Large Fractional Free Volume Research Articles

MONTE CARLO SIMULATION OF THE SORPTION OF PURE AND MIXED ALKANES IN POLY[1-(TRIMETHYLSILYL)-1-PROPYNE

The sorption of alkanes (methane, ethane, propane, and n-butane) and alkane mixtures in poly[1-(trimethylsilyl)-1-propyne] (PTMSP) has been studied by means of Grand Canonical Monte Carlo (GCMC) simulations at low pressure. Sorption isotherms for the four alkanes in PTMSP were obtained by fixed-pressure GCMC simulation at several temperatures. The sorption isotherms followed dual-mode sorption behavior, often observed for gases and some vapors in glassy polymers; however, sorbate concentrations were consistently lower than experimental values with increasing pressure. On the other hand, solubility coefficients calculated by extrapolating the ratio, cell loading/p, to zero pressure were in good agreement with those reported from gravimetric measurements. Heats of sorption determined from plots of simulation solubility coefficients against reciprocal temperature were large and negative and, at least in the case of methane where data was available, there was good agreement with experiment. In the case of the alkane mixtures, GCMC simulation showed that the more condensable component was preferentially sorbed. The high free volume and large free-volume distribution which contribute to the high solubility of gases in PTMSP were investigated by the Voorintholt method. The large fractional free volume of 0.35 obtained from this simulation was consistent with positron annihilation measurements and with Bondi group contribution calculations. Simulation views of the free volume sites revealed a broad distribution of small and large elements.

Free Volume in Epoxide Oligomers

Free volume studies are reviewed for the oligomers of the diglycidyl ether of bisphenol-A (DGEBA) before cross-linking. The Williams-Landel-Ferry (WLF) method was used to analyse the macroscopic free volume in the DGEBA oligomers in comparison with that in the crosslinked DGEBA network. The crosslinked system has relatively large fractional free volume at the Tg. The DGEBA oligomers have bulky bisphenol-A structure. The cross-link sites in the network structure does not always provide a suitable environment for close packing of rigid molecules. The WLF type equations are applicable to the temperature dependencies of the melt viscosity, the dielectric relaxation time, and the ionic conductivity for the uncured DGEBA oligomer. These three viscoelastic properties are discussed in terms of the movements of three moving units: the chain segment, the dipole, and the ionic charge carrier. The mobility difference between the three moving units is clarified from the standpoint of the free volume concept.

Relationships between the chemical structures and the solubility, diffusivity, and permselectivity of propylene and propane in 6FDA-based polyimides

The solubility, diffusivity, and permselectivity of propylene and propane in 40 different polyimides synthesized from 2,2-bis(3,4-decarboxyphenyl)hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of the chemical structures on the physical and gas permeation properties of the 6FDA-based polyimides was studied. The solubility of propylene in an unrelaxed volume of a polymer matrix mainly contributes to the total solubility of propylene for various 6FDA-based polyimides. The diffusivity, the permeability of propylene, and the permselectivity in the propylene/propane mixed-gas system depend on the solubility of propylene. This is thought to be associated with the penetrant-induced plasticization effect. 6FDA-based polyimides, which have a high glass-transition temperature and a large fractional free volume, exhibit a high permeability with a relatively low permselectivity. Changing the number of-CH3 substituents in the phenylene linkage and changing the connectivity in the main chain are good ways of controlling the solubility of propylene and the corresponding permselectivity in the propylene/propane mixed-gas system. Some 6FDA-based polyimides restrict the solubility of propylene through the introduction of a -CONH- linkage between the phenylene linkage; the -Cl substituent in the phenylene linkage at the diamine moiety exhibits a high separation performance in the mixed-gas system. The polyimides are potentially useful membrane materials for the separation of propylene and propane in the petrochemical industry.

95/05468 Multimedia for all

This paper reports comprehensive characterization and detailed analyses of microporous structure and fundamental gas transport properties for a series of new pentiptycene-containing polyimide gas separation membranes prepared from custom-synthesized pentiptycene-based diamines and 4,4′-hexafluoroisopropylidene bisphthalic dianhydride (6FDA) to identify the molecular origins for fast and selective gas transport. Both experimental characterizations of inter-chain spacing and microporosity and molecular modeling analysis of chain conformations and rigidity suggested that rigid H-shape pentiptycene units effectively disrupted chain packing, resulting in large fractional free volume and consequent high gas permeabilities in these membranes. Atomic-level detection of free volume architecture by positron annihilation lifetime spectroscopy (PALS) analysis revealed a bimodal microcavity size distribution with cavity sizes of d4~7–8 Å and d3~3–4 Å in this series of membranes. The microcavity size and size distribution were found to be sensitively affected by the substituent groups in the pentiptycene monomer structure based on the mechanism of “partial filling” of internal molecular cavities defined by the shape of pentiptycene units. Analysis of fundamental gas transport properties in terms of diffusivity (D) and solubility (S) coefficients demonstrated that size sieving mechanism (diffusivity contribution) dominates the gas transport in these polymers and bimodal microcavity size distribution with ultrafine microporosity is responsible for the excellent H2-related gas separation performance. Superior resistance against physical aging was observed for these high-free-volume polymers, which is ascribed to the stable, configuration-induced microcavity structure constructed by the rigid pentiptycene units.

Gas transport properties of polycarbonates and polysulfones with aromatic substitutions on the bisphenol connector group

AbstractThe effect that substitution of aromatic groups on the bisphenol connector unit of bisphenol‐A based polycarbonate and polysulfone materials has on their gas transport properties was assessed. Replacement of a methyl group by a phenyl ring (bisphenol acetophenone polycarbonate, PC‐AP, and bisphenol acetophenone polysulfone, PSF‐AP) gives a small increase in permeability coefficients with similar or slightly higher selectivity for all gases compared to bisphenol‐A polycarbonate, PC, or polysulfone, PSF. Substitution of two locked phenyl rings (fluorene bisophenol polycarbonate, FBPC, and fluorene bisphenol polysulfone, FBPSF) in place of the methyl groups in the connector unit leads to permeability and solubility coeffcients that are about twice those observed for PC or PSF. Increases in permeability for the polycarbonate and polycarbonate and polysulfone materials with aromatic substitutions are related to their larger fractional free volume. FBPC and FBPSF have the largest fractional free volume and the largest permeability coefficients. Thermal measurements show that the fluorene based polycarbonate and polysulfone materials have the highest thermal and oxidative stability. Such aromatic substitutions can be useful for developing gas separation membranes to be used in harsh thermal or oxidative environments. © 1993 John Wiley & Sons, Inc.

Cavities in molecular liquids and the theory of hydrophobic solubilities.

Thermal configurational data on neat liquids are used to obtain the work of formation of hard spherical cavities of atomic size in six molecular solvents: n-hexane, n-dodecane, n-undecyl alcohol, chloroform, carbon tetrachloride, and water. These results are used to test a recent suggestion that the differences between nonaqueous solvents and liquid water in solvation of inert gases are not principally due to the hydrogen-bonded structure of liquid water but rather to the comparatively small size of the water molecule. The frequencies of occurrence of cavities in liquid water can be meaningfully distinguished from those in the organic solvents. Liquid water has a larger fractional free volume, but that free volume is distributed in smaller packets. With respect to cavity work, water is compared to a solvent of the same molecular density and composed of hard spheres of the same size as the water molecule. That comparison indicates that the hard-sphere liquid finds more ways to configure its free volume in order to accommodate an atomic solute of substantial size and thus, would be more favorable solvent for inert gases. The scaled particle model of inert gas solubility in liquid water predicts cavity works 20% below the numerical data for TIP4P water at 300 K and 1.0 g/cm3 for cavity radii near 2.0 angstroms. It is argued that the sign of this difference is just the sign that ought to be expected and that the magnitude of this difference measures structural differences between water and the directly comparable hard-sphere liquid. In conjunction with previous data, these results indicate that atomic sized cavities should be considered submacroscopic.

Molecular motions in bisphenol a polycarbonates as measured by pulsed NMR techniques. I. Homopolymers and copolymers

Molecular motions in homopolymers and copolymers based on bisphenol A polycarbonate (BAP) were investigated over a wide temperature range by means of two pulsed nuclear magnetic resonance methods: the spin-lattice relaxation time T1 and the spin-lattice relaxation time in rotating frame, T1ρ. Nuclear magnetic relaxation measurements of bisphenol A polycarbonate indicated the existence of four transitions in the solid amorphous state. This included the glass transition Tα, two transitions associated with localized motions of the main chain, Tβ and Tγ, and a transition due to side-group motion, Tδ. The results offer new evidence for the transition Tβ, which is ascribed to localized segmental motions of the polycarbonate chain. The physical properties of the polymer (high impact strength and a large fractional free volume in the glassy state) can be explained by the presence of the two secondary transitions Tβ and Tγ, which are both associated with main-chain motions. The presence of the bulky bromine on a bisphenol A unit of a polymer chain in a position ortho to the carbonate groups restricts all motions of the main chain, thus increasing Tα, Tβ, and Tγ. As a result of an increase in the secondary transition temperatures, impact strength is reduced at least 20-fold as compared with the unsubstituted material. The results for tetrabromobisphenol A polycarbonate and copolymers of bisphenol A with tetrabromobisphenol A and bisphenol A with tetrachlorobisphenol A polycarbonate confirm the idea that the impact strength of the polymer is related to the secondary transitions (T < Tg) arising from the main-chain motions. Copolymers of bisphenol A polycarbonate with tetrabromobisphenol A polycarbonate showed a single glass transition temperature whose value lay between those of the homopolymers. The results indicated multiple secondary transitions which corresponded to the transitions of the homopolymers with magnitudes proportional to the comonomer content.