Free Volume Elements Research Articles

A Simulation Study on OH-Containing Polyimide (HPI) and Thermally Rearranged Polybenzoxazoles (TR-PBO): Relationship between Gas Transport Properties and Free Volume Morphology

Recently, high free volume polymer materials have been regarded as high potential candidates for gas transport/separation membranes, since the amount of free volume in polymeric membrane can improve the diffusivity and solubility of gas molecules. In this study, we focused on how local changes in polymer structure can affect the performance of a membrane at the molecular level. The transport behavior was theoretically analyzed, and then the differences in the amount and morphology of free volume were characterized. Finally, we suggested how the "evolution of microcavities" affects the gas transport properties of hydroxyl-containing polyimide (HPI) and thermally rearranged (TR) polymers. In particular, using image analysis, we intuitively demonstrate the morphological difference between HPI and TR polymers that have been indirectly explained by experimental analyses using a wide-angle X-ray diffractometer (WAXD) and positron annihilation laser spectroscopy (PALS). Solubility results using the grand canonical Monte Carlo (GCMC) method showed marginal improvement in thermally rearranged polybenzoxazoles (TR-PBOs) from its precursor HPI, which is in good agreement with the experimental tendency. Moreover, higher diffusivities but lower selectivities of TR-PBO models compared with those of HPI models were observed, as reported experimentally. The difference in gas transport abilities between HPIs and TR-PBOs originates from the difference in their diffusion behavior, and this is strongly related to the free volume amount and morphology of polymeric materials. In addition to the higher amount of total free volume in TR-PBO, our image analysis revealed that TR-PBO has a higher amount of interconnected "hourglass-shaped free volume elements", which consist of larger and more elongated cavities with bottlenecks than the HPI model. In particular, the bottleneck diameters in the TR-PBO models are wider than those in the HPI models, enabling the larger gas molecules to diffuse through the cavities faster. However, the narrower and smaller bottleneck diameters in the HPI model can induce better selectivity for large gas molecules.

Virtual Synthesis of Thermally Cross-Linked Copolymers from a Novel Implementation of Polymatic

Because of the complex connectivity of cross-linked polymers, generating structures for molecular simulations is a nontrivial task. In this work, a general methodology is presented for constructing post-cross-linked polymers by a new two-stage implementation of the Polymatic simulated polymerization algorithm, where linear polymers are first polymerized and then cross-linked. It is illustrated here for an example system of thermally cross-linked octene-styrene-divinylbenzene (OS-DVB) copolymers. In the molecular models, the degree of cross-linking is ranged from 0 to 100%, and the resulting structural and thermal properties are examined. The simulations reveal an increase in the free volume with higher cross-linking degrees. Shifts in the peaks of the structure factors, which are assigned to contributions from the backbone and side-chain atoms, correspond to the formation of larger free volume elements. Furthermore, the glass transition temperatures increase with higher degrees of cross-linking, while the thermal expansivity decreases. Comparisons with experimental results for similar systems are made when available. As demonstrated here, the presented methodology will provide an effective route to simulating post-cross-linked polymers for a variety of applications, which will enable an improved understanding of their structure-property relationships.

Application of negative retention in organic solvent nanofiltration for solutes fractionation

For the first time, the utilization of negative retention in organic solvent nanofiltration (OSN) for solute fractionation was proposed and realized. Using membranes made of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and ethanol as a solvent, this principle was successfully proved for two different pairs of dyes: (i) Orange II (anionic; 350Da) and Solvent Blue 35 (neutral; 350Da), (ii) Remazol Brilliant Blue R (anionic; 626Da) and Oil Red O (neutral; 408Da). The solutes ratio in the permeate after one step filtration with 80% of liquid recovery was achieved as follows (initial ratio in the feed – 1:1): 1:10 for Orange II to Solvent Blue 35 and 1:26 for Remazol Brilliant Blue R to Oil Red O. With compromising of liquid recovery, the solutes ratio in the permeate can be significantly improved; for example, 15% liquid recovery may provide the ratio in favor of neutral solute as 1:260 and 1:340, respectively. Pronounced difference in solutes retention was interpreted by membrane–solute interaction expressed in terms of equilibrium solute distribution coefficient K between the membrane and liquid and difference in solubility parameters δ. Low values of solute distribution coefficient K for Orange II and Remazol Brilliant Blue R (less than 1) revealed that both anionic dyes have a tendency to be concentrated in liquid phase resulting in high solute retention (more than 90%). Better rejection of Remazol Brilliant Blue R (1258Å3) in contrast to Orange II (297Å3) was explained by difference in solute size. High K values (more than 10) observed for Solvent Blue 35 and Oil Red O indicated that these two neutral solutes have a high affinity to membrane material and can be easily accommodated within interconnected free volume elements of PTMSP. Such solubility determined solute transport resulted in negative retention. Analysis of solubility parameters δ of components showed that neutral solutes have a stronger affinity to the membrane material rather than ethanol; meanwhile, PTMSP has a preferential affinity to solvent molecules over selected anionic dyes. These experimental observations allowed to conclude that the principle of one step solutes fractionation based on combination of negative and positive retention could be realized with PTMSP membranes for OSN applications.

Molecular modeling of poly(benzoxazole-co-imide) membranes: A structure characterization and performance investigation

The molecular simulation technique was adopted to investigate the structure and transport performance of thermally rearranged poly(benzoxazole-co-imide) membranes. A molecular dynamics (MD) technique was used to construct three models: a poly-benzoxazole (PBO) membrane with high free volume; a polyimide (PI) membrane with dense structure; and their co-polymer, PBO-PI membrane. The MD simulation was performed to characterize the membrane models to understand how the very rigid benzoxazole segments affect the micro-structure, free volume, cavity size, and gas diffusion of the membrane models. A Monte Carlo method was adopted to investigate the gas sorption behaviors in the three types of membranes. The torsional angle and wide-angle X-ray diffraction analyses suggest that the benzoxazole segments stiffened the polymeric chains, leading to the formation of a looser structure. In free-volume and cavity-size studies, the PBO membrane exhibited the highest free volume and largest cavity size, which can be attributed to the presence of the benzoxazole structure constructed by thermal rearrangement. The enlarged free volume in the membranes with benzoxazole segments provided more space for gas sorption and diffusion, which effectively enhanced the gas permeability. In addition, increasing the benzoxazole segments in the membrane structure enhances the gas sorption in accordance with Henry's law, as the PBO membrane provides more inter-polymeric chain space and allows for the larger free volume elements. Fabrication of the poly(benzoxazole-co-imide) membrane with an appropriate PBO/PI composition would help optimize the gas permeability and selectivity in the gas separation process. The results from the simulation agree with the experimental data, indicating that the molecular simulation technique is a useful method in the field of materials design and development for the membrane separation process.

Synthesis, characterization, and gas transport properties of novel iptycene-based poly[bis(benzimidazobenzisoquinolinones)

Two novel iptycene-based tetramine monomers were successfully synthesized by nucleophilic aromatic substitution of triptycene-1,4-diol and pentiptycene-6,13-diol with 5-chloro-2-nitro-aniline, followed by reduction, respectively. These monomers were polymerized with 4,4-binaphthyl-1,1,8,8-tetracarboxylic dianhydride to obtain two novel iptycene-based poly[bis(benzimidazobenzisoquinolinones)]s (PBIBI–TPD and PBIBI–PPD) by a one-step, high-temperature solution polycondesation. Incorporation of iptycene groups especially the pentiptycene group in the polymer backbones improved their solubility and thermal stability. The resulting membranes exhibited good gas permeability owing to the high internal free volume elements introduced by the iptycene groups as well as high gas selectivity due to the restricted local segmental mobility arising from the interlocking of these groups in the polymer backbone. The membrane of PBIBI–PPD showed high CO2 permeability (112 barrer) and moderately good CO2/N2 and CO2/CH4 selectivity (22 and 31) for mixed gas separation.

Gas sorption, diffusion, and permeation in thermally rearranged poly(benzoxazole-co-imide) membranes

Thermally rearranged polymer membranes exhibit extraordinary gas permeability based on a rigid polymer structure with a high free volume element. In this study, TR poly(benzoxazole-co-imide) membranes from 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4-4′-diamino-biphenyl (HAB), 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF), and 2,4,6-trimethyl-m-phenylenediamine (DAM) were prepared to improve their gas separation properties. Copolymer membranes of polyimides and TR polybenzoxazoles may be desirable to generate efficient gas transport properties as well as to process polymers into fiber or film forms. Gas permeability, diffusivity, and solubility of the precursor polyimide and TR poly(benzoxazole-co-imide) membranes were investigated to characterize gas transport properties for small gas molecules including H2, O2, N2, CH4, and CO2. Thermal rearrangement process resulted in an increase in polymer free volume, which improved the diffusion and sorption coefficients.

Free volume characterization of sulfonated styrenic pentablock copolymers using positron annihilation lifetime spectroscopy

A series of sulfonated styrenic pentablock copolymers was characterized using positron annihilation lifetime spectroscopy (PALS). Ortho-positronium (o-Ps) lifetime was measured as a function of the polymer's degree of sulfonation and the environment in which the polymer was equilibrated. Samples ranged from an un-sulfonated styrenic pentablock copolymer to one with an ion exchange capacity (IEC) of 2.0meq/g(dry polymer), and the samples were equilibrated in either air having 50% relative humidity, de-ionized (DI) water, or 2000mgL−1 aqueous NaCl solution environments. The o-Ps lifetime, and thus, the average free volume element size, decreased as the polymer's degree of sulfonation increased and as the amount of water sorbed into the polymer increased, similar to reported decreases in o-Ps lifetime accompanying increased water sorption in uncharged cross-linked poly(ethylene glycol) hydrogels. The o-Ps lifetimes of the DI water equilibrated acid counter-ion form polymers were quite similar to those of the salt solution equilibrated sodium counter-ion form polymers, suggesting that the PALS data are not particularly sensitive to the counter-ion form of these hydrated polymers. Previously reported water and salt permeability data in polymers were correlated with the PALS data. Correlating water and salt permeability with free volume data, estimated using o-Ps lifetime, was complicated by the micro-phase separated nature of the block copolymers. The water permeability of polymers whose hydrophilic micro-domains are believed to be well connected correlated with free volume data, based on o-Ps lifetime. Salt permeability, however, did not correlate with free volume data estimated from o-Ps lifetime, according to free volume theory. In general, contributions of the hydrophobic micro-domain free volume to the PALS measurement frustrate the correlation of water and salt transport properties with PALS parameters in a manner that is not observed in other sulfonated and uncharged polymers.

Positron lifetime study of poly(dimethylsiloxane) based polymer-nanoparticle composites

Positron annihilation lifetime spectroscopy was employed to study the changes in the size of the local free volume elements (holes) of poly(dimethylsiloxane) (PDMS) based nanocomposites incorporating two different types of nanoparticles, meta-carborane (m-CB) and iso-octyl polyhedral oligomeric silsesquioxane (POSS). Materials were prepared with various loading levels of each nanoparticle and the temperature dependence of the o-Ps lifetime, and thus the free volume, and its distribution were measured from 100–300 K. It was observed that m-CB reduces the size of the local free volume elements while POSS has the opposite effect, in each case the change is only seen in the rubbery state below the so-called "knee temperature" (Tk ~ 210 K).

Influence of the addition of water to amorphous switching domains on the simulated shape-memory properties of poly(l-lactide)

It is well known that the environmental conditions e.g. humidity can alter the shape-memory properties of polymers. In this work we applied an atomistic molecular dynamics simulation approach to model the influence of the addition of 1 wt% and 2 wt% water on the simulated shape-memory behavior of the amorphous switching domains of poly(l-lactide) (PLLA) with a molecular weight of Mn = 52,000 g mol−1. For the dry as well as the water swollen PLLA-models, two subsequent uniaxial thermomechanical test cycles have been calculated, whereby the applied uniaxial deformation εm was varied between 50% and 150%.All simulated PLLA models showed high shape fixity ratio of Rf ≥ 84%. The shape-memory properties obtained at εm = 100% for the dry PLLA and the models containing 1 wt% water were almost identical with a shape recovery ratio in the first and second test cycle around Rr = 61%–64%. In contrast PLLA with 2 wt% water exhibited higher values of Rr = 76% during the first test cycle, but a lower Rr = 52% in the subsequent second cycle. Furthermore, increasing the applied εm resulted in a decrease of Rr from 82% to 42% for the dry PLLA, whereas PLLA with 2 wt% water did not show a dependence of Rr on εm. We anticipate that these observations can be attributed to differences in the initial structure of the various simulated PLLA models e.g. the different distribution of the free volume elements.

Morphology and free volume of nanocomposite Teflon AF 2400 films and their relationship to transport behavior

Free volume is a key property regulating penetrant (gas phase) or solute (solution phase) permeability in polymeric membranes. Thus, we investigated the origins of free volume in two types of Teflon AF 2400 nanocomposites. In nanocomposites from glassy Teflon AF and fluorophilic, solid silica nanospheres, the soft matrix shows exceptionally high apparent fractional free volume (aFFVP)—reaching over 50% at high particle loading. Adding the higher density FNPs to polymer films actually decreases film density. Nanocomposites from plasticized Teflon AF show similarly high aFFV at high particle loading. However, the nano-scale morphologies of the films are very different. The nanocomposites with glassy polymer exist as a polymer matrix with entrained particles. At higher FNP loadings, where the characteristic distance between particles is smaller than the radius of gyration of the polymer, the polymer exists as a vine-like network entraining the particles. Nanocomposite films with plasticized polymer have a packed, core–shell particle morphology with FNPs as the core and plasticized Teflon AF as the eight-nm shell. While the aFFVPs of the two types of films at high FNP loading are similar, systematic studies reveal different mechanisms for the formation of the apparent free volume elements. Distinctive free volume properties lead to remarkably different transport behavior.

Liquid permeation through PTMSP: One polymer for two different membrane applications

The extra high free volume glassy polymer poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is a perspective membrane material for two inherently different applications: (i) organic solvent nanofiltration (OSN), when a high solvent flux through the membrane is expected, and (ii) high pressure membrane contactors for CO2 capture that requires the absence of liquid permeation (no liquid leakage) through the membrane. The transport of wetting/non-wetting liquids through PTMSP was studied. Filtration experiments of binary water–ethanol mixtures revealed that no liquid permeation through PTMSP took place at trans-membrane pressures up to 30bar if the alcohol content is 40wt% or lower. The further increase of the ethanol concentration (50wt% and higher) provides the hydrodynamic flux through PTMSP; its value increases with increasing ethanol content. No single transport model available in the literature can satisfactorily describe the water–ethanol permeation through PTMSP. In this work, a new semi-empirical model was developed and it was successfully applied for describing the experimental data on liquids permeation through PTMSP dense membranes at pressure differences up to 150bar. The model takes into account the fractional free volume of polymer, swelling degree of the polymer in solvent environment, membrane compaction under elevated pressure difference across the membrane and physical–chemical macroscopic parameters of the liquid and polymer such as surface tension, viscosity and molar volume. The sorption and swelling degree of PTMSP were measured for the water–ethanol mixtures under study. The sorption and swelling data and liquid permeation results for binary ethanol–water mixtures revealed that, in spite of remarkable content of the liquid in the free volume elements of PTMSP (i.e. sorption is up to 0.42g/g and swelling degree is up to 0.12), there is no liquid flux through PTMSP at pressure differences of 10–30bar. Thus, the threshold values of the PTMSP swelling degree was estimated on the level of 0.12–0.19. It was shown that PTMSP assures zero liquid flux (no liquid leakage) for the PTMSP/liquid system the corresponding parameters of which are below the threshold region (important for high pressure membrane contactor applications). When sorption and swelling for a PTMSP/liquid system are above the threshold levels, the PTMSP membrane exhibits a certain hydrodynamic flux for the corresponding liquid (important for OSN applications).

Relation of gas-transport parameters of amorphous glassy polymers to their free volume: Positron annihilation study

Correlations of transport parameters (diffusion coefficients D and permeabilities P of gases) and thermodynamic parameters (solubility coefficients S and parameters C′ H of sorption isotherms) with the sizes of free-volume elements, v h, as estimated via positron annihilation lifetime spectroscopy are analyzed for the first time on the basis of the data array obtained for glassy polymers. Correlations of logD and logP with 1/v h that agree with the free-volume model under the condition of a weak change in the concentration of free-volume elements in different polymers are ascertained. Certain deviations from linear correlations with 1/v h for polymers with high free volumes are interpreted as evidence that the connectivity (openness) of pores increases with the sizes of free-volume elements. For solubility coefficients and Langmuir parameters of sorption capacity C′ H , good linear correlations with the value of v h are demonstrated.

Local chain mobility dependence on molecular structure in polyimides with bulky side groups: Correlation with gas separation properties

The effect of bulky side groups on the molecular mobility and chain packing of three polyimides derived from dianhydrides with m-terphenyl moieties and 2,4,6-trimethyl-m-phenylenediamine has been studied. Moreover, structure–properties relationships in terms of gas transport have been established. One of the polyimides has no side groups, while the other two have a t-butyl group in 5′ (meta) position on the central ring of the m-terphenyl moiety and one of them has also a pivaloylimino group placed in the 2′ (ortho) position. The changes in chain packing of the membranes were evidenced by wide angle X-ray diffraction (WAXS), whilst the molecular mobility was evaluated from the dielectric sub-ambient secondary relaxation, γ relaxation. The WAXS pattern of the polyimide containing both t-butyl and pivaloylimino side groups evidenced a greater degree of packing regularity than in the other two polyimides. Moreover, the additional pivaloylimino group strongly restricted the local molecular mobility associated with the phenylene-imide ring flips. On the other hand, the gas permeability of the polyimides strongly increased with the introduction of side groups. This increase was mainly related to the increase in the gas diffusivity, which could not be explained by an increase in fractional free volume (FFV). The results presented in this work indicate that the difference in gas diffusivity of the substituted polyimides is consistent with a difference in size distribution of free volume elements.

Nanocomposite membranes of Nafion and Fe3O4-anchored and Nafion-functionalized multiwalled carbon nanotubes exhibiting high proton conductivity and low methanol permeability for direct methanol fuel cells

Fe3O4 magnetic nanoparticle-anchored and Nafion-functionalized multiwalled carbon nanotubes (MWCNT–MNP–Nafion) have been prepared and used as a nano-additive for Nafion. The addition of 0.1 wt% of MWCNT–MNP–Nafion to Nafion results in the membranes showing a reduction of methanol permeability from 30.1 × 10−7 cm2 s−1 to 6.8 × 10−7 cm2 s−1 and an increase of proton conductivity from 31 mS cm−1 to 78 mS cm−1. The presence of MWCNT–MNP–Nafion reduces the ionic cluster sizes and decreases the free volume element concentration of the modified Nafion membrane. Both have been verified with small angle X-ray scattering spectroscopy and positron annihilation lifetime spectroscopy and contribute to the reduction of the methanol permeability of the membrane. The increase in the proton conductivity originates from the formation of proton-conducting pathways on the MWCNT bundles. For the membrane fabricated under a magnetic field, its proton conductivity further increases to 85 mS cm−1 due to the magnetically-driven alignment of MWCNT–MNP–Nafion in the Nafion matrix (A-Nafion-NM-0.1). Compared to the plain recast Nafion membrane, the A-Nafion-NM-0.1 membrane has a 15-fold membrane selectivity, an 8.7-fold maximum power density, and a 6.6-fold current density at an operating voltage of 0.4 V in the tests for direct methanol fuel cells (DMFC). The high performance of the membrane in the DMFC tests demonstrates that the MWCNT–MNP–Nafion is a highly efficient additive for the preparation of Nafion-based membranes for DMFCs.

Simulated swelling during low-temperature N2adsorption in polymers of intrinsic microporosity

Polymers of intrinsic microporosity (PIMs) are a promising alternative in gas separation applications; however, their industrial applicability is greatly reduced by their susceptibility toward sorption-induced plasticization. In this study, the effects of swelling during nitrogen adsorption isotherms at 77 K for five PIMs (PIM-1, PIM-1c, PIM-SBF, PIM-SBF-Me, and sPIM-1) were examined through predictive molecular simulations. The structural changes and adsorptive implications of swelling each PIM from 0 to 15% were analyzed by generating swollen "snapshots" of the PIM during the swelling process by means of an effective swelling procedure. The size of the free volume elements increased with the simulated swelling percentage, while the closely packed polymer chains remained associated. By comparing the simulated swollen and available experimental isotherms, this investigation shows that the amount of swelling observed during low-temperature nitrogen adsorption is strongly correlated with the interaction strength of the polymer matrix. Therefore, this study provides further support that PIMs should be designed with functionalities to increase the associative interactions of adjacent polymer chains while simultaneously prohibiting efficient packing to construct a gas separation membrane with limited low-temperature N2 swelling and high free volumes.

Tailoring the Chain Packing in Ultrathin Polyelectrolyte Films Formed by Sequential Adsorption: Nanoscale Probing by Positron Annihilation Spectroscopy

Depth profiling experiments by positron annihilation spectroscopy have been used to investigate the free volume element size and concentration in films assembled using the layer-by-layer (LbL) adsorption method. Films prepared from strong polyelectrolytes, weak polyelectrolytes, hydrogen-bonding polymers, and blended polyelectrolyte multilayers have different chain packing that is reflected in the free volume characteristics. The influence of various parameters on free volume, such as number of bilayers, salt concentration, solution pH, and molecular weight, has been systematically studied. The free volume cavity diameters vary from 4 to 6 Å, and the free volume concentrations vary from (1.1-4.3) × 10(20) cm(-3), depending on the choice of assembly polymers and conditions. Films assembled from strong polyelectrolytes have fewer free volume cavities with a larger average size than films prepared from weak polyelectrolytes. Blending the weak polyanion poly(acrylic acid), PAA, with the strong polyanion poly(styrene sulfonate), PSS, to layer alternately with the polycation poly(allyamine hydrochloride), PAH, is shown to be a viable method to achieve intermediate free volume characteristics in these LbL films. An increase in salt concentration of the adsorption solutions for films prepared from strong polyelectrolytes makes these films tend toward weaker polyelectrolyte free volume characteristics. Hydrogen-bonded layered films show larger free volume element size and concentration than do their electrostatically bonded counterparts, while reducing the molecular weight of these hydrogen-bonded polymers results in slightly reduced free volume size and concentration. A study of the effect of solution pH on films prepared from weak polyelectrolytes shows that when both polyelectrolytes are substantially charged in solution (assembly pH = 7.5), the chains pack similarly to strong polyelectrolytes (i.e., lower free volume concentration), but with smaller average cavity sizes. These results give, for the first time, a clear indication of how the free volume profile develops in LbL thin films, offering numerous methods to tailor the Ångström-scale free volume properties by judicious selection of the assembly polymers and conditions. These findings can be potentially exploited to tailor the properties of thin polymer films for applications spanning membranes, sensing, and drug delivery.

Characterization of the spray drying behaviour of emulsions containing oil droplets with a structured interface

The aim of this study was to characterize the process of atomization and drying of layer-by-layer emulsions containing lecithin (single layer emulsion) and lecithin/chitosan (bilayer emulsion) and the oxidative stability of the microcapsules during storage. For this purpose, the analysis of the emulsion spray droplet size during two-fluid nozzle and rotary atomization was carried out to identify suitable process parameters. The drying behaviour of single and bilayer emulsions was investigated by calculation of the volume flow density during single-droplet drying during acoustic levitation. In spray-dried solid particles, the oxidative stability in the single layer microcapsules was higher than in the bilayer microcapsules. This was partly attributed to lower microencapsulation efficiency in the bilayer microcapsules compared to the single layer microcapsules. Furthermore, it could be shown, that excess chitosan in the bulk carrier matrix affects the free volume elements and thus oxygen diffusion.

Solvent nanofiltration through high permeability glassy polymers: Effect of polymer and solute nature

The effect of solute nature on the retention performance of membrane materials based on PTMSP, PMP and PIM-1 has been investigated in ethanol media. Using the examples of PMP and PIM-1, it was shown that membrane swelling could enhance the solvent transport across the membrane, regardless of the initial gas permeability. All polymers show the same level and order of retention for low molecular weight dyes (350Da) regardless of the difference in polymer nature; in particular, negative retention of the neutral dye Solvent Blue 35 was observed for PTMSP, PMP and PIM-1: −17%, −11% and −4%, respectively. This retention behavior was explained by a “blocking effect” due to high dye-membrane interaction and comparable size of the solute (Vcalc=685Å3) and available free volume elements within the polymer (5–12Å). An increase of dye retention is attributed to a decrease of solute-membrane interaction; for example, PIM-1: Solvent Blue 35 (K=1107; |δPIM−1−δsolute|=4.2)<Safranine O (K=47.0; |δPIM−1−δsolute|=6.1)<Orange II (K=3.3; |δPIM−1−δsolute|=9.7). The retention of higher molecular weight solutes is expected to be limited mainly by free volume size. Thus, it is possible to control nanofiltration parameters by proper selection of the polymer and the variety in the chemical nature of high permeability glassy polymers provides additional flexibility in solvent resistance and further modifications (e.g. cross-linking). A new possible application of PIM-1 as a polymeric sorbent with selectivity towards dissolved neutral compounds is proposed.

Macrochain configuration, stucture of free volume and transport properties of poly(1-trimethylsilyl-1-propyne) and poly(1-trimethylgermyl-1-propyne)

The relationship between poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-trimethylgermyl-1-propyne) (PTMGP) microstructure, gas permeability and structure of free volume is reported. n-Butane/methane mixed-gas permeation properties of PTMSP and PTMGP membranes with different cis-/trans-composition have been investigated. The n-butane/methane selectivities for mixed gas are by an order higher than the selectivities calculated from pure gas measurements (the mixed-gas n-butane/methane selectivities are 20–40 for PTMSP and 22–35 for PTMGP). Gas permeability and n-butane/methane selectivity essentially differ in polymers with different cis-/trans-composition. Positron annihilation lifetime spectroscopy investigation of PTMSP and PTMGP with different microstructure has determined distinctions in total amount and structure of free volume, i.e. distribution of free volume elements. The correlation between total amount of free volume and gas transport parameters is established: PTMSP and PTMGP with bigger free volume exhibit higher n-butane permeability and mixed-gas n-butane/methane selectivity. Such behavior is discussed in relation to the submolecular structure of polymers with different microstructure and sorption of n-butane in polymers with different free volume.

Effect of thermal treatment on gas transport properties of hyperbranched polyimide–silica hybrid membranes

Physical and gas transport properties of thermotreated hyperbranched polyimide and its silica hybrid membranes were investigated. Thermal treatment at 450 °C resulted in partial degradation and rearrangement of molecular chains. Gas permeabilities of the hyperbranched polyimide membrane were remarkably enhanced by thermal treatment at 450 °C, compared to those of linear-type polyimide membrane with a similar chemical structure. The enhanced gas permeabilities were considered to be due to local deformation of molecular packing caused by thermal degradation and rearrangement of molecular chains, which provided an increase in free volume elements. In addition, for the hyperbranched polyimide–silica hybrid membrane treated at 450 °C, hybridization with silica brought about increases in gas permeabilities and improvement of CO2/CH4 separation ability. The prominent CO2/CH4 separation ability was attributed to the enhanced free volume elements induced by facilitated thermal degradation and rearrangement of molecular chains, as well as created around interfacial region between polymer and well-dispersed silica domains with a large surface area.