Single-molecule study on the force-induced melting in syndiotactic polypropylene single crystals

Highly stereoregular syndiotactic polypropylene (s-PP) with high crystallinity and melting temperature has unique physical properties such as high impact resistance and transparency, which is closely related to the chain conformation and crystal structure of s-PP. However, the molecular mechanism underlying the mechanical properties remains unclear, because the complex amorphous-crystalline network and the diverse crystal structure limit the classical characterizations. Here, we investigate the single-molecule force-induced melting in the melt-prepared and solution-prepared s-PP single crystal by using atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS). Our results show that the mechanical stability slightly increases with the increase of crystal thickness and the crystal thickness also has a significant impact on the melting pathway. The comparison of the melting pathways of s-PP and isotactic polypropylene (i-PP) suggests that the syndiotactic structure and lower tacticity of s-PP will increase the chain friction during slide and therefore suppresses the αc relaxation in the crystal region. Moreover, the difference of the chain folding trajectory in sectors (100) and (010) shows no effect on the mechanical stability. However, the mechanical stability of form I structure was higher than that of form II.