Eur. Phys. J. E 3, 185-194
What can polymer crystal structure tell about polymer crystallization processes?
B. Lotz
Institut Charles Sadron (CNRS - ULP), 6 rue Boussingault,
F-67083 Strasbourg, France
lotz@ics.u-strasbg.fr
Received: 5 May 2000
Abstract
Contrary to most or all other materials, crystallization
of chiral but racemic polymers such as isotactic polypropylene is
accompanied by a conformational rearrangement which leads to
helical geometries: the building units of the crystal are helical
stems,
-20 nm long, which can be either
right-handed or left-handed. Helical hand cannot be reversed within
the crystal structure: it is therefore a permanent marker
and an indicator of molecular processes (in particular
segregation/selection of helical hands) which take place during
crystal growth, and more precisely during the crucial step of
"efficient'' helical stem deposition. The issue of proper helical
hand selection during polymer crystal growth is mainly illustrated
with isotactic polypropylene. Its various crystalline polymorphs
(
,
,
and smectic) display virtually all
possible combinations of helical hands, azimuthal settings and even
non-parallel orientation of helix axes in space. Furthermore, a
specific homoepitaxy which generates a lamellar branching in the
phase "quadrites'' and
composite
structures makes it possible a) to determine the helical hand and
associated azimuthal setting of every stem in the crystalline
entities and b) to determine the impact on the crystal structure
and morphology of "mistakes'' in helical hand of the depositing
stem. Analysis of these morphologies demonstrates that the
crystallization of isotactic polypropylene (and by implication of
other achiral, helical polymers) is a highly sequential and
"substrate-determined'' process, i.e. that the
depositing stem probes the topography of the growth face prior to
attachment. These observations appear difficult to reconcile with
crystallization schemes in which molecules (helical segments) are
prearranged in a kind of pseudo-crystalline bundle (and as such,
are not subjected to the high constraints of crystal symmetry)
before deposition as a preassembled entity on the substrate.
PACS
61.41.+e Polymers, elastomers, and plastics
Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag
