Paracrystallinity of hot stretched linear polyethylene cut in slices parallel to the fiber axis and graft copolymerized with styrene

Linear polyethylene (PE), hot stretched 10-fold at 80°C and cut into 0.1 mm-thick slices parallel to the fiber axis was annealed for 102 min at 120°C and then graft copolymerized in styrene by varied doses of 60Co up to 0.4 Mrad. The ungrafted styrene was washed out and the sample then vaccum dried for 24 hr. Again, by a combined analysis of small and wide angle x-ray scattering (SAXS and WAXS), characteristic changes can be observed during the first 40% increase of weight ΔG. The diffraction analysis proves with the help of the theory of paracrystals, that microslices parallel to the plane of cutting are produced, within them the g13-value of the paracrystalline superlattice decreases till ΔG = 20% by 4% and reaches its old value g13 = 28% again at ΔG = 45%, while g23, orthogonal to the slices, till ΔG = 20% increases by 2% and then decreases by 4% to the value g23 = 30% at ΔG = 45%. The graft copolymerization up to ΔG = 20% mainly takes place in the “amorphous” layers of the ultrafibrils, until the shortest PE tie molecules are stretched to such an amount that families of adjacent microparacrystals build up common backfolding surfaces. Above ΔG = 20% all styrene graft copolymerizes in the lateral grain boundaries of the ultrafibrils. At ΔG = 40% they reach a main thickness of ∼ 30 A. Then the adhesive forces between adjacent microparacrystals are so weakened that they are pushed back into their old position with larger g13-values, by which g23 automatically reaches a 4% smaller values. At ΔG = 200% the lateral grain boundaries are 140 A thick in the average and can not grow more at higher doses, since now the PE tie molecules between lateral microparacrystals are stretched out too. The importance of these grain boundaries between adjacent ultrafibrils is pointed out. Though contrary to “amorphous” layers they practically have the same electron density as the microparacrystals they play an important role for the physico chemical behavior of the polymers which can not be understood by the conventional two phase model. Some new observations of distortions in the microparacrystals depending on the degree of graft copolymerization are described. They give hints about kinks and kink blocks in the microparacrystallites.