Rheological characterization of storage and loss moduli and relaxation moduli of thin-film polymers exposed to low earth orbit

Abstract

The materials made available through the LDEF satellite provide a set of specimens that can be well characterized and have a known exposure history with reference to atomic oxygen and ultraviolet radiation exposure. Mechanical characteristics measured from control samples and exposed samples provide a data base for predicting the behaviour of polymers in low earth orbit. Samples of 1.0 mil thick low density polyethylene were exposed to the low earth orbit environment for a period of six years. The viscoelastic characteristics of these materials were measured and compared to the viscoelastic characteristics of control samples. To further explore the sensitivity of the experimental techniques used to characterize these specimens, crystallinity was varied in a known manner by annealing samples in a vacuum oven, The viscoelastic properties measured for the control, annealed, and exposed specimens were the storage and loss modulus as a function of frequency and temperature. From these experimentally measured values the storage and loss modulus master curves were constructed by employing the time-temperature superposition principle. Once the master curves were constructed the relaxation modulus as a function of time was calculated using the method of Ninomiya and Ferry. Storage and loss modulus master curves and relaxation modulus curves are compared to examine the difference in the directionality of the material and the effect of isothermal annealing and exposure to low earth orbit on resulting properties.