Polymer Characterization

Progress in ThFFF instrumentation and methodology has allowed a systematic study on the thermal diffusion of polymer solutions. The success of these studies is provided by the ability of ThFFF to produce accurate values of thermal diffusion parameters using small quantities of polymer (a few hundred micrograms). The values of the thermal diffusion coefficients (parameters equal to the thermo-phoretic mobility) have been obtained for 17 polymersolvent systems and are about 10~8-10~7 cm2/s• K, and their molecular masses are from about several tens to about hundred Daltons. The results show the correlation of the polymer thermophoretic mobility with several polymer and solvent parameters, the thermal conductivity of the polymer and solvent, the polymer density, and the viscosity and viscous activation energy of the carrier liquid. Studies also demonstrated a correlation of the polymer thermophoresis parameters with the solvatation properties of the solvent. Though conventional diffusion in polymer solutions is well defined, the thermal diffusion of poly mers in liquids is not exactly understood and not well characterized. Although there are equations relating retention to experimental parameters and transport coefficients of polymers, values of polymer ther-mophoretic mobility are not commonly available, and a model for predicting them from physicochemi-cal parameters is in progress only. Therefore, a calibration is necessary for characterizing the molecular weight distribution (MWD) of polymers by ThFFF (although a single calibration point can be used, when the dependence of the diffusion coefficient on molecular weight is known). Calibration is simple in the analysis of homopolymers because well-characterized molecular weight standards are available for a variety of polymers.

The characterization of copolymers presents more problems because of the overlapping effects of composition and molecular weight distribution (MWD). Often it is necessary to characterize both the MWD and the compositional distribution. In this case the commonly used method of size exclusion chromatography (SEC) is not adequate because the separation is governed by size alone. Thus, molecular weight fractions with different compositions may be eluted in SEC simultaneously. In contrast, ThFFF may separate polymers by both chemical composition and size, and is therefore capable of yielding both size and compositional information on copolymers. Separation by size in ThFFF is governed by differences in the diffusion coefficient of the polymers, while separation by chemical composition may result from differences in the thermophoretic mobility.

The results on ThFFF of random and block copolymers of polystyrene (PS) and polyisoprene (PI) in tetrahydrofuran and cyclohexane show that for random copolymers and block copolymers with a random configuration in solution, the ther-mophoretic mobility is a linear function of monomer composition. It may be a basis for obtaining compositional information on such copolymers by ThFFF. For copolymers with a radial segregation of monomers, thermophoresis is determined mainly by monomers located in the outer region of the polymer coil. The dependence of retention on the radial distribution of monomers provides a basis for evaluating bonding arrangements in copolymers. The further progress in copolymer characterization by ThFFF is related to progress in the theory of the polymer thermophoresis.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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