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Joseph A. Gardella, Jr.'
Department of Chemistry and Industry University Center for Biosurfaces University at Buffalo. SUNY Buffalo, NY 14214
Jean-Jacques Pireaux
Facult& Universitaire Notre Dame de la Paix Laboratoire lnterdisciplinaire de Spectroscopie Electronique Rue de Bruxeiles 61 5 5 0 0 0 Namur Belgium
When materials science emerged as one of the important research areas in the 19809,chemists were encouraged to become more involved in interdisciplinary and multidisciplinary efforts in the field (I).Today, interest continues unabated and chemists are making contributions in areas ranging from material synthesis and characterization to the details of development and device engineering. One emphasis in materials science involves surface and interfacial concerns, and this is an area in which advances in basic science rapidly transfer to technology (2). Materials scientists and engineers are now focusing more attention on polymers because of their increasing usefulness in the marketplace. Polymers are an inexpensive alternative for traditional structural materials: in fact, the volume of polymers produced worldwide has surpassed that of steel. High-technology polymers tailored for Current address: Chemistry Division, National Science Foundation, 1800 G St.. N.W.,Washington,DC 20550 1
0003-2700/90/0362-645A/502.50/0
@ 1990 American Chemical Society
specific applications are continuously being developed, although the production volume is relatively small. Applications are often based on particular properties (e.g., special dielectric, mechanical, chemical, thermal, or interfacial properties) that can be achieved throueh the svnthesis of the volvmer itself."Many oithe specific appiicaiions of volvmeric materials with interfacial propeities are well known (e.g., wetting, grafting, or adhesion enhancement). A major challenge in polymer interfacial science is understanding the relationship between hulk structure and composition and the resulting surface structure and composition. X-ray photoelectron spectroscopy (XPS), also
need more precise information about surface structure than that available from ESCA analyses. For example, the presence and concentration of surfaceactive functional groups can routinely he detected with ESCA, especially in concert with IR or Raman vibrational soectroscow. .. For biocomvatibilitv or chromatography applicatih, however. information about the orientation and reactivity of these functional groups would be helpful. In addition, information about the monomer arrangement along a chain in copolymers and intrachain interactions at the surface would also be important data not routinely available. In this article, we will focus on the experimental aspects of three tech-
INS7RUMENTATION called electron spectroscopy for chemical analysis (ESCA), has been the workhorse ultra high vacuum (UHV) method for polymer applications. It provides chemical bonding information, exhibits high surface sensitivity, results in minor damage to the sample, and is relatively insensitive to the insulating properties of the sample. ESCA applications for polymer surfaces have involved quantitative analysis of surface species and detection of impurities or modifications a t the surface, as described previously in this JOURNAL (35). A recent review describes the many types of information available from ESCA of polymers (6). To meet today's polymer engineering requirements, however, scientists
niques that can provide information complementary to that provided by ESCA high-resolution electron energy loss spectroscopy (HREELS), lowdamage (macro)molecular secondary ion mass spectrometry (SIMS), and low-energy ion scattering spectrometry (LEIS or ISS) for polymer surface science applications. Selected examples will illustrate the limitations and interrelationships of these methods with other available ones. Table I summarizes the analytical characteristics of useful methods for polymer surface science applications. Instrumentation
An UHV environment is required for the spectroscopies described here he-
ANALYTICAL CHEMISTRY, VOL. 62. NO. 11, JUNE 1, 1990 * 6 4 5 A
INSTRUMENTATION cause of the electron or ion beam used to probe the surface and the need to avoid contamination of the surface. An ultrahighvacuumisnecessary; typically, 10-7-10-1" mbar pressure must be maintained.
HREELS. In a HREELS experiment (Figure l), a low-energy, monochromatic electron beam (typically, 110 eV with an energy defined to -5 meV) is focused onto the sample surface. There the beam is backscattered
Monochromator
alyrer
Figure 1. Schematlc of a hlgh-resolution electron energy loss experiment In an ulba high Y B C U U environment. ~ a Iow-~nergyelectron beam emitted from a hn c a t h a is SpatiallY dispersed (monochomata) to select B very narrow slice 01 i*i energellc distribution. This beam is then accelerated toward the target wlth me desired imeraCtionpotential. The backscaneredbeam is men energy analyzed (analyzer). and a loss spectrum is produced.
Characterlstlc
ESCA (XPS)
EhnentaI and mdeo- No H detection UWlnformatlon Chemical shitts
IR and Raman
7
HREELS
(= 80 cn-') ?
into an electron energy analyzer whose function is to separate the electrons that suffered inelastic collisions in the target. Low-energy electrons can excite vibrations of molecular functional groups present on the (polymer) SUIface. In principle, the energy loss corresponding to vibrational excitations are easily detected between 25 and 600 meV or more. Thus the technique has a large dynamic range of 2005000 cm-I (1meV = 8.066 cm-') that is available at once from a single scan in a single experiment. The electron monochromator and analyzer are based on a 1fW hemispherical or 1 2 4 O cylindrical sector or cylindrical mirror design. The electron beam produced by a hot fdament is transported, focused, and (de)accelerated with standard electron optics and, after being backscattered from the sample surface, detected with a channeltron multiplier. The signal amplification is processed by standard electronic modules that typically are computer interfaced. Many HREELS systems are built by individual researchers, but several commercial surface science manufacturers such as Leyhold Heraeus, McAllister, Vacuum Generators, and LK Technologies provide svstem comDonents and build spectrometers. SIMS. SIMS can be considered a family of methods, and the different
SlMS
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ANALYTICAL CKMISTRY. VOL. 62, NO. 11, JUNE 1, 1890
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components depend on the type of information desired. The basic block diagram (Figure 2 ) of an ion/atom beam based SIMS experiment involves the generation of a keV ion or atom beam focused on the sample surface as well as the collection and mass analysis of secondary ions generated from desorption or sputtering. Two sets of configurations commonly used involve a combination of two types of primary ion or atom beam sources and two types of mass analpem. In the first type, the ion or atom source uses a noble-gas ion beam (suhsequently neutralized if it is an atom source) with a low current density that is achieved via defocusing. The second type of source uses a focused ion beam that is produced from liquid metals (Ga) or desorption from a solid-state source (Cs). In this type of ion beam, the ion current is low although the current density is quite high. After secondary ions are generated from the sample, they are collected by ion optics and focused into a mass spectrometer for analysis. Every type of MS detector has been used for SIMS ex-
periments; for polymers, the two common approaches have heen quadrupole and time-of-flight (TOF) detectors. ISS. Basic instrumentation for lowenergy ion scattering (Figure 2 ) involves a charged-particle energy analyzer and low-current ion sourcecomponents common to ESCA/ HREELS and SIMS, respectively. Only a change in polarity of the energy analyzer is needed to perform rudimentary ISS experiments. Typical commercial instrumentation exists as an afterthought to ESCA or Auger spectrometers. However, component geometries and alignments are usually optimized for electron spectrometry, not ISS. ISS involves the two-body conservation of momentum and energy in a collision between a noble-gas ion beam and the sample surface and the subsequent energy analysis of the loss that results from the “billiard-hall” collision. The ion beam is typically at a few hundreds to thousands of electron volts and at a low current density. This experiment requires the Same type of noble-gas ion sources discussed above for
n source
Flwuro 2. Schematic of ion beam-polymer interactions ieadinl lnwts show
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 1 1 , JUNE 1, 1990
MS and ISS.
static SIMS and sector or mirror-type energy analyzers commonly used for ESCA or Auger spectroscopy. However, the cylindrical mirror analyzer (CMA) is preferable because of the limited signal levels at low-current densities and the low-energy resolution requirements. The CMA offers the advantage of higher signal throughput at lower energy resolutions over the hemispherical sector (7)and a 360° solid acceptance angle when the ion source is coaxial with the CMA. Unfortunately, commercial instruments do not incorporate this design. Normally the hemispherical analyzer of an ESCA or part of a CMA (with the ion gun off axis) is used. For more 80phisticated ISS studies, researchers generally change the ion optics and the polarity of the analyzer. Retardation used in ESCA electron optics results in a loss of intensity, and the increased energy resolution obtained is generally not needed. History
HREELS. This technique is particularly helpful in the study and interpre-
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INS’TRUMEN’TA7lON tation of vibrational states of gases adsorbed on clean metallic substrates. HREELS is also valuable in the study of chemical (catalytic) reactions and chemical structures on solid surfaces. It is even possible to follow the kinetics of adsorption and reaction with times). resolved spectrometers (These studies indicate that the dipolar selection rule dictates which vibrational modes can be excited, as it does for optical spectroscopy. HREELS is sensitive to the different sites of adsorption of the molecules and can disclose the structure or morphology of the adsorbate-substrate system (8). Although HREELS has a specific surface sensitivity to the uppermost 20 8, layer of a polymeric material, it has only recently been applied to very large molecules and polymer surfaces (9). In such applications, information about the chemical composition, the morphology or structure, and the intrinsic (collective) vibrations of polymer surfaces can be obtained. SIMS and ISS. Both techniques are based on the interaction of low-energy ion beams with condensed-phase surfaces. SIMS involves the mass analysis of sputtered or desorbed secondary ions, and the basic principles have been extended to molecular secondary ions. This allows the derivation of molecular structure information based on molecular ion and fragmentation information similar to that inherent in traditional MS. Recently workers have concentrated on minimizing the sputtering rates produced by the primary ion beam as well as the damage that results from high-current densities and energies; the goal has been to produce “static” or low-damage conditions (10-13). In early work (14) the necessity of lowering ion beam damage conditions was pointed out, and the results from ISS and SIMS were compared with those of ESCA. SIMS analysis of polymers has been further refined (15) using low-mass positive ion fragmentation patterns and negative ion analysis (16).Most recently, Benninghoven, Hercules, and co-workers ( I 7) generated high-mass ions, suggesting the possibility of direct molecular weight distribution determination. In contrast to SIMS experiments in which the results of low-energy ions that implant and transfer their momentum are analyzed, the ion-scattering experiment involves analysis of the fraction of primary ions that scatter with momentum and energy conserved. The scattering can be described by a ratio of the energy after collision E to the energy before collision Eo that is related to the masses of the primary ion 650 A
( M I )and target atom ( M z )and the laboratory scattering angle (0)by the following equation.
ISS has been widely applied in the field of catalysis (18), but because of damage concerns similar to SIMS it has only recently been extended to polymer surface structure and orientation analysis (14,19,20). The primary attribute of the technique is the high surface sensitivity, especially to orientations of surface atoms. ISS provides complementary information to that obtained with ESCA and allows scientists to build depth profiles and sense differences in orientation. Problems
Polymer materials surface analysis. For several reasons, polymers are inherently more difficult to analyze than metals, alloys, semiconductors, and even glasses or catalysts. First, they are ill-defined materials both chemically and structurally. They are rarely “clean” or “pure,” often contain additives and stabilizers, and are composed of a distribution of molecular weights. Amorphous and disordered phases in contact with microcrystalline structure are much more common than high crystallinity. Second, they are generally insulating materials. When chargedparticle beams-for example, low-energy (1-10 eV) electrons in HREELS or low-energy ion beams (500 eV to keV) in SIMS or ISS-impinge on polymers, the surface generally builds up an electrostatic charge that rapidly deflects the probe beam, impeding the recording of any spectrum. Finally, polymeric materials are quite radiation and heat sensitive. HREELS. If the polymer can be prepared as a thin film on a conducting substrate (e.g., by spin or melt casting from a solution, or by using a Langmuir-Blodgett type technique), the resulting film conductivity may be sufficient to eliminate the charging problem. The charging effect on insulating polymers can be controlled, but the socalled “flood gun” technique is mandatory for studying thick insulators (21). Current HREELS experience has shown no evidence of electron-induced damage: The electron beam current hitting the sample is typically in the 10-11-10-10 A range. Even after a cumulative dose required for high-resolu-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
tion spectra, no influence on the vibrational spectra could be detected. Energy resolution is a problem specific to HREELS. The number of vibrational modes in polymers that can be excited is so great that the bands in the electron-induced vibrational spectra of polymers will never be completely separated. Indeed, the highest instrumental resolution now achieved is in the 7-10 meV (56-80 cm-’) range, incomparably worse than that routinely obtained in optical spectroscopy. Moreover, this resolution limit does not seem to be dictated by instrumental factors and probably results from intrinsic sample properties or the surface preparation. Until now it has not been possible to use HREELS spectral intensities from polymers in a quantitative manner. This point will be developed later. SIMS and ISS. Polymer and organic materials present particular concerns for ion beam analysis because of the difficulty in understanding the physical and chemical processes underlying, in the case of SIMS, the ejection of secondary molecular ions representative of macromolecular bonding orientation and interactions. For ISS of polymers, predicting and verifying regular functional group orientations that inhibit or encourage scattering (socalled shadowing and shielding) is necessary. Information from SIMS and ISS analysis of the uppermost few molecular layers complements the power of ESCA and vibrational spectroscopic analysis of polymer surfaces. Because sputtering is an unavoidable aspect of the ion beam/solid interaction (and in fact the basis for SIMS), concerns about the sampling depth of each method are related to the effects of ion beam based radiation damage. Even under mild ion beam conditions, damage involving breaking and reforming molecular bonds occurs, and it is these bonds that impart the important structure and properties in polymers. A description of the potentially powerful orientation and bonding information in ISS and SIMS must be tempered by a clear knowledge of the extent and mechanisms of damage. Each technical advance in minimizing damage from the ion beam has led to dramatic advances in the types of information available in SIMS and ISS. In evaluating the data, one must consider the following questions. For both SIMS and ISS, what is the role of damage of the native polymer structure on the depth of sampling? For SIMS, what are the formation mechanisms and escape depths of structurally related secondary ions when damage is strictly controlled? For ISS, how is the
uppermost atomic layer sensitivity realized under low damage, and how are specific regular surface orientations realized and predicted? Current status HREELS Surface spectroscopic information. To be useful, electron-induced vibrational spectra recorded from polymer surfaces must provide information that will identify the material studied. Dozens of polymers have been analyzed by HREELS, and spectra of simple materials [e.g., polyethylene, polystyrene, and poly(methy1 methacrylate)], model compounds such as Langmuir-Blodgett layers, and more complex systems such as poly(ethy1ene terephthalate), polyimide, and polymer physical mixtures have been published (32). Figure 3 shows the high-resolution spectrum recorded from a clean, fully cured polyimide thin film. The sample was a 200 A layer deposited by spin coating from a solution onto a gold decorated silicon wafer. Identifying the peaks recorded between 0 and 500 meV Le., 0 and 4000 em-') required the help of the very large data bank avail-
Figure 3. Elemon-induced vibrational spectrum of a clean, cured polyimide surface. Insel SJWWan IR fingerprim born the 681118 material.
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INS7RUMENTA7lON
--CH,
scissor
m Wavenumber ( Figure 4. ene.
misol
R absorptioi
(b) HREEL spectra from polyethyl-
(Nom dinerent energy scales.)
able for IR and Raman spectra. When such an optical absorption chart is superimposed on the electron loss fingerprint (Figure 3). the hand assignment is almost straightforward Indeed, there is a one-to-one correspondence between the electronically and optically excited vibrational bands. Figure 4 compares the electron and optical vibrational spectra of the simplest polymer, polyethylene. The transmission IR spectrum of polyethylene is very simple, with a low-intensity C-H stretch band around 2950 cm-L (not shown) and two major peaks at 1472 cm-' and 720 cm-'. In contrast, the HREEL spectrum of the same material presents a more complicated fingerprint. The electron-induced vibrational bands correspond to both IR and Raman active modes; the CH, stretches are particularly intense. This suggests that the excitation mechanisms are different in the electronic and optical spectroscopies. 852A
HREEL spectra contain invaluable information that is not available by ESCA analysis. HREELS can detect hydrogen or deuterium present in the material via the CH,, OH, or NH stretch bands. Its energy resolution is sufficient to distinguish aromatic and aliphatic CH, species; even methyne, methylene, and methyl groups can be separated. Finally, carbon-carbon bonds can be fingerprinted; and single, ethvlenic. and acetvlenic bonds can be disthguished (22): Surface mnsitivitv. HREELS offers a way to examine iibrational spectra of polymer surfaces with high sensitivity and a much more limited sampling depth than all the conventional optical spectroscopies. Although researchers to date have not agreed on a preciseestimationofthe probed depth, experimental results suggest that the sampling depth must he extremely small, perhaps < 20 A. Stearic and oleic layers (Langmuir-
ANALYTICAL CHEMISTRY, VOL. 62. NO. 11. JUNE 1. 1990
Blodgett films) of -25 A thickness lack any discernible carboxyl stretch vibrational features around 1650 em-', the band that is usually dominant in IR spectra (23). This result suggests that coherent electron diffusion from an organic surface is restricted to the last few angstroms. For the fatty acids studied, the sampling depth was limited to the outermost -CH2 and -CH3 groups. In effect, the uppermost CH3 layers dominate the spectra (24). Results from hexatriacontane (&HY2) thin films and polyethylene surfaces also showed a large signal from the methyl vibrations relative to the methylene. The vibrational spectra of polyethylene (Figure 4) clearly shows two bands attributed to -CH3 and -CH2 scissor modes at 1370 cm-' and 1440 em-', respectively. Considering that in the hulk this high-density polyethylene contains very few -CH3 groups (which terminate both ends of the polymeric chains), the ratio of the CH3/CH2 groups should be extremely small. The high intensity observed for the CH3scissor modes (1370em-') suggests that the terminating -CH3 groups .lie preferentidy on the polymer extreme surface and that electron-induced spectra gather information only from the uppermmt layer(s) (25). More evidence of the usefulness of the surface sensitivity of HREELS came from a combined IR and electron study of polymer physical mixtures. Normal and perdeuterated polystyrene solutions of the same average molecular number were mixed according to ascribed molecular ratios. Vibrational intensities from the U G H )and dCD)stretch bands at 3000 cm-' were recorded from the solutions with an IR transmission spectrometer and from thin-film surfaces cast from the same solutions with a HREEL spectrometer (Figure 5). Although the optical transmission results show a linear relationship between the band intensities and the inverse of the deuterium molar fraction, the HREELS results clearly present some kind of saturation effect: The film surfaces prepared from low deuterated polystyrene concentrations in the solution disclose a net enrichment in deuterated species. This evidently must be related to selective surface diffusion processes of the perdeuterated chains, during the film casting from the solution, the air drying, or the measurement in UHV. HREELS is the sole electron spectroscopythat could detect this type of effect at the very surface of the composite polymeric films; moreover, it is probable that this effect could not be evidenced by any optical spectrcscopy (26).Both dynamic depth
profiling SIMS and high-energy ion scattering have detected this phenomenon in blends of polystyrene and deuterated polystyrene (27,223). The above results justify the enthusiasm to continue to develop HREELS applications on polymer surfaces. Unique and complementary information on the extreme surface chemical composition is indeed available. Surface morphology o r structure. Studies on clean metal surfaces with HREELS have helped to disclose adsorption and reaction sites. By varying experimental parameters such as the scattering geometry or the electron impact energy, it is possible to gain structural information, especially when highly symmetric adsorption sites are considered. However, polymers are not ideal candidates for such investigation because, on the atomic molecular scale, they present quite rough surfaces and several topology variations. As a result, the electron scattering is very diffuse. This was evidenced for stearic acid Langmuir-Blodgett systems deposited
I
I
onto substrates (Ag, Au, Al, Ge) with different surface finishes (23). As a consequence, for all the polymers studied up to now, the reflected beam intensity in the elastic and inelastic channels is spread over a range of loo20' around the specular direction. This spread did not prevent the successful study of the surface morphology of a well-cured and (probably) crystallized polyimide film. Ratios of the intensities of the C-H stretch versus the C=O stretch were measured for different scattering geometries. Results showed that the C-H stretch signal is more intense when electrons are collected in a direction close to normal at the surface, whereas the C=O band is enhanced in a direction close to the grazing collection angle. This evidence suggested that the CH and CO present different orientations on the surface of the film as prepared, a finding that is in agreement with other structural analyses of polyimide films (29). Quantification a n d scattering mechanisms. To study the relation-
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
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INSTRUMENTATION ship between HREELS band intensities and the effective number of different chemical functionalities on the polymer surface, one must consider the excitation mechanisms that play an effective role in the electron loss process on polymers: dipolar, impact, and resonance scattering (8,30).Until recently, this situation prevented direct application of symmetry rules and use of (dynamic) dipolar moment strength to predict band-relative intensities. However, on metal substrates the so-called surface normal selection rule (of the dipolar interaction) is substantially relaxed for at least some of the molecular vibrations. Systematic investigations
of possible image dipole effect from the substrate, polarizability contributions from substrate contaminants, and very low energy excitations through the polymer film (31)will be needed. In addition, careful cross-section measurements (hand intensities vs. electron impact energy) on well-characterized polymers and model compounds will be required to elucidate this problem. Currently, HREELS studies of polymer surfaces can produce only qualitative results, except when the studies are performed using standard or reference materials. Metal-polymer interfaces. Detecting minute surface phenomena such as
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Figure 6. HREELS evaluation of polymer metallization. (a) HREEL spectrum of clean polyimide sv1acB. (b) SDectrUm alter deD06nlon 01 1.2 X 10" amms 01 AI. and (c)difference s p e m (b - a).
(1.54,.
ANALYTICAL CHEMISTRY, VOL. 62. NO. 11, JUNE 1, 1990
the incipient phases of a new interface formed on the polymer surface is within the capability of HREELS. This was illustrated by the study of chemical reactions and sites developed during the metallization of polymers (32). Figure 6 shows representative HREELS data collected during aluminum condensation onto PMDA-ODA (pyromelletic dianhydride-4,4'-oxydianiline) polyimide (1,2,4,5-benzene tetracarboxylic anbydride-4,4'-oxydianiline). The metal atoms are evaporated from a Knudsen cell (1A h i n ) , and Figure 6b shows the spectrum of a clean polyimide surface covered with 1.2 X 10" Al atoms/emz. Around 3000 cm-' and between 1100 and 1800 cm-', eight vihrational bands are resolved and assigned, indicating that some hands are more affected hy A1 deposition than others. Figure 6c shows a difference spectrum that enhances this observation. Severely attenuated bands at 1720 and 1120 em-' clearly localize the A1 atoms near C = O sites. The Y (C-N) band is attenuated simultaneously, but not as severely. The C-0-C ether linkage (-1250 em-') does not appear to be affected by metal deposition, hut a large intensity increase over the whole 1400-1600 cm-' loss region shows major intensity transfer between vihrational modes. Either the phenyl ring stretches of the ODA segment in the polyimide are enhanced or the Y (C=O) and Y (C-N) we red and blue shifted, respectively. Finally, the u(C-H) band above 3000 cm-I is significantly enhanced in Figure 6b. These spectral features suggest that the initially deposited AI atoms preferentially interact with carbonyl oxygen atoms and that the C-N group is also perturbed via electronic delocalization on the imide ring. This perturhation might then affect the ODA segments, introducing a conformational change. The C-H bonds may be changing with respect to the surface, increasing the intensity of C-H bands at 1400-1600 cm-', whereas the CsH2moieties in the PMDA are not affected. This explanation is also consistent with ESCA results on the same system. Lowsnergy Iss Homopolymer surface structure. Low-energy ISS wm first applied to polymers in a study that involved developing low-damage conditions to preserve slight changes in surface conformation of methacrylates (14). This work showed that the surface composition of glassy methacrylates was equivalent to the bulk composition, except for three polymers with bulky side chains that might he sterically hindered.
c
e
3He-- C
1.45 min 60 nAJcm2
aHe--N
L ,ure 7. Structures of vinylpyridlne polymers derived from molecular met..-...-. (a) Top down view of poiyi2-vinyipyridIne) (PPVP) atactic chaln. (b) side view of P2VP aIactic chain (note N is shadowed by C in backbone). (c) view of five mom. m lwklng down at backbone. and (d) ion-ocatterlng spectra lor P2VP and poly(4-vinylpyridine) (P4VP). Note me lack of signal hom N in the P2vP spenrum. (Adaptedfrom Reference20.)
ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1. 1990
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INSTRUMEN7A7ION This observation remained unexplained until recent studies showed that ISS could distinguish between conformations of stereoregular poly(methyl methacrylate) (PMMA) and slight amounts of poly(methacry1ic acid) (PMAA) copolymerized randomly with PMMA (19). Very different C/O scattered ion intensity ratios were measured in the ISS experiments. Angular-dependent ESCA results were
unable to distinguish between any of these polymers. Atactic, syndiotactic, and isotactic PMMA and PMMA/ PMAArandom copolymersall have the same atomic composition and no differences in monomer bonding. Molecular models were used to explain the differences in measured surface atomic concentrations that result from shielding and shadowing of functional groups in different conformations. Similar re-
T
Unmodified P2VP
3.2
I
4.8
Souner time lminl
i rigure 8. ISS results from (a)P2\rr ana WVP
ana (DJ moartreaana unmoaitiea pzvp. Note the change in intensity vwsw time. whlch indicates very low levers of damage. Reaction Wnh alkyl iodide in 801Ulion promotes rotetion Of nw-n in P2VP from beneath me polymer backbont. (Adapted from RBterenCB 20.) 656A
ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
sults were obtained with methyl- and phenyl-substituted polysiloxanes, where the Si10 ratios were different. We have since extended this work (20) to probe differences in the shielding of nitrogen functionalities in different isomers of vinylpyridine polymers (Figure 7). Rather than explaining the results after the experiment with models, we studied polymers whose structures were previously confirmed by molecular mechanics calculations and NMR spectra. Based on this information, we predicted that different N/C scattered ion intensity ratios would be measured for these polymers. Because the nitrogen in poly(2-vinylpyridine) (PZVP) is shielded by the backbone carbons, no initial nitrogen scattering signal should be measured; this was confirmed experimentally. We also observed that the N/C ratio increases upon sputtering to the equivalent of poly(Cviny1pyridine) (P4VP), which always shows nitrogen scattering. The quaternization of the nitrogen in P2VP via methyl, ethyl, or tert-butyl groups causes nitrogen to rotate, because of steric considerations, away from the backbone. This rotation yields very different N/C signals (Figure &?),indicating the sensitivity to the orientation of this particular functional group. Multitechnique depth profiles of multicomponent polymers. Integrating the ISS method into the multitechnique analysis of polymer materials allows ita superior surface sensitivity to complement ESCA and FT-IR sampling at different depths. This was illustrated by the analysis of random block copolymers of bisphenol A polycarbonate (BPAC) and poly(dimethy1siloxane) (DMS) (33): Quantitative surface compositions determined from angle-dependent ESCA and ISS were combined with scanning electron microscopy (SEM) to develop a morphological model of the domain segregation approaching the air-facing interface. The same approach was then used on polymer hlends created from homopolymers of BPAC and DMS. These results allow researchers to compare the forces driving segregation to the near-surface region in a blend of the aame components (as in copolymer systems under study) and to observe effects of surface energy differences and blend component compatibility (i.e., interchain bonding interactions) in the overall free energy of mixing. We have heen able to show that in blends of < 11%(by weight) DMS, the surface comp~itionas measured by angular-dependent ESCA and Iss was always 85%(by weight) DMS (34). Al-
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though these results suggest that a compatible mixture exists at that concentration, this system is considered to be incompatible. At higher concentrations (25% DMS), visible phase separation occurred and the surface was 100% DMS. We are currently studying a variety of incompatible and compatible blends, attempting to use ISS to complement angle-dependent ESCA. Another application is in the area of surface-modified polymers (35-37). In a series of studies of different treatments of PMMA, we used the higher surface sensitivity of ISS to develop specific models of orientation of modified functional groups. ISS results showed a great increase in surface oxygen, yet ESCA results showed a decrease in oxidized carbon concentrations (35, 36). These data were completely consistent with the adsorption of water vapor rather than permanent oxidation. Different treatments that promoted specific oxidation were then developed (37).Contact angle methods were also used to differentiate functionality in the latter study. One outcome of that work has been attempts to determine the relationships between ISS conformational sensitivity and contact angle techniques. Static SIMS Homopolymer structure sensitivity. We will consider three different types of ions (Figure 2) generated in the static SIMS experiment from polymers. They can be considered to be roughly divided by mass range (i.e., low, high, and in between), but more generally are divided by the mechanism of formation (fragmentation and rearrangement, desorption and cationization of oligomers, and simple bond breaking/ charge stabilization). The most complex of the three are low mass (Le., < 300 amu) fragment ions, the original focus of much static SIMS polymer characterization. In the first applications of static SIMS to the analysis of polymer surfaces, low mass (0-150 amu) fragment ions (14, 15) were used to distinguish structure and isomerism in a series of methacrylate polymers with different side chain structures. The surfaces were distinguished primarily from positive ion spectra via their fragmentation pattern from the side chain, within a common pattern characteristic of the backbone. A significant result from these studies was the differentiation of isomeric structures of butyl methacrylate, which could not be distinguished by ESCA core-level analysis and by the analysis of slight reactive surface degradations of the tert-butyl methacrylate (38). Subsequent improvement in SIMS
ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
experimental conditions to lower surface damage induced by the primary ion bombardment has allowed for the analysis of higher mass fragments that are related to the original structure of the polymer (15).In the course of these studies, the commonly accepted description of static or low-damage conditions evolved from the original current density (1nA/cmL)criterion based on sputter yields to preserve a surface for 1 h (10) to minimum ion dosages (e.g., ions) (15,ZO). In addition, other criteria for minimal damage have been proposed, some of which are based on the theory that chemical and structural disruption result from inefficient dissipation of the primary charge input attributable to the inherent low conductivity of the polymer sample (16). The use of a primary atomic (neutral) beam was suggested in this case to remove the input of charge to the sample, with similar spectral results as derived from primary ion beam analysis and better negative ion spectra (16). However, precise spatial and low current control of the atomic beam is much more difficult compared with an ion beam (16). It is instructive to compare results from the ISS work on nitrogen-containing polymers discussed earlier with the typical conditions for static SIMS. In the ISS work, damage to the native polymer structure as assessed by changes in N/C signal intensities (Figure 8) was apparent after 2 min of irradiation at 2 nA/cm2 (i.e., 1.5 X lo-" ions) with 2 kV He. This dosage is at or below static conditions, in part because the sputter yield of He is lower than that of the Ar or Xe primary ions used in a typical static SIMS experiment. The ion beam induced changes were not detectable in comparisons of ESCA spectra taken before and after analysis. Furthermore, simple sputtering theory at the beam currents used would not allow for significant ion beam depth profiling (10). Clearly in this case chemical modification and damage of the surface of the polymer is occurring, and sputter-induced consumption of the material is not necessarily involved. The problem of considering the higher damage cross sections in the determination of primary ion beam conditions for organic species, as opposed to sputter yields, has been recognized by Benninghoven's group (17, 39). They have designed a TOF secondary ion mass spectrometer that incorporates a pulsed ion source of very low beam density (< 1 pAlcmL). Lower dosages are possible because of the simultaneous detection of all ions from a pulse of primary ions in the TOF experiment. As conditions and instru-
mentation have improved, more information has been gained. Because of the simplicity of assignment for higher mass ions, this work has provided information to characterize homopolymer and multicomponent polymers. Applications of the TOF-SIMS experiment to nylons, polystyrenes, poly(dimethylsiloxanes) and surface-modified polymers have presented information beyond the characterization of the monomer identity (17, 39, 40). For instance, high-mass oligomeric ions can generate an accurate molecular weight distribution (17). The potential here is great; a surface-sensitive molecular weight distribution would be important for many polymer processing problems. High-mass fragmentation patterns have been postulated to be sensitive to intrachain bonding in nylons (17) and composition in complex polyurethanes (40). Differentiation of isomeric composition is also evident from high-mass ions (17). Recently attention has focused on the ions generated from fragmentation along the chain into charge-stabilized ions, which we have termed “n-mer ions” (41). These ions could provide a great deal of information about intrachain bonding and chemical reaction sites. N-mer ions have been observed from a variety of polymers: Teflon (39), homopolymer and copolymer nylons, and segmented poly(ether urethanes) (42). In our most recent work, we discussed charge stability in ion formation from functional polymers (e.g., the vinylpyridines and substituted polystyrenes), with emphasis on the effects of charge and reaction site and structure determination via analysis of the n-mer ions (41). The potential impact in qualitative and quantitative surface analysis of a variety of multicomponent polymer applications where functional and nonfunctional polymers are combined is obviously great. However, more attention to the simple ability of static SIMS to differentiate the surfaces of polymer materials with equivalent atomic compositions and difficult ESCA chemical shift information has meant even greater excitement about fragmentation pattern analysis. Multicomponent polymers. A few studies of multicomponent polymers have successfully used model polymers to explore the effects of fragment ion formation (42-44). Block and random copolymers were analyzed, and fragment ions unique to each structure were identified. Quantitative ion intensity ratios were correlated to composition, and sampling depths were estimated via comparison with ESCA data. However, many questions generated by this type of study remain to be an-
swered. Low-mass fragment ions originate from a variety of processes, and in unknown multicomponent polymers it would be impossible to assign ions unambiguously to particular structures or components. Besides the dynamic SIMS depth profile work on labeled polymers mentioned earlier (27), a series of very exciting experiments using dynamic SIMS depth profiling on block copolymer thin films by Russell et al. were recently described in an overview (45). This work shows the power of focusing strictly on elemental ions in determining profiles; where unique isotropic tags can be used to identify one component, complex different ion formation mechanisms can be ignored. One advantageous aspect of the work by Russell is the correlation to ESCA and light and X-ray scattering. However, even with these advances, much work remains to be done to identify exact conditions that describe sampling depth issues, and instrumental improvements are needed to further enhance the probability of formation and detection of high mass ions. Summary What does the future hold for HREELS, SIMS, and ISS? Even with better knowledge about the mechanisms of vibrational energy loss, there are three major challenges to the use of HREELS for vibrational spectroscopy of the surface of polymers: charge compensation, limited resolution, and knowledge of the sampling depth. Progress is being made on experimental charge compensation to allow full use of the method, and studies on thin films can be performed to resolve other issues. With the best conditioned polymer surface, a clear understanding of the contributions of roughness and electrical properties on the broadening of the elastic peak in HREELS may allow the deconvolution procedures used in IR, Raman, and electron spectroscopy to be widely applied (46, 47). This approach is simple in concept for HREELS because the model lineshape that would be deconvolved is available directly in the experimental spectrum. Line narrowing from this approach would provide information on the relative intensities and identification of bands under a complex envelope. To resolve sampling depth issues, more knowledge about the mean free path of low-energy electrons will have to be gained for model insulating polymers; our laboratories are now collaborating to perform such transmission experiments on thin films. We hope that these efforts will more broadly define the technique so that it will be more
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Applications in Agricultural, Pharmaceutical, and Envimnmental Chemistry
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applicable in concert with other surface science methods to study polymer surfaces and interfaces. One application involves metalpolymer interfaces. Determining the initial stages of metal deposition and the functional groups at the surface that are important to adhesion promotion are the goals of these studies. In the area of ion beam methods (specifically, low-energy ISS), two important applications to polymers should be possible with further developments in instrumentation. The socalled impact collision ISS experiment (ICISS), developed by Aono and Souda (48) to identify bond orientations from shadow cone formation considerations, would allow the determination of exact bond angles in highly oriented regular surface structures. For some types of systems this information would be highly desirable. In addition, electronic structure-related effects on charge neutralization are completely unknown in ISS of polymers; these fundamentals should be investigated. A better understanding of the surface sensitivity and structurelorientation sensitivity will be extremely useful in complementing ESCA for analysis of the uppermost surface interface. It appears that such data will help resolve the details of contact angle measurements with ESCA and ISS. This information could be useful in quality control and on-line analysis. Although contact angle measurements can be used more readily, detailed spectroscopic information to monitor surface processes such as corona discharge treatments would be invaluable. Finally, we are excited about the potential of SIMS for polymers. In addition to the current lsw-mass uses, there are unanswered and controversial questions about the limitation in ion formation at high mass. If these questions can be answered, the molecular weight distribution of the near-surface region may be determined. When this information is compared with the bulk molecular weight, segregation and processing effects can be known. Important surface properties related to segregation of low molecular weight species plague a variety of polymer interfacial adhesion and compatibility issues. The determination of chain orientation and intrachain bonding in the near-surface region may be available from n-mer ion distributions. Hydrogen bonding in proteins in different conformations may be deduced from the probability of formation of specific ions. The average length in monomer units of the folds and loops in a polymer chain can then be determined from the distribution of n-mer ions.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 11, JUNE 1, 1990
All of these possibilities complement surface compositional information that is available from other methods and give hope that the future of applications of these methods will be bright for polymer analysis. The contributions of J. H. Magill, R. L. Schmitt, J. H. Wandass, P. A. Cornelio, K. J. Hook, T. J. Hook. L. Salvati, R. L. Chin, T. G. Vargo, C. Gregoire, M. Rei W a r , M. Vermeersch, Y. Novis, and P. A. Thiry are gratefully acknowledged. Most of the HREELS work was performed while JeanJacques Pireaux was research associate at the Belgian National Fund .for Scientific Research, and we acknowledge the financial support provided by that institution. Work at Buffalo is sponsored by the National Science Foundation Polymers Program of the Division of Materials Research. This joint research on electron and ion spectroscopies is sponsored by NATO project 0563/88.
References (1) a. Biotechnology and Materials Sci-
ence: Chemistry for the Future; Good, M. L., Ed.; American Chemical Society: Washington, DC, 1988. b. Polymeric Materials: Chemistry for the Future; Alper, J.; Nelson, G. L., Eds.; American Chemical Society: Washington, DC, 1989. (2) Research Briefings 1986: Interfaces and Thin Films; Committee on Science, Engineering and Public Policy of the National Academy of Sciences, National Academy of Engineering and Institute of Medicine; National Academy Press: Washington, DC, 1986, pp. 2-15. (3) Nebesney, K. W.; Masschoff, B. L.; Armstrong; N. R. Anal. Chem. 1989, 61, 469 A. (4) Gardella, J. A., Jr. Anal. Chem. 1989, 61,589 A. (5) Dilks, A. Anal. Chem. 1981,53,802 A. (6) . . Pireaux. J. J. J. Electron. SDectrosc. Relat. Phenom., in press. (7) Barrie, A. In Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden: London, 1977, pp.-7-9-119. (8) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. (9) Pireaux, J. J.; Gregoire, C.; Vermeersch, M.; Thiry, P. A,; Caudano, R. Surf. Sci. 1987.189f190.903. (10) a.'Benninghoven, A. J. Vac. Sci. Technol. A 1985, A3(3),451. b. Benninghoven, A.; Rudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications and Trends; Wiley: New York, 1987; Chapter 5, p ,672-753. (11) Winograd, N. {canning Electron Microsc. 1985,III, 919-26. (12) Pachuta, S. J.; Cooks, R. G. Chem. Rev. 1987,87, 647. (13) Colton, R. J. Nucl. Instrum. Methods Phys. Res. 1983,218,276-86. (14) a. Gardella. J. A.. Jr.: Hercules. D. M. Anal. Chem. 1980, 52, 226. b. Gardella, J. A., Jr.; Hercules, D. M. Anal. Chem. 1981,53,1879. (15) Briggs, D.; Hearn, M. J. Vacuum 1986, 36,1005. (16) Brown, A.; van den Berg, J. A+;Vickerman, J. C. Spectrochim. Acta 1985, 40E, A71
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(lyj Bletsos, I. V.; Hercules, D. M.; Grei-
fendorf, D.; Benninghoven, A. Anal. Chem. 1985, 57, 2384. b. Bletsos, I. V.; Hercules, D. M.; vanleyen, D.; Benninghoven, A. Macromolecules 1987,20,407.
(18) Horre1l.B. A.;Cwke. D. L. Cotal.Reu.
Sci. Eng. 1987,29(4),447-91. (19) Hook, T. J.; Schmitt, R. L.; Gardella. J. A,. Jr.;Salvati, L., Jr.;Chin, R. L.Anol. Chem. 1986.58,1285. (20) Hook. K. J.; Cardella. J. A,, JT.; Salvati. L.. Jr. Macromolecules 1987, 20, 2112. (21) Liehr. M.: Thirv. P. A,; Pireaux. J. J.; Caudano, R. Phys.-Reu. B 1986,33,5682. (22) Wandass, J. H., HI;Gardella. J. A., Jr. Surf. Sei. 1985,150,L107. (23) Wandass, J. H., III; Gardella, J. A.. Jr. Langmuir 1986.2.543. (24) Wandass, J. H., Ill; Gardella, J. A,, JT. Lanzmuir 1987.3.183. (25) Pireaux, J. J.; Thiry, P. A,; Caudano, R.; Pfluger, P. J. Chem. Phys. 1986, 84, 6452. (26) Rei Vilar, M.; Schott, M.; Pireaux, J. J.; Gregoire, C.; Caudano, R.; Lapp, A.; Lopes da Silva, J.; Botelho do Rego. A. M. Surf. Sci. 1989.211-12, 782. (27) Valenty. S. J.; Chera, J. J.; Olson, D.R.; Webb, K. K.; Smith, G.A.; Katz, W. J.Arn. Chem. Soe. l984,106.6155. (28) Jones, R.A.L.; Kramer, E. J.; Rafailovich. M. H.; Sokolov, J.; Schwarz, S. A. Phys. Rev. Lett. 1989,62,280. (29) Pireaux, J. J.; Vermeersch, M.; Gregoire. C.; Thiry, P. A,; Caudano. R.; Clarke, T.C. J. Chem. Phys. 1988, 88, 3353. (30)Thiry, P. A.; Liehr, M.; Pireaux, J. J.; Caudano, R. Phys. Scripta 1987.35.68. (31) DiNardo, N. J.; Demuth, J. J. Chem. Phys. 1986,85,6739. (32) Pireaux. J. J.; Vermeersch, M.; Degosserie, N.; Gregoire, C.; Novis, Y.; Chtaib, M.; Caudano, R. In Adhesion and Frielion; Grunze, M.; Kreuzer. H. J., Eds.; Springer Verlag: Heidelberg, 1989. pp. 53-66. (33) Schmitt, R. L.; Gsrdella, J. A,, Jr.; Magill, J. H.; Salvati, L., Jr.; Chin. R. L. Moeromoleeules 1985,18,2675. (34) Schmitt, R. L.; Cardella, J. A., Jr.; Salvati, L.. Jr. Macromolecules 1986,19,648. (35) Hook. T. J.; Gardella, J. A., Jr.; Salvati, L..Jr. J. Mater. Res. 1987.2(1), 117. (36)Hook, T. J.; Gardella. J. A., Jr.; Salvati, L.. Jr. J. Mater. Res. 1987,2(1),132. (37) Vargo, T. G.; Gardella, J. A,, Jr.; Salvati, L., Jr. J. Polym. Sci. Port A: Polyrn. Chem. 1989,27,1267. (38) Cardella, J. A., Jr.; Novak, F. P.; Hercules, D. M. Anal. Chem. 1984.56.1371. (39)Bletsos, 1. V.; Hercules, D. M.;Magill, J.H.; vanleyen. D.; Niehus, E.; Benninghoven, A. Anal. Chem. 1988,60,938. (40)Rletsos, 1. V.; Hercules, D. M.; vanLeyen. D.; Benninghoven. A,; Karakatsank, C. G.; Rieck, J. N. Anal. Chrrn. 1989,61,2142. (41) Hook, K. J.; Hook, T. J.; Wandass. J. H., III; Gardella, J. A., Jr. Appl. Surf. Sei. 1990.44, 29. (42) (a) Briggs, D. 0%. Moss Spectrom. 1987, 22, 91. b. Hearn, M. J.; Ratner, B. D.; Brig@, D. Macromolecules 1988. 91
Y,Y . Jusrpk A . Gnrdrlla, J r . ( l u f f ) is on leave from t h e S t a f r 1 , ' n r r r r s i t ~~ f X wYork at
B u f f a l o a n d currentlyserces as programofficerforanalytical and surfacechernistry i n t h e chemistry division of t h e National Science Foundation. He received a Ph.D. from t h e University of Pittsburgh (1981) and joined the faculty at Buffalo i n 1982after postdoctoral research at t h e Uniuersity of Utah. His research interests involve surface chemistry and structure of organic and biological t h i n films, multicomponent polymers, and biomaterials. Special emphases include ion beam and electron microscopies applied to these systems. Other interests include t h e philosophy of science, curriculum and teaching, basketball, baseball, and bowling ( t h e sport o f kings or philosophers). Jean-Jacques Pireaux (right) is professor of physics at t h e Facult& Uniuersitaire Notre Dame d e la Paix i n Namur, Belgium. H e received his Ph.D. i n physics f r o m F U N D P N a m u r (1976) andsince t h e n has been associated with t h e Laboratoire Interdisciplinaire d e Spectroscopie Electronique at Namur. There h e pursues research i n electron spectroscopies ( X P S I E S C A and electron-induced uibrational spectroscopy) t o characterize polymer surfaces, polymer-metal interfaces, and modified polymers. Pireaux has authored more than 100 papers in these fields and is currently European editor of t h e Journal of Electron Spectroscopy and Related Phenomena.
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