I am trying to perform rheology experiments on a cross-linked polymer gel to measure its shear modulus (G) and its G' G" response to an oscillatory stress on an oscillatory rheometer. My polymer gels are swollen, and therefore I have little or no control of the height of the sample. I use a 40 mm Al parallel plate to perform the experiment on the TA instruments Rheometer AR 2000. This geometry has an preset gap of 5 mm. Even if I set a gap manually, prior to running the experiment as I have to direct the instrument to the geometry gap my gel gets squished. As far as I understand, the gap is crucial to the calculations the instrument makes for the measurement. What is a suggested work around for the same?
Dear Uma
There are two ways for measuring the dynamic moduli G', G'' for your gels without non-defined deformation history impact, I suggest:
1.) you may try preparing (cross linking) your polymer system in the prefixed plate/plate gap to have a "stress/deformation-free" starting condition guaranteed.
2.) you may try to get access to a rheometer which enables you to fix the gap height in a normal force controlled manner. This would allow you to have a well defined minimal pre-compression force fixed, which guarantees full plate contact of your gel sample and no destructive compressive deformation but only well defined, small elastic deformation.
Regards
Erich
Dear Uma
There are two ways for measuring the dynamic moduli G', G'' for your gels without non-defined deformation history impact, I suggest:
1.) you may try preparing (cross linking) your polymer system in the prefixed plate/plate gap to have a "stress/deformation-free" starting condition guaranteed.
2.) you may try to get access to a rheometer which enables you to fix the gap height in a normal force controlled manner. This would allow you to have a well defined minimal pre-compression force fixed, which guarantees full plate contact of your gel sample and no destructive compressive deformation but only well defined, small elastic deformation.
Regards
Erich
Thanks for your reply! I was wondering if you were saying that one cannot preset the gap on the AR 2000
The easiest way around your issue is to setup your sample as you currently and either use Prof. Windhab's suggestion of a nominal compressive normal force to ensure contact (or whatever method you are currently employing to ensure the initial contact) and then run your experiment. The software (I'm assuming TRIOS) will then flag a pop-up along the lines of 'the instrument is not currently at the measuring gas, do you want to proceed to measuring gap' and you just click no - the rheometer will then run and use the current gap (and not the preset gap) in its calculations. I would suggest, if 5mm isn't large enough you may want to consider using a 60mm parallel plate since, as a rough rule of thumb, you want the maximum gap to be around a tenth of the plate diameter ...
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