Molecular Mobility and Gas Transport Properties of Mixed Matrix Membranes Based on PIM-1 and a Phosphinine Containing Covalent Organic Framework

Polymers with intrinsic microporosity (PIMs) are gaining attention as gas separation membranes. Nevertheless, they face limitations due to their pronounced physical aging. In this study, a covalent organic framework containing λ5-phosphinine moieties, CPSF-EtO, was incorporated as a nanofiller (concentration range 0–10 wt %) into a PIM-1 matrix forming dense films with a thickness of ca. 100 μm. The aim of the investigation was to investigate possible enhancements of gas transport properties and mitigating effects on physical aging. The incorporation of the nanofiller occurred on an nanoaggregate level with domains up to 100 nm, as observed by T-SEM and confirmed by X-ray scattering. Moreover, the X-ray data show that the structure of the microporous network of the PIM-1 matrix is changed by the nanofiller. As molecular mobility is fundamental for gas transport as well as for physical aging, the study includes dielectric investigations of pure PIM-1 and PIM-1/CPSF-EtO mixed matrix membranes to establish a correlation between the molecular mobility and the gas transport properties. Using the time-lag method, the gas permeability and the permselectivity were determined for N2, O2, CH4, and CO2 for samples with variation in filler content. A significant increase in the permeability of CH4 and CO2 (50% increase compared to pure PIM-1) was observed for a concentration of 5 wt % of the nanofiller. Furthermore, the most pronounced change in the permselectivity was found for the gas pair CO2/N2 at a filler concentration of 7 wt %.


S2 Synthesis
Synthesis The synthesis of the sample was carried out according to the procedure below, based on that reported by Du et al 1 : Scheem S1: Reaction scheme of PIM-1.
After washing with acetone, the product was stirred in 1,4-dioxane for 30 min to remove low molecular weight oligomers and cyclic products, before washing again with acetone.The sample was then refluxed overnight in deionized water, stirred in methanol for 20 min and then dried at 100 o C for two days.The final yield of PIM-1 obtained was 22.06 g (95.9 %).

FTIR spectroscopy (ATR-IR)
ATR-FTIR spectroscopy was performed with a Nicolet 6700 equipped with SmartOrbit diamond module and DTGS detector.The spectra were recorded in a wavenumber range of 4500 to 450 cm -1 , averaging 32 scans with a resolution of 2 cm -1 .The obtained data were processed with ATR correction available with OMNIC 9 software (Thermo Fischer Scientific, Karlsruhe, Germany).

S4
FTIR spectroscopy was performed to study the possible incorporation and interaction of the fillers in the PIM-1 matrix (Figure S2).H-bonds between the alkoxy group (O-CH 2 CH 3 ) in EtO-CPSF and cyano ( C N) group can be formed.
-≡ Such bonds are much weaker than the covalent bonds but stronger than the typical intermolecular van der Waals interactions 2 .The formation of H-bonds can shift the characteristic peaks to lower wavenumbers 3 .Nonetheless herein no significant changes were observed between the FTIR spectra of PIM/CPSF-EtO and neat PIM-1 membranes.This is possibly due to the presence of mostly proton accepting bonds in both filler and matrix. 4gure S2.FTIR spectra of PIM-1, CPSF-EtO nanofiller and PIM-1/CPSF-EtO with 5wt% filler concentration.

Thermogravimetric analysis (TGA)
Thermogravimetric analysis was performed using a STA7000 Thermogravimetric Analyzers (Hitachi, Chidoya, Japan), employed with a horizontal dual balance.In the first TGA measurement samples of 3-5 mg were heated in alumina pans from room temperature to 1000°C (1273 K) at a heating rate of 10 K min -1 .Nitrogen was used as a purge gas up to approximately 600 °C (873 K) to avoid oxidation reactions.From 600 to 1000 °C (873 -1023 K), oxygen was used as purge gas to oxidize the sample completely.

S6
According to the TGA results neat PIM-1 and the MMMs exhibit similar thermal stability in the studied temperature window (Figure S5).The small weight loss below 733 K corresponds to the removal of moisture and volatile organic compounds 5 , whereas the significant weight loss at approximately 1000 K originates from the chemical degradation 6,7 .
The TGA curve for pure CPSF-EtO reveals a lower thermal stability than PIM-1.

Gas Transport properties
Table S1.Gas permeability (P), diffusion coefficient (D) obtained with the time-las setup for membranes at 35°C at 5bar.

Figure S4 .
Figure S4.Exemplary time-lag curve for of Nitrogen gas: downstream pressure p 2 vs. time for pure PIM-1 at 35 °C and 3 bar.

Figure S10 .
Figure S10.Permeability at 5 bar vs. kinetic diameter for all membranes.