Polyoxymethylene single crystals. III. Deformation behavior

Solution grown single crystals of polyoxymethylene have been drawn uniaxially on Mylar to elongations of 90 %. Dark field electron micrographs suggest that these crystals are composed of a mosaic block structure with lateral dimensions of about 300 Å. Deformation occurs by a combination of mechanisms: plastic draw, “brittle” fracture (and formation of fibers), shear along distinct slip lines above 30 % draw, and wrinkling of the lamellae parallel to the draw direction. Striated structures observed along wrinkles and fibers may be an unexplainable artefact of the replicating process rather than a representation of true structure. However, the direct observation of coalescing fibers from an interface region (along crack edges), strongly suggests local melting and subsequent recrystallization. A method was also developed for studying the formation of these crystals under uniaxial compression. Wrinkles developed normal to the compression direction. When the sample is reextended to its original elongation, no trace of the compressive deformation can be found.