Mechanical Properties and Morphology of Propene–Pentene Isotactic Copolymers (original) (raw)
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Macromolecules, 2021
The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4−5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.
Mechanical Properties and Elastic Behavior of Syndiotactic Propene− Butene Copolymers
2009
The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4−5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.
Macromolecules, 2017
A study of the nucleating effects of isotactic poly(trimethylallylsilane) (iPTMAS) and poly(vinylcyclohexane) (iPVCH) for isotactic polypropylene (iPP) and their influence on the mechanical behavior of iPP is reported. The particles of catalyst are covered by poly(trimethylallylsilane) or poly(vinylcyclohexane) by polymerization of the corresponding monomers before polymerization of propylene. The method guarantees that the nucleating agents are perfectly dispersed into the iPP samples, with their consequent high efficiency. Isotactic poly(vinylcyclohexane) and poly(trimethylallylsilane) favor crystallization from the melt of α and β forms, respectively. Both nucleating agents affect the morphology greatly reducing the size of shperulites. This in turn affects the mechanical properties improving ductility and flexibility. All nucleated samples crystallize by rapid cooling from the melt in the α form and not in the mesophase, as occurs in the pure iPP. The resulting α form presents nodular crystals and behaves as a stiff material with high deformability.
Polymer, 2006
Syndiotactic propylene-ethylene copolymers have been synthesized with a single-center C s -symmetric syndiospecific metallocene catalyst. A study of the effect of the presence of ethylene comonomeric units on the polymorphic behavior of syndiotactic polypropylene (sPP) and on the structural transitions occurring during stretching is reported. For copolymer samples with low ethylene contents, in the range 2-7 mol%, crystals of the helical form I, present in the melt-crystallized samples, transform into the trans-planar form III by stretching at high deformation. Form III transforms in part into the helical form II by releasing the tension, as it occurs for sPP. Samples with ethylene contents in the range 8-10 mol% are crystallized from the melt as a mixture of crystals of helical form I and form II. Both helical forms transform by stretching at low values of deformation (lower than 300%) into the trans-planar mesomorphic form, which transforms into the trans-planar form III by further stretching at higher deformations (higher than 500%). For these samples form III transforms back into the mesomorphic form, rather than into the helical forms, by releasing the tension. Unoriented samples of copolymers with ethylene content in the range 13-18 mol% are mainly crystallized in the helical form II, which transforms into the trans-planar mesomorphic form by stretching. Upon releasing the tension the mesomorphic form remains stable and no polymorphic transition is observed. The presence of ethylene comonomeric units stabilizes the trans-planar forms in fibers of the copolymer samples. This has been confirmed by the result that for high ethylene contents the trans-planar form III and the mesomorphic form do not transform in helical forms by annealing of fibers stretched at high deformations. q
A New Mesophase of Isotactic Polypropylene in Copolymers of Propylene with Long Branched Comonomers
Macromolecules, 2010
Isotactic propylene-1-octene (iPPC8) and propylene-1-octadecene (iPPC18) copolymers have been synthesized with a highly isospecific C 2-symmetric metallocene catalyst. iPPC8 copolymers crystallize in the R form of iPP for octene concentration up to 12-13 mol %, whereas iPPC18 copolymers crystallize in the R form for very low octadecene concentration and in a new mesomorphic form for octadecene contents higher than 2-3 mol %. In both copolymers crystals of R form transform into the new mesophase by stretching at high deformations. This mesophase is different from the quenched mesomorphic form of the iPP homopolymer. In fact, the diffraction patterns of the new mesophase of iPPC18 copolymers present a strong equatorial reflection at 2θ = 20°, which is absent in the diffraction pattern of the mesophase of iPP. The new mesophase is characterized by parallel chains in 3/1 helical conformation packed at average interchain distances of about 6 Å , defined by the self-organization of the flexible side groups, and high degree of disorder in the lateral packing of the chains.
Polymer, 2005
A set of copolymers of propene and alpha olefins (1-hexene, 1-octene and 1-octadecene) and the corresponding homopolymer (sPP) have been synthesized using a syndiotactic metallocene catalyst. The effect of either the incorporation or length of these conomomeric units on the structure and final properties exhibited has been analyzed. As expected, there is a considerable decrease in crystallinity with the increase of comonomer content. Thus, a completely amorphous copolymer is obtained if the molar fraction is high enough. The structural variations drastically influence the viscoelastic and mechanical behaviors of these copolymers. Three or four relaxation processes can be observed depending on composition and length of comonomer. At temperatures above the process associated with the glass transition (b relaxation), a deep drop (in one or two steps) in E 0 and a shoulder in E 00 , overlapped with that b mechanism, are observed. The existence of a relaxation involving crystalline regions is postulated because of its variation with crystal characteristics. Moreover, a relaxation related to internal motions within the comonomeric units is seen in the range of very low temperatures. This process, primarily ascribed to movement of methylenic segments within the comonomer, is strongly depending on composition and length of the pendant aliphatic chains on incorporated units. On the other hand, a decrease in stiffness and microhardness as well as the brittle-ductile transition are observed by simply varying composition when deformation takes place at room temperature.
European Polymer Journal, 2007
A new metallocene catalyst applied to propene polymerization provides an expansion of the properties of polypropylene (PP) by generating semi-crystalline materials having 0-60% crystallinity. The purpose of the present study was to investigate such low crystalline elastomeric polypropylene (ePP) samples in order to compile further information on their crystal structure and the related phase transitions in the temperature range of 298-423 K.
Poly(propylene-co-1-pentene-co-1-heptene) terpolymers: Mechanical and rheological behavior
Polymer, 2018
This work describes the important effects of microstructural features on crystalline structure, mechanical behavior and molten state response for poly(propylene-co-1pentene-co-1-heptene) terpolymers. The study was carried out in a wide range of compositions, varying the 1-pentene/1-heptene ratio and the thermal history applied. Molecular features, as propylene sequences and compositional triads, allow predicting the material behavior. Moreover, rheological response has demonstrated the influence of molecular weight, comonomer content and short-chain branches on the properties. The flow activation energy can justify the importance of the material density over the whole spectrum of properties.