Abstract
The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EA-TB(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.
Highlights
Green key technology for the separation of gasses is membrane processes employing glassy polymers as materials for the active separation layer
The research has led to novel classes of polymers characterized by an exceptional high free volume as the polymers of intrinsic microporosity (PIMs) [12,13]
Detailed comparison is made to the VDOS of PIM-1 [56] and that of microporous polynorbornenes [54]
Summary
Green key technology for the separation of gasses is membrane processes employing glassy polymers as materials for the active separation layer. For these polymers from the group of the polyacetylenes, the permeability is orders of magnitude higher compared to conventionally glassy polymers but with low permselectivity values Based on these developments, the research has led to novel classes of polymers characterized by an exceptional high free volume as the polymers of intrinsic microporosity (PIMs) [12,13]. Based on the solution diffusion model the permselectivity of high-performance polymers for membranes can be discussed in a frame of discrimination to the effective diameter of the considered gas molecule. Quasi-elastic neutron scattering studies have been reported for PIM-1 [31] and for highly permeable polynorbornenes [32] In these investigations the role of localized mobility is emphasized for gas transport through microporous polymers. Detailed comparison is made to the VDOS of PIM-1 [56] and that of microporous polynorbornenes [54]
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