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