J . Phys. Chem. 1990, 94, 6123-6125
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Scanning Tunneling Microscopy Images of Poly(ethylene oxide) Polymers: Evidence for Helical and Superhelical Structures R. Yang, X. R . Yang, D. F. Evans,* Department of Chemical Engineering and Materials Science, Unicersity of Minnesota, Minneapolis, Minnesota 55455
W. A . Hendrickson, and J. Baker 3M Company, Corporate Research Laboratory, 3M Center, Bldg. 208-1 -01, S t . Paul, Minnesota 55144 (Receiued: January 17, 1990)
Scanning tunneling microscopy images of single, double, and multistranded helical structures of bulk poly(ethy1ene oxide) polymers (PEO) are presented. The single-stranded helix appears to contain seven PEO monomer units with two turns per repeat unit consistent with the model proposed by Tadokoro; individual monomer units are visible. The multistranded STM images are similar to those obtained by transmission electron microscopy, but smaller in scale. A hierarchy of superhelical structures is proposed to account for the observed images.
The elucidation of macromolecular structures at interfaces is important in understanding adsorption, adhesion, and compatibility in polymeric and protein systems. Recent measurements with scanning tunneling microscopy have established the ability of this technique to characterize macromolecular structures at solid-air interfaces. For example, the evolution of complex structures associated with conducting polymers adsorbed on can bc followed. The structure of nucleic and proteins5 can be visualized. In this paper we describe scanning tunneling microscope (STM) measurements on poly(ethy1ene oxide) (PEO) adsorbed on graphite. We present the first images of single- and double-helical structures with a resolution that permits individual monomer units to be descerned as well as images of more complex PEO structures. The S T M samples were prepared by applying a drop of an ethanol solution containing 0. I wt % of PEO (Polymer Laboratories Inc.) to the surface of highly orientated pyrolytic graphite. After air-drying, samples were analyzed in the constant current mode by using a Nanoscope I1 S T M made by Digital Instruments Inc. Typical S T M settings were bias 100-150 mV, tunneling current 0.4-1 .O nA, and scan rate 4.3 Hz. W e studied two PEO samples, M W = 14200 and 21 800, but detected no structural difference in the S T M images. In all cases multiple images from independently prepared samples were obtained: two like those shown in Figure 1, four examples of the braided double-back structure shown in Figure 2, and four examples of the images shown in Figure 3. As a check that the STM tip was well formed and imaging only simple atoms, we also obtained atomic resolution of the graphite surface adjacent to the PEO structures shown in Figures 1-3. It has been known since the early 1960s that PEO forms helical structures. X-ray,6 IR,’ and NMR8 data support the 7/2 skeletal model proposed by Tadokoro et aL6 shown in Figure l a . This structure consists of seven monomer units -(CH2CH20)- with two turns per repeat unit. The only previously published image of PEO is that by Deryagin et aL9 showing an electron micrograph of a high molecular weight sample (4 X IO6) deposited on a carbon support. They observed very large double-helical structures with a pitch of 0.6 pm and a diameter of 1.2 pm. The S T M image of two simple PEO structures is shown in Figure 1 b. The one on the left is a helical structure containing two polymer chains while the one on the right contains a single polymer chain. An enlargement of part of a single PEO strand and a superposition of seven monomer units from the molecular modcl (Figure la) on this S T M image are shown in Figure I C . The average measured length of the seven unit sequences is 2.0 *To whom correspondence should be sent.
f 0.13 nm in good agreement with the value of 1.93 nm predicted
by the Tadokoro model. The average width of the single-strand hclix is 0.76 nm with a variation in height of the monomer units consistent with a helical structure. A PEO polymer ( M W = 21 800) with this helical configuration would have a length of 137 nm and a volume of 6 1.5 nm3. An enlargement of the doublestranded helix and a proposed helical model superimposed on it arc given in Figure Id. These are the first images of nonconducting polymer structures which permit individual monomer units to be vi sua I i zed. The S T M image of a more complex PEO structure is shown in Figure 2a. There are two distinct regions, one with a diameter of 23 nm (segment I ) and a second with a diameter of 33 nm (scgment 11) corresponding to a ratio of cross-sectional areas of l:2. The S T M image suggests that segment I1 is formed by the chain in segment I folding back on itself to form a twisted doublet. This observation is consistent with the measured diameter of 23 nm for both of the single chains in the twisted doublet. We have observed several such structures. The structure in segment I 1 is reminiscent of that reported by Deryagin, but on a much smaller scale. We now consider in more detail the structure of these complex polymer assemblies. Visual inspection of segment I indicates the presence of a periodic structure. From traces of height vs distance along the axis of the segment I (Figure 2b), we obtain periodicities of 53, 27, and 18 nm by direct measurement and by one-dimensional Fourier transform analysis. We attribute these distances to a distorted helical-like structure. In addition, from a trace along thc edge of segment I, we also detect a repeat distance of 4.3 nm. Similar measurements along segment I1 give two periodicities (see Figure 2c); the largest one at 39.1 nm is a distance associated with thc twisted braid while the smaller one at 4.3 establishes a fine structure common to both segments. ~_____
( I ) Yang, R.; Dalsin, K. M.; Evans, D. F.; Christensen, L.; Hendrickson, W . A. J . Phys. Chem. 1989, 93, 5 1 1 . ( 2 ) Yang, R.;Evans, D.F.; Christensen, L.; Hendrickson, W. A . J . Phys. Chem., in press. (3) Lee. G.: Arscott, P. G.; Bloomfield, V . A,; Evans, D. F. Science 1989, 244, 475.
( 4 ) Arscoti, P. G.; Lee, G.; Bloomfield, V . A,; Evans, D. F. Nature 1989, 339. 485. ( 5 ) Edstrom, R.D.: Meinke. M. H.; Yang, X . R.: Yang, R.; Evans, D. F. Biochemistry 1989, 28, 939.
(6) Tadokoro, H.; Chateri. Y.; Yoshihara, T.; Tahara, S.; Murahashi, S. Makromol. Chem. 1966, 73. 109. (7) Bailey, F. E.; Koleske, J . N. Polyethylene Oxide; Academic Press: New York. 1976; Chapter 3. (8) Conner. T. M.: McLauchlan. K. A . J . Phws. Chem. 1965.69. 1888. (9) Deryagin, B V , Lyashev. K F , Novik, 0 G Dokl Akad Nauk SSSR 1975. 218 599
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The Journal of Physical Chemistry. Vol. 94. No. 1.5. 1990
Yang et al.
la
d
Figure 1. STM images of single- and double-stranded helices of poly(ethylene oxide) (PEO). (a) Molecular model of the single-stranded helix of PEO consisting of seven monomer units with two turns per repeat unit. (b) STM image of doublc-stranded helix of PEO on the left and a singled-stranded helix on the right. (c) Enlargement of the singled-stranded helix (which is 2.0 f 0. I3 nm and 0.76 nm in diameter) and a supcrposition of the 7/2 helical structure. (d) Enlargement of the double-stranded helix and a proposcd double-helical modcl superimposed on it.
S T M Images of Poly(ethy1ene oxide)
The Journal of Physical Chemistry, Vol. 94, No. 15, I990 6125
Figure 3. STM image of bulk PEO showing discernible structure.
d
Figure 2. STM images of a complex multistranded PEO structure. (a) Segment I is superhelical coil, diameter 23 nm, formed from a PEO strand which is 4.3 nm in diameter. Segment 11, diameter 33 nm, is formed by segment I twisting back on itself. The insert at the left corner is a schematic illustration of this complex multistranded PEO structure. (b) Plot of height vs distance along segment I giving periodic distances of 54, 27, 17, and 4.3 nm. (c) Plot of height vs distance along segment II showing periodic distance of 39 and 4.3 nm. (d) A proposed model for polymer superhelices structure.
We believe that the basic structural unit of segments I and 11 is a superhelix formed by the coiling of a polymer strand which
is 4.3 nm in diameter (see Figure 2d). This 43-nm polymer strand is in turn formed by the association of several simpler polymer helices like those shown in Figure 1. Dividing the total crosssection area of this strand ( D = 4.3 nm, A = 14.5 nm2) by the corresponding area of a single-chain helix ( B = 0.76 nm, A = 0.45 nm2) gives a maximum of 32 PEO chains per strand, Le., a strand made up of 32 PEO chains clumped together to form a thick rope. At this time, we do not have very detailed analysis of this structure. The other explanation of this strand could be a certain number of PEO chains clumped together and then coiled to form a large structure. We estimate that there are -430 PEO polymers (MW = 21 800) contained in the image shown in Figure 2a. This estimate is obtained by adding the values of 1.8 X lo4 and 8.5 X IO3 nm3 associated with 21 and 10 helical turns in segments 1 and I 1 and dividing by the value of 61.5 nm3 for single polymers given above. In the single supercoil, interactions between coils are likely to be weak resulting in a distorted structure as seen in segment I, Figure 2b. In more complex structures formed by the braiding of super coils (segment 11, Figure 2a and the TEM image of Deryagin et al.), more ordered structures are expected as a result of packing constraints. A similar hierarchy of helical structures is also observed with polypyrrole and polythiopene conducting polymers near solid surfaces, but not in thicker more bulklike samples.* Finally we show an STM image containing larger aggregates of PEO (Figure 3). Such images contain parallel polymer structures packed together with different orientations. Such criss-crossing structures were also observed with the conducting polymers. However, it is somewhat surprising that images of large aggregates of nonconducting materials can be obtained. We have no explanation of why such images can be obtained, but note that we have observed similar STM images of other nonconducting macromolecules such as polysaccharides, which will be described in future publications.
Acknowledgment. Support by the Center for Interfacial Engineering (CI E), a National Science Foundation Engineering Research Center, The 3M Company, a CIE Sponsor Company, and the National Institutes of Health, Grant Number GM3434 103, is gratefully acknowledged. Registry No. PEO,25322-68-3.
