Structure of Copolymers of Syndiotactic Polypropylene with Ethylene

Abstract

A structural characterization of copolymers of syndiotactic polypropylene containing ethylene comonomeric units (PPET) in a wide range of concentrations (0.4-60 mol %) is presented. The copolymer samples are crystalline up to an ethylene content of 18-20 mol %. Ethylene units are partially included in the crystals of both as-prepared and melt-crystallized samples. As-prepared PPET samples basically crystallize in conformationally disordered modifications of form II of syndiotactic polypropylene (sPP) containing kink bands. The amount of kink bands defects, represented mainly by T 6G2 sequences, increases with the ethylene content. These modifications are metastable and transform by crystallization from the melt into the most stable form I or form II, depending on the ethylene concentration. PPET samples having low ethylene content (up to 6-7 mol %) crystallize from the melt into the antichiral form I, even though disordered modifications of form I are always obtained. For higher ethylene contents, mixtures of crystals of forms I and II are obtained by crystallization from the melt. The fraction of form II increases with increasing the crystallization temperature and the ethylene content. The form II obtained by melt crystallizations does not present kink-band disorder, all chains being in the more stable (T 2G2)n helical conformation. This is the first example of crystallization of the isochiral form II of sPP from the melt at atmospheric pressure.

Propene‐Ethene Copolymers Synthesised with Cs‐Symmetric Metallocenes and Different Cocatalysts

“…Syndiotactic polypropene presents a complex polymorphic behaviour with four crystalline forms described so far. [8] The first, lower, melting peak values correspond to the melting of the primary and secondary crystals formed at the corresponding crystallisation temperature. The second, higher, melting peak represents the melting of the recrystallised crystals formed during a subsequent heating endotherm.…”

“…[6,7] In copolymers of syndiotactic propene with low amounts of ethene (up to 18 mol-%), the ethene units are partially included in the crystals of the P-E polymer samples. [8] The crystalline form is dependent on the amount of ethene in the copolymer. In addition to the ratio of crystalline and amorphous segments in P-E copolymers, the molecular weight of the copolymer is one of the main features defining the mechanical properties.…”

Nonisothermal crystallization and melting behavior of syndiotactic polypropylenes of different microstructure

“…In general, sPP has lower crystallinity and higher impact strength than isotactic polypropylene,3 and at least four limited‐ordered crystal structures or polymorphism 4, 5. The presence of either ordered or disordered polymorphism is highly dependent on the crystallization conditions and the stereoregularity of polypropylene 6–8…”

Effect of the polymer microstructure on the behavior of syndiotactic polypropylene/organophilic layered silicate composites

“…2). These changes in T m and T c were explained by the fact that the stereodefects acted as comonomers being rejected from the polymeric crystals,22, 34–36 at least partially 37. Flory, several decades ago, explained this phenomenon on the basis of thermodynamic reasons 38.…”

“…Form I is the most stable crystalline structure, and it is characterized by one orthorhombic unit cell with chains in the s(2/1)2 antichiral helical conformation, having either ordered or disordered structures. The metastable form II has a similar structure as form I but with an isochiral conformation, whereas forms III and IV present chains in the transplanar and (T 6 G 2 T 2 G 2 ) n conformations, respectively 22, 34…”