Small Angle X-ray Scattering Studies of Phase Transitions in Polymeric and Ol1gomeric Systems

Small angle x-ray scattering (SAXS) is a valuable method for studying structure changes which take place during phase transitions in polymeric and oligomeric systems. Measurements of the dependence of SAXS on temperature yield information on the changes in morphology or of the structure on a supermolecular level. The technique can be applied to various problems: glass transition in highly crystalline polymers, solid state phase transitions in crystalline polymers, rotational transition in n-paraffins, premelting phenomena in crystals of chain molecules, and partial melting of homoand co-polymers. The glass transition temperature of the amorphous regions in highly crystalline polymers can be measured by SAXS since the scattered intensity depends on the thermal expansion coefficient of the two phases. This technique was applied to so-called single crystals grown from solution of polybutene-1, branched and linear polyethylene. The amorphous surface layer of linear polyethylene single crystals exhibits a 7, of — 125° ± 4°C. Solid state phase transitions of polymer crystals are often accompanied by characteristic changes in morphology. As examples, the annealing behaviour of trans-1,4-polybutadiene and isotactic polybutene-1 is described. The rotationa1' transition in paraffins studied with n-tritriacontane as an example also leads to a pronounced structure change of the surface of the paraffin lamellae, speaking in favour of a transition theory which is based on the assumption of a sudden change in the concentration of kinks within the crystals. With increasing temperature an increase of the roughness and depth of the surface region of paraffin crystals is observed. This phenomenon can be described as 'surface premelting of chain molecular crystals' and SAXS yields quantities suitable for a thermodynamic explanation of that process. In homoand co-polymer crystals partial melting during heating is observed. The SAXS studies show that this effect can be due to a 'boundary premelting of polymer crystals' which manifests itself by a reversible increase of the thickness of the intercrystalline layers. This phenomenon can be observed in many polymers, it can be described quantitatively by analysis either of the scattering curves or of the correlation functions y(x) obtained from the intensity distribution.