Deformation X-ray study of propylene-based elastomers with controlled sequence distributions

- Morphology of three PBEs made with different catalysts dictate microstructure and subsequent morphology.
- These morphology differences result in changes in elastic properties.
- Different strain induced morphologies for blocky PBE than random PBE or alternating PBE.

Three iso-specific organometallic catalysts for ethylene and propylene copolymerization with varying products of ethylene and propylene reactivity ratios were utilized to synthesize equal-molecular-weight propylene-based elastomers (PBEs) of alternating (aPBE), random (rPBE), and blocky (bPBE) backbone sequences. To compensate for the variations in catalyst iso-specificity, the ethylene content of PBEs was varied between 11 wt % and 16 wt % in order to maintain constant amount of 50% isotactic propylene trimer concentration. Their microstructural evolutions during the initial extension-retraction cycle at a low strain ranging from 0% to 100% were investigated using 2D Wide-Angle X-ray Diffraction (WAXD) and Small-Angle X-ray Scattering (SAXS) techniques. WAXD results revealed that the crystallinity values of these PBEs changed little during the extension-retraction cycle. However, SAXS results indicated that bPBE formed distinctly ordered stacked lamellae morphologies, whereas the crystalline-amorphous repeat units in aPBE and rPBE were more disordered. In addition, the long period of the bPBE lamellar structure exhibited elastic affine deformation. The analysis suggested that initially randomly arranged crystalline lamellae in PBE become partially oriented, but not destroyed, with increasing strain, where over half of the lamellar structures were orientated parallel to the stretching direction when the strain reached 100%. Upon cessation of stress, these orientated crystalline structures could recover back to their isotropic state.

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