From the melt via mesomorphic and granular crystalline layers to lamellar crystallites: A major route followed in polymer crystallization?

Abstract

Based on broad and detailed evidence from a large variety of experiments on several polymer systems carried out by other authors and ourselves, a novel concept for understanding the crystallization of polymers from the melt is developed. The experiments generally indicate that the formation and growth of the lamellar crystallites is a multi-step process passing over intermediate states. We suggest a specific route which is compatible with the observations. It is proposed that the initial step is always the creation of a mesomorphic layer which spontaneously thickens, up to a critical value, where it solidifies through a cooperative structural transition. The transition produces a granular crystalline layer, which transforms in the last step into homogeneous lamellar crystallites. The model leads to predictions about the temperature dependencies of the crystal thickness and the growth rate which are at variance with conventional views but in agreement with findings in recent experiments.

A two-dimensional IR correlation spectroscopic study of the conformational changes in syndiotactic polypropylene during crystallization

“…This suggests that, before the close packing of the 4 1 helical chains of sPP, the local molecular conformation is already changed and arranged into some ordered short helical sequences. Strobl and colleagues [4][5][6]41 proposed a four-state scheme for treating polymer crystallization and suggested that the primary crystallization from the melt produces an imperfect 'native' crystal in the first step, which is subsequently stabilized by structural relaxation processes. However, direct evidence for the occurrence of a mesophase or 'native' crystal between the equilibrated melt and ordered crystallites is absent during the isothermal crystallization of sPP.…”

“…[1][2][3][4][5][6][7][8][9][10][11] Several concepts for polymer crystallization have been described in the last century, of which the classical nucleation and growth theory (Hoffman-Lauritzen theory), [1][2][3] the mesomorphic phase concept [4][5][6] and the spinodal concept [7][8][9][10][11] are the most important. In recent decades, structural formation during the induction period before crystal growth has attracted increasing interest.…”

A Review on Polymer Crystallization Theories

“…The concept of the hexagonal phase that is similar to the liquid crystalline phase with high mobility was subsequently generalized by Strobl [119], who argued that it was a common intermediate state during polymer crystallization. This is the so-called "multi-stage model," and we will discuss it in the following section.…”

“…In addition, in order to explain the difference between the re-crystallization line and the melting line observed in experiments, Strobl [119,[178][179][180] also proposed the concept of the "mesomorphic phase." He argued that there was a mesomorphic inner structure between the crystalline and amorphous phase, which was stabilized by epitaxial force.…”

Combined time-resolved SAXS and X-ray Spectroscopy methods

“…A fine example of this is the debate that ranged for some time early this century in the polymer crystallisation literature. Here the issue was that depending on the simultaneous or time-staggered occurrence of a scattering signal in the SAXS data or the earliest moment of detection of diffraction peaks in the WAXS regime, in a sample that was quenched below the crystallisation temperature, one would be able to distinguish between a crystallisation model that could be described by a spinodal decomposition like mechanism or by a more accepted nucleation and growth mechanism (11)(12)(13)(14). Obviously it is much more difficult to prove that something is not there then to prove that a diffraction peak is present.…”