I have been trying to measure the Rg value of poly acrylate acid with a molecular weight of 5000 using SLS instrument. However, I got several negative KC/R values, which made me not be able to calculate Mw or Rg value.Almost all KC/R values are positive, so I really don't know the reason of it. Can anyone give me advice?
Thanks.
Check the following two issues:
(1) The concentration of the solution has to be dilute (especially for strong polyelectrolytes)
(2) Ideally Rg should be larger than the wavelength/30. It seems that the MW you are using is too low. You may want to try SANS, which is available at CAS as well.
Another point is that, due to polyelectrolyte effects, the coil radius will increase as you dilute, if you have not fixed the salt concentration (& pH) by adding extra salt (buffer). If the coil radius changes with dilution, the standard data analysis will not work.
In my opinion, the best papers in this area are those from Prof. Wayne Reed's lab at Tulane University, New Orleans. See in particular those by Sorci and Reed, in Macromolecules.
Which is you specific dn/dc?
Is it positive or negative?
If the dn/dc of your system was positive, the only reason for getting negative Kc/R values would be to have a light scattered lower than that from the solvent (by the way: is it water?). The concentration must be adjusted to the diluted range but with a clear excess from the solvent (I mean, low but not too low ...; see Mu-Ping Nieh comment). In general, it is a good idea to start not far away from 100% of excess (double signal than the solvent). But now too high, because you will fall in none dilute none linear range.
Which is the light source (wavelength and intensity) of your equipment? It will be critical if you want to determine a very small Rg ...
Kc/R reads not R but actually Delta R the excess scattering, so if you do not treat your solvent in the right way the solvent scatters more than the solute and you get negative numbers. K and c are by definition positive. You should use X-ray scattering or SANS to determine Rg for PAA 5000 and extrapolate to zero angel and concentration. I guess as it is probably too small for light scattering (dependent on wavelength and sensitivity of your equipment).
You probably have problems with concentration detector. Check its baseline. If for some reasons signal falls below the baseline, you will get negative concentration and therefore negative KC/R.
If you have sure all the parameters (i.e. concentration, calculation of q2, etc) were correct, check the following factors one by one:
1) choose one data series, plot 1/intensity vs. q2, see the intercept is negtive or not, also see if all the plots are in a line well.
2) plot 1/(intensity at 90degree at every concentration) vs. concentration, see the intercept is negtive or not.
If both intercepts above are positive, then the problem is in the Zimm plot calculation. If 1) intercept is negative, or even ,the plot is like y=x2 in low q, there may be aggregations or other large particles in the solutions. if 2) intercept is negative, well, the concentration must be wrong.
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