Gas permeability and permselectivity of polyimides prepared from phenylenediamines with methyl substitution at the ortho position

Polyimides were prepared from diamines: 2,4,6-trimethyl-1,3-phenylenediamine (3MPDA) and 2,3,4,5-tetramethyl-1,4-phenylenediamine (4MPDA). 1,4-Bis(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), 3,3'-4,4'-diphenylsulphone tetracarboxylic dianhydride (SO(2)PDA), 3,3',4,4'-diphenylsulphide tetracarboxylic dianhydride (SPDA), pyromellitic dianhydride (PMDA), and 2,2'-bis(3,4-dicarboxyphenyl)hexa-fluoroisopropane dianhydride (6FDA) were used as dianhydride. The gas permeabilities of H-2, O-2 and N-2 through the polyimides were measured at temperatures from 30 degrees C to 90 degrees C. The results show that as methyl and trifluoromethyl substitution groups densities increase from 7.73 x 10(-3) molcm(-3) to 13.50 x 10(-3)molcm(-3), the peameability of H-2 increases 10-fold at 60% loss of permselectivity of H-2/N-2 however, the permeability of O-2 increases 20-fold at 20% loss of permselectivity of O-2/N-2. For O-2/N-2 separation, PMDA-3MPDA has similar performance to 6FDA-3MPDA and 6FDA-4MPDA; all have higher permeabilities for O-2 than normal polyimides, and the P(O-2)/alpha(O-2/N-2) trade-off relationships lie on the upper bound line for polymers. (C) 1999 Society of Chemical Industry.

Effect of crosslinking distribution on gas permeability and permselectivity of crosslinked polyimides

Crosslinking modification of polyimide, prepared from 3,3'4,4'-benzophenone tetracarboxyl dianhydride (BTDA) and 2,3,5,6-tetramethyl-1,4-diphenylenediamine (4MPDA), was performed by two methods, putting the polyimide in ambient environment for 4 months, and under UV irradiation for 2 or 8 h. The gas permeation properties of the crosslinked polyimides were investigated. The crosslinked polyimides induced by UV irradiation, in which crosslinking occurs in the surface layer, are of much higher gas permselectivity of hydrogen to nitrogen than that induced by the ambient environment, in which crosslinking takes place in the whole matrix, but their gas permeabilities for hydrogen are similar. (C) 1999 Elsevier Science Ltd. All rights reserved.

Gas permeability and permselectivity of photochemically crosslinked copolyimides

A series of copolyimides were prepared from 2,4,6-trimethyl-1,3-phenylenediamines (3MPDA), 3,3',4,4'-benzophenone tetracarboxyl dianhydride (BTDA), and pyromellitic dianhydride (PMDA). Modification of the copolyimides by ultraviolet irradiation were carried out. Gas permeabilities of H-2, O-2, and N-2 through the copolyimides and photochemically crosslinked copolyimides were measured at temperatures from 30 to 90 degrees C. The relationships between gas permeabilities and temperature are in agreement with the Arrhenius equation. The structure of photochemically crosslinked copolyimides were characterized by Fourier transform infrared and gel measurement methods. Linear relationships between both log P and E-p and the volume fraction of PMDA-3MPDA exist. Photochemically crosslinking modification result in a decrease in gas permeability and an increase in E-p and alpha(H-2/N-2) for all the copolyimides. For H-2/N-2 separation, photochemically crosslinked copolyimides are of higher gas permeabilities and permselectivities simultaneously than normal polyimides. (C) 1999 John Wiley & Sons, Inc.

GAS PERMEABILITIES AND PERMSELECTIVITY OF PHOTOCHEMICALLY CROSS-LINKED POLYIMIDES

Photosensitive polyimide BTDA-3MPDA was modified by UV irradiation. The structure of UV-irradiated polyimides was investigated by FTIR and gel fraction measurements. The results showed that longer UV exposure time resulted in a higher extent of crosslinking. The gas permeabilities of hydrogen, oxygen and nitrogen through UV-irradiated polyimides were characterized in a temperature range from 30 degrees C to 90 degrees C. Photocrosslinking resulted in a sharp decline in gas permeability for hydrogen, oxygen, and nitrogen through polyimide in the initial stage of photocrosslinking. Then, as the crosslinked benzophenone percentage amounted to 28-38% for hydrogen, 17-31% for oxygen and 3-28% for nitrogen, the gas permeabilities showed another sharp decline. Gas permselectivity increased significantly with the progress of photocrosslinking, and it can be adjusted in a wide range by controlling the extent of crosslinking. Arrhenius plots of gas permeability for hydrogen and oxygen through UV-irradiated polyimides are straight lines; for nitrogen, however, change in the slope of the straight line is observed and activation energies for hydrogen and oxygen permeation show abrupt increases when crosslinked benzophenone percentage amounts to about 30%. UV-irradiated polyimides with simultaneous high gas permeability and permselectivity make them ideal candidate materials for gas separation. (C) 1995 John Wiley & Sons, Inc.

Factors affecting the estimation of gas hydrate and free gas saturation

Attenuations of different types of gas hydrate cementation in fluid-saturated porous solids are discussed. The factors affecting estimation of gas hydrate and free gas saturation are analyzed. It is suggested that porosity of sediment, the P wave velocity model and methods of calculating elastic modulus are key factors in the estimation of gas hydrate and free gas saturations. Attenuation of gas hydrate-bearing sediment is closely related with the cementation types of gas hydrate. Negative anomalies of quality factors indicate that gas hydrate deposits away from grain as part of fluid. Positive anomalies of the quality factors indicate that gas hydrate contacts with solid and changes the elastic modulus of matrix. Low frequency velocity and high frequency velocity models are used to estimate gas hydrate and free gas saturation in the Blake Ridge area according to the well log data of the hole 995 in ODP leg 164. The gas hydrate saturation obtained by low frequency velocity is 10% similar to 20% of the pore space and free gas saturation is 0.5% similar to 1% of the pore space. The gas hydrate saturation obtained by high frequency velocity is 5% similar to 10% of the pore space and free gas saturation is 1% similar to 2% of the pore space.

Multicomponent gas-mixture measurements using an array of gas sensors and an artificial neural-network

An Electronic Nose is being jointly developed between the University of Greenwich and the Institute of Intelligent Machines to detect the gases given off from an oil filled transformer when it begins to break down. The gas sensors being used are very simple, consisting of a layer of Tin Oxide (SnO2) which is heated to approximately 640 K and the conductivity varies with the gas concentrations. Some of the shortcomings introduced by the commercial gas sensors available are being overcome by the use of an integrated array of gas sensors and the use of artificial neural networks which can be 'taught' to recognize when the gas contains several components. At present simulated results have achieved up to a 94% success rate of recognizing two component gases and future work will investigate alternative neural network configurations to maintain this success rate with practical measurements.

A comparison of the gas-blast and centrifugal-accelerator erosion testers: The influence of particle dynamics

The gas-blast and centrifugal-accelerator testers are the two most commonly used erosion testers. An experimental and analytical study was made of the effect of particle characteristics (size, shape and concentration) on particle dynamics in each of these testers. Analysis showed that in the gas-blast tester both particle velocity and the dispersion angle of the particle jet were relatively sensitive to the particle characteristics. Particle characteristics, within the ranges studied, had little influence in the centrifugal accelerator tester. Consequently, during an erosion test, the range of particle velocities and dispersion angles in the gas-blast tester ismuch wider than in the centrifugal-accelerator tester. It was concluded that the centrifugal-accelerator tester gave closer control of the important erosion test parameters and therefore more consistent erosion test measurements. However, one drawback of the centrifugal-accelerator tester is the need to account for erosion effects associated with the impact of rotating particles, an inherent feature of this tester.

The manufacture and characterisation of hot-melt extruded enteric tablets

The aim of this highly novel study was to use hot-melt extrusion technology as an alternative process to enteric coating. In so doing, oral dosage forms displaying enteric properties may be produced in a continuous, rapid process, providing significant advantages over traditional pharmaceutical coating technology. Eudragit (R) L100-55, an enteric polymer, was pre-plasticized with triethyl citrate (TEC) and citric acid and subsequently dry-mixed with 5-aminosalicylic acid, a model active pharmaceutical ingredient (API), and an optional gelling agent (PVP (R) K30 or Carbopol (R) 971P). Powder blends were hot-melt extruded as cylinders, cut into tablets and characterised using powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and dissolution testing conducted in both pH 1.2 and pH 6.8 buffers. Increasing the concentration of TEC significantly lowered the glass transition temperature (T,) of Eudragit (R) L100-55 and reduced temperatures necessary for extrusion as well as the die pressure. Moreover, citric acid (17% w/w) was shown to act as a solid-state plasticizer. HME tablets showed excellent gastro-resistance, whereas milled extrudates compressed into tablets released more than 10% w/w of the API in acidic media. Drug release from HME tablets was dependent upon the concentration of TEC, the presence of citric acid, PVP K30, and Carbopol (R) 971P in the matrix, and pH of the dissolution media. The inclusion of an optional gelling agent significantly reduced the erosion of the matrix and drug release rate at pH 6.8; however, the enteric properties of the matrix were lost due to the formation of channels within the tablet. Consequently this work is both timely and highly innovative and identifies for the first time a method of producing an enteric matrix tablet using a continuous hot-melt extrusion process.