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Methods for Ultrahigh Mn determination?

Hello group- I study synthesis of a biological polyester (PHB) that is typically 5 x

10^5 to 2 x 10^6 da or more, and have been trying for a couple years to optimize a SEC/MALLS system I have for determination of Mn and Mw. I've finally come to the conclusion that I'll never get the chromatographic resolution necessary for accurate Mn determination, and yet I want to know!! Every one else in the field uses GPC, but I want something better! Seems like osmometry and viscometry is out because of my polymer size, ditto for endgroup NMR. Does anyone have any bright ideas? Anyone ever heard of derivatizing polymer endgroups with a highly absorbant molecule so it shows up in a UV detector? Any help much appreciated. Adam MIT
Reply to
Adam Lawrence
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Consider flow field fractionation (FFF) and its various embodiments (thermal, ...)

John Aspen Research, -

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Reply to
john.spevacek

Isn't there a light scattering instrument that will do this kind of determination?

Reply to
Colin Cook

Dynamic light scattering will give you a particle size distribution, assuming the particles (proteins) are spherical, using Stokes-Einstein equation. With a solution of appropriate concentration in a suitable solvent, you can obtain the particle size distribution in minutes. Then there's the problem of relating particle size to MW, not so easy.

Even more preferred is static light scattering to obtain information about MW of a polymer -do a google search on "Zimm plot". Note that Kc/R_theta is directly related to scattering intensity. You get M_w. This is more complicated than dynamic but gives you more inherent information, it may be well worth the effort to try to understand scattering theory as this is a major polymer characterization technique.

Reply to
rekuci

Quasielastic light scattering does not give you directly a size. The primary output is a time correlation function. Then you have to do a lot of math and include some assumptions to get size values. Usually you are looking for the size of particles (assuming a spherical shape) but the dynamics of the system might be dominated by other things than simple translational diffusion.

QELS is a wonderful way of producing completely meaningless results.

Regards, Oliver

Reply to
Oliver 'Ojo' Bedford

Colin is right. Light scattering is the preferred way for determining the weight average molecular weight of very high molecular weight polymers. This is the technique preferred by industry for measuring ultrahigh molecular weight PE and PS. It is even used for highly branched polymers as well. There are several companies that make light scattering equipment that will handle all the "nasty" math and do your transformations for you. A couple are Waters and Viscotek. Both instruments are additions to standard GPC units.

Reply to
Larry Effler

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