Let's assume that for some reason Russians wants to have a space station on their own, thus to separate their part from the rest of ISS. Would that be even possible without much of work?
The two segments are tightly intertwined. But the Russian segment is more independent and could potentially survive on its own.
The US Segment would need a new control module to replace the Russian segment which does not exist today. Whereas the Russian segment would be able to survive on its own, just at reduced power levels, since it gets a lot of its power from the US segment arrays.
Going forward with real plans, versus hypotheticals, the plan for Russia is with the Nauka module (MLM) launch summer 2021 (was supposed to launch as far back as the early 2000's so we shall see if it happens) using the nadir (down facing port) on Zarya (where Pirs is right now) to form the core of their new space station.
Those new modules could separate on their own and act as an independent station. When the ISS is officially retired, the Russian plan is to separate and operate Nauka/OM as the base of their next station.
It is possible if it has to be done, but it's very unlikely since the modules are becoming old, like Zarya, the first-ever module launched as part of ISS back in 1998 has the wear and tear during operations all these years. ROSCOSMOS is planning to launch its own space station in the next few years.
I think that it is possible because the ISS was assemble in multiple parts meaning that each parts should have a way of closing itself to avoid exposing the interior to the vacuum of space during the assembly. Furthermore i once heard that the russian part might become an independent station when NASA will retire the iss. So i'm pretty sure it's possible but i couldn't find an article as a source so feel free to edite this post if you find an article discussing this subject.