyethylene. Another advantage of SAMs on silicon wafers is their planar geometry that allows the use of optical ellipsometry6 to measure the layer thickness and its eventual modification during the plasma treatment: the sensitivity is such that a change of 0.1 nm can be detected. (1) Zisman, W. A. Adv. Chem. Ser. 1964, 43, 1. (2) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-496. (3) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367-4373. (4) Parikh, A. N.; Allara, D. L.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. (5) Clouet, F.; Shi, M. K.; Prat, R.; Holl, Y.; Marie, P.; Le´onard, D.; De Puydt, Y.; Bertrand, P.; Dewez, J. L.; Doren, A. In Plasma Surface Modification of Polymers; Strobel, M., Lyons, M., Mittal, C. K. L., Eds.; VSP: Utrecht, The Netherlands, 1994; pp 65-97. (6) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, The Netherlands, 1977.
10.1021/la030378l CCC: $27.50 © 2004 American Chemical Society Published on Web 10/29/2004
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Langmuir, Vol. 20, No. 24, 2004
Tatoulian et al.
Figure 1. Schematic representation of the successive steps leading to self-assembled monolayers of n-alkyltrichlorosilanes on silicon wafers [adapted from ref 3]. The chains in the solid state are represented with a tilt of 11° relative to the vertical axis, according to ref 4.
To the best of our knowledge, there is only one report in the literature of plasma treatment on self-assembled monolayers.7,8 Low-pressure oxygen direct-current plasma was used to introduce oxygen groups in the SAM in order to improve the adhesion of metallic films. In this paper, we present an in-depth comparison of the functionalization of bulk PE films and octadecyltrichlorosilane SAMs, using low-pressure low-frequency NH3 plasmas. We show that it is possible to introduce up to one nitrogen group per chain in the SAMs, without inducing significant degradation. Several varieties of nitrogen-containing groups can be created, and their respective ratios are a sensitive function of the plasma treatment time. We also show that these groups are accessible to later reduction or derivatization. The differences in the plasma surface modification of SAM and PE films are then emphasized. 2. Experimental Section 2.1. Materials. 2.1.1. PE Films. Large sheets of PE films were obtained from BASELL and ATOFINA and used as received. The films were composed of three layers and had a total thickness of 150 microns. The outer surfaces are low-density polyethylene, whereas the inner core is of high-density polyethylene. 2.1.2. OTS Silicon Wafers. Octadecyltrichlorosilane monolayers have been deposited onto oxidized silicon wafers by using the self-assembly mechanism. The procedure has been described in detail by Brzoska et al.,3 based on the earlier work of Sagiv.2 The solvent used for the silanization reaction was a mixture of 70% hexadecane and 30% carbon tetrachloride. The concentration of octadecyltrichlorosilane in that solution was 10-3 M. The substrates were 1 in. (100) silicon wafers purchased from Siltronix (France). They are covered by a 1.7 nm thick natural layer of amorphous SiO2 bearing silanol surface groups onto which the trichlorosilane headgroups can chemically react. The selfassembly mechanism is displayed schematically in Figure 1. It is based on the lateral rearrangement of the octadecyltrichlorosilane molecules following their adsorption on the hydrated oxidized substrate. The molecules are attracted to the surface by (7) Unger, W. E. S.; Lippitz, A.; Gross, Th.; Friedrich, J. F.; Wo¨ll, Ch.; Nick, L. Langmuir 1999, 15, 1161-1166. (8) Unger, W. E. S.; Lippitz, A.; Friedrich, J. F.; Koprinarov, I.; Weiss, K.; Wo¨ll, Ch. In Metallized Plastics 5&6: Fundamental and Applied Aspects; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1998; pp 147-168.
the hydrophilic interaction between the polar trichloro headgroups and the physisorbed water layer. They hydrolyze upon contact and are converted to trihydroxyl groups. The water layer also permits in-plane lateral diffusion, and therefore monolayer reorganization, following adsorption. If the temperature of the reacting bath is sufficiently low (
