Synergistically Improved PIM-1 Membrane Gas Separation Performance by PAF-1 Incorporation and UV Irradiation

Super-glassy polymer membranes have suffered from the trade-off relationship between permeability and selectivity for gas separation applications, despite the fact that membrane technology exhibits remarkable energy efficiency advantages over other...


Gel Permeation Chromatography
We used a Waters Alliance e2695 liquid chromatograph equipped with a Waters 2414 differential refractometer and 3× mixed C and 1 mixed E PL gel columns (each 300 mm × 7.5 mm) to obtain Gel Permeation Chromatography (GPC) for PIM-1 and UV irradiated PIM-1 from our laboratories. The eluent was tetrahydrofuran (THF) at 30 °C (flow rate: 1 mL min -1 ). Number (Mn) and weight-average (Mw) molar masses were evaluated using Waters Empower Pro software. The GPC columns were calibrated with low dispersity polystyrene (PSt) standards (Polymer Laboratories), and molar masses are reported as PSt equivalents. A third order polynomial was used to fit the log Mp vs time calibration curve, which was linear across the molar mass range 2 × 10 2 to 2 × 10 6 g mol −1 .

Table S1
Gel

Isotherm Adsorption Curves
BET surface areas were calculated from nitrogen isotherms at 77 K. The BET surface area for PAF-1 is 3435 m 2 /g. Figure S1: N 2 adsorption isotherm for the prepared PAF-1

Thermal Gravimetric Analysis (TGA)
Thermalgravimetric analysis of membranes was carried out using a Mettler Toledo TGA 2 STAR e System thermogravimetric analyser from 50 °C to 800 °C at 10 °C/min under 50 ml/min nitrogen.

Membrane gas performance before and after UV irradiation -Single Gas
Pure gas permeabilities for H 2 , N 2 , CH 4, and CO 2 were calculated by using the constant volume and variable pressure method. The gas permeability is determined from the rate of permeate pressure increase (dp/dt) once permeation reaches a steady state, according to equation 1. Where: P refers to the permeability of a membrane to a gas and its unit is in Barrer (1 Barrer = 1 × 10 -10 cm 3 (STP)cm/cm 2 seccmHg); V is the volume of the permeate chamber (cm 3 ), L is the film thickness (cm). A is the effective membrane area (cm 2 ); T is the temperature (K), and P 2 is the feed gas pressure (psia).
Ideal selectivity (α A/B ) is stated as the ratio of single gas permeability for a given gas pair.

Mixed Gas Performance
Mixed gas permeabilities for CO 2 and CH 4 (50/50 mole ratio) at a partial pressure of 2 bar (total pressure 4 bar) and 35 °C were calculated by using the constant pressure method as illustrated in previous work. 6 A custom-built permeation cell, which contains a flow distributor, was used to prevent the concentration polarization at the upstream face of the membrane. The downstream pressure was atmospheric (0 psig), and a carrier gas (Helium) was used to sweep the permeate gas molecules away from the membrane surface to the gas chromatograph (GC). The gas composition stream was determined by an Agilent 6890 GC (Agilent Technologies, Santa Clara, CA, USA) with a thermal conductivity detector (TCD). All data from these measurements were collected when the steady-state transmembrane flux was reached, and the stage cut (i.e., the ratio of the feed flow rate to the permeation rate) was less than 0.1 %. Permeability was calculated using the following equation respectively; is the mole fraction of helium in the permeate stream, is the feed stream 2 pressure, is the permeate stream pressure, A is the area of the membrane, and L is the membrane 1 thickness. Due to the different test methods, instruments, and conditions between single gas (constant volume, 25 °C) and mixed gas (constant pressure, 35 °C), the performance of single gas and mixed gas cannot be compared directly. However, this didn't take away the merits of PAF-1 and UV irradiation on PIM-1 membrane from mixed gas performance as similar synergistic effect observed compared to the single gas, that is, M3.0 exhibited a higher selectivity (66% up, 14 vs. 8.5) and retained permeability (6700 Barrer) compared to that of the pure PIM-1 membrane under mixed gas measurements.

Positron Annihilation Lifetime Spectroscopy (PALS)
Average pore size and their relative abundance were obtained using Positron Annihilation Lifetime Spectroscopy. The membrane samples were cut and stacked into two 2 mm bundles and placed on either side of the sealed positron source in a Mylar envelope ( 22 NaCl, 1.8 MBq). The samples were placed in a vacuum cell (5 x 10 -6 torr) between two EG&G Ortec fast-fast coincidence spectrometers. The timing resolution of the system was 240 ps, and a minimum of 5 files of 4.5 x 10 6 integrated counts were collected. The spectra were analysed using LT-v9 software 7 and fitted to 4 component lifetimes and a source correction (1.48 ns, 3.42%). The first lifetime (τ 1 ) was fixed to 0.125 ns and attributed to parapositronium (bound state of a positron and an electron with opposite spin) annihilation. The second component (τ 2 ) was due to free annihilation of the positron with free electrons within the sample. The longer lifetimes (τ 3 , τ 4 ) were due to ortho-positronium annihilation of the positron in a bound state of an electron in the same spin state. These longer lifetimes are due to annihilation within the free volume of the membranes and indicate the presence of a bimodal porosity in the PIM-1 and the composite samples. The lifetimes were calculated using the Relative Tao-Eldrup relationship. [8][9][10] The pore size distribution was a visual representation adapted using the PAScual software. 11 The fractional free volume (FFV) calculation 12 was based on the equation below.

Equation S2
= where Equation S3 = 4 Here, C is the empirical constant, 0.0018 Å, V PALS is the average volume of the pore elements calculated using the radius, R PALS determined from the PALS lifetime and I PALS is the associated Intensity. Separate pore size FFV (FFV 3 and FFV 4) and total FFV are listed in Table 1

X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Nova spectrometer (Kratos Analytical Inc., Manchester, UK) with a monochromated Al K α source at a power of 180 W (15 kV  12 mA), a hemispherical analyser operating in the fixed analyser transmission mode, and the standard aperture (analysis area: 0.3 mm × 0.7 mm). The total pressure in the main vacuum chamber during analysis was typically between 10 -9 and 10 -8 mbar. Survey spectra were acquired at a pass energy of 160 eV and step size of 0.5 eV. To obtain more detailed information about chemical structure, oxidation states, etc., high resolution spectra were recorded from individual peaks at 20 eV pass energy and step size of 0.1 eV, typically yielding a FWHM for the ester peak in PET of less than 0.85 eV during performance tests.
Each specimen was analysed at an emission angle of 0° as measured from the surface normal. Assuming typical values for the electron attenuation length of relevant photoelectrons, the XPS analysis depth (from which 95 % of the detected signal originates) ranges between 5 and 10 nm for a flat surface.
Depth profiling experiments were conducted using an Ar Gas Cluster Ion Source (GCIS; Kratos Analytical Inc. Minibeam 6) operated at a cluster size of Ar 1000 + with an impact energy of 10 keV, equating to a partition energy of 10 eV per atom. For the ion beam, a raster size of 1.4 x 1.4 mm 2 was employed. A stable beam current was confirmed prior to depth profiling by measuring the sample current on the earthed sample platen. Samples were etched five times for the following amount of time: 10 s, 20 s, 30 s, 90 s and 120 s.
Data processing was performed using CasaXPS processing software version 2.3.15 (Casa Software Ltd., Teignmouth, UK). All elements present were identified from survey spectra. The atomic concentrations of the detected elements were calculated using integral peak intensities and the sensitivity factors supplied by the manufacturer. A three-parameter Tougaard background 13 was employed using the default parameters for polymers (Cross Section: 551, 436, 3). A generalised Voigt lineshape was employed for synthetic components used in peak fitting, represented by LA() and LF() in CasaXPS, specifically LF(1,1,10,300), for all components with the exception of N1 -LF(1,1, 15,200). Binding energies were referenced to the C 1s peak at 284.8 eV (aromatic hydrocarbon).
Precision (ie. reproducibility) depends on the signal/noise ratio but is usually much better than 5%. The latter is relevant when comparing similar samples.

Membrane Long -term Study
The aging study of the polymer films involved storing the samples under ambient conditions after initial membrane performance measurements for as-cast samples. To reflect membrane intrinsic physical aging properties, the long-term study compared the performance of membranes aged 15 days and 60 days. The typical fast physical aging behaviour during the first two weeks is due to the rapidly collapsed excess free volume resulting from methanol soaking, as described in the work of Piannu et al. 17 Permeation measurements were carried out on these aged membranes after evacuating overnight to remove any adsorbed air and any other potential impurities from storage. For each test, single gas measurements were recorded sequentially using H 2 , N 2 , CH 4 , and CO 2 at 2 bar feed pressure, in duplicate (deviation with ± 10%) at 25 ± 1 °C. Before changing gases for permeation testing, the membrane and permeation system was evacuated under a low vacuum for at least 2 hours to completely remove prior gas and ensure the measurement accuracy.
As seen in Figure S10, either PAF-1 incorporated PIM-1, or UV treated PIM-1, or samples that combined these two functions, demonstrated a slower physical aging rate than the native PIM-1 membrane, which was consistent with previous studies. 18