Slow dynamics of polymer crystallites revealed by solid-state MAS exchange NMR

One- and two-dimensional exchange NMR methods that include a T-tpc relaxation filter were applied to investigate chain dynamics, material properties, and crystal-crystal transformation mechanisms of isotactic-poly(I-butene) (iPB) and isotactic-poly(4-methyl-1-pentene) (iP4M1P) crystallites in natural abundance. Results revealed that iP4M1P performs helical jump motions in the disordered 72 helix, and their correlation times show non-Arrhenius behavior with a strong slowing down near the glass transition temperature (T-g = 304 K). Such a unique dynamics in the crystalline region is related to the rigidity of amorphous segments below T-g. On the other hand, it was found that iPB does not show any chain dynamics up to 370 K near the melting temperature. The obtained dynamics in two crystalline polymers are closely related to the bulk material properties such as drawability and thermal parameters. Furthermore, it was found that unusual forms, form III of iPB and of iP4M1P, which commonly show crystal-crystal transformations spontaneously, perform large amplitude motions around their helical axes before the transformations. These results indicate that the large amplitude motions play an important role for crystal-crystal transformations.