TIT# Oriented quiescent crystallization of polyethylene studied by USAXS.
Part 2: The liquid scattering of stacks generated from random placement
of lamellae
AUT# Stribeck, Norbert;
SOU# Macromol. Chem. Phys. (2004), submitted
LOC# xv084
CLA#
COM#
APP#
MAT#
ABS# In part 1 of this series results of an in-situ small-angle X-ray
scattering (SAXS) study of polyethylene crystallization are presented.
They show that crystallite placement is basically a random process, from
which some order is growing. This paper presents a first survey
concerning the required change of paradigm.
No distortion of a lattice is to be studied, but order grown on the
nanometer scale must be distinguished from the stochastic case, i.e. the
"car parking problem" from the field of random sequential adsorption
(RSA). RSA is explored by computer simulation. The results concerning the
corresponding liquid scattering are required to identify short-range
order (cf. part 3). Processing of simulated scattering patterns verifies
that the features of quasi-random arrangement are preserved in the
interface distribution function (IDF), if only the crystals are shielded
by some transition layer.
In a condensed random nanostructure almost only next neighbor
correlations are present. The distribution of the widths of amorphous gaps
between the crystals, $h_{a}$, is a truncated exponential distribution.
In the scattering pattern the stochastic nanostructure can hardly be
distinguished from a system with short-range order. On the other hand,
in the IDF the features of order become clear. In random systems there
is no convolution polynomial based on crystalline and amorphous
distributions. Only two shifted images of $h_{a}$ are occurring. We find
that packing correlations collapse, if the crystallite thickness
distribution is wide enough. In this case the pure particle scattering
is a fair approximation in the IDF. This criterion is, in general, valid
for technical polymer materials.