Single-crystal polymer specimens

Electron Microscopy can be used for resolution of smaller objects the practical limit of resolution being a few angstrom units. Electron Microscopy has been used in the study of the morphology of crystalline polymers. The usual techniques of replication, heavy-metal shadowing, and solvent etching are widely used. The direct observation of thin specimens, like polymer single crystals, is also possible and permits the observation of the electron-diffraction pattern of some specimen area, which is invaluable for... 

The refinement (2) proceeded in the same way as for the x-ray work, except that o and S q were refined as additional variables. We assumed that the scattering was kinematic. The cross sectional dimensions of the microfibrils are 200xl0C)X and our previous work on synthetic polymer single crystals showed that the kinematic approximation was adequate for such small crystallites. Intensity measurement presented considerable difficulty in that multiple film exposures could not be obtained. Sequential exposures of the same area of the specimen led to problems of beam damage, and patterns from different areas were not comparable due to differences in the preferred orientation. As a result, only the 28 strongest non-meridional intensities could be measured. These were all for reflections which could be indexed by the Meyer and Misch unit cell, and thus the two chain unit cell was used for the refinement. 

Anisotropic friction was observed for all samples discussed in this paper. The anisotropy is directly correlated with the directionality of the polymer molecules at the surface of the specimens. The directionality was either confirmed experimentally by AFM (for uniaxially oriented polymers and the transcrystallized PEO) or reported in the literature in numerous diffraction studies for extended-chain polymer single crystals. The existence of regularly packed folds at the surface of lamellar polymer crystals is still a matter of discussions. 

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

When an X-ray beam passes through such a fibre perpendicular to its length, the pattern produced is of the same type as that given by a single crystal rotated about a principal axis. All orientations perpendicular to the fibre axis are already present in the specimen, so that the effect of rotation is produced. Examples are shown in Plate X. The reflections are less sharp than those produced by single crystals, for two reasons firstly, the orientation of the crystals in the fibre is not perfect, so that each spot is drawn out to the form of a short arc, and secondly, in most polymer fibres the crystals are so small that the reflections are inevitably more diffuse than those of la rge crystals (see p. 437). 

In fibres of some polymers, made under certain conditions, the crystalline regions are found to be tilted with respect to the fibre axis in a well-defined crystallographic direction. This is a very valuable feature, because the diffraction patterns of specimens in which this type of orientation occurs are of precisely the same form as tilted crystal diffraction patterns of single crystals rotated round a direction inclined to a principal axis. The unit cell cannot be obtained directly, for 90° oscillation tilted crystal photographs are required for direct interpretation, but unit cells obtained by trial can be checked by the displacements of diffraction spots from the layer lines this is a severe check, and consistent displacements would leave no doubt of the correctness of a unit cell. This procedure played an effective part in the determination of the unit cell of polyethylene terephthalate (Daubeny, Bunn, and Brown, 1954). 

The nanotensilometer is just beginning to produce useful information about single-crystal polymer specimens. In view of the difficulty of sample preparation and characterization at the present time, we must be... 

Powdered crystalline samples can also be studied by XRD. The sample is loaded onto the specimen holder, which is placed in the X-ray beam in a setup similar to that used for single crystal XRD. The sample must be powdered by hand or by mechanical grinding and is pressed into a sample holder to form a flat surface or packed into a thin glass or polymer capillary tube. After mounting, the specimen is rotated relative to the X-ray source at a rate of (degrees 0)/min. Diffracted radiation comes from the sample according to the Bragg equation. The detector is simultaneously rotated at 20/min. 

The structure of the crystalline regions of polymers can be deduced from wide-angle X-ray diffraction patterns of highly stretched specimens. When the stretching is uniaxial the patterns are related to those obtained from fully oriented single crystals. The crystal structure of polyethylene was determined by Bunn [7] as long ago as 1939 (Figure 1.10). 

If the scattered beam is a sharp spot diffracted from a single crystal, the phase contrast image when it is recombined is an image of the crystal lattice. This specialized phase contrast technique is applied to the study of atomic scale structure in crystalline specimens of metals and ceramics. It has only rarely been applied to the study of polymer materials due primarily to their instability in the electron beam. Lattice images have been obtained from radiation stable aromatic molecules, such as the liquid crystalline polymers (Section 5.6). They have shown important information regarding the ordered structure. 

Vadimsky [23] described a useful method for preparation of thin films from the melt or solution. The method involves the evaporation of carbon onto freshly cleaved mica or fractured NaCl crystal substrates. The thin polymer film is cast onto the carbon coated substrate or the substrate is dipped into a polymer solution. After solvent evaporation, the film is scored and removed from the glass by floating it onto a water surface. The specimen can also be deposited directly onto the substrate followed by carbon coating. Geil [24,25], in a variation of this method, deformed PE single crystals by deposition on a Mylar substrate and drawing it before carbon coating and TEM examination. 

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