2 research outputs found
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 CH<sub>2</sub> 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