A MODEL FOR NONLINEAR CREEP AND PHYSICAL AGING IN POLY(VINYL CHLORIDE)

The creep behavior of polymers depends on the physical age of the mate rial at the time of stress application. Creep curves shift to longer t imes for more highly aged material and, in poly(vinyl chloride), (PVC) , this can be modeled by an empirical equation in which the magnitude of an effective mean retardation time parameter is dependent upon, and increases with, the age of the polymer. Creep compliances for PVC als o depend on the magnitude of the applied stress when this exceeds abou t 4 MPa. This nonlinear behavior is caused, at short creep times, by a reduction in the value for the retardation time parameter on applicat ion of the creep stress. Specimens appear therefore to be initially de -aged by elevated stresses. Subsequently, this parameter increases wit h creep time implying that physical aging has been reactivated, but th e rate of increase also depends on the stress level. These influences of elevated stresses can be described by an extension of the creep mod el, and parametric expressions have been derived which relate creep co mpliance values to time, stress, and the age of the polymer. It is sho wn how the parameters can be determined from a short series of creep e xperiments and thus how creep deformations can be calculated over wide ranges of time, stress, and age.