Nucleation and Crystallization of Low-Crystallinity Polypropylene Followed in Situ by Hot Stage Atomic Force Microscopy

The isothermal crystallization of the ether-soluble fraction (ES) of elastomeric stereoblock polypropylene (ePP) was investigated in situ by hot stage atomic force microscopy (AFM) at temperatures between 30 and 60 °C. Owing to the low average tacticity of 21% ([ mmmm ]), this material possesses a very low degree of crystallinity (≤1% as determined by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS)) in fully crystallized samples. Despite this low degree of crystallinity, hot stage AFM allowed us to study the nucleation and growth processes of crystallization in thin ES films in real time with nanometer resolution. The crystallization occurs in the form of lamellar crystals that develop from stable nuclei. Both metastable and stable primary nuclei have been visualized. Many crystals can be assigned to the α-phase of isotactic polypropylene (iPP) based on the observation of crosshatching. The lamellar growth rates are <3 × 10 -3 μm/min and show a maximum between 40 °C and 45 °C. These results form the basis for a better understanding of the crystallization of ultralow crystallinity polypropylenes and likely other low crystallinity polymers.