Diffraction study of crystallite orientation in a stretched polychloroprene vulcanizate

Abstract

Abstract Crosslinked polychloroprene samples were allowed to crystallize from the melt at various fixed elongations. X‐ray diffraction studies revealed that the c axis becomes oriented toward the stretching direction, while the a and b axes are randomly distributed about the c axis. The alignment of the crystallite c axes toward the drawing direction is much more perfect than that of the amorphous statistical chain segments prior to crystallization. This can be explained if we assume that the formation of a stable nucleus for crystallization requires the simultaneous alignment of v amorphous segments. Thus, such measurements offer a means by which the critical size of the crystallization nuclei can be determined. The sizes deduced for polychloroprene vary with elongation from a cube 30 A. on a side at relative elongation α = 1.45 to a 17 A. cube at α = 7.40. Although our samples were not crystallized isothermally, these results are in qualitative agreement with nucleation theory. If the variation of v can, indeed, be estimated theoretically, then it will be possible to predict the entire crystallite orientation distribution in materials of this type from first principles. The orientation of crystallites formed in a sample at relative elongation α = 2.92 was found to be essentially independent of temperature from 24° C. to the melting point. For low degrees crystallinity, ω it is shown that a plot of 1/ T versus 1/(1 − ω) 2 is linear, thus permitting an evaluation of the melting point by extrapolation. Finally, the crystallite orientations were compared for a sample crystallized from the melt at α = 2.86 and for the same sample after further drawing to α = 3.50 at room temperature. The distribution in the latter case was considerably broader, which demonstrates the primary role played by interactions between crystallites in the cold drawing of partially crystallized materials.