Light scattering photometry

The light scattering from liquids results from local density fluctuations in the liquid. In the case of solutions, the solute also gives rise to scattering. In order to assess the amount of light scattered from the solute it is necessary to take the difference between the Rayleigh ratio for the solution and the solvent. This difference was shown by Debye to be 

Equation (2.68) shows that when c- 0,Kc/AR tends towards the value of 1 /M. If Kc/AR is plotted against c, then 1 /M is obtained by extrapolation of c to zero. In the case of a polymer solution, M = M since the scattering phenomenon is a function of the mass Qi of a given molecule of size and 

In arriving at equation (2.68), it has been assumed that the solute polymer molecules and the solvent molecules are point scattering sources. If the size of the scattering molecules is comparable with the wavelength of the incident light, it is possible for a phase difference to exist between the electromagnetic waves incident at different parts of the same molecule leading to interference of the waves scattered by different parts of the same molecule. The main consequences of interference are  

The influence of the size of the solute molecule on the scattering behaviour becomes significant when the dimensions of the molecule are greater than 2/20. 

Although not a simple task, it is possible to extend equation (2.68) in order to address (a)-(d). One of the main consequences of the distortion of the 

Light is scattered all around us— the fact that the sky above us appears blue, the clouds white, and the sunset shades of reds and oranges is a consequence of preferential scattering of light from air molecules, water droplets, and dust particles. This scattered light carries messages about the scattering objects. 

The measurement of light scattering is the most widely used approach for the determination of M . This technique is based on the optical heterogeneity of polymer solutions and was developed by Nobel laureate Peter Debye in 1944. 

For light-scattering measurements, the total amount of scattered light is deduced from the decrease in intensity of the incident beam as it passes through a polymer sample. This can be described in terms of Beer s law for the absorption of light as follows  

The intensity of scattered light or turbidity (t) is proportional to the square of the difference between the index of refraction (n) of the polymer solution and of the solvent ( o), to the molecular weight of the polymer (M ), and to the inverse fourth power of the wavelength of light used (A). Thus  

For polymer solutions containing polymers of moderate to low molecular weight is 1, giving Equation 3.16. At low polymer concentrations Equation 3.16 reduces to Equation 3.17, an equation for a straight line y = b + mx) where the c -containing terms beyond the 2Bc term are small  

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Low Angle Light Scattering Photometry Static Envelopes. We shall first consider the final scattering envelopes obtained after gelation is complete. The next subsection will be devoted to the kinetic evolution of the scattering envelopes ... 

Bulk properties associated with large deformations, such as viscosity and toughness, are most closely associated with M . The values of are most often determined by light-scattering photometry. 

However, melt elasticity is more closely related to the third moment known as the z-average molecular weight M. The values of are most often determined using either light-scattering photometry or ultracentrifugation. It is shown mathematically as ... 

For polydisperse polymer samples, measurements that lead directly to the determination of the molecular weight, such as light-scattering photometry and membrane osmometry, are referred to as absolute molecular weight methods. Techniques such as viscometry are not absolute molecular weight methods because they require calibration using an absolute molecular weight technique. 

As noted earlier, certain techniques such as colligative methods, light-scattering photometry, special mass spectrometry (MS) techniques, and ultracentrifugation allow the calculation of specific or absolute molecular weights. Under certain conditions some of these also allow the calculation of the MWD. 

SEC-MALLS and SEC-LALLS Coupled chromatography and light-scattering photometry that allows the determination of a number of important values along with chain length distribution, sedimentation equilibrium experiment Ultracentrifugation technique that allows chain length information to be determined. 

Zimm plot Type of double extrapolation used to determine the weight-average molecular weight in light-scattering photometry. 

What kind of molecular weight do you generally get from light-scattering photometry ... 

Capillary hydrodynamic chromatography Fraunhofer diffraction Light-scattering photometry Phase Doppler anemometry Ultrasonic spectroscopy... 

Light Scattering Photometry. We will limit our discussion to LALLS because it constitutes a net improvement as compared to other methods of Mw determination. It has the advantage of providing absolute values of the Rayleigh ratio by direct comparison of scattered and transmitted light. 

Low Angle Laser Light Scattering Photometry. Weight-average molecular weights were determined with a KMX-6 photometer (LDC/Milton Roy). The light source was a 2 mW vertically polarized helium-neon laser (A =... 

Molecular weights are determined by end-group analysis (Mn), membrane osmometry (Mn), viscometry (Mv), size exclusion chromatography (Mm), light scattering photometry, and sedimentation (Mw). Any molar mass computed by these methods must be evaluated critically, in view of a dependence on methodology. 

Viscometry is the most widely used method for the characterization of polymer molecular weight because it provides the easiest and most rapid means of obtaining molecular weight related data and requires a minimum amount of instrumentation. A most obvious characteristic of polymer solutions is their high viscosity, even when the amount of added polymer is small. Even so, because viscometry does not yield absolute values of M, one must calibrate the viscometry results with values obtained for the same polymer and solvent by using an absolute technique such as light scattering photometry. 

Molecular weights were obtained employing light scattering photometry utilizing a Brice-Phoenix OM-3000 Universal Light Scattering Photometer. 

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