Molecular weight determination light-scattering method

Unfortunately, the intractable nature of PPy has made molecular weight determinations virtually impossible. Methods such as GPC and other techniques including light scattering, viscosity measurements, and vapor pressure osmosis, all require the polymer to be dissolved completely. Because PPy prepared by conventional methods does not dissolve, these analyses have not been possible. 

The polymerization kinetics were followed by varying the reaction times and determining the molecular weights by light scattering photometry employing the serial dilution method. Attempts were made to follow the reactions at shorter times by UV/Visible spectrophotometry. The monomers and polymers were scanned from 900-190 nm and in all cases the bands of the monomers and polymers were found to overlap such that the reaction could not be easily monitored by this method. 

Absolute calibration is of critical importance for the determination of molecular weights by the light-scattering method. The scattered intensity is so low compared with the incident beam at a distance of a few cm. being of the order of 10 lo) that a reliable direct comparison of the two beams using the same photocell is impractical. Several methods have been used, with results which are not always comparable, however. 

In contrast to these thermodynamic methods, the viscosity molecular weight determination depends on the interference in the flow of the solvent caused by the dissolved molecules. In contrast to osmometry and light scattering, it has not been possible to develop the viscosity effect into an absolute measure of molecular weight. Rather, it must be calibrated, preferably by light scattering measurements. 

The determination of molecular weights by the light scattering method is based on the principle that the intensity of light scattered at some angle, 6, is a... 

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