Free Volume Size Distribution Research Articles

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The cover image is based on the Research Article Effect of hexyl‐branched backbone size on the size distribution and coalescence of free volume around an amphiphilic molecule by Sajjad Kavyani et al., https://doi.org/10.1002/pen.26705. image

Effect of hexyl‐branched backbone size on the size distribution and coalescence of free volume around an amphiphilic molecule

AbstractWe used molecular dynamics simulation to study the size distribution and coalescence of free volume around an amphiphilic molecule (nonyl ethoxylate (NE)) at a concentration of 0.5 wt% in blends of linear and branched polyethylene that contained a small four‐arm alkane (7,12 hexyl octadecane). The branched polyethylene chains had 10 and 82 hexyl branches/1000 backbone carbons. The coalescence dynamics (fluctuation of free volume in time) was quantified by the number of Fourier frequencies and the corresponding power (amplitude). In our previous work, we hypothesized that cavitation, the first step of environmental stress cracking observed experimentally, starts from the free volume coalescence at the interface between NE and polyethylene molecules and showed that hexyl branches in the branched polyethylene molecules suppress such coalescence, suggesting that cavitation, a much longer time scale process, could also be slowed down. The current work showed that the behavior of hexyl branches in branched polyethylene in a blend with linear polyethylene was similar to that in pure branched polyethylene, but that the hexyl branches in the 7,12 hexyl octadecane exhibited the opposite behavior. In particular, they intensified free volume coalescence, especially around the hydrophilic ethylene oxide segments of NE. The addition of 7,12 hexyl octadecane to branched polyethylene alone or blended with linear polyethylene does not seem to slow down free volume coalescence (cavitation), leading us to conclude that the effect of hexyl branches on the free volume coalescence around NE depends on the size of the backbone to which the branches are attached.Highlights Hexyl branches reduce free volume coalescence activities. The longer the backbone is, the stronger the hexyl branch effect. More coalescence activities occur on the hydrophilic segment of NE.

Enhanced gas separation by free volume tuning in a crown ether-containing polyimide membrane

Tuning coupled free volume and pore size distribution is of great significance for membrane-based gas separation. In the present work, a polyimide membrane containing bulky -CF3 group and crown ether ring structure was developed using 4, 4′ - (hexafluoroisopropylidene) diphthalic anhydride as dianhydride monomer, 2, 2′-bis [4-(4-aminophenoxy)phenyl] -hexafluoropropanane and di(aminobenze)-21-crown-7-ether as diamine monomers. Diamine monomer with large bulky -CF3 grant the polyimide membrane more free volume, which was helpful for gas permeability enhancement. Besides, crown ether could act as a pores size distribution tuning monomer, which endowed membranes with improved gas selectivity. Results showed that the introduction of crown ether into polyimide backbone exhibited a significant impact on free volume and pores size distribution. Typically, with the increasing crown ether content, the free volume decreased, whereas the relative available free volume ratio (FAVi/FAVj) achieved a remarkable increment. It’s also worth noting that the pores size distribution was changed by copolymerization with crown ether. The intensity of macro-pore gradually decreased accompanied with an increase in micro-pore structure. As a result, the gas selectivity significantly increased. Compared the membrane without crown ether, the maximum selectivity enhancement was achieved at 30 mol% di(aminobenze)-21-crown-7-ether content. It exhibited remarkable selectivity increments about 204.8%, 198.8%, 123.2%, and 115.4% for He/CH4, H2/CH4, CO2/CH4, and CO2/N2, respectively. These results took a huge step forward which nearly hit up to the 2008 Robeson’s upper bound, especially for CO2/CH4.

Free volume structure at interphase region of poly (ethylene oxide)-Al2O3 nanorods composites based solid polymer electrolyte and its direct correlation with Li ion conductivity

Poly (ethylene oxide), PEO, based solid polymer electrolytes (SPEs) filled with nanosize passive fillers are shown to have higher ionic conductivity compared to pure PEO based SPEs. The enhancement in ionic conductivity has been attributed to an alternate path of ion transport through larger size free volumes/voids available for ionic conduction in the vicinity of nanofillers i.e. at the interphase region without experimental evidence. In order to investigate the exclusive role of molecular packing/free volume at interphase region on ionic conductivity, PEO based SPEs filled with different amounts (0–10 wt%) of Al2O3 nanorods have been prepared using solvent casting method. The variation in semicrystalline morphology has been investigated using differential scanning calorimetry and X-ray diffraction. Fourier transform infrared spectroscopy and scanning electron microscopy have been used to investigate the interfacial interaction, fracture morphology and nanofiller dispersion in SPEs. Free volume size distribution determined using positron annihilation lifetime spectroscopy becomes broader with the loading of nanorods confirming the creation of an interphase region with larger size free volumes. In case of 5 wt% Al2O3 nanorods, a bimodal free volume size distribution attributed to bulk and interphase region is observed to be directly correlated with the enhancement in ionic conductivity. The results have been compared with PEO-Al2O3 nanoparticles based SPEs. The lower ionic conductivity in case of PEO-Al2O3 nanoparticles based polymer electrolytes was observed to be consistent with the limited modifications observed in the free volume structure at the interphase region. A direct correlation between the free volume structures at interphase region with Li ion conductivity is obtained. The present study provides an experimental evidence of an alternate ion-transport path through the interphase region in polymer composite based SPEs.

Estimation of linear, ring, and star polyethylene viscosity through proper orthogonal decomposition and Voronoi tessellation analysis of molecular dynamics data

AbstractWe carried out molecular dynamics (MD) simulations on polyethylene (PE) with linear, ring, and four‐arm symmetrical star structures at various molecular weights (420 − 5700 g ⋅ mol−1) and temperatures (410−450 K). We then analyzed the MD data using the technique of proper orthogonal decomposition (POD) to obtain the time correlation functions of different eigenmodes, thereby calculating the viscosity of the aforementioned macromolecules. The time correlation functions show that the ring and star PEs relax much faster than their linear counterpart. Free volume size distributions and the mean fractional free volume of the equilibrated PE melts were determined by Voronoi tessellation (VT). The mean fractional free volume and the corresponding effective pressure were then used to calculate viscosity using a polymer free volume theory recently developed in our lab. The POD and VT approaches yielded similar viscosity values. Furthermore, they both successfully predicted the crossover in the molecular weight dependence of viscosity. It is interesting to note that the difference in the free volume parameters (ϕ+ − F) of various structures always fall within the range of 0.02−0.06. Here, F and ϕ+ signify the probability of a bead having enough free volume for the activation of diffusive motion or momentum transfer and the minimum required fraction of such beads, respectively. The temperature dependence of viscosity as obtained from the POD and VT approaches gave comparable apparent activation energy () values of 5.8−6.4 and 5.4−6.4 kcal ⋅ mol−1, respectively, for the three structures.

Free volume characteristics of 2,2‐bistrifluoromethyl‐4,5‐difluoro‐1,3‐dioxole‐co‐tetrafluoroethylene copolymers: Effect of composition and molecular weight

Abstract2,2‐bistrifluoromethyl‐4,5‐difluoro‐1,3‐dioxole‐co‐tetrafluoroethylene (PDD‐TFE) copolymer is a good candidate to prepare gas separation membranes with excellent permeability due to its free volume characteristics. However, the influence of PDD‐TFE copolymer structure on its free volume characteristics is less studied. In this paper, PDD‐TFE copolymers with different compositions and molecular weights were synthesized, and their free volume characteristics were analyzed by positron annihilation lifetime spectroscopy and a molecular dynamics simulation. It indicated that the molar fraction of PDD in copolymers had a significant effect on free volume characteristics, while the molecular weight of copolymers exerted a slight influence on free volume when the molecular weight exceeded a critical region (intrinsic viscosity [η] > 68 ml g−1). PDD‐TFE copolymers with greater PDD molar fractions (i.e., 72% and 84%) showed bimodal distributions in positron lifetime and free volume size distributions, while PDD‐TFE copolymers with lower PDD molar fractions (i.e., 27% and 35%) exhibited a single peak. The long‐lifetime parameter τ3 was assigned to micro‐cavities formed by [‐(TFE)y‐PDD‐] segments and τ4 was attributed to micro‐cavities formed by [‐(PDD)x‐TFE‐] segments. The cis and trans transitions of PDD led to a local multilayer spiral structure with a 2.6–4.3 Å layer spacing, which would also increase the free volume of copolymers.

Novel semi-fluorinated poly(ether imide)s with benzyl ether side groups: Synthesis, physicochemical characterization, gas transport properties and simulation

The present work reports the preparation of several new poly(ether imide)s (PEIs) from the benzyl ether substituted, fluorinated diamine monomer, 4,4′-((2′,5′-bis(benzyloxy)-3,3′'-bis(trifluoromethyl)-[1,1′:4,4′'-terphenyl]-4,4′'-diyl)bis(oxy))dianiline (TADBE) with different fluorinated and non-fluorinated aromatic dianhydrides as the materials for membrane-based gas separation applications. The PEIs were readily dissolved in numerous organic solvents and cast into freestanding flexible films with tensile strength up to 159 MPa and elongation at break up to 106%. The films were optically transparent and showed dielectric constant as low as 2.47 at 1 MHz and 30 °C for the polymer derived from 6FDA (PEI-b). The structural elucidation of the polymers was done by 1H NMR and FTIR studies. The weight average molecular weight of the polymers was as high as about 255000 g/mol, and dispersity values were with the range as expected for condensation polymers (2.1–2.8). The thermal stability of PEIs was tested carefully to judge their ability for membrane-based application. The polymers showed 10% weight loss temperature above 440 °C in the air with high Tg values (as high as 252 °C) under nitrogen. Three independent methods were used to estimated the gas permeability coefficients of the PEIs. The structure-property correlations on gas transport properties and chemical structure and different physical properties of these PIEs were established. The best combination of selectivity and gas permeability (P) value is seen for the polymer with 6FDA fragment (PEI-b). The group contribution method predicts the permeability coefficients exceptionally well in all three PEIs and for different gases. Molecular dynamics provides a better insight on free volume, free volume size distribution, and morphology of the PEIs.

Structure and Properties of High and Low Free Volume Polymers Studied by Molecular Dynamics Simulation

Using molecular dynamics, a comparative study was performed of two pairs of glassy polymers, low permeability polyetherimides (PEIs) and highly permeable Si-containing polytricyclononenes. All calculations were made with 32 independent models for each polymer. In both cases, the accessible free volume (AFV) increases with decreasing probe size. However, for a zero-size probe, the curves for both types of polymers cross the ordinate in the vicinity of 40%. The size distribution of free volume in PEI and highly permeable polymers differ significantly. In the former case, they are represented by relatively narrow peaks, with the maxima in the range of 0.5–1.0 Å for all the probes from H2 to Xe. In the case of highly permeable Si-containing polymers, much broader peaks are observed to extend up to 7–8 Å for all the gaseous probes. The obtained size distributions of free volume and accessible volume explain the differences in the selectivity of the studied polymers. The surface area of AFV is found for PEIs using Delaunay tessellation. Its analysis and the chemical nature of the groups that form the surface of free volume elements are presented and discussed.

Polymer of Intrinsic Microporosity (PIM-1) Membranes Treated with Supercritical CO2

Polymers of intrinsic microporosity (PIMs) are a promising membrane material for gas separation, because of their high free volume and micro-cavity size distribution. This is countered by PIMs-based membranes being highly susceptible to physical aging, which dramatically reduces their permselectivity over extended periods of time. Supercritical carbon dioxide is known to plasticize and partially solubilise polymers, altering the underlying membrane morphology, and hence impacting the gas separation properties. This investigation reports on the change in PIM-1 membranes after being exposed to supercritical CO2 for two- and eight-hour intervals, followed by two depressurization protocols, a rapid depressurization and a slow depressurization. The exposure times enables the impact contact time with supercritical CO2 has on the membrane morphology to be investigated, as well as the subsequent depressurization event. The density of the post supercritical CO2 exposed membranes, irrespective of exposure time and depressurization, were greater than the untreated membrane. This indicated that supercritical CO2 had solubilised the polymer chain, enabling PIM-1 to rearrange and contract the free volume micro-cavities present. As a consequence, the permeabilities of He, CH4, O2 and CO2 were all reduced for the supercritical CO2-treated membranes compared to the original membrane, while N2 permeability remained unchanged. Importantly, the physical aging properties of the supercritical CO2-treated membranes altered, with only minor reductions in N2, CH4 and O2 permeabilities observed over extended periods of time. In contrast, He and CO2 permeabilities experienced similar physical aging in the supercritical treated membranes to that of the original membrane. This was interpreted as the supercritical CO2 treatment enabling micro-cavity contraction to favour the smaller CO2 molecule, due to size exclusion of the larger N2, CH4 and O2 molecules. Therefore, physical aging of the treated membranes only had minor impact on N2, CH4 and O2 permeability; while the smaller He and CO2 gases experience greater permeability loss. This result implies that supercritical CO2 exposure has potential to limit physical aging performance loss in PIM-1 based membranes for O2/N2 separation.

Transport Diffusion of Light Gases in Polyethylene Using Atomistic Simulations.

We explore the temperature dependence of the self-, corrected-, and transport-diffusivities of CO2, CH4, and N2 in a polyethylene (PE) polymer membrane through equilibrium molecular dynamics simulations. We also investigate the morphology of the polymer membrane based on the intermolecular radial distribution function, free volume, and pore size distribution analysis. The results indicate the existence of 1.5-3 Å diameter pores in the PE membrane, and with the increase in the temperature, the polymer swells linearly with changing slope at 450 K in the absence of gas and exponentially in the presence of gas. The gas adsorption isotherms extracted via a two-step methodology, considering the dynamics and structural transitions in the polymer matrix upon gas adsorption, were fitted using a "two-mode sorption" model. Our results suggest that CO2 adsorbs strongly, whereas N2 shows weak adsorption in PE. The results demonstrate that CO2 is more soluble, whereas N2 is least soluble. Further, it is found that an increase in the temperature negatively impacts the solubility of CO2 and CH4 but positively for N2; this reverse solubility behavior is due to increased availability of pores accessible to N2, which are kinetically closed at the lowest temperatures. The reported self-diffusivities of the gases from our simulations are on the order of 10-6 cm2/s, consistent with the experimental evidence, whereas transport-diffusivities are 2 orders of magnitude higher than self-diffusivities. Furthermore, the temperature dependence of the self-diffusivity follows Arrhenius behavior, whereas the transport-diffusivity follows non-Arrhenius behavior having different activation energies in low and high temperature regions. Also, it is seen that loading has little effect on the self- and corrected-diffusion coefficients of all gases in the PE membrane.

Permeation and separation of SO2, H2S and CO2 through thermally rearranged (TR) polymeric membranes

Abstract The permeabilities of sulfur dioxide and hydrogen sulfide are reported here for two copolymer based TR membranes, as a function of feed partial pressure and temperature. The two TR membranes are formed from the precursor co-polyimide 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4,4′-diaminobiphenyl (HAB) and 2,4,6-trimethyl-m-phylenediamine (DAM) (6FDA-HAB-DAM), where one of the membranes undergoes crosslinking with 3,5-diaminobenzoic acid (DABA) during TR. It was observed that SO2 permeability decreased with increasing partial pressure and temperature in both TR membranes. This is indicative that SO2 permeability is dominated by SO2 solubility within the TR membranes, and is attributed to the high condensability of SO2. Both polymeric membranes displayed selectivity for SO2 against CO2, with separation performance similar to other classes of polymeric membranes. H2S permeability was observed to decrease with increasing partial pressure and temperature for the cross-linked TR (XTR) membrane. The non-cross-linked TR membrane displayed constant H2S permeability with pressure and permeability increased with increasing temperature. Hence, permeability within the XTR membrane was H2S solubility dominated, while in the non-crosslinked TR membrane permeability was H2S diffusivity dominated. This was attributed to the difference in free volume and cavity size distribution in the TR membrane as a result of crosslinking. It was observed that both membranes displayed H2S/CO2 selectivity of less than one, indicative that they favor CO2 over H2S.

Positron annihilation characterization of free volume in micro- and macro-modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics

Free volume and pore size distribution size in functional micro and macro-micro-modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics are characterized by positron annihilation lifetime spectroscopy in comparison with Hg-porosimetry and scanning electron microscopy technique. Positron annihilation results are interpreted in terms of model implication positron trapping and ortho-positronium decaying. It is shown that free volume of positron traps are the same type for macro and micro modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics. Classic Tao-Eldrup model in spherical approximation is used to calculation of the size of nanopores smaller than 2 nm using the ortho-positronium lifetime.

Synthesis and properties of antibacterial polyurethane with novel Bis(3-pyridinemethanol) silver chain extender

To synthesize antibacterial polyurethane (PU) films, a novel chain extender, bis(3-pyridinemethanol) silver (BPDS), and a PU prepolymer were also synthesized through coordination and covalent reactions. The effects of incorporating different amounts of BPDS were investigated by characterizing the BPDS/PU films using various instruments: FTIR, TGA, DSC, DMA, XRD, EDX, universal testing machine, contact angle goniometer, and positron annihilation spectroscopy (PALS). With a higher BPDS content, the glass transition temperature, final degradation temperature, and Young's modulus were all enhanced; however, the tensile strength and elongation at break were both reduced. PALS analysis revealed that the more rigid and randomly packed structure of the BPDS/PU films at a higher BPDS content was characterized by a broader free volume size distribution that consisted largely of much smaller free volume holes. Quantitative tests with Staphylococcus aureus and Escherichia coli indicated excellent antibacterial properties, suggesting that the PU films could be used repeatedly.

Hole size distributions in cardo-based polymer membranes deduced from the lifetimes of ortho-positronium

To clarify the free volume size distributions of the cardo-based polymer membranes, where ortho-positronium (o-Ps) undergoes pick-off annihilation, the o-Ps lifetime distributions were analyzed by the LT9 programme. It was found that the cardo-based polysulfone membrane has much narrower o-Ps lifetime/hole size distributions than the cardo-based polyimide membranes with the 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) moiety. Further, the lifetime/hole size distributions of the cardo-based polymer membranes are appreciably broadened with increasing temperature. This suggests that in these membranes there are holes not only of different sizes but also of different thermal expansion coefficients. It is also shown that in a membrane with a wider hole size distribution the average o-Ps lifetime tends to be longer than would be expected from the correlation between the o-Ps lifetime and the total free volume for common polymers.

Physical Selectivity of Molecularly Imprinted polymers evaluated through free volume size distributions derived from Positron Lifetime Spectroscopy

The technique of imprinting molecules of various sizes in a stable structure of polymer matrix has derived multitudes of applications. Once the template molecule is extracted from the polymer matrix, it leaves behind a cavity which is physically (size and shape) and chemically (functional binding site) compatible to the particular template molecule. Positron Annihilation Lifetime Spectroscopy (PALS) is a well known technique to measure cavity sizes precisely in the nanoscale and is not being used in the field of MIPs effectively. This method is capable of measuring nanopores and hence suitable to understand the physical selectivity of the MIPs better. With this idea in mind, we have prepared molecular imprinted polymers (MIPs) with methacrylicacid (MAA) as monomer and EGDMA as cross linker in different molar ratio for three different size template molecules, viz. 4-Chlorophenol (4CP)(2.29 Å), 2-Nephthol (2NP) (3.36 Å) and Phenolphthalein (PP) (4.47Å). FTIR and the dye chemical reactions are used to confirm the complete extraction of the template molecules from the polymer matrix. The free volume size and its distribution have been derived from the measured o-Ps lifetime spectra. Based on the free volume distribution analysis, the percentage of functional cavities for the three template molecules are determined. Percentage of functional binding cavities for 4-CP molecules has been found out to be 70.2% and the rest are native cavities. Similarly for 2NP it is 81.5% and nearly 100% for PP. Therefore, PALS method proves to be very precise and accurate for determining the physical selectivity of MIPs.

Enhanced plasticity in Zr–Cu–Ag–Al–Be bulk metallic glasses

The dependences of bending plasticity on compositional adjustment and structural relaxation in Zr–Cu–Ag–Al–Be bulk metallic glasses (BMGs) were systematically studied. We revealed that with increasing Zr content, the plasticity was enhanced at the expense of glass forming ability and the shear transformation zone (STZ) volume became larger, while structural relaxation resulted in the embrittlement accompanied with the further enlarged STZ volume. Average positron lifetime, representing average free volume content, dropped with the increased Zr content and structural relaxation. These results indicate that neither STZ volume nor average positron lifetime can solely describe plastic deformability of BMGs. Analyzing positron annihilation lifetime spectra, a bimodal distribution of free volume size was observed. The smaller free volume shrinks with increased concentration and the larger free volume expands with decreased concentration due to the increase in the Zr content, while structural relaxation mainly reduces smaller free volume. The effects of free volume size distribution on the STZ activation and shear band propagation have been discussed. It has been demonstrated that combining the STZ volume with free volume size distribution can explain the plasticity evolution with composition and structural relaxation, which may shed light on understanding the deformation mechanism of BMGs.

Influence of hydrostatic pressure on dielectric properties of polyethylene/aluminum oxide nanocomposites

The effect of hydrostatic pressure on the dielectric characteristics of polyethylene/aluminum oxide nanocomposites is reported in the present work. Nanocomposite specimens with a thickness of 0.2 mm were placed in a hydrostatic press at pressures ranging from 50 to 200 MPa. Then, ac and dc breakdown and dielectric spectroscopy measurements were carried out on the original and treated specimens. In comparison to the dielectric properties of the original specimens, the results of the treated specimens demonstrate some interesting dielectric behaviors. For example, the breakdown strengths of the treated specimens are enhanced, and the permittivity and dielectric loss are reduced after the hydrostatic pressure treatments. It is found that the movements of molecular chains and polar groups are restricted especially in the interfacial region, leading to the reduction in permittivity and dielectric loss of specimens after hydrostatic pressure treatment. The improvement in breakdown properties is due to the reduction in free volume or a more uniform size distribution of free volume in the nanocomposites after hydrostatic pressure treatments.

Molecular dynamics simulation and positron annihilation lifetime spectroscopy: Pervaporation dehydration process using polyelectrolyte complex membranes

The micro-structure of various novel polyelectrolyte complex membranes (PECMs) was investigated by means of molecular dynamics (MD) simulation and positron annihilation lifetime spectroscopy (PALS). These PECMs that differed in their chemical structure design were applied to dehydrate a 90wt% ethanol aqueous solution by pervaporation, and their separation performance was correlated with their microstructure. Free-volume size and free-volume size distribution were determined both by the PALS and MD simulation techniques. To describe the free-volume shape and the polymer chain flexibility and stiffness, MD simulation analysis in terms of radial distribution function and mean square displacement was also conducted. Results obtained from PALS and those from MD simulation were in agreement with each other. These results were highly consistent with the chemical structure of the PECMs designed in this study and were demonstrated to correlate well with the membrane pervaporation separation performance.

Preparation and Characterization of Molecularly Imprinted Polymer for Selective Adsorption of 4-Chlorophenol Molecules by Physical Selectivity Method

Molecular imprinted polymer (MIP) with methacrylate acid (MAA) as monomer and ethylene glycol dimethacrylate (EGDMA) as cross-linker in different molar ratios has been prepared. The free volume size and distribution have been derived from the measured positron lifetime spectra in MIP. The free volume size distribution observed clearly indicates the influence of the amount of cross-linker in the sample. Present results suggest that the optimum monomer cross-linker ratio is 1:5 for this MIP. Fourier tranform infrared (FTIR) results confirm complete extraction of the 4-chlorophenol template molecules (4-CP) from the MIP with the absence of stretching frequencies at 648 and 825 cm–1 for the C–Cl group. The free volume distribution analysis indicates that the percentage of accessible cavities to 4-CP molecules in this MIP is only 85% and less than those values reported in earlier studies.

Preparation and gas separation properties of partially pyrolyzed membranes (PPMs) derived from copolyimides containing polyethylene oxide side chains

Abstract A new family of partially pyrolyzed membranes (PPMs) based on copolyimides containing poly(ethylene oxide) side chains (PI-PEOs) as precursors has been prepared. The synthesis of PI-PEOs was carried out by an esterification reaction of a copolyimide containing carboxylic acid groups and poly(ethylene glycol) monomethyl ethers of differing molecular weights. The degree of modification and the PEO content were determined by 1 H-NMR and some properties such as crystallinity, water uptake and thermal behavior were investigated. The PI-PEOs membranes were pyrolyzed at a relatively low temperature (385 °C) to prepare the corresponding PPMs. During the pyrolysis the PEO and carboxylic acid groups were removed, increasing the free volume in the PPMs while the copolyimide framework was maintained. The study of the gas separation properties of both families of polymers revealed the positive effect of removing the PEO side chains because the gas permeability of the pyrolyzed membranes increased by two to six times compared to the corresponding PI-PEOs, depending on the gas tested, without significant loss of ideal selectivity.The membranes were characterized also by positron annihilation lifetime spectroscopy (PALS) to better understand the gas transport properties. Pyrolysis of the PEO groups was shown to increase the free volume element size and concentration whilst broadening the free volume size distribution. The increase in free volume element size and concentration is deemed responsible for the increase in permeability and diffusivity.