Langmuir 1994,10, 1667-1669
1667
Anisotropic Friction at the Surface of Lamellar Crystals of Poly(oxymethylene) by Lateral Force Microscopy Rozalia Nisman, Paul Smith, and G. Julius Vancso* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 1A1, Canada Received January 27, 1994. I n Final Form: March 30, 1994@ Lamellar crystals of poly(oxymethylene), POM, grown from dilute solutions in bromobenzene, were studied in simultaneouscontact mode atomic force microscopy,AFM, and lateral force microscopy, LFM, experiments. AFM scans showed the expected hexagonal crystal habit with clearly resolved chain fold domains at the surface of the lamellae defined by the diagonals and the edges of the crystals. The frictional force within the domains, obtained in LFM experiments,was found to depend on the relative scan direction. This result is explained by the presence of molecular loops arising from chain folds at the folded surface, which are oriented predominantlyparallel with the edges of the crystalsin the different chain fold domains. Early studies by Staudinger on oligomers and polymers of formaldehyde showed the presence of a crystalline phase in the solid state.l The possibility of chain folding in crystallites of a polymer, gutta percha, was first suggested by Storks.2 The pioneering studies of Keller concluded that macromolecules with long, regular chains crystallize from dilute solutions in folded macroconformations forming lamellar crystals. The polymer chains in these crystals are folded essentially perpendicular to the surface of the lamellae.3 The major unsolved problem related to lamellar crystals of polymers has remained the nature and the atomic arrangement at the fold surface^.^ Regular sharp folds in adjacent position^,^ loose loops with adjacent r e e n t r ~and , ~ random folds (switchboard modeV have been suggested. In addition, it has been proposed that the existence of physically adsorbed macromolecules on the fold surface could be responsible for the significant amorphous component in lamellar crystal^.^ These models inevitably result in different surface structures at the atomic level. The use of various forms of scanning probe microscopy, SPM, including atomic force microscopy, AFM,8 allow direct visualization of the macromolecular architecture, as well as studies of the morphological characteristics of the surface of polymers from the nanometer to the micrometer scale in one experiment.9 In addition, early AFM experiments on the fold surface of linear polyethylene by AFMlO indicated the presence of “periodicarrangements of high points of the folds” on the nanometer scale which might be related to the presence of sharp chain folds. A feature common to all SPM techniques is that the sample surface is studied by an integrated microcantilever tip-probe. In AFM the deflection of the cantilever is measured as a function of the tip position relative to the Abstract published in Advance A C S Abstracts, May 1, 1994. (1) Staudinger, H.; Johner, H.; Signer, R.; Mie, G.; Hengstenberg, J. 2. Phys. Chem. 1927,126, 425. ( 2 ) Storks, K. H. J. Am. Chem. SOC.1938, 60, 1753. (3) Keller, A. Philos. Mag. 1957, 2, 1171. For a review on polymer crystals see Keller, A. Rep. Prog. Phys. 1968, 31, 623. (4) For a review see Encyclopedia of Polymer Science and Engineering; Mark, H., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; John Wiley: New York, 1987; Vol. 10, pp 26-94. (5) Zachmann, H. G. 2. Naturforsch. 1964,19A, 1937. (6) Flory, P. J. J. Am. Chem. SOC.1962, 84, 2857. (7) Hoffmann, J. D.; Davis, G. T. In Polymer Surfaces; Clark, D. T., Feast, W. J., Eds.; John Wiley: Chichester, 1978; Part 13, pp 259-268. (8) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Reo. Lett. 1986,56,1930. (9) See, e.g., Sngtivy, D.; Rutledge, G.; Vancso, G. J. Macromolecules 1992, 25, 7037. Magonov, S. N.; Cantow, H. J. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1992,51, 3. (10) Reneker, D. H.; Patil, R.; Kim, S. J. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1992, 33, 790. @
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Figure 1. AFM scan of a POM lamellar crystal (deflectionmode, scan size 12.5 pm X 12.5 pm). The orientation of the cantilever and the scan direction (1)during imaging are shown in Figure 3.
surface. This technique yields images of the topography of the sample surface. In lateral force microscopy, LFM, the horizontal “twist” rather than the vertical deflection of the cantilever is measured during a scan.ll This lateral force indicates the strength of the “frictional forces”acting between the tip and sample. LFM measurements provide additional information on structure and can be used to identify components in heterogeneous systems.12 The newest generation of SPM equipment, including the NanoScope I11 (Digital Instruments) setup used in this work, allows for dual imaging of the same area at the sample surface with simultaneous LFM and AFM scans. In this study we report on the first observation of direction-dependent friction in the different chain fold domains at the surface of lamellar poly(oxymethy1ene) crystals obtained by simultaneous AFM and LFM measurements. The consequences of this result for chainfolding models are discussed. (11) Mate, C. M.; McClelland,G. M.; Erlandsson,R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. (12) Overney, R. M.; Meyer, E.;Frommer,J.; Brodbeck, D.; Luthi, R.; Howald,L.; Guntherodt,H. J.;Fujihira, M.;Takano, H.;Gotoh,Y. Nature 1992,359, 133.
1994 American Chemical Society
Letters
1668 Langmuir, Vol. 10, No. 6, 1994
Figure 2. (a, left) AFM scan of a section of the crystal shown in Figure 1 (height mode, scan size 1.5 pm X 1.5 pm). The orientation of the cantilever, the scan direction (2) during imaging and the imaged area shown in Figure 3. (b, right) LFM scan of a section of the crystal shown in Figure 1 (scan size 1.5 p m X 1.5 pm). The orientation of the cantilever, the scan direction (2) during imaging, and the imaged area are shown in Figure 3.
POM crystals were obtained from a 0.015% (w/v) solution of poly(oxymethylene), POM, in bromobenzene in isothermal crystallization. The POM sample used was a commercial product of Du Pont Chemical Co. (Delrin 500 NC 10) with = 90 000 g mol-’. The polymer was dissolved in the boiling solvent. The hot solution was then transferred into a crystallization vessel which was maintained at the crystallization temperature of 136 “C in an oil bath. The crystals were grown in the crystallization vessel on muscovite mica for a period of 20 h. The crystallization setup used was similar to that described for PE crystallization by Keller.13 A NanoScope I11 multimode SPM equipped with a “D” head was used to obtain AFM and LFM images. The probe used was a NanoProbe microcantilever, made of Si3N4, with 100 pm long, thin legs and a force constant of 0.38 N m-l. All images were obtained in air at room temperature. Figure 1 displays an AFM micrograph (deflection mode) of a regular POM crystal with a hexagonal crystal habit. The scan size of this image is 12.5 pm X 12.5 pm, the scan rate was 5 Hz, and all filters were turned off during imaging. Boundaries are clearly visible between the chain fold domains along the diagonals of the crystal. The crystal thickness, determined from cross-sectional profiles of images obtained in height mode, was 8.5 f 1.0 nm. The expected thickness at the crystallization temperature of 135 “C, obtained from SAXS experiments, was 8.8 nm.14 Height-mode AFM and LFM scans of a 1.5 pm X 1.5 pm area of the crystal are displayed in parts a and b of Figure 2, respectively. These micrographs were obtained in the “dual imaging mode” at a scan rate between 1 and 2 Hz by using the same Cantilever. The orientation of the V-shaped cantilever with respect to the crystal and the scan directions are shown in Figure 3. The micrograph shown in Figure 1was obtained by scanningin the direction (1) while the dual images (shown in Figure 2) were captured after a rotation of the scan direction by 90°, i.e. by scanning in the direction (2). Thus, LFM scans (see Figure 2b) were performed parallel to the edge of the domain shown
aw
(13) Bassett, D. C.; Keller, A. Philos. Mag. 1962, 7, 1553. (14) Bassett, D. C.; Dammont,F. R.; Salovey, R. Polymer 1964,5,579.
Figure 3. Schematics of the relative orientation of the POM crystal imaged and the AFM/LFM cantilever. The V-shaped cantilever and the projection of the square-pyramidal tip are shown on the right. The image shown in Figure 1 was obtained by using scan direction (1). The AFM and LFM scans displayed in parts a and b of Figure 2, respectively,were obtained in direction (2).
in the middle of the nanograph. The area imaged using LFM is outlined in the crystal schematic shown in Figure 3.
On AFM images color correspondsto height. The height increases from purple (low features), through blue and green to yellow (high features). On the LFM image different frictional forces are marked with different colors. The measured lateral force increases from purple, through blue, green, and yellow. The fold domain boundaries on both images are clearly visible. These are shown on the AFM image (Figure 2a) by the yellow features at ca. 60” clockwise and counterclockwise from the horizontal. Interestingly, at the chain-folding boundaries the crystal surface protrudes out and forms “wrinkles”. From Figure 2a it is obvious that the averageheight of the crystal surface is the same in the different chain fold domains. This is in contrast with the frictional forces obtained in the different fold domains of the crystal, as shown in Figure 2b. In the two triangle-shaped sections at the bottom-left and bottom-right corner, the lateral force is higher than in the central area of the LFM image. This indicates that
Letters the lateral force depends on the relative orientation of the fold domain and the scan direction. Since the same material is scanned in the different domains, the only possible explanation to account for this observation is to assume anisotropic friction at the surface of the different domains. It can be assumed that a disordered surface of a polymer lamellar crystal, e.g., with random, irregular chain folds or with physically adsorbed macromolecules of random coils, would not yield such a frictional anisotropy. If folding occurs predominantly parallel to the growing edge of the crystal during crystallization, then the crystal surface will consist of directionally ordered “loops” of the folded chain^.^ Depending on the scan direction, the outermost atoms of the LFM tip can experience low friction (sliding along the loops of the folds, parallel with the edge of the crystal) or higher friction, if
Langmuir, Vol. 10, No. 6,1994 1669 during scanning the tip atoms “bump” into the loops of the folds instead of “sliding” along the folds. Thus, our LFM results can be explained by the presence of directionally ordered chain folds at the surface of the POM crystal which are aligned “predominantly parallel to the growing edge of the corresponding fold domains. These results contradict the chain folding models that produce irregular surface structures in POM since these structures would have no frictional anisotropy. A detailed study including other lamellar crystals in underway.
Acknowledgment. The authors thank the Natural Sciences and Engineering Research Council of Canada for financial support and Ms. Charlene Ng for her help with the preparation of the polymer crystals.
