A recent comment on an older post raised a good question: can intrinsic viscosity be negative?
Surprisingly, it can. I'm not up on that area, but there are a 17 Google Scholar hits for "negative intrinsic viscosity" and many are published in good journals, so it seems to be real. Or as I would argue, the data is reproducible. But as always in science, it interpreting the results that is the challenge.
I haven't read any of the articles yet, so here's my big chance to insert my foot in my mouth. I will start collecting the references and will post what I learn, so public humiliation is a real possibility here. My guess is that the polymer is breaking up some sort of weak network in the solvent (hydrogen bonding, polar-polar interactions...) so that the solvent has a lower viscosity, thus leading to the negative intrinsic viscosity - and the need for a new theory to explain it. Yes, go with a new theory since the old theory made a certain set of assumptions that are no longer valid.
That said, I am now wondering of the possibility of this effect happening in reverse but not being noticed. i.e., the polymer forces more hydrogen bonding or what have you that would lead to the solvent having a higher visocosity, but because the polymer is also leading to a higher solution viscosity, this "addition" is not noticed since it's easy to just explain the viscosity increase strictly to the polymer. How could you determine this?
First steps first: to the literature. As is often stated, 6 months in the lab will save you an afternoon in the library.