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Crystallization of Long-Spaced Precision Polyacetals I: Melting and Recrystallization of Rapidly Formed Crystallites

Zhang, Xiaoshi, Zuo, Xiaobing, Ortmann, Patrick, Mecking, Stefan, Alamo, Rufina G.
Macromolecules 2019 v.52 no.13 pp. 4934-4948
cooling, crystal structure, crystallites, crystallization, differential scanning calorimetry, endothermy, heat, melting, polyethylenes, temperature, wide-angle X-ray scattering
Rapidly melt-crystallized polyethylenes with acetal groups (−O–CH₂–O−) precisely spaced by 12, 18, 19, or 23 methylene backbone carbons exhibit two reorganizations and three melting endotherms on heating at relatively low heating rates. Real-time wide-angle X-ray diffraction (WAXD) experiments further confirm that these transitions are associated with a reorganization to a different crystalline structure. The differential scanning calorimetry endotherms are associated with melting of the initially formed disordered (or hexagonal) phases, followed by melting of Form I and by melting of Form II crystals with increasing temperature. In parallel with these structural changes, the long spacing undergoes a discrete step increase at the polymorphic transitions. Upon the transition from disordered to Form I crystals, the core crystal thickness increases by one repeating unit, whereas the crystal thickness remains basically unchanged during the transition from Form I to Form II crystallites. The effect of even vs odd CH₂ spacer on the observed melting temperatures is prevalent for spacer <10 CH₂ units. The even or odd number of methylenes between acetals also affects the type and kinetics of packing assembly in layered crystallites. Although a disordered mesomorphic-like structure develops under fast cooling in odd-spaced polyacetals (PA-19, PA-23), the formation of Form I crystals cannot be bypassed in PA-12 and PA-18. Furthermore, some differences are also found in the WAXD patterns of Form II between odd- and even-spaced polyacetals, denoting that staggering of the acetals in the crystallites is affected by the configuration of consecutive acetals with respect to the methylene sequence.