That might be a bit of a stretch — Materials Testing System

In much of our research in the polymer lab, we’re looking to find molecular weights using the SEC-MALLS (Size Exclusion Chromatography — Multiple Angle Laser Light Scattering) and CIV (Corrected Intrinsic Viscosity). I’ve already written a blog post about CIV and I’ll save SEC-MALLS for later. But sometimes, we look at more than just the molecular weight — we’re looking at the mechanical properties of a sample of aged or fresh polymer.

Materials Testing System (MTS) is what we use to do just that. Of course, not any regular old sample of polymer will work. We have a lot of shapes in lab — punches, beads, but also dogbones. Dogbones are called dogbones because….well…its easier to just post a picture.

A dogbone with corresponding label

 

They’re shaped a bit like dogbones. Pretty self-explanatory. This particular one is PA-12 on day 60 of aging in de-ionized water. ‘DO’ stands for ‘Dissolved Oxygen’ — the solution in which this was aged is completely anaerobic. The pH is the measured pH of the solution — a bit finicky to measure with an electronic probe because the water is de-ionized (and the probe has no ions to detect), but noted anyway.

The first step to running an MTS on a particular dogbone is measuring the width and thickness of the sample with callipers at 6 different points along the thinnest area, as well as the width of the ends. This data creates a modulus for the program that analyzes our results. This is probably the most tedious part and it isn’t even all that tedious (depending on how nice your pair of callipers is, of course).

Once this is all done, its time to go to the polymer testing room. Its just a few steps down the hall from lab, hooray! The MTS is in this room. It’s a pretty big machine with a semi-complex start up procedure.

First, the cooling water has to be turned on. Much how brake pads get hot after driving, this machine would get hot without cooling water because of the sheer amount of force and mechanical energy involved.

After the cooling water is turned on and the computer file is set to the correct procedure and prepared to run (this part is not super interesting, nor do I have handy-dandy screenshots of it, so I’ll skip over it) it is time to load the sample into the machine. I imagine the MTS as two little jaws, which, to be fair, is more or less exactly what they look like. Or, sideways, I kind of imagine it like a crab.

 

The MTS System

The MTS System

 

Loading the sample into the machine involves making sure the dogbone is perfectly secure. The dogbone sits between these plates that the “jaws” of the MTS clamp onto. Extra C-clamps are added so that the dogbone sample does not twist out of the plates.

The plates with a dogbone sample in them:

And a loaded sample, with clamps, not yet stretched:

 

Ready to go!

Ready to go!

 

 

Now comes the fun part, and I’m not being sarcastic — this part is seriously fun to watch. I start the run and the bottom clamp starts to pull the sample. And it stretches for a loooooooooonnngg time. Eventually, it breaks (and I usually jump a little, not expecting it).

This is a sample that has broken:

 

However, if the extra C-clamps aren’t added the dogbone can wiggle out of the plates without breaking, resulting in a really long dogbone. I made this mistake once — it looks like this and the data cannot be analyzed very easily:

 

This was an unfortunate circumstance, but 99.99% of the time the dogbone does break and the data goes on to be analyzed.

From there on, the MTS file is transferred to our lab folder and analyzed using the original measurements of the dogbone and the datafile, through a MATLab program that tells us ultimate strain, yield, and the break point. The remaining bits of the dogbone go on to become CIV samples, MALLS samples and TGA samples.

So that’s how its done, and this is what it means:

The above plot is a graph of Ultimate Strain, the % Elongation at the dogbone’s breaking point against the Correct Inherent Viscosity, which I have posted about before. The data points to the far right are not our focus here. The cluster between 0.4 dL/g and 1.6 dL/g, however, is of interest. There is little trend if you try to look at this graph from a linear perspective, but a quadratic/parabolic fit slows trend. From 0.4 to 1 dL/g, there is an upward trend — as the viscosity is higher, the % elongation is higher. Remember that a higher viscosity indicates a higher molecular weight — therefore, the higher molecular weight (less degradation) means the polymer can stretch farther under more force without experiencing a breakage.

A few things affect the data from the MTS. The data we get includes yield strain, ultimate strain, and the break point. The dogbone will always break at its weakest point so the shape of the sample is important. Roughness also influences the break point in the MTS. There is often roughness on the edges of the dogbone. High contrast photographs show this roughness.

 

A previous project in Kranbuehl’s Lab, called Jotun, studied the effect of roughness on the stretching. The Jotun Study was a field study, so these polymer samples were taken from oil pipe coupons from oil fields and oil companies.

The difference, between two comparable (as identical as possible with the exception of a roughness variable) polymer samples, one rough and one smooth, is fairly evident just from a photograph:

High Contrast Dogbone Comparison

The dogbone on the top is nice and smooth — and stretched very far. The dogbone below it is the same one pictured one photograph above — its very rough, and broke early.

The smooth dogbone is referred to as Dogbone A. The graph of its elongation vs. physical stress is seen below:

MTS Data for Dogbone A

The Ultimate Strain for the smooth Dogbone (A) is a whopping 484% with a Yield Strain of 55%.

Now lets look at the rough Dogbone, referred to as Dogbone B:

MTS Data for Dogbone B

 

The Ultimate Strain for the rough Dogbone (B) is a comparably small 196% with a Yield Strain of 4%.

Because of all the tiny imperfections created by roughness on the side of the dogbone, espicially roughness perpendicular to the stretching direction, the tiny little imperfections become weak points, which become early break point. Because we know this effect, we know to sand the dogbones to smoothness to remove imperfections to get the best data for each polymer sample — data that represents the ageing of the polymer rather than the unpredictable roughness of the dogbone’s surface. However, this data shows the importance of the condition of an oil pipe — while our data could indicate a high molecular weight and no danger of breaking for a smoothed sample, it is important that the condition of the pipes considered — if the polymer used to make the pipe has an extremely rough outside, the danger of breakage becomes a concern much earlier on in any ageing process.