MOLECULAR-BASES OF STRESS-RELAXATION AND VISCOSITY OF POLYMERS .1. NEW EMPIRICAL RELATIONS

Experiments with several polymers led to new relations which describe stress relaxation at small shears and show the influence of the molecular structure on relaxation. Bubble-free melts of anionically polymerized polystyrenes with the molecular masses (relative molecular mass is referred to as molecular mass throughout this paper)M=6 102 to 1.8 106 g mol-1 and their blends are studied at several temperatures and at times t>3 ms. As the stress σ (102 to 106 Pa) deforms the rheometer, a new mathematical method was developed to correct the relaxations. The energy-elastic stress is separated from the entropy-elastic stress. The relaxations of the latter do not confirm the Rouse theory as σ is proportional to exp (-|t1/2|) for M<4000. For M>4000 the relaxations deviate from the theoretical functions as well. The initial modulus G(0) then depends on M, and σ is not proportional to |t-1/2|. A new and simple function describes the relaxation of melts with entangled molecules at long times. The influences of concentration and chemical structure on relaxation are formulated for weak intermolecular forces. Published data for the constant D of self-diffusion are used in calculating values of D for a constant free volume making use of the initial velocity of relaxation and its dependence on temperature. Then D∼M-1 holds for M<1.3 104 and D∼M-2 for larger M. Correct values of viscosity are calculated from the empirical functions of stress relaxation. © 1990 Steinkopff.