Environmental Effects on Low Temperature Crazing of Crystalline Polymers
The ductile behavior of crystalline polymers between liquid nitrogen and room temperatures is primarily due to enhancement of crazing by the presence of gases (N2, A2, O2, CO2) of high thermodynamic activity, i.e., close to their condensation point. Without the presence of such a gas, at temperatures sufficiently above its condensation point so that its activity has dropped to a few percent, the crystalline material is rather brittle although some very limited crazing can be detected. As expected, the ductility and the amount of crazing are very nearly the same in helium as under vacuum. The crazes in smectic PP are extremely long, often traversing the entire width (0.5 cm) of the sample, and generally very similar to crazes in glassy amorphous polymers. They are thin and become thicker at higher temperatures. In monoclinic PP with well developed spherulitic structure, crazing occurs along spherulite diameters that are perpendicular to the stress direction but not along the boundaries between spherulites. In general, the length of a craze equals the spherulite diameter. At low temperatures all crazes originate on the outer surface of the sample which is in direct contact with the gas.
The action of the gas is due to two mechanisms. First, the adsorbed gas reduces the surface energy of the polymer, facilitating the creation of a new surface in the holes and voids of the craze. Second, the gas becomes highly absorbed at the tip of an incidental flaw or of an existing craze, since these are regions of high dilatant stress. The locally absorbed gas acts as a plasticizer easing the flow involved in the nucleation and growth of the craze, i.e., in the formation of fibrillar material of the craze.