Conformational variation and crystalline phase transformation of low syndiotactic polypropylene films in stretched and stress‐relaxed states

Abstract

Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 °C (sample S0) and 90 °C (sample S90) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction were used to characterize the structure transformation and orientation behavior of samples S0 and S90 at both stretched and stress‐relaxed states. It was found that stretching (λ = 0–700%) induces the transformation of the chain conformation from helical to trans‐planar form for both S0 and S90 films. The stretched S0 and S90 samples show well oriented trans‐planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched S0 sample, form III with trans‐planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S90, however, stretching might induce the formation of both the form III crystals and mesophase with trans‐planar chains; releasing the tension, form III again gets converted into trans‐planar mesophase and form I crystals. In the stretched and stress‐relaxed states of samples S0 and S90, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat‐treating methods of the low syndiotacticity sPP. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2924–2936, 2005

Crystallization, Recrystallization, and Melting Lines in Syndiotactic Polypropylene Crystallized from Quiescent Melt and Semicrystalline State Due to Stress-Induced Localized Melting and Recrystallization

“…This is assigned to the fact that all thermodynamic parameters included in eq that govern the crystalline lamellar thickness are related mostly to the mesomorphic and crystalline phase only independent of the state of amorphous phase. Hints of existence of mesomorphic phase in sPP during stretching have been reported extensively. ,, The observed slight increase in the limiting temperature for stretching induced crystallization in sPP can be understood as a result of the influence of global orientation of polymer chains on the thermal dynamic parameters of the mesomorphic phase.…”

“…Hints of existence of mesomorphic phase in sPP during stretching have been reported extensively. 21,24,25 The observed slight increase in the limiting temperature for stretching induced crystallization in sPP can be understood as a result of the influence of global orientation of polymer chains on the thermal dynamic parameters of the mesomorphic phase.…”

Stress induced reversible crystal transition in polymers

“…An in situ X‐ray diffraction study showed that this transition was a fast and direct process, occurring nearly instantaneously as the material was strained . Rheo‐FTIR studies also confirmed the transition …”

Conformation Orientation of Syndiotactic Polypropylene Induced by Deformation at Different Temperatures

“…The initial structures of S 0 and S 90 films have been described in detail in our previous paper . In brief, the initial structure of sample S 0 consists of form I crystals with a helical conformation and trans-planar mesophase, while sample S 90 allows the prevalent crystallization of form I.…”

“…The results showed that the helical conformation does not orient appreciably at higher temperature, though it is present beyond the yield point, while the trans-planar chains show a significant increase in dichroism. The orientation of different structures is related to the content of structure during stretching, which is controversial compared with other results. , Therefore, this research is not enough to provide comprehensive understanding of the orientation behavior of different structures in sPP and their dependence on the initial morphology and the stretching temperature.…”