The influence of side branches on the structure of crystalline phase in ethylene-1-alkene copolymers

The influence of side branches on the unit cell dimensions in ethylene-1-alkene copolymers was investigated both at room temperature as well as during heating of the samples up to melting. Time resolved X-ray diffraction investigations revealed that in the case of ethylene-1-octene copolymers with low and moderate comonomer content, the transversal expansion of the unit cell occurs due to the limited thickness of lamellae, however the samples with higher comonomer content (above 5 mol %) exhibit distinctly larger expansion resulting from the stresses imparted on the basal surfaces of crystals by the excluded branches. At high temperatures, independently on the thermal expansion, an additional increase of the unit cell volume occurs, caused probably by a penetration of the end-parts of side branches, residing near the crystals surface, into the surface cells. Similar phenomenon is observed for ethylene-1-butene copolymer. On contrary, the ratio of the average unit cell volume in ethylene-1-propene copolymer to the unit cell volume in linear polyethylene at the same temperature, decreases starting from about 50 °C. Such a behavior is a new confirmation of a partial inclusion of methyl branches into the crystalline phase. Most probably, when the temperature increases, the volume of included branch becomes a decreasing fraction of the thermally expanding unit cell volume.