Structural Evolution Mechanism of Crystalline Polymers in the Isothermal Melt-Crystallization Process: A Proposition Based on Simultaneous WAXD/SAXS/FTIR Measurements

Time-resolved simultaneous measurements of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) (and FTIR spectra) were performed for various kinds of crystalline polymers in isothermal melt-crystallization processes, from which the common features of the structural evolution process as well as the different behaviors intrinsic to the individual polymer species were extracted. The polymers targeted here were polyethylene, isotactic polypropylene, polyoxymethylene, aliphatic nylon, vinylidene fluoride copolymer, trans-polyisoprene, and poly(alkylene terephthalate). A universal concept of the microscopically viewed structural evolution process in isothermal crystallization may be described as follows: (i) the small domains composed of locally regular but more or less disordered helical chain segments are created in the melt (this important information was obtained by the IR spectral data analysis); (ii) these domains grow larger as the length and number of more regular helical segments increase with time; (iii) the correlation among the domains becomes stronger and they approach each other; and (iv) they merge into the stacked lamellar structure consisting of the regularly arranged crystalline lattices. The inner structure of the domains is different depending on the polymer species, as known from the IR spectral data.