Determination of the molecular architecture of synthetic and natural rubber by the use of thermal field-flow fractionation and multi-angle laser light scattering

The processability of elastomers is largely governed by molecular architecture and hence the influence of molecular weight distribution, branching and gel content is of great interest. Analysis by conventional size exclusion chromatography (SEC) has limitations which may distort the molecular weight distribution. However, the combination of Thermal Field Flow Fractionation (ThFFF) and Multi-angle Laser Light Scattering (MALLS) has allowed absolute molar mass and size distribution to be obtained without the need for calibration, standards or assumptions. ThFFF is a separation technique that enables the physical structure and composition of complex macromolecules to be determined and relies on diffusive transport as the principal mechanism of separation. An open channel geometry minimises shear effects, making it possible to separate fragile, high molecular weight polymers, whilst the absence of a stationary phase means that adsorption effects can be ignored. Consequently, complex mixtures of polymer, micro-gel and macro-gel can be studied in a single run without the need for filtration. By combining all the information derived from ThFFF/ MALLS a more comprehensive molecular weight distribution, including levels of branching, can be determined. Light scattering profiles and absolute molecular weight distributions were determined by ThFFF/MALLS for a number of synthetic and natural rubbers and comparisons have been made with results obtained from conventional SEC. For example, the molecular weight distribution of natural rubber has been shown to extend up to 109 g/mol with approximately 20% of the rubber having a molecular weight greater than 107 g/mol. MALLS can also provide information on the size distribution of species and this is discussed further in relation to both synthetic and natural rubber.