On the volume organisation of thermoplastic vulcanisates (TPVs) as revealed by scanning transmission electron microscopy (STEM) tomography

The phase organisation of thermoplastic vulcanisates (TPVs) has been analysed in detail by applying scanning transmission electron microscopy (STEM) tomography. High contrast between the crosslinked rubber (EPDM) and the isotactic polypropylene (iPP) phases has been achieved without staining the samples. STEM tomography at low-converge-angle conditions allows for the investigation of micrometre thick samples enabling analysis of dispersion and (possible) connectivity of the phases in the TPVs for ultra-large (>50 μm3) volumes. EPDM is dispersed in the iPP phase in a TPV with low EPDM content, whereas it forms a co-continuous phase in a TPV with high EPDM content. The latter could up to now not been demonstrated unambiguously and has impact on the rheological behaviour of the TPV. Moreover, it is demonstrated that zinc oxide and talc particles used as additives in the TPV formulation are located exclusively in the iPP phase of the TPV blend, which suggests that wetting of the solid particles by the relatively low viscous iPP phase during dynamic vulcanisation determines the location of these inorganic additives.