I am calibrating a viscotek omnisec GPC with DMF as a solvent. While my PS105K standard seems to calibrate properly, my PS245K standard's values upon calibration and execution of the analysis program do not match up at all. I have tried using fresh calibration standards, as well as sonication to avoid aggregation for the samples, but the problem persists.
The temperature used is a critical parameter (esp with high Mw samples). Make sure you are dissolving the compound fully into solution and running the detector as per the manufacturer suggests. If you still have problems, contact the manufacturer for assistance,
The addition of 0.05M LiBr is often considered when running with DMF/DMAc solvents. This helps prevent interactions with the solvent which might be more likely with PU's or PI's than PS but I would usually run DMF with a salt as 'standard' conditions.
Check with sample preparation may be you can filter couple of times more. You can try the same standard in THF solvent, if still there is a problem u can take help of manufacturer. You can also try with standard samples from other manufacturer to cross check the efficiency of standard you used.
0.6-1.0 mL/min flow rate, DMF+0.01M LiBr or 5 mM NH4BF4, 50-60 *C.
See page 9 of SI for Gody, G.; Maschmeyer, T.; Zetterlund, P. B.; Perrier, S. Macromolecules 2014, 47, 3451–3460.
Good luck!
I have 2 points to add:
1. sonication can break polymers, especially high molecular ones. Therefore, you should avoid sonication whenever possible.
2. the samples can be dissolved at significantly higher temperature than the measurement temperature, if necessary. I had this case before: we could not get a new copolymer in solution in chloroform at RT. My postdoc found out that chloroform at 40°C works though. So we dissolved it forst at 40°C and then injected it at RT into the GPC. This is not ideal, but in some cases when e.g. the operator does not want to change the tempeature, it might be a good way out.
In general, I would say that PS should be well dissolvable. Maybe, the solution was not prepared long enough in advance.
In general the use of Viscometer on a GPC with DMF as solvent is not very easy and leads to inaccurate results due to the solvent.
When using DMF as a solvent you need to use special columns for polar solvents and a flow rate of 0,6ml/min (the flow rate depends also on the columns). Also you need a system that can function in higher temperatures, the suggested temperature is 75C. After all that you need to use a mixture of DMF and a salt like LiBr in a concentration between 0,05-0,1M. Take care to always use the same salt and concentration because it changes dramatically the hydrodynamic volume of the polymers diluted in it and especially PS type of polymers are greatly affected. Make sure that you GPC's valves can handle a solvent like DMF with a salt diluted in it. Because of the characteristics of DMF and the presence of salt you cannot run your system on recycle when it is idle. You always need to wash your system and columns with pure DMF after analyzing a group of samples to protect the, from the negative effects of the salt.
A crutial parameter for the analysis is to use also fresh HPLC grade DMF and dried salt.
In any case with that type of solvent it will be very difficult to acquire accurate results on the Viscometer.
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