ORIENTATION MECHANISM DURING THE BIAXIAL STRETCHING OF ISOTACTIC POLYPROPYLENE FILMS AT VARIOUS TEMPERATURES

Orientation mechanism during the two-step biaxial stretching with film width unrestrained were studied mainly by means of x-ray and DSC methods. Four cases of the stretching and the restretching temperatures were examined. One of the cases gives the Type III orientation and the others the Type II orientation.<br>When the restretching proceeds (in the Type II) the fibrils oriented to pp direction rotate gradually toward ps direction (in the range of v₂<4_??_5), while some fibrils oriented to ps direction are newly developed over v₂=3. The long period in the newly developed fibrils are larger than that in the original fibrils at v₂=1.1_??_1.3. It suggests that the tie molecules between crystal brocks in the fibrils seem to be elongated. The long period also depends on T₂.<br>As T₂ is higher than T₁ about 20°C (Type III) premelting of the lamellae proceeds significantly to the crystal core. Then, at the lower restretching ratio (v₂=1.3_??_4) some intermolecular crystals oriented to ps direction are developed with the chains unfolded from the premelting lamellae. At the higher restretching ratio (v₂=4_??_9), the remaining fibrils in which the crystals have become stable by the annealing effect during the restretching may be not only unfolded but also destructed into small folded chain brocks. Then the brocks may be arranged in the fibrils newly developed to ps direction in addition to the unfolded chains. Such fringed-like structure seems to be observed in the films restretched at 150°C (about v₂=4).<br>Change in dimensions of the films during the restretching at the various temperatures can be explained in relation to the change of the fine structure above mentioned. In general, change in dimension is small when the rotation of the fibrils mainly occurs.<br>Each of DSC melting curves in the Type II orientation has a shoulder below T_m2 at the higher restretching ratio (v₂>4). The shoulder may correspond to the melting of the fibrils which are newly developed in ps direction. Changes in T_m2, T_m3 and ΔH during the restretching at the various temperatures are considered to depend not only on thermal stability of the crystals, but also on many other factors. Especially at the higher restretching ratio, the strain of the tie molecules between crystal brocks has an important effect. After consideration of the effect, the results of the thermal analysis support the interpretation that the difference of the two types of orientation occurs from the crystal state of the premelting in the original fibrils.