Report Alan Dilks Xerox Webster Research Center Xerox Square W-114 Rochester, N.Y., 14644
POLYMER Paints, glues, building materials, fabrics, packaging, electrical components and devices, power cable insulation, photographic film, magnetic tape, optical fibers, boxes, casings, biomedical implants, and so on—the number of applications of polymers in our modern-day world has grown to gigantic proportions. Polymers are light in weight and easily molded, extruded, or machined, often into quite intricate shapes. They have a wide variety of useful chemical and physical properties, and the combined effect of these qualities has resulted in their dramatic emergence as one of the most important classes of material on which modern society is built. The choice of polymer is governed by the particular application. In some cases the polymer is specifically selected for its surface characteristics. For example, polytetrafluoroethylene is often selected for its extremely low surface free energy or "nonstick" characteristics. Most often, however, a polymer is chosen for its bulk properties—superior strength, or optical or dielectric properties, for example. Whatever the reason for its choice, the polymer must be compatible with its surroundings. What's the use of a paint that soon peels from the painted object? What's the use of a tough, flexible photographic film backing that the emulsion will not coat uniformly? What's the use of a biomedical implant that the body rejects? Since a given polymer always interfaces with its surroundings via its surface we must be able to examine polymer surfaces and ask: • What is the structure of the surface? • Is the surface composition the
same as that of the bulk material? • What contaminants are present at the surface? • How do reactions at the surface differ from those occurring in the bulk material? Classically, chemical information about polymer surfaces has been inferred through physical characterization. For example, fluid contact angles measure wettability (surface free energy), peel testing measures adhesive strength, and profilometry measures surface roughness. While these methods measure quantities that may be related to chemical structure, they do not provide direct chemical information. More recently the use of multiple attenuated total reflectance infrared (MATR-IR) spectrometry has greatly improved matters in this direction, although the signals are typically averaged over a depth of many hundreds of angstroms and a true surface characterization on the scale of one to five monolayers is not yet possible. More recently still, the surface-enhanced Raman technique has been shown to provide detailed chemical information within a depth scale of ~100 Â. This latter technique may well be of great importance in the near future, but at the present time it is not routinely available. The surface-sensitive electron and ion spectroscopies based on high vacuum systems, therefore, are the major candidates for the chemical analysis of polymer surfaces. However, we must first consider the issue of sample degradation before making a final choice of technique. Of all the surface analysis techniques available today, only a limited number are of use when studying organic materials. This arises from the
802 A · ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
somewhat fragile nature of organic materials exposed to energetic particles or ionizing radiation. Generally speaking, the damage induced in a polymer surface by an incident beam increases in the order: photons < electrons < ions. By far the vast majority of surface characterizations of polymers have therefore been accomplished using photons as the incident radiation, with the detection of ejected electrons. X-ray photoelectron spectroscopy (XPS or ESCA), ultraviolet photoelectron spectroscopy (UPS), and X-ray-induced Auger electron spectroscopy (XAES) are the primary surface analysis tools available for polymers, of which ESCA is the most widely used due to its versatility and the comparative ease of interpretation of the data obtained. The interaction of a beam of energetic electrons with a polymer surface causes severe degradation. Detailed chemical information on such surfaces is therefore not forthcoming from conventional, electron-induced Auger electron spectrometry (AES), although this technique has been used on occasion, giving gross elemental information for organic systems. A similar situation might be anticipated for secondary ion mass spectrometry (SIMS), where energetic ions are the incident radiation. Indeed this is precisely the case in the conventional, dynamic SIMS experiment where extensive cross-linking and molecular rearrangement, preferential sputtering, and migration of fragments into the subsurface are found when studying polymers. However, a greater ultimate sensitivity than ESCA, and the ability to detect hydrogen and molecular fragments not readily identi0003-2700/81 /0351-802AS01.00/0 © 1 9 8 1 American Chemical Society
SURFACES
I , ,ι»
ι
.im.iimii.ii . « « . Μ ·
Binding Energy (eV)
Figure 1 . C o r e level s p e c t r a of N y l o n - 6
fied by ESCA, have provided an impe tus for the development of a less de structive form of SIMS, namely static SIMS. In this mode, low primary ion energies (0.5-2 kV) and an extremely low ion current ( ~ 1 0 - 8 A cm - 2 ) mini mize surface damage. Under these conditions, it can be shown that while the interaction of an ion with the or ganic material causes damage, the probability of a second ion impacting in close proximity to the damaged area, during a typical analysis time, is close to zero. This approach has proved successful with condensed small organic molecules but is still in its infancy in the study of polymer molecules. As will emerge from the examples discussed later in this article, ESCA is most often the technique of choice for studying the structure and reactivity of polymer surfaces. Surface Analysis of Polymers by ESCA. Due to several considerations, polymers are more straightforwardly studied by ESCA than are many inor ganic materials. First, it is a fact that in a high vacuum system (or else where) the sticking coefficients for po tential contaminant species (e.g., oxy gen, water, hydrocarbons) are general ly somewhat smaller at organic sur faces than at inorganic surfaces. There are therefore much less stringent vac uum requirements for the study of polymer surfaces. Thus, while pres sures of 1CT10 or ΙΟ" 11 torr (10- 8 -10- 9 Pa) are routinely employed when studying, for example, metal surfaces, we may satisfactorily study polymers in a vacuum of only ~ 1 0 - 8 torr ( 1 0 - 6 Pa). That is, less elaborate pumping systems may be used, and samples can be introduced from the atmosphere to the sample analysis chamber via inser tion mechanisms based on Viton or Teflon seals. These points are very im portant when throughput and cost of analysis are being considered. Second, the lineshapes obtained in
the ESCA analysis of the core levels of the first- and second-row elements, commonly found in polymers, tend to be symmetric and the linewidths are not usually dependent on chemical en vironment. This is in contrast to the situation found in studying inorganic materials. For example, the spectra of transition metal compounds are often quite asymmetric due to multiplet splitting and shake-up phenomena, both of which depend on chemical en vironment. This well-behaved nature of signals derived from polymers al lows the component signals to be ap proximated by Gaussian lineshapes, and quite detailed curvefitting be comes possible. This is important since ESCA possesses an inherently small chemical shift/linewidth ratio, which usually results in only partially resolved spectra. Finally, for electrically insulating samples such as polymers, it is gener ally the case that the electron flux photoejected from the sample is only partially balanced by the flux of lowenergy secondary electrons arriving at the sample surface from its surround ings. A pseudoequilibrium condition results in which the sample usually possesses an overall positive charge. The magnitude of this charge is of the order of 2-15 V for spectrometers employing a nonmonochromatic X-ray source, and is manifested in a shift in the energy scale in the spectrum by an amount equal to the charge. To accu rately determine binding energies, some form of referencing is necessary. The most widely used method for polymers involves the monitoring of the Cis level of the extraneous hydro carbon contamination, which builds up on the sample to detectable levels when it has been in the spectrometer for many hours. The binding energy of this reference is commonly found to be 285.0 eV. This ability to correct for and measure charging effects for poly mers has led to the use of sample
charging as an extra information level. Let us now briefly consider the ESCA experiment in a little more de tail before discussing examples of its impact on polymer surface analysis. ESCA, in which the binding ener gies of electrons in molecules are mea sured, is accomplished by photoejecting the electrons with a monoenergetic beam of soft X-rays and measuring their kinetic energies. The binding en ergy for a given electron is then de rived from the difference between the photon energy and the electron's ki netic energy. In this way the binding energies of both core and valence elec trons can be determined, but, because of the low cross sections for photoionization of the valence levels, the pre dominant emphasis has been in the study of core levels. The absolute binding energies of the electrons in a given element are characteristic of that element, and ESCA in its sim plest form can be employed to pro duce an elemental analysis of a sur face. Differences in electronic environ ment of a given atom give rise to a small range of binding energies (i.e., chemical shifts) that are often repre sentative of a particular structural feature. Figure 1, for example, shows high resolution spectra in the Oi s , Ni s , and Ci s regions for a Nylon-6 film. While the Oi s and Ni s spectra show symmetrical peaks containing single components, the Ci s spectrum exhib its a distinct fine structure. The Ci s spectrum can be resolved into three components at binding energies of -287.8, 286.3 and 285.0 eV, with rela tive intensities of 1:1:4, respectively. These components are assigned as shown in the figure to particular fea tures in the polymer. Although the incident X-ray may penetrate several thousands of ang stroms into the polymer, the mean free paths of the photoemitted elec trons in the energy range of interest (i.e., 300-1500 eV) are only of the order of a few tens of angstroms. It is only those photoemitted within the outermost layers of the material, therefore, that escape the sample without energy loss. This provides ESCA with its extreme surface sensi tivity. For example, for Ci s levels irra diated with MgK„i,2 photons, 95% of the observed signal intensity derives from about the top 50 Â of the sample, corresponding to an electron mean free path of ~ 1 5 Â. For a homogeneous material the signal intensity observed for a given core level can be described by the equation /, = F^N^Xi
(1)
where F is the X-ray flux, a is the cross section for photoionization in the direction of the analyzer, Ν is the number of atoms in a given volume el-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981 · 805 A
Analyzer
Figure 2. Substrate/overlayer model showing the effects of θ
ement, k is a spectrometer factor and λ is the electron mean free path, which depends on the kinetic energy of the photoemitted electron as well as the material through which it passes. For an inhomogeneous sample a sit uation commonly encountered is that shown in Figure 2, in which a homoge neous substrate has an overlayer of thickness d. The expressions for the signal intensity of a core level either in the substrate or the overlayer can be shown to be as follows: For the surface layer: 1° = FatiNikiMl
- exp(-d/\i
cos Θ)] (2)
For the substrate or bulk: /? = FctiNikiXi exp(-d/\i
cos Θ) (3)
d/cos θ is the effective overlayer thick ness in the direction of the analyzer, where θ is the electron take-off angle.
The signal intensities derived from features at the very surface of the sample are therefore relatively en hanced in going to grazing electron take-off angles (0 -» 90" in Figure 2). What is the Structure of the Sur face? As we have seen in Figure 1 the determination of the surface structure of polymers by ESCA is relatively straightforward and bioengineers, among others, have taken advantage of this. Many hydrated, cross-linked hydrophilic polymers (hydrogels) have been found to exhibit biocompatibility. The poor mechanical strength of these materials, however, has dictated that for most applications they must be covalently grafted to the surface of polymers with more suitable mechani cal properties. Applications have been proposed in which the hydrogel is in troduced into the tissue and subse quently allowed to swell and hydrate in situ. Since events that occur in the first few seconds (e.g., protein adsorp tion) may well determine the success or failure of the implant, a detailed knowledge of the surface structure under conditions of hydration, dehy dration, and rehydration is clearly of considerable importance. Figure 3 shows the Ci s spectra of a sample in which poly(2-hydroxyethyl methacrylate) (HEMA) has been ra diation-grafted to the surface of a Silastic film, both in the hydrated and dehydrated states. Also shown are the Si/C and O/C elemental ratios derived from the total SÎ2P, Oi s and Ci s intensities. As the hydrogel (HEMA) is dehydrated, the amount of silicon at the surface increases. Since silicon is only present in the Silastic substrate this observation suggests some rearrangement in which more of the Silastic is
C 1 s Spectra
Elemental Ratios
H yd rated 0.18
0.42
Surface Interior
Dehydrated
0.61
0.52
Surface Interior
Binding Energy (eV)
Figure 3. ESCA data for HEMA grafted on Silastic 806 A · ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
exposed after dehydration. The upper Cis spectrum in Figure 3 corresponds to a convolution of the spectra of HEMA, Silastic, and a small amount of hydrocarbon contamination. The component at ~285.0 eV is due to the contaminant and Silastic, whereas the higher-binding energy components are due to Ç—Ο (~286.6 eV) and 0 = C — Ο (~289.0 eV) in HEMA. On dehydration the contribution to the spectrum of these high-binding energy components is reduced to a small level. This is consistent with the con clusion that the concentration of HEMA graft at the surface of the Silastic film decreases upon dehydra tion. The conclusions drawn from the ESCA analysis can be represented by the models shown in Figure 3. The hy drated graft polymers are found to have a significant quantity of the hy drogel at the surface intermixed with silicone polymer chains. Upon dehy dration the experimental measure ments can be rationalized by graft chains penetrating ~30-50 Â into the silicone rubber. Rehydration of the film failed to reproduce the original surface, but the rehydrated film showed the same trend as the original film on dehydration. This type of information is very useful when considering dehydration of these systems and rehydration in vivo. A class of materials for which investigation by ESCA is particularly advantageous over most analytical techniques is that produced when organic vapors are passed through an electrical discharge, namely plasma polymers. ESCA has been found to be complementary to infrared studies on these thin films. ESCA is particularly suited to the study of fluorinated plasma polymer systems. Infrared spectrometry, however, often reveals only broad unresolved bands for these materials, and is better suited to the study of hydrocarbon-based polymers. For plasma polymers containing other hetero-atoms (e.g., Ν or O), valuable structural information can be ob tained from both techniques. ESCA, however, has the additional advantage of being able to routinely study very thin films in situ, for applications in thin-film technology and device fabri cation. Furthermore, in the construc tion of multilayer systems, a knowl edge of the surface chemistry is im portant to overcome problems of ad hesion. The combination of ESCA analysis of the solid phase and mass spectrometric analysis of the gas phase in fluorocarbon plasmas has made it possible to identify processes that lead to film formation in some systems. Figure 4 shows the Fi s and Ci s spec tra for the plasma polymers formed in three separate experiments in which
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Figure 4. F 1s and C 1 s spectra of three plasma-polymerized fluorocarbons
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Figure 5. Total C1s intensity •*• shake-up intensity, vs. n for the styrene-methylene copolymers. Horizontal lines represent surface CIRCLE 145 ON READER SERVICE CARD
808 A · ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
fluoroethylene, 1,1-difluoroéthylene and trifluoroethylene were the injected gases or monomers, respectively. These spectra are very complex, and this is typically the case when studying plasma polymerized fluorocarbons. The discussion of data of this complexity is aided by the use of curve fitting procedures, and the blue curves in Figure 4 represent a lineshape analysis of the original data (red curves), obtained using an analog curve resolver. This is the most convenient and reliable method, and the peak assignments are shown. Each spectrum reveals a variety of carbon environments present in the respective plasma polymers, and their structures are very unlike their conventionally polymerized linear counterparts. The relative quantities of CF 3 , CF 2 , CF, and carbon with no fluorine nearest neighbors are readily derived from the ESCA data, as are the F/C stoichiometries. This allows the film structure to be monitored as a function of the injected material and, as is apparent even from a cursory examination of Figure 4, the surface chemistries of the three plasma polymers are very different. This affords them different surface properties. As we go along the series of fluoroethylenes we can observe distinct trends in the structure of the polymer. Thus, as the fluorine content of the injected material increases so does that of the polymer, and the CF2 and CF3 contents increase. These types of measurements on thin plasma polymer films and coatings allow materials to be rapidly screened for a particular application and, with the use of additives in the plasma, allow materials to be specifically modified to give the desired surface characteristics. Is the Surface Composition the Same as the Bulk Material? For homopolymers having short repeat units the answer to this question is generally yes, if we neglect physical effects such as segregation of low molecular weight material to the surface of a solvent-cast film. However, many examples arise in polymer science where the composition of the surface does indeed differ from t h e bulk material. These are, generally speaking, found when the system is composed of long repeat units or segments. Let us first consider a series of copolymers derived from styrene and methylene units of general formula, [CH0—CH 2 —CH 2 — CH0—(CH 2 )„] m where η = 1, 3, 5, 6, 10. Since the parent polymers of these systems, polyethylene and polysty rene, are purely hydrocarbon, their ESCA spectra show only one direct photoionization peak at 285.0 eV in the Cis region. The same is also true for the copolymers. Polyethylene and polystyrene can be distinguished by
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981 · 809 A
ESCA, however, since polystyrene ex hibits a shake-up satellite centered ~6.6 eV higher on the binding energy scale than the direct photoionization peak. The intensity of the polystyrene shake-up satellite is ~8.1% of the in tensity in the Ci s spectrum due to the core levels of atoms that are part of or directly attached to the phenyl ring (i.e., ~7.1% of the total Ci s intensity). This shake-up arises from the finite probability of simultaneous excitation of the valence electrons from π to π* levels with photoionization of these carbon atoms. Since these transitions are effectively localized on a given pendant phenyl group, the intensity of the shake-up satellite can be used as a direct measure of the relative number of styrene units at the surfaces of the copolymers. Figure 5 shows a plot of the intensi ty of the direct photoionization peaks at 285.0 eV divided by the shake-up intensity, vs. the length of methylene chain, n. The white line represents the calculated values derived from the shake-up intensity of polystyrene and assuming an additive model. The dif ference in the slopes of the two straight lines is explained in terms of specific orientation effects of the poly mer chains in the outermost ~100 Â. For η > 1 the samples show less sty rene units at the very surface than an ticipated, and the deviation from the expected values increases with n. In the absence of detailed information on the surface structure of these systems we may only speculate as to the possi bilities that might give rise to these observations. The most likely situa tion is that in which the surface con sists of a folded chain structure having the poly-methylene segments specifi cally oriented at the surface. This is shown schematically in Figure 5 for η = 3, 6 and 10. For the copolymers with long repeat units, therefore, the surface is effectively less rich in sty rene units than the bulk material. Differences in surface and bulk composition have also been noted in block copolymer systems. It is known that the components of polystyrenepolyethylene oxide (PS-PEO) copoly mers are highly incompatible and that they undergo microphase separation and domain formation. This bulk mor phology is strongly influenced by the solvent from which the films are cast, since some solvents are in fact only a solvent for one component and a nonsolvent for the other. For the exam ples of PS-PEO diblock copolymers, ethylbenzene is a preferential solvent for PS, nitromethane a preferential solvent for PEO and chloroform a common solvent for both phases. Casting of films from these solvents therefore causes profound differences in the bulk morphology and domain
0 1 s Region
C 1s Region
From CHCl3
From C2H5*
From CH3NO2
Binding Energy (eV)
Figure 6. Core level spectra of the PS/PEO diblock copolymer, 21.4 mole % PS in the bulk, cast from different solvents
structure. The question then arises of what effects can be seen at the copoly mer surfaces. Figure 6 shows the ESCA spectra in the Ois and Ci s regions of a PS-PEO diblock copolymer, 21.4 mole % PS, cast from these three solvents. The signal at 285.0 eV in each spectrum is due to the carbon atoms in the purely hydrocarbon PS component while the Cis component at ~286.5 eV and the Ois signal are due to the core levels in the PEO component. The mole ratio of the two components at the surface can be directly obtained from the rela tive intensities of the Ci s signals (cor rected for the numbers of carbon atoms in the repeat units) and also from the Oi s /Ci s total intensity ratios. As can be seen from Figure 6, the mole % of PS at the surface is in each case very different from that anticipated from the bulk composition, and shows a dependence on the solvent from which the film is cast. A careful analy sis of the dependence of the ESCA spectra on the electron take-off angle shows the data cannot be explained in terms of an overlayer of the PS com ponent, but rather by the model shown schematically in Figure 6, in which columns of one component are
810 A · ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
embedded in a continuous phase of the other. What Contaminants Are Present at the Surface? It is well known that under certain process and/or environ mental conditions—heat, radiation, pressure, etc.—additives may undergo diffusion from a polymer. This phe nomenon is of considerable impor tance in three respects. First, the loss of a particular additive from the mate rial might cause serious degradation of the bulk properties of the polymer. Loss of plasticizer will make the poly mer more brittle, while loss of a stabi lizing additive will result in accelerat ed structural degradation and oxida tion. Second, the fate of the lost addi tive is of deep concern in, for example, food packaging applications and where curing agents are potentially ir ritating to the eyes or skin. Finally, the accumulation of only monolayer quantities of additives at the polymer surface may affect properties such as adhesion and printability. ESCA pro vides a reliable method for detecting additives at a polymer surface, and the ability of ESCA to study solid samples allows the materials to be studied in a situation close to their working environment.
Binding Energy (eV)
Figure 7. Core level spectra of a polyamide-cured paint
Figure 7 shows the core level spectra of a polyamide-cured, epoxy-based paint. Since the epoxy resin is based on bisphenol-A we might expect to see a distinct shake-up satellite to the low kinetic energy side (high binding energy side) of the Ci s signal of carbon atoms in the benzene ring structure. That this is not observed for the stoved paint indicates that ESCA does not sample the epoxy-based material. On scraping the sample, however, the shake-up satellite becomes apparent in the spectrum. Clearly the paint has a surface contaminant. Indeed, the stoichiometry of the contaminant film, derived from the ESCA spectra, corresponds to a somewhat oxidized form
of the polyamide curing agent. The heat curing has therefore resulted in the diffusion of this material to the paint surface. Probably the most common form of surface contamination of polymers is hydrocarbon material picked up during handling (e.g., a thumbprint or solvent residue). This type of contamination is readily detected, even in submonolayer quantities, by the presence of a signal at ~285.0 eV in the ESCA spectrum. Many examples of this can be found. A potential contaminant equally as important, however, is that comprised of silicone material. The relatively low surface free energy of silicones provides them with a consid-
Binding Energy (eV)
Figure 8. Spectra of polyethylene films adhered to double-sided tape for 11 days
erable driving force to coat all surfaces in their vicinity in monolayer quantities. For this reason the use of silicone greases, for example, is not recommended in systems used for preparing polymer films whose surface properties are important. This phenomenon is also of concern in preparing samples for ESCA analysis. The most common method employed to mount free-standing films and powders is the use of double-sided adhesive tape. Thus the tape is first adhered to the sample probe tip, and then the sample is adhered to the tape. However, since these tapes often contain silicone-based release agents, care must be exercised. It is often found that while the desired spectrum is obtained if the data is taken immediately, after the sample has remained in contact with the tape for several days the spectrum will show significant contributions in the Ci s , Oi s , and Si2P regions arising from the release agent. T h a t is, the silicone material will migrate around the edges of the sample and coat its surface. Furthermore, experiments have been carried out which demonstrate that this form of contamination may even pass through the polymer film under certain conditions. The main features of the ESCA spectra of two ~50-Mm-thick low density polyethylene films are shown in Figure 8. Each had double-sided adhesive tape adhered to its underside for 11 days, and was sealed by a circular "knife edge" in a pot to prevent migration along the surface. The first sample was stored at ~20 °C while the second was stored at ~75 °C. The spectrum of the sample stored at the lower temperature consists essentially of one core level peak at ~285.0 eV associated with the polyethylene backbone, with only trace amounts of silicon and oxygen at its surface. In contrast, the sample stored at the higher temperature exhibits significant signals in the Oi s and Si2P regions, and it is clear that this sample has an accumulation of the silicone release agent at its surface. Interestingly, this diffusion of silicone material has not been observed for high-density polyethylene. How Do Reactions at the Surface Differ from Those Occurring in the Bulk Material? As we noted at the outset, polymers are often chosen in a particular application for their bulk properties. A number of methods are then available to modify the surface chemistry, if necessary, to provide the required surface properties (adhesive, chemical, or wetting properties, etc.) Among these methods are chemical etching by acids or bases, surface grafting, and treatments in electrical discharges. In general the modified
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981 · 813 A
layer must be extremely thin so that the bulk properties, for which the ma terial was originally chosen, remain unchanged. Modification by electrical discharges, for example, can dramati cally alter the chemistry of the sur face, but the modification is usually confined to a few molecular layers. Let us consider three examples of oxida tion of hydrocarbon polymers by elec trical discharges for which bulk ana lytical techniques show no apparent change and even MATR-IR spectrom etry provides little or no information. Fluid contact angle and adhesion mea surements, however, exhibit dramatic changes for all three films. Indeed, oxygen discharge treatments are com monly used to raise the surface energy of polymers to increase wetting and adhesion strength. But what chemical modifications occur at the surfaces and to what depth do the modifica tions occur? Figure 9 shows the ESCA intensity data for three discharge-treated poly mer films: (a) polyethylene, corona ox idized in air, (b) polyethylene, oxi dized in the effluent of a microwave discharge in oxygen, and (c) polysty rene, oxidized in a low-power, induc tively coupled radiofrequency dis charge excited in a low pressure of oxygen. Figure 9 lists the total Oi s /Ci s intensity ratios at electron take-off angles, 0, of 35° and 70° (see Figure 2). At grazing electron take-off angles (e.g., θ = 70°) surface features are rel atively enhanced in the spectra. For the corona-treated sample, the Ois/Ci s intensity ratio is independent of the take-off angle, suggesting the degree of oxidation is homogeneous within the ESCA sampling depth (~50 Â). This is illustrated schematically on the right of the figure as a plot of degree of oxidation vs. depth into the film. This is consistent with an oxidation mechanism involving energetic species that may penetrate more than 100 À into the polymer (i.e., oxygencontaining ions). For the sample treated in the effluent of a microwave discharge in oxygen, the 0] S /Ci s intensity ratio increases by ~25% on going to the near grazing electron take-off angle. This indicates that the degree of oxidation within the top ~50 A of material is inhomogeneous, with a greater degree of oxidation at the very surface. Again this is shown schematically and is consistent with an oxidation mechanism based on singlet molecular oxygen, which has a mean free path of ~100 Â in the polymer. Finally, for the rf plasma-oxidized film, the Oi s /Ci s intensity ratio shows the greatest dependence on the electron take-off angle. For this sample the oxidation is confined to the outermost monolayer of the film as shown in the schematic. The oxidation mechanism
Figure 9. ESCA data for three hydrocarbon polymers treated by different electrical discharge methods. PE = polyethylene; PS = polystyrene
Fully Exposed
Under Pyrex
Shaded
Starting Material
Binding Energy (eV)
Figure 10. S2P and C 1 s spectra of a polysulfone surface, initially and after weather ing under different conditions
in the rf plasma is therefore dominated by reactions of atomic oxygen, which has an extremely short mean free path in hydrocarbon polymers. Although this discussion of the depth of modification has been kept deliberately qualitative, quantitative data can be derived from the relevant expressions (Equations 2 and 3) assuming an overlayer model. Further-
more, the mechanistic conclusions are substantiated by detailed considerations of the Cis and Oi s lineshapes, which reveal the structural features produced upon oxidation. Our last example in Figure 10 illustrates an example of the study of a polymer surface under conditions of natural weathering. While a volume of literature exists on the bulk modifica-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981 · 815 A
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.100--
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tion of many polymers, caused by en vironmental conditions, relatively lit tle attention has been paid to surfaces. Modification at the polymer surface is likely to be different, however, for sev eral reasons. At the surface, • the solar radiation is unattenuated; • there are no diffusional con straints on O2, H 2 0 , or pollutants; • the volatile products are lost more readily; • the polymer chains may have greater mobility; and • the possibility exists of abrasion by windborne particulates. Figure 10 shows the data for a polysulfone surface weathered from Feb ruary to May, under various condi tions, in San Jose, Calif. The bottom spectrum is that of the starting mate rial and is quite representative of the polymer repeat unit. Thus the peak centered at ~285.0 eV in the Ci s spec trum corresponds to all carbon atoms attached only to hydrogen and other carbon atoms. Those in and attached to the phenyl rings have a shake-up structure associated with them, and this is apparent, centered at ~291.7 eV on the binding energy scale. The signal at ~286.7 eV is due to carbon singly bonded to oxygen and that at ~286.0 eV is due to carbon singly bonded to sulfur. The S2P spectrum consists of a partially resolved spinorbit split doublet at the correct bind ing energies for the sulfone group. After three months' exposure, shaded from sunlight, the polymer surface is unchanged, reflecting the superior thermal stability of this material. For the samples either exposed under a 1-mm-thick Pyrex glass slide or fully exposed, a high degree of photooxidative degradation has occurred, and this is consistent with the fact that polysulfone is known to absorb strong ly in the region between 320-340 nm wavelength when partially degraded. Thus, the appearance of components in the Ci s spectra, due to Ç—Ο at ~286.6 eV, C = 0 at ~287.9 eV and O — C = 0 at ~289.2 eV, is quite strik ing. An interesting situation arises
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varian 816 A · ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
when we consider the S2P spectra. The fully exposed sample shows a decrease in the S2P signal intensity when com pared to the starting material, which is consistent with a small loss of SO2 upon photooxidation at the surface. In contrast, the sample exposed under Pyrex glass shows à three- to fourfold increase in the S2P signal intensity, and the centroid of the peak is shifted to higher binding energy. This observation can be rationalized, in part, by the formation of sulfate esters at the surface via reaction of SO2, permeating from the bulk material, with hydroperoxide groups at the surface. This implies that the hydroperoxides are somewhat longer-lived in the sample having a Pyrex filter between it and the solar radiation. Clearly, much additional information can be obtained on natural weathering from surface analysis of the material. The Message. Although several techniques are available for the surface analysis of polymers—ESCA, UPS, XAES and, recently, surface enhanced Raman spectrometry—ESCA stands alone in the ease of sample preparation, accumulation of data, and spectral interpretation. This is clearly apparent from the current literature, where the vast majority of surface analyses of polymers are achieved by ESCA. The information derived from studies of polymer surfaces is important in many areas of application. It is hoped that this REPORT has stimulated additional interest in this exciting area of research, which is so crucial to our understanding of the interaction of polymers with their surroundings. Further Reading (1) "Characterization of Polymer Molecular Structure by Photon, Electron, and Ion Probes"; Thomas, H. R.; Dwight, D.; Fabish, T., Éds., American Chemical Society: Washington, D.C., 1981. (2) Dilks, A. In "Electron Spectroscopy"; Brundle, C. R.; Baker, A. D., Eds.; Academic Press: London, 1981; Vol. 4. (3) Dilks, A. In "Developments in Polymer Characterization-2"; Dawkins, J. V., Ed.; Applied Science Publishers Ltd.: Barking, U.K., 1980.
Alan Dilks is a research scientist with Xerox Corporation in Rochester, Ν. Υ. His major research interests include the structure and reactivity of poly mer surfaces, and polymer surface modification and synthesis by radiofrequency plasma techniques. He earned his PhD at the University of Durham, England, in 1977.
