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Metrology & Instrumentation: Polymer Imaging


Standard Test Samples

Users of atomic force microscopes are aware about a need of checking the calibration of the piezo-scanners in lateral and vertical dimensions using different gratings and other calibration samples. There are also test samples, which are applied for checking sharpness of AFM probes. In addition, it is useful to have various samples, imaging of which will allow demonstration of advantages of various AFM modes and can help to train beginners. For this purpose, we suggest several polymer samples, which are described below.


Oriented film of isotactic polypropylene, which is also known as the microporous membrane Celgard.
Surface of this film seen in an optical microscope is characterized by one-dimensional orientation and AFM images shown characteristic pattern, Figure 1, formed by alternating fibrilar and lamellar regions. Both structural elements have dimensions in the 5-30 nm range. Stacks of lamellae are slightly elevated over the fibrilar regions and a precise profiling of isolated nanofibrils and pores in-between, whose dimensions are comparable with AFM probe radius, is a challenging task for AFM whose probe radius has dimensions comparable with these structures. Additional difficulties are caused by a relatively soft nature of this material, which makes difficult the non-destructing imaging of this film in the contact mode. The film surface is best imaged in the tapping mode with low-tip sample forces.


Ultra thin layers of normal alkane C60H122 on graphite.
The adsorbates of C60H122 alkanes prepared on graphite are composed of large isolated patches up to 100 microns in lateral dimensions, which are clear seen in an optical microscope. The patches are separated by featureless areas, which are covered by layers of C60H122 lamellae. High-resolution images of these layers performed in tapping mode reveals fine structure associated with individual lamellae, Figure 2. Linear edges of the individual lamellae, where mobile -CH3 end groups are located, are typically seen as dark lines. Individual lamellae are ~ 7.5 nm in width and ~ 0.4 nm in height. It has been shown that the lamellae layer, which is lying immediately on the substrate, exhibit high temperature stability and can be observed at the temperatures up to 50篊 higher than melting temperature of bulk C60H122 crystals (see Magonov S. N., Yerina N. A. "High Temperature Atomic Force Microscopy of Normal Alkane C60H122 Films on Graphite" Langmuir 2003, 19, 500-504.). The regular spacing of this layer is identical to the extended length of C60H122 molecules of 7.4 nm. Some deviations of this spacing might happened in top alkanes layers of the multi-layered adsorbates. These samples are very useful to practice in high-resolution imaging and checking the piezo-scanners' calibration in the nanometer range.


Thin film of triblock copolymer: poly (styrene)-b-poly (butadiene)-b-poly (styrene) spin cast of Si substrate.
A micro-phase separated pattern consisting of alternating blocks of polystyrene (PS) and polybutadiene (PB) is seen in AFM images of this sample, Figure 3. Averaged spacing of this pattern is ~ 35 nm. The micro-phase separation, in which PS and PB blocks are forming short cylinders, can be further improved by vacuum annealing of this material at temperatures above 100篊. It also should be taken into account that a top ~ 20 nm layer of this film is enriched in PB blocks, which exhibit lower surface energy than PS blocks. Therefore images are obtained at elevated forces � the condition, which is more favorable for the probe penetration and visualization of the microphase-separated pattern (Magonov S. N., Elings V., Cleveland J., Denley D., Whangbo M.-H. Surface Science 1997, 389, 201.


Rubbed patches of polydiethylsiloxane on Si.
Rubbing of polydiethylsiloxane (PDES) on Si substrate leads to stretched patches of this polymer, which are separated by the substrate areas. At room temperature, this polymer is in mesomorphic state in which lamellae coexist with amorphous material in rubber-like state. Imaging of PDES patches in tapping mode at room temperature requires use of high amplitudes to overcome the tip adhesion to this soft material. Typical images obtained on this material are shown in Figure 4. Phase image distinctively shows areas of Si substrate (most bright features), amorphous polymer (most dark features) and mesomorphic lamellar aggregates, which have a specific cigar-like shape of contrast between those of the substrate and the amorphous material. This sample can be also useful for demonstration of AFM imaging at low temperature because it crystallizes at sub-ambient temperatures (Magonov S. N., Elings V., Papkov V. S. Polymer 1997, 38, 297-307. Godovsky Yu. K., Papkov V. S., Magonov S. N. Macromolecules 2001, 34, 976-990).


Film of multi-layered PE
A multi-layered film with alternating PE layers of different density is prepared by a co-extrusion technique. AFM imaging of a cross-section of this film, which is prepared by cryo-microtomy in direction perpendicular to the layers, is extremely useful for demonstration of tip-sample force interactions in different modes (contact mode, lateral force mode, force modulation, phase imaging, etc.). Figure 5 shows height and phase images obtained on this sample in hard tapping (Magonov S. N., "AFM in Analysis of Polymers" Encyclopedia of Analytical Chemistry, (R. A. Meyers, Ed.), pp. 7432-7491, John Willey & Sons Ltd, Chichester, 2000.). The phase contrast helps visualizing PE layers of different density (brighter strips correspond to stiffer layers and darker strips � to softer layers). These layers are barely distinguished in the height image.


Layers of syndiotactic polypropylene
The sample of semicrystalline polymer � syndiotactic polypropylene (sPP), which is prepared by deposition of the melted polymer on mica or Si substrate, is a good example for imaging of spherulites and lamellae at different temperatures (Zhu, W., Cheng, S. Z. D., Putthanarat, S., Eby, R. K., Reneker, D. H., Lotz, B., Magonov, S., Hsieh, E. T., Geerts, R. G., Palackal, S. J., Hawley, G. R., Welch, M. B. Macromolecules 2000, 33, 6861-6868.). Height images in Figure 6 shows spherulites growing while imaging of sPP sample after its temperature was dropped from 160篊 (melt) to 130篊.


Film of thermoplastic vulcanizate (a multi-component material consisting of isotactic polypropylene, rubber and carbon black filler)

This material shows electric conductivity due to a percolation network formed by carbon black particles. It is a useful sample for demonstration of different AFM methods applied for compositional imaging, including electric force microscopy, surface potential microscopy, etc. The height and phase images of this film recorded in EFM mode are shown in Figure 7. Height image shows surface topography and one can distinguish lamellar structure of isotactic polypropylene domains. Phase image, which is recorded in the lift mode, emphasizes conductive locations, which are seen as dark spots. They are related to carbon black particles contributing to the percolation network (Yerina N. A., Magonov S. N. Rubber Industry and Technology, 2003 to appear in October).