I've been synthesizing PNIPAAM these days.However, when I characterized it by GPC, whose eluent is THF, I found the retention time was kind of wired(which is extremely long) and the shape of the peak is trailed. I heard that if there are some functional groups like -OH or -NH2 on the polymer chains, the sample will adsorbed on the column, which makes an abnormal result of GPC. Is that the case? and how can I determine the Mn of my sample? Thanks.
I had the same problem and even worse because PNIPAAm clogged my column. What helps me, was the addition of 5% of triethylamine (according to this document http://www.chem.agilent.com/Library/applications/SI-02381.pdf).
Dear Chao,
I do not think the solvent is the problem. I found this paper: DOI: 10.1002/pola.21256
They use THF as solvent for GPC of PNIPAAM. As column, they use a Viscotek I-BMMU 3078 which is described as a polar organic column. You probably don't need to use this exact column but probably also a column described as "polar organic". Also, in the cited paper they use different detectors (RI and light scattering). Comparing retention time to PS standards alone may work for relative mol wt. but not for absolute determination of Mn. Hope that this is helpful and keep us informed of how it is going. Regards,
S.C. Ligon
Actually the -OH and -NH2 should be involved in inter-molecular H-bond formation.
Which cases the effect on hydrodynamic volume of the polymer in solvent systems. Accordingly the hydrodynamic volume changes effects on the retention time of sample in GPC/SEC analysis.
To over come this problem the -OH and -NH2 groups of the polymer protection before the SEC analysis is required. Generally polyacryli acid is protected before SEC analysis in so many reports...
MALDI can help without group protection. I hope it will help. All the best. Krystyna
Hi Chao,
It may help a little to see your GPC traces. Long tails are not necessarily a anomaly - it all depends on your polymer. Is your polymer very big? Are your columns suited to your polymer's molecular weight?
Are you using controlled radical polymerization? How good is the control?
What temperature are you running the GPC at?
PMMA standards may be more applicable to you than polystyrene. If you have access to a DMF GPC calibrated with PMMA standards, that will be ideal.
Cheers,
J
I had the same problem and GPC result was always end up with low molecular weight and high PDI value.
I had the same problem and even worse because PNIPAAm clogged my column. What helps me, was the addition of 5% of triethylamine (according to this document http://www.chem.agilent.com/Library/applications/SI-02381.pdf).
This behaviour is understood. The leading edge of the peak is adsorbing onto the column and being retained much longer leading to a flat front (probably) and a long tail. To confirm this inject the same twice, at two different concentrations, and in excel deduct the less concentrated eluogram from the more concentrated one. If it's a column effect the tail should essentially dissapear. If so then there are things we can do about it....
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