synopsisA polypropylene film was stretched a t 1O&16O0C., quenched to room temperature, and then restretched a t the same temperature perpendicularly to the first stretching. The reorientation behavior was investigated by using optical and x-ray methods. During the restretching the monoaxiaI orientation caused by the stretching is converted into a new monoaxial orientation through a balanced state, where npp = npi > ns.. The more or less parallel orientation to the film surface of the polypropylene molecules, brought about by the first stretching, proceeds further on restretching. nss is a h e a r function of the degree of stretching in area VA. The inclination of this line is independent of the type of deformation, stretching, or restretching, provided the temperature is kept constant. At 160OC. the plot of nss versus thickness is less steep than it is a t 100 or 130OC. The overall reorientation apparently proceeds according to Kratky's first deformation law. The x-ray pattern of a restretched film is a four-point diagram which indicates the existence of a pair of reorientation axes inclined symmetrically against the stretching axis. The inclination grows larger with restretching, and the axes merge into the restretching axis at extreme restretching. This phenomenon is less pronounced when the restretching is carried out a t higher temperatures. The density of the restretched film is determined mainly by the stretching temperature, but extreme restretching has a tendency to lower it very slightly.
-ku, Tokyo, J a p a n synopsis Polypropylene film was stretched polyaxially at 1O0-16O0C., and the orientation behavior was studied by means of optical and x-ray method. The molecular chains oriented progressively to the film surface with an increase in stretching area V A in the range 1-16, and the (040) selective uniplanar orientation developed at the extreme stretching. The plot of orientation versus V A was less steep when the stretching was carried out at higher temperature, but the final degree of orientation was independent of the temperature, because the final V A increased with temperature. At 160%. premelting occurred to such a degree that the high stretching and, consequently, the high orientation could not be obtained. The orientation of the amorphous chains was always behind that of the crystalline region. In the initial stage the polyaxial stretching was not as effective in attaining high biaxial orientation as the two-step biaxial stretching, but the final orientation was the same in both types of stretching because V A reached a value of 16 in the polyaxial stretching while it was only 2 in biaxial stretching.
Load-extension behavior of a biaxially balanced polypropylene film which has been prepared by the two-step biaxial stretching method is not always isotropic, i.e., the Young's modulus and the yield stress along the pp-axis (the first stretching direction) are smaller than those along the Ps axis (the restretching direction), while the tenacity and the ultimate elongation which relate to large deformation, do not differ according to the direction of the test.Such an anisotropy appears more remarkably on the thermal shrinkage. When a piece of biaxially balanced film is shrunk freely by raising progressively its temperature, the 1p, and lvS, the dimen sions along the pp and ps-directions respectively, recover along the restretching curve, which indicates the 1,P vs l, relation during the restretching, until the film is heated to the stretching temperature.On further heating above this temperature, the 1z,p and fps change reversely along the first stretching curve. Both the optical index and x-ray diffraction in reference to the pp axis orientation change correspondingly with this dimensional change.
Polypropylene films of various isotacticities and crystallinities were stretched biaxially in one step in air at 140–152°C or polyaxially in poly(ethylene glycol) at 130–160°C, and the morphological changes were studied by electron microscopy (replica). In the initial stage of stretching, with vA = 1.4, the spherulites of one of the films used for the experiment were broken both from the centers and boundaries, and those of another film were broken mainly from the center. This difference in the deformation behavior seems to be characteristic of the film properties and independent of the method of stretching, although the factors involved are still unknown. On further stretching (vA = 22), well annealed spherulites were broken into many small blocklike fragments with unfolded fibrils running among them, particularly at the low stretching temperature (140°C), and fibrillation proceeded at the expense of the residual fragments. In the case of quenched or slightly crystallized material, the fragments were dendritic and divided into finer and finer fibrils on stretching. At elevated temperature, however, even for well annealed spherulites, the deformation behavior resembles that of the quenched material, and at a high degree of stretching the spherulites take on the fibrillar net structure in every case. In films containing a high amount of atactic fraction, radial, tangential, and boundary cracking occurred more easily, and broad fibrils were observed across the cracks.
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