10-22 MOLECULAR ORIENTATION AND MORPHOLOGY

10-22 MOLECULAR ORIENTATION AND MORPHOLOGY

Deformation of a polycrystalline film results in an orientation of both crystalline and noncrystalline (amorphous) regions with respect to the deformation direction. As the polymer is deformed, spherulites change from a spherical to a spheroidal shape. When the spherulites are deformed, the substructures of crystalline and noncrystalline regions reorient. As the deformation continues, crystal reorganization proceeds by the process of lamellar slip, orientation, and separation, until the crystal lamellae in all regions of the spherulite become aligned with their helix axis direction nearly parallel to the deformation direction. Further extension of the polymer results in a new deformation mechanism, crystal cleavage. The substructure is no longer spherulitic but has evolved into a fibrillar structure.

It has been a well-established fact that orientation of the polymer enhances its tendency for crystallization as, for example, crystallization of natural rubbers. Thus, on stretching, the stiffness and the strength of the polymer in the stretch direction are increased about 25 times of the original value. However, in the direction perpendicular to the stretch direction, the oriented polymer is weak and tends to split. This behavior is of little importance in fibers where stress is usually applied in one direction but, in films, more uniform properties are required. This is achieved by stretching the film in two directions at right angles to each other so as to orient the molecules randomly within the plane.

Films. The rate of cooling and the degree of stretching determine the structure and final properties of the produced film. Rapid cooling results in very fine spherulites or amorphous regions, depending on the nature of the polymer. This gives a hazy appearance to the film. The film then may undergo further crystallization and increase the size of spherulites, which may make it opaque.

The phenomenon of the retractive force caused by stretching of the polymers is used in the production of shrinkable films. The films are biaxially stretched and then used for packages. Then, on application of heat, the retractive forces relax, causing shrinkage of the film that will tightly fit the package material. Such films may exhibit shrinkage up to 20%. Much greater shrinkage and stronger films for packaging can be obtained by irradiation. The film is irradiated so that a certain degree of crosslinking is produced. The film is heated and stretched biaxially, cooled, and rolled up. Then such a film is used for the packaging and, after wrapping. it is again reheated. This causes relaxation of stresses and return to its original dimensions at the time of irradiation. The shrinkage may be as high as 70% but, usually, films of average shrinkage from

30 to 50% are produced.

Fibers.

A fiber is defined as a unit of matter having a length at least 100 times greater than its width or diameter. A filament, on the other hand, is an individual strand of continuous length. In order to impart to the polymer fibers desirable mechanical characteristics, drawing is peformed after spinning. A draw ratio (or drawing conditions) is selected according to the properties of the polymer and desired properties of the fiber. 1f fiber that was essentially unoriented is drawn to different tensions at 80°C (176°F), two processes occur: (1) the noncrystalline chains become oriented, and (2) crystallization occurs. The noncrystalline chains seem to decrease in orientation with increasing temperature by an amount of shrinkage. At high temperatures, a high draw ratio can be obtained because the crystallites fracture under the combined action of heat and stress. Actually, spherulites also fracture during cold drawing but, at this temperature. they do not have

a chance to align themselves. An increase in temperature results in an increased crystallinity of the fiber. The natural draw ratio is not affected by strain rate. Young’s modulus is determined more by molecular orientation than by the number of interlamellar ties. The modulus increases with the draw ratio. For high tensile strength, high draw ratios (about 6 to 10) are used. After drawing, cold drawn fibers are usually annealed in order to increase their crystallinity and to remove the residual stresses within the fiber.