J. Phys. Chem. 1994, 98, 9391-9395
9391
Atomic Force Microscopic Imaging of the Ethylene-Propylene Copolymer on Mica Jianping Yang,' Tris Laurion? Tze-Chi Jao? and J. H. Fendler*,' Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, and Texaco R&D Department, P.O. Box 509, Beacon, New York 12508 Received: April 8, 1994; In Final Form: July 14, 1994@
Hexane solutions of a maleic anhydride-grafted ethylene-propylene copolymer (containing 2% (w/w) maleic anhydride in 60 mol % ethylene/40 mol % propylene), E-P(60/40), in several concentrations, have been deposited onto freshly cleaved mica surfaces and imaged thereon, in air and under 1-propanol, by atomic force microscopy. The diameters of aggregated copolymers on the mica have been found to increase with the E-P(60/40) concentration.
Introduction Investigations of polymers adsorbed on surfaces are inherently interesting3 and highly relevant to many practical applications, such as surface modification and l~brication.~Atomic force microscopy (AFM) is shown, in the present paper, to provide a convenient means for imaging polymers on solid surfaces. The approach is illustrated by the AFM imaging of a maleic anhydride-grafted ethylene-propylene copolymer, E-P(60/40),5 on mica surfaces, in air and in propanol. The method of preparation developed permits the deposition of a submonolayer of E-P(60/40) which, in turn, can be imaged by AFM. AFM has been shown to be a powerful method in the investigation of monolayer surface m o r p h ~ l o g y . ~Particularly ,~ relevant have been the recent measurements of force profiles and friction forces of ethylene oxide-polystyrene block copolymers on mica surfaces by A m . * In air, imaging of the copolymer was difficult since, even at moderate forces, the
copolymers were damaged and/or moved at slow tip scanning speeds. Satisfactoryimaging could be obtained, however, under propanol.8 This approach to E-P(60/40) samples, deposited on mica, provided a convenient means for characterizing copolymer morphologies as a function of their concentration.
Experimental Section Purification and characterization of ethylene-propylene (60 mol % ethylene) copolymers, grafted with 2% (w/w) maleic anhydride, E-P(60/40), have been de~cribed.~ Hexane (Aldrich, 99+%) and 1-propanol (Sigma, HPLC grade) were distilled prior to use. Stock solutions of E-P(60/40) samples were prepared in hexane by weight. Mica was obtained from Structure Probes Inc. Samples for air imaging were prepared by depositing one drop of a 5.8 x g/L hexane solution of E-P(60/40) onto freshly cleaved mica. Subsequent to evaporation, the sample
Figure 1. A three-dimensional image of a 300 nm x 300 nm sample of E-P(60/40). The sample was prepared by placing one drop of a 5.8 x lov5 g/L hexane solution of E-P(60/40) on freshly cleaved mica which was tilted at about 45". Subsequent to quick runoff of the E-P(60/40) solution mm). The image was obtained by using a silicon from the mica surface, the substrate was dried (5 min in air and 30-60 min in vacuum at tip with a spring constant around 0.5 N/m.
0022-3654/94/2098-9391$04.50/0
0 1994 American Chemical Society
9392 J. Phys. Chem., Vol. 98, No. 38, 1994
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Figure 3. Images of E-P(60/40) under 1-propanol. The sample was prepared by placing 1 drop of a 1.0 g/L hexane solution of E-P(60/40) on freshly cleaved mica which was tilted at 45" and drying it (in air, then in lop4mm vacuum for 1 h). Images were obtained by using a SiiN4 tip with a spring constant of 0.12 N/m. (top) Section analysis of the 560 nm x 560 nm image shown at the bottom. The arrows indicate the heights at which diameters of the particles were taken.
was dried in a vacuum chamber mm) for 1 h. Samples for imaging under 1-propanol were prepared by dropping one drop of the E-P(60/40) solution (at the appropriate concentration) in hexane onto freshly cleaved mica which was tilted at about 45". Subsequent to the quick runoff of the E-P(60/40) solution from the mica surface, the substrate was dried (5 min in air and 30-60 min in vacuum at lop4 mm). Images were taken on a Digital Instrument Nanoscope I11 instrument using pyramidal Si3N4 or silicon tips, mounted on a triangular cantilever with a spring constant in the 0.1- 1.O N/m range and a type D scanner with a 15 x 15 pm2 scanning range. The X - Y piezo was calibrated by a standard gold grating while the Zpiezo was calibrated by standard pits with a depth of 193.6 3~ 6.3 nm. A commerical Nanoscope liquid cell was used for imaging in 1-propanol. The entire cantilever and sample were immersed into the solvent so that no surface tension effect acted on the tip. Prior to a given experiment, the liquid cell and
cantilever were cleaned by rinsing with acetone and 1-propanol. Silicon gel and quartz were the only materials which came into contact with the components of the liquid cell. Imaging of mica under 1-propanol (five separate samples) revealed no impurities on the subnanometer scale. The AFM was operated in the repulsive force mode (Le., the tip was always touching the surface). Typical scanning rates were 3-10 Hz.
Results and Discussion No satisfactory images could be obtained in air of the E-P(60/ 40) samples, deposited on mica, by using the Si3N4 tips due to the strong interaction between the tip and the sample. Using silicon tips (Digital Instruments) with spring constants in the 0.1- 1.O N/m range revealed mostly streaky images, indicating tip-induced movements of the copolymers during scanning. This behavior has been recognized for molecules which are weakly
9394 J. Phys. Chem., Vol. 98, No. 38, 1994
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Letters attached to substrates9 Indeed, satisfactory images of ethylene oxide-polystyrene block copolymers could not be obtained on mica in air.8 In the present work, it was occasionally found to be possible to image the E-P(60/40) in air at the first scan when using the silicon tip. A typical example of an image which was taken at the first scan of a sample, prepared by letting one drop of 5.8 x g/L E-P(60/40) in hexane solution evaporate on mica, is illustrated in Figure 1. The presence of wellseparated spherical particles with diameters around 30 nm is clearly discernible. Recognizing the possible distortion of images by the AFM tip geometry, the “tip effect”,1°-12 diameters of the particles were taken at heights which are indicated by the arrows in Figures 2 and 3. Adherence of E-P(60/40) to mica was found to be sufficiently strong under a “poor” solvent, such as 1-propanol, so that the AFM tip did not dislocate the copolymer during scanning. Images of 0.08 g/Lhexane solution of E-P(60/40) deposited on freshly cleaved mica and viewed under 1-propanol are shown in Figure 2. Well-separated aggregates are seen to spread uniformly in the area under view. Grain size analysis, assuming the presence of spherical particles, revealed the particle diameters to be 41.4 i 4.5 nm. Depositing increasingly higher concentrations of E-P(60/40) solutions in hexane onto mica resulted in images, as expected, which showed the presence of larger and aggregated E-P(60/ 40). Thus, the AFM image of a 1.0 g/L E-P(60/40) solution indicated the presence of a few 89 & 12 nm spherical particles interspaced between much larger, elongated (Figure 3). AFM images of samples prepared from 5.0, 12, and 25 g/L hexane solutions of E-P(60/40) revealed the presence of increasingly larger aggregates and more dense coverage of the mica surface (Figure 4). At these higher concentrations of E-P(60/40), assessment of the size of the aggregates becomes more difficult. In general, the heights of the E-P(60/40) aggregates are about ‘/g of their diameters. This is likely to be due to the flattening of the aggregates subsequent to solvent evaporation.
Conclusion Development of a simple technique for depositing copolymers onto mica, at different stages of their concentration-dependent aggregation, is the most significant accomplishment of the present work. The submonolayer coverage of the copolymers on the mica surface can be readily imaged under propanol by AFM. This approach provides, therefore, a convenient means for the visualization of sizes, size distributions, and morphologies of the copolymeric aggregates on solid substrates. Refinement of resolution will permit the elucidation of polymer structures and conformation on surfaces under a variety of experimental conditions. Acknowledgment. Support of this work by grants from the National Science Foundation and Texaco, Inc., is gratefully acknowledged. References and Notes (1) Syracuse University. (2) Texaco R&D Department. (3) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (4) Strate, G. V.; Shuglinski, M. J. In Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symposium Series Vol. 466; American Chemical Society: Washington, DC, 1991; p 256. Rubin, I. D.; Sen, A. In Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symposium Series Vol. 466; American Chemical Society: Washington, DC, 1991; p 273. (5) Nbmeth, S.; Jao, T.-C.; Fendler, J. H. Macromolecules, in press. (6) Sarid. D. Scanning Force Microscouv: . _Oxford University Press: new^ York, 1991. (7) Schwarz, D. K.: Garneas, J.; Viswanathan, R.; Zasadzinski, J. A. Science 1992, 257, 508. (8) O’Shea, S. J.; Welland, M. E.; Rayment, T. Langmuir 1993, 9, 1826. (9) Lea, A. S.; Pungor, A,; Hlady, V.; Andrade, J. D.; Herron, J. N.; Voss, E. W. Langmuir 1992, 8, 68. (10) Markiewicz, P.; Goh, M. C. Lamgmuir 1994, IO, 5 . (1 1) Griffith, J. E.; Grigg, D. Q.; Vasile, M. J.; Russell, P. E.; Fitzgerald, E. A. J . Vac. Sci. Technol. A 1992, 10 (4), 674. (12) Woodward, J. T.; Zasadzinski, J. A. Langmuir 1994, IO, 1340. I
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Abstract published in Advance ACS Abstracts, September 1, 1994.
