Elastic Properties and Polymorphic Behavior of Fibers of Syndiotactic Polypropylene at Different Temperatures

Abstract

An analysis of the elastic properties of syndiotactic polypropylene at different temperatures is reported. Fibers stretched at low temperatures are in the trans-planar form III, which transforms into the helical form II upon the release of the tension, whereas fibers stretched at high temperatures are in the stable helical form I. The mechanical analysis of these fibers has shown that only at room temperatures the specimens show good elastic properties. The elastic recovery of the samples is in part lost also at room temperature in annealed fibers, which are in the stable form I, in both the stretched and unstrained states. When the structural transition from the trans-planar form III into the helical form occurs upon removing the tension, a nearly total elastic recovery is observed. Only a partial elastic recovery is instead observed when this transition is absent. The relative amount of crystalline phase in unoriented films and in stretched and stress-relaxed fibers is rather constant (≈40−43%) and does not depend on the stretching temperature in the range 25−80 °C. These data indicate that the elastic behavior of syndiotactic polypropylene is in part linked to the enthalpy gain achieved when the tension is removed, due to the metastability, in the unstrained state, of the trans-planar form, which transforms into the stable helical form. On the other hand, also an entropic factor must play a role, due to the entangled amorphous chains which experience a reversible conformational transition between the disordered (coil) and extended conformations during the stretching and relaxing process.

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

“…Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior (the first part of this section on s-PP modifications is reported in the publication [203][204][205][206][207][208][209][210][211][212]). s-PP chains crystallize to various polymorphic crystalline phases and, depending on the degree of stereoregularity and the mechanical and thermal history, different degrees of statistical disorder are present in the crystal packing.…”

“…Form III is obtained by stretching at room temperature the compression-molded samples, as found by Natta et al It is characterized by chains in zigzag trans-planar conformation. Its crystal structure has been reported recently [203][204][205][206][207][208][209][210][211][212] and belongs to the space group P2 1cn . In this form, the trans-planar chains are packed in orthorhombic unit cells with axes a ¼ 5. , Cu Kα).…”

“…Form I, the thermodynamically stable form of s-PP is obtained under the most common conditions of crystallization, from the melt and/or precipitation from solution, in powder samples and single-crystals of s-PP. Form II is a metastable polymorph of s-PP and has been obtained only in oriented fibers of s-PP and, recently, by melt-crystallization at elevated pressure [203][204][205][206][207][208][209][210][211][212]. It can be also obtained by room temperature stretching of compression-molded specimens of low stereoregular s-PP samples prepared with vanadium-based Ziegler-Natta catalysts [10][11][12][13][14][15][16][17] or upon releasing the tension in stretched fibers of highly stereoregular s-PP samples initially in form III.…”

Diffusion and morphological properties of syndiotactic polypropylene (sPP) films

“…The improved sPP has high crystallinity and melting temperature and has outstanding physical properties. The important physical properties are high mechanical strength and high toughness in spite of the high crystallinity and relatively high glass transition temperature [4][5][6][7].…”

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

“…The importance lies on its excellent toughness and elastic properties. Previous investigations [1][2][3][4][5][6][7] indicated that sPP presents a complex polymorphic behavior, and four crystalline forms have been found so far. The most stable form I and the metastable form II are characterized by chains in s (2/1)2 helical (t 2 g 2 ) n conformation, packed in orthorhombic unit cells.…”