Hello, I am a french phd student and I'm interested in caracterization of polylactic acid by SEC, but I have a problem with the choice of the good eluant : chloroform or THF ?? For information, I have a mixed-c PLgel columns from polymers laboratories. Thanks a lot.

You should use the best solvent for your sample. In a better solvent, the polymer will tend to maximize exposure to the solvent thus maximizing the size. In poorer solvents, the polymer will tend to ball up to minimize solvent exposure, minimizing the size. Since SEC is based on size, all other things being equal, you would want a solvent that will maximize the size of a polymer with a given molecular weight. However, do not use a solvent that is not compatible with the column you have. You can measure visosities in different solvents to determine which are better for your polymer.