PHYSICALLY MEANINGFUL DISCRETE RELAXATION SPECTRUM - RHEOLOGICAL BEHAVIOR OF MONODISPERSE POLYMER MELTS

A discrete relaxation spectrum using a small number of modes is derive d to describe the relaxation behaviour of monodisperse polystyrene mel ts. Attention is focused on keeping the physical meaning of the relaxa tion times. Therefore, molecular dynamics theories are used in the ter minal, plateau and transition zones and in the case of entangled melts . The spectrum is divided into three parts corresponding to three assu med different relaxation mechanisms: Rouse mechanism at short times, m odified Rouse for tube equilibration and reptation with tube fluctuati on in the long time range. This description only requires the knowledg e of a limited number of parameters, namely the molecular weight of th e polymer under investigation, the scaling law for Newtonian viscosity , the plateau modulus and the molecular weight between entanglements. The method is successfully applied to entangled monodisperse polystyre ne melts and enables the recovery of final spectra containing only a f ew relaxation modes.