VISCOELASTICITY AND PHYSICAL AGING IN PEN FILMS. Maria L. Cerrada and Gregory B. Mckenna. Polymer Division, NIST, Gaithersburg, MD 20899, U.S.A.,

Poly(ethylene terephtalate) or PET, is currently the most widely used polymeric substrate material for magnetic recording tapes. Alternative substrates being considered include poly(ethylene naphthalate) or PEN, a polyimide (PI), an aromatic polyamide (ARAMID) and poly(benzoxazole) or PBO. The main objective of this research was to measure viscoelastic characteristics (stress relaxation) of one of these alternative polymers: PEN.

Stress relaxation experiments were performed on PEN film specimens in a wide range of temperatures. The tests were run on a MTS system servo-hydraulic machine with a linear tensile head. This head was equipped with clamps especially designed to hold the thin films of this study. The applied strain level was 0.005 in uniaxial extension. In addition, work was performed in both, machine and perpendicular, directions to explore how much the balance in the films would vary with relaxation time.

The results, as expected, show a relaxation moduli decrease when temperature was increased in both directions. We observe that the lower temperature (short time) behaviors are different from those exhibited at the higher temperatures. At the lower temperature, the relaxation modulus (logarithmic) variation is nearly linear on the logarithmic time scale. At higher temperature the behavior exhibits significant curvature. This effect can be observed as the aging time increases at constant temperature in aging experiments using this material.

As conclusions of these results, it can be said that due to the semicrystalline nature of PEN the stretched exponential KWW equation (valid for amorphous polymers) is inadequate as a model. Secondly, PEN seems to have two relaxation mechanisms in the time and temperatures windows of interest.