Lots of heat/cold. The sections are baked in an oven so you cannot dammage them with heat.
Get an electric paint stripper and heat by rotating the tip of the largest section (3/4") first, for say 2 -3 min then let it cool down naturally. Then do it again, and again and again. Leave it overnight ( go to bed and switch off) and then try to gently pull it apart.Or if your impatient have a go at pulling it apart (on the cooler bits or it will burn you) on the 2nd or 3rd heat application. If it doesnt work repeat the process until it does, It will work just give it time
Its the gentle heat expansion and contraction that does it. I suppose you could put it under a tap to speed up the cooling down.
The first and second sections are the weakest and will take lots of patience, so dont force them or you may break them with just very firm hand pressure.
If it still doesnt work you can then try wd40 as well, but the heat should work. Let me know
dont rush and it will save you having to buy new sections.