Crystalline Polymers: Imaging by AFM

figure 1 [Polyethylene crystal]shows a stacked array of lamellar polyethylene single crystals.

By F. Lin, N. V. Gvozdic and D. J. Meier, Michigan Molecular Institute, 1910 West St. Andrews Rd., Midland, MI 48640-2696

Introduction

The development of the atomic force microscope has now enabled direct observation of the topological properties of non-conductive materials at the molecular level. This unprecedented ability has presented new opportunities to characterize the shape of polymeric molecules and the morphological features developed when these molecules aggregate to form more complex structures. The use of AFM for imaging of polyethylene, a typical crystalline polymer, has proven to be very successful, as described in this Applications Note.

figure 2 [Molecular chains]shows the molecular chains of highly-oriented polyethylene, clearly showing not only the individual polymer chains, but even the methylene groups (CH 2).

Sample Preparation

The single crystals of polyethylene were formed by cooling a 0.01% solution of high density (linear) polyethylene in xylene from 120C to room temperature. A drop of the cooled solution was placed on a freshly-cleaved mica substrate and the solvent was allowed to evaporate for two hours. The mica was then glued to an AFM sample holder for scanning.

Orientation of the second sample was accomplished by drawing a bar of high density polyethylene in an Instron Tensile Tester. Drawing was done at 110C to a draw ratio of approximately 30 times its original length, highly orienting the molecular chains. A short length of the drawn material was then glued to an AFM sample holder and a fresh surface was exposed by peeling away a section of the sample with a surgical knife. A few stray fibrils on the surface were removed with tweezers.

Scanning

TopoMetrix' exclusive Step and Scan (tm) system was used to locate a single polyethylene crystal stack for subsequent detailed scanning, utilizing the SuperTranslator to search a large area with user-defined parameters. A 75 m scanner was used in constant force mode with a scan rate of 9 x EE3 microns/sec.

The highly oriented polyethylene sample was scanned with a 1 micron scanner in constant force mode and high z gain to locate a region in which the maximum z height of an area 100 angstroms was below 30 nm. After the desired location was found, atomic level scanning was performed in variable mode at low z gain. Atomic resolution required a very stable environment, a fresh sample surface and a clean probe tip. The scan rate was 6.5 EE4 nm/sec.

Discussion

It is known that many crystallizable polymers will form lamellar single crystals from dilute solution. Such single crystals typically have a characteristic lamellar thickness of 10-20 nm and have required electron microscopy for detailed study. Although such imaging is possible, it requires extensive sample preparation and possible sample damage in the electron beam. In contrast, the image shown in Figure 1 required minimal preparation and was obtained in just a few minutes. Lamellar step heights are easily distinguishable and can be easily measured from the AFM data.

Figure 2 is a truly remarkable image in that it not only clearly shows individual polyethylene chains and methylene groups, but also shows what can be interpreted as a folded polyethylene chain. Chain folds have been postulated in order to rationalize the fact that the polyehtylene chain axis is normal to the lamella surface but the thickness of the lamella is less than the polyethylene chain length, often by more than a factor of 10. Hence the chains in the lamella must fold back on themselves and the surfaces must consist of these folded chains. Crystal surfaces, however, have never been imaged on a molecular scale and it is believed that this image is the first of its kind. The image confirms the predictions of a molecular model of a chain fold, in that at least six methylene groups are required to make a fold. In the future it is hoped that the surface of a single crystal can be imaged to help answer present questions concerning the details of chain folding in single crystals.

TopoMetrix Applications Note #2-1092-001, October, 1992 TopoMetrix Corporation

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