The γ form of crystalline, isotactic polypropylene

Awaya, Hiroshi

doi:10.1002/pol.1966.110040209

Cited by 32 publications

(12 citation statements)

References 2 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…1 In 1966, a paper appeared in the literature showing that the γ form of iPP may be obtained also in uniaxially oriented specimens, in the case of low molecular weight iPP samples. 5 As pointed out in ref 5, the X-ray fiber diffraction patterns of oriented specimens of iPP in the γ form are very similar to those of uniaxially oriented samples of iPP crystallized in the R form containing a nonnegligible portion of daugther lamellae, tilted by ≈81°w ith respect to the parent lamellae, the parent lamellae being oriented with the c axes parallel to the stretching direction, the daughter lamellae having the a axes parallel to the c axis of the parent ones. [6][7][8][9] The only difference is the presence in the fiber pattern of the γ form, of an equatorial reflection at d ) 4.42 Å (2θ ) 20.1°).…”

Section: Introductionmentioning

confidence: 99%

The Oriented γ Form of Isotactic Polypropylene

et al. 2001

View full text Add to dashboard Cite

A structural analysis of the oriented γ form of isotactic polypropylene (iPP) is presented. The X-ray diffraction intensity distribution in the oriented γ form is quantitatively evaluated for the first time. The structural analysis is performed on a new, poorly stereoregular ([mmmm] = 35%) polypropylene material, prepared with a novel C 2-symmetric zirconocene based catalytic system, showing elastomeric properties. The sample, initially amorphous, partially crystallizes upon aging. In the oriented crystalline materials, obtained by stretching the aged sample, the crystallites tend to assume a preferred orientation with the chain axes parallel to the draw direction. Disorder is present in the structure. For low draw ratios (less than twice the initial length) disordered crystalline modifications, very close to the γ form of iPP, are obtained; increasing the draw ratio, crystalline modifications, closer to the α form of iPP, are observed. The structural disorder occurring in our stretched samples originates from the random succession of bilayers of chains with chains parallel (like in the α form) or tilted ≈81° each other (like in the γ form). The results of this analysis are the follows. For the sample stretched at a low draw ratio, ≈75% of the bilayers of chains are faced each other like in the γ form so that the fraction of bilayers with chain axes pointing preferentially along the draw direction is on the order of 60−65% (the remaining 35−40%, being oriented with the chain axes tilted ≈81° to the draw direction). For the samples stretched at higher draw ratios, the fraction of bilayers facing each other like in the γ form is reduced to 20−30%, so that only 10−15% of the bilayers results oriented with the chain axes nearly normal to the draw direction. This is as if, in all cases, the regions of the crystals with chains arranged like in the α form tend to be oriented with the chain axes parallel to the stretching direction rather than tilted ≈81°. A continuum of disordered modifications intermediate between the pure γ and α forms could exist. The possible inclusion in the crystalline domains of stereo defects (mainly of the kind mrrm), which would favor the γ -like packing situations on the local scale, is discussed.

show abstract

Section: Introductionmentioning

confidence: 99%

The Oriented γ Form of Isotactic Polypropylene

et al. 2001

View full text Add to dashboard Cite

show abstract

“…[13][14][15][16] Usually, the ␤ phase is observed only in a small amount, although in the presence of selective ␤-nucleating agents, 16 -20 the content of ␤ can be significantly increased. The ␥ modification occurs even more rarely but may form in degraded iPP, 10,21 in low molecular weight iPP, 7,[22][23][24] or under high-pressure conditions. [25][26][27][28] The self-nucleation and recrystallization of iPP synthesized with conventional Ziegler-Natta catalysts for the ␣ and ␤ phases were investigated by Fillon and coworkers 17,29,30 with differential scanning calorimetry (DSC), optical microscopy, and electron microscopy.…”

Section: Introductionmentioning

confidence: 99%

“…Usually, the β phase is observed only in a small amount, although in the presence of selective β‐nucleating agents,16–20 the content of β can be significantly increased. The γ modification occurs even more rarely but may form in degraded iPP,10, 21 in low molecular weight iPP,7, 22–24 or under high‐pressure conditions 25–28…”

Section: Introductionmentioning

confidence: 99%

Thermal analysis and X‐ray scattering study of metallocene isotactic polypropylene prepared by partial melting

Dai

Cebe

Capel

2002

J Polym Sci B Polym Phys

View full text Add to dashboard Cite

In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m-iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic ␣ and ␥ phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self-nucleation temperatures (T n 's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160°C. The relative amounts of ␣ and ␥ crystals are determined from the area under the unique wide-angle X-ray reflections. The lower and upper endotherms are caused by the melting of ␥ and ␣ crystals, respectively. Four distinct regions of T n were identified on the basis of the thermal and structural parameters of m-iPP. In region I, T n is below the peak melting temperature of the ␥ phase. Here, ␥ crystals are annealed and ␣ crystals are barely affected by T n . In region II, T n is above the peak of the lower endotherm but below the peak of the upper endotherm. ␥ crystals melt, and ␣ crystals anneal. In both regions I and II, the portion of the sample melted at T n recrystallizes epitaxially with existing parent ␣ lamellae as the substrates, and the amount of ␣ always exceeds the amount of ␥. In region III, T n is above the peak of the upper endotherm, and all ␥ crystals and some or all ␣ crystals are melted at T n . The number of ␣-crystal nuclei steadily decreases as T n increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, T n exceeds the upper endotherm, and only small self-nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of ␣ and ␥ is observed during cooling from T n . Because of the effective nucleating ability of ␣ toward ␥, as the temperature drops, the amount of ␥ increases and then exceeds the amount of ␣. With subsequent reheating, the reverse crossover occurs because of the lower melting point of ␥.

show abstract

“…The crystallization behavior of isotactic polypropylene (i-PP) is very complex. Numerous groups have aimed their research on a better understanding of the different crystal modifications (α, β, γ, and smectic). − At atmospheric pressure the γ-modification is commonly observed as a minor constituent in bulk-crystallized i-PP. , Large contents of the γ-modification are obtained when i-PP is crystallized at elevated pressures , or when very low molecular weight samples (between 1000 and 3000 g/mol) are used. − These samples can be produced by thermal degradation of high molecular weight i-PP. , Also random copolymers of propene with 2.5−20 wt % of other 1-olefins may crystallize preferably in the γ-modification. − More recently, it has been shown that high molecular weight i-PP samples prepared by employing metallocene catalyst systems may tend to crystallize in the γ-modification. , Similar to propene copolymers with 1-olefins, the steric irregularities of these samples have been proposed to be responsible for the formation of the γ-modification , The orthorhombic unit cell of the γ-modification is formed by bilayers composed of two parallel helices. − The direction of the chain axis in adjacent bilayers is tilted with an angle of 80°. − This is a unique packing arrangement for polymers but has been known e.g.…”

Section: Introductionmentioning

confidence: 99%

“…[11][12][13][14] These samples can be produced by thermal degradation of high molecular weight i-PP. 15,16 Also random copolymers of propene with 2.5-20 wt % of other 1-olefins may crystallize preferably in the γ-modification. [17][18][19][20][21][22] More recently, it has been shown that high molecular weight i-PP samples prepared by employing metallocene catalyst systems may tend to crystallize in the γ-modification.…”

Section: Introductionmentioning

confidence: 99%

On the γ-Phase of Isotactic Polypropylene

et al. 1996

View full text Add to dashboard Cite

The crystalline morphology of a relatively high molecular weight isotactic polypropylene (i-PP) (M w = 18000 g/mol) with well-defined regio- and stereoirregularities is studied by means of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and light microscopy. This i-PP crystallizes under certain crystallization conditions nearly exclusively in the γ-modification. The growth of the γ-phase is often influenced by the initial growth of the α-phase, but also the neat γ-phase is observable. The formation of the γ-phase can be related to stereo- and regioirregularities in the polymer chain caused by the polymerization mechanism using metallocene catalysts. WAXS measurements show that at large supercoolings the formation of α-phase i-PP is preferred, whereas at low supercoolings this sample crystallizes nearly exclusively in the γ-modification. Light microscopy and AFM reveal for the γ-phase i-PP morphologies different from the well-known spherulites formed by the α- or β-modification. However, it can be shown that the morphology development of γ-phase i-PP can be widely influenced by very small amounts of α-phase i-PP. A lamella thickness of about 4 nm and a long period of 12 nm of γ-phase i-PP isothermally crystallized at 100 °C are obtained from SAXS measurements. At very low supercoolings, supermolecular structures of the neat γ-modification are formed in competition with mixed (α and γ) morphologies. In thin films the neat γ-modification shows network structures, which are formed by triangular entities. Etched bulk samples reveal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extended chain lamellae with periodic distances of rows of methyl groups of 3.6 Å is measured. This value is in excellent agreement with the distance of the methyl group rows on the crystallographic (010) plane according to the nonparallel chain packing model by Brückner and Meille.

show abstract

The γ form of crystalline, isotactic polypropylene

Cited by 32 publications

References 2 publications

The Oriented γ Form of Isotactic Polypropylene

The Oriented γ Form of Isotactic Polypropylene

Thermal analysis and X‐ray scattering study of metallocene isotactic polypropylene prepared by partial melting

On the γ-Phase of Isotactic Polypropylene

Contact Info

Product

Resources

About