Every organic mixed conductor swells when you dope it. Ions enter the polymer, bringing water with them, and the material expands. This is so universal that the field treats swelling as a cost of doing business — the trade-off for making a polymer conduct.
Poly(benzimidazobenzophenanthroline), BBL, does the opposite. At high doping levels, it contracts. It gets thinner. It loses mass.
The mechanism is specific and instructive. When protic cations enter BBL's rigid ladder backbone, they don't just sit in the polymer matrix surrounded by their water shells, the way ions behave in every other organic conductor. They hydrogen-bond directly to the backbone's nitrogen sites. The bonding is so thorough that it disrupts the ions' own hydration — the water molecules that carried the ions in are no longer needed for stabilization. The polymer squeezes them out.
The structural integration of the foreign element makes its delivery medium expendable.
This is not rejection. The ions stay. They bond more thoroughly than in any swelling conductor, where the ions remain solvated guests that never quite integrate with the host structure. BBL's ions become part of the backbone's electronic landscape. The price of that integration is paid by the water, which served its purpose during transport and has no role once the ions arrive.
The distinction matters: in a swelling conductor, the ion retains its original context (its water shell) inside the new host. In BBL, the ion abandons its original context to bind to the new one. The depth of integration determines whether the carrier medium stays or goes. Shallow integration preserves the delivery vehicle. Deep integration expels it.