X-ray diffraction from single crystals of polyacrylonitrile shows that the diffuse nature of the nonequatorial scattering maxima cannot be accounted for by crystal size. The theory of paracrystals may be used to set both an upper and a lower limit to the amount of lattice distortion and yields a plausible explanation for the absence or extreme weakness of higher-order reflections. Finally, it is shown why the diffuse maxima cannot be used directly in measuring lattice parameters and the method which must be used to obtain a more definite crystal structure for polyacrylonitrile is indicated.
SynopsisNylon 6 can exist in either of two stable crystal structures, the a crystal with hydrogen bonds between antiparallel chains or the y crystal with hydrogen bonds between parallel chains. Both structures have been observed in the same highly annealed fiber, suggesting that the polymer should not he regarded as a pure compound hut a multicomponent mixture. Nylon 6 can also exist in a series of metastahle crystal structures which vary continuously in size, perfection, and structural parameters from a pseudohexagonal structure to either of the two stable forms. A single equatorial reflection in the wide-angle x-ray pattern (i.e., pseudohexagonal structure) will not distinguish which of the two stable forms will result upon annealing. Measurement of the 0,14,0 reflection in oriented fibers will distinguish the major trend, but it is still uncertain to what extent annealing conditions can favor one stahle form over the other. The variation in the unit cell parameters as a function of annealing has been accurately measured, and the observed phenomena can he qualitatively described by postulating various balances between the basic forces which hold the crystal together (i.e., hydrogen bonding, dipole-dipole interaction, van der Waals attraction, and covalent bonding).
The formation of spherulites from concentrated solutions of polyacryloiiit,rile (PAN) has previously been reported in this Journal.' It was shown
SynopsisThe complete orientation distribution of the a and b crystallographic directions have been measured for a series of tubular extruded polyethylene films as a function of blow ratio. The use of pole figures as a means of representing orientation is emphasized and it is shown that the usual technique of using flat plate x-ray diffraction photographs can lead to erroneous conclusions. Increasing the blow ratio has a distinctly different effect upon the orientation of high density polyethylene film in comparison with low density polyethylene film. In contrast with previously published information our results clearly indicate that the c axis does not orient preferentially perpendicular to the extrusion direction. I n particular, it is shown that one cannot in general deduce the maximum in the orientation distribution of the c axis from qualitative estimates of the a and b axes distributions. The ultimate tensile strength, elongation, modulus and tear strengths of these films were measured and the results correlated with the orientation of the crystallographic directions.
Cooling curve investigations show that the transition between the two crystalline forms of tetranitromethane occurs at −99.8°C and its character suggests an order-disorder transformation. The infrared spectra of tetranitromethane were obtained at 25°C (vapor), 18°C (liquid), −40°C (solid I), −88°C (solid I), −104°C (solid II) and −126°C (solid II). The spectra of the vapor are best interpreted by a molecule with S4−4̄ symmetry whereas reported Raman spectra indicate D2d−4̄2m symmetry in the liquid. Satisfactory agreement between these and the x-ray diffraction data require molecules of both symmetries. A complete lack of selection rules was observed in the spectra throughout the solid range.
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