Rate of damage of polymer crystals in the electron microscope: Dependence on temperature and beam voltage

Abstract

Irradiation in the electron microscope causes many changes in crystalline polymers, but the most easily measurable of these is the change in the diffraction pattern from single crystals. It has been found that the intensity of a {1010} reflection from a polyoxymethylene single erystal decays exponentially after a short period, of reproducible duration, when the form of decay is variable. The intensity of all electrons emerging from the column except those assosciated with the directly transmitted beam behaves in the same way. The period of variable decay corresponds to the period when diffraction contrast remains visible on the dark-field image. The decay of diffraction from polyethylene is more complex because radiation damage produces long-range disorder in the lattice, and this is accompanied by motion of the crystal, even when it is firmly supported on a carbon film. Measurement of these decay curves have provided the basis for comparison of radiation damage rates at different beam voltages and different specimen temperatures. Cooling to 18°K decreased the damage rate of polyethylene by a factor of 3, and did not affect that of polyoxymethylene. Changing the beam voltage between 30 and 125 kv affected all specimens in the same way: the damage rate was proportional fo (electron velocity)⁻².