Chemistry Reference
In-Depth Information
Molecular Topology Fractionation of Polystyrene
Stars and Long Chain Branched Polyethylene Fractions
David M. Meunier,
*
1
Theodore M. Stokich Jr.,
1
David Gillespie,
2
Patrick B. Smith
1
Summary: Control of long chain branching (LCB) architecture is an area of consider-
able interest in materials science because LCB can have a dominating effect on
polymer rheology and properties. Currently no analytical technique provides a
quantitative description of the LCB topologies in these materials beyond a basic
estimation of the average number of branch points per molecule. Neither the
molecular weight of the branch, nor the shape of the branched molecule (e. g. star,
comb, ''H'' or other) can be determined using current state of the art methodology
such as size exclusion chromatography (SEC) with molecular weight sensitive
detectors or nuclear magnetic resonance spectroscopy.
In our laboratory, we have developed a fractionation method that sorts polymer
solutes based on LCB topology. The approach, which we term molecular topology
fractionation (MTF), utilizes a separating medium comprising channels having
dimensions similar in size to the dimensions of the macromolecules being analyzed.
An applied flow field provides the driving force for the separation. Although the
details of the separation mechanism are not well understood at this time, two
possible mechanisms are being considered. In one, dissolved solute molecules are
restricted by the channels such that the relaxation modes for reorientation determine
the rate of transport. In the second, pinning (or entanglement) of molecules on
the stationary phase determines the rate of transport. Both mechanisms result in the
largest molecules eluting latest (opposite to the sequencing in SEC), and produce
significant additional retardation for LCB chains above that of linear chains. This
additional retardation leads to fractionation of an LCB distribution even if the
hydrodynamic radii of the components are the same.
In this paper, an overview of the MTF experiment will be provided. MTF fractionation
of PS stars is presented to demonstrate the separation of LCB chains from linear
chains and LCB chains based on topology. The application of MTF for characterizing
LCB polyolefin fractions will be shown. The paper will also include a brief discussion of
the coupling of MTF and SEC in an on line two dimensional approach for determi-
nation of LCB distributions.
Keywords: branching distribution; characterization; entanglement; lightly cross-linked
homogeneous ethylene octene copolymer; long chain branching; pinning; star polystyrene;
topology fractionation
Introduction
Molecular topology fractionation
[1]
(MTF)
is a relatively new dilute solution polymer
separation technique developed within the
Dow Chemical Company.
[2]
The development
