Morphology of Thermoplastic Elastomers: Elastomeric Polypropylene

Abstract

The morphology of low-density (0.86 g/cm3), 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 (Mw: 147−432 kg/mol) and polydispersity (Mw/Mn: 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 α-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 °C in all of the fractions is most consistent with a stereoblock microstructure of atactic and isotactic sequences in ePP.