SynopsisThe most striking feature of the mechanism of thermal annealing of doubly oriented samples of low-density polyethylene (LDPE) and probably of high-density polyethylene (HDPE) is a progressive tilt of lamellar crystals around their crystallographic b axis. Such a rotation does not occur on thermal annealing in doubly oriented nylons. However, this rotation mechanism occurs during the thermal annealing of doubly oriented samples of nylon 11 in contact with a solvent below its dissolution temperature. As for oriented samples of polyethylene (PE), a correlation between the changes of macroscopic dimensions and long spacing obtained from the small-angle x-ray pattern is difficult to establish. In doubly oriented samples of nylon 11, the basal faces of the lamellar crystals are parallel to the a axis of the unit cell. Nevertheless, simple Miller indices cannot be assigned to the basal planes of the lamellae. On thermal annealing in formic acid, the basal planes of the lamellar crystals are, in some cases, parallel to (001) planes. Annealing in formic acid at room temperature induces a phase transition: the chain c axis remains oriented along the rolling direction and the (001) planes become parallel to the limiting planes of the lamellar crystals. Bulk doubly oriented samples of nylon 11 annealed in formic acid just below the "dissolution temperature" have the same texture of orientation as filter mats of single crystals grown from dilute solution; moreover, as these bulk specimens remain doubly oriented, they can be used for further physicochemical investigations. The usual interpretation of the small-angle x-ray pattern is also discussed on the basis of the results reported in this paper. adopted the following approach. Samples of low-density polyethylene (LDPE) are prepared so as to possess as simple a texture as possible; thereby some individual structural processes can first be identified, then studied in detail. Thus Hay and Keller4 prepare double-textured samples by drawing followed by rolling of melt-pressed sheets of LDPE. The macroscopic rolled sheet is defined by directions OX, OY, and 02. In these samples, the molecular axes are parallel to the rolling direction OY and the (100) planes are parallel to the rolling plane YOZ. The chains are folded in the lamellar crystals: the lamellar structure gives rise to discrete x-ray reflections at low angles. In the simple crystal-type texture