The Electron microscopy of crystalline polymers

This thesis describes the micro-structure of crystalline polymers, particularly as revealed by electron microscopy, and also the effect of the electron beam on the radiation sensitive specimens under various conditions. Polymer single crystals grown from dilute solution were used for the radiation damage studies, as they are thin (100 andAring;), and a single preparation may contain 108 crystals of uniform properties. Preparations of polyethylene (PE), polyoxymethylene (POM), poly-4-methylpentene-1 (P4MP), nylon 6 and polyethylene oxide were characterized in the transmission electron microscope, confirming previous work for the most part. The scanning electron microscope, operating in its normal mode, showed even the thinnest single crystals clearly, when they were sedimented onto a metal coated substrate. Attempts to observe the 3-D pyramidal structure of PE single crystals were unsuccessful. The crumpling seen when crystals floating on glycerol, or freely suspended from a fine mesh, are irradiated in an electron microscope indicates that even if the structure were preserved, by freeze drying or however, it would be difficult to observe. Thicker, complex, crystals seemed more stable in the scanning electron microscope, and cannot be seen well in the transmission microscope. A chart recorder, picoammeter and special faraday cup were fitted to a Siemens Elmiskop 1 for accurate beam current measurements, and dynamic recording of the varying intensity of polymer crystal images. Reproducible curves for the variation of intensity of one spot in the diffraction pattern were obtained for POM and PE. There was a period, p, of very variable intensity, followed by an exponential decay. The very variable intensity is due to the diffraction contrast of detail, so after p no information could be gained from the crystal, although the diffraction pattern shows it to be largely crystalline at that point. For the particular preparations described, at 100 kV, andnbsp; PE POM andnbsp; p 43 andplusmn; 5 34 andplusmn; 4 values in Coulombs/sq. m. Diff. Pattern Decayed 92 andplusmn; 10 70 andplusmn; 7 It is shown that in the electron microscope, heating and ionic bombardment are negligible effects compared to the electron irradiation. An energy loss analysing microscope applied to PE crystals showed that 1 Coulomb/sq.m. is equivalent to 42 Mrads. at 100 kV in agreement with theory. The density of PE increases sharply on irradiation, and the final product seems much like amorphous carbon. Radiation damage mechanisms are discussed, with special reference to polymers and to conditions in an electron microscope. For POM, PE and P4MP the variation in radiation damage rate with beam voltage between 30 kV and 100 kV was found to be accurately andprop;andnbsp;(electron velocity)andminus;2 as predicted by theory, both simple and complex. With exactly similar samples of PE and POM in the 1 MV electron microscope at U.S. Steel, the trend was the same up to 1 MV. Between 700 kV and 1 MV there was a deviation from proportionality towards even slower damage. Absolute measurements differed by 30andndash;50%. Adapting the scanning electron microscope to transmission, the damage of POM could be studied down to 3 kV. There was a steady increase of damage rate as the voltage was reduced, but it was not possible to compare the results with those at higher voltages. The effect of specimen temperature on damage rate was investigated with a liquid helium cooled specimen stage. In going from room temperature to andlt;andnbsp;30anddeg;K the rate was reduced by a factor of 2 (2.2 andplusmn; 0.8). This is not enough to outweigh practical difficulties. There is a fundamental noise limit to resolution with a decaying crystal. This has been calculated, and found to be well below anything practically realized. For example random structure 50 andAring; across, and periodic structure of 20 andAring; should be resolvable in a POM crystal, 85 andAring; thick imaged in bright field at 100 kV and 13,000 times magnification. The real limits are image instability and focus, and there is no good way round either. There is no advantage to be gained from higher beam voltages. Ease of rapid operation of the microscope is the most important factor. Image intensifiers, if easy to use, may be a great help, allowing more manipulation of specimens, and presenting more of the information that is obtained. Thin films of PE and POM were cast from slurry and observed in the electron microscope. PE gives complex crystallographic diffraction contrast which rapidly disappears but another, stronger, contrast builds up showing another structure, and this is retained however large the dose. This beam-induced image looks just like what one might expect from undamaged material, in this case, banded spherulites. Detailed study of the changes, combined with optical microscopy and experiments on single crystals leads to the following conclusion: The beam-induced contrast is due to anisotropic deformation caused by radiation damage. Each crystalline unit expands along a, and contracts along c, while the third dimension, along b, remains constant. This explanation supports the current view of spherulite structure, a collection of radiating twisting lamellae, and may well apply to other polymers and structures. Thin films of PE were drawn at room temperature and annealed at 120anddeg;C. On irradiation, the drawn regions retracted by around 30%a, but this is not a measure of contraction along c (predicted as andgt;andnbsp;25%) because of the unknown behaviour of interlamellar material. The fibrils seen in drawn PE after irradiation were found to be the radial lamellae, homogeneously deformed, whenever the spherulite structure persisted enough for measurements to be made. An ultra-thin PE film which became in the limit, an open network, was produced in an uncontrolled way. Normal spherulite structure was not observed. Areas seemingly undrawn gave a single crystal type of diffraction pattern, and drawn ribbons looked like a 'shish-kebab', transverse platelets on a fine thread. The plates were 200 andplusmn; 25 andAring; thick, separated by 200 andplusmn; 75 andAring;. The centre to centre period was 380 andplusmn; 40 andAring;. Diffraction indicated that the orientation was very good, with b and c in the specimen plane, and c along the ribbon. The film was quite stable, and diffracting units could be identified with individual platelets. This was a fascinating demonstration of an ideal drawn PE, but it could not be made to order, or investigated systematically. The POM films degraded to a featureless film in the electron microscope with andlt; 20% of the mass remaining. Optical microscopy showed coarse textured spherulites, beginning to organise into banded spherulites with radial period andgt; 10andmu;. Initial contrast in the electron microscope showed a radiating structure from well spaced centres. The radiating units gave a single crystal type of diffraction pattern, with a {101and#773;0} growth direction. It seems that because of the coarse texture of POM spherulites, a thicker film is needed to give information about the bulk. This would need a high voltage microscope.