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

Abstract

In this paper we extend the study of polymorphism and crystallization kinetics of aliphatic polyacetals to include shorter (PA-6) and longer (PA-26) methylene lengths in a series of even long-spaced systems. On a deep quenching to 0 °C, the longest even polyacetals, PA-18 and PA-26, develop mesomorphic-like disordered structures which, on heating, transform progressively to hexagonal, Form I, and Form II crystallites. Shorter polyacetals, such as PA-6 and PA-12 cannot bypass the formation of Form I. In these systems a mixture of this form and disordered structures develops even under fast deep quenching. A prediction from melting points that Form II will not develop in polyacetals with eight or fewer methylene groups between consecutive acetals was further corroborated with data for PA-6. The temperature coefficient of the overall crystallization rate of the two highest temperature polymorphs, Form I and Form II, was analyzed from the differential scanning calorimetry (DSC) peak crystallization times. The crystallization rate of Form II shows a deep inversion at temperatures approaching the polymorphic transition region from above. The new data on PA-26 confirm that at the minimum rate the heat of fusion is so low that crystallization becomes basically extinguished. The rate inversion and dramatic drop in the heat of fusion irrespective of crystallization time are associated with a competition in nucleation between Forms I and II. The latter is due to large differences in nucleation barriers between these two phases. As PA-6 does not develop Form II, the rate data of this polyacetal display a continuous temperature gradient. The data of the extended polyacetal series demonstrate the important role of methylene sequence length on polymorphism and crystallization kinetics.

Unlayered–Layered Crystal Transition in Recyclable Long-Spaced Aliphatic Polyesters

“…Often, different rates of cooling result in different polymorphic structures, but a change from a layered to an unlayered crystalline structure with the increased cooling rate is unprecedented. For instance, it is well known that highly isotactic polypropylene (iPP) forms a mesomorphic structure on rapid cooling from the melt and a monoclinic phase under slower crystallizations. , Similarly, mesomorphic forms were found recently in fast quenched long-spaced polyacetals. , Other common polymers, such as syndiotactic polystyrene, polyamide 66, and polyamide 6, also form different polymorphs depending on the rate of cooling. In contrast, for a series of polyhydroxyalkanoates, the lamellar thickness and spherulitic morphologies were highly dependent on the cooling rate, but the crystal structure remained unchanged …”

“…The presence of ester layer reflections after relatively slow crystallization conditions is expected since layered crystallites have been found previously in other aliphatic polyesters, ,,− and furthermore in most precision polyethylenes. ,,,,− However, the reduction in ester layer periodicity after rapid quenching for PE-32,32, PE-19,19, and PE-18,18, and especially the complete disappearance of the layer reflection in PE-48,48 is quite remarkable, since all other reported fast quenched systems have been found to be layered. Even the recently studied long-spaced polyacetals that adopt a disordered, mesomorphic-like structure under fast crystallization were found to be layered. ,, …”

“…Even the recently studied long-spaced polyacetals that adopt a disordered, mesomorphic-like structure under fast crystallization were found to be layered. 39,40,47 The fact that no layer reflection is evident in PE-48,48 on rapid quenching indicates one of two structural possibilities. Either the core lamellae thickness, l c , under these crystallization conditions is less than the all-trans distance between consecutive esters and the ester groups remain in the amorphous region or the ester groups reside inside the crystals but are not layered.…”

“…48,49 Similarly, mesomorphic forms were found recently in fast quenched long-spaced polyacetals. 39,47 Other common poly-mers, such as syndiotactic polystyrene, 50 polyamide 66, 51 and polyamide 6, 52 also form different polymorphs depending on the rate of cooling. In contrast, for a series of polyhydroxyalkanoates, the lamellar thickness and spherulitic morphologies were highly dependent on the cooling rate, but the crystal structure remained unchanged.…”

Crystallization Rate Minima of Poly(ethylene brassylate) at Temperatures Transitioning between Quantized Crystal Thicknesses

“…The impact of large differences in nucleation rates on the crystallization rate at the transition between two crystalline forms was shown in a subsequent study of the crystallization kinetics of long-spaced precision polyacetals that also display temperature-induced polymorphism. − A deep minimum was observed in the overall crystallization rate at the transition between the two polymorphs, but not in the growth of the most stable phase. Although self-poisoning could not be ruled out, the minimum was mainly attributed to the sharpness of the transition and to a large difference in nucleation barriers between the two forms …”