Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering

Reiner Zorn; Paulina Szymoniak; Mohamed A. Kolmangadi; Richard Malpass-Evans; Neil B. McKeown; Madhusudan Tyagi; Martin Böhning; Andreas Schönhals

doi:10.3390/cryst11121482

Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering

Reiner Zorn, Paulina Szymoniak, Mohamed A. Kolmangadi, Richard Malpass-Evans, Neil B. McKeown, Madhusudan Tyagi, Martin Böhning, Andreas Schönhals^*

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

Abstract / Description of output

The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EATB(CH₃ ) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H₂ ) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m²/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH₃ ) is higher than that for the demethylated counterpart PIM-EA-TB(H₂ ). The additional contribution found for PIM-EA-TB(CH₃ ) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH₃ ) has methyl groups where PIM-EA-TB(H₂ ) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules.

Original language	English
Article number	1482
Journal	Crystals
Volume	11
Issue number	12
Early online date	29 Nov 2021
DOIs	https://doi.org/10.3390/cryst11121482
Publication status	E-pub ahead of print - 29 Nov 2021

Keywords / Materials (for Non-textual outputs)

Boson peak
Methyl group rotation
Neutron scattering
Polymers of intrinsic microporosity

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10.3390/cryst11121482

20220318_McKeown_crystals-11-01482-VORFinal published version, 2.53 MBLicence: Creative Commons: Attribution (CC-BY)

Cite this

@article{05a42b7b08aa4d8eb3f7d89830249e86,

title = "Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering",

abstract = "The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EATB(CH3 ) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H2 ) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m2/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH3 ) is higher than that for the demethylated counterpart PIM-EA-TB(H2 ). The additional contribution found for PIM-EA-TB(CH3 ) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH3 ) has methyl groups where PIM-EA-TB(H2 ) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules.",

keywords = "Boson peak, Methyl group rotation, Neutron scattering, Polymers of intrinsic microporosity",

author = "Reiner Zorn and Paulina Szymoniak and Kolmangadi, {Mohamed A.} and Richard Malpass-Evans and McKeown, {Neil B.} and Madhusudan Tyagi and Martin B{\"o}hning and Andreas Sch{\"o}nhals",

note = "Funding Information: Acknowledgments: Access to HFBS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under agreement no. DMR-1508249. Publisher Copyright: {\textcopyright} 2021 by the authors. Licensee MDPI, Basel, Switzerland.",

year = "2021",

month = nov,

day = "29",

doi = "10.3390/cryst11121482",

language = "English",

volume = "11",

journal = "Crystals",

issn = "2073-4352",

publisher = "Multidisciplinary Digital Publishing Institute (MDPI)",

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TY - JOUR

T1 - Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering

AU - Zorn, Reiner

AU - Szymoniak, Paulina

AU - Kolmangadi, Mohamed A.

AU - Malpass-Evans, Richard

AU - McKeown, Neil B.

AU - Tyagi, Madhusudan

AU - Böhning, Martin

AU - Schönhals, Andreas

N1 - Funding Information: Acknowledgments: Access to HFBS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under agreement no. DMR-1508249. Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

PY - 2021/11/29

Y1 - 2021/11/29

N2 - The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EATB(CH3 ) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H2 ) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m2/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH3 ) is higher than that for the demethylated counterpart PIM-EA-TB(H2 ). The additional contribution found for PIM-EA-TB(CH3 ) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH3 ) has methyl groups where PIM-EA-TB(H2 ) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules.

AB - The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EATB(CH3 ) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H2 ) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m2/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH3 ) is higher than that for the demethylated counterpart PIM-EA-TB(H2 ). The additional contribution found for PIM-EA-TB(CH3 ) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH3 ) has methyl groups where PIM-EA-TB(H2 ) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules.

KW - Boson peak

KW - Methyl group rotation

KW - Neutron scattering

KW - Polymers of intrinsic microporosity

U2 - 10.3390/cryst11121482

DO - 10.3390/cryst11121482

M3 - Article

AN - SCOPUS:85120703047

SN - 2073-4352

VL - 11

JO - Crystals

JF - Crystals

IS - 12

M1 - 1482

ER -

Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering

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Keywords / Materials (for Non-textual outputs)

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