Report Alan Dilks Xerox Webster Research Center Xerox Square W-114 Rochester, N.Y., 14644
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 ofpolymers 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
.
.
802A
same as that of the bulk material? What contaminants are present a t the surface? How do reactions a t the surface differ from those occurring in the hulk 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 he 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 A. This latter technique may well be of great importance in the near future, but a t 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
* ANALYTICAL CHEMISTRY, VOL. 53, NO. 7. JUNE 1981
somewhat fragile nature of organic materials exposed to energetic partcles or ionizing radiation. Generally speaking, the damage induced in a polymer surface hy 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 precisel'y 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 identi0003-2700/81/0351-802A$O1.00/0 @ 1981 American Chemical Society
When everything f i t s together...
In o f ? performance spectrophotometer coupled ucvice capable of storing more than 200 spectra on one at least IO components! very 0.10 nm for precise, accurate resolutioi guided by a simple question and answer routine which can be understood by ev
on of hardware, software and intelligent systems design makes e, a multi-componentanalysis approach to meet his needs.
" .
lor0 ers .ncredlbly Compact Kontron offers a choice of two spectrofluorometers, uding a special version for liquid chromatography. units feature an advanced optical system with holo-
pect
...
phic concave grating, digital display, pushbutton ration, and two independent drive motors which
-
536
1
t
~
g
532
8
~
1
g
r
528404
400
396
' ' ' 288'
' 284 ' ' '
'
Binding Energy (ev)
fied hy ESCA, have provided an impetus for the development of a less destructive form of SIMS, namely static SIMS. In this mode, low primary ion energies (0.5-2 kV) and an extremely low ion current (-10-8 A cm-2) minimize surface damage. Under these conditions, it can he shown that while the interaction of an ion with the organic 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 hut 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 inorganic materials. First, it is a fact that in a high vacuum system (or elsewhere) the sticking coefficients for potential contaminant species (e.g., oxygen, water, hydrocarbons) are generally somewhat smaller a t organic surfaces than a t inorganic surfaces. There are therefore much less stringent vacuum requirements for the study of polymer surfaces. Thus, while pressures of 10-10 or 10-11 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 -10-8 torr (10-6 Pa). That is, less elahorate pumping systems may be used, and samples can he introduced from the atmosphere to the sample analysis chamber via insertion mechanisms based on Viton or Teflon seals. These points are very important 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 he symmetric and the linewidths are not usually dependent on chemical environment. 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 environment. This well-behaved nature of signals derived from polymers allows the component signals to he approximated by Gaussian lineshapes, and quite detailed curvefitting becomes possible. This is important since ESCA possesses an inherently small chemical shiftninewidth ratio, which usually results in only partially resolved spectra. Finally, for electrically insulating samples such as polymers, it is generally the case that the electron flux photoejected from the sample is only partially balanced by the flux of lowenergy secondary electrons arriving a t the sample surface from its surroundings. A pseudoequilihrium 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 accurately determine hinding energies, some form of referencing is necessary. The most widely used method for polymers involves the monitoring of the CI. level of the extraneous hydrocarbon contamination, which huilds 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 he 285.0 eV. This ability to correct for and measure charging effects for polymers has led to the use of sample
charging as an extra information level. Let us now hriefly consider the ESCA experiment in a little more detail before discussing examples of its impact on polymer surface analysis. ESCA, in which the binding energies of electrons in molecules are measured, is accomplished hy photoejecting the electrons with a monoenergetic h e m of soft X-rays and measuring their kinetic energies. The binding energy for a given electron is then derived from the difference between the photon energy and the electron's kinetic energy. In this way the binding energies of both core and valence electrons can he determined, but, because of the low cross sections for photoionization of the valence levels, the predominant 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 simplest form can he employed to produce an elemental analysis of a surface. Differences in electronic environment of a given atom give rise to a small range of binding energies (i.e., chemical shifts) that are often representative of a particular structural feature. Figure 1, for example, shows high resolution spectra in the Ol., N,,, and CI, regions for a Nylon-6 film, While the 01.and N1, spectra show symmetrical peaks containing single components, the CISspectrum exhihits a distinct fine structure. The CIS spectrum can he resolved into three components a t binding energies of -287.8, 286.3 and 285.0 eV, with relative intensities of 1:1:4, respectively. These components are assigned as shown in the figure to particular features in the polymer. Although the incident X-ray may penetrate several thousands of angstroms into the polymer, the mean free paths of the photoemitted electrons in the energy range of interest (Le., 300-1500 eV) are only of the order of a few tens of angstroms. I t 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 sensitivity. For example, for C1, levels irradiated with MgKrrl,2photons, 95%of the observed signal intensity derives from about the top 50 A of the sample, corresponding to an electron mean free path of -15 A. For a homogeneous material the signal intensity observed for a given core level can he described by the equation I , = FaiNikiXj (1) where F is the X-ray flux, a is the cross section for photoionization in the direction of the analyzer, N is the number of atoms in a given volume el-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
-
805A
v.er
-
Y
source
3 COS
e
L
J
ement, k is a spectrometer factor and Xis 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 situation commonly encountered is that shown in Figure 2, in which a homogeneous 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: I f = FaiNjkiX;[l - exp(-d/Xj cos 0 ) ] (2) For the substrate or hulk:
I:
The signal intensities derived from features at the very surface of the sample are therefore relatively enhanced in going to grazing electron take-off angles (0 90° in Figure 2). What is the Structure of the Surface? As we have seen in Figure 1the determination of the surface structure of polymers by ESCA is relatively straightforwardand 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 mechanical properties. Applications have been proposed in which the hydrogel is introduced into the tissue and subsequently allowed to swell and hydrate in situ. Since events that occur in the first few seconds (e.g., protein adsorption) may well determine the success or failure of the implant, a detailed knowledge of the surface structure under conditions of hydration, dehydration, and rehydration is clearly of considerable importance. Figure 3 shows the C1. spectra of a sample in which poly(2-hydroxyethyl methacrylate) (HEMA) has been radiation-grafted to the surface of a Silastic film, both in the hydrated and dehydrated states. Also shown are the SilC and O K elemental ratios derived from the total Sip,,. 01,and CISintensities. As the hydrogel (HEMA) is dehydrated, the amount of silicon a t the surface increases. Since silicon is only present in the Silastic substrate this observation suggests some rearrangement in which more of the Silastic is
= FajNjk;X;exp(-dlX;
me)
(3)
dlcos 0 is the effective overlayer thickness in the direction of the analyzer, where 0 is the electron take-off angle.
M
Elemental Ratios
S i / c - o/c
I \
-
0.18
0.42
: a 0.61
0.52
S U W
lntsrior
295
290
285
280
Blnding Energy (eV)
Figure 3. ESCA data for HEMA grafted on Silastic 806A
ANALYTICAL CHEMISTRY, VOL. 53. NO. 7, JUNE 1981
exposed after dehydration. The upper CI, spectrum in Figure 3 corresponds to a convolution of the spectra of HEMA, Silastic, and a small amount of hydrocarbon contamination. The component a t -285.0 eV is due to the contaminant and Silastic, whereas the higher-binding energy components are due to c-0 (-286.6 eV) and O=c-O (-289.0 eV) in HEMA. On dehydration the contribution to the spectrum of these high-hinding energy components is reduced to a small level. This is consistent with the conclusion that the concentration of HEMA graft a t the surface of the Silastic film decreases upon dehydration. The conclusions drawn from the ESCA analysis can he represented hy the models shown in Figure 3. The hydrated graft polymers are found to have a significant quantity of the hydrogel a t the surface intermixed with silicone polymer chains. Upon dehydration the experimental measurements can be rationalized h graft chains penetrating -30-50 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 uiuo. 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 he 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., N or 01,valuable structural information can he ohtained 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 fabrication. Furthermore, in the construction of multilayer systems, a knowledge of the surface chemistry is important to overcome problems of adhesion. 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 F1, and C1, spectra for the plasma polymers formed in three separate experiments in which
1
Introducing the Model 560 ESCA/SAM. A complete surface chemistry laboratory.
Most analytical and research laboratories face a wide range of surface chemistry problems, so multiple surface analysis techniques are needed to do the job. That's why PHlodesigned the new Model 560 ESCA/SAM for optimum, computerized ESCA and SAM performance. Each technique offers unique advantages for different types of samples. Used together, they provide a more detailed analysis and a higher probability of getting the correct answer to a problem. PHI'S new SlMS I1 accessory can also be added to the Model 560. Software for the integral computer system covers all of these techniques practically and efficiently. Features include: Maximum Sample Throughput A sample introduction system, combined with the integral computer system, ensures the highest
sample throughput and lowest cost per sample. High ESCA Count Rates A new, high flux, dual anode x-ray source in combination with a cylindrical-mirror analyzer provides higher count rates for excellent ESCA sensitivity. Improved SAM Performance Redesigned coaxial electron gun provides beam diameters below 2 pm for improved SAM resolution and SEM imaging. Total System Automation Computerized system control and data manipulation increase sample throughput. Available software provides maximum spectral information for a more thorough analysis. Optional Transfer Vessel A special transfer vessel allows sample pre-treatment and transfer to the analysis chamber without exposure to outside atmosphere.
For More Information To find out how this unique new instrument can help you solve your materials problems, contact the office nearest you. Minneapolis, MN, USA: PerkinElmer, Physical Electronics Div., 6509 Flying Cloud Drive, Eden Prairie, MN, USA, 55344; Telephone (612) 941 -5540. Munich, West Germany: PerkinElmer GmbH, Physical Electronics Div., Bahnhofstrasse 30, D-8011 Vaterstetten, Germany. Hong Kong: Perkin-Elmer, Surface Sciences Div., 303 Fourseas Building, 208-21 2 Nathan Road, Tsimshatsui, Kowloon, Hong Kong. Japan: Contact Ulvac Corporation, Tokyo.
Physical Electronics
@
PERKIN-ELMER Responsive Technology CIRCLE 170 ON READER SERVICE CARD
Automatic Particle Size Distribution
c.. :F CH
i
/I\
J
n
Plasma-polyirifluoroethylene
\
A --
Plasma-polyfluoroemyiene
a
Rapid, accurate sedimentation analyses from 100 to 0.1 u m diameter.
L-
d 692
690 688
686
684
1
1
294
1
1
292
1
1
290
1
.
288
1
.
1
286
1
1
284
1
282
Binding Energy (eV) SediGraph 5000D Particle Size Analyzer: Provides the precision, speed and flexibility needed for process control, quality assurance and RaD. The SediGraph auto. mates classical, sedimentation particle size analysis. By using low energy x-rays to determine the particle concentration at depths decreasing with time, particle equivalent spherical diameters from 0.1 to 100 pm are measured and data automatically plotted as cumulative mass percent versus equivalent spherical diameter. The SediGraph operates unattended and yields results in minutes, requires no calibration or data reduction and has no narrow range of operation, no liquid-liquid or liquid-surface interfaces and no easily-clogged, fragile orifices. The SediGraph can use both aqueous and organic liquids. Resuits are compatible with other sedimentation data and accuracy and repeatability are better than 1%.
*
For more information, contact Micromeritics Instrument Corp., 5680 Goshen Springs Road, Atlanta. Georgia 30093 U.S.A. (404) 448-8282; Telex: 70-7450
gure 4. F,. and Ci. spectra of three piasma-polymerized fluorocarbons
20
10
L
.
,
4
5
6
7
Y
9 1 0
Methylene Chain Length, n
Ei"micrumrerrtn=s .
3
I
Solving Your Materials Analysis Problems
igure 5. Total Ci, intensity + shake-up intensity, vs. n f o r the styrene-methylene )polymers. Horizontal lines represent surface CIRCLE 145 ON READER SERVICE CARD
8088
ANALYTICAL CHEMISTRY, VOL. 53. NO. 7. JUNE 1981
fluoroethylene, 1,l-difluoroethylene and trifluoroethylene were the injected gases or monomers, respectively. These spectra are very complex, and this is typically the c,wewhen 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 CF3, CF2, CF, and carbon with no fluorine inearest neighbors are readily derived frlom 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 suirface does indeed differ from the bulk material. These are, generally speaking, found when the system is composed of long repeat units or segments. Lei, us first consider a series of copolymers derived from styrene and methylene units of general formula, [CH~-CI-IZ-CH~CH0-(CHz),], where n = 1, 3, 5 , 6 , 10. Since the parent polymers of these systems, polyethylene and polystyrene, are purely hydrocarbon, their ESCA spectra show only one direct photoionization peak at 285.0 eV in the C1, region. The saine is also true for the copolymers. Polyethylene and polystyrene can be distinguished by
YOU DON'T KNOW WHAT YOUR R&D CAN DO TILL YOU W E IT SPACE. Imaginea laboratory that's free of such constraints as hydrostatic pressure. Convection.Sedimentation, There is such a research lab. It's called the Space Shuttle. And you can use it to send your experiments into Space. OPPORTUNITIESAS VAST AS SPACE ITSELF
Space Shuttle experiments can be either pure or applied research leading to the development of new and improved products, processes and materials.Whatever your project, it can take advantage of near zero gravity,
almost perfect vacuum. Any process that can be affected by exposure to such an environment may be greatly improved -and may, in turn, greatly improve your competitive position. The decisive factor could be your use of Space to experiment in areas such as cryogenics.Crystal growth. Containerless processing. Solidification.Or mixing.
CAN YOUR PRODUCTS OR PROCESSES BE ENHANCED BY SPACE?
Rockwell International has answers that may be profitable to you. We operate a User Service Center to assist in the planning necessary for you to take advantage of the opportunities in Space. Call us a t (213) 922-3344 or write: STS User Service Center, FC15- B Space Operations & Satellite Systems Division, North Ameriran Space Operations, Rockwell International, 12214 Lakewood Blvd,, Downey, CA 90241.
ROCKWELL INTERNATIONAL KNOWS SPACE.. FROM THE GROUND UP.
.
Rockwell International ,..wherescience gets down to business
CIRCLE 186 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
809A
ESCA, however, since polystyrene exhibits a shake-up satellite centered -6.6 eV higher on the binding energy scale than the direct photoionization peak. The intensity of the polystyreni shake-up satellite is -8.1% of the intensity in the CI. 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 CISintensity). This shake-up arises from the finite probability of simultaneous excitation of the valence electrons from ?r to ?r* 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 a t the surfaces of the copolymers. Figure 5 shows a plot of the intensity of the direct photoionization peaks a t 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 difference in the slopes of the two straight lines is explained in terms of specific orientation effects of the polymer chains in the outermost -100 A. For n > 1the samples show less styrene units a t the very surface than ar 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 possibilities that might give rise to these observations. The most likely situation is that in which the surface consists of a folded chain structure having the poly-methylene segments specifically oriented at the surface. This is shown schematically in Figure 5 for n = 3,6 and 10. For the copolymers with long repeat units, therefore, the surface is effectively less rich in styrene 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) copolymers are highly incompatible and that they undergo microphase separation and domain formation. This bulk morphology 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 anonsolvent for the other. For the examples of PS-PEO dihlock 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 810 A
-+
CHzCH4~CHnCH20
n O,,Region
CHCIJ
From
C,. Region
A
I
Mole % PS at Surface
46
Binding Energy (ev)
Figure 6. Core level spectra of the PSlPEO diblock copolymer, 21.4 mole the bulk, cast from different solvents
structure. The question then arises of what effects can he seen at the copolymer surfaces. Figure 6 shows the ESCA spectra in the 01. and C1. 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 CI, component at -286.5 eV and the 01. signal are due to the core levels in the PEO component. The mole ratio of the two components at the surface can he directly obtained from the relative intensities of the C1, signals (corrected for the numbers of carbon atoms in the repeat units) and also from the 01JC1, 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 hulk composition, and shows a dependence on the solvent from which the film is cast. A careful analysis 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 component, hut rather by the model shown schematically in Figure 6, in which columns of one component are
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
7o PS
in
embedded in a continuous phase of the other. W h a t Contaminants Are Present a t t h e Surface? It is well known that
under certain process andlor environmental conditions-heat, radiation, pressure, etc.-additives may undergo diffusion from a polymer. This pbenomenon is of considerable importance in three respects. First, the loss of a particular additive from the material might cause serious degradation of the hulk properties of the polymer. Loss of plasticizer will make the polymer more brittle, while loss of a stahilizing additive will result in accelerated structural degradation and oxidation. Second, the fate of the lost additive is of deep concern in, for example, food packaging applications and where curing agents are potentially irritating to the eyes or skin. Finally, the accumulation of only monolayer quantities of additives a t the polymer surface may affect properties such as adhesion and printability. ESCA provides a reliable method for detecting additives at a polymer surface, and the ability of ESCA to study solid samples allows the materials to he studied in a situation close to their working environment.
..
31
r
I "'1 out with Chromowrb to :compoundsto molecular components.
and a wide range~ofdiatomite supports. For more infdrmation on the Chromosorb solution. includ-
r Filtration and Minerals Division, ohns-Manville,Ken-Catyl Ranch, DenvecCO 80217, (3O3)979-1000.
gasandliquidseparation. ..
The growing Chromosorb line includesten productsfor LC. our Century Seriesof productsforGC.
Johns-lHtaw:Ele CIRCLE 113 ON READER SERVICE CARD
I
It makes great GC so much easier, there's no comparison.
Ilk-"
Ll
The VISTA 6000 is the first microcomputer-CRT-basedGC. It makes it easier than ever before to use all the power of gas chromatography. You get more done. And you have more fun doing it. R makesthetoughest analysis seem simple. The helpful CRTcontinuouslydisplays instrumentconditions. Helps you build methods. Prompts and leads you. Makes GC so responsive, iVs a whole new experience. Proven c h m m a t o g r a p h y c o m p g i v e ywexceilent results. TheVlSTA 6000utilizes ali the high performancecomponentsdetectors, injectors. columns and capillary Systems-proven for excellence in theVarian3700 Series
It'sthetirstexpandableGC.TheVlSTA6000iseasyandeconomical loexpand. It will fully control aVISTA6500 Dual-columnSatellite Chromatographdoubling your GC capability. TheViSTA 6500 is identical to theVISTA6000except it takesless space and costsmuch less becausethe 6500 doesn't have its own controls. There's nothing like It. Compare ihe VISTA 6000, feature for feature,with the other gaSchrOrnatOgraphSand we believe you'll agree, there's no comparison.
Clip and save this checklist of features to commre different models
01more informal on on me incornparahle VISTA 6wO
,erv ce h-mow 235 To na e a 'epre5eilai
circ e RPnder
,e cor ldcl 10- c rc c'
leader Service Number 236
varian FOllmme4l.lt)a~IMncecontact * Florham Park, NJ (201)822-3700. ParkRidge. IL (312)825-7772* HwslOn.TX (713)783-18M. L0~AIfOE,CA(415)968~8141 Orwrite 611 Hansen M y , PaloAll0,CA 94303. In Europe Sleinnauserstrarre.Cn.6300Zug.Swiflerland.
812A * ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
~
536
-
532
1
404
~
l
292 Binding Energy (eV) 4M)
~
~
288
284
~
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. 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 a t -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-
---
rrwrrrk*umu*
534
530
288
284
104
loo
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 I ltape. 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 he 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 CIS,01,,and Silp regions arising from the release agent. That 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-pm-thick low density polyethylene films are shown in Figure 8. Each had double-sided adhesive tape adhered to its underside for 11days, and was sealed by a circular “knife edge” in a pot to prevent migration along the surface. The first sample was stored a t -20 “C while the second was stored at -75 OC. The spectrum of the sample stored a t the lower temperature consists essentially of one core level peak a t -285.0 eV associated with the polyethylene hackbone, with only trace amounts of silicon and oxygen a t its surface. In contrast, the sample stored a t the higher temperature exhibits significant signals in the 01. and Sizpregions, and it is clear that this sample has an accumulation of the silicone release agent a t 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 a t 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
813A
-
New VISTA 401 simply outsmarts the other chromatography data systems
method, the prompted dialog you have with the big interactive CRT~S so relaxed and friendly it's almost human.And you can build methods on the CRTwhile the printerlplotter is reporting an analysis. You don't have to wait. TheVlSTA 401 even has a "learn''mode in which it scans a chromatogram and automatically selects the best peak measuring parameters for each peak. A more useful record. On one piece of paper theVlSTA 401's fast, fixed-head dual-channel printerlplotterwill give you a fully annotated chromatogram and report, both with compound names. Results can be automatically reported in any units of measure desired: ppb, mole %, BTUs, etc. and versatile editing functions help you get clean, clear reports. Because It's smarter you get bettcr data. In addition to performing all the standard calculations, theVISTA401 has unique featuresthat help you get better data. It will actually draw the computed baseline on the chromatogram so you can see if each peak is accurately integrated and correct the baseline whenever necessary. To improve the accuracy of data on runs where you get drifting baseline, the VISTA 401 will subtract baseline drift. See Figure 1
I To help you mprove data. VISTA 401 Subtracts drifting baseline and draws io the computed baseline.
Floppydlaks give you newcapabllltles.Built-in floppydisks provide dataStorage with randomaccess. Much faster than tape, and smarter. The VISTA 401 will store, directly on disk, all the raw data for an entire chromatogram for reploning and recalculation. It will even replot a standard chromatogram, saved on "
..h c =
__
--
-
8 -
llwill grow with you. VISTA 401 provides smart data handling for any four chromatographsin your lab-GC or LC. You can use the VISTA 401 to fully control up to four VISTA chromatographsand it has a standard interfaceto lame and small comouters. To helo vou aet the most fromyourVISTAi01we offer regularly scheduled ke; training courses at our regional offices and provide strong service and applications support. For full information call us (phone numbers below), drop us a line or check these Reader Service Numbers:238 to have aVarian Representative contact you; 239 for literature.
.I "...." ...d . "-4% _..-. -c.minis* ....... ~
t
To help you confirmdata, the VISTA401 replals a storedstandardalongside an analysis plot.
n
vaian f o # l r l m m . d l ~ ~ ~ I ~ E O n Florham t . n : Park.NJ (X)1)822-37M ParkRidge, lL(312)825-7772 Hou~,lon,TX(713)783-lsw* LOEAIIOE,CA(415)9688141 Orwrite611 Hans0 Way. Pal0 Allo, CA 94303. In Europe: SleinhaUSerSlraSSe.C H - ~ ~ W Z USwitlerland. Q,
814A
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
layer must he extremely thin so that the bulk properties, for which the material was originally chosen, remain unchanged. Modification hy electrical discharges, for example, can dramatically alter the chemistry of the surface, hut the modification is usually confined to a few molecular layers. Lei us consider three examples of oxidation of hydrocarbon polymers by electrical discharges for which hulk analytical techniques show no apparent change and even MATR-IR spectrometry provides little or no information. Fluid contact angle and adhesion measurements, however, exhibit dramatic changes for all three films. Indeed, oxygen discharge treatments are commonly 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 modifications occur? Figure 9 shows the ESCA intensity data for three discharge-treated polymer films: (a) polyethylene, corona oxidized in air, (h) polyethylene, oxidized in the effluentof a microwave discharge in oxygen, and (e) polystyrene, oxidized in a low-power, inductively coupled radiofrequency discharge excited in a low pressure of oxygen. Figure 9 lists the total O1$Cl. intensity ratios a t electron take-off angles, 8, of 35' and '70' (see Figure 2). At grazing electron take-off angles (e.g., 8 = 70°)surface features are relatively enhanced in the spectra. For the corona-treated sample, the O1,/Cl1 intensity ratio is independent of the take-off angle, suggesting the degree of oxidation is homogeneous within the ESCA sampling depth (-50 A). 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&. into the polymer (i.e., oxygencontaining ions). For the sample treated in the effluent of a microwave discharge in oxygen, the O1,/C1. intensitj ratio increases by -25% on going to the near grazing electron take-off angle. This indicates that the de ree of oxidation within the top -50 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 A in the polymer. Finally, for the rf plasma-oxidized film, the 01$C1, 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
Sample
Relative O1,/C1. Intensity at e = 35" at e = 70'
(a) Corona-Oxidized PE
0.24
(b) Microwave
0.31
Schematic
0.24
d
I 0.39
Plasma-Oxidized PE
Figure 9. ESCA data for three hydrocarboo polymers treated by different electrical discharge methods. PE = polyethylene; PS = polystyrene
s2p
(X3.3)
Fully
Exposed
Shaded
I
Starting Material
A
172
168
-d
--.-.cLlc1
292
Binding Energy(eV)
1
1
288
1
1
1
1
1
1
284
1
.
Figure 10. Spp and CISspectra of a polysulfone surface, initially and after weathering 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 heen kept deliherately qualitative, quantitative data can he derived from the relevant expressions (Equations 2 and 3) assuming an overlayer model. Further-
more, the mechanistic conclusions are substantiated hy detailed considerations of the CI, and 0 1 , 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 hulk modifica-
ANALYTICAL CHEMISTRY, VOL. 53. NO. 7, JUNE 1981
815A
Are you analyzing
in the worknlace? Wavelength: 193.7 nm Background Corrected
is4.5% RSD
0
20
50
100
Weight of AS. ng
Here’s help for you from Varian. Instrument Systems. A large selection of gas chromatography,atomic absorption, UV-Visible and liquid chromatography inslrumenls designed to meet the latest standards in air and water pollution analysis. Appllcation Reports. An ongoing series of Varian application reportson environmental pollution analysis. Local Support. Hundreds of Varian sales and service representatives and applications scientists located throughout the world. They’reon call to give you the help you need, before and after you buy. Workshops &TrainingCourses. Local workshops, ranging fromelementary training in the analysis of carcinogensto special problem-solvingcourses tailored specificallyfor your needs. Lots 01 instrument training courses, too. Parts &Supplies. Everything you need to create a total GC. AA or LC system for environmental pollution analysis. For literature,circle240onlhereplycard if you’d be interested in a local “handson” workshop circle 241. Circle 242for a representative to call. n Varian Instrument Group 61 1 Hansen Way, D-070 Palo Alto. CA 94303
tion of many polymers, caused by environmental conditions, relatively little attention has been paid to surfaces. Modification a t the polymer surface is likely to he different, however, for several reasons. A t the surface, the solar radiation is unattenuated; there are no diffusional constraints on 0 2 , HzO, or pollutants, the volatile products are lost more readily; the polymer chains may have greater mobility; and the possibility exists of abrasion by windhorne particulates. Figure 10 shows the data for a polysulfone surface weathered from Fehruary to May, under various conditions, in San Jose, Calif. The bottom spectrum is that of the starting material and is quite representative of the polymer repeat unit. Thus the peak centered a t -285.0 eV in the CI, spectrum corresponds to all carbon atoms attached only to hydrogen and other carhon atoms. Those in and attached to the phenyl rings have a shake-up structure associated with them, and this is apparent, centered a t -291.7 eV on the binding energy scale. The signal a t -286.7 eV is due to carbon singly bonded to oxygen and that at -286.0 eV is due to carhon singly bonded to sulfur. The SzPspectrum consists of a partially resolved spinorbit split doublet at the correct binding 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 ahsorb strongly in the region between 320-340 nm wavelength when partially degraded. Thus, the appearance of components in the CtSspectra, due t o C-0 at -286.6 eV, C=O a t -287.9 eV and O-C=O a t -289.2 eV, is quite striking. An interesting situation arises
W vafian
816A
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7. JUNE 1981
when we consider the SzPspectra. The fully exposed sample shows a decrease in the SZpsignal intensity when compared to the starting material, which is consistent with a small loss of SO? upon photooxidation a t the surface. In contrast, the sample exposed under Pyrex glass shows B three- to fourfold increase in the Szpsignal intensity, and the centroid of the peak is shifted to higher binding energy. This observation can he rationalized, in part, by the formation of sulfate esters at the surface via reaction of SOz, permeating from the hulk material, with hydroperoxide groups a t 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 he ohtained on natural weathering from surface analysis of the material. T h e 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. I t 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., Eds., American Chemical Society: Washington, D.C., 1981. (2) Dilks, A. In “Electron Spectroscopy”; Brundle. C. R.: Baker. A. D.. Eds.: Eds.; Academlr I’ress: I*ond*”, 19RI. Vol. 4. (3) IXlks. A. In “l)rrelopmenrsin Polymer Characterization-’L“; Dawkinn. J. V.. Ed.; Applied Scirnce I’ublishrrs l.td.: HarkAmlied ing, U.K., 1980. ink
