Form Of Isotactic Polypropylene Research Articles

Crystallization Behavior of Isotactic Propylene−Ethylene and Propylene−Butene Copolymers: Effect of ComonomersversusStereodefects on Crystallization Properties of Isotactic Polypropylene

Isotactic propylene−ethylene (iPPEt) and propylene−butene (iPPBu) copolymers have been prepared with different metallocene catalysts. The different influences of stereodefects (isolated rr triads), ethylene and butene comonomeric units on the crystallization of the α and γ forms of isotactic polypropylene (iPP) have been discriminated. Both iPPEt and iPPBu copolymers crystallize from the melt as mixtures of the α and γ forms. The amount of the γ form increases with increasing crystallization temperature, comonomer concentration, and content of rr stereodefects. In iPPBu copolymers, the amount of the γ form decreases for concentration of butene units higher than 10−14 mol % and is always lower than that crystallized in iPPEt copolymers. Butene units, therefore, favor crystallization of the γ and α forms at low and high concentrations, respectively. These data have indicated that the crystallization of the γ form of iPP is not only related to the value of the average length of the regular propylene sequences 〈L iPP〉, but is also related to the inclusion of stereodefects and constitutional defects in the crystals of iPP. Very different proportions of ethylene and butene units are included in crystals of the α and γ forms of iPP. Butene units are included indifferently in crystals of the α and γ forms, but probably more easily in the α form at high concentrations. Therefore, at low butene concentration, up to nearly 10 mol %, the effect of shortening of the length of regular isotactic propylene sequences prevails and induces crystallization of the γ form. For butene concentrations higher than 10 mol %, the effect of inclusion of butene units in crystals of the α form prevails, producing a decrease of the amount of the γ form and crystallization of the pure α form for butene contents higher than 30 mol %.

Crystal Structure of Isotactic Propylene−Hexene Copolymers: The Trigonal Form of Isotactic Polypropylene

A study of the structure of isotactic propylene−hexene random copolymers, prepared with single-center metallocene catalysts, is reported. For low concentrations of hexene comonomeric units, up to nearly 10 mol %, copolymers crystallize in the α form of isotactic polypropylene. Copolymers with hexene contents higher than 10 mol % crystallize in a new polymorphic form that melts at nearly 50 °C. The hexene units are partially included in both crystals of α form and of the new form. The crystallization of the new crystalline form allows incorporation of a high amount of hexene units. The melting temperature and the crystallinity decrease rapidly with increasing hexene content. A slower decrease of melting temperature and crystallinity is observed at high hexene concentrations because of the crystallization of the new form. The crystal structure of the new form has been studied by analysis of X-ray powder and fiber diffraction patterns of samples containing hexene concentration higher than 10 mol %. Chains in 3-fold helical conformation of propylene−hexene copolymers are packed in a trigonal unit cell according to the space group R3c or R3̄c. The values of a and b axes of the unit cell depend on hexene concentration, and values of a = b = 17.5 Å and c = 6.5 Å have been found for the sample with 26 mol % of hexene. The structure contains high degree of disorder due to the constitutional disorder of the random copolymer chains that produces disorder in the positioning of the lateral groups in the unit cell. Moreover, statistical disorder in the up−down positioning of the helical chains and slight disorder in the orientation of chains around the 3-fold axes are present. The inclusion of hexene units in the crystals induces a suitable increase of density that allows crystallization of 3-fold helical chains in the trigonal form, where the helical symmetry of the chains is maintained in the crystal lattice. The structure is similar to that of form I of isotactic polybutene. This form does not crystallize, and has never been observed so far for the polypropylene homopolymer because, in the absence of bulky side groups, it would have a too low density. This structure represents an example of entropy-driven phase formation.

The β‐crystalline form of isotactic polypropylene in blends of isotactic polypropylene and polyamide‐6/clay nanocomposites

AbstractBlends of isotactic polypropylene and polyamide‐6/clay nanocomposites (iPP/NPA6) were prepared with an internal batch mixer. A high content of the β‐crystalline form of isotactic polypropylene (β‐iPP) was observed in the injection‐molded samples of the iPP/NPA6 blends, whereas the content of β‐iPP in the iPP/PA6 blends and the iPP/clay composite was low and similar to that of neat iPP. Quiescent melt crystallization was studied by means of wide‐angle X‐ray diffraction, differential scanning calorimetry, and polarized optical microscopy. We found that the significant β‐iPP is not formed during quiescent melt crystallization regardless of whether the sample used was the iPP/NPA6 blend or an NPA6 fiber/iPP composite. Further characterization of the injection‐molded iPP/NPA6 revealed a shear‐induced skin–core distribution of β‐iPP and the formation of β‐iPP in the iPP/NPA6 blends is related to the shear flow field during cavity‐filling. In the presence of clay, the deformation ability of the NPA6 domain is decreased, as evidenced by rheological and morphological studies. It is reasonable that the enhanced relative shear, caused by low deformability of the NPA6 domain in the iPP matrix, is responsible for β‐iPP formation in the iPP/NPA6 blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3428–3438, 2004

Structure and Properties of Elastomeric Polypropylene from C2 and C2v-Symmetric Zirconocenes. The Origin of Crystallinity and Elastic Properties in Poorly Isotactic Polypropylene

A comparative analysis of the structure and mechanical properties of two elastomeric amorphous and semicrystalline polypropylenes samples, produced with C2v-symmetric and chiral C2-symmetric ansa-zirconocene catalysts, respectively, is presented. The poorly isotactic polypropylene (iam-PP) of medium molecular weight, prepared with the chiral C2-symmetric catalyst, slowly crystallizes, by aging at room temperature or by stretching, in a continuum of disordered modifications intermediate between α and γ forms of isotactic polypropylene. Structural and morphological transformations occur during stretching. Disordered α/γ modifications, containing a high fraction of perpendicular chains, present in the unoriented film, transform by stretching at high deformations into structures more similar to the α form with a high content of parallel chains. The development of the small level of crystallinity induces elastic properties. The small crystalline domains in the amorphous matrix, indeed, act as physical knots of the elastomeric lattice. This poorly isotactic iam-PP sample shows, instead, poor elastic properties and viscous flow at high deformations in the amorphous state, before crystallization, because of the not high molecular weight. The unique crystallization mode of the iam-PP sample explains the remarkable enhancement of elastic properties in low stereoregular polypropylene upon development of crystallinity. The fully amorphous high molecular weight atactic polypropylene (a-PP), prepared with the C2v-symmetric catalyst, presents elastic behavior only in a small deformation range (up to 300% strain) and experiences viscous flow of the chains at high deformations. The elasticity arises from the physical network generated by the high degree of entanglement of the polymeric chains generated by the high molecular weight of the sample. The poorly isotactic iam-PP sample, before crystallization, presents much poorer elastic properties than the a-PP sample because of the lower molecular weight. After crystallization, it shows better elastic properties in a large range of deformation with higher strength, notwithstanding the lower molecular weight, due to the presence of crystallinity.

Stereoblock Polypropylene from a Metallocene Catalyst with a Hapto-Flexible Naphthyl−Indenyl Ligand

Stereoblock polypropylenes have been obtained performing the polymerization of propene at different temperatures in the presence of [1-methyl-1-naphthylethyl-2-inden-1-yl]zirconium(IV) trichloride and methylaluminoxane. The stereoblock microstructure probably arises from to fact that the zirconium complex exists in equilibrium among structures having the naphthyl group coordinated to the metal, corresponding to chiral and/or pseudo-achiral forms, and a structure having the naphthyl group not coordinated, corresponding to a semimetallocene form. The chiral form produces regular isotactic sequences, whereas the pseudo-achiral and the semimetallocene forms produce atactic sequences, giving rise to isotactic−atactic stereoblocks. The length of the isotactic block depends on the polymerization temperature. A structural characterization and an analysis of the thermal behavior of these stereoblock polypropylenes have been performed. The samples crystallize from the melt in mixtures of α and γ forms of isotactic polypropylene. In the samples polymerized at low temperatures (17 and 30 °C), the amount of γ form is lower than that observed in samples prepared at higher temperatures (50 °C). This indicates that the regular isotactic sequences are longer in samples prepared at low temperatures, according with the interconversion mechanism of the catalyst among isospecific and aspecific forms. In these samples the amount of γ form is, in any case, lower than that observed in the literature for samples having higher stereoregularity and a random distribution of stereodefects, confirming the stereoblock microstructure of the samples prepared with the flexible catalyst.

Effects of mineral additives on the β‐crystalline form of isotactic polypropylene

AbstractNineteen kinds of minerals, alone and in bicomponent mixtures with LaC (a mixed ternary complex of trivalent lanthanum stearate and stearin), were added to isotactic polypropylene (iPP). The influences of the minerals on the crystallographic forms of iPP were investigated. A wide‐angle X‐ray diffraction examination demonstrated that no mineral or LaC acting alone could induce the occurrence of the hexagonal β‐form, whereas the bicomponent mixtures could when the mineral was a calcium compound or contained calcium compounds, whether the calcium compounds had a hexagonal crystallographic form or not. We surmise that the actual β‐iPP substrate in such a system might be some binuclear complexes of calcium and rare earth elements with some specific ligands. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1742–1748, 2002

The Oriented γ Form of Isotactic Polypropylene

A structural analysis of the oriented y 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 y form of iPP, are obtained; increasing the draw ratio, crystalline modifications, closer to the a 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 a form) or tilted 81° each other (like in the y 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 y 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 a 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 a forms could exist. The possible inclusion in the crystalline domains of stereo defects (mainly of the kind mrrm), which would favor the y -like packing situations on the local scale, is discussed.

The Flame Retardant for Polypropylene using Magnesium Hydroxide with Intumescent Components

In this work we present results of our study on flammability of polypropylene samples containing various fire retardants. Moreover, the influence of the flame retardants added to PP on the structure of polypropylene matrix was investigated. The effect of magnesium hydroxide on thermal properties of isotactic polypropylene was measured on a cone calorimeter. The measurements were performed in air at heat flux of 50kW/m2. Melamine polyphosphate, when added to MagShield-PP system, shows the highest fire-retarding effect compared to other intumescent compounds studied, such as pentaerythritol, urea polyphosphate and zinc borate. The weakest effect was recorded for zinc borate-containing composite. Results of the structural study allow to conclude that fire retardants based on magnesium compounds in practice do not change the crystalline structure of isotactic polypropylene. The intumescent fire retardants act as nucleants of hexagonal form of isotactic polypropylene.

Crystal structures of the α and β forms of isotactic polypropylene: a Monte Carlo simulation

Isotactic polypropylene has various crystalline modifications (α, β, γ, smectic). Molecular conformations in these modifications are common 3/1 helix of (TG)3 or (TG∗)3. However crystal structures of the modifications, that is the modes of chain packing, are quite different and are subjects of intensive investigations nowadays. We study here, the detailed modes of chain packing in the α and β forms by Monte Carlo simulation. We assume that the molecular conformation is a rigid 3/1 helix, and that the chain axes are placed either in a monoclinic lattice (α form) or in a hexagonal lattice (β form). Most stable modes of chain packing are investigated, with respect to chain chirality (right or left handedness), chain rotation and translation, and the methyl-group direction (up or down positioning). We find that an initial random state in the monoclinic lattice converges to the α1 or to the α2 form, both with alternating rows of right-handed and left-handed helices; we also investigate a molecular process of growth of the α2 crystal in the matrix α1 crystal. On the other hand, the arrangement of the molecular axes in the hexagonal lattice is found to give rise to the β form structure, in which the chains form chiral domains surrounded by boundaries parallel to the (110) and (100) planes. It is also shown that detailed chain rotations (setting angles) have a unique superstructure, which conforms to the incommensurate packing recently proposed by Lotz and Meille.

?- and ?-crystalline forms of isotactic polypropylene investigated by nanoindentation

Under special crystallization conditions from the melt, both α- and β-forms of isotactic polypropylene were produced simultaneously. The α- and β-spherulites of polypropylene were differentiated under optical microscope, allowing the nanoindentation of individual spherulites of each crystallographic form. Elastic modulus and hardness of β-spherulites were found to be 10 and 15% respectively lower than in α-spherulites. The higher stiffness of α may be related to the particular interlocked structure with cross-hatched lamellae, and to a lower molecular mobility in the crystallites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 195–200, 1999

On row structures, secondary nucleation and continuity in α-polypropylene

The morphology of row structures in the α form of isotactic polypropylene has been investigated, with lamellar resolution, in a homopolymer and three of its fractions, by transmission electron microscopy following permanganic etching. In longitudinal view, dominant radial lamellae were not always perpendicular to the strain-induced central nucleus with which they shared a common c axis; lamellae parallel to the core increased in number at greater radial distances. The underlying reason is related to novel phenomena observed in transverse sections showing that lattice continuity is not maintained if its loss allows crystallization to occur in otherwise inaccessible regions. If there were lattice continuity, the lathlike habit of α-polypropylene lamellae would lead to gaps developing between neighbouring laths with increasing radial distance. In fact, such gaps are generally filled, frequently by lamellae which are edge-on, i.e. parallel to the rows, and rapidly, implying that a loss of continuity is not a significant barrier to growth. Space-filling nucleation increased as the crystallization temperature decreased, leading to finer structures at lower temperatures, but was severely inhibited by a large drop in tacticity in which case the gaps remained largely unfilled. Different mechanisms to effect this infilling have been identified with and without loss of continuity but principally incoherent nucleation; the implications for the properties of melt crystallized polymers in general are discussed.

On variable nucleation geometry and segregation in isotactic polypropylene

Lamellar thicknesses, cross-hatching frequencies and growth rates have been compared for spherulites, transcrystalline layers and row structures formed from the melt in the α (monoclinic) form of isotactic polypropylene. These three morphologies relate to point, surface and linear nucleation, respectively. The measured quantities are independent of the type of nucleation confirming that morphological statistics compiled from row structures are relevant to general condition of melt crystallization. This independence also implies that the same supercooling must be effective for growth in the three cases notwithstanding the very different geometries of their crystal/melt interfaces and the consequent locations and concentrations of any fractionated molecular species. With no detectable depression of supercooling at the growth faces the conditions for cellulation to develop do not exist. Segregation of ‘impurities’ can thus have no influence on the establishment of physical texture in this high crystallinity polymer.

Simultaneous in-situ WAXS/SAXS and d.s.c. study of the recrystallization and melting behaviour of the α and β form of iPP

The behaviour of the α and β form of isotactic polypropylene (iPP) was studied during heating. The β form is usually found in iPP specimens that have been subjected to mechanical deformation, e.g. in injection moulded and extruded products. A sample containing a large fraction of the β form was obtained by doping with small amounts of quinacridone. To enable the study of recrystallization and melting of α and β iPP, simultaneous wide-angle X-ray scattering and small-angle X-ray scattering were used. Differential scanning calorimetry was used to facilitate correlations with previous work. Using this combination of techniques, several partial processes could be distinguished. Melting behaviour depended on the formation history. In samples formed at high cooling rates, best comparable to processing conditions, the melting of β crystals is followed by an absolute increase in α crystallinity. In the slowly cooled samples, α and β crystal lamellae melt independently on heating. Also the recrystallization of α crystals, resulting in lamellar thickening, was detected.

Temperature dependence of the thermodynamic stability of the two crystalline α forms of isotactic polypropylene

Journal of Polymer Science Part B: Polymer PhysicsVolume 28, Issue 2 p. 139-147 Article Temperature dependence of the thermodynamic stability of the two crystalline α forms of isotactic polypropylene Roberto Napolitano, Roberto Napolitano Istituto di Chimica, Università della Basilicata, via Nazario Sauro 85, 85100 Potenza, ItalySearch for more papers by this authorBeniamino Pirozzi, Corresponding Author Beniamino Pirozzi Dipartimento di Chimica, Università di Napoli, via Mezzocannone 4, 80134 Napoli, ItalyDipartimento di Chimica, Università di Napoli, via Mezzocannone 4, 80134 Napoli, ItalySearch for more papers by this authorVincenzo Varriale, Vincenzo Varriale Dipartimento di Chimica, Università di Napoli, via Mezzocannone 4, 80134 Napoli, ItalySearch for more papers by this author Roberto Napolitano, Roberto Napolitano Istituto di Chimica, Università della Basilicata, via Nazario Sauro 85, 85100 Potenza, ItalySearch for more papers by this authorBeniamino Pirozzi, Corresponding Author Beniamino Pirozzi Dipartimento di Chimica, Università di Napoli, via Mezzocannone 4, 80134 Napoli, ItalyDipartimento di Chimica, Università di Napoli, via Mezzocannone 4, 80134 Napoli, ItalySearch for more papers by this authorVincenzo Varriale, Vincenzo Varriale Dipartimento di Chimica, Università di Napoli, via Mezzocannone 4, 80134 Napoli, ItalySearch for more papers by this author First published: 30 January 1990 https://doi.org/10.1002/polb.1990.090280203Citations: 34AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume28, Issue230 January 1990Pages 139-147 RelatedInformation

The molecular origin of lamellar branching in the α (monoclinic) form of isotactic polypropylene

AbstractThe specific lamellar branching characteristic of isotactic polypropylene (iPP) in its α monoclinic modification, first investigated in detail by Khoury, is analyzed in molecular terms on the basis of the geometrical and structural models of Padden and Keith and of Binsbergen and de Lange, respectively. As suggested by these authors the branching corresponds, structurally, to an homoepitaxy of the daughter lamellae on the lateral (010) faces of the parent lamellae; the epitaxy is favored by a satisfactory interdigitation of the molecular subgroups (methyl side chains) of facing planes and by the near identity of a and c parameters of the α monoclinic unit cell of iPP. On a molecular basis, the branching is initiated on a lateral (010) face made up of chains of a given hand by the deposition of chains of the same hand, whereas the crystal structure of the α modification would call for chains of opposite hand. Favorable interactions of side chains of helices of the same hand is realized only when the helix axes are at a substantial angle (in the present case, 100°) to one another. Related behavior is observed for the packing of α helices in the structure of globular proteins, in which molecular interactions, but no crystallographic interactions, are involved. On the basis of this molecular picture, the widespread occurrence of lamellar branching in the crystallization of the α form of isotactic polypropylene is accounted for, especially for high growth rates. At the same time, the specificity of the branching (only observed for the α crystal modification of this polymer) is explained by stringent geometrical and structural requirements that must be fulfilled at the contact plane.

X‐ray analysis on unoriented and oriented samples of the quenched form of isotactic polypropylene

X‐ray analysis on unoriented and oriented samples of the quenched form of isotactic polypropylene

Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient

AbstractSamples of isotactic polypropylene (PP) were zone‐solidified in temperature gradients up to 300°C/cm at growth rates down to 3 μm/min. Oriented α‐type spherulites were obtained only by nucleation. While β nucleation is extremely rare, the β phase is easily initiated by growth transformations along the oriented α front. Since the β phase was found to grow considerably faster than the α phase, the α‐to‐β transformation points diverge across the sample, interrupting growth of the oriented α fibrils. This causes subsequent nucleation to yield teardrop‐shaped α spherulites.Differential scanning calorimetry (DSC) studies of zone‐solidified PP show the β‐phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt—all of which tend to suppress nucleation. Differential thermograms of largely β‐PP obtained at a heating rate of 1°C/min show the actual melting and recrystallization of the β spherulites into the α form.