Molecular topology fractionation of polystyrene stars and long chain branched polyethylene fractions

Control of long chain branching (LCB) architecture is an area of considerable 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 determination of LCB distributions.