Branched polymers have much smaller hydrodynamic volume than the linear analogues with same molar mass, so molar mass can not be determined based on viscosity.
@karou and @gergely
Thanks for your reply. I have also made the same observation based on different literature. Is there any other method where we can use viscosity data to calculate the molar mass.
Fair warning, I am giving my best guess since I don't know all the details. If the main chain polymer is a well characterized polymer, then I would study it using well known Mark Houwink parameters from literature. Then, after grafting, I might look at the NMR spectrum of the product, and try to determine the main chain to grafted chain ratio, and estimate the molecular weight that way.
When using viscosity data, you either need literature data (Mark Houwink parameter of the exact linear polymer), or else make a calibration data set yourself using known molecular weight samples, which you do not have. So that is why I think you need a more absolute measurement technique like outlined above (i.e. NMR).
Dear Kaoru Aou,
if it is a two step synthesis than actually thats a great idea! Should work.
In General determination of copolymers is still a demanding problem.
Using GPC with online viscometry and/or light scattering detection you might be able to obtain reasonable molar masses. However, even if light scattering is commonly known to be an absolute method of molar mass determination it is not as easy for copolymers as for homopolymers. The reason is that different chains will/might have a different chemical compositions (cehmical heterogeneity) This it might be in your case if you use e.g. a grafting onto approach. This Variation in chemical composition will result in a variation of the refractive index increment for the individual chains. Similar holds true for GPC viscometry detection.
Depending on the appraoch you applied in the synthesis and on the information available on the precursors (Backbone) it might be possible to estimate the molar mass from the molar mass of the backbone and the composition of the copolymer (NMR).
Best regards
Wolfgang
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