AVS1997 Session AS-WeM: Polymer Films and Modified Polymer Surfaces

Wednesday, October 22, 1997 8:20 AM in Room J2

Wednesday Morning

Start	Invited?	Item
8:20 AM		AS-WeM-1 An In Situ IR+visible Sum Frequency Spectroscopic Study: Polymer Surface Structure and Composition Changes in Response to an Aqueous Environment D. Zhang (Lawrence Berkeley National Laboratory); R. Ward (Polymer Technology Group, Inc.); Y.R. Shen, G.A. Somorjai (University of California, Berkeley) IR+visible sum frequency vibrational spectroscopy has been used to monitor changes in the structure and chemical composition of polymer surfaces when in contact with air and water, and striking spectral changes were observed as the interface was altered. The polymer studied is of polyurethane type, grafted with poly(dimethylsiloxane) (PDMS) as end groups. Our results show that in air, hydrophobic PDMS segments segregate at the polymer surface. In contrast, when the polymer is immersed in water, the hydrophilic segment of the polymer chain outdiffuse to the polymer surface and the hydrophobic PDMS tails migrate into the polymer bulk. The hydration process takes about 25 hours to reach equilibrium. These changes are reversible upon dehydration but only take about 3 hours. Our findings demonstrate a very dynamic picture of the polymer surface and point to the need to characterize the polymer surface in its working environment in order to properly understand its surface properties.
8:40 AM		AS-WeM-2 Surface Composition and Morphology of Polyimidesiloxane Copolymers with Short Polydimethylsiloxane Segments Studied by ESCA and TOF-SIMS J. Zhao (State University of New York, Buffalo); S. Rojstaczer (Occidental Chemical Corporation); J.A. Gardella, Jr. (State University of New York, Buffalo) Due to their attractive properties, such as good adhesion, low dielectric constant, low moisture absorption, etc., polyimidesiloxane copolymers (PIS) are becoming increasingly important materials in the microelectronics industry for applications, such as encapsulants and die attach adhesives. The characterization of the surface structure and composition as well as the resultant adhesive properties of polyimidesiloxanes is the focus of the present work. Specifically, the surface composition and morphology of a series of polyimidesiloxane copolymers have been determined by angle-dependent ESCA and Time of Flight SIMS. The copolymers are based on α,ω-aminopropylpoly(dimethylsiloxane) (PDMS), 2,2-bis(4-[4-aminophenoxyl]phenyl)propane (BAPP) and 4,4’-oxydiphthalicanhydride (ODPA). The effects of PDMS segmental length and PDMS content on the surface composition and morphology were investigated. Experimentally, Electron Spectroscopy for Chemical Analysis (ESCA) was used to acquire composition information at the copolymer surface. Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) was used to describe the segment length distribution of the copolymer. In all cases, PDMS was segregated to the top 100Å at the film (c.a. 15 micron thick) even with a short segmental length and low siloxane concentrations. It was also found that, with the same siloxane content, a longer siloxane segment gives a surface richer in siloxane. For a given siloxane segment length, varying the siloxane content does not change the surface composition.
9:00 AM		AS-WeM-3 The Role of Surface Chemistry on the Intercoat Adhesion Performance of Polyester/Melamine - Epoxy Paint Systems L.P. Haack, J.W. Holubka (Ford Motor Company) An X-ray photoelectron spectroscopy (XPS) study was conducted to determine the effect of the environment within a paint bake oven on the surface chemistry and subsequent intercoat adhesion performance of a model polyester/melamine - epoxy paint system. Two types of bake ovens are utilized in manufacturing operations to cure paint: 1) direct-fired ovens, which cure by using the hot combustion exhaust of natural gas, and 2) indirect-fired ovens, which cure by using air heated through heat-exchangers. Consistently better intercoat adhesion performance is observed when the epoxy coating is cured in a direct-fired oven prior to application of the polyester/melamine-based coating. Surface analysis using XPS has confirmed that crater-control additives present in certain epoxy-based coatings segregate to the surface during cure, where they form overlayers that are not amenable to painting. Nitrogen oxides and water, formed as a result of combustion processes within a direct-fired oven, are found to reduce or remove this overlayer, likely in oxidative, hydrolytic processes, respectively, exposing a chemically modified surface which is more reliably painted. This talk will attempt to elucidate the chemical mechanisms governing the surface modifications induced by direct-fired ovens, as well as describe briefly the role of polyester/melamine chemistry and its influence on intercoat adhesion performance.
9:20 AM		AS-WeM-4 Quantitative Uses of the Valence Band Region in the Analysis of Polymer Blends E.A. Thomas, J.E. Fulghum (Kent State University) The valence band region can be used like a fingerprint spectrum in the XPS analysis of polymers. Valence band spectra can be particularly useful in the analysis of blends of polymers which may be difficult to distinguish based on the core spectra alone. This region has been underutilized, however, due to long acquisition times resulting from signal intensities which can be a factor of 30x lower than in the core spectral region. Recent XPS instruments with high intensity monochromatic x-ray sources, efficient collection optics and multichannel detection systems have dramatically reduced the time required to collect valence band spectra. This talk will focus on the determination of surface composition using valence band spectra. Several different systems will be discussed, including: binary blends of miscible poly(butyl methacrylate) isomers used in a study of liquid crystal alignment layers; PVC/PMMA blends which phase separate; and acrylate copolymers whose surface composition has been quantified using TOF-SIMS. This work has been supported by NSF CHE-9631702, NSF DMR89-20147, and 3M.
9:40 AM		AS-WeM-5 Trifluoroethanol Derivatization of Acrylic Acid-Methacrylic Acid Copolymers I. Yoon, D.G. Castner, B.D. Ratner (University of Washington) Derivatization reactions are used to enhance surface analysis techniques and to functionalize surfaces. When using derivatization reactions the extent of reaction, diffusion limitations, and stability of the derivatized surface must be evaluated. For example, vapor phase trifluoroethanol (TFE) derivatization of the carboxyl acid (COO) groups in poly(acrylic acid) (pAAc) goes to 100% completion within 10 hrs. In contrast, TFE derivation of the COO groups in poly(methacrylic acid) (pMAAc) only reaches 60% completion after 48 hrs. TFE derivatization of a series of random AAc:MAAc copolymers (90:10, 70:30, 50:50, 30:70, 20:80, and 10:90) was examined with XPS to determine whether the extent of reaction difference between pAAc and pMAAc was due to steric effects or different reactivities of the COO groups in the two polymers. Complete derivatization of the 90:10 and 70:30 AAc:MAAc copolymers was observed at both the 10 and 48 hr time points. This indicates that the COO groups of MAAc, when dispersed with AAc, have a similar reactivities to the COO groups in AAc. For copolymers with ≥ 50% concentrations of MAAc, the extent of reaction was significantly below 100%, especially at the 10 hour time point. However, the extent of reaction for all copolymers at both 10 and 48 hrs was always higher than the weighted average calculated using the extent of reaction observed on the two homopolymers. Thus, it appears the assessability of COO groups to TFE in MAAc is constrained compared to the assessability in AAc. Angle-dependent XPS and ToF static SIMS experiments were done to confirm diffusion limitations were not responsible for the lower extent of TFE derivatization in pMMA. In addition, experiments with a COO terminated self-assembled monolayer also exhibited incomplete reaction with TFE. Steric constraints should also be present in this film since the COO groups are closely packed into a two-dimensional array.
10:00 AM	Invited	AS-WeM-6 Polymer Surface Chemistry: How Characterization Drives Synthesis and Structure Property Development J.A. Gardella, Jr. (State University of New York, Buffalo) In our laboratory the study of fundamentals and applications of polymer surface chemistry are driven by developments in electron and vibrational spectroscopy and secondary ion mass spectrometry of polymer surfaces and organic and polymeric thin films. New developments in data analysis are also necessary to push polymer surface analysis to make measurements of relevance to polymer scientists. This paper will emphasize studies in four areas; all dealing with aspects of surface structure-property relationships for polymers. The effects of polymer chain length and end groups on the surface composition and adhesion properties of siloxane copolymers with urethane and polyimide structures are under study. Angle dependant XPS (ESCA) depth profiling and ATR-FTIR allow quantification of the extent of surface segregation in multicomponent polymers. ToF-SIMS is used to describe the segregation of surface segment length distributions. ToF-SIMS studies of the surface reactivity of hydrolytically degradable polymers will point out the need for quantitative information about molecular weight distributions of the polymer surface. Fundamental studies of SIMS ion formation mechanisms from thin polymer films formed by Langmuir Blodgett techniques reveal that the tertiary structure of polymers can be revealed by ion formation mechanisms. Polymer surfaces synthesized by lithographic covalent bonding of peptide based receptors or drugs are designed for specific cellular bioadhesion. Fluoropolymer surface chemistry developed in our laboratory relies on characterization by SIMS, XPS and FTIR.
10:40 AM		AS-WeM-8 Secondary Ion Emission from Polymer Surfaces under Ar⁺, Xe⁺ and SF₅⁺ Bombardment F. Kötter (Universität Münster, Germany); E. Niehuis (ION-TOF GmbH, Germany); A. Benninghoven (Universität Münster, Germany) We investigated characteristics of the molecular secondary ion emission from polymers under 10 keV Ar⁺, Xe⁺ and SF₅⁺ bombardment. Samples of PET, PP, PTFE, PS, PC, PMMA and PEG were investigated under static SIMS conditions. For PS and PC damage cross sections were determined, too. We used a time-of-flight mass spectrometer equipped with a pulsed EI-source, allowing the generation of mass separated primary ion beams. Spin coated multilayers or/and surfaces of compact pieces of the polymer materials have been studied. Changing from Ar⁺ to Xe⁺ and to SF₅⁺ bombardment we found for the characteristic molecular secondary ions a strong increase in the usful yield Y_u (up to a factor of 1000) in parallel with a much smaller increase in the corresponding damage cross section σ (up to a factor of 6). Both effects are more pronounced in the higher mass range. The increases in the corresponding transformation probabilities ranges between a factor of 5 and 50, depending on the polymer species, the sample preparation and the mass range. We did not find a comparable effect for monomolecular overlayers of organic molecules on Ag or Si surfaces. Considering the wide range of molecular ions which can be produced by electron impact the results presented here open new possibilities in yield control and yield optimization in static molecular SIMS, through an appropriate modification of the impact cascade.
11:00 AM		AS-WeM-9 Plasma-Phase Reactions and Plasma Polymer Product R.D. Short (University of Sheffield, United Kingdom) Inductively coupled, radiofrequency-induced plasmas of a number of organic compounds (propanoic and propenoic acids, allyl and propyl amines and allyl alcohol), operated at low electrical power (< 10 W), have been investigated using mass spectrometry (MS) and deposition rate measurements.¹ The solid, plasma-polymers were collected on silicon substrates and analysed by X-ray photoelectron spectroscopy (XPS). The positive-ion MS data indicate that species of the general form: (nM+H)⁺, and / or (nM-H)⁺, where M represents a unit of the starting material, are present, depending on compound. The relative abundance of these species was monitored as a function of the electrical power supplied to the plasma. No neutral oligomeric species were detected. XPS analysis reveals an inverse relationship between plasma power and retention of functionality in the solid product. Comparison of the plasma-MS and XPS results suggests that the above cationic oligomers are responsible for functional retention, whereas fragmentation events lead to the introduction of new functional groups. Mechanistic schemes are proposed. For allyl alcohol, the scheme is substantiated by means of a selected ion flow tube.² ¹1. L. O'Toole, R. D. Short et al.: J. Chem. Soc., Faraday Trans., 1995, 91, 3907-3913; J. Chem. Soc., Faraday Trans., 1997, 93 (6), 1141-1145; Macromolecules, 1996, 29, (15) 5172-1577. ²2. R. D. Short, L O'Toole, C. Mayhew, J. Chem. Soc., Faraday Trans., 1997, 93 (10), 1961-1964
11:20 AM		AS-WeM-10 Polymeric Surface Modification by Vacuum Ultraviolet Radiation From Low Pressure Plasmas A.C. Fozza, A. Bergeron, J.E. Klemberg-Sapieha (École Polytechnique, Canada); M. DiRenzo (Université de Montréal, Canada); M.R. Wertheimer (École Polytechnique, Canada) Low-pressure glow discharge plasmas are increasingly used as an effective method for the surface modification of polymers. Although vacuum-ultraviolet (VUV, λ < 200 nm) is an important component of the plasma environment, it's effects during plasma surface treatment of polymers are little explored and still not well understood ¹². We are using low-pressure microwave plasmas as broad-band VUV/UV light sources (H₂ plasmas), and as almost monochromatic VUV sources (Ar-H₂ and Ar-O₂ mixture plasmas), for the study of VUV/UV effects on various polymers. The lower wavelength limit of the radiation investigated is λ = 112 nm, the cutoff wavelength of the magnesium fluoride used as a window to separate the sample chamber from the plasma light source. We employ a quartz crystal microbalance (QCM) to measure, in situ and in real time the net mass change of polymeric (polyethylene, polymethylmethacrylate, polystyrene), or of high molecular weight oligomeric (hexatriacontane) films exposed to various VUV radiation fluxes. Measurements are 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 VUV-generated atomic oxygen (AO), parallel to the radiation flux. Following irradiation the samples are analysed by several surface-sensitive techniques: we determine the types of chemical groups on the polymeric surfaces by photoacoustic infrared (PA-FTIR), while X-ray photoelectron spectroscopy (XPS) is used to study the evolution of the oxygen content and of various functional groups. Atomic force microscopy (AFM) is used to investigate topological changes of the treated surfaces. Finally, spectroscopic ellipsometry is used to confirm etch rates by thickness measurements, and to study the evolution of optical properties which are related to morphological changes such as crosslinking in a subsurface layer ³. ¹A. Holländer, J.E. Klemberg-Sapieha and M.R. Wertheimer, J. Polym. Sci., Part A: Polym Chem., 33, 2013 (1995) ²Z.A. Iskanderova, J.I. Kleiman, Yu. Gudimenko and R.C. Tennyson, Journal of Spacecraft and Rockets, 32, 878 (1995) ³S. Vallon, R. Brenot, A. Hofrichter, B. Drévillon, A. Gheorghiu, C. Sénémaud, J.E. Klemberg-Sapieha, L. Martinu, F Poncin-Epaillard, J. Adhesion Sci. Technol., 10, 1313 (1996).