Effect of tensile strain on the use of the WLF equation

The effect of finite elongation on superposed infinitesimal torsional oscillations has been determined on two propellants, a carbon black-filled rubber and Solithane 113 (Galcit I), as a function of temperature at various fixed frequencies. Torsional storage modulus—temperature data for carbon black-filled rubber and propellant show that the effect of the imposed tensile elongation cannot be explained by any simple temperature–elongation shift relationship. The shift factors for the torsional moduli of these two polymeric systems have been calculated as a function of temperature at various tensile elongations. The WLF constants C₁ and C₂ have been computed for these systems as a function of the elongation. The constants decrease with increasing elongation. The values of the constants at 0% elongation are larger than those commonly found in unfilled materials. The temperature dependence of the shift factor of the torsional storage modulus was found to differ from that of the loss modulus in the cases of carbon black-filled rubber and propellant. This difference is slight for the rubber and large for the propellant. The effect of increased elongation is to increase the difference in the shift behavior of the moduli for each of these filled polymers. The shape of the loss tangent curve of the propellants examined indicates that these propellants are not thermorheologically simple.