I wanted to determine the Mw and Rg of PNIPAAM with static light scattering(SLS) in aqueous solution.The Mn of PNIPAAM is about 10000g/mol according to HNMR and GPC results. But somebody says that the molecular weight of my sample is too small for SLS to measure. Are there anyone who has experience in this technique? and is it possible to measure Mw and Rg of a 10000g/mol polymer with SLS? Thanks!
Dear Chao,
Before reading your question today, I just completed a lab experiment where I measured the "Zimm plot" by light scattering of a ~ 5,500 g/mole Mw (weight average) polyelectrolyte with a Mw/Mn of about 2.5. As expected for single chains in solution the radius of gyration will be too small for determination by light scattering (sub ~ 10 nm) but the weight average molecular weight and 2nd virial coefficient were easy to determine.
I had good optical contrast in the solvent/polymer system (dn/dc ~ 0.15 ml/gl) so that aided me greatly in the experiment.
Trust me, I am not saying what I am reporting is spectacular science done by an industrial giant of chemistry--- I am just giving you a concrete example to base your thinking on. :)
So you are not likely to get the Rg, but with care you should get the Mw and 2nd virial coefficient in the chosen polymer/solvent system. I anticipate your optical contrast will be sufficient, also.
Best regards,
Mike Bender
Good question - I've not thought about the lower limit before.
NIPAAM monomer molar mass is 113 g/mol, so you have deg.of polym. N ~ 88. The link below mentions a PNIPAAM persistence length of N ~ 10. So the Kuhn length would be 2x that (for worm-like chains), or N~20. I think you need at least 2 Kuhn units (so N~40) to have a random walk chain. A random walk chain is assumed by the original formulation of the theory of polymer static light scattering by Zimm (link attached). So I think your polymer is long enough for SLS.
http://pubs.acs.org/doi/abs/10.1021/jp810685g
http://scitation.aip.org/content/aip/journal/jcp/16/12/10.1063/1.1746738
Hello dear, the hydrodynamic dimensions of your polymer are enough for determinating Mn by SLS. Your polymer is be able to scatter the light.
Dear Chao,
Before reading your question today, I just completed a lab experiment where I measured the "Zimm plot" by light scattering of a ~ 5,500 g/mole Mw (weight average) polyelectrolyte with a Mw/Mn of about 2.5. As expected for single chains in solution the radius of gyration will be too small for determination by light scattering (sub ~ 10 nm) but the weight average molecular weight and 2nd virial coefficient were easy to determine.
I had good optical contrast in the solvent/polymer system (dn/dc ~ 0.15 ml/gl) so that aided me greatly in the experiment.
Trust me, I am not saying what I am reporting is spectacular science done by an industrial giant of chemistry--- I am just giving you a concrete example to base your thinking on. :)
So you are not likely to get the Rg, but with care you should get the Mw and 2nd virial coefficient in the chosen polymer/solvent system. I anticipate your optical contrast will be sufficient, also.
Best regards,
Mike Bender
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