ABSTRACT:The molecular structure and morphology grown from isotactic polypropylene (iPP)/o-dichlorobenzene solution were investigated by thermal analysis and high-resolution solid-state 13 C NMR. The morphology varied depending on incubated temperature. A gelation occurred below 70• C, while the crystallization took place over 80• C. The crystal obtained was identified with the α-form, which is the most stable crystalline modification of iPP. In CP/MAS 13 C NMR spectrum, the chemical shift of methylene, methine, and methyl carbons provided almost the same shift for both the gel and α-form, but the characteristic doublet for methylene and methyl signals of the gel was different in intensity ratio with the α-form. These results indicate that the gel-forming crystal is identified with α 1 -form, being the disordered α-form. Indeed, the lower melting temperature and smaller melting enthalpy than the crystal were observed by thermal analysis. This result connects to the incompleteness of crystal formed in the gel.KEY WORDS Isotactic Polypropylene / Gelation / Crystallization / Morphology / Thermal Analysis / Solid-State 13 C Nuclear Magnetic Resonance (NMR) /Isotactic polypropylene (iPP) shows polymorphs as a function of crystallization condition. As for the molecular conformation, all crystal modifications take a regular trans-gauche (tg) conformation with 3/1 helix. The most thermodynamically stable form takes a monoclinic structure called α-form. 1 The pseudohexagonal β-form is obtained by high cooling rate or by adding specific nucleating agents. 2-8 The orthorhombic γ-form has unique chain structure which is featured by nonparallel chain axes to one another. [8][9][10][11][12] This form can be obtained by crystallization for low molecular weight iPP or by dealing at high pressure. There is also a mesomorphic structure which can be obtained by quenching from the melt. The investigation of this form has been performed by lots of researchers, who proposed the interpretations such as smectic phase, 2 paracrystal, 13, 14 microcrystallite, [15][16][17][18] and so on. The recent study seems to reach the conclusion that the ordered helical chain distributes without longitudinal order, however the crystalline unit is close to the structure of the monoclinic α-form. 19 In order to investigate these crystalline modifications, high-resolution solid-state 13 C NMR is a powerful tool. First Bunn et al. reported the characteristic doublet with 2 : 1 intensity ratio for methylene and methyl signals in cross polarization (CP) / magic angle spinning (MAS) 13 C NMR. 20 After that, lots of investigations using high-resolution solid-state 13 C NMR were carried out. 7,16,17,21 Caldas et al. investigated the relationship between the crystalline form and the intensity ratio of doublet, and established the origin of different intensity ratio for the lower crystalline order form (α 1 ) with C2/c and the higher crystalline order form (α 2 ) with P2 1 /c space group. [16][17][18] Two different environments giving doublet exist in the cryst...