RAPID COMMUNICATION
Near-Edge X-Ray Absorption Fine Structure (NEXAFS) Microscopy of a
Polycarbonate/Poly(acrylonitrile/butadiene/styrene) Blend
C. C. SLOOP,1* H. ADE,2 R. E. FORNES,2 R. D. GILBERT,3 A. P. SMITH1,†
1
North Carolina State University, Raleigh, North Carolina 27695-8209
2
Physics Department, North Carolina State University, Raleigh, North Carolina 27695-8209
3
Fiber and Polymer Science Program, North Carolina State University, Raleigh, North Carolina 27695-8209
Received 14 June 2000; revised 11 December 2000; accepted 20 December 2000
Published online 00 Month 2001
Keywords: polycarbonate/acrylonitrile– butadiene–styrene (ABS) blends; near-edge
X-ray absorption fine structure (NEXAFS) microscopy; reprocessing
INTRODUCTION
Blends of polycarbonate (PC), poly(styrene/acrylonitrile) (SAN), and polybutadiene (PB), commonly called
PC/acrylonitrile– butadiene–styrene (ABS) blends, are
complex mixtures. The ABS component consists of free
SAN copolymer and SAN grafted onto PB (SAN-g-PB).
PC/ABS blends are materials that typically require
heavy metal staining to differentiate the separate
phases at a high spatial resolution in an electron microscope. Our eventual goal is the characterization of
blends of PC and ABS as a function of increasing thermomechanical cycles. Because heavy metal staining is
not directly sensitive to potential compositional
changes in these polymers, we explored the characterization of PC/ABS blends with a directly sensitive imaging technique: near-edge X-ray absorption fine structure (NEXAFS) microscopy. Here we report NEXAFS
spectra of the carbon K shell of PC, SAN, and SAN-gPB, and we evaluate the contrast in a PC/ABS blend
Correspondence to: C. C. Sloop (E-mail: ccsloop@us.
ibm.com)
†
Present Address: National Institute of Standards and
Technology, Gaithersburg, MD
*Present Address: IBM Corporation, Research Triangle
Park, NC
Journal of Polymer Science: Part B: Polymer Physics, Vol. 39, 531–535 (2001)
© 2001 John Wiley & Sons, Inc.
across an energy range of 280 –295 eV in the presence
of TiO2 additives. We unambiguously observed free
SAN in the PC matrix.
NEXAFS spectroscopy exhibits spectral variations
that are sensitive to numerous chemical functionalities
and permits the characterization of the composition of
organic materials.1,2 The information is analogous to
what can be obtained in the near edge of core loss
features in electron energy loss spectroscopy.3,4 During
the last few years, the combination of NEXAFS and a
high spatial resolution of about 50 nm has been
achieved. The resolution has not reached a fundamental limit and is expected to approach 10 nm in the
future.5 NEXAFS microscopy has already been used to
image biological systems6 and examine the morphology
and orientation of polymer systems near the K-shell
absorption edge of carbon, oxygen, and nitrogen.2,7–18
NEXAFS spectroscopy without spatial resolution has
also been successfully used to investigate various polymer surfaces (see refs. 19 –26). One advantage of NEXAFS microscopy in imaging polymer systems is that
materials of similar electron density and elemental
chemistry can be differentiated and complex compositions can be quantified at much higher spatial resolutions than are possible with IR or Raman microscopy.
In addition, organic materials can be differentiated
directly on the basis of differences in chemical composition rather than through the indirect and aggressive
531
532
SLOOP ET AL.
chemical staining techniques commonly used in scanning and transmission electron microscopy.
Although NEXAFS results in some radiation damage, imaging with this technique produces significantly
less beam-induced radiation damage than electron
beams.27 This is particularly important in biological
and polymeric samples where constituents are susceptible to radiation damage. NEXAFS spectroscopy produces relatively unique spectra from different polymer
species and moieties. However, the usefulness of NEXAFS microscopy for the characterization of any particular material or system cannot always be ascertained a
priori from reference spectra because the range of compositional differences may be too small to be detected.
In addition, density and thickness variations of the
sample can reduce or enhance contrast arising from
spectroscopic differences, requiring complex quantitative analysis.2
EXPERIMENTAL
Samples of PC, SAN (75/25 styrene/acrylonitrile), a
PC/ABS blend in pelletized form, and a SAN-g-PB in
powder form were obtained from General Electric Plastics. SAN-g-PB was mixed with Dexter Hysol RE 2038
epoxy potting compound (Hysol HD-3416 curing agent)
in a 50/50 wt % ratio to prepare samples suitable for
microtoming ultrathin sections. A PC/SAN blend was
prepared by the addition of isopropyl alcohol to a methylene chloride solution of equal weights of PC and SAN.
The PC/SAN precipitate was dried at 70 °C under vacuum for 48 h, extruded at 300 °C and a 22-kg load, and
microtomed. An SAN/PS (polystyrene) blend was prepared in a similar manner. A Reichert Jung FC 40
Ultra Cut E with a diamond blade was used for microtoming. SAN-g-PB samples were sectioned under
cryogenic conditions. PC, SAN, PC/SAN, and PAN/PS
were sectioned at ambient room temperature. PC/ABS
and epoxy/SAN-g-PB were microtomed at ⫺125 °C with
a knife temperature of ⫺92 °C and a block temperature
of ⫺125 °C.
These samples were investigated with the Stony
Brook Scanning Transmission Electron X-Ray Microscope (STXM) at the National Synchrotron Light
Source, Brookhaven National Laboratory.28 Carbon Kshell NEXAFS spectra of PC, SAN, and SAN-g-PB were
acquired in the 280 –295-eV energy range (Fig. 1) and
used as a guide for the selection of energies promising
good contrast in images for the various components. (In
actual images, differences in density and thickness also
influence the relative contrast observed.) PC exhibits
several pronounced absorption peaks: a C 1s(COH) 3
␲*CAC component at approximately 285 eV, a C
1s(COR) 3 ␲*CAC component at approximately 286.5
eV, and a C 1s(COR) 3 ␲*CAO peak at 290.3 eV. In
this notation, the core level is indicated parenthetically
before the arrow (i.e., COH), and the nature of the
Figure 1. Carbon K-edge NEXAFS spectra of PC,
SAN, and SAN-g-PB.
upper level of a given transition is indicated by the final
subscript (i.e., CAC for phenyl and CAO for carbonyl).
SAN, because of the presence of the nitrile group, exhibits a distinct ␲* absorption peak at 286.6 eV and has
a strong C 1s(COH) 3 ␲ *CAC component at approximately 285 eV. The virtual absence of the nitrile absorption peak in SAN-g-PB is attributed to the low
acrylonitrile (AN) content in the graft. PC, SAN, and
SAN-g-PB all exhibit strong absorption near 285 eV
due to the unsaturated bonds in each component. As
expected, the 285-eV absorption is greater for the components with the larger aromatic or unsaturation content.
RESULT AND DISCUSSION
To investigate the potential of NEXAFS to differentiate
SAN copolymers of different AN contents, the limiting
case of the azeotropic composition of SAN (25 wt % AN)
and PS (0 wt % AN) were imaged. A NEXAFS image of
the SAN/PS blend was acquired at 286.8 eV to verify
that the chemical specificity of the AN absorption was
sufficient to differentiate a blend where the AN content
of the two components differed by 25 wt % (PS, AN ⫽ 0;
SAN, AN ⫽ 25). In the NEXAFS transmission images
acquired at the AN peak energy (Fig. 2), SAN appears
as a dark phase relative to the PS with very good
contrast. We estimate that we should be able to detect
as little as a few weight percent AN.
NEAR-EDGE X-RAY ABSORPTION FINE STRUCTURE
533
Figure 2. NEXAFS image of a 50/50 wt % SAN/PS
blend imaged at the AN absorption peak at 286.7 eV.
SAN appears as the dark phase.
NEXAFS images of the PC/SAN 50/50 wt % blend
are shown in Figure 3. The unique spectral features at
286.9 and 286.7 eV for PC and SAN, respectively, were
used to provide contrast between the blend components. Because PC has a higher absorption at 286.9 eV,
it appears dark relative to SAN in Figure 3(a). With the
AN absorption at 286.7 eV, a reversed contrast image
can be acquired in which SAN appears dark compared
with PC [Fig. 3(b)]. Small, occluded PC and SAN domains on the order of 0.5 ␮m can be clearly distinguished. The ability to achieve contrast inversion at
different chemically specific energies in the fine structure of the carbon K shell permits elucidation of the
morphology of blends containing finely dispersed species such as the PC/ABS blends.
The PC/ABS blends were imaged at selected X-ray
energies from 280 to 295 eV. A series of NEXAFS
images at several energies are shown in Figure 4. All
images are as-collected transmission images; only the
display limits have been adjusted for contrast enhancement. At 282.4 eV, below the carbon K-shell absorption
edge, the dark spots observed in the image [Fig. 4(a)]
are titanium dioxide particles and clusters that have a
higher mass absorption coefficient at this X-ray ener-
Figure 3. NEXAFS images of a 50/50 wt % PC/SAN
blend acquired at (a) 286.9 and (b) 286.7 eV. At 286.9
eV, PC is the dark phase, whereas at 286.7 eV, SAN is
the dark phase because of AN absorption (see Fig. 1
and the text for a discussion of NEXAFS absorption
peaks).
Figure 4. NEXAFS images of a PC/ABS blend at an
energy selected to explore and enhance the chemical
contrast of the PC, SAN, and SAN-g-PB phases: (a)
282.4 (below the carbon absorption edge), (b) 284.85, (c)
285.45, (d) 285.5, (e) 286.6, and (f) 286.8 eV.
gy.29 PC, SAN, and SAN-g-PB are all very transparent
to X rays, and no contrast between the polymeric
phases is observed. The titanium dioxide particles
serve as registration points in subsequent NEXAFS
images.
In Figure 4(b), additional features are observed at
284.85 eV, which corresponds to the low-energy shoulder of the aromatic/unsaturated absorption peak. The
larger titanium dioxide particles appear as bright spots
because of the relatively high absorption of the polymeric species at the carbon K-shell absorption edge.
SAN-g-PB is less absorbing because of its relatively low
unsaturation and lower density and appears as lightgray inclusions within the darker gray zones, which are
the SAN copolymer. At this energy, the SAN copolymer
has the highest mass absorption cross section because
of a slightly lower C 1s(COH) 3 ␲ *CAC energy and,
therefore, appears dark. The horizontal striations in
the image are due to thickness variations in the ultrathin sections. At 285.45 eV [Fig. 4(c)] on the highenergy slope of the aromatic peak, the SAN-g-PB and
534
SLOOP ET AL.
titanium particles are still observed as bright spots for
the same reasons given previously. However, the high
PC aromatic absorption results in a contrast reversal
between the SAN and PC phases of the material, with
the SAN copolymer now appearing as an intermediate
gray adjacent to the SAN-g-PB particles. A slightly
higher contrast between the polymeric phases is obtained at 285.5 eV [Fig. 4(d)].
The contrast mechanisms and NEXAFS images at
the aromatic absorption energies are relatively sensitive to instrument noise and calibration when polymeric phases are imaged that all contain aromatic
structures. Therefore, the SAN copolymer nitrile absorption at 286.7 eV was also investigated for imaging
the PC/ABS blend [Fig. 4(e)]. The carbonyl absorption
at 290.3 eV of PC was also used to selectively image PC.
Because we were predominantly interested in selectively imaging SAN and the PC/ABS blend is predominantly PC (80 wt %), we did not explore the 290.3-eV
energy. The image in Figure 4(e), acquired at the rising
edge of the nitrile spectral feature, exhibits maximum
SAN contrast with respect to SAN-g-PB and PC. SANg-PB occlusions are clearly shown to be surrounded by
SAN copolymer. ABS (SAN/SAN-g-PB) is dispersed
within the PC matrix in the form of discrete domains.
At 286.8 eV, the relative absorption of SAN and PC is
close to unity, and the contrast between PC and SAN
disappears. The SAN-g-PB and titanium dioxide particles appear as bright spots. In comparison to Figure
4(a), the SAN-g-PB particles can be clearly distinguished in Figure 4(f). These images also unambiguously show that there is free SAN in the PC matrix.
NEXAFS microscopy could thus be used to monitor the
morphology of a PC/ABS blend as a function of increasing thermomechanical cycles. An increase in the volume/weight percent of free SAN would indicate degrafting of the SAN-g-PB due to thermal and mechanical
stresses. A decrease of free SAN moieties in the PC
matrix would be indicative of additional grafting of free
SAN onto PB.
CONCLUSIONS
PC, SAN, and SAN-g-PB have unique chemical structures that make NEXAFS a feasible tool for elucidating
the phase morphology of PC/ABS blends. Submicrometer structures in blends of SAN/PS, PC/SAN, and PC/
ABS were successfully imaged with NEXAFS microscopy. Some free SAN was observed in the PC matrix of
the PC/ABS blend. This free SAN was isolated and
distinguishable from SAN-g-PB. The free SAN content
as a function of increasing thermomechanical processing cycles was monitored with NEXAFS microscopy.
The free SAN content was used to infer degradation
mechanisms of free SAN and SAN-g-PB as a function of
increasing thermomechanical cycles.
As the NEXAFS resolution improves, the technique
will approach 10-nm resolution and be comparable to
electron-beam resolution. The elimination of aggressive
staining techniques and selective chemical etching
techniques reduces sample preparation time and the
potential for affecting the morphology, particularly in
materials such as PC/ABS blends in which SAN-g-PB is
present. The reduced beam-induced radiation damage
compared with electron-beam instruments is advantageous for polymer samples in which a radiation-sensitive component such as SAN-g-PB is present.
Data were collected with the Stony Brook STXM developed by the groups of J. Kirz and C. Jacobsen at SUNY
Stony Brook with financial support from the Office of
Biological and Environmental Research, U.S. Department of Energy (DE-FG02-89ER60858), and the National Science Foundation (DBI-960-5045). H. Ade and
A. P. Smith were supported by the National Science
Foundation Young Investigator Grant (DMR-9458060).
REFERENCES AND NOTES
1. Stöhr, J. NEXAFS Spectroscopy; Springer-Verlag:
Berlin, 1992.
2. Ade, H.; Urquhart, S. In Chemical Applications of
Synchrotron Radiation; Sham, T. K., Ed.; World
Scientific: River Edge, NJ, 2001.
3. Isaacson, M. J Chem Phys 1972, 56, 1813.
4. Rez, P. In Transmission Electron Energy Loss
Spectroscopy in Materials Science; Ahn, C. C.;
Fultz, B.; Disko, M. M., Eds.; Minerals, Metals and
Materials Society: Warrendale, PA, 1992; p 107.
5. Ade, H.; Zhang, X.; Cameron, S.; Costello, C.; Kirz,
J.; Williams, S. Science 1992, 258, 972.
6. Zhang, X.; Balhorn, R.; Mazrimas, J.; Kirz, J. J
Struct Biol 1996, 116, 335.
7. Cossy-Favre, A.; Diaz, J.; Anders, S.; Padmore, H.;
Liu, Y.; Samant, M.; Stohr, J.; Brown, H.; Russell,
T. P. Acta Phys Pol A 1997, 91, 923.
8. Cossy-Favre, A.; Diaz, J.; Liu, Y.; Brown, H.; Samant, M. G.; Stöhr, J.; Hanna, A. J.; Anders, S.;
Russell, T. P. Macromolecules 1998, 31, 4957.
9. Ade, H.; Hsiao, B. Science 1993, 262, 1427.
10. Ade, H. Trends Polym Sci 1997, 5, 58.
11. Ade, H.; Winesett, D. A.; Smith, A. P.; Anders, S.;
Stammler, T.; Heske, C.; Slep, D.; Rafailovich,
M. H.; Sokolov, J.; Stöhr, J. Appl Phys Lett 1998,
73, 3773.
12. Ade, H.; Winesett, D. A.; Smith, A. P.; Qu, S.; Ge,
S.; Rafailovich, M.; Sokolov, J. Europhys Lett 1999,
45, 526.
13. Ade, H.; Smith, A.; Cameron, S.; Cieslinski, R.;
Costello, C.; Hsiao, B.; Mitchell, G.; Rightor, E.
Polymer 1995, 36, 1843.
14. Smith, A. P.; Ade, H. Appl Phys Lett 1996, 69,
3833.
NEAR-EDGE X-RAY ABSORPTION FINE STRUCTURE
15. Smith, A. P.; Spontak, R. J.; Ade, H.; Smith, S. D.;
Koch, C. C. Adv Mater 1999, 11, 1277.
16. Smith, A. P.; Bai, C.; Ade, H.; Spontak, R. J.; Balik,
C. M.; Koch, C. C. Macromol Rapid Commun 1998,
19, 557.
17. Zhu, S.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.;
Gersappe, D.; Winesett, D. A.; Ade, H. Nature
1999, 400, 49.
18. Smith, A. P.; Ade, H.; Koch, C. C.; Smith, S. D.;
Spontak, R. J. Macromolecules 2000, 33, 1163.
19. Stöhr, J.; Samant, M. G.; Cossey-Favre, A.; Diaz,
J.; Momoi, Y.; Odahara, S.; Nagata, T. Macromolecules 1998, 31, 1942.
20. Samant, M. G.; Stöhr, J.; Brown, H. R.; Russell,
T. P.; Sands, J. M.; Kumar, S. K. Macromolecules
1996, 29, 8334.
21. Sutherland, D. G. J.; Carlisle, J. A.; Elliker, P.; Fox,
G.; Hagler, T. W.; Jimenez, I.; Lee, H. W.; Pakbaz,
K.; Terminello, L. J.; Williams, S. C.; Himpsel,
F. J.; Shuh, D. K.; Tong, W. M.; Jia, J. J.; Callcott,
T. A.; Ederer, D. L. Appl Phys Lett 1996, 68, 2046.
22. Keil, M.; Rastomjee, C. S.; Rajagopal, A.; Sotobayaski, H.; Bradshaw, A. M.; Lamont, C. L. A.; Gador, D.; Buchberger, C.; Fink, R.; Umbach, E. Appl
Surf Sci 1998, 125, 273.
535
23. Liu, Y.; Russell, T. P.; Samant, M. G.; Stöhr, J.;
Brown, H. R.; Cossy-Favre, A.; Diaz, J. Macromolecules 1997, 30, 7768.
24. Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang,
J. G.; Korner, H.; Xiang, M. L.; Char, K.; Ober,
C. K.; DeKoven, B. M.; Bubeck, R. A.; Chaudhury,
M. K.; Sambasivan, S.; Fischer, D. A. Macromolecules 2000, 33, 1882.
25. Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang,
J. G.; Xiang, M.; Char, K.; Ober, C. K.; Bubeck,
R. A.; Fischer, D. A.; Graupe, M.; Colorado, R.;
Shmakova, O. E.; Lee, T. R. Macromolecules 2000,
33, 6068.
26. Xiang, M. L.; Li, X. F.; Ober, C. K.; Char, K.; Genzer, J.; Sivaniah, E.; Kramer, E. J.; Fischer, D. A.
Macromolecules 2000, 33, 6106.
27. Rightor, E. G.; Hitchcock, A. P.; Ade, H.; Leapman,
R. D.; Urquhart, S. G.; Smith, A. P.; Mitchell, G.;
Fisher, D.; Shin, H. J.; Warwick, T. J Phys Chem B
1997, 101, 1950.
28. Feser, M.; Carlucci-Dayton, M.; Jacobsen, C.; Kirz,
J.; Neuhäusler, U.; Smith, G.; Yu, B. SPIE Proc
1998, 3449, 19.
29. Henke, B. L.; Gullikson, E. M.; Davis, J. C. At Data
Nucl Data Tables 1993, 54, 181.