Dependence of Mechanical Relaxation on Morphology in Isotactic Polypropylene

The shear modulus and logarithmic decrement at approximately 1 c/s have been measured from -180 °C to about 150 °C for a series of samples of isotactic polypropylene prepared by various thermal treatments. The samples had varying densities (degrees of crystallinity) and morphologies as characterized by spherulite size. All of the samples exhibited the three relaxations characteristic of polypropylene. The maxima in the three measures of relaxation (logarithmic decrement, G″, and J″) occur at about -60 °C, 0-10 °C, and 50 to 100 °C for the three processes. However, the relation between crystallinity and the magnitude of the peak value of the particular measure of loss depends upon the relaxation and the function used to measure the loss. The loss is almost independent of density and morphology for all three relaxations when G″ is used to characterize the loss, whereas the loss increases monotonically as the density decreases when using J″ to characterize the loss behavior. The logarithmic decrement behaves in a more complicated manner. The implications of this behavior are discussed, and it is shown that the primary effect of changing density is to change the equilibrium modulus rather than the relaxation processes.