As it is a known fact that in order to determine the sol-gel point in a hydrogel by rheology we should look at a point where there is a crossover between G' and G''. But when I do a temperature sweep at the sol-gel transition temp[erature both G' and G'' increases but they never cross. Is there any explanation for this?
Dear Mithra Kishore,
This seems to be an interesting problem in rheology.
The problem seems like, the amplitude might be out of the LVE region (Higher than LVER limit) and therefore it causes increase in G" too (causes shearing of the gel)...check that the amplitude(shear strain %) is lower or within the LVER and run it.. And also the frequency.
Alternatively , you could also plot the viscosity against temperature, and once you obtain a curve, you could try applying differential function (similar to differential in TG/DTA) to get the accurate sol-gel transition temperature.
If you could post the temp sweep graph, it might be useful to check.
Thank you and keep updated..
The gel point is usually defined as gel TIME, i.e. the time point, where a viscous gel is transformed into an elastic solid. Viscosity increases to infinity, whlile modulus becomes finite form zero. I would suggest you to perform ISOTHERMAL tests at various constant temperatures and wathc the change of G' and G".
From the graph i understand that , your material, liquifies at around 32 to 37 C.. Is this correct?
And if the above statement is correct, then I would presume you had ran the tests with constant amplitude and constant frequency. And ya, you got a good temp Vs Viscosity curve. Plot it separately and apply a differentiation to that curve, you will get the exact value of the transition temperature.. Thats kind of the simplest way although there are lot of other ways to find.
From the graph i understand that , your material, liquifies at around 32 to 37 C.. Is this correct?
And if the above statement is correct, then I would presume you had ran the tests with constant amplitude and constant frequency. And ya, you got a good temp Vs Viscosity curve. Plot it separately and apply a differentiation to that curve, you will get the exact value of the transition temperature.. Thats kind of the simplest way although there are lot of other ways to find.
Ad Mithra. This is a delicate question. You meet the same problem in taking isothermal DSC curves of chemically reactive resins. The problem is to define the starting point in kinetics. The problem is that the recation proceeds even during the warm-up period. it can be eliminated only if the test sample mass is very small, i.e. the thermal inertia is slow. If you wait with starting the measruement until the thermal equilibrium is achieved, your reference point ((the first one taken) will not be the real "zero" situation. I would suggest you to try to measure the rheological properties during the warm-up period, as then you may have superfluous data which can be discarded later. The ideal situation is to find a temperature (or a set of temperatures) where the warm-up period is definitely shorter than the thermal eqilbiratio time. (You should record the course of the temperature even during the warm-up period).
I found a very good tutorial from TA on the rehology of thermosets. I attach it, not because I do nos suppose that you know it, but only as ana additional, well-visualized information.
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