Separation and characterization of polymers by thermal field-flow fractionation

Thermal field-flow fractionation (thermal FFF) is primarily used for separation and molecular weight characterization of synthetic polymers. The principal theoretical guidelines that help elucidate retention and the thermal diffusion phenomenon in thermal FFF are presented. The important features that govern polymer analysis between size exclusion chromatography (SEC) and thermal FFF are discussed. Thermal FFF is shown to be an effective tool characterized with its high selectivity for the analysis of high molecular weight polymers. The open channel design helps to minimize shear degradations and polymer interactions with the column packing often associated with SEC. Retention in thermal FFF is governed by the thermal diffusion factor ? which decreases with temperature and increases with molecular weight, but is independent of deltaT. With the flexibility of the operation and the help of the formulated equations, one has easy access to an optimum separation, and molecular weight characterization can be carried out by using the FFFRC-developed program. A calibration procedure is widely used to obtain the molecular weight curve and polymer polydispersity for the unknown polymer. This method is based on the plot of log(?.dT/dx·)w versus log(M/10 to the 6 power) with slope n (relevant to the effective selectivity), and intercept log?6 ( a function of temperature). A power programmed delaT method is utilized to greatly reduce analysis time with sufficient peak resolution. The results determined for various samples are compared to the manufacture-provided references measured using independent methods and are found to be in reasonable agreement. To better understanding the molecular weight calibration, two extensive studies have been carried out: temperature extension (Tc ranging from 15°C to 70°C) and molecular weight extension (up to molecular weight of 30 million Daltons). The n values are highly consistent over a variety of experimental parameters, implying a possibility of universal calibration of thermal FFF. For calibration of the ultrahigh molecular weight (10 million Daltons or higher) polymers, flowrate-induced hydrodynamic perturbation and sample overloading effect become significant. The results are examined. Thus thermal FFF can be used for the ultrahigh molecular weight polymer characterization.