Morphology of thermoplastic elastomers:: Elastomeric polypropylene

The morphology of low-density (0.86 g/cm(3)), low-crystallinity (10%) elastomeric polypropylene (ePP) derived from a bis(2-arylindenyl)hafnium catalyst was investigated using a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), Fourier transform infrared (FT-IR) spectroscopy, and tapping mode atomic force microscopy (TM-AFM). This thermoplastic elastomer, when crystallized isothermally from the melt, exhibits morphologies reminiscent of classical semicrystalline polymers. The presence of lamellae, crosshatching, hedrites, and spherulites was revealed by high-resolution TM-AFM. POM confirmed the presence of hedrites and spherulites. The parent ePP can be fractionated into components of different average tacticities ([mmmm]%: 21%-76%) and crystallinities (1%-40% as determined by DSC and WAXS) but more similar molecular mass (M-w: 147-432 kg/mol) and polydispersity (M-w/M-n: 2.1-2.5). The analysis of the morphologies of these fractions revealed large hierarchical structures for all but the lowest crystallinity fraction and crosshatching typical of the a-modification of crystalline isotactic polypropylene for all fractions. The solubility of the corresponding fractions in ether and heptane combined with new evidence for crystals with melting temperatures higher than 100 degreesC in all of the fractions is most consistent with a stereoblock microstructure of atactic and isotactic sequences in ePP.