As polyurethane cures (or hardens), it undergoes a chemical bonding reaction, linking the mono- and oligomer strings in the components into long polyurethane chains. The chemical reaction is exothermic, it creates heat.
So, we have a process that heats up the polyurethane mixture as it hardens, but how much? Well, it's hard to find numbers for it, but I suspect it can easily reach 30 to 40 °C, depending on the mixture (fast curing) it could easily go higher. To combat the effects of heat softening of the PLA/ABS model inside the mold, I strongly suggest printing with extra shells and extra infill. While most items can get away with 10 %, in this case, I suggest 20-30 %. ABS would be the superior choice above PLA as it starts to deform at a higher temperature. PLA can start to deform at around 60 °C, ABS only at about 80 °C.
The temperature of the PU curing depends on the speed of the curing process - it is safer for the masters to choose a slower curing mix as the heat is generated over a longer time and the maximum temperature is thus lower as a result (as excess heat is lost to the room)
To reduce the sticking to the surface from the material creeping into the gaps of the model, it has to be as smooth as possible and best also sealed. If you choose ABS, a quick acetone vapor bath would do the trick in this case. PLA should be lacquer sealed as it doesn't like to stick to most waxes.
Adding a mold release agent isn't necessarily needed, but could help in removing the masters from the mold.
ABS might be the better choice in this application. It is advisable to use extra-thick walls (3+), a lot of infill (20-30 %) and a vapor smoothed surface.