Effect of Self-Poisoning on Crystallization Kinetics of Dimorphic Precision Polyethylenes with Bromine

High molar mass polyethylenes with bromine atoms placed on each and every 21st, 19th, 15th, or 9th backbone carbon crystallize into two distinctive layered polymorphs by changing undercooling. Crystallization at low temperatures produces Form I, a planar <i>all-trans</i> conformation, while at higher temperatures <i>gauche</i> conformers set for backbone bonds adjacent to the methine due to a close intermolecular staggering of bromines resulting in a herringbone Form II structure. In this work, the sharp range of isothermal crystallization temperatures for the transition between Form I and Form II is first identified via WAXD and melting behaviors for all members of the series. Furthermore, the temperature dependence of the isothermal linear spherulitic growth rates of Form II has been studied for a wide range of crystallization temperatures. The linear growth rates display a discrete minimum with decreasing temperature at a crystallization temperature near the melting point of Form I, a feature which is reminiscent of the minimum found in the crystallization rate of long-chain <i>n</i>-alkanes. Changes in spherulitic morphology and the growth rate minima are analyzed on the basis of self-poisoning at the growth front resulting from frequent but unstable Form I depositions on the growth surface of Form II. The similarity with the behavior observed in the growth of long-chain <i>n</i>-alkanes crystallites supports a polymer crystallization process controlled by events that take place at the crystal growth front

Kinetic Control of Chlorine Packing in Crystals of a Precisely Substituted Polyethylene. Toward Advanced Polyolefin Materials

The crystallization of a polyethylene with precise chlorine substitution on each and every 15th backbone carbon displays a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a sharp increase in melting temperature, a change in WAXD patterns, and a dramatic increase in TG conformers around the Cl substitution while the main CH₂ sequence remains with the all-trans packing. These changes correlate with the formation of two different polymorphs characterized by a different packing and distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the chains assemble in an all-trans planar packing (form I) with a layered Cl distribution that presents some longitudinal disorder, while slower crystallization rates favor a more structured intermolecular halogen staggering consistent with a herringbone-like nonplanar structure (form II). The drastic change in morphology is enabled by the precise halogen placement in the chain and appears to be driven by the selection of the nucleus stem length in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows models for generating new materials based on nanostructures at the lamellar and sublamellar level not feasible in classical branched polyethylenes