Structural characterization of isotactic polypropylene films of ultrahigh orientation. I. Determination of crystalline and amorphous orientation

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Abstract
Nearly perfect crystalline orientation is found in a set of four polypropylene films prepared under different stretching conditions. The crystalline orientation function fc, determined by X-ray diffraction, ranges from 0.9917 to 0.9968, exceeding the highest orientation function previously reported for a polymer: 0.996 for polyethylene crystallized under shear and pressure in a capillary viscometer. This demonstrates that neither shear nor pressure is essential to the attainment of ultrahigh crystalline orientation. The key feature appears to be the existence of a tractional gradient, i.e., of a large velocity gradient in the direction parallel to the velocity vector. Sonic modulus measurements give values from 0.38 to 0.69 for the amorphous orientation function fam. Sonic modulus is especially useful for two reasons: (a) the method averages out short-range fluctuations in fam, and (b) the method is insensitive to crystalline orientation when fc is close to 1. The average orientation function ranges from 0.78 to 0.90, placing all four samples in the fibrillar regime using Samuels's criterion. The results indicate that certain variations in structure are possible in polymers with ultra-high crystalline orientation and suggest that careful work is required to delineate these differences.