Crystallization of Low-Molecular-Weight Polypropylene Fractions

Abstract

Samples of very low molecular-weight fractions of isotactic polypropylene have been crystallized by means of a solvent-evaporation technique as well as by melt recrystallization. The crystalline entities obtained by these methods have been examined in terms of their internal structure and associated morphology. The particular polymorphic form observed for these fractions is dependent on both the length of the molecule and the particular crystallization conditions employed. The morphological characteristics of the samples have been found to be qualitatively similar to those previously reported for unfractionated polypropylene and high-molecular-weight fractions, in spite of the fact that the molecular lengths of the samples investigated are sufficiently short to prevent chain folding. Selected-area electron-diffraction patterns have been obtained from the γ form and from mixed α and γ form of the isotactic polymer. The diffraction data have resulted in a modification of the previously proposed triclinic unit cell for the γ phase, and a conclusion that the γ→α transformation in isotactic polypropylene is martensitic in nature. The mechanism of this solid-solid transformation involves relative motion between the (040) planes which are found to be invariant in the two crystal phases.

The effect of molecular mass on the polymorphism and crystalline structure of isotactic polypropylene

“…The k-value of β-LICO sample is 0.81. In contrary to the literature sources, which have reported that γ-modification forms in low molecular mass iPP grades [18], γ-iPP could not be detected by WAXS technique in the samples studied. The supermolecular structure of LICO sample formed at 130°C is demonstrated in Figure 2.…”

“…We suppose that iPP produced using recent superactive catalyst systems, which provide highly stereoregular structure (isotacticity is usually higher than 98%), is not inclined to the formation of γ-form. The formation of γ-iPP observed in the pioneering works on this topic [18] might be explained by the lower stereoregularity of the earlier iPP grades. The supermolecular structure of peroxide degraded samples is demonstrated in Figure 11.…”

“…The high impact resistance of β-form resulted in increased industrial interest in the past decades and simultaneously the number of available β-nucleating agents increased as well. The early studies revealed that iPP with low molecular mass (M w < 6000 g/mol) results in the formation of samples rich in γ-iPP [16][17][18]. The effect of molecular mass on the crystalline morphology in non-nucleated and β-nucleated peroxide degraded iPP has been studied by Wang et al [19].…”

Crystallization of α and γ Phases in Isotactic Polypropylene with Low Ethylene Content: Isothermal Crystallization and Secondary Crystallization

“…In the secondary crystallization, the remaining low molecular weight portion crystallizes. For low molecular weight, the molecules tend to crystallize into the phase rather than the phase by nature (13,14). Especially, in the case of ethylene copolymer, the ethylene monomer may accumulate in the low-molecular weight portion.…”

“…A large amount of it is obtained by crystallization under high pressure (11, 12), by using low molecular weight materials (13,14) and by co-polymerization with low content ethylene (13,(15)(16)(17)(18). The crystal structure of the phase had long remained a puzzle.…”