STRESS-RELAXATION OF PVC BELOW THE YIELD-POINT

Stress relaxation of commercial poly(vinyl chloride) (PVC) is measured at strains below 3% and at different temperatures below the glass tra nsition temperature. First it is shown that below the yield point the material follows a linear viscoelastic behavior. Then the data at a fi xed deformation level (0.03) are fitted by considering a lognormal dis tribution function of relaxation times. Furthermore, from the measured stress-strain curves, the temperature dependence of the elastic tensi le modulus is determined. The temperature dependence of the elastic mo dulus, the relaxation strength, and the parameters of the distribution : mean relaxation time, tau(m), and half-width, beta, are given. Moreo ver, the distribution function and the temperature dependence of its c haracteristic parameters are discussed in terms of a cooperative model of the mechanisms involved in the mechanical relaxation of glassy pol ymers. Finally, the relationship proposed between the tensile modulus and the free volume helps explain the temperature dependence of the re laxation strength. (C) 1996 John Wiley & Sons, Inc.