Why should I add lithium bromide to my GPC eluent? I'm running gel permeation chromatography (GPC) of a polar, uncharged polymer (acrylamide-based) in a polar solvent (dimethylformamide). The literature recommends adding a salt to the eluent; for my particular experiment, I'm using 0.05 M LiBr (example). Other salts are reported (Sodium Sulfate, for example) in the literature, both for this eluent/polymer mixture, and for similar polymers and eluents. Why is this done? Most papers simply report the eluent and the additives they used, with no mention of their reasoning. One paper just identifies the eluent, with no mention of any added salts; given the prevalence of the use of salts, I'm not sure if this is an oversight on their part, or if they just didn't use a salt.

If I were working with a polyelectrolyte, I would know that adding a salt would screen the charges along the polymer, reduce the $A_2$ effect, and make my data much easier to interpret. One GPC column manufacturer claims (under "What additives are important...") that dipole-dipole interactions can have a similar effect, but I'm not not sure how much to trust that claim. In addition to confirming or denying that this is the reason, I would also appreciate pointers to any relevant literature.
The answer to my question appears to be in a 1977 paper by Nancy D Hann (DOI: 10.1002/pol.1977.170150604; full citation below). In it, she compares GPC of polyurethane in DMF with and without added LiBr. The chromatograms with no added LiBr show multiple peaks. The chromatagrams with added LiBr show just a single peak. The addition of LiBr also shifts the peak to later elution volume. 

Hann explains these results by the polymer having a strong polar interaction with the pure solvent. This causes a higher effective hydrodynamic radius, resulting in the earlier elution time. The polar interactions also case aggregation of the polymer, giving multiple peaks. Adding a salt breaks up the polar interactions, giving just a single, later, peak.



Hann, N. D. (1977). "Effects of lithium bromide on the gel-permeation chromatography of polyester-based polyurethanes in dimethylformamide." Journal of Polymer Science: Polymer Chemistry Edition 15(6): 1331-1339.