Relationship between molecular configuration and stress induced phase transitions

In semi-crystalline polymers, polymorphic transitions between different crystalline forms can be triggered not only by thermal treatments but also by mechanical deformation. The transformations related to phase changes of the crystals, and those occurring at lamellar length scales by the effect of tensile deformation are studied in detail, focusing on a set of isotactic polypropylene samples, having a molecular mass in the range 100–200 kg/mol, polydispersity index close to two, and different concentration stereodefects along the chains with a uniform distribution. This enables the effect of the microstructure of the chains on the deformation behaviour to be illustrated. The transformations which occur by effect of deformation are followed in real time during stretching through wide and small angle X-ray scattering measurements, made using the high flux of X-rays available at a Synchrotron light sources. The stream of data obtained in the measurements performed in continuum are analysed in the framework of our current understanding of the deformation mechanism of semi-crystalline polymers. This study shows that, during the transformations of a spherulitic morphology into a fibrillar morphology, the stress-induced phase transitions which occur during plastic deformation are regulated by the same factors that govern the textural and morphological changes that is the ability of the entangled amorphous chains to transmit the stress and the intrinsic stability of the lamellar crystals. Since the relative stability of the different polymorphic forms involved in the structural transformations and the intrinsic flexibility of the chains depend on the stereoregularity, we are able to make precise correlations between the stereoregularity of the chains, and the deformation behaviour, paving the way for understanding the material properties at molecular level.