Can Polycarbonate be used as heated bed
Since Polycarbonate (PC) has glass transition point of 147°C (according to wikipedia) where it starts to melt, you could in theory use it as a heated bed for PLA or even PETG. BUT, there are other characteristics:
PC is quite good heat insulator, which would result in uneven heat distribution in the heated bed.
Also it expands quite a lot with rising temperature, which could result in warped bed.
And as per usual with thermoplastic polymers such as PC, heating them and cooling them repeatedly can cause material degradation. That would result in the material becoming brittle, deforming, or changes of other properties....
Next problem would be that it is not stiff enough. Depending on the size of the heated bed you would have to support it on multiple points (I would say at least 5x5 grid for 300x300mm bed) and even then it would be unpredictable.
To sum it up: yes, you probably could use PC as heated bed, but it is much better to stick to traditional materials like aluminium or glass fibre sheets (PCB material), because PC would be very inconsistent and therefore hard to level.
I hope this helps.
Note: I am basing this on my theoretical knowledge, I have not tested it and thus do not know if my assumptions are correct or not.