Importance of lateral grain boundaries in synthetic polymers

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Abstract
Combined wide-and small-angle x-ray scattering studies (WAXS and SAXS) of line profiles of polymers prove the existence of small mosaic blocks (50-1500 Å in size). They all have liquid like distortions quantitatively defined by a g value and are identified therefore as microparacrystallites (mPC's). Unlike crystals, they can crystallize only in coherent units of not more than N net planes with N = (α*/g)2. Electron micrographs of strain-crystallized transpolyisoprene show directly the grain boundaries between adjacent mPC's within one lamella which appear in temperature range III. The free lateral surfaces of mPC's at the lamella ends are mostly responsible for the long-period growth during annealing in temperature range II. Annealing experiments with poly-1-butene prove that no premelting occurs but solely a solid-state diffusion. This apparently occurs mostly starting from the lateral grain boundaries on account of their larger excess free energy. In stretched isotactic polypropylene films, these lateral boundaries are so pronounced that equatorial interference maxima occur in SAXS. The lateral grain boundaries in mats of polyethylene single crystals are responsible for many phenomena, for instance degradation experiments with HNO3 and gel-permeation chromatographs after Co60 irradiation and further degradation with O3.