Hi all,

I performed relaxation measurements on 1 mm thick gels using a rheometer (parallel plates, 25 mm diameter). I apply a strain of 10%, and a normal force of 2N to ensure good contact between the gel and the geometry during measurement.

I repeat this measurement at different temperature, from 80 to 30°C.

The relaxation modulus is negative during my first measurement, at 80°C (the graph is attached).

During this measurement, the gap decreases from 0.8 to 0.7 mm to maintain a normal force of 2 N. This is not the case in further measurements at lower temperatures.

Could this significant gap decrease explain the negative relaxation modulus values? How can this problem be remedied during measurements at 80°C?

Thank you!