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| Home > Publications database > Determination of the cooperativity length in glass forming liquids and polymers |
| Conference Presentation (Invited) | FZJ-2024-01595 |
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2023
Abstract: Although the idea of a ‘characteristic’ or ‘cooperativity’ length scale ξ related to the glass transition is now wide-spread, there is much less consensus on whether this length scale can be related to thermodynamic fluctuations and, if yes, whether one has to consider temperature fluctuations δT. The crucial experiment to this end has to compare values of ξ from ‘thermodynamic’ formulae to independent values from structural-dynamics experiments.An experiment with the aim of determining the cooperativity length ξ in glass forming materials was proposed some time ago by Ernst Donth [1]. The basic idea of this experiment is to assign a length scale to the AC-calorimetric relaxation time using the spatial resolution of quasielastic neutron scattering. The main problem is to find a range of relaxation times that is accessible by both methods. From the very beginning it is clear that only neutron-spin echo (NSE) is suited for this task. This raises the additional problem that incoherent scattering has to be measured. Therefore, significant progress in the performance of NSE as well as AC calorimetry was required to conduct this experiment.A first experiment of this kind was realised on a glass-forming liquid, propylene glycol (PG) [2]. The result was that agreement with the thermodynamic calculations was better if temperature fluctuations were accounted for. Nevertheless, in PG the difference between the two thermodynamic estimates is small and the dynamics of the methyl groups overlaps with the α relaxation. Therefore, a material with a higher ‘contrast’ between the alternatives was of interest and used in a second experiment, poly(ethylmethacrylate) (PEMA). In addition, neutron backscattering experiments showed that the methyl group dynamics is better separated in PEMA. Finally, the experiment benefitted from the upgrade of J-NSE with superconducting coils. The new results show a clear agreement with the thermodynamic calculation involving temperature fluctuations.[1] E. Donth, Eur. Phys. J. E 12, 11 (2003).[2] Y. Z. Chua, R. Zorn, O. Holderer, J. W. P. Schmelzer, C. Schick, E. Donth, J. Chem. Phys. 146, 104501 (2017).
Keyword(s): Basic research (1st) ; Soft Condensed Matter (2nd)
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