Studies on biaxial stretching of polypropylene film. X. High‐temperature x‐ray and optical study of type III orientation

Change of orientation and crystalline state of uniaxially stretched polypropylene film during subsequent restretching with the film width unrestrained was studied by means of x‐ray, optical, and calorimetric methods. Uniaxially stretched film immediately after 5 min of preheating at 130°C barely suffers premelting. When the preheating temperature rises above 150°C, the premelting proceeds gradually and the x‐ray pattern becomes a halo around 160°C, which, however, returns nearly to the original crystalline pattern after cooling to room temperature. The fraction premelted in the preheating amounts to about ½–⅔ under the condition yielding type III orientation at room temperature, as previously reported. The 130°C restretching brings type II orientation already at that temperature, similar to what has been observed at room temperature. When the restretching is performed above 155°C, the crystalline pattern remaining after the preheating converts to a halo during the restretching, which, however, converts again to the crystalline pattern of type III orientation when it is rapidly cooled. This suggests that the restretching at higher temperatures breaks up lamellae into smaller‐sized crystallites. Upon cooling, the smaller‐sized crystallites reorganize lamellae, the deformed lattice recovers its ordinary state, and the pulled‐out chains crystallize into intermolecular crystallites, aligning in the direction of restretching. Concurrently, disorientation proceeds fairly rapidly at such high temperatures, hence, type III orientation cannot be observed even at room temperature unless the film is quickly cooled after restretching. It is concluded that type III orientation results from restretching when thermal motion of the chains within the crystalline phase becomes so violent that the unfolding occurs easily as compared with lamellar rotation.