I make polymer coating on the elastic substrate, and twist the surface of the coating to observe the allochromasia processing and then measure the precise pressure or stain applied on the polymer coating that trigger the color transferring.
There are two problems:
1. If I twist directly on the coating surface, the coating polymer is easy to fall off
2. I put a piece of glass sheet over the coating, and twist on the glass. If I apply this method, how to measure the strain by the twist?
I suggest to review the TAInstruments page, I remember they have some equipment able to perform this testing, -namely rheometry- one of these includes a CCD, which must be helpful to you. Getting access to such equipment is another problem...
Do you want to measure the strain applied to an individual polymer strand? If an individual molecule, would optical tweezers work?
Thermal Machanical Analysis (TMA) or Dynamic Machanical Analysis (DMA) for Lab scale
Instron for industry
As mentioned above, TMA is a suitable method for the measurement of strain and stress on twisted polymer. It means you don't need to use the elastic substrate of glass sheet.
The quantity of the strain can determine your procedure. you can use both "Tensile test or DMTA on free fims of your coatings" or "Bending test on the coated substrate". In the second method you can evaluate the adhesion along with the starin that the coatings tolerate under the defined condition.
I suggest that you use strain Strain gauge.
http://www.hbm.com/en/menu/products/strain-gages-accessories/strain-gages-for-stress-analysis/
The reason that your polymer coating is getting off from the substrate is, I guess, the interaction between the film and the substrate is not strong enough at the interface. Additionally the elasticity of both material (substrate and polymer coating) should be close to each other. If you want to keep the polymer coating on the surface for your application you should first check how strongly they interact at the interface. On the other hand, if you are only interested in measuring the strain at which your polymer coating switch the color, you should prepare a free standing polymer film and use one of the techniques suggested above.Because when you apply a strain on a polymer film coated elastic substrate, the result will be mainly coming from the substrate, since the polymer coating is so thin it will have almost no contribution.
And if you measure the strain before only on elastic substrate and after measure on complete part with polymer coated ?
Coat a release paper or a PET film and peel of the coating from the substrate and measure strain on the pure coating with DMA or with tensile testing.
@ Mr. Cruz. The approach you suggested (first measuring the blank material, then subtracting or comparing with the result after the addition of analayte or material of interest) is valid for almost all scientific measurements if blank and analayte are in the same range... In this case I just want to draw your attention that the contribution of polymer film can be negligible because it is very thin comparing the substrate.... I may explain it in a different way. Imagine that you want to measure the breaking force of a piece of paper and you use an Iron or a Steel stick as a blank. First you measure the breaking force of Steel stick and then cover it with paper and measure the breaking force again. The force measured in later case wouldn't be different (or will not be detectable) than the first measurement. The contribution of paper in the total measured breaking force would be so small, and can not be compared in this way. I hope now it is more clear now.
Interaction between coating polymer and the substrate in your case is so weak, may be measureing with atomic force microscopy (AFM) is suitable to solve your problem.
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