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

Abstract

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.

Crystallization of Long-Spaced Precision Polyacetals III: Polymorphism and Crystallization Kinetics of Even Polyacetals Spaced by 6 to 26 Methylenes

“…Prior works demonstrated that crystallites of polyacetals are layered, in other words, two or more polyacetal repeating units participate in the crystallite with inter-chain staggering of the acetal groups (-O–CH 2 –O-) in planes that are about normal to the chain axis [ 8 ]. The number of layers in the crystal increases with increasing acetal content in the chain [ 8 , 9 ]. Since the melting point and heat of fusion decrease also proportionally to the increasing content of acetal, the trends of Figure 1 c,d reflect the role of the acetal group as a defect in the crystal, restricting crystallization.…”

“…Corrected SAXS intensities ( I ( q )) were used to obtain the normalized one-dimensional correlation function of the electron density fluctuations normal to the lamellar stacks according to [ 16 , 17 ], where q is the scattering angle, and r the correlation length. The long period ( L ) and crystal thickness ( l c ) were obtained as described in prior works [ 8 , 18 ].…”

“…In two preceding works [ 8 , 9 ], we studied the polymorphic transformations on heating and the isothermal crystallization kinetics of a set of polyacetals with the acetal unit precisely spaced by 12, 18, 19, or 23 backbone carbons. In the first work of the series [ 8 ], we demonstrated that rapidly crystallized systems with acetal groups spaced by an even (12, 18) or odd (19, 23) number of methylenes exhibit multiple melt-recrystallization events associated with polymorphic transformations on heating. These polyacetals also exhibit differences in melting points and a trend of the X-ray patterns with increasing temperature that differs between odd and even-spaced polyacetals.…”

“…N -alkanes experience an analogous even-odd effect due to a different setting of the methyl end group at the surface of the extended crystallites [ 10 , 11 ]. In precision polyacetals, packing of the crystalline acetal layers in even or odd spacers confers the same effect as does the methyl end-group in n -alkanes [ 3 , 8 ]. As discussed in our prior work, for CH 2 spacing > 12, the odd-even effect on melting disappears as van der Waals interactions of CH 2 sequences in the crystal become more prevalent to the packing structure than the interactions between acetal groups [ 8 ].…”

Polyethylene-like materials from plant oils

“…Polyacetals deviate from this picture in that accommodation in the crystals is hindered due to the conformations of acetal groups not being compatible with an all-trans arranged methylene chain. [28,30] [22]. Published by the Royal Society.…”