DAMPING FUNCTIONS AND CHAIN RELAXATION IN UNIAXIAL AND BIAXIAL EXTENSIONS - COMPARISON WITH THE DOI-EDWARDS THEORY

Damping functions for polystyrene melts with narrow and broad molecula r weight distributions were obtained from step-strain stress relaxatio n experiments in shear and biaxial extension. A lubricated squeezing f low method was used to obtain the biaxial relaxation modulus. Followin g the Wagner method, we calculated the damping function in uniaxial ex tension from experimental data on the stress growth coefficient. The c alculated damping function was independent of strain rate. In all defo rmation modes, strain dependences of the damping functions for narrow distribution polystyrene were stronger than those for broad distributi on polystyrene. Experimental damping functions for narrow distribution polystyrene were compared with predictions of the Doi-Edwards theory. The predicted shear damping function agreed well with the experimenta l data. Fair agreement between the predictions and the data was observ ed in uniaxial and biaxial extensions. Compared at the same Hencky str ain, the component of the radius of gyration tenser in the stretching direction is greater for uniaxial extension than for biaxial extension . However, stretching of the primitive chain along the contour is more prominent in biaxial extension than in uniaxial extension, giving a s tronger damping (contraction of the primitive chain) in biaxial extens ion. This explains the stronger strain dependence of the biaxial dampi ng function.