We used fluorescence nonradiative energy transfer (NRET) to detect the interchain distances r at the overlapping regions of polymer chains. Freeze-dried polystyrene (PS) is found to have an increased r value and thus a reduced chain packing density, which shows an increased segmental mobility under uniaxial compression. We clarify that it is the interchain coupling that restricts the chain motion in a glassy oligomer in which chain entanglement is absent.