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A basic study of fibrillation and void development during drawing of melt spun polypropylene filaments is presented. The filaments are characterized by small angle x-ray scattering (SAXS), density, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), wide angle x-ray diffraction (WAXS), and birefringence. The latter two methods were used to characterize the molecular orientation produced by processing while SEM studies of peeled filaments revealed, qualitatively, the development of the fibrillated internal superstructure. The volume fraction of microvoids was computed from the intensity of SAXS and from a combination of DSC and density measurements. The microvoid fractions determined by these two techniques were in good agreement. The volume per cent of microvoids varied from 0.04 to 2.8%. It was found that the fibrillation observed qualitatively by SEM and the measured volume fraction and number density of microvoids increased with (i) increase in draw ratio, (ii) decrease in draw temperature, (iii) increased orientation of melt spun filaments, and (iv) increased molecular weight of the polypropylene. Using Guinier analysis of the SAXS data it was found that the voids had dimensions of 25–40 nm, parallel to the fiber axis and of order 15 to 30 nm perpendicular to the fiber axis. As the drawing temperature increased the SAXS void size increased and void number density decreased. The longitudinal mechanical properties of the fibers were found to be mainly a function of the orientation and were not very much affected by changes in fibrillation or void structure per se.

The Fibrillar Structure of Melt Spun and Drawn PP Filaments