Papers

AVS1996 Session AS-FrM: Polymer Interfaces and Adhesion

Friday, October 18, 1996 8:20 AM in Room 105B

Friday Morning

Start	Invited?	Item
8:20 AM		AS-FrM-1 The Adsorption Isotherms of PMMA on Oxidised Metal Substrates by XPS S. Leadley, J. Watts (University of Surrey, England); C. Blomfield, D. Surman, S. Page (Kratos Analytical, United Kingdom) The manner in which ionic or molecular species adsorb on to solid surfaces can provide valuable information about surface properties and surface adsorbate interactions. The traditional way of following such processes is the construction of adsorption isotherms, originally devised to study gas phase adsorption but also derivable from solution adsorption using surface analysis methods. The use of XPS to construct adsorption isotherms was first reported for the adsorption of silane adhesion promoters onto hydroxylated iron surfaces\super 1\. More recently this method has been extended to determine the acidity of carbon fibre surfaces through exposure to silver and magnesium ions\super 2\. Adsorption isotherms have also been generated for PMMA on conducting polymers using Tof-Sims\super 3\. This study will show how XPS has been used to construct adsorption isotherms for a basic polymer (PMMA) and an acidic polymer (poly(acrylic acid)) on oxidised metal substrates of differing IEPS. It will be shown that the gradients of the Langmuir plots of the isotherms are related to the acid-base interactions between the organic probes and the oxidised metal substrates/super 2/. Using a Kratos AXIS 165 instrument high resolution spectra have been acquired of PMMA and poly(acrylic acid)) adsorbed on to the oxidised metal substrates. The small changes in binding energy shifts within the C 1s spectra have been attributed to acid-base interactions between the polymer and the substrate. References 1. R Bailey, JE Castle, J.Mater.Sci, 12 4647 (1977) 2. CA Baillie, JF Watts, JE Castle, J.Mater.Chem 2 939 (1992) 3 M-L Abel, MM Chehimi, AM Brown, SR Leadley, JF Watts, J.Mater.Chem, 5 845 (1995).
8:40 AM		AS-FrM-2 Time Resolved Transient Current Measurements during Metal/Polymer Tribological Loading L. Scudiero, S. Langford, J. Dickinson (Washington State University) An important component of friction during rubbing of two surfaces arises in the rapid, transient making and breaking of adhesive bonds between asperities. When metals are drawn across polymers and inorganic crystalline materials, the continuous detachment between the two surfaces generates charge separation due to contact electrification. We have devised fast, sensitive circuits for detecting transient currents generated by this process while simultaneously measuring the normal and lateral forces as a conducting tip is moved across these surfaces. The experiments are performed in vacuum as well as under controlled atmosphere. We relate these currents to the extent of damage to the substrate as well as the contribution of adhesion to the frictional force. Recent measurements on the contact of oxide coated Al with thin layers of Fromblin-Zdol (a lubricant used on disk surfaces in magnetic storage devices to combat stiction) indicates substantial charge transfer of electrons to the lubricant. These measurements along with simultaneous detection of energetic electron emission support an earlier proposed lubricant degradation mechanism involving electron-molecule interactions.
9:00 AM		AS-FrM-3 Vapor Deposited PMDA/ODA Polyimides: Relationship of Adhesion to Interface Structure R. Dillingham, F. Boerio, W. Zhao (University of Cincinnati) PMDA/ODA polyimides are typically deposited using spin-coating techniques. They may also be deposited using a solventless vapor phase technique by sublimation of the monomers in vacuo, condensation onto a substrate, and subsequent curing to a high MW polyimide. Films of PMDA, ODA, and PMDA/ODA of various thicknesses were deposited onto clean aluminum surfaces and analyzed using Relection-Absorption Infrared Spectroscopy, XPS, and Surface Enhanced Raman Scattering. The adhesion of these films was measured using a circular blister test. An embedded cohesive zone finite element analysis was used to decouple plastic deformation processes from the work of adhesion to allow determination of the fracture energy required to delaminate the film. The failure surfaces were analyzed for composition and structure using XPS and RAIR. The structure of the polymer-substrate interface region was shown to be significantly different for vapor phase deposited PMDA/ODA films than for solvent-cast films. Monomer films were shown to adsorb in specific orientations. Films of the co-deposited monomers on the order of a few monolayers in thickness consist of unreacted monomer. Thicker films show condensation to polyamic acid. Co-deposited monomer films cure readily to form polyimides without the carboxylate formation at the interface that is seen in thin spin-coated films. Approximately 50% of the energy of fracture during the blister test is expended in plastic deformation of the films. Failure analysis shows crack propagation through a well-cured zone close to the metal oxide surface which is devoid of the carboxylates seen in spin coated film interfaces on many substrates.
9:20 AM		AS-FrM-4 Plasma Sprayed Coatings as Treatments for Aluminum, Steel, and Titanium Adherends G. Davis (Dacco Sci, Inc.) Plasma sprayed coatings are being developed as pretreatments for aluminum, steel, and titanium adherends. The plasma spray process allows a wide variety of coatings to be deposited onto an equally wide variety of substrates and, hence, the coatings can be engineered for different applications. Because there are no liquid or vapor wastes for disposal, the process has several environmental advantages to conventional treatments that use chromates, strong acids or bases, or other hazardous materials. Bonds prepared using plasma spray coatings on aluminum adherends perform better under humidity conditions than those prepared by the Forest Products Laboratory (FPL) process and similar to those prepared by the phosphoric acid anodized (PAA) process, at least for some adhesives. Bonds prepared using plasma sprayed coatings on titanium perform equivalently to the best chemical treatments under moderate conditions with failure within the adhesive. Following exposure to high temperatures, the plasma sprayed joints exhibit performance superior to conventional treatments. For steel substrates, the plasma sprayed coatings protect from corrosion in addition to providing a good bonding surface. Rubber-steel and epoxy-steel bonds fail within the polymer even if the steel has been previously exposed to salt fog or marine beach environments. Additionally, the plasma sprayed steel bondline is less susceptible to surface contamination than bondlines prepared using grit blasted steel. Some of this work was performed under contract F33615-93-C-5324 with the Wright Laboratory Materials Directorate at Wright-Patterson AFB, OH. It was partially funded by the Strategic Environmental Research and Development Program (SERDP).
10:00 AM		AS-FrM-6 In Situ Vibrational Analysis of a Lubricant/Silver Interface J. Miragliotta, R. Benson, T. Phillips (Johns Hopkins University) Results are reported from an in-situ vibrational investigation of a lubricant layer/silver (Ag) flake system which is used in Ag-filled conductive adhesives. The investigation focused on the interaction between stearic acid and the surface of an Ag flake using the temperature conditions that are employed in the curing of polymer adhesives. The characteristics of the stearic acid/Ag surface were probed with surface enhanced Raman scattering (SERS) and IR-visible sum-frequency generation (SFG). The room temperature SERS spectra exhibited vibrational bands that were consistent with an adsorbed carboxylate rather than a carboxylic acid species which was bound via the oxygen atoms in the terminal group. The adsorbate remained stable for temperatures up to 150\super o\C. Above this temperature, modes associated amorphous carbon became evident in the spectra, indicating a partial decomposition of the carboxylate species. The C-H stretching modes of a stearic acid layer adsorbed on a smooth Ag surface were also examined with SFG. These C-H modes, associated with the terminal methyl and adjacent methylene group, exhibited a similar temperature dependence as that observed for the carboxlyate end group in the SERS data, i.e., their intensity was markedly and irreversibly decreased for temperatures above 150\super o\C.
10:20 AM		AS-FrM-7 Plasma Polymerized Acetylene as a Primer for Rubber to Metal Bonding Y. Tsai, F. Boerio (University of Cincinnati) Plasma polymerized (PP) acetylene primer films have been shown to be excellent primers for rubber to metal bonding. Lap joints prepared using natural rubber as an "adhesive" for steel adherends primed with plasma polymerized acetylene films show breaking strengths of about 31 N/mm\super2\ with 100% cohesive failure in the rubber. Reactions occurring in the "interphase" between the plasma polymerized primers and natural rubber were simulated using a model "rubber" based on squalene. Steel samples, primed with PP acetylene or deuterated acetylene, were immersed in the model rubber mixtures. Subsequently the samples were analyzed by reflection absorption infrared spectroscopy (RAIR). The plasma-polymerized deuterated acetylene film provided a frequency shift in the infrared spectra showing the nature of the complex reactions in the model rubber system. Pendant perthiomercaptide groups were found in the interphase. These groups disappeared as the reaction progressed, introducing crosslinks between the plasma-polymerized primer film and squalene. The reaction mechanism between the primer film and the model rubber compounds involved reactions through pendant methyl groups in the plasma polymerized acetylene film. A similar structure, possibly pendant -CH\sub3\ groups and double bonds alpha to the tertiary carbon atom, exist in both plasma-polymerized acetylene film and natural rubber. As a result, the reaction mechanism between the plasma-polymerized primer films and natural rubber compounds is similar to the self-vulcanization of rubber.
10:40 AM		AS-FrM-8 Study of Poly(ether sulfone)/Metal Interfaces by High Energy XPS and X-ray Absorption Spectroscopy N. Tegen (University of Surrey, United Kingdom); J. Watts (University of Surrey, England) The metallisation of high temperature resistant polymers like Poly(ether sulfone) (PES) is important especially for microelectronic purposes. The adhesion of metals to these polymers can be greatly improved by using ion implantation. The low ion doses required to improve adhesion, however, make it difficult to determine the mechanism of the adhesion enhancement. This work studies the buried interfaces of ion implanted and as-deposited metal films (Al and Cu) on PES. The UK Synchroton Radiation Source, SRS, at Daresbury Laborotory was used to provide tunable photon energies in the range of 2100 eV up to 4600 eV. Thus, the interface can be probed to greater depth than usual without being destroyed by peeling or sputter profiling. Adhesion improvement as measured by mechanical testing can thus be linked to the chemical changes occuring at the interface of the system. We expect these findings to be complemented by additional X-ray absorption spectroscopy (NEXAFS and SEXAFS) studies) of silver-covered PES films. These experiments are designed to give a deeper understanding of the interaction of individual metal atoms with the polymer surface.
11:00 AM		AS-FrM-9 Irradiation of Polymers by Vacuum-Ultraviolet Radiation from Low Pressure Plasmas A. Fozza (Ecole Polytechnique, Canada); A. Holl\um a\nder (Fraunhofer - Institute for Applied Polymer Research, Germany); J. Klemberg-Sapieha (Ecole Polytechnique, Canada); A. Kruse (Fraunhofer - Institute for Applied Materials Research, Germany); J. Roch, M. Wertheimer (Ecole Polytechnique, Canada) The emission from low-pressure microwave plasmas of various gases and gas-mixtures, in the VUV (\lambda\ < 200 nm) and UV (200 nm < \lambda\ < 400 nm) regions was investigated in order to use these plasmas as light sources for the study of the VUV/UV effects on various polymers (polyethylene, polypropylene, polyethyleneterephtalate, polymethylmethacrylate, etc.), or high molecular weight oligomers (hexatricontane). We have employed a quartz crystal microbalance (QCM) in order to study in-situ the net mass change of the polymeric films exposed to UV/VUV radiation originating from various plasmas. The lower wavelength limit of the radiation investigated was \lambda\ = 112 nm, the cutoff of magnesium fluoride used as a window to separate the sample chamber from the plasma light source. Measurements were made with the specimens in vacuum, or immersed in low-pressure oxygen, directly exposed to the VUV/UV (perpendicular to the radiation flux), or only to the VUV-generated atomic oxygen, AO (parallel to the radiation flux). We have observed that the mass loss (etching rate), dM/dt, depends strongly on the radiation energy and on the oxygen pressure in the sample chamber, for the case of plasmas that have intense emissions in the spectral region where O\sub 2\ possesses a strong absorption (130 nm < \lambda\ < 170 nm). The etch rate under VUV/UV alone or AO alone was found to be much less than that observed under simultaneous action, which provides conclusive evidence of the synergistic effect proposed by Holl\um a\nder et al. [1]. Following irradiation, samples were analyzed by X-ray photoelectron spectroscopy (XPS) and by surface-sensitive FTIR techniques, in order to study the evolution of the oxygen content and of various functional groups, and by AFM for surface topological changes. [1] A. Holl\um a\nder, J.E. Klemberg-Sapieha and M.R. Wertheimer, J. Polym. Sci., Part A: Polym Chem., 33, 2013 (1995).
11:20 AM		AS-FrM-10 Surface Characterization of the Modification of Polymer Surfaces using UV and Ozone Treatments N. Dontula, C. Weitzsacker, L. Drzal (Michigan State University) Surface modification of polymer surfaces resulting in better wettability has been fount to improve the adhesion between polymer surfaces and coatings such as paint and adhesives. Various methods of surface modification are possible, such as flame treatment, corona discharge, wet chemical modification, UV light and ozone treatments. The goal of the study was to explore the effects of short wavelength UV and optimize the treatments for best results under conditions adaptable to fast manufacturing. Variables such as exposure times, distance from UV source and environmental conditions such as humidity were explored. Time studes were undertaken to explore surface modification stability in ambient conditions. The effects of the treatments were characterized by both XPS and contact angle measurements. Experimental difficulties in both the treatments and analysis, and how they were overcome, will be discussed.