# Tensile, Stress Relaxation, and Dynamic Mechanical Behavior*

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 *a*_{t} as a function of the reduced volume *v*^{19, 20}
and the Hartmann equation of state^{21-23} 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 *a*_{t} = 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.

Maintained by
Christopher S. Own.