Yu.M. Boiko
Physico-Mechanical Laboratory, Plastpolymer Okhta Research and Production Association, Polustovsky Prospekt 32, 195108 St. Petersburg, Russia
Witold Brostow and Anatoly Ya. Goldman
Dept. of Materials Science, University of North Texas, Denton, TX 76203-5308, USA
A.C. Ramamurthy
Plastic and Trim Products Division, Ford Motor Co., 24300 Glendale Avenue Detroit, MI 48239, USA
ABSTRACT
High-density polyethylene (HDPE) specimens were obtained by standard extrusion and also by a procedure of solidification from a highly deformed melt (So-dem) in wide ranges of temperatures T, times t, and draw ratios l from 1.0 to 12.2. Tensile tests were conducted isothermally between 20°C and 120°C and stress relaxation at constant tensile strain studied as a function of time also isothermally at several temperatures in the range from -50°C to + 100°C. Dynamic mechanical testing was similarly conducted in the range from -150°C to + 120°C. The time-temperature equivalence principle, an equation for the temperature shift factor at as a function of the reduced volume v19, 20 and the Hartmann equation of state21-23 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: ln at = 1/[a + cl] + B/[v - 1]; a, c, and the Doolittle constant B are characteristic for a given material but independent of T and l. v depends on T via Eq. (7) and on l via Eq. (8). Drawing causes a decrease in the number of available chain conformations - what is reflected in the first term; it also changes intersegmental interactions, as reflected in the second term through Eq. (8). The Sodem procedure improves mechanical properties of HDPE. Specimens with the highest draw ratio l = 12.2 exhibit the highest elastic modulus, the highest tensile strength as well as high relaxation rates during long-term testing.
*Polymer 1995, 36, 1383.