Two types of poly(vinyl chloride) (PVC) samples are studied by mechanical spectroscopy, namely (i) a commercially available PVC that is submitted to different heat treatments and (ii) the same PVC but slightly crosslinked in order to compare the effect of this chemical crosslinking with the effect of the physical crosslinking obtained after ageing at T> T(g). The comparison is made by means of a theoretical approach for the non-elastic deformation of polymers.