Langmuir 1992,8, 1199-1203
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Force Microscopy of Ion-Containing Polymer Surfaces: Morphology and Charge Structure F. Saurenbach, D. Wollmann, B. D. Terris, and A. F. Diaz* IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Received November 19, 1991. In Final Form: February 10, 1992 A force microscope is used to detect charge structure on the surfaces of films of the insulating polymers polystyrene and poly(styrene-co-butylmethacrylate). In contrast, films of the ionomer styrene-co-Nmethyl-4-vinylpyridinium toluenesulfonate with 6% ionic monomer (520pmol/g ions) do not show such a charge structure by force microscopy, and charge deposited on the surface decays rapidly. The film of a copolymer/ionomerblend, with 26 pmol/g ions, shows intermediate behavior where the surface charge decays over a period of ca. 1h. Annealingthe films does not change the observed behavior of the insulating polymer films or the ionomer films but does affect the behavior of the blend. In the latter case, annealing makes the surface of the film sufficiently conductive that the film does not hold a charge. X-ray photoelectron spectroscopyresults reveal that the change in the behavior corresponds to a change in the surface composition where the surface is enriched with ions from the ionomer.
Introduction Ions are frequently incorporated into polymers to enhance a variety of their performance characteristics, both mechanical and electrical. While the bulk properties of such systems have been studied extensively, the surface properties are only beginning to be explored. One reason for the increasing interest in these surfaces is the importance of ions in contact charging of polymer surfaces, which is of great technological interest, especially in electrophotographic printing and copying. In particular, such ioncontaining polymers are used to control the contact electrification properties of xerographic toner particles. Recent studies with polymers have implicated electron,610 and protonl1-l3 transfer in the generation of charge. Regardless of the mechanism of charging, the charge is strongly affected by ion type, mobility, and distribution. Both molecular salts and ionomers, which are random copolymers of ionic and nonionic monomers, have been used as the source of ions in polymer blends. There is a large difference in the ion mobilities associated with these two types of ionic materials. Molecular salts have independently mobile anions and cations while in the case of ionomers, either the cation or the anion may be immobilized by binding it to the polymer backbone, (1) Birkett, K. L.; Gregory, P. Dyes Pigm. 1986, 7, 341. (2) Macholdt, H.-T.; Sieber, A. Dyes Pigm. 1988,9, 119. (3) Diaz, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. J. Polym. Sei., Polym. Lett. 1990,28, 75. (4) (a) Diaz, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.;
Eisenberg, A.; 5th International Congress on Non-Impact Printing Technologies, San Diego, CA, 1989. (b) Diaz, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wpllmann, D.; Eisenberg, A. 63rd Colloid and Surface Science Symposium, ACS, Seattle WA, 1989. (5) Mizes, H. A.; Conwell, E. M.; Salamida, D. P. Appl. Phys. Lett. 1990,56, 1597. (6) Gibson, H. W. J. Am. Chem. SOC.1975,97,3832. (7) Anderson, J. H.; Bugner, D. E. 4th International Congress on Non-
Impact Printing Technologies,New Orleans, LA;The Societyfor Imaging Science and Technology: Springfield, VA, 1988; p 79. (8)Bugner, D. E.; Anderson, J. H. Polym. Prepr. ( A m . Chem. SOC., Diu. Polym. Chem.) 1988, No. 29, 463. (9) Anderson, J. H.;Bugner,D. E.;DeMejo,L. P.;Sutton,R.C.; Wilson, J. C. US Patent 4,837,392, June 6, 1989. (10) Nanya, T.; Tsubuko, K. Japan Hardcopy '88; SOC.Electrophotography Japan, 1988,9. (11) Folan, L. M.; Arnold, S.;OKeeffe, T. R.; Spock, D. E.; Schein, L. B.; Diaz, A. F. J . Electrost. 1990, 25, 155. (12) Mataui, N.; Oka, K.; Inaba, U., The 6th International Congress on Aduances in Non-Impact Printing Technologies; The Society for Imaging Science and Technology: Springfield, VA, 1990; p 45. (13) Watanabe, M.; Nagase, H. US Patent 4,883,735, Novenber 28, 1989.
leaving only the counterion mobile. Contact charging results with molecular salts and with ionomers (either the "anchored" cation or the "anchored" anion) have been r e p ~ r t e d , ~and ~ ~ Jthe - ~ charge is shown to be strongly affected by the nature of the ion source.3-9 Although the electrical properties of these surfaces are very important to charging, the surfaces are difficult to characterize because of the small amount of ions involved. Electrostatic force microscopy is a novel method for studying the distribution of localized surface charge. As described p r e v i o ~ l y , ~the ~ -electrostatic ~6 forcemicroscope (EFM), which is a modified force microscope, offers promise for obtaining high lateral resolution and, in addition has the sensitivity to detect the charge from a few electrons. In the EFM, a weak lever is scanned close to a sample surface. The lever responds to any changes in the forces acting on it, such as from surface charge, and the lever deflection is measured. In addition to measuring the sample charge, the end of the lever can be touched to the surface in order to deposit charge. In this study, the effect of ions on the electrostatic behavior of a series of polymer surfaces was examined. The polymers ranged from nonionic materials, i.e. polystyrene and poly(styrene-co-butyl methacrylate), to an ionomer, poly(styrene-co-N-methylvinylpyridiniumtoluenesulfonate). Intermediate materials, comprised of blends of poly(styrene-co-butyl methacrylate) and the ionomer, were also studied. While bulk dielectric studies of both nonionic polystyrene and styrene-based ionomers have shown these materials to be insulators, they exhibit different behavior in contact charging experiments, suggesting that their surface electrostatic behavior might also differ. X-ray photoelectron spectroscopy, XPS, was used to determine the elemental composition of the surface region (ca. 50 A penetration). XPS complements the results from the EFM by ascertaining the overall composition of the film surfaces, although the available XPS equipment does not have the spatial resolution attained by the EFM. In addition, scanning electron microscopy, SEM, images of the surfaces were taken and compared to the surface topography imaged by the EFM. (14) Stern, J. E.; Terris, B. D.; Mamin, H. J.; Rugar, D. Appl. Phys. Lett. 1988, 53, 2717. (15) Terris, B. D.; Stern, J. E.; Rugar, D.; Mamin, H. J. Phys. Reu. Lett. 1989, 63, 2669. (16) Terris, B. D.; Stern, J. E.; Rugar, D.; Mamin, H. J. J . VQC.Sci. Technol., A 1990, 8 , 374.
0743-1463/92/2408-1199$03.00/00 1992 American Chemical Society
1200 Langmuir, Vol. 8, No. 4, 1992 +CHz
-CH j ( CHz-CH
1
I
CH3 poly(slyrene-c~Kmethylvinylpyridinium toluenesutfonate)
S-XMPV OTs. where x = 0.06or 0.1 1
Experimental Section The materials used for the film surfaces in these experiments belong in one of three categories: nonionic, ionomer, or a blend of the two. The nonionic polymers,polystyreneand poly(styreneco-butyl methacrylate), S-BMA, were available commericially. The synthesis of the ionomer poly(styrene-co-N-methylvinylpyridinium toluenesulfonate),S-xMVP OTs, has been previously reported.3J' The mole fraction of ions in the ionomer is given byx andiseither0.06or0.11. BlendsofS-MVPOTsandS-BMA were prepared as follows. The desired amounts of the two polymers were first dry-mixed, then melt mixed, cooled, and ground to a fine powder.3J7 The particles obtained were typically ca. 10 pm in diameter. To prepare the films, ca. 0.05 g of the polymer or polymer blend was dissolved in 1 mL of chloroform, and the solutions were spun cast onto 24 X 30 mm microscope cover slips at 30 000 rpm for 30 s. The samples were air-dried for 2-3 days to remove any traces of solvent. In the case of the polystyrene film, the substrate was coated with gold prior to spin casting. Some films were thermally annealed by heating at 85 "C for 18 h under a partial vacuum of 6-10 Torr. Any solvent loss which may have occurred during this treatment did not alter the visual appearance of the surface. The operation of a force microscope in the noncontact, or attractive, mode has been described in detail previously.15J6Js For these experiments, the lever/tip is formed by bending an etched tungsten wire. The lever is mounted on a piezoelectric bimorph and is oscillated just above its natural resonant frequency. As the tip scans the surface, changes in the tip-tosurface force gradient will shift the resonant frequency of the lever, and thus change the amplitude of oscillation. The lever motion is detected with an optical fiber based interfer~meter,'~ and a feedback loop adjusts the tip height so as to maintain a constant oscillation amplitude. By monitoring the feedback voltage, contours of constant force gradient are measured. The force gradient measured as described above will include all forces felt by the tip, not just electrostatic forces. In addition, the force gradient between the tip and any surface charge depends on the product of the charge and its image charge in the tip and is thus insensitive to the polarity of the surface charge. In order to distinguish electrostaticforces from other forces,and to determine the sign of any surface charge, an ac voltage is applied between the tip and an electrode located on the back of the insulating ~ a m p l e . ~ ~IfJthere 6 is charge present on the sample surface, the force gradient between the surface charge and the charge due to the ac voltage will cause a tip oscillation at the ac voltage frequency. The total tip motion will therefore consist of an oscillation at the bimorph frequency, with an envelope at the ac voltage frequency. This envelope is detected by a lock-in amplifier, and the phase of the signal is determined by the sign of the charge. This output is referred to as the "charge signal". This technique has been used to image a variety of local charge distributions, including tribocharged samples*5and ferroelectric domain walls,20with spatial resolution better than 200 nm and sensitivity approaching the charge of a single electron. The surface compositions of the samples were determined by XPS using a HP ESCA Model 5950 spectrometer. The films were prepared by spin coating and allowed to air-dry as previously described. The heat annealing was carried out for 19 h at 60 "C and 6-1OTorr. Low energy electronswere supplied to the samples (17) Diaz, A.; Fenzel-Alexander, D.; Wollmann, D.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 1991, 29,1559. (18) Martin, Y.; Williams, C. C.; Wickramasinghe, H. K . J . Appl. Phys. 1987, 61, 4123. (19)Rugar, D.; Mamin, H. J.;Guethner, P. Appl. Phys. Lett. 1989,55, 2588. (20)Saurenbach, F.; Terris, B. D. Appl. Phys. Lett. 1990, 56, 1703.
Saurenbach et al. as required to neutralize surface charging, and the spectra were referenced to the main carbon peak at 284.6 eV. The peaks used for analysis were (position and cross-section in parentheses): C ls(284.6eV,1.000),Ols(532.0eV,2.492),Nls(399.0eV,1.678), andS 2p (164.0eV,1.794). Contaminationby Si (100.0eV,1.128) and F (686.0 eV, 3.328) was also monitored. A Hitachi S-800 scanning electron microscope was used to observe the surface of the films. The films were prepared for analysis by the deposition of a 200-A layer of gold, and the films were mounted at 45" to the incident beam.
Results and Discussion Two types of polymer films were studied, films with no added ions and ion-containing films consisting of either t h e unblended ionomer or blends of a nonionic copolymer and the ionomer. Ionomers were selected as a source of ions because i t has been shown t h a t in these S-co-xMVP OTs/S-BMA blends, the surface ion composition of the milled powders resembles the bulk ion composition.3~4J7 This is certainly not the case with molecular salts, where t h e surface ion content can be 3-10 times greater than the b ~ l k , reflecting ~.~ the strong tendency for t h e salts to accumulate on t h e surface. However the ionomer/polymer blends are not completely uniform since the ionic segments are largely insoluble in nonpolar matrices, as is often the case for dissimilar polymers which d o not have strong interchain interactions. T h e ions thus often remain associated and form small aggregates of ion pairs due to the absence of ion-solvating interactions. T h e aggregates probably exist within larger domains of t h e ionomer which itself is phase separated from t h e S-BMA resin. Phase separation was observed in the bulk of compression molded samples of blends containing 0.6-14% S-xMVP OTs ion0mer.l' T h e domains were ca. 500 nm in size. This phase separation has been proposed to be responsible for the 20-50% variations observed in the contact charge of polymer blends with the same total ion content b u t containing ionomers with different amounts of the ionic monomer." T h e samples selected for this study have different ion contents and t h e contact charge characteristics of the corresponding powders are known. Particles (8-10 pm size) of t h e blend containing 1.4 pmol/g ions have the same charge as t h e resin alone. Particles of the blend containing 26 pmol/g ions develop a controlled positive charge as expected for a mechanism involving the transfer of the mobile anion t o the second surface and t h e charge is in the range +60 t o +75 pC/g depending on t h e ion fraction in the ionomer ~ s e d . ~T*h~e pure J ~ ionomer with 520 pmollg ions only charges at 42 pC/g, which is lower than the value extrapolated from the monotonic relationship observed for the blends with lower ion content. T o compare the charge levels in t h e powder and the tip/film experiments, we calculate that t h e typical powder charges of 20-80 pC/g correspond to ca. 1to 4 charges for the area of the tip (500 A radius). Furthermore, since the powder charge is the equilibrium charge after many contacts, the corresponding charge for a single contact (as with the tip/film) is much smaller. Therefore, in the tip/film experiments, a voltage is used to reliably deposit charge and the charge level is estimated a t tens of charges/contact.l5 With the tip/film we also observe changes in the resultant charge as i t decays or spreads away from the local spot. Although the initial charge levels and the charging mechanism is different in the two techniques, a comparison of the charge behavior helps in understanding the variation in the charge stability with the ionic composition of the blends. Shown in Figure 1are the total force gradient and charge images for the polystyrene film spun cast onto gold. In
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'I
Figure 1. Constant force gradient (a, top) and charge images (b, bottom) of a polystyrene film on gold. The gold region is to the left and the polystyrene to the right. The scan area is 8 pm X 8 pm.
the totalforcegradient image,severalsteps near the center of the image are seen. The left side is gold, while the right side is the polystyrene-coated gold. In the charge image, the gold appears featureless while the polystyrene shows a rich structure. The structure apparently is due to electrical charge which is either on the film surface or trapped in the bulk. Figure 2 showsthe total forcegradient and chargeimagesof a S-BMA film. The charge structure is similar to the one seen with polystyrene and has also been seen in a variety of other insulating polymer samples.21 There are also small depressions in the total force gradient image which are most likely small surface depressions. Note the lack of a correspondingstructure in the charge image. The charge structure in the pure S-BMA and polystyrene films was stable and no change was seen over a period of hours. Films of S-BMA were also thermally annealed in an attempt to remove the trapped charge, but this was unsuccessful. The annealed and as-prepared films contained similar charge structure. Figure 3 shows the S-BMA film after the tip was used to deposit a local region of charge. The charge was deposited by touching the tip to the surface with the tip biased at -1 V with respect to the sampleelectrode.22This charge,isstable over a period of hours and is much larger in magnitude than the background charge structure. In general, sampleswhich containeda charge structure could be locally charged by contactingthe tip. Scanningelectron (21) Terris, B. D. Unpubliihed results. (22) Saurenbach, F.; Tems, B. D. Trans. IEEEIAS, in press.
micrographstaken of S-BMA films as prepared and after annealing show no structure on the surface. The compositions of the surfacesof S-BMA films, before and after annealing, were determined by XPS. As can be seen in Table I, the surface composition of the as-prepared film agreeswell with the bulk composition, while the annealed film showed a small increasein the oxygen content. Signals for nitrogen and sulfur were not observed for either film. In contrast, both the as-preparedand annealed films of the S-O.06MVP OTs ionomer containing 520 pmol/g ions showed no such charge structure. In addition, when the tip was touched to the film surfaces,no charge (effectively zero) could be observed. The decay time was too short for any charge to be imaged by the microscope. The powder of S-O.06MVP OTs, however, charges positive and the charge is 2-3 times smaller than the value expected for the ion concentration in the material. Nevertheless, the powder still has a significantcharge and it is stable. The lower contact charge has been attributed to excessive ion transfer to the second ~urface,~ although it may also be due to a rapid charge dissipation as observed here. The surface composition of the S-O.06MVP OTs film, as prepared, agrees with the bulk composition. The S signal is again greater than the N signalas previouslyr e p ~ r t e d . ~ , ~ After annealing,the surfaceshows an increasein the oxygen and sulfur content. Furthermore, the O/S atom ratio becomes ca. 3, which is appropriate for the atomic stoichiometry in the OTs-, and the N/S ratio indicates that the anion contentis twice the cation content on the surface.
Saurenbach et al.
1202 Langmuir, Vol. 8, No. 4,1992
Table I. Elemental Composition of the Film Surfaces from XPS atom % C 0 N Styrene-Butyl Methacrylate (S-BMA, no ions) as-prepared 91.37 8.63 nda annealed 90.29 9.71 nd calcd 91.95 8.05 0.00 Styrene406 Methylvinylpyridinium OTs (520 pmol/g ions) as-prepared 95.86 2.59 0.67 annealed 92.31 5.63 0.69 calcd 96.56 2.06 0.69 S-BMA/StyreneO.OG Methylvinylpyridinium OTs (26 pmol/g ions) 7.61 nd as-prepared 92.39 8.50 nd annealed 91.41 0.04 calcd 92.19 7.74 S-BMA/Styrene-O.ll Methylvinylpyridinium OTs (26 pmol/g ions) as-prepared 91.05 8.79 0.13 annealed 88.65 11.10 nd calcd 92.02 7.91 0.04 a
Figure 2. Constant force gradient (a) and charge images (b) of the S-BMA film before charging. The scanned area is 2 pm X 2 pm, and the deepest depressions in (a) are ca. 20 nm deep.
Figure 3. Constant force gradient (a) and charge images (b)of the S-BMA film after touching the surface with a -1 V tip. The scanned area is 2 pm x 2 pm.
Thus the surface appears to be highly enriched with the mobile ion. In the EFM charging results reported here, the surface is charged to the same sign as the tip voltage,i.e., a negative tip resulfs in a negative surface charge. The process
S nd nd 0.00 0.87 1.37 0.69 nd nd 0.04
0.03 0.25 0.04
Not detected, below detection limit.
appears to be voltage driven and is most easily interpreted as electron transfer.22 From our previous work,21 we estimate that tens of charges are deposited per contact. The question to be asked here is where do the electrons reside in the film? In the polystyrene and styrene-butyl methacrylatefilms, the most likely electron acceptorsare the antibonding orbitals of the benzene groups and the ester carbonyls (copolymer). There is also the possibility that the electrons do not react directly with the polymer but instead are accepted by 02 in the vicinity of the film surface to produce 0 2 - or by H20 to produce hydrogen atoms and OH-. The stability of the 0 2 - and OH-ions will then depend the nature of the polymer or the presence of other ions. With the introduction of ions, such as the methylpyridinium and toluenesulfonate ions, additional electron-acceptorcenters become available. The methylpyridinium cation, for example, is known to be a good acceptor. In contrast with these conditions,in the contact charging experiments with the powders, there was no applied voltage and ion transfer was Ion transfer would be difficult to observe in these EFM contacts due to the low probability of contactingan ion in these samples with low ion content, plus ion transfer would be masked by the greater number of electrons transferred by the applied field. In order to vary the charge mobility of the samples, S-O.06MVPOTs ionomer was added to the S-BMA. With a blend containing 0.25 % ionomer, 1.4 pmol/g ions in the film, the charging behavior of the film was unchanged from that of the pure S-BMA. This result completely parallels the contact charging results with the powders. With the 5% ionomer blend, 26 pmol/g ions in the film, the behavior was markedly different. A charge structure was present in the as-preparedfilms and it slowly changed with time. If the surface was charged with the tip, the charge region was not stable, but decayed to zero over a period of approximately1h. A priori, we had not expected this low level of ions to make the film so conductive. The X P S results indicatethat the surfacecompositionis similar to the bulk, and thus the enhanced conductivityis not due to phase separation which increases the ionomer content on the surface. Nitrogen and sulfur were not detected in these analysesand are outside the detection limits.3~~ After thermal annealing, no charge structure was observed for
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Langmuir, Vol. 8, No. 4,1992 1203
quite short (ca.2-3 h). ABseen inTable I,annealingcaused some enhancement of the OTs- concentration on the surface of the film. Cratering was also seen in the SEM photographs of some of the unannealed films, but was barely visible on others. The craters had three distinct sizes within the range 0.03-0.6 pm. The films with the faintly visible craters were imaged using backscattering electrons to determine if the craters were compositionally different relative to the flat areas of the film. Backscattered electrons provide compositional information for depths greater than 50 below the film surfaceand permit discrimination (although not identification) between elements with different atomic numbers. The backscattering results indicate that the film is phase separated intoa largelylow atomicnumber matrix (C, 0)surrounding smallareas containingenriched with atoms of higher atom number, e.g. sulfur. This is consistent with phase separation into S-BMA and ion-richdomainsand corroborates the EFM results.
Figure 4. Constant force gradient (a) and charge images (b) of the 3 % S-O.11VMP OTs/S-BMA blend. The scanned area is 3 pm X 3 pm and the crater walls are ca. 100 A high.
this blend. If the tip was touched to the surface, no local charge could be detected. The decay time had decreased and was now too short to observecharge. The film surface became ionomer-like in its charge properties, and small changeswere observed in the surfacecompositionby XPS. However, the increase in surface ions was not sufficient to produce a N and S signal. The film of the third ionomer blend, also containing 26 pmol/g ions, but prepared with 3% of S-O.11MVP OTs, displayed the most interesting topography. In the EFM total force gradient images, both the as-prepared and the annealed films showed craters on the surface (Figure 4a). The craters in the annealed films had smaller lips (2-3X) than those in the as-prepared. In the charge images of both films, the craters appeared to be charged possibly indicating a compositional difference. The craters are mostly negatively charged as seen in Figure 4b. Some craters have a positive charge nearby. The rough surface topography made scanning difficult, and thus it was not possible to reliablychargeand imagethe unannealed fiis. On the other hand, the annealed films could be charged by contacting them with the tip, but the decay time was
Conclusion By use of a force microscope, the surface charging properties of a variety of polymer filmshave been studied. For the insulting samples, S-BMA and polystyrene, a charge structure is observed and a stable charge can be deposited by the tip of the EFM lever. In contrast, the pure ionomer films of S-0.06MVP OTs (520 pmol/g ions) show no charge structure nor can they be locally charged. It appears that surface charge mobility is much higher in these films; i.e., they have a higher surface conductivity. When 0.25 % of the ionomer was blended with S-BMA for a net ion content of 1.4 pmol/g, the film behaved like unblended S-BMA. When 5 % ionomer was blended with S-BMA for a net ion content of 26 pmol/g, the film again holds charge but the slightly higher surface conductivity results in a moderate charge decay rate. Upon annealing, this film (26 pmollg ions) did not hold charge and behaved like the ionomer, although XPS did not detect a significant change in the surface composition. The results of the 3 wt % S-O.11MVP OTs blend are less straightforward to interpret because of the coexistence of charge and mass structures. In the as-prepared films, craters are present and they appear charged. This charge may be the result of a compositionaldifference between the craters and the remainder of the surface. The annealing enhances the ion content on the surface and also smooths the topography.
Acknowledgment. We thank Jose Vazquez, Dolores Miller, and Debra Feme1 for their technical assistance. Registry No. (MVP)(OTs) (copolymer), 139408-72-3; PS, 9003-53-6; S-BMA (copolymer), 25213-39-2.
