Molecular Structure of Isotactic Polypropylene Formed from Homogeneous Solution. Gelation and Crystallization

Abstract

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...

Morphological reorganization and mechanical enhancement in multilayered polyethylene/polypropylene films by layer multiplication or mild annealing

“…Here, we instead observe by WAXD a conversion of PP mesophase to α crystal phase, in a manner consistent with the change in mechanical properties. SSNMR spectra acquired on the same samples are compatible with the mesophase reported for iPP films crystallized with very large overcooling (100 °C) or where disorder is inserted by the presence of solvents but do not show significant amount of crystallographically pure α phase.…”

Aggregation Behavior of the Styrene Units in Oil Gel of Styrene-Butadiene-Styrene Triblock Copolymer

“…[7][8][9] SBS oil gel is applied as constructing materials and shock absorber for its rubbery elasticity. The gel networks of SBS in oil is formed by the aggregative interaction of the styrene units in a selective solvents which is a good solvent for butadiene units, but a poor solvent for styrene unit, and seggregative interaction between the styrene and butadiene units.…”

Melting Behavior of Bound Solvent in Isotactic Polypropylene/o-Dichlorobenzene Gel

“…The gel-forming crystal possibly takes a disordered form. 4 Most investigations on gel have focused on details of the sol-gel transition, but some are concerned with the coagulation and melting behavior of the solvent in the gel. Higuchi et al investigated the melting behavior of water in a polyvinyl alcohol (PVA) swollen gel, and showed the existence of free water and freezable bound water.…”

“…Gelation of a crystalline polymer such as isotactic polypropylene (iPP) is closely associated with the formation of crystal as a cross-linking point. [1][2][3][4][5] Okabe et al reported a solgel transition and the morphology of iPP gel by thermal analysis and polarizing microscopy, 6,7 and found that the gel melting temperature is fitted as a function of polymer concentration by a Eldridge-Ferry plot and many spherulites exist in the gel. There exist three types of crystalline modification.…”

“…Studies on the crystalline structure formed in iPP/ o-dichlorobenzene gel have been reported by high resolution solid state 13 C NMR measurements. 3,4 A cross polarization (CP)/magic angle spinning (MAS) spectrum showed the characteristic doublets for methylene and methyl resonance lines with an intensity ratio of 1:2 from downfield, whereas the stable crystalline -form [12][13][14][15][16][17][18] provided a spectrum with the opposite intensity ratio. The gel-forming crystal possibly takes a disordered form.…”