Alberto Tena

Poly(ether–amide) vs. poly(ether–imide) copolymers for post-combustion membrane separation processes

This work is focused on the comparison between the commercial polyamide PEBAX® MH 1657 and a new set of synthetized polyimides with different polyethylene glycol lengths. The samples were synthesized with the same poly(ethylene oxide) (PEO) content (57 wt%) for comparison with the commercial polymer. All polymers have been characterized by several techniques revealing a direct relationship between crystallinity, PEO length and permeability properties. Results at temperatures lower than the Tm of the polyether blocks confirm that lower PEO crystallinity corresponds to higher permeability. At temperatures higher than the Tm of the PEO block, no significant differences were found between the commercial polyamides and the synthesized polyimides. This confirms that the aliphatic phase controls the separation while the hard block provides mechanical strength. Remarkable are the results for the CO2/N2 separation. These new copolyimides are promising materials for post-combustion processes.

Claisen thermally rearranged (CTR) polymers.

Second generation of thermally rearranged polymers presents low temperatures for a complete rearrangement. Thermally rearranged (TR) polymers, which are considered the next-generation of membrane materials because of their excellent transport properties and high thermal and chemical stability, are proven to have significant drawbacks because of the high temperature required for the rearrangement and low degree of conversion during this process. We demonstrate that using a [3,3]-sigmatropic rearrangement, the temperature required for the rearrangement of a solid glassy polymer was reduced by 200°C. Conversions of functionalized polyimide to polybenzoxazole of more than 97% were achieved. These highly mechanically stable polymers were almost five times more permeable and had more than two times higher degrees of conversion than the reference polymer treated under the same conditions. Properties of these second-generation TR polymers provide the possibility of preparing efficient polymer membranes in a form of, for example, thin-film composite membranes for various gas and liquid membrane separation applications.

Polymeric films based on blends of 6FDA–6FpDA polyimide plus several copolyfluorenes for CO2 separation

Three emitting copolyfluorenes, based on 2,7-(9,9-dihexyl)fluorene and different aryl groups (1,4-bencene, PFH-B; 1,4-bencen-1,2,5-thiadiazole PFH-BT; 1,4-naphthalen-1,2,5-thiadiazole, PFH-NT), showing diverse acceptor character, in different proportions were blended with a polyimide 6FDA–6FpDA to make a series of films. These copolyfluorene–polyimide blends were prepared and characterized in the solid state, using several techniques. The fluorescence of conjugated polymers can be used as a tool to understand the formation of the membrane and also to increase permeability and selectivity in comparison to films which do not fluoresce. The relationship between the intrinsic fluorescence of conjugated polyfluorenes and their gas separation properties has been explored in order to establish the influence of the composition and the nature of the aryl group in the conjugated polymer, on the gas separation performance. In all cases, a low proportion of copolyfluorene (<1% weight) gives better CO2/CH4 permselectivity properties than the original pure polyimide matrix. The best results were found for the samples that contain PFH-NT. These samples gave over 23% increase in the CO2 permeability with a 15% increase in CO2/CH4 selectivity. Finally, the loss of efficiency in conjugation mechanisms of absorption and emission of the samples could be explained on the basis of the π-stacking of the polymer chains, produced when a certain low percentage of conjugated polymers in the blend is surpassed. When this π-stacking starts, the gas permeation properties start to decline too.

Gas Separation Properties of Polyimide Thin Films on Ceramic Supports for High Temperature Applications

Novel selective ceramic-supported thin polyimide films produced in a single dip coating step are proposed for membrane applications at elevated temperatures. Layers of the polyimides P84®, Matrimid 5218®, and 6FDA-6FpDA were successfully deposited onto porous alumina supports. In order to tackle the poor compatibility between ceramic support and polymer, and to get defect-free thin films, the effect of the viscosity of the polymer solution was studied, giving the entanglement concentration (C*) for each polymer. The C* values were 3.09 wt. % for the 6FDA-6FpDA, 3.52 wt. % for Matrimid®, and 4.30 wt. % for P84®. A minimum polymer solution concentration necessary for defect-free film formation was found for each polymer, with the inverse order to the intrinsic viscosities (P84® ≥ Matrimid® >> 6FDA-6FpDA). The effect of the temperature on the permeance of prepared membranes was studied for H2, CH4, N2, O2, and CO2. As expected, activation energy of permeance for hydrogen was higher than for CO2, resulting in H2/CO2 selectivity increase with temperature. More densely packed polymers lead to materials that are more selective at elevated temperatures.

Thermal rearrangement of ortho-allyloxypolyimide membranes and the effect of the degree of functionalization

Aromatic polyimides containing an ortho-allyloxy group with different ratios (in the range of 10–100%) of the ortho-allyloxy to ortho-hydroxy units were synthesized. The allyl ether synthesis was done via a post-polymerization Williamson etherification reaction. Thermally induced Claisen-rearrangement of the allyloxy-phenyl unit was conducted in the solid-state, followed by isothermal treatments at 250 °C leading to a crosslinked ortho-hydroxy containing polyimide. Further thermal annealing at 350 °C was employed to achieve a high imide-to-benzoxazole conversion, commonly described as the thermal rearrangement process (TR). The influence of the degree of modification on the crosslinking reaction as well as the imide-to-benzoxazole conversion temperature and the rate were studied by means of TG-FTIR, DSC and dielectric spectroscopy. A nearly linear change of the material properties, such as film density, d-spacing and gel-fraction, with an increasing number of allylated units was observed. Additionally, an incline of the permeability, due to an increase of the free volume elements, was observed. Moreover, the polymer chain mobility in terms of relaxation times was demonstrated to depend on the degree of allylation, which in turn led to a reduction of the TR temperature of about 80 °C compared to the pristine polyimide. The thermally induced imide-to-benzoxazole rearrangement occurred already to a large extent of 77% at 350 °C. In comparison, the pristine polymer showed only a conversion of 20%. Furthermore, the observed HPI-to-PBO conversions at 350 °C surpassed those of various other reported TR polyimides treated at even higher temperatures of 400 to 450 °C. Side-reactions and degradation that usually accompany treatments at 400 °C and above might be avoided at lower treatment temperatures of 350 °C.

Novel functionalized polyamides prone to undergo thermal Claisen rearrangement in the solid state

A new way to obtain functionalized polymers by 3,3′-sigmatropic Claisen rearrangement has been developed and identified. The process was identified for three polyamides and three allyl-derivatives with different functional groups and rigidity. The Claisen rearrangement process seems to be independent of the polymeric structure, and it is only affected by the allyl functionality. The polymeric structure conditioned the rate of the process. The influence of polymer chain and allyl-derivatives on a cyclodehydration process was also studied. The cyclodehydration reaction is mainly affected by the rigidity of the polymer chain; therefore, it is influenced by the corresponding alkenyl structure. This work demonstrates the great potential of this approach that allows, by a simple and well-known process, the alkene functionalization of the polymeric chain in order to create a great variety of new high-performance materials for the application of these polymers in multiple fields.