PVAc (M WT 80000 Da), composite with nanocellulose fibrils prepared by solvent casting.
I observe a drastic decrease in the Tg of nanocomposites, 1 day after they are prepared; after which the Tg begins to recover slowly. Could both these phenomenon (sudden decrease and subsequent recovery) be explained by physical ageing? What experiments need to be carried out to ensure this?
The neat PVAc Tg is around 30 C and the composite Tg is 12 C, as measured from DSC. In the first thermal scans, I do not see a significant relaxation enthalpy.
Hi Ioannis, thanks a lot for your inputs. Carrying this forward, I would clarify a few points.
1) I understand that reduction of Tg is expected but we observe a drop of ~30 degrees, while literature reports no more than 5 degrees (although for different systems). The huge drop in our case is what intrigues me !!!
2) The 'equilibrium state' argument is also perfectly valid, but in our case the Tg trend was: day 0 - 35C, day 1- 10C, day 3 - 12 C, day 7 - 14 C. With my limited knowledge, I would assume that structural inequilibrium will force the system to move towards a more stable conformation, which would give rise to a constant trend in Tg (increasing or decreasing). Could the huge drop followed by slow recovery be explained (in theory) by molecular mobility? We know for a fact that the PVac/cellulose interactions are poor.
Since your area of expertise seems to be Physics, I will ask this additional naive question. Assuming there is an aged material and I do the following dynamic scans in DSC (a) heat well above Tg (b) cool rapidly (c) heat again over Tg. Now, if we compare the data from scans (a) and (c), shouldn't curve (c) be exactly the same as 'unaged' material?. Is it wrong to assume that the affect of ageing (whether decrease or increase in Tg) should be seen only in the scan (a).
Thank you very much for the discussion.
Thanks again. One quick question to follow up on the last point. Let's say, we do a DSC scan on the same material but aged for 1, 3, 5, 10 days. We further assume that ageing causes reduction in Tg of this material. Now, in the DSC run, the first scan of this different aged material will show us that the Tg is reducing (?) but the second scan for all the different aged materials should give us the exact same Tg, right?
Ioannis, thank you very much for your help with this. It was an extremely helpful discussion. Please let me know if I can be of any help to you.
Good luck !!
The explanation of 2-part Tg response may be due the phase change.
Drop of Tg: dispersion of unstable nano-cellulose fibrils phase
Recovery of Tg: forming of new phase (unknown composition)
To verify, you may try X-ray diffraction experiment during aging to estimate amount of crystalline component in blend.
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