Cavity size distributions in high free volume glassy polymers by molecular simulation

Two very permeable polymers, poly(1-trimethylsilyl-1-propyne) (PTMSP) and a random copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (TFE/BDD), have very similar and large fractional free volumes (FFV), but very different permeabilities. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamic and Monte Carlo methods are consistent with the observation that PTMSP is more permeable than TFE/BDD. The average spherical cavity size in PTMSP is 11.2 Å whereas it is only 8.2 Å in TFE/BDD. These cavity size distributions determined by simulation are also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy.     

Enhanced gas transport properties and molecular mobilities in nano-constrained poly[1-(trimethylsilyl)-1-propyne] membranes

Interfacial constraints in ultrathin poly(l-trimethylsilyl-1-propyne) (PTMSP) membranes yielded gas permeabilities and CO₂/helium selectivities that exceed bulk PTMSP membrane transport properties by up to three-fold for membranes of submicrometer thickness. Maximum permeability coefficients of 110 × 10³ Barrer and 27 × 10³ Barrer for carbon dioxide and helium, respectively, were found to occur in membranes of ～750 nm thickness. Indicative of a free volume increase, a molecular energetic mobility analysis (involving intrinsic friction analysis) revealed enhanced methyl side group mobility. This was evidenced by a minimum in the activation energies of ～4 kcal/mol in thin PTMSP membranes with maximum permeation, compared to ～5.5 kcal/mol in bulk films. Aging studies conducted over the timescales relevant to the conducted experiments signify that the free volume states in the thin film membranes are highly unstable in the presence of sorbing gases such as CO₂. These results are discussed and contrasted to PTMSP bulk membrane systems, which were found to be unaffected by aging over the equivalent experimental time scale.     

Pure gas and vapor permeation properties of poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA) and its desilylated analog, poly[diphenylacetylene] (PDPA)

The permeabilities of He, H₂, N₂, O₂, CO₂, CH₄, C₂H₆, C₃H₈, and n-C₄H₁₀ in poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA) and poly[diphenylacetylene] (PDPA) are presented and compared to those of poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(1-phenyl-1-propyne) (PPP), and polysulfone. Like PTMSP, PTMSDPA, a disubstituted glassy acetylene-based polymer, exhibits higher permeabilities to organic vapors than to permanent gases due to its rigid polyacetylene backbone and bulky side groups, which provide a relatively high fractional free volume (FFV) value of 0.26. Desilylation was performed on PTMSDPA. The resulting material, PDPA, is totally insoluble in common organic solvents, so it has much higher chemical resistance than PTMSDPA. Additionally, due to its insolubility in polymerization solvents, desilylation provides the only known route to high molar mass PDPA. The FFV of the resulting membrane (PDPA) is reduced by approximately 12% relative to that of PTMSDPA. This leads to a decrease in gas permeability values and selectivity of organic vapors relative to nitrogen. For example, the oxygen permeability is reduced from 1200 to 500 Barrers upon desilylation. The pure gas selectivities decrease from 9 to 3 for n-C₄H₁₀/N₂ and from 26 to 9 for C₃H₈/N₂.     

Synthesis and stable high oxygen permeability of poly(diphenylacetylene)s with two or three trimethylsilyl groups

We synthesized two new poly(diphenylacetylene)s having two or three trimethylsilyl groups and found these membranes having extremely high oxygen permeabilities of more than 1000 barrers which are of the same order as that for poly[1-(trimethylsilyl)-1-propyne]. Whereas oxygen permeability of poly[1-(trimethylsilyl)-1-propyne] was reported to decrease largely with time, these high oxygen permeabilities were stable for several months. These membranes also showed ethanol permselectivities because of their hydrophobicity. It was found that the introduction of two or three trimethylsilyl groups to poly(diphenylacetylene) was very effective for obtaining stable high oxygen permeable and ethanol permselective membranes.     

The Effect of Process Parameters on the Pervaporation of Alcohols through Organophilic Membranes

Abstract

Several organophilic membranes were utilized to selectively permeate ethanol, n-butanol, and t-butanol from dilute aqueous mixtures using pervaporation (PV). Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes were utilized to investigate the effect of temperature, pressure, and start-up/transient time on the separation of aqueous ethanol mixtures. Results indicate optimal ethanol selectivity and flux at the lowest permeate-side pressure. Increased temperature significantly enhanced the productivity of PTMSP, but extended operation of the PTMSP membranes at high temperatures resulted in flux degradation. Two other hydrophobic membranes, poly(dimethyl siloxane) (PDMS) and a poly(methoxy siloxane) (PMS) composite, were used to separate n-butanol and t-butanol from dilute aqueous mixtures. The effect of feed concentration on the flux and selectivity was investigated. Both membranes were found to be more permeable to n-butanol than t-butanol. The PDMS membrane was found to be more effective than the PMS membrane in terms of flux and selectivity. The effect of membrane thickness on water permeation and on organic selectivity was also studied using the PDMS membrane.     

评估混合气在PDMS膜中渗透性能的操作模型

An operational model is developed to evaluate and predict the permeation performance of mixed gas through poly（dimethylsiloxane） （PDMS） membranes by combining the ideal gas permeation model with the experimental analysis of the mixed gas transport character. This model is tested using the binary and ternary mixed gas with various compositions through the PDMS membranes, and the predicted data of the permeation flux and the compositions of the permeated gas are in good agreement with the experimental ones, which indicates that the operational model is applicable for the evaluation of the permeation performance of mixed gas through PDMS membranes.     

Transport and stability enhancement in interfacially and dimensionally constrained CO2 selective polymers embedded in nanoporous sieve membranes

Nanocomposite polymer and ultrathin film membranes have shown great promise in enhancing gas permeation and selectivity properties by interfacially straining polymer matrices, yielding structures of higher free volume. However, undesired particle aggregation and short temporal stability remain a big challenge. In the present study, an “inverse” architecture to conventional polymer nanocomposites was investigated, in which the polymer phase poly(l-trimethylsilyl-1-propyne) (PTMSP) was interfacially and dimensionally constrained in nanoporous anodic aluminum oxide (AAO) membranes. While with this architecture the benefits of nanocomposite and ultrathin film membranes could be reproduced and improved upon, also the temporal stability could be enhanced substantially. Gas permeabilities of helium, nitrogen and carbon dioxide were increased over five-fold, and selectivities of CO₂/He and CO₂/N₂ could be enhanced by 40% compared to the pristine bulk phase, while physical aging, caused by free-volume collapse, was reduced twenty-fold compared to ultrathin membranes.     

Sorption and Permeation of Aqueous Alkyl Piperazines through Hydrophilic and Organophilic Membranes: A Transport Analysis

ABSTRACT

A detailed analysis of separation of N-methyl piperazine (NMP), N-ethyl pipera-zine (NEP), and water was undertaken by the pervaporation technique. A systematic study of sorption and permeation of the aqueous alkyl piperazines through poly(dimethylsiloxane) (PDMS), styrene-butadiene rubber (SBR), PDMS filled with zeolites NaX and silicalite (SA-5), polyimide (PI), and poly(acrylonitrile-co-acrylic acid) (PAN-co-AA) was carried out at different concentrations and temperatures. Organophilic membranes showed higher selectivity toward alkyl piperazines during sorption, but permeation was in favor of water. Hydrophilic membranes, however, showed higher affinity toward water during both sorption and permeation. PI membrane showed higher selectivity for water than PAN-co-AA. A model was used to estimate the diffusion coefficients of the various permeants. It was found that the transport selectivity for water in organophilic membranes was due to high diffusion selectivity (for water) although sorption selectivity favored the piperazines.     

Characterization and pervaporation of chlorinated hydrocarbon–water mixtures with fluoroalkyl methacrylate‐grafted PDMS membrane

It is desirable to enhance the selectivity of a polydimethylsiloxane (PDMS) membrane for chlorinated hydrocarbons. In this study, the PDMS membranes were improved by graft polymerization of 1H,1H,9H‐hexadecafluorononyl methacrylate (HDFNMA), which has the effect of increasing the selectivity for chlorinated hydrocarbons. The PDMS membrane and HDFNMA were irradiated simultaneously by a ⁶⁰Co source. The grafted membranes had a microphase‐separated structure, that is, a separated structure of PDMS and grafted HDFNMA. In the grafted PDMS membrane, a great separation performance for a TCE–water mixture was shown due to the introduction of the hydrophobic polymer, poly(HDFNMA). For the permeation of the grafted PDMS membrane, the permeability of molecules in the PDMS phase was significantly great, and that in the poly(HDFNMA) phase was too low to affect the whole permeation of the grafted PDMS membrane directly. However, the permeation of molecules at the interface of poly(HDFNMA) and PDMS played an important role because poly(HDFNMA) had a much stronger affinity for TCE than water. At a low feed concentration of the TCE solution, the diffusivity of TCE molecules must be much lower than that of water due to the larger molecular size of TCE. At a high concentration of TCE solution, TCE was sufficiently sorbed into the membrane so that the diffusion of water was prevented by TCE molecules; in turn, the permselectivity of TCE was increased significantly. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 273–287, 1999     

Pure and mixed gas CH4 and n-C4H10 permeability and diffusivity in poly(1-trimethylsilyl-1-propyne)

Pure and mixed gas n-C₄H₁₀ and CH₄ permeability coefficients in poly(1-trimethylsilyl-1-propyne) (PTMSP) are reported at temperatures from −20 to 35 °C. CH₄ partial pressures range from 1.1 to 14.6 atm, and n-C₄H₁₀ partial pressures range from 0.02 to 1.8 atm. CH₄ permeability decreases with increasing n-C₄H₁₀ upstream activity (f/f_sat) in the feed. For example, at −20 °C, CH₄ permeability decreases by more than an order of magnitude, from 52,000 to 1700 Barrer, as n-C₄H₁₀ activity increases from 0 to 0.73. In contrast, n-C₄H₁₀ mixed gas permeability is essentially unaffected by the presence of CH₄. The depression of CH₄ permeability in mixtures is a result of competitive sorption and blocking effects, which reduce both CH₄ mixture solubility and diffusivity, respectively. Diffusion coefficients of n-C₄H₁₀ and CH₄ in mixtures were calculated from mixture permeability and mixture solubility data. The CH₄ concentration-averaged diffusion coefficient generally decreases as n-C₄H₁₀ activity increases. On the other hand, the n-C₄H₁₀ diffusion coefficient is essentially unaffected by the presence of CH₄. Pure and mixed gas activation energies of permeation and diffusion of CH₄ and n-C₄H₁₀ are reported. The mixed gas n-C₄H₁₀/CH₄ permeability selectivity increases with increasing n-C₄H₁₀ activity and decreasing temperature, and it is higher than pure gas estimates would suggest. Mixture diffusivity selectivity also increases with increasing n-C₄H₁₀ activity. The difference between pure and mixed gas permeability selectivity arises from both solubility and diffusivity effects. The dual mode mixed gas permeability model describes the mixture permeability data reasonably well for n-C₄H₁₀. However, the model must be modified to accurately describe the methane data by accounting for the decrease in methane diffusivity due to the presence of n-C₄H₁₀ (i.e., blocking). Even though the penetrant concentrations are rather significant at some of the conditions considered, no evidence is observed for phenomena such as multicomponent coupling that would require a model more complex than the binary form of Fick's law. That is, Fick's law in its simplest form adequately describes the experimental data.     

Composite membranes of poly(1-trimethylsilyl-1-propyne) and poly(dimethyl siloxane) and their pervaporation properties for ethanol–water mixture

A poly(1-trimethylsilyl-1-propyne) (PTMSP) membrane was systematically modified to prevent flux decline over time by incorporating poly(dimethyl siloxane) (PDMS) in three different ways: (1) semi-interpenetrating polymer network (I series), (2) PDMS sorption (S series), and (3) PDMS sorption and crosslinking (X series). The PTMSP and PDMS phases were partially mixed in the I series, which was confirmed by the measurement of density and glass transition temperature. The flux and separation factor in pervaporation of an ethanol–water mixture decrease with time for the I series, analogous to the behavior of pure PTMSP. However, the flux and separation factor remained steady with time in the case of the S and X series. The sorption method appears to be a good means for maintaining a time-unvarying flux and separation factor at a minimum expense. © 1994 John Wiley & Sons, Inc.     

Poly(1-trimethylsilyl-1-propyne)/MFI composite membranes for butane separations

The separation of equimolar mixtures of i-butane and n-butane through poly(1-trimethylsilyl-1-propyne)/MFI composite membranes was studied. Membranes were characterized by XRD and SEM. Addition of 50 wt% MFI particles into a PTMSP matrix showed increased permeability and simultaneous improved selectivity in the temperature range 25–200 °C. The best improvement was seen at 150 °C for the composites, giving almost threefold increase in permeability and 56% higher n-butane/i-butane selectivity over the pure polymer. To our knowledge, this is the first successful demonstration of the incorporation of a molecular sieve into a polymer matrix for butane isomer separations. The composite membranes were also tested for separations of n-hexane/2,2-dimethylbutane and p-xylene/o-xylene.     

The influence of crosslinking and fumed silica nanoparticles on mixed gas transport properties of poly[1-(trimethylsilyl)-1-propyne]

Crosslinking poly[1-(trimethylsilyl)-1-propyne] (PTMSP) films with 3,3′-diazidodiphenylsulfone, a bis(azide) crosslinker, rendered the films insoluble in common solvents for PTMSP such as toluene. At all temperatures, mixed gas CH₄ and n-C₄H₁₀ permeabilities of crosslinked PTMSP were less than those of uncrosslinked PTMSP, which correlates with lower free volume in the crosslinked material. The presence of fumed silica (FS) nanoparticles in both uncrosslinked PTMSP and crosslinked PTMSP increased mixed gas CH₄ and n-C₄H₁₀ permeabilities, consistent with the disruption of polymer chain packing by such nanoparticles. Mixed gas CH₄ permeabilities of all films were significantly less than their corresponding pure gas CH₄ permeabilities. For example, at 35 °C, the mixed gas CH₄ permeabilities were approximately 60–80% less than their pure gas values. The greatest decrease was observed for uncrosslinked PTMSP, while nanocomposite PTMSP films showed the least decrease. The mixed gas n-C₄H₁₀/CH₄ selectivities of crosslinked PTMSP and nanocomposite PTMSP films were less than those of uncrosslinked PTMSP at all temperatures. For example, at 35 °C, the mixed gas n-C₄H₁₀/CH₄ selectivities of uncrosslinked PTMSP, crosslinked PTMSP containing 10 wt% crosslinker, and uncrosslinked PTMSP containing 30 wt% FS were 33, 27, and 17, respectively, when the feed gas contained 2 mol% n-C₄H₁₀ and the total upstream mixture fugacity was 11 atm. For all films, as temperature decreased, mixed gas n-C₄H₁₀ permeabilities increased, and mixed gas CH₄ permeabilities decreased. Consequently, the mixed gas n-C₄H₁₀/CH₄ selectivities increased substantially as temperature decreased and the mixed gas selectivity of uncrosslinked PTMSP increased from 33 to 170 as temperature decreased from 35 °C to ?20 °C when the feed gas contained 2 mol% n-C₄H₁₀ and the total upstream mixture fugacity was 11 atm.     

Fibers spinning from poly(trimethylsilylpropyne) solutions

This study for the first time directs in assessment of the necessary conditions for spinning fibers from poly[1-(trimethylsilyl)1-propyne], one of the best for gas separation. It includes a search of appropriate solvents, investigation of rheological properties of solutions, a preparation of dopes with reasonable polymer content and a choice of effective coagulants based on their solubility parameters in frames of wet fiber spinning. The fibers were obtained with diameter of 7 ± 1 μm and strength of up to 200 MPa. The morphology of the surface and core of the PTMSP fibers was distinctively different: dense skin and friable core. From the viewpoint of membrane properties, it looks like asymmetrical membrane. In addition, the hollow fibers we prepared by dry spinning method. Overall, the stable fiber spinning process from PTMSP solutions was developed for the first time, and monolith and hollow PTMSP fibers of good quality were obtained. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48511.     

Effect of Fumed Silica Nanoparticles on the Gas Permeation Properties of Substituted Polyacetylene Membranes

Summary The gas permeability of three substituted polyacetylenes, poly(1-chloro-2-phenylacetylene) (PClPA), poly[1-phenyl-2-(4-trimethylsilyl)phenylacetylene] (PTMSDPA), and poly[1-(trimethylsilyl)-1-propyne] (PTMSP), increased systematically with increasing content of nonporous fumed silica (FS) nanoparticles. For instance, the oxygen permeability coefficient (PO₂) of PClPA containing 30 wt % FS was 86 barrers, which was 10 times higher than that of the unfilled polymer (PO₂=8.6 barrers). The extent of permeability increase with the addition of FS was smaller when the permeability of the original polymer was higher. The order of the permeability increase in FS-filled polymers was as follows: PClPA > PTMSDPA > PTMSP. The addition of FS resulted in the decrease of O₂/N₂ permselectivity of these polymers. The H₂/CH₄ permselectivity largely decreased with increasing FS content in PClPA, while it hardly changed with FS loading in PTMSP. The gas solubility of PClPA was practically independent of FS content, and the increase in gas permeability in filled PClPA resulted from an increase in diffusivity with the addition of FS.     

Chemical modification of poly(substituted-acetylene). VI. Introduction of fluoroalkyl group into poly(1-trimethylsilyl-1-propyne) and the improved ethanol permselectivity at pervaporation

In order to improve the separation characteristics of membranes for pervaporation, the introduction of fluoroalkyl groups into poly(1-trimethylsilyl-1-propyne) (PTMSP) was achieved by two methods. First, 3,3,3-trifluoropropyldimethylsilylated PTMSP was prepared via metalation of PTMSP followed by treating with 3,3,3-trifluoropropyldimethylchlorosilane. About 6 mol % of 3,3,3-trifluoropropyldimethylsilyl group was introduced by the polymer reaction. Second, the copolymerizations of 1-trimethylsilyl-1-propyne (TMSP) with 1- (3,3,3-trifluoropropyldimethylsilyl)-1-propyne (FPDSP) or 1- (1H,1H,2H,2H-tridecafluorooctyldimethylsilyl)-1-propyne (FODSP) were carried out to afford TMSP/FPDSP or TMSP/FODSP random copolymers. The ratio of TMSP monomer unit and the comonomer unit, x/y, was in the range of 99/1-85/15. All the chemically modified PTMSP membranes showed ethanol permselectivity for pervaporation of aqueous ethanol solution. In particular, the introduction of less than about 5 mol % of fluoroalkylsilylated units into PTMSP effectively enhanced the selectivity. However, excess introduction of FODSP comonomer unit caused a decrease of the selectivity, with the value being smaller than that of PTMSP membrane. Furthermore, tetrahydrofuran, acetone, acetonitrile, dioxane, and isopropanol were efficiently separated from their dilute aqueous solutions using a TMSP/FPDSP copolymer membrane.     

Magnetic Mixed Matrix Membranes Consisting of PPO Matrix and Magnetic Filler in Gas Separation

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and magnetic neodymium powder particles MQP-14-12 have been used for the preparation of magnetic mixed matrix membranes. Permeability diffusion and sorption coefficients of O₂, N₂, and synthetic air components were estimated for homogeneous and heterogeneous membranes using the Time Lag method based on dynamic experiments in a constant pressure system. The influence of magnetic field and magnetic powder particles on the gas transport properties of MMMs was studied. The results showed that the membrane permeation properties were improved with the magnetic neodymium particle filling. It was observed that the magnetic ethylcellulose and poly(2,6-dimethyl-1,4-phenylene oxide) membranes showed higher gas permeability, while their permselectivity and solubility were rather maintained or slightly increased. The results also showed that the magnetic powder addition enhanced gas diffusivity significantly in EC and PPO membranes.     

Gas Diffusion and Solubility in Poly(organophosphazenes): Results of Molecular Simulation Studies

A combination of quantum chemistry, molecular dynamics, and Monte Carlo methods have been used to investigate gas diffusion and solubility in three isomeric poly[di(butoxyphosphazenes)] and in amorphous and crystalline states of poly[bis(2,2,2-trifluoroethoxyphosphazene)] (PTFEP). In this review of recently published studies reported from our laboratory, conclusions are reached in regards to the relationship between polymer structure and gas diffusion and sorption in poly(organophosphazenes). These conclusions also serve to validate our current understanding of the nature of gas transport in other polymers. Specifically, gas diffusivity has been shown to increase with increasing side-chain and main-chain mobility as determined from vectorial autocorrelation function analysis; however, high diffusivity is accompanied by a loss in diffusive selectivity resulting in decreasing permselectivity with increasing permeability. Simulation of crystalline supercells of PTFEP indicate that gas diffusion is unrestricted in the crystalline state as has been reported only for a few other polymers, principally poly(4-methyl-1-pentene). Gas solubility in poly(organophosphazenes) correlates well with gas condensability as measured by the Lennard–Jones potential well depth parameter, ɛ/k. Exceptions are cases where specific interactions can occur between gas molecules and the polymer chain such as is the case of CO₂ and PTFEP. High-level ab initio calculations of the interaction of CO₂ with low-molecular-weight fluoroalkanes indicate the presence of a weak quadrupole–dipole interaction. Association of CO₂ with the trifluoromethyl groups of the trifluoroethoxy side chain of PTFEP has been confirmed by radial distribution function (RDF) analysis of MD trajectories. Comparison between solubility coefficients obtained from Grand Canonical Monte Carlo (GCMC) simulations of amorphous cells with experimental values of microcrystalline PTFEP indicates that gas solubility in polyphosphazenes such as PTFEP that exhibit a mesophase/crystalline state is greatly reduced. This paper is dedicated to Prof. Harry Allcock for his scientific contributions to inorganic and organometallic polymers.     

Molecular dynamics simulation of diffusion of gases in pure and silica-filled poly(1-trimethylsilyl-1-propyne) [PTMSP]

Nanocomposites have been extensively applied, and molecular dynamics simulation techniques have been applied to study the diffusion of gases (H₂, O₂, N₂, CO₂, CH₄, n-C₄H₁₀) through pure and filled with silica particle poly(1-trimethylsilyl-1-propyne) [PTMSP]. The aim for this research is to explore and investigate the effect of silica particle on the diffusion of gases in polymer. The diffusion coefficients of gases were determined via NVT molecular dynamics simulation using the COMPASS force field up to 500 or 1000 ps simulation time. We have focused on the effect of the concentration and the size of the silica particles on diffusion coefficients of gases and the changes of free volume and translational dynamics and intermolecular energies. It has been found that the addition of silica particle to PTMSP increased the diffusion coefficients of gases by enhancing the free volume of polymer.     

Effect of the casting solvent on the free‐volume characteristics and gas permeability of poly[1‐(trimethylsilyl)‐1‐propyne] membranes

The effect of the casting solvent on the structure of poly[1‐(trimethylsilyl)‐1‐propyne] (PTMSP) membranes was investigated experimentally. The PTMSP membranes were cast from solutions of cyclohexane, toluene, and tetrahydrofuran; the membranes were characterized by the positron annihilation lifetime spectroscopy (PALS) technique and by gas‐permeation measurements of O₂, N₂, and CO₂. The decay curves from the positron annihilation lifetime spectroscopy gave the best fit when two long‐life components (τ₃ and τ₄, τ₃ < τ₄) were employed. This suggests that two types of free volume existed in the PTMSP membranes. The size and number density of τ₄, which was characteristic for PTMSP, decreased in the following order of the casting solvents: cyclohexane > toluene > tetrahydrofuran. The order was consistent with the order of gas permeability. A good correlation was observed between the permeability and the structural parameter that denoted the free‐volume size and the number density of τ₄. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 497–501, 2003