Denis Roizard

Preparation and study of crosslinked polyurethane films to fractionate toluene–n-heptane mixtures by pervaporation

Abstract Poly (ether urethane) and poly(ester urethane) segmented copolymers (noted PEG and PCL) were prepared from aliphatic diisocyanate and ether or ester oligomer diols in one or two synthetic steps either from end capped isocyanates (route 1) or hydroxyl (route 2) functionalised oligomers. The ratio of NCO to OH groups used for the copolymerisation led to the preparation of chemically crosslinked polyurethane films. Their microphase-separated structures were characterised by DSC and the T g and T m of the soft blocks was recorded. The separation properties of the polymer films were investigated by sorption, diffusion and pervaporation (PV) experiments with toluene– n -heptane mixtures. The experiments demonstrated strong toluene selectivity for all the films carried out on the whole range of concentration. The membranes prepared according to route 1 exhibited lower sorption degrees and fluxes than the ones prepared by route 2, differences that could be easily related to the lower crosslinking density achieved by route 2; on the other hand the selectivity of both membrane types was almost the same. The diffusion coefficients at infinite dilution of both pure solvents were determined from experiments carried out in the transient state and the toluene coefficient was found to slightly exceed the n -heptane one in the less crosslinked urethane networks. The preferential pervaporation of the aromatic compound was basically shown to be due to the sorption step; nevertheless in all cases the sorption selectivity was higher than the pervaporation one which means that the diffusion step was in fact more favourable to the aliphatic component.

Experimental evidence and implications of an imperfect upstream pressure step for the time-lag technique

Abstract The time-lag technique is one of the workhorses for the study of gas permeation through dense polymers. Through the years, numerous experimental results have been accumulated based on this technique primarily for diffusion and permeability of gases, but also for sorption coefficients determination. Many refinements have been developed to the original set of equations in order to take into account more complex situations (e.g. non-constant diffusion coefficient, heterogeneous matrix, simultaneous chemical reaction within the polymer matrix …). Nevertheless, attempts to apply the time-lag technique to organic vapors are scarce and, in that case, experimental results have often shown significant discrepancies compared to other techniques. We describe in this work for the first time how the non-instantaneous upstream pressure step condition, imperative for time-lag framework build-up, can hardly be achieved when working with an organic vapor. This phenomenon can lead to major errors if a classical data treatment of the time-lag results is performed. A rational transfer analysis based on Laplace transform enables however the changing upstream boundary conditions to be taken into account. This strategy leads to a simple correction term which gives a better data consistency. The application of this strategy is illustrated by a series of experiments performed with dichloromethane, toluene and acetone vapor diffusion through a dense PDMS membrane.

Solubility and polarity parameters for assessing pervaporation and sorption properties. A critical comparison for ternary systems alcohol/ether/polyurethaneimide

This paper considers the comparative use of two semi-empirical parameters δ and ET(30) for the correlation of PV and sorption properties in complex ternary systems. First, the PV properties of various polyurethaneimides PUI were investigated for the separation of the azeotropic mixture ethanol/ethyltertiarybutylether (ETBE). A new parameter called the interaction selectivity Y was derived from the solubility parameters of the different species involved and allowed the PV selectivity to be correlated quantitatively. The separation selectivity was also successfully related to the PUI soft segment polarity parameter ET(30). The results of an analysis of the properties of various systems involving a given PUI and different alcohols and ethers in mixtures EtOH/ether or alcohol/ETBE were then reported. The liquid chemical structure was shown to strongly affect the mass transfer properties in such systems. Inaccuracy of the values of ether solubility parameters led to a relative failure of the first correlative approach for EtOH/ether mixtures. The same approach was however successful for correlating the properties obtained with different alcohol/ETBE mixtures. The use of semi-empirical polarity parameters revealed interesting structure-property relationships to describe the sorption properties of the different liquids in binary or ternary systems. The PV selectivity could be correlated in the same way providing that the permeant molecular size did not influence very significantly the diffusion process.

Polymer design for pervaporation membranes: influence of the soft segment size of block copolymers (polyurethaneimides or polyureaimides) on their pervaporation features

Abstract Three sets of new block copolymers PUI with increasing soft segment lengths were obtained by a method which allows the control of the material structure by a step-by-step polycondensation: polyetherurethaneimides (PEG-MDI-AP), polyetherureaimides (JFA ED-MDI-AP) and polyesterurethaneimides (PCL-MDI-AP) with soft block molecular weights in the range of 300–2000. At 30°C, all the PUI exhibited selectivity towards ethanol during the pervaporation of EtOH/ethyl tert -butylether mixtures close to the azeotropic composition and the PV fluxes were related to the polymer structure in a quantitative way. They were shown to vary exponentially with the soft segment weight fraction in the case of PUI polyethers whereas linear relationships described the partial and total fluxes obtained with PUI polyesters. The second part of the investigations dealt with the influence of the soft segment length on the PV features of PUI at higher temperatures. Despite one particular polymer leading to a significant deviation from the usual behaviour, the PV mass transfer of both permeants in all the PUI investigated obeys an Arrhenius type law in the temperature range of 30–60°C. The activation energies could be related to the material structure qualitatively, showing an important decrease of these parameters for the polymers with the longest soft segment.

Pervaporative transport modelling in a ternary system: ethyltertiarybutylether/ethanol/polyurethaneimide

Abstract The study deals with the analysis of diffusion and mass transfer modelling during pervaporation in a true ternary system involving a polar liquid mixture (ETBE/EtOH) and a polar block copolymer (polyurethaneimide or PUI). A survey of methods of pervaporative transfer modelling in ternary systems is first developed. From differential permeation experiments carried out with both pure liquids, it appears that both permeants obey a Fickian law. Moreover, the diffusional behaviour is consistent with Longs model, which has thus been assumed for the related ternary system. An extension of the Bruns model is then derived, which takes into account the diffusion coupling as well the significant deviation from sorption ideality. From a practical point of view, the calculated values of fluxes show generally good agreement with the experimental results, although a small deviation occurs for mixtures of low ethanol content. Diffusion coefficients of both pure solvents corresponding to transient or steady state are compared. A very good agreement is found for the aprotic permeant (ETBE). whereas the diffusion coefficient of ethanol in transient state is only the quarter of the value corresponding to steady state. The results are discussed in comparison with related investigations in the literature, involving specific liquid-polymer interactions.

Synthesis, characterization and transport properties of a new siloxane-phosphazene copolymer. Extraction of n-butanol from water by pervaporation

A new type of siloxane-phosphazene copolymer system has successfully been synthesised via hydrosilylation of a high molecular weight polyaryloxyphosphazene, with oligo-H-polysiloxanes of different molecular weight, allowing the preparation of different materials having an increasing PDMS content up to 80 wt%. The starting hydrogeno siloxanes used were quantitatively grafted and led to cross-linked structures characterized as a microphase-separated copolymer system. Thin films of good mechanical properties up to 60 wt% of PDMS were prepared. The transport properties of the films for the permeants n-butanol, water and their saturated mixture have been determined by sorption, desorption and pervaporation measurements. It was established that all the tested materials have a stronger affinity for butanol than for water and that the difference in transport properties in the series is mainly based on the type of continuous phase. Up to 40 wt% of PDMS, the continuous phase consisted of poly(phosphazene) having a lower diffusion ability, whereas for higher values, the continuous matrix turns to PDMS phase which leads to a considerable permeability increase. For a PDMS content of 60 wt%, the selectivity reached is similar to that of polydimethylsiloxane itself.

1-Methyl-2-Pyrrolidinone : A Well-Adapted Solvent in the Benzothiazoles Synthesis

Abstract We describe the synthesis of a series of 2-aryl and 2-alkyl benzothiazoles with high yields using 1-Methyl-2-pyrrolidinone as reaction solvent.

Alcohol/ether separation by pervaporation. High performance membrane design

Several routes were investigated to design high performance membranes for the separation of tert-butyl ethers (octane enhancers) from alcohols by pervaporation. These routes aim at incorporating Lewis base groups into good film-forming polymers with different structures. The Lewis base groups showed a high affinity to alcohols in screening tests, thus imparting high pervaporation selectivity to the polymer materials. They led to several membranes able to extract pure ethanol out of the azeotropic mixture, but with very low permeation rates. Further modifications of the polymer structure allowed us to synthesize materials with greatly enhanced transfer rates and with acceptable selectivity for industrial applications. Structure–property relationships were derived from sorption and pervaporation data for a qualitative prediction of the effect of polymer structure on the flux and selectivity. For these solvent–polymer systems the diffusion phenomenon appears to further improve the pervaporation selectivity f...

Synthesis of polysiloxane-imide membranes — application to the extraction of organics from water mixtures

Abstract The synthesis of new copolysiloxane-imide polymers was carried out from a,w-dimethylsiloxane oligomers and aromatic di-anhydrides (PMDA and 6FDA) to obtain soluble siloxane-imide based polymers (PSI) endowed both with organophile properties and good physical membrane properties. Synthesized PSI block copolymers had high weight contents of siloxane residue varying from 70 to 94 wt.%; their pervaporation properties were studied towards ethanol-water feed mixtures (EtOH: 10 and 50wt.%) at 40 °C. The two block copolymer series-fluorinated and not fluorinated — were both highly selective for ethanol removal from water. The registered pervaporation properties demonstrated that the membrane flux and the selectivity could be tailored with the polymer microstructures; in particular, much higher permeabilities were obtained with the fluorinated series than with the PDMA series whereas the alcohol selectivity was kept constant or slightly increased. These results showed that fluorinated PSI materials are promising candidates for the extraction of organic compounds from water mixtures.

Contribution of sorption to global mass transfer during pervaporation of ethyl tert-butyl ether–ethanol mixtures through a polyurethaneimide film

The pervaporation of ethyl tert-butyl ether–ethanol mixtures through a polyurethaneimide block-copolymer film has been investigated over the entire composition range. Experiments revealed the system to be unstable at a critical feed mixture, inducing strong flux oscillations. Pervaporation selectivity towards ethanol is higher than that of the sorption method. The features of sorption were examined in relation to the corresponding pervaporation properties and analysed in terms of the activity of each penetrant to take account of the system non-ideality. Original sorption isotherms could be expressed in terms of a single linear relationship for ethyl tert-butyl ether and two linear relationship in the case of ethanol. The discontinuity in the EtOH isotherm corresponds to the composition of the critical feed mixture and was ascribed to the complete monosolvation of the polymers most basic sites, i.e. urethane groups, by the protic penetrant.