| Semi-crystalline polymers undergo drastic structural changes from random coils in the melt states to folded chains in the crystalline lamellae during crystallization. One fundamental question in this process is the long-standing controversy about the chain-folding structures in semi-crystalline polymers. The isotactic polypropylene (iPP) is one of the most important polymer materials in commercial application as well as in academic field. Also, the iPP has different crystalline forms, in which the chain packing structures, such as fraction of different forms and spatial heterogeneity, were not fully understood previously. When cooled from melt, iPP crystallize into alpha form, which can be further divided into ordered alpha2 and disordered alpha 1 depending on the methyl group orientations. Using modern high-resolution solid-state NMR (SS-NMR), the chain-packing structure of alpha1 and alpha2 forms can be quantitatively separated by the dramatic lineshape differences. The effects of chemical structures of iPP on the chain-packing structure in the crystalline region have been studied. The higher stereo regularity sample forms more alpha2 form at Tc > 135°C. Moreover, 1H spin diffusion experiments indicated that the alpha1 and alpha2 form will form domain structures at average side of ca. 40 nm when iPP crystallized at 150°C Furthermore, high-resolution NMR spectra demonstrated that the stereo regularity defects are excluded in the crystalline region at Tc = 150°C, whereas 4% of the defect remains in the crystalline region quench annealed alpha1 form. Finally, the chain-folding structures and ensemble average of successive chain-folding number are revealed by 13C-13C double quantum (DQ) NMR. The average chain-folding number is determined to be 5 to 7 for both alpha1 and alpha2 forms. However, different chain-folding structures are concluded based on the chain-packing structures and conformational constrains. Through obtained packing and folding structures, crystallization mechanism was discussed at molecular levels. |
