The condensed behavior of polymer fluids were studied based on the van der Waals interaction. By comparing the phase diagram of pure polymer fluids and that of van der Waals non-ideal fluids, it is found that (1) pure polymer fluids show a similar phase diagram to van der Waals non-ideal fluids; (2) the differences between the phase diagrams were caused by the difference of chain lengths. It is explained that why the vapor pressure of polymer is hard to detect and why the negative pressure can exist in polymer liquids. It is also found that some scaling relations exist between the reduced critical pressure and the chain length, and the reduced critical volume and the chain length.
