Syndiotactic polypropylene (sPP) was melt-quenched in an ice-water bath and directly drawn at 0 °C up to a draw ratio of 6. The oriented sample was kept in the cold bath for 10 days under tension, and then analyzed at room temperature either fixed or after releasing tension. The fully extended fiber showed by X-ray diffraction essentially the trans-planar crystalline form III. Upon relaxation, the sample transformed in high proportion to the trans-planar mesomorphic form, free of reflections of helical crystal forms, and became elastic. In this process, there is a reduction of both crystallinity and crystallite orientation. Since the crystalline and mesomorphic chain conformation is trans in both the stretched and relaxed fiber, the elastic behavior in the present sample does not involve a conformational transition in the crystalline domains. At variance, in the infrared spectrum of the relaxed but not of the stretched sample, one of the bands diagnostic of the helical conformation appears. Thus, when the fiber is unhooked, chains tend to disorder reversibly and some segments adopt the helical conformation, even if they must reside in the amorphous component connecting the more ordered mesomorphic domains. Reversible crystallization of the mesophase and of the partially oriented, intercrystalline, amorphous chains into highly oriented metastable trans-planar form III appears to be responsible for the elastic behavior of sPP fibers in the present study. Models of the structural organization in the trans-planar mesophase and the implications of our results with respect to the general mechanism leading to the elastic behavior of sPP are discussed.

Two syndiotactic polypropylene (sPP) samples were obtained by quenching the melt in a bath at 0 °C, for 1 min (sample 1A) and 3 days (sample 1B). In these conditions sample 1A crystallized in the disordered form I, having the chains in helical conformation, whereas sample 1B formed a mesophase with the chains in trans-planar conformation, as evident by X-rays and FTIR analysis. Samples 1A and 1B were drawn at room temperature, obtaining fibers with a draw ratio between 4 and 7, which were analyzed under tension. The fibers, with a draw ratio 4 and 5, obtained from sample 1A, show a well oriented helical form and a fraction of oriented trans-planar form. When the draw ratio reaches the value of 6 and 7, the crystalline form III with chains in trans-planar conformation is obtained. At variance, the fibers obtained from sample 1B show a progressive orientation of the trans-planar mesophase, and the appearance of form III at draw ratio of 6 and 7. Therefore, the fibers drawn at λ = 6 and 7, coming from the two samples 1A and 1B, are very similar, as shown by X-rays and FTIR analysis. Despite this similarity, upon releasing the tension, they undergo different transformations, going toward the oriented helical form or the oriented trans-planar mesophase, respectively.

Thermal transitions of the three crystalline phases (γ, δ, and ε) of syndiotactic polystyrene (s-PS), presenting s(2/1)2 helices, have been compared by X-ray diffraction, differential scanning calorimetry (DSC), and dynamic-mechanical analyses. These analyses have been conducted on crystalline (δ and ε) films, obtained by similar solvent sorption and desorption procedures, starting from a same γ-form film. The Fourier transform Infrared (FTIR) spectra of the three films have also been compared. The obtained results indicate that the recently discovered ε-phase, as the already known δ-phase, is transformed in γ-phase by heating above 100 °C. However, the ε → γ transition occurs directly without the formation, for intermediate temperatures, of a helical mesomorphic phase, as instead observed for the δ → γ transition. DSC studies and FTIR measurements also suggest that the crystalline packing of the ε-form could be rather similar to that one of the γ-form.

Two syndiotactic polypropylene (sPP) samples were obtained by quenching the melt in a bath at 0 °C for 1 min (sample 1A, crystallized in the helical form I), and for 3 days (sample 1B, in the trans-planar mesophase). Samples 1A and 1B were drawn at room temperature obtaining fibers with draw ratios λ = 5, 6, and 7. The fibers were analyzed under stress by X-rays and FTIR, successively unhooked, and again analyzed. Different structural organizations were found in the fibers fixed or relaxed. In sample 1B, the drawing produced a progressive orientation of the trans-planar mesophase ending with the formation of the crystalline trans-planar form III. On releasing the tension, form III transformed again into the trans-planar mesophase, but very weak helical reflections appeared in the X-ray patterns. Infrared spectra confirmed that one of the helical bands increases on releasing the tension. In sample 1A, the helical form I was first oriented and then, by progressive drawing, transformed into the crystalline trans-planar form III. Releasing the tension, form III again converted into the helical form I and partially into the trans-planar mesophase. Despite the different structural organizations, all the relaxed fibers showed a good elasticity and the same qualitative behavior. It is characterized for all the fibers by a low hysteresis and a small permanent set after a 50% deformation. However, the elastic fibers show a much higher modulus than the conventional elastomers.

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