Refraction effects and structural changes of hard elastic polypropylene (HEPP) during stretching

Abstract

Small angle X-ray diffraction from hard elastic polypropylene 5M-45B (HEPP) of Celanese shows a sharp long streak along the equator indicating total reflection and refraction effects equivalent to ≳ 2000 A long cylindrical parts within the fibre. The well known amorphous phase therefore does not exist in high modulus fibres similar to 11-times stretched samples of PE with fully extended chains. A new method is described which explains the observed scattering by means of shape functions. One term depends on the lamellae, the other one on the fibrils, whilst the third takes care of the correlations between terms 1 and 2. The latter does not interfere with the calculation because it is simply another well separated component of the equatorial scattering analysis previously described by Guinier. Fibrils exist between the lamellae for extensions greater thanγ= 40%. Below this value the diffuse equator scattering is explained by 100 A long gaps between the chain molecules; from 5 vol. % packing density at γ=0 % they increase to about 50 vol. % atγ=40 %. Here the gaps change their functions to become the surroundings of the fibrils. This thermodynamically-induced metamorphosis explains the metastable nature of the lamellae, the gaps and the fibrils, observed atγ=40 %. The pseudo-Hook domain disappears gradually during iterated strain-relaxation operations after 100 cycles. The stress-strain curve is similar, then, to that of a sulphur vulcanized natural rubber except that theE-modulus is now 100 times larger. The small angle scattering changes according to these phenomena: The number of lamellae becomes smaller and their mean distance increases to 2000 A or more along the chain direction.