TENSILE, STRESS-RELAXATION AND DYNAMIC-MECHANICAL BEHAVIOR OF POLYETHYLENE CRYSTALLIZED FROM HIGHLY DEFORMED MELTS

High-density polyethylene (HDPT) specimens were obtained by standard extrusion and also by a procedure of solidification from a highly deformed melt (Sodem) in wide ranges of temperature T, time t, and draw ratio lambda from 1.0 to 12.2. Tensile tests were conducted isothermally between 20 and 120-degrees-C and stress relaxation at constant tensile strain studied as a function of time also isothermally at several temperatures in the range from -50 to +100-degrees-C. Dynamic mechanical testing was similarly conducted in the range from -150 to +120-degrees-C. The time-temperature equivalence principle, an equation for the temperature shift factor a(T) as a function of the reduced volume v and the Hartmann equation of state were applied to the properties so established, including the stress relaxation and the mechanical loss tangent. The earlier shift factor equation has been generalized so that it now includes the draw ratio in two ways: In a(T) = 1/[a + clambda] + B/[v - 1]; a, c and the Doolittle constant B are characteristic for a given material but independent of the degree of orientation and of temperature. The reduced volume v depends on temperature T via equation (7) and on lambda via equation (8). Drawing causes a decrease in the number of available chain conformations, which is reflected in the first term; it also changes intersegmental interactions, as reflected in the second term through equation (8). The Sodem procedure improves mechanical properties of HDPE. Specimens with the highest draw ratio lambda = 12.2 exhibit the highest elastic modulus and the highest tensile strength as well as high relaxation rates during long-term testing.