Inelastic and quasielastic neutron scattering on microporous polymers for green separation processes

by Dr Andreas Schönhals (Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12489 Berlin, Germany)

Monday 20 Nov 2023, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. These polymers are characterized by a high permeability and reasonable permselectivity. The latter point is somehow surprising because for microporous systems a more Knudson-like diffusion is expected then a size dependent temperature activated sieving process. It was argued in the framework of a random gate model that molecular fluctuations on a time scale from ps to ns are responsible for the permselectivity [1].

Here series of polymers of intrinsic microporosity (PIMs) as well as microporous polynorbornenes with bulky Si side groups and a rigid backbone are considered. The polymers have different microporosity characterized by high BET surface area values.

First inelastic time-of-flight neutron scattering measurements were carried out to investigate the low frequency density of state (VDOS). The measured data show the characteristic low frequency excess contribution to the VDOS above the Debye sound wave level, generally known as the Boson peak in glass-forming materials. It was found that the frequency of the maximum position of the Boson peak correlates with the BET surface area value [2].

Secondly elastic scans as well as quasielastic neutron scattering measurements by a combination of neutron time-of-flight and backscattering have been out [3,4]. A low temperature relaxation process was found for both polymers. This process was assigned to the methyl group rotation. It was analysed in terms of a jump diffusion in a three-fold potential. The analysis of the dependence of the elastic incoherent structure factor on the scattering vector yields the number of methyl groups which might be immobilized.

The neutron scattering experiments were accompanied by fast scanning calorimetry and broadband dielectric investigations as well as atomistic molecular dynamic simulations.

 

 

[1] R. Inoue, T. Kanaya, T. Masuda, K. Nishida, O. Yamamuro Macromolecules 45, 6008 (2012)

[2] R. Zorn, P. Szymoniak, M. A. Kolmangadi, A. Wolf, D. Alentiev, M. Bermeshev, M. Böhning, A. Schönhals Physical Chemistry Chemical Physics 22, 18381 (2020)

[3] A. Schönhals, P. Szymoniak, M. A, Kolmangadi, M. Böhning, M. Zamponi, B. Frick, M. Appel, G. Günther, M. Russina, D. Alentiev, M. Bermeshev, R. Zorn, R. Journal of Membrane Science 642 119972 (2022)

[4] R. Zorn, W. Lohstroh, M. Zamponi, W. J. Harrison, P. Budd, M. Böhning, A. Schönhals, Macromolecules 53 6731-6739 (2020)