ma9b00922_si_002.avi (10.83 MB)
Crystallization of Long-Spaced Precision Polyacetals I: Melting and Recrystallization of Rapidly Formed Crystallites
media
posted on 2019-06-19, 14:04 authored by Xiaoshi Zhang, Xiaobing Zuo, Patrick Ortmann, Stefan Mecking, Rufina G. AlamoRapidly 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.
History
Usage metrics
Categories
Keywords
even-spaced polyacetalsPA -1223 methylene backbone carbons exhibitForm II crystalscore crystal thickness increasesmesomorphic-like structurescanning calorimetry endothermspolymorphic transitionsForm II crystallitesReal-time wide-angle X-ray diffractionCH 2 spacerLong-Spaced Precision Polyacetalsmethylene sequencePA -18.Crystallites Rapidly melt-crystallized polyethylenesForm IIcrystal thicknessheating ratesodd-spaced polyacetalsstep increaseWAXD patterns
Licence
Exports
RefWorks
BibTeX
Ref. manager
Endnote
DataCite
NLM
DC