The Role of Stretching-Induced Phase Transformations in the Mechanical Properties of Isotactic 1-Butene-ethylene Copolymers from Ziegler–Natta Catalyst

The crystallization behavior and phase transformations occurring during deformation of random isotactic 1-butene-ethylene (C4C2) copolymers, synthesized with a Ziegler–Natta catalyst and characterized by an ethylene (C2) content between 1.7 and 16.3 mol %, were investigated using a combination of tensile testing and in situ wide-angle X-ray diffraction (WAXD) with synchrotron radiation. The phase transformations were correlated to the mechanical behavior. Samples with C2 content ≤7.6 mol % were basically crystallized in form II in their unstretched state, whereas samples with higher C2 contents were crystallized as a mixture of form II and form I′. During stretching, form II present in all the initial unoriented samples rapidly transforms into form I, indicating that uniaxial deformation significantly accelerates form II–form I transition, since in quiescent conditions, this transformation required longer times to complete. However, the effect of stretching on the form II–form I transition differs among the various copolymers. Specifically, the critical strain at which the form II–form I transition begins (εc) and the strain at which 50% of the initial form II transforms into form I (ε0.5) increase with increasing C2 content. The quantitative analysis of WAXD data enabled the construction of a phase diagram for C4C2 copolymers, which identified the regions of stability of the various polymorphic forms as a function of ethylene content and deformation. In addition to the form II–form I transition, further crystallization of form I′ from the amorphous phase was observed during stretching of some samples. This study provides valuable structure–property information that is helpful for the design and application of this class of semicrystalline materials.