Sandia LabNews

Silly Putty probe yields non-silly results about time-dependent material properties

Silly Putty probe yields non-silly results about time-dependent material properties

Leave it overnight on your papers and it’s sticky as bubblegum. Pat it into a sphere and it bounces like a tennis ball. Hit it sharply with a hammer, and it separates with edges flat as crystal.

Silly Putty™. The words should strike fear in the hearts of experimentalists dealing with visco-elastic materials. The polymeric stuff creeps forever under a static load, so conditions of the experiment continually change. Worse, the stuff strongly adheres to probes: Blind sensors register little data.

However, Sandia researcher Jack Houston (1114) sees the solid liquid merely as the most extreme representation of a common modern material: particles encased in a polymer matrix. These include golf club shafts and rocket fuel, polymers bonding steel to cement, and rubber bands. All these polymers (and many others), elastic at first, weaken over time, leaving golf clubs creaky, compressing rocket fuel into a bomb, weakening cement, and causing aged rubber bands to snap when stretched.

What’s needed, Jack thought, are better methods to locally measure the effect of time on these polymer chains and to set up a chart of expected deterioration rates. Methods used today either involve bulk probes or don’t have quick enough response to map time-dependent effects. Jack felt he had a better product: the interfacial force microscope, which he invented 15 years ago and on which he and Bill Smith (1114) recently obtained a patent for an updated sensor.

Like the show-business phrase about making it in New York, Jack figured if he could measure the time response of silly putty — the most experimentally difficult of all polymer matrices — he could make such measurements on any polymer matrix.

Characterizing matrix deterioration in the laboratory could alert manufacturers or users before unpleasant outcomes come to pass. And timetables of matrix decay could be established, if suitable measurements could be made to examine the pace at which a particular matrix changes.
“We started out wanting to study adhesion in a more fundamental way by actually watching the bond form and subsequently fail,” says Jack of his efforts of a decade ago. At that time no technique was available to make such measurements.

And it wasn’t as though anyone recently radioed Jack to say, “Houston, we have a problem.” But it occurred to him that his microscope could solve this increasingly widespread problem of measuring local changes in matrix behavior better than any other tool.
His results, supported by DOE’s Office of Basic Energy Sciences, have been accepted for publication by the Journal of Polymer Science B (Physics).

The IFM is unique in being able to obtain quantitative and mechanically stable data of both the adhesive interaction and a material’s time-dependent mechanical properties. It’s like the atomic force microscope, or AFM, but that popular technique suffers from being mechanically unstable, says Jack. It snaps in and out of contact, like trying to bring two kitchen magnets, one in each hand, together controllably.

The IFM, on the other hand, has a tip located on one end of a very small “teeter totter,” which is supported by torsion bars above two tiny capacitor pads. When a sample is brought very near the tip, the force of attraction between the tip and sample causes the teeter to totter, increasing the capacitance of one and decreasing the other. The key to the IFM concept is that this rotation is forced to zero by a feedback system, which places the proper voltages on the capacitor pads. Forces are thereby measured quantitatively by the amount of voltage necessary to achieve balance without tip motion.

By pushing on its target, the probe deforms the material. The measurable force changes with time and depends on the nature of the material. Suddenly advancing the tip into Silly Putty results in a spring-like deformation and a large initial force, which rapidly decays as the material creeps away from the tip in a viscous flow, leaving behind a dent.

“This tells you how much stress the material can tolerate and over what period of time the stress can be maintained, which can be translated into the material’s frequency response,” says Houston. The microscope measures this stress response in a few seconds, with results that matched 10 to 12 individual frequency tests by a classical rheometer.

Rheometer tests are done on bulk samples and consist of a series of measurements over a range of frequencies. Such measurements can take several minutes, during which time the sample can creep and change the experimental conditions.

The IFM measurement gives the details on how the material reacts to being deformed and is done in a time frame where the experimental conditions remain the same.

There are currently 17 IFM machines in use at Sandia and various universities around the US and in Canada.

Houston expects more next year when a newly patented laser interferometric measuring system replaces the simple radio-frequency bridge system that can’t be scaled to smaller sensor dimensions. The new system achieves greater sensitivity.