SynopsisX-Ray diffraction pole figure measurements have been made on six films blown from high-density polyethylene. Five were prepared on an experimental unit; the sixth, blown on commercial equipment, had a higher degree of overall orientation thus enabling more extensive results for the a-, b-, and c-axis distributions to he obtained. The results provide information on the orientation present, how it may be related to the stress crystallization process of Keller and Machin, and the dependence of the tear strengths in the machine and transverse directions on the orientation. The typical hehavior, with the a and c axes a t 90" and both a t an angle to the plane of the film, is shown to be the result of a composite process involving several types of stress during the blowing operation and a partial relaxation arising a t one or possibly two stages of the process. The predicted relative tear strengths, obtained from the concentration of c axes inclined towards the machine direction, are in line with the experimental values. There is some evidence that transcrystallization in the surface layers of the films may lead to a better balance of the tear strengths in the machine and transverse directions.

SynopsisX-Ray diffraction pole figure measurements have been made on a series of films, blown under various conditions from three high-density polyethylenes. The results are interpreted in terms of two distinct types of orientation. The first, and probably the more normal, is the result of the type of stress crystallization process described by Keller and Machin and has the a and c axes inclined at an angle to the plane of the film. The second type of orientation is crystallographically analogous to that found in cold drawn polyethylene in having the c-axis distribution substantially along the machine direction. This is termed high-stress orientation. The type of orientation obtained is dependent both on the blowing conditions and the particular polyethylene. With an experimental Rigidex grade and with Shell LPPE 040 there are always substantial amounts of the conventional low-stress orientation although certain combinations of machine conditions predispose towards the high-stress form. This latter type forms readily in the case of Hostalen GM 9955F over a rather wide range of machine conditions and appears to be favored by slower cooling conditions.

SynopsisX-Ray diffraction orientation measurements have been made on a wide range of films blown from three high-density polyethylenes, to determine more precisely the conditions which lead to the high-stress crystallization type of orientation. The most extensive measurements relate to films from Hostalen GM 9955F; the results show that there is a very wide range of orientational behavior.Under very low-stress conditions there is almost pure a-axis orientation; with very high stress there is substantial c-axis orientation, both with reference to the machine direction. Commercial blowing conditions give rather high stress and the a axis is inclined a t 60" to 70" to the machine direction in the sheet-normal-machine-direction plane. Calcium stearate, used to improve the surface finish, increases the stress for a given set of machine conditions and, of these, a high draw ratio and a low extrusion temperature are most effective in promoting high-stress crystallization. The less extensive results for an experimental Rigidex grade and Shell LPPE 040 fit into this overall pattern; for a given set of blowing conditions they have lower stress than the Hostalen polymer. Commercial blowing conditions give an a-axis inclination of about 45".

X‐ray diffraction scans and infrared absorption spectra for the C‐CI stretching region were obtained for a highly syndiotactic poly(vinyl chloride) (PVC) sample made by the urea clathrate method. When the polymer was annealed at a series of increasing temperatures up to 180°C, x‐ray diffraction measurements showed that the crystallinity increases steadily with annealing temperature. Even at 50°C an increase above the original value of 63% was detectable and by 180°C it had reached 70% with a further increase to 78% after cooling to ambient temperature. There is a concurrent significant increase in the lateral crystallite dimensions. However, the infrared spectrum did not change, in agreement with the recent prediction of Moore and Krimm that there is no observable band splitting from interchain interaction in crystalline regions, thus indicating that these C‐CI bands cannot be used as a measure of crystallinity. The results from computer curve fitting of the spectra suggest that at least 85% of the polymer consists of long planar syndiotactic sequences and there is therefore substantial order along the chain direction. A mechanism for the increase in crystallinity on annealing, involving the lateral ordering of these regular chains, is discussed. Furthermore, as the temperature is raised some amorphous material is converted to a nematic phase, and this may crystallize during the subsequent cooling.

No abstract