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Technique to Predict Ultraviolet Radiation
Embrittlement of Polymers in Space
In the low-Earth-orbit environment, solar ultraviolet (UV) radiation embrittles polymer materials
through bond breaking and crosslinking. This UV embrittlement increases the surface hardness
of the polymer. Before the durability of polymer materials in the low-Earth-orbit environment
can be predicted, the extent of UV embrittlement needs to be determined. However, traditional
techniques for measuring the microhardness of materials cannot be employed to measure changes
in the hardness of UV-embrittled surfaces because traditional techniques measure bulk hardness
and are not sensitive enough to surface changes. A unique technique was used at the NASA
Lewis Research Center to quantify polymer surface damage that had been induced by UV
radiation. The technique uses an atomic force microscope (AFM) to measure surface
microhardness.
An atomic force microscope measures the repulsive forces between the atoms in a microscopic
cantilevered tip and the atoms on the surface of a sample. Typically, an atomic force microscope
produces a topographic image of a surface by monitoring the movement of the tip over features
of the surface. The force applied to the cantilevered tip, and the indention of the tip into the
surface, can be measured. The relationship between force and distance of indentation, the
quantity force/distance (newtons/meter), provides a measure of the surface hardness. Under
identical operating conditions, direct comparisons of surface hardness values can be made.
This technique has been used to evaluate small changes in the surface hardness of fluorinated
ethylene-propylene (FEP) Teflon (E.I. du Pont de Nemours & Company, Wilmington, Delaware)
samples that received varying solar exposures during 3.6 years on the Hubble Space Telescope.
The figure shows the increase in surface hardness (represented as indent force/indent distance)
with increasing solar UV radiation of Hubble Space Telescope Teflon.
Increase in surface hardness (represented as indent force/indent distance) with increasing solar
UV radiation of Hubble Space Telescope Teflon.
Because ground test facilities do not exactly simulate the radiation spectrum of the Sun in space,
ground-to-space correlation factors need to be determined for in-space durability predictions that
are based on ground testing. The force-versus-distance technique can be used to determine the
correlation between ground-testing durability predictions and actual in-space durability. This is
achieved by determining what ground-test exposure produces the same UV damage as a
particular in-space exposure. The Hubble Space Telescope Teflon data will be compared with
ground-test-exposed Teflon data to determine the ground-to-space correlation factor of Teflon.
Once the ground-to-space correlation factors for materials in a particular facility are determined,
long-term, in-space durability can be more accurately predicted on the basis of ground testing.
Find out more about the atomic force microscope and its applications to space durability.
Lewis contact: Kim K. de Groh, (216) 433-2297
Headquarters program office: OSAT
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Last updated April 26, 1996
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