Morphology of bulk-crystallized linear polyethylene: Study by small-angle x-ray scattering, Raman spectroscopy, and gel-permeation chromatography

Measurements of the longitudinal acoustic (LA) Raman mode have been combined with small-angle x-ray scattering (SAXS) and nitric acid etching, followed by gel-permeation chromatography (GPC) to examine the morphology of bulk-crystallized linear polyethylene (LPE). It has been shown that in some slow-cooled samples, Raman spectroscopy can provide a direct measure of the all-trans molecular length and hence the actual thickness of the crystalline regions. For other samples (high molecular weight, slow-cooled and quenched) the Raman data taken in conjuction with those from SAXS and GPC give a direct indication of the bulk morphology provided that the differences between the different techniques are interpreted in terms of the degree of coupling between amorphous and crystalline regions. Secondly, comparison between SAXS and GPC data alone suggests that the distribution of the amorphous phase follows a completely different pattern in the two classes of samples investigated. The slow cooled samples exhibit a highly uniform morphology with all the amorphous phase distributed in series with crystals of “infinite” lateral size; the quenched samples are characterized by a heterogeneous morphology with part of the amorphous material either interrupting the lateral continuity of the crystals (crystals of finite lateral size), or being segregated at the spherulite boundaries. The insight which the present results provide on the effects of molecular weight and thermal treatment on the morphology of bulk LPE, is of value in obtaining further understanding of the drawing behavior of this polymer.