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Different length of a polymeric string at a certain temperature in the cooling and heating phases
2

Different length of a polymeric string at a certain temperature in the cooling and heating phases

Different length of a polymeric string at a certain temperature in the cooling and heating phases

(OP)
Dear Engineers,

I am currently working on finding a model for a specific type of polymeric actuators. A linear motion is generated by heating a string made of Nylon 6/6. I heat my actuator and then let it cool down to the room temperature and measure its length. When I plotted the temperature-length trajectory, I observed that the actuator had gone a different trajectory in the cooling phase than the heating. As the temperature change rate was almost same for the both phases, I expected that it should have had the same length at a certain temperature in either phase.


Do you have any idea what is the cause? Is it the case for all polymers?

Thank you very much in advance.

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

First, how high did you heat it, and how slowly did you cool it?

One explanation might be that the nylon was plasticized by ambient moisture, and the heating phase dried the nylon. It would have been stiffer during the cooling phase once the moisture was removed.

Another explanation might be that the higher temperature increased the creep rate, and the string underwent permanent deformation.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

(OP)
@rickfischer51
Thank you for your reply.
I went to 90C which is beyond the glass transition of Nylon 6/6 in 15 seconds and then let it cool down in still air. It took around 30 seconds for the string to reach its initial temperature.
Regarding the moisture effect, I repeated the test for three cycles each one right after another but I observed the same results. I suppose that the moisture content would be removed at the first heating cycle and then the heating and cooling cycles exactly match each other unless the string could absorb it again in one minute which is the time interval between two actuation cycles. am I correct?
The start and points of the test are exactly same and I can say the permanent deformation is negligible. I am going to attach the result I am talking about.

@IRstuff
Thank you very much.

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

Polymer fibers are stretched when made in order to orient the polymer molecules and greatly increase the strength and stiffness of the fiber. Heating above Tg will allow this orientation to relax and thus the length of the fiber will shrink, permanently.

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

So the the deflection is increasing with each cycle? Am I reading your plot correctly?

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

(OP)
@rickfischer51
let me explain the figure a little bit. It shows only one actuation cycle with a heating phase in the top part of each trajectory and the cooling phase in the bottom. each plot, with its particular color, corresponds to a different actuation current, as they are indicated in the legend. So, the deflection is different in the cooling phase than the heating but it is same for different actuation cycles with a fixed actuation current.

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

Those deflections are a result of different temperature settings, by my reading. The curves look very similar to the hysteresis curves in at least one of the papers on the subject:
https://www.researchgate.net/profile/Jun_Zhang199/...

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RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

(OP)
@IRstuff

Thank you for the paper. I went through the paper.But it just proposed a model for hysteresis without a physical explanation.

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

How accurately is the temperature being measured? Would temperature lag between the reading instrument and the actual polymer temperature explain the hysteresis?

RE: Different length of a polymeric string at a certain temperature in the cooling and heating phases

(OP)
@btrueblood

If it was the case, I guess there would be the same lag for both the cooling and heating, so it wasn't such a difference in the Temp-Disp plot. Am I correct?

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