Flow and Rubber Elasticity in Polymer Melts [4,45, 46, 48–57]

In addition to those relaxation processes which have been discussed up to now (γ- process, glass transition) there is an additional process, which occurs in all uncrosslinked polymers: the flow relaxation process. By this process the polymer melt exhibits a recoverable strain if the stress is released. In dynamic mechanical investigations, this corresponds to a paraelastic relaxation process taking place over the same frequency range as that where J=l/ωη. This process occurs on the temperature scale above the glass transition, whereas on a frequency scale it occurs below this transition. Figure 47 shows, as an example, the frequency dependent master curves of the dynamic shear compliance and the shear modulus for polyisobutylene at a reference temperature, T₀ = 273K, obtained using the frequency-temperature superposition principle. In this graph one can see (from left to right): flow, the rubber plateau, the glass transition and the modulus and compliance plateaus of the glassy, frozen solid. From a comparison of the two curves it is obvious that the maximum loss modulus at the glass transition occurs at a frequency some six orders of magnitude greater than that of the loss compliance.