A simple method of estimation of the polydispersity index of narrow molecular weight distributions by using quasielastic light scattering data

1991 ◽  
Vol 56 (8) ◽  
pp. 1642-1652
Author(s):  
Jaromír Jakeš
Keyword(s):  
Molecular Weight ◽  
Light Scattering ◽  
Least Squares Method ◽  
Relaxation Times ◽  
Polydispersity Index ◽  
Scattering Data ◽  
Simple Method ◽  
Quasielastic Light Scattering ◽  
Molecular Weight Distributions ◽  
Weight Distributions

A simple method of estimation of the polydispersity index of narrow molecular weight distributions using quasielastic light scattering data has been suggested. The method approximates the relaxation times distribution by using the Pearson V distribution and iterates its parameters by the least squares method to an experimental correlation function. For the usual unimodal molecular weight distributions the polydispersity index estimate is good (with an error up to 0.1) at a determined value up to 1.2, rough at a determined value up to 1.5, while above this value the method is virtually impracticable. If the type of the molecular weight distribution is known (Schulz-Zimm, lognormal, Pearson V), the polydispersity value thus determined can be corrected, which would extend the applicability limits of the method. it is shown that for an adequate determination of the polydispersity index the noise level ought to be about 10-4.

Evaluation of polydispersity index Mw/Mn by quasielastic light scattering

1987 ◽  
Vol 52 (5) ◽  
pp. 1235-1245 ◽  
Author(s):  
Petr Štěpánek ◽  
Zdeněk Tuzar ◽  
Čestmír Koňák
Keyword(s):  
Molecular Weight ◽  
Light Scattering ◽  
Decay Time ◽  
Sampling Time ◽  
Polydispersity Index ◽  
New Method ◽  
Molecular Weights ◽  
Quasielastic Light Scattering ◽  
Good Agreement

The response of quasielastic light scattering to the polydispersity of scattering objects has been investigated. A new method of the polydispersity index determination has been suggested, suitable for the range 1.02 ⪬ Mw/Mn ⪬ 2.0 and consisting in the measurement of the dependence of the apparent decay time on the correlator sampling time. The polydispersity index can be determined by comparing these dependences with the theoretical ones obtained using correlation curves simulated for various values of the polydispersity index, assuming lognormal and Schulz-Zimm distributions of molecular weights. The test measurements on polystyrene standards having molecular weights in the range 9 103 – 20.6 106 give polydispersity index values Mw/Mn that are in a good agreement with those given by the manufacturer. The polydispersity index for polystyrene having the molecular weight Mw = 20.6 106 thus determined was Mw/Mn = 1.35.

Determination of Copolymer Molecular Weight

1968 ◽  
Vol 41 (1) ◽  
pp. 245-253 ◽  
Author(s):  
Paul Rempp ◽  
Henri Benoit
Keyword(s):  
Molecular Weight ◽  
Light Scattering ◽  
Density Gradient ◽  
Compositional Heterogeneity ◽  
Precise Information ◽  
Molecular Weights ◽  
Molecular Weight Distributions ◽  
Weight Distributions ◽  
Binary Copolymers

Abstract From this brief review it appears that determinations of molecular weight averages, of molecular weight distributions, and of compositional inhomogeneity of binary copolymers, require care in the choice of techniques and methods. Some of the most commonly used techniques for molecular weight determinations on homopolymers of various kinds are inadequate for the same determinations on copolymers. Others are more sensitive to fluctuations in composition than in molecular weights. Osmotic methods are the only one which are really insensitive to inhomogeneity, and which yield molecular weights. Ultracentrifugation in a density gradient yields precise information only on fluctuations in composition. Viscosity determinations require calibration, but even so, they may lead to erroneous values of the molecular weight in the case of copolymers. GPC is less sensitive to compositional heterogeneity, but cannot be applied for nonlinear copolymers. Finally, light scattering is a very powerful tool for studies on copolymers, since it leads to molecular weight averages and its helps characterize polydispersity and fluctuations in composition.

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