Electron micrographs of highly crystalline PTFE crystallized by slow cooling from temperatures not far above the melting point show well‐marked bands, often with striations perpendicular to the bands; optical evidence shows that the chain molecules are parallel to the striations. The structure is in marked contrast to the spherulitic structure of most polymers; it appears that in PTFE the molecules are straight and parallel for much greater distances than in other polymers, and it is suggested that the width of the bands is a measure of molecular length. The unusual structure is attributed to the unusual stiffness of the fluorocarbon chain; the molecules in the liquid are straighter and less tangled than in other polymer melts. Heating to 500°C., followed by slow cooling, gives a modified structure approaching the spherulitic type found in other polymers; it is suggested that this is due to increased molecular tangling in the melt at higher temperatures. One type of polymer shows bands of remarkably uniform thickness, suggesting unexpected uniformity of molecular length. PTFE wax made by thermal or radiation degradation of the high polymer shows well‐defined spiral growth steps due to dislocations; the step heights suggest an unexpected uniformity of molecular length.
It has been discovered that when fuming nitric acid oxidises bulk samples of polythene crystallised from the melt, preferential attack on the links between the lamellar crystals occurs with the consequent release of fragments of these crystals. The reaction is not confined to the surface of samples and lamellar crystals can be separated from relatively massive specimens for further study.Selected area electron diffraction results are presented to show that the molecular chains are oriented approximately perpendicular to the plane of the lamellae and that the sheets are substantially single crystals.The results of an examination, by a variety of physical methods, of the lamellar crystals and reaction products from samples with different thermal histories treated for different times are given. Analysis of the results confirms that the attack occurs preferentially at the available amorphous regions, which in the case of the linear polymer are almost exclusively located between the lamellar crystals.
The orientation in the crystalline and amorphous regions of a series of polyethylene films prepared by the tubular films extrusion process has been studied by a combination of x‐ray diffraction and optical methods. All of these films show a relaxation orientation differing both from the fully relaxed type of orientation previously proposed by the authors, and from the more complicated type of orientation suggested by Keller. In all films there is cylindrical symmetry about the machine direction and the b‐axes of the crystallites are preferentially oriented in the plane perpendicular to this direction. The preferred directions of the a and c crystallite axes depend both on the extrusion conditions and on the nature of the polymer; at low blow‐up ratios and high haul of speeds the c axes tend to lie more nearly parallel to the machine direction, but as the blow‐up ratios is increased and the haul‐off speed decreased rotation of the crystallites about their b axes occurs, and eventually the a axes tend to lie more nearly parallel to the machine direction. Both the preferred crystallite orientation and its angular distribution have been measured for a large number of films made from various ethylene polymers, and a linear relation between a function of the orientation angle and the optical birefringence has been established. As the c axes tilt away from the machine direction towards the plane perpendicular to this direction the birefringence steadily decreases, and eventually becomes negative. An expression is given for the birefringence of such a cylindrically symmetrical array of crystallites tilted to various angles, and is used to predict the orientation at which the crystallite birefringence contribution will be zero and also the extreme values of the crystalline birefringence contribution in the fully drawn and fully relaxed conditions. By comparing the theoretical and experimental relations between crystallite orientation and birefringence, it has been deduced that in these films the amorphous material appears to provide a nearly constant small positive contribution to the birefringence irrespective of the birefringence contribution of the crystallite regions; this means that in the negatively birefringent films some of the molecules in the amorphous regions are crossed with respect to those in the crystalline regions. When allowance is made for this amorphous contribution, crystallinities calculated from the extrapolated observed and theoretical extreme values of the birefringence are in reasonable agreement with the x‐ray measured crystallinity of 45%. The mechanism resulting in the formation of this unusual orientation is considered in reference to the manufacturing process.
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