Chapter 5
Plasma- and Corona-Modified Polymer Surfaces Characterization by Static Secondary Ion Mass Spectrometry 1
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W. J. van Ooij and R. S. Michael
Department of Chemistry, Colorado School of Mines, Golden, CO 80401
The use of static SIMS for the characterization of surfaces of polypropylene (PP), PTFE and a PMDA-ODA type polyimide is described. Interfaces between evaporated copper or chromium films onto PTFE and polyimide were also analyzed. Some of the polymer substrates were modified by ion beams, corona discharge in air or plasma treatments in air, Ar and H . It is demonstrated that SIMS is highly complementary to XPS for the analysis of such modified surfaces, in that effects such as crosslinking, unsaturation and formation of low-molecular weight material at surfaces can be detected. 2
There are many different methods for modifying polymer surfaces to improve their adhesion and wetting properties. They include chemical etching and oxidation, ion bombardment, plasma treatments, flame treatment, mechanical abrasion and corona-discharge treatments (1*2). Especially flame and corona treatments are widely used for the modification of polyolefin surfaces to enhance, for instance, their printability. Despite the widespread use of such processes in industry, the understanding of the fundamental processes which occur at the polymer surface is very limited. This is undoubtedly due to the shallow depth to which the polymer is modified, typically 5 nm or less. The application of modern surface analysis techniques, such as XPS, to the analysis of modified polymer surfaces, has demonstrated that in most of the above processes the polymer is oxidized. Many functional groups such as hydroxyl, carbonyl, ether, carboxyl, ester, peroxide, epoxide, etc., have been detected by direct X P S analysis or after derivatization of functional groups. The interaction between evaporated metal films and several of such functional groups has been clearly demonstrated (3). Although X P S has provided a wealth of information on the surface chemistry of treated polymer surfaces, several possible processes are beyond the capabilities of the technique. Some of these are: crosslinking, formation of double bonds, reorientation of surface molecules and the formation of 1
Current address: Armco Research & Technology, Middletown, OH 45043 Current address: GE Plastics, Pittsfield, MA 01201
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0097-^156/90/0440-0060$08.00/0 © 1990 American Chemical Society
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Plasma- and Corona-Modified Surfaces 61
VAN OOJJ AND MICHAEL
low-molecular weight material. Especially in processes which involve a free radical mechanism, such as corona discharge in the presence of air, reaction between oxygen and polymer radicals results in chain scission (4). This low-molecular weight material is highly oxidized and contains most of the incorporated oxygen functionalities. The presence of such material cannot be detected directly by XPS, but can be inferred from analyses of the surface before and after rinsing in water. A surface analysis technique that has the potential to detect structural chemical changes in polymer surfaces, including low-molecular weight mate rial formation, is static SIMS. Its capabilities for characterizing polymers by virtue of their fingerprint spectrum nave been amply demonstrated in recent years (5J>). The technique is more surface sensitive than X P S and can detect structural differences, even in hydrocarbons (7). It is, therefore, highly complementary to XPS. Nevertheless, only very few applications to the study of modified polymer surfaces have been published. Among these are reports on SIMS analysis of flame-treated polypropylene and plasmafluorinated polyolefin surfaces (8.9Λ. In this paper some applications of static SIMS to a variety of modi fied polymer surfaces are described. They include plasma treatments in reactive and inert gases, corona treatment in air, as well as thermal and ion beam modifications of polymer-metal interfaces. The examples presented and discussed here primarily serve to illustrate the capabilities of static SIMS for the study of such surfaces and interfaces. More detailed discus sions of the actual chemical processes that proceed in several of the systems cited will be published elsewhere. Experimental Materials. Biaxially oriented polypropylene (PP) films of 50 um thickness were obtained from 3 M and have been described (9). P M D A - O D A (PI) was Kapton H polyimide from Dupont. Copper-plated P T F E films were obtained from Spire Corporation (Bedford, M A ) . They were prepared using the Ion Beam Enhanced Deposition (IBED) process in which a 100 nm thick C u film was vapor-deposited onto a P T F E substrate in the presence of a beam of 400 e V A r ions of 25 w A / c m (10). Shortly before SIMS analysis, the C u film was removed slowly by peeling at 90° in ambient conditions. Metal-coated PI films were prepared by sputtering 50 nm C r and 1 um C u onto a 50 um thick Kapton film on both sides. Thermal annealing was performed in a vacuum chamber at 2xl0" torr using a quartz lamp as the heating source. The samples were held for 15 min at the desired temperature and then cooled down to ambient temperature inside the chamber for about 2 hours. Just prior to SIMS analysis, the metal films were peeled slowly at 90° and then immediately introduced into the vacuum chamber of the instrument. +
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Plasma and Corona treatments. PP was treated in a glow discharge of air, A r or H at 820 V , 60 H z and 13 Pa pressure for various lengths of time. The glass bell jar was cleaned by the glow discharge for several hours prior to exposure of the films (11). The treated films were exposed to the atmosphere for several hours and then analyzed. The PI films were plasma treated for several minutes in argon at 200 mbar, 40 V and 60 Hz. They were stored for two days in air before SIMS analysis. Corona treatment was performed on an industrial unit as described previously (4). Normalized energies of 0.8 J / c m and 12.7 J / c m were used and treatments at these energies were done at high and low relative humidities (>75% and
