Dynamic Mechanical Analysis part 11 pps

©1999 CRC Press LLC

Another point of this test is that any mechanical or environmental noise in the

vicinity will be seen in the test. This allows us to find and remove sources of error

that have nothing to do with the test. A lot of the oddities in data are caused by

environmental effects including mechanical vibration, impure gases, poorly con￾trolled gas rates, improper cooling, or noisy power. All of these sources of error

must be eliminated before a test can be considered valid.

As mentioned above, creep–recovery testing is also done to see how time affects

the polymer. This is done to determine the linear region for creep–recovery curves

and to measure relaxation times. One first applies creep stresses to the sample in

increasing amounts and plots this as compliance, J, versus time. In the linear region,

J becomes independent of the stress, so the curves overlay. A fast way to check this

is to start at a very low stress and increase it by doubling the stress for each run.

When the strain stops doubling, we are out of the linear region.

8.9 CHECKING THE TEMPERATURE RESPONSE

Using the information from above, we now check the response of the material to

temperature by running a temperature scan. One normally scans the widest range

possible within instrumental limits. The end of the linear region from the static

stress–strain curve tells us the maximum total force and the maximum from the

dynamic stress–strain run tells us the maximum dynamic force. Then, we pick a

dynamic force strong enough to give us a strain between 2–12 mm. If we are running

a solid sample, we adjust the static or clamping force to keep the specimen in good

contact with the probe. For a hard glassy sample, this is normally 110% of the

dynamic force. As the sample becomes soft and more rubbery, this increases as

necessary. For fluid samples in a torsional rheometer, we set the gap to where the

sample is able to keep a smooth edge. Obviously, for some geometries such as the

cone-and-plate, the fixture is designed to run at a fixed gap.

We adjust the positioning of the solid sample to get as low a phase angle as

possible at those stresses. (Remember, we are concerned about the stress, not the

force.) This is especially important in cantilever and extension geometries, where it

is easy to misalign the specimen. The specimen needs to be set up at the lowest

temperature, as otherwise the forces may not be sufficient. The sample is then run

at a heating rate slow enough to give even heating to the specimen. This should

allow one to identify the transitions in the material. It is not uncommon to need

multiple runs or special control to collect all these data, as some materials will

change so much at the Tg that the specimen will fail.

8.10 PUTTING IT TOGETHER

Now that we have collected the data, we need to apply the analysis given in the

previous chapters to determining what it means for our sample. We should now be

able to determine the linear region, the effects of temperature, stress, and frequency,

and how time-dependent the material is. In addition, we should be aware of where

transitions occur and to what of type of behavior they correspond.